advanced drying technologies.pdf
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Second Edition
AdvancedDryingTechnologies
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© 2009 by Taylor & Francis Group, LLC
CRC Press is an imprint of theTaylor & Francis Group, an informa business
Boca Raton London New York
Tadeusz KudraArun S. Mujumdar
Second Edition
AdvancedDryingTechnologies
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© 2009 by Taylor & Francis Group, LLC
CRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742
© 2009 by Taylor & Francis Group, LLCCRC Press is an imprint of Taylor & Francis Group, an Informa business
No claim to original U.S. Government worksPrinted in the United States of America on acid-free paper10 9 8 7 6 5 4 3 2 1
International Standard Book Number-13: 978-1-4200-7387-4 (Hardcover)
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Library of Congress Cataloging-in-Publication Data
Kudra, Tadeusz, 1945-Advanced drying technologies / authors, Tadeusz Kudra, Arun S. Mujumdar.
-- 2nd ed.p. cm.
“A CRC title.”Includes bibliographical references and index.ISBN 978-1-4200-7387-4 (hardcover : alk. paper)1. Drying. I. Mujumdar, A. S. II. Title.
TP363.K755 2009660’.28426--dc22 2008052002
Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.com
and the CRC Press Web site athttp://www.crcpress.com
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© 2009 by Taylor & Francis Group, LLC
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Contents
Preface to the First Edition ...............................................................................xiPreface to the Second Edition....................................................................... xiiiAuthors ..............................................................................................................xv
Part I General Discussion: Conventional andNovel Drying Concepts 1
1 Need for Advanced Drying Technologies ............................................ 31.1 Why New Drying Technologies? .................................................... 31.2 1.3 Multistage Dryers .............................................................................. 7References .................................................................................................... 9
2versus Novel Technologies .................................................................... 11References .................................................................................................. 17
3 Innovation and Trends in Drying Technologies ............................... 193.1 Introduction ...................................................................................... 193.2 Innovation: Types and Common Features................................... 193.3 Development of Improved Drying Technologies ....................... 233.4 Trends in Drying Research and Development............................ 24References .................................................................................................. 26
Part II Selected Advanced Drying Technologies 27
4 Drying on Inert Particles ....................................................................... 294.1 Introduction ...................................................................................... 294.2 Drying Mechanism and Process Considerations ....................... 294.3 Modeling and Scale-Up .................................................................. 384.4 Selected Dryers with Inert Particles ............................................. 42References .................................................................................................. 51
5 Impinging Stream Drying ..................................................................... 555.1 Basic Features ................................................................................... 55
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Intensification of Drying Rates ........................................................ 4
Classification and Selection Criteria: Conventional
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5.2 Hydrodynamics and Heat Transfer .............................................. 625.2.1 Penetration Depth and Oscillation Time ......................... 635.2.2 Pressure Drop ...................................................................... 655.2.3 Heat Transfer........................................................................ 66
5.3 Drying ............................................................................................... 675.4 Some Recent Studies on ISD .......................................................... 71References .................................................................................................. 73
6 Drying in Pulsed Fluid Beds................................................................. 77References .................................................................................................. 87
7 Superheated Steam Drying ................................................................... 897.1 Introduction ...................................................................................... 897.2 Basic Principles of Superheated Steam Drying ........................... 91
7.4 Quality Considerations ................................................................... 947.5 Superheated Steam Drying of Selected Products ....................... 97
7.5.1 Drying of Sludges................................................................ 977.5.2 Drying of Coal ..................................................................... 987.5.3 Drying of Beet Pulp .......................................................... 1017.5.4 Drying of Lumber ............................................................. 1037.5.5 Drying of Pulp................................................................... 1067.5.6 Drying of Peat .................................................................... 1087.5.7 Drying of Paper and Tissue ..............................................1117.5.8 Drying of Wood Particles and Wood Wafers ................ 1127.5.9 Low-Pressure Drying with Superheated Steam ............1137.5.10 Miscellaneous Applications ..............................................113
7.6 Utilization of Exhaust Steam ........................................................1147.7 Closing Remarks .............................................................................117References .................................................................................................118
8 Airless Drying ........................................................................................ 123References ................................................................................................ 127
9 Drying in Mobilized Beds ................................................................... 129References ................................................................................................ 140
10 Drying with Shock Waves ................................................................... 143Appendix: Process Calculations and Equipment Design ................. 153References ................................................................................................ 159
11 Vacu Jet Drying System.........................................................................161References .................................................................................................167
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7.3 Classification and Selection of Superheated Steam Dryers ....... 93
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12 Contact-Sorption Drying ..................................................................... 16912.1 General Characteristics ............................................................... 16912.2 Mechanism of Contact-Sorption Drying .................................. 17112.3 Characteristics of Sorbents/Carriers ......................................... 17912.4 Technology of Contact-Sorption Drying .................................. 184
12.4.1 Drying of Particulate Materials onInert Sorbent .................................................................... 184
12.4.2 Drying in a Fluid Bed of Inert Sorbent........................ 18512.4.3 Sorption Drying Using Filler Materials ....................... 18712.4.4 Contact-Sorption Freeze-Drying .................................. 194
References ................................................................................................ 197
13 Sonic Drying ........................................................................................... 19913.1 Basic Characteristics of Sound ................................................... 19913.2 Sound Generation ........................................................................ 20313.3 Mechanism of Sonic Drying ....................................................... 20513.4 Drying Kinetics ............................................................................ 21013.5 Sound-Assisted Dryers................................................................ 215References ................................................................................................ 221
14 Pulse Combustion Drying ................................................................... 22514.1 Principle of Pulse Combustion ................................................... 22514.2 Pulse Combustors: Design and Operation ............................... 229
14.2.1 Basic Types of Pulse Combustors ................................. 22914.2.1.1 Quarter-Wave (Schmidt) Pulse
Combustor ........................................................ 22914.2.1.2 Helmholtz Pulse Combustor ......................... 23014.2.1.3 Rijke-Type Pulse Combustor ......................... 230
14.2.2 Valve and Valveless Pulse Combustors ....................... 23014.2.2.1 Flapper and Reed-Type Valves ...................... 23114.2.2.2 Rotary Valves ................................................... 23214.2.2.3 Aerodynamic Valves....................................... 234
14.3 Pulse Combustion in Drying ..................................................... 23614.4 Numerical Simulation of Pulse Combustion Drying ............. 250References ................................................................................................ 258
15 Heat Pump Drying ................................................................................ 26315.1 Introduction .................................................................................. 26315.2 Principle of Heat Pump Operation ............................................ 26315.3 Low-Temperature Heat Pump Drying ...................................... 27215.4 Chemical Heat Pump Drying..................................................... 27815.5 New Developments and Trends in Heat Pump Drying ......... 286References ................................................................................................ 288
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16 Fry-Drying .............................................................................................. 293References ................................................................................................ 297
Part III Selected Techniques for Dryingand Dewatering 299
17 Mechanical Thermal Expression ........................................................ 301References ................................................................................................ 306
18 Displacement Drying ............................................................................ 307References .................................................................................................314
19 Vapor Drying .......................................................................................... 315References ................................................................................................ 319
20 Slush Drying .......................................................................................... 321References ................................................................................................ 325
21 Atmospheric Freeze-Drying ................................................................ 327References ................................................................................................ 335
22 Spray-Freeze-Drying............................................................................. 337References ................................................................................................ 341
23 Refractance Window ............................................................................. 343References ................................................................................................ 346
24References ................................................................................................ 350
Part IV Hybrid Drying Technologies 351
25 Microwave–Convective Drying with Cogeneration ...................... 353References ................................................................................................ 362
26 Microwave–Vacuum Drying ............................................................... 363References ................................................................................................ 369
27 Filtermat Drying .................................................................................... 371References ................................................................................................ 373
28 Spray-Fluid Bed–Vibrated Fluid Bed Dryer..................................... 375Reference .................................................................................................. 377
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Carver-Greenfield Process................................................................... 347
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29 Combined Filtration and Drying ....................................................... 37929.1 Introduction .................................................................................. 37929.2 Pressure Filtration and Drying .................................................. 38129.3 Centrifuge Dryer .......................................................................... 38429.4 Vapor Pressure Dewatering........................................................ 38529.5 Steam Pressure Filtration ............................................................ 39029.6 Jet Stream Centrifugation ........................................................... 39229.7 Microwave-Assisted Filter Drying ............................................ 393References ................................................................................................ 395
30 Radio-Frequency Drying with 50 Ω Technology............................ 397References ................................................................................................ 405
31 Radio Frequency–Assisted Heat Pump Drying .............................. 407References ................................................................................................ 412
32 Radio Frequency–Vacuum Drying ..................................................... 413References ................................................................................................ 417
33 Miscellaneous Hybrid Technologies ................................................. 41933.1 Combined Microwave and Superheated Steam Drying ........ 41933.2 Combined Infrared and Convection Drying ........................... 42033.3 Infrared Heat Pump Drying ....................................................... 42033.4 Heat Pump Superheated Steam Drying ................................... 42133.5 Infrared–Microwave Freeze-Drying ......................................... 422References ................................................................................................ 423
Part V Other Techniques 425
34 Special Drying Technologies .............................................................. 42734.1 Drying by Alternating High- and Low-Pressure
Operations ..................................................................................... 42734.2 Through-Air Rotary Dryer ......................................................... 42934.3 Drying with Bed Mixing ............................................................. 43434.4 Drying at Ambient Air Temperature ........................................ 43834.5 Vacuum-Superheated Steam Drying and Granulation .......... 44134.6 Drying in Rotating-Jet Spouted Bed .......................................... 44534.7 Drying with Sol–Gel Transformations........................................ 44834.8 Supercritical Fluid Technology to Produce Dry Particles...... 452References ................................................................................................ 453
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Preface to the First Edition
Drying is a ubiquitous operation found in almost all industrial sectors, ranging from agriculture to pharmaceuticals. It is arguably the oldest, most common, most diverse, and most energy-intensive unit operation—coincidentally it is also one of the least understood at the microscopic level. Drying technology is an amalgamation of transport phenomena and material science as it deals not only with the removal of a liquid to produce a solid product but also with the extent to which the dry product meets the necessary quality criteria.
About two decades ago, developments in drying occurred at a remark-
of progress. Spurred by the energy crisis, consumer demand for better quality, and initiation of the biennial International Drying Symposium series, advances on both the fundamental and applied fronts began by leaps and bounds. Literally, thousands of technical papers of archival interest were published and made widely available. This had a synergistic effect on promoting further advances in the truly inter- and multidisci-
This book is a direct outcome of the phenomenal growth in drying lit-erature as well as in new drying hardware. It is now virtually impossible for academic and industry personnel to keep abreast of the developments and evaluate them logically. Therefore, the main objective of this book is to provide an evaluative overview of the new and emerging technol-ogies in drying, which are not readily accessible through conventional literature. We have attempted to provide a glimpse of the developments that have taken place in the past two decades and the directions toward which we see these technologies heading. We have included some well-established new technologies that are already commercialized, such as
bed dryers, and laboratory curiosities such as the displacement drying of wood (displacing water with the more volatile alcohol). We hope that some of the laboratory curiosities of today will lead to truly revolutionary
evaluation of current technologies will hopefully lead to new ideas.
coin. It is important to know what drives innovative ideas to the market-place. Here, we have also tried to look at the process of innovation and compare the innovative technologies with the more conventional ones, noting that novelty per se is not the goal of innovation.
As can be seen readily from a cursory look at this book’s Contents, we have included dryers for all types of materials—from slurries and
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ably slow pace. Indeed, one wondered if the field showed any visible signs
plinary field of drying technology.
novel drying technologies in the future; a systematic classification and
the superheated steam drying of pulp in flash or pressurized fluidized
Innovation and knowledge are often called the flip sides of the same
© 2009 by Taylor & Francis Group, LLC
xii Preface to the First Edition
suspensions to continuous sheets such as paper and textiles. We have covered from low-tech, low-value products such as waste sludge to high-tech advanced materials, biotechnology products, and ceramics. We have included production rates that range from fractions of a kilogram per hour (some pharmaceuticals) to tens of tons per hour (paper, milk, etc.). Furthermore, we have dealt with drying processes that are completed in a fraction of a second (e.g., tissue paper) to several months (certain species of wood in large-dimension pieces). Thus, the scope is broad and, as the
Finally, no new technology will see the light of the day without being appropriately supported by research and development (R&D). We have therefore tried to identify loopholes in our current knowledge regarding drying and dryers, which will provide new challenges to the new genera-tion of academic and industrial researchers, eventually leading to better drying technologies.
Dr. Tadeusz KudraDr. Arun S. Mujumdar
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reader will find out, the range of innovations is truly breathtaking.
© 2009 by Taylor & Francis Group, LLC
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Preface to the Second Edition
As noted in the “Preface to the First Edition” of this book, drying is a ubiquitous operation found in almost all industrial sectors ranging from agriculture to pharmaceuticals. Drying technology involves the coupling of transport phenomena and material science, as it deals not only with the removal of liquid to produce a solid product, but also with the devel-opment of necessary quality criteria in the dried product. Often, what is optimal for heat and mass-transfer rates is not appropriate for drying wet material. Selection of optimal dryers or, more appropriately, drying systems, is a complex task because of the diverse physical and chemical characteristics of both the wet material and the dry product; possibilities of heat supply by convection, conduction, radiation; and radiation includ-
well as the quality, costs, energy, and environmental constraints. Several hundred types of dryers have been examined in the literature; thus, select-ing the right dryer is very challenging indeed.
The rising global population, combined with their aspirations for enhanced standards of living, will continue to place very high demands on
all its operations. As a particularly energy-intensive unit operation, dry-ing technologies can be expected to be re-examined in the years to come. Because most dryers still use fossil fuels, there is a serious environmen-tal impact of very large-scale drying operations. The rapidly escalating energy costs and the potential for energy shortages provide incentives for increased attention to use renewable energy sources such as solar, wind energy, and biomass for agricultural and industrial drying operations.
This second edition is a result of the increased industry interest in the technical literature on drying technologies. Over the past three decades, there has been an explosive growth in the technical literature dealing with both basic and applied aspects of thermal drying. Therefore, as with
form, an evaluative overview of the new and emerging technologies in drying, which is not readily accessible in the conventional literature such as handbooks or textbooks. Here, we have attempted to provide to the reader a glimpse at the key developments that have taken place in the past two decades. We have included some well-established relatively new technol-ogies that are already commercialized, such as the superheated steam
tory curiosities such as the displacement drying of wood where water is displaced with the more volatile alcohol. We hope that both incremental
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ing volumetric heat generation by microwave or radio frequency fields, as
energy resources. This will require industry to be more energy-efficient in
the first edition, the main objective of this book is to provide, in a capsule
drying of pulp in flash- or pressurized-fluidized bed dryers, and labora-
© 2009 by Taylor & Francis Group, LLC
xiv Preface to the Second Edition
and radical innovations in drying technologies will lead to improved drying technologies in the coming decades.
whole range of wet materials ranging from liquids through semisolids to solids. Thus, the scope of coverage is broad and, as the reader will
national industrial energy consumption attributed to industrial drying in developed nations, it is very important for industry and academia alike to seek better ways of thermal dehydration. As the quality of living in the populous nations such as China, India, Indonesia, and Brazil rises, the energy demand for agricultural and industrial drying will necessar-
This book provides a panorama of ideas that can be applied toward this effort.
We hope that this new edition of Advanced Drying Technologies will also help trigger further industry interest in supporting R&D in this important area and also in joining hands with academia to carry out cost- effective R&D in a collaborative manner. Such interaction is key to the effective and rapid transfer of technology from R&D laboratories to industrial practice.
Dr. Tadeusz KudraDr. Arun S. Mujumdar
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As was the case with the first edition, we have covered dryers for the
find out, the range of innovations is truly breathtaking. With 10–20% of
ily rise. We hope that this will also lead to more efficient drying systems.
© 2009 by Taylor & Francis Group, LLC
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Authors
Dr. Tadeusz Kudra is senior research scientist at CANMET Energy Technology Centre, Varennes, Quebec, Canada; formerly associate pro-fessor at Lodz Technical University (Lodz, Poland), adjunct professor at McGill University (Montreal, Canada), Laval University (Québec, Canada), and University of Ottawa (Ottawa, Canada). He is the author of more than 200 papers, 40 conference presentations, 28 book chapters, and 4 books including Drying: Principles, Applications and Design and Thermal Processingof Biomaterials. He is the holder of 12 patents and the recipient of numer-ous honors, including the prestigious International Drying Symposium (IDS) awards for Distinguished Professional Contribution to Drying Technology (IDS’96) and for Excellence in Drying: Transferring Fundamental Results into Practice (IDS’2004). He delivered more than 30 keynote andinvited lectures at conferences and professional courses. He is also anhonorary professor at Tianjin University of Light Industry (China) andTambov State Technical University (Russia). Dr. Kudra is a member of several international committees, advisory panels, and boards for jour-nals and conferences in the area of drying. His research has focusedon advanced drying technologies and energy aspects in drying. Dr. Kudra received his MSc (1969) and PhD (1975) from the Lodz TechnicalUniversity in Poland.
Dr. Arun S. Mujumdar is professor of mechanical engineering anddirector of the Minerals, Metals and Materials Technology Centre at theNational University of Singapore (NUS). He is also an adjunct professor atMcGill University, Montreal, Canada, where he was professor of chemicalengineering before moving to NUS in 2000. Dr. Mujumdar is well known worldwide for his extensive and impactful contributions to drying R&D,professional service to the community through the establishment of theIDS series, and for mentoring more than 45 PhD students and 30 post-doctoral fellows. He is the editor-in-chief of Drying Technology—an inter-national journal. He has edited 50 books on drying as well as on heatand mass transfer including the Handbook of Industrial Drying (Taylor &Francis/CRC Press, Boca Raton, Florida, 2007), now in its third edition.He has authored more than 360 papers and 100 book chapters and hasdelivered more than 300 conference presentations and 60 keynote andplenary lectures. Being the winner of numerous international awards,Dr. Mujumdar was recently conferred with Doctor Honoris Causa by theTechnical University of Lodz, Poland, and awarded a Platinum JubileeDistinguished Visiting Professorship by the University of Mumbai
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xvi Authors
Institute of Chemical Technology (MUICT). His research theme has
of innovation in drying technologies through mathematical modeling.Dr. Mujumdar earned his BChemEng in Chemistry with distinction from the University of Mumbai, India, and obtained his MEng and PhD degrees from McGill University, Montreal, Canada.
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focused on the development of novel dryers including intensification
© 2009 by Taylor & Francis Group, LLC
Part I
General Discussion:Conventional and Novel
Drying Concepts
This part provides a general discussion of the need for new (advanced) drying technologies, objectives of drying research and development
drying technologies, as well as some thoughts on innovation and R&D
will have to refer to the literature cited for details. The objectives of this part of the book are to provide a concise introduction to our philosophy and to assist in using the information provided here.
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(R&D), classification and selection criteria for conventional and advanced
needs. All of these topics are covered briefly; hence, the interested reader
© 2009 by Taylor & Francis Group, LLC
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1Need for Advanced Drying Technologies
1.1 Why New Drying Technologies?
Authors of a book such as this must answer a fair question. It is true that we already have scores of conventional dryers with well-established records of performance for drying most materials. However, not all of these dry-ing technologies are necessarily optimal in terms of energy consumption, quality of dried product, safety in operation, ability to control the dryer in the event of process upsets, ability to perform optimally even with large changes in throughput, ease of control, and minimal environmen-tal impact due to emissions or combustion of fossil fuels used to provide energy for drying. Most drying technologies were developed empirically over sustained periods of time, often by small vendors of drying equip-ment with little access to research and development (R&D) resources—
environmental considerations as well as quality demands were not very stringent. Indeed, many have been upgraded satisfactorily to meet the leg-islative and competitive restrictions. Perhaps most are already designed and operated at their asymptotic limit of performance. However, if for any reason we wish to exceed their current performance in a cost-effective way, we need to look for alternative technologies with a higher asymptotic
by thermodynamic constraints.The majority of novel drying technologies, which evolved through a
process of evolutionary incremental improvements, were built to offset some or all of the limitations faced in operating conventional dryers. The
new technologies may even start at a performance level below that of a conventional dryer. From this point of view, it is not a fair comparison: novel versus conventional might be like comparing apples and oranges. We urge our readers not to be judgmental at this stage and rule on novel
mance at this time, since not much effort has yet been devoted to a greater study of such technologies. Rather, they should study their potential and compare their predicted asymptotic limits of performance. Even on this scale, some of these technologies may not turn out to be commercially suc-cessful in the long run and may disappear. However, we must give new
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human or financial. They were also designed at a time when energy and
limit to performance, which is necessarily below the maximum defined
benefits are typically also incremental rather than dramatic. Some of the
dryers simply because they do not have a significantly superior perfor-
© 2009 by Taylor & Francis Group, LLC
4 Advanced Drying Technologies
30–40 years; a lifetime cost is the only way to really make a proper choice
criterion in the selection of a dryer.
whereas another may need one to look outside the conventional set of dryers. One must set the criteria for selection and then see which one suits them better and is more cost-effective. There is a cost associated with the
the full scale. Most companies shun this risk and are prepared to pay a higher cost for a conventional technology—the premium is often consid-ered an insurance premium rather than a cost.
In some cases, new drying technologies are sought simply because the current technologies have a limit in terms of the production rates possible. For example, today’s modern newsprint machine is limited by the dryer speed. One can make the wet paper sheet faster than it can be dried cost effectively on the current multicylinder dryers. For higher speeds, entirely new drying concepts are being evaluated. This is a complex and expen-sive task in view of the very high investment and operating costs required at the mill scale.
In the following sections, we will review two evolutionary types of
rates and multistaging of convective dryers.
It is obvious that reduction of the size of the dryer will lead to a reduction in the initial capital cost. Although this should not be a deciding factor in the selection of an individual dryer, as only 10–15% of the life-cycle cost of a direct dryer is typically due to the initial capital cost of the drying system, it is still an important consideration as it can reduce the space requirement, duct sizes, size of ancillary equipment, etc., as well. One must intensify the drying rates without adversely affecting product qual-ity to make the equipment smaller.
Reduction of capital and operating costs of dryers clearly depends on the feasibility to enhance drying rates within the limits of product quality requirements. Higher drying rates translate into smaller physical size of the dryer as well as the associated ancillary equipment. Generally, it is also
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ideas a chance—some of them definitely will emerge as victors and those
between conventional and new dryers. Novelty should not be the chief
choosing them will be the beneficiaries. Note that dryers have a lifetime of
A conventional dryer may be admirably suited for a specific application,
risk accompanying the technology not verified at the pilot scale and even
advances in drying technologies, specifically the intensification of drying
1.2 Intensification of Drying Rates
intensified heat and mass transfer results in high volumetric evaporation
reflected in lower running costs. An example is drying of liquid feeds in a fluidized or spouted bed of inert particles (see Chapter 4) where highly
© 2009 by Taylor & Francis Group, LLC
Need for Advanced Drying Technologies 5
to the conventional spray dryer of the same throughput.In general, the feedstock to be dried contains both surface and internal
moisture. The rate at which the surface moisture can be removed depends only on the external heat and mass transfer rates since the controlling resis-tance to drying rate lies outside the material being dried. Thus, enhanc-ing external convective heat and mass transfer rates by increasing the gas velocity and gas temperature or reducing gas humidity will lead to increased drying rates for a purely convective (or direct) dryer. Any action that enhances external (gas-side) resistance will yield an increase in the drying rate. Of course, there are exceptions, for example, intense drying may cause case-hardening and reduce drying rates or it may cause extreme shrinkage and cracking, which are undesirable phenomena. Thus, an increase of free-stream turbulence, application of mechanical vibration, or
responsible for the augmentation are different (see Chapter 13).Above a critical temperature, commonly termed the inversion tempera-
ture, the rate of evaporation of the surface moisture is higher in super-heated steam (SHS) drying than in hot-air drying (see Chapter 7). This is due to the superior thermal properties of SHS. At lower temperatures, the reduced temperature difference between the drying medium and the dry-ing surface for SHS results in a lower drying rate for the latter. In purely convective air-drying, the surface temperature is equal to the wet bulb temperature corresponding to the air humidity and dry bulb tempera-ture, whereas for SHS drying, it is the saturation temperature of steam, that is, 100°C for atmospheric pressure.
Enhancement of the falling rate period of drying, which requires faster
achieve. In general, attempts to do so result in a change in product qual-
pulsation resulting in cavitation; the successive generation of high- and
(e.g., microwave [MW] or radio-frequency radiation) can heat up volu-metrically the polar liquid to be vaporized (e.g., water). This practically eliminates the resistance to transfer heat into the material; the transport of moisture out through the material is also enhanced to some extent due to the higher mobility of moisture at higher temperatures as well as due to internal pressure gradient toward the material surface. The same mecha-nism is responsible for the marginally increased drying rates observed in SHS drying.
Another possible way of intensifying the drying rate involves increas-ing the effective interfacial areas for heat and mass transfer. For example,
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rates, so that the dryer volume can be reduced significantly as compared
sonic fields is also known to increase the drying rates, but the mechanisms
transport of heat and moisture through the material, is more difficult to
ity. Application of an ultrasonic field can cause high-frequency pressure
low-pressure fields causes rapid vaporization and enhanced transport of the liquid through the material. The use of an electromagnetic field
in an impinging stream configuration, the impingement zone generated
oscillation of flow yields higher drying rates. Application of ultrasonic or
© 2009 by Taylor & Francis Group, LLC
6 Advanced Drying Technologies
by the collision of opposing gas–particle streams is one of high shear and high turbulence intensity (see Chapter 5). If a pasty or sludge-like material
and increase the interfacial area of drying. The drying rate is further inten-
proportional to the particle or droplet size, all other things being equal. When it is permissible, use of mechanical dispersers or mixers within the dryers results in more rapid drying.
An obvious means of intensifying drying rates is to increase the convec-
can yield very high drying rates. It may be possible to use adsorbent particles so that the heat-transfer medium can also effectively enhance the mass-transfer potential by lowering the gas humidity concurrently (see Chapter 12).
been tested. These are discussed in some detail in this book. It should be noted that not all ideas might be applicable in a given situation as most of these also result in changes in product quality. There is an increase in the complexity of the equipment, as well. A careful technoeconomic evalua-tion is necessary before one may justify use of enhancement techniques in a given application. The application areas for some of these enhancement techniques are given in Table 1.1.
TABLE 1.1
Techniques for Enhancement of Drying Rates
Drying Period
Constant Rate Only Both Falling Rate Period
Enhance free-stream turbulence Increase interfacial area forheat and mass transfer
Apply oscillation, vibration Dielectric heating Dielectric heatingTwo-phase (gas–particle) drying medium
Superheated steam drying Electrokinetic phenomena
pressure levelSynergistic effects
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is dispersed in it, the turbulence field tends to de-agglomerate the lumps
sified by the fact that the heat and mass transfer rates are nearly inversely
rate severalfold while removing surface moisture. A gas–solid suspen-
drying (SD), recirculation of fines can result in better drying rates.
immersion drying (e.g., mixing hot inert particles with wet particles)
Finally, as particle-to-particle heat transfer is more efficient (provided sufficient contact area) than between a gas and particles, the use of
Most of the drying-rate intensification concepts mentioned here have
Apply ultrasonic field
Acoustic field of high sound
tive heat/mass transfer rate when feasible. Use of impinging flow configu-ration rather than a parallel flow configuration can increase the evaporation
sion flow yields higher heat-transfer rate than a single-phase gas flow. For
and geometric parameters as well as particle loading in the gas. In spray
impinging gas–particle flows, the heat-transfer rate is two to three times higher than for gas flow alone; the enhancement ratio depends on the flow
© 2009 by Taylor & Francis Group, LLC
Need for Advanced Drying Technologies 7
1.3 Multistage Dryers
If a material has both surface and internal moistures, that is, both the so-called constant and falling rate periods exist in batch drying, it is logical to believe that for optimal drying, the drying conditions, and even the type of a dryer in some cases, should be different to remove these two distinctively different types of moisture. For cost reasons, often it is pref-erable to choose a single dryer to accomplish the entire drying by vary-ing the drying conditions spatially for continuous dryers and temporally for batch dryers, that is, the dryer type is the same. Zoning of the dryers
beds, continuous vibrated beds, tunnel dryers, etc., to ensure optimal dry-ing; this is especially true for heat-sensitive materials that could be dried under intense conditions only while surface moisture is being removed. In the falling rate, the drying conditions must be made less intense to ensure that the material temperature remains below the critical tempera-ture above which the material starts to deteriorate (change its color, tex-ture, activity, solubility, etc.). However, for large production rates and for certain materials, it is cost-effective to employ two different dryer types for removal of surface and internal moistures.
Removal of surface moisture is generally a more rapid process requir-ing shorter dwell time in the dryer, whereas internal moisture removal is a slower process requiring a longer dwell time and hence a larger dryer.
ers, etc. For longer residence times, one could employ through circulation,
reduce the overall cost of drying. Indeed, this is a well-established com-mercial process for drying coffee, detergents, skim milk, etc. SD is an expensive drying process requiring a very large spray chamber size if the entire drying is to be accomplished in the spray dryer alone. On the contrary, if all of the surface moisture is removed along with a small part of the internal moisture in the spray chamber, one can employ a small
For successful multistage drying, it is important that the wet feed
the drying times for the two-stage dryer concept become attractive. In some
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bed (or vibrated bed) can be used to instantize (agglomerate) the fine
material has both the types of moisture in significant amounts, so that
cases, the first stage may be used simply to remove the surface moisture,
along their length is commonly used in conveyer, continuous fluidized
Dryers suited for surface moisture removal are fluid bed, flash, spray dry-
fluid bed, packed bed (or tower), continuous tray dryers, etc. Relative to spray or flash dryers, which have residence times on the order of 1–45 s, fluid bed or vibrated bed dryers have much longer dwell times. Thus, a spray dryer can be followed with a fluid or vibro-fluidized bed dryer to
fluid bed—even as an integral part of the conical bottom of the spray chamber—and the overall dryer becomes cost-effective. Indeed, the fluid
powder produced by the spray dryer. Such hybrid dryers are presented briefly elsewhere in this book.
© 2009 by Taylor & Francis Group, LLC
8 Advanced Drying Technologies
so that the product becomes nonsticky and suitable for processing in a
and hot air impingement as the second stage is used to obtain softer paper although both stages have comparable drying rates and comparable dry-ing times (in fractions of a second).
TABLE 1.2
Selected Examples of Two-Stage Drying
Stage 1 Stage 2 Advantages Applications
Spray dryert ∼ O 10 s
Fluid bed dryer t ∼ O10 min
Reduces overall sizeof dryer, hence better technoeconomics
Spray Fluidizer (Niro)
Product is granulated (instantized), if necessary
Drying of slurries, forexample, coffee, detergent, and milk
Spray dryert ∼ O 10 s t ∼ O 10 min
Reduces overall sizeof dryer, hence better technoeconomics
Drying of coffee, milk, etc.
Product is granulated (instantized), if necessary
Spray dryert ∼ O 10 s
Through circulationconveyer dryer withtemperature zoning
Drying at moderateconditions forheat-sensitive materials; high-sugar content sticky solids
Filtermat—commercialname—can handle drying of juices, forexample, orange
Flash dryert ∼ O 1–10 s
Fluid bed dryer t ∼ O10 min
Surface moisture
dryer; internalmoisture removed inlong-residence time
Fluid beddryer t ∼ O1 min
Tower/packed beddryer t ∼ O 10 h
Surface moisture
residence time obtained in a tall tower
Polymer suspension
Throughdryer t ∼ O0.1 s
Impingement dryert ∼ O 0.1 s
Through dryer helps produce a structureof tissue paper that issoft
Drying of tissue paper,exceptional applicationfor two-stage drying.The same order of residence times and drying rates in each stage
Note: O, on the order of; t, dwell time in dryer.
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paper drying, a two-stage process with through drying as the first stage conventional fluid bed, for example. In some special cases such as tissue
Vibrofluid bed dryer
removed in flash
removed fast in a
fluid bed
fluid bed—long
© 2009 by Taylor & Francis Group, LLC
Need for Advanced Drying Technologies 9
Sometimes, a long residence time is needed to accomplish some physical or chemical reactions, which are much slower than the drying kinetics, for example, crystallization of polyethylene terephthalate (PET) resin is accomplished at a tall tower, whereas the initial drying of surface moisture
process.Table 1.2 lists selected commercially viable two-stage drying tech-
nologies. Some of these technologies and three-stage dryers are covered elsewhere in this book. It is important to note that the multistage dry-ers represent nothing but an intelligent combination of well-established conventional technologies. However, such a combination usually offers unique advantages not possible with the component technologies sepa-rately (Table 1.3) (Mujumdar and Passos, 2000).
Reference
Mujumdar, A. S. and Passos, M. L. 2000. Drying: Innovative technologies and trends in research and development. In: Developments in Drying. A. S. Mujumdar and S. Suvachittanont (Eds.). Kasetsart University Press, Bangkok, Thailand, pp. 235–268.
TABLE 1.3
Multistage Drying in the Dairy Industry: Combination of Conventional Technologies
Dryer Energy Savings Powder Characteristics
One-stage: spray dryer Reference valueNonagglomerated (∼0.2 mm)Wide size distribution
Two-stage: spray dryer + ∼18%Instantized agglomerated powder
Nondusting
Three-stage: spray dryer + ∼30%Agglomerated and granulated
Narrow size distribution
Source: From Mujumdar, A. S. and Passos, M. L., Developments in Drying., Kasetsart University Press, Bangkok, Thailand, 2000, pp. 235–268.
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Significant fraction of fines
Small fines fractioninternal fluid bed
fluid bed + external fluid bedGood flowability
is done in a small fluid bed dryer in a two-stage drying–crystallization
© 2009 by Taylor & Francis Group, LLC
11
2
Conventional versus Novel Technologies
Mujumdar and Menon (1995) as well as Mujumdar (2000) provided
criteria that are important in making an appropriate selection. It is noted that one should select a drying system—including predrying and post-
as its operating conditions. More information on this subject along with detailed information on numerous conventional industrial dryers can be found in the handbook by Mujumdar (2007).
Table 2.1 summarizes the key criteria often used in classifying dryers.
Table 2.2 is a typical checklist for selection of industrial dryers. In addi-tion, the following information should be considered in specifying pos-sible dryer types for a given application.
As a minimum, the following quantitative information is necessary to arrive at a suitable dryer:
Dryer throughput; mode of feedstock production (batch/• continuous)Physical, chemical, and biochemical properties of the wet feed •
feed characteristicsUpstream and downstream processing operations•Moisture content of the feed and product• Drying kinetics, sorption isotherms• Quality parameters (physical, chemical, and biochemical)•
biohazardsValue of the product• Need for automatic control• Toxicological properties of the product•
Type and cost of fuel, cost of electricity•
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Classification and Selection Criteria:
detailed classification schemes for industrial dryers along with numerous
A finer classification is also possible, but is not relevant here.
as well as desired product specifications; expected variability in
drying equipment—that can influence the choice of the dryer itself as well
• Turndown ratio, flexibility in capacity requirements
• Safety aspects, for example, fire and explosion hazards,
© 2009 by Taylor & Francis Group, LLC
12 Advanced Drying Technologies
Environmental regulations• Space in plant•
Mujumdar (2000) presents cases where the choice of dryer is also gov-erned by the quantity produced and the quality of the dried material.
Table 2.3 compares possible types of conventional and new drying tech-nologies for various physical forms of wet materials. This list is not all-inclusive; it is given only for illustrative purposes. One can arguably place some of the dryers from the new to the conventional category, as their use becomes more commonplace.
As expected, there is a preference by industry to use conventional dryers due to their mature status and familiarity. Dryer vendors also prefer such technologies due to the low risk factor in design and scale-up. Also, the cost of developing new technologies may discourage offering quotes involving
TABLE 2.1
Criterion Types
Mode of operation BatchContinuousa
Heat input type Convection,a conduction, radiation,
heat-transfer modesIntermittent or continuousa
Adiabatic or nonadiabaticState of material in dryer Stationary
Moving, agitated, or dispersedOperating pressure Vacuuma
AtmosphericDrying medium (convection) Aira
Superheated steamFlue gases
Drying temperature Below boiling temperaturea
Above boiling temperatureBelow freezing point
Relative motion betweendrying medium and drying solids
ConcurrentCountercurrent
Number of stages Singlea
MultistageResidence time Short (<1 min)
Medium (1–60 min)Long (>60 min)
a Most common in practice.
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Classification of Dryers
electromagnetic fields, combination of
guaranteed performance. New drying technologies must offer significant advantages over the existing ones to find industrial acceptance.
Mixed flow
© 2009 by Taylor & Francis Group, LLC
13
Legislative requirements may change this picture in the future in many parts of the world. For example, the imposition of carbon tax and severe restrictions on the emission of greenhouse gases—particularly CO2—will force industry to consider superheated steam (SHS) drying where it is feasible. High fuel costs and high insurance rates reigning safe operation may also make steam drying more attractive in the future.
frames include combinations of well-known conventional technologies as noted earlier. Use of heat pumps, multistage operation, better control
TABLE 2.2
Typical Checklist for Selection of Industrial Dryers
Physical form of feed Granular, particulate, sludge, crystalline, liquid, pasty, suspension, solution, continuous sheets, planks, oddshapes (small/large)
Sticky, lumpyAverage throughput Kilograms per hour (dry/wet); continuous
Kilograms per batch (dry/wet)Expected variation in throughput (turndown ratio)
SmallHigh
Fuel choice OilGasElectricity
Predrying and postdrying operations (if any)
Preforming, backmixing, grinding, milling, screening,standardizing
For particulate feed products Mean particle sizeSize distributionParticle densityBulk densityRehydration properties
Inlet/outlet moisture content Dry basisWet basis
Chemical/biochemical/microbiological activity
ActiveInactive
Heat sensitivity Melting pointGlass transition temperature
Sorption/desorption isotherms Shape, hysteresisEquilibrium moisture content
Drying time Drying curvesEffect of process variables
Special requirements Material of constructionCorrosionToxicityNonaqueous solutionFlammability limitsFire hazardColor/texture/aroma requirements (if any)
Foot print of drying system Space availability for dryer and ancillaries
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Classification and Selection Criteria
New technologies that are likely to find acceptance over shorter time
at optimum conditions, model-based control, etc., will find—and indeed
© 2009 by Taylor & Francis Group, LLC
14 Advanced Drying Technologies
have already found—many applications. With government incentives, use of renewable energy for drying may prove cost-effective, especially if the fossil fuel costs double in the next decade as is currently projected. Indeed, use of solar energy and wind energy to offset the need for energy from fossil fuels and electricity may be worth considering even now at several geographical locations around the world.
The selection criteria for new technologies remain the same as those for conventional ones with the possible exception of risk management. With time, the risk factor will decrease and such technologies will become mainstream technologies.
each type of gas–solid contactor. With the new devices, often the data available in the literature are obtained at laboratory scale—only in a few
tain. One must objectively evaluate the potential offered by the new tech-
TABLE 2.3
Conventional versus Innovative Drying Techniques
Feed Type Dryer Type New Techniquesa
Liquid suspension Drum Fluid/spouted beds of inertparticles
SprayVacuum belt dryerPulse combustion dryers
Paste/sludge Spray Spouted bed of inert particlesDrum Fluid bed (with solids backmixing)Paddle Superheated steam dryers
Particles RotaryFlash Vibrated bedFluidized bed (hot air or combustion gas)
Ring dryer
Jet-zone dryerYamato rotary dryer
Continuous sheets(coated paper, paper,textiles)
Multicylinder contact dryers Combined impingement/radiation dryers
Impingement (air) Combined impingement and through dryers (textiles,low-basis-weight paper)
Impingement and microwave or radio frequency
a New dryers do not necessarily offer better technoeconomic performance for all products.
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Tables 2.4 and 2.5 compare the features of conventional and modified
them, one must know and compare the specific merits and demerits of
cases, it may be pilot scale. The scale-up is, therefore, difficult and uncer-
nology and, if justified, carry out a systematic pilot-scale study. Often, it
Spray/fluid bed combination
Superheated steam fluid bed dryer
Pulsated fluid bed
fluidized bed and spouted bed dryers, respectively. To choose between
© 2009 by Taylor & Francis Group, LLC
15
may be possible to scale up the heat and mass transfer characteristics.
experimental testing is therefore a necessity. One advantage of the so-called "Digital Big Bang" is that advanced mathematical models of vari-ous dryers can be used as a tool to foster innovation. Such models, once validated, can be used to evaluate novel designs and optimize operating parameters to reduce the cost as well as the risk involved in using novel drying technologies. Since we do not yet have a general drying theory and
to precise mathematical description, this approach has a long way to go
TABLE 2.4
Fluidized Bed Dryers: Conventional versus Innovative Concepts
Conventional Innovative
Convective heat transfer Convection + conduction (immersedheaters in bed)
Constant gas temperature Variable gas temperature
(vibration/agitation)Used for drying of particles Drying pastes, slurries using inert mediaAir/combustion gas as drying medium
Air drag resisted by gravitygenerated by rotation)
Multistage with different dryer types
bed
TABLE 2.5
Spouted Bed Dryers: Conventional versus Innovative Concepts
Conventional Innovative
Pneumatic spouting Mechanical spouting (screw, vibration)Single spout Multiple spouts
spoutingConstant gas temperature Variable gas temperatureDrying particles Drying pastes, slurries using inert
mediaMoving spout (rotation, oscillation)
Convective drying Combined convection and conductionAxisymmetric Two-dimensional, annular,
hexagonal, etc.
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Classification and Selection Criteria
Spatially fixed spout
However, the quality of the dried product is difficult to predict: actual
before industry can benefit from it.
Steady gas flow Pulsed gas flow
Pneumatic fluidization Mechanically assisted fluidization
Superheated steam for fluidization/drying
Centrifugal fluid beds (artificial gravity
Single-stage/multistage fluid bedsSimultaneous fluidization of entire Moving fluidization zone (pulsating
fluidized bed)
Constant gas flow/continuous Variable gas flow/pulsed gas flow
the flow conditions in most dryers are extremely complex to be amenable
© 2009 by Taylor & Francis Group, LLC
16 Advanced Drying Technologies
It is important to reemphasize that general statements regarding the superiority of otherwise different dryers, whether conventional or novel, are not possible. A dryer may be better for one product but not for another.
relative cost of fossil fuel and electricity can change the cost-effectiveness of some dryers. Hence, readers should guard against copying dryers or drying systems from one geographic location to another as their cost-ef-
requirements.Finally, Table 2.6 lists the attributes of the conventional conveyor (or
apron) dryer and compares them with some innovative concepts. Note
provide some potential advantages, but need to be tested at both laboratory and pilot scales. For a more detailed discussion, the reader is referred to Mujumdar (2000).
TABLE 2.6
Conveyor (or Apron) Dryers: Conventional versus Innovative Concepts
Conventional Innovative
Fixed layer thickness Variable layer thickness along length (between zones)
Fixed (within each zone) temperature Variable gas temperatureSuperheated steam as drying medium
zonesFix bed—no mixing along bed depth
nonhomogeneity in drying rates)Single-stage conveyor dryer
moisture followed by conveyor dryer (reduceattrition, etc.)
Continuous heating Tempering zone between heating zones(interrupted drying when internal heat/masstransfer resistance is high)
Purely convective heating Combined convective and microwave heating to reduce drying time
Atmospheric pressure Vacuum or high pressure (with steam drying)Fixed total pressure Oscillating pressure between low and
atmospheric (when convective heat issupplied)
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two fixed zones)
Operating conditions chosen affect dryer performance significantly. The
that many of the new concepts are proposed here for the first time; they do
Fixed gas flow (within each zone) Variable gas flow along length
Hot air or flue gases as drying medium
Mix or mechanically agitate bed between fixed
Unidirectional gas flow Reverse drying air flow direction between
bed zones (e.g., vibrated or fluid bed between
Air flow in bed thickness direction only Air flow in cross-flow direction between zonesof conventional axial flow (to reduce
Use of flash or fluid bed to remove surface
fectiveness is typically influenced by local conditions including legislative
© 2009 by Taylor & Francis Group, LLC
17
References
Practical Guide to Industrial Drying. S. Devahastin (Ed.). Exergex, Brossard, QC, pp. 23–36.
Mujumdar, A. S. (Ed.). 2007. Handbook of Industrial Drying. 3rd edition. CRC Press, Boca Raton, FL.
and selection of dryers. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). Vol. 1, 2nd edition. Marcel Dekker, New York, pp. 1–39.
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Classification and Selection Criteria
Mujumdar, A. S. 2000. Classification and selection of dryers. In: Mujumdar’s
Mujumdar, A. S. and Menon, A. S. 1995. Drying of solids: Principles, classification,
© 2009 by Taylor & Francis Group, LLC
19
3Innovation and Trends in DryingTechnologies
3.1 Introduction
As an operation of prehistoric origin, drying is not normally associated with innovation. As whatever products need to be dried currently are dried using existing technologies, it is often hard to justify the need for innovation and the concomitant need for research and development (R&D) in drying and
that drying is able to attract as opposed to some of the exotic bioseparation processes, which on an economic scale may be an order of magnitude less
titles of which contain the words “dryer,” “drier,” or “drying”—are issued
patents is being issued per year in some of the other key unit operations such as membrane separations, crystallization, adsorption, and distillation. A negative correlation appears to exist between the current level of indus-trial interest and the level of academic research activity, at least as measured by the number of publications in the archives of literature.
describing types of innovation, and then identifying the need for innova-tion in drying as well as the features common to some of the novel drying technologies. At the outset, it is important to recognize that novelty per se
nically superior and cost-effective compared to the current technology. In some instances, the newer technologies may offer advantages over the con-
3.2 Innovation: Types and Common Features
It is interesting to begin with Webster Dictionary’s meaning of innovation,which is as follows:
Innovation, n.• The introduction of something new• A new idea, method, or device
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significant. It is interesting to note, however, that about 250 patents—the
by the U.S. Patent Office every year. Only 10% or less of this number of U.S.
It is instructive to start this discussion with a definition of innovation,
is not adequate justification for embracing new technology; it must be tech-
ventional ones only for specific products or specific rates of production.
dewatering. This is reflected in the relatively low level of R&D resources
© 2009 by Taylor & Francis Group, LLC
20 Advanced Drying Technologies
Notice that it does not use adjectives such as “better,” “superior,” “improved,” “more cost-effective,” and “higher quality” to qualify as an innovation. In our vocabulary, however, we are not interested in innova-tion for the sake of novelty or even originality of concept but for the sake of some other positive technoeconomic attributes.
be more appropriate here: “A process that begins with an invention, pro-ceeds with development of the invention, and results in the introduction of the new product, process or service in the marketplace.”
To make it into a free marketplace, the innovation must be cost-effective. What are the motivating factors for innovation? For drying technologies, one or more of the following attributes may call for an innovative replace-ment of existing products, operations, or processes:
• New product or process not made or invented heretofore• Higher capacities than current technology permits• Better quality and quality control than currently feasible• Reduced environmental impact• Safer operation
• Lower cost (overall)
ucts, etc.
Innovation is crucial for the survival of industries with short time scales (or life cycles) of products/processes, that is, a short half-life (less than 1 year, as in the case of most electronic and computer products). For longer half-lives (e.g., 10–20 years—typical of drying technologies), innovations come slowly and are less readily accepted.
The management of innovation depends on the stage it is at. Thus,
• Initially, value comes from rapid commercialization.• Later, value comes from enhancing the product, process, or service.• At maturity, value may come from discontinuing and embracing
newer technology. It is important to recognize when a current technology is due for replacement.
Note that management must be agreeable to discontinue a currently via-ble technology in the interest of the company’s future if the technology has reached its asymptotic limit of performance. This principle applies to all technologies.
Numerous studies have appeared in the literature on the fundamental aspects of the process of innovation. One of the models of the innovation process assumes a linear progress from (a) discovery of laws of nature to
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The following definition given by Howard and Guile (1992) appears to
• Better efficiency (resulting in lower cost)
• Better control, more flexibility, ability to handle different prod-
© 2009 by Taylor & Francis Group, LLC
Innovation and Trends in Drying Technologies 21
(b) invention to (c) development of a marketable product or process, in this order. It is well known, however, that some of the truly remarkable revo-lutionary technologies evolved well before the fundamental physics or chemistry responsible for their success was worked out. True innovation is most likely to be a nonlinear—even chaotic—trial-and-error, serendipi-
although one could presumably encourage creativity or try to remove blockages in the process of creativity.
istics. A list of the quality parameters of innovations in general (Howard and Guile, 1992) is as follows:
• Innovation establishes an entirely new product category.
existence.
technology.• Innovation is a modest improvement in existing product/
process.
Innovations trigger technological changes, which may be revolutionary or evolutionary. From our experience, we know that the latter are more common. They are often based on adaptive designs, have shorter gesta-tion periods and shorter times for market acceptance, and are typically a result of “market-pull”—something the marketplace demands, that is, a need exists currently for the product or process. These usually result
tion of the dominant design is an example). Revolutionary innovations, on the contrary, are few and far between, have longer gestation periods, may have larger market resistance, and are often a result of “technology-push,” where the development of a new technology elsewhere prompts the design of a new product or process for which market demand may have to be created. They are riskier and often require larger R&D expen-ditures as well as sustained marketing efforts. The time from concept to market can be very long for some new technologies. It is well known that
copter took to the air. The idea of using superheated steam (SHS) as the drying medium was well publicized over 100 years ago, yet its real com-
not fully. In fact it is not fully understood even today. A recent example of this long gestation period is the Condebelt drying process for high-ba-sis-weight (thick grades) paperboard proposed and developed by the late Dr. Jukka Lehtinen of Valmet Oy in Finland (Lehtinen, 1998). It took a full 20 years of patient, expensive, and high-quality R&D before the process
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tous process. Therefore, it is difficult to teach innovation in a logical sense
What may be classified as innovation can represent different character-
• Innovation is the first of its type in a product category already in
• Innovation represents significant improvement in existing
from a linear model of the innovation process (an intelligent modifica-
the concept of a helicopter appeared some 500 years before the first heli-
mercial potential was first realized only about 50 years ago, and that too
© 2009 by Taylor & Francis Group, LLC
22 Advanced Drying Technologies
teams of such organizations must be truly farsighted to permit successful implementation of a revolutionary process.
It is natural to inquire whether it is possible to predict or even estimate the best time when the marketplace requires an innovative technology or the mature technology of the day is ripe for replacement. Foster’s well-known “S-curve”, shown in Figure 3.1 (Foster, 1986), which gives a sig-moid relationship between product or process performance indicators and resources devoted to develop the corresponding technology, is a valu-able tool for such tasks. When the technology matures (or is saturated in some sense), no amount of further infusion of R&D resources can enhance the performance level of that technology. When this happens (or even somewhat sooner), the time is right to look for alternate technologies—which should not be incremental improvements on the dominant design but truly new concepts—that, once developed to their full potential, will yield a performance level well above that of the current one. As proven by Foster with the help of real-world examples, the performance-versus-effort (resources) curve occurs in pairs when one technology is replaced by another. They represent discontinuity when one technology replaces another and industry moves from one S-curve onto another. As indicated in Figure 3.1, most well-established drying technologies are very close to their asymptotic performance level if they are well designed and operated under optimal conditions.
Table 3.1 lists examples of some new drying technologies that were developed through technology-push versus market-pull. In some cases, a sharp distribution of grouping in just two types is not possible because a “market-pulled” development may require a “technology-push” to succeed.
Current level(most conventional dryers)
Saturation level
Efforts (resources)
Perfo
rman
ce
FIGURE 3.1Foster’s S-curve.
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was first deployed successfully. The vision required by the management
© 2009 by Taylor & Francis Group, LLC
Innovation and Trends in Drying Technologies 23
3.3 Development of Improved Drying Technologies
tionary or revolutionary process. Most developments follow the evolu-tionary path involving incremental improvements to offset one or more of the limitations of the contemporary technology. Such technologies are more readily accepted by industry since the risk associated with the adoption of such technologies is generally minimal and the cost-
combinations of traditional technologies necessitated by changes in the marketplace.
The following list illustrates the evolutionary developments that have
be traced for most other dryer types, as well.Rotary dryer
2. Internal heaters (tubes or coils) or external heating of the shell to
3. Direct drying by air injection into the rolling bed of particles in the rotating shell through tubes connected to a central header (Yamato dryer)
TABLE 3.1
Examples of New Drying Technologies Developed through Technology-Push andMarket-Pull
Technology-Pusha Market-Pullb
Microwave/RF/induction/ultrasonic drying
Superheated steam dryers—enhanced energy
environmental impact, safety, etc.Pulse combustion drying—PC developed for propulsion and later for combustionapplications
Impulse drying/Condebelt drying of paper(also need technology-push)
Vibrating bed dryers—originally developed for conveying solids economics of spray drying
Impinging streams (opposing jets)—originally developed for mixing,combustion applications
a Technology originally developed for other applications and applied to drying.b Developed to meet current or further market demand.
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efficiency, better-quality product, reduced
Intermittent drying—enhance efficiency
New developments in any field may occur as a result of either an evolu-
to-benefit ratio is favorable. Often, the new technologies are intelligent
occurred over the past five decades in two commonly used industrial dry-
improve efficiency and capacity
Combined spray-fluid bed dryers—to improve
ers: rotary and flash dryers. Similar evolutionary development trends can
1. Purely convective, axial gas flow
© 2009 by Taylor & Francis Group, LLC
24 Advanced Drying Technologies
Flash dryer
1. Single-pass, vertical, round, insulated tube (adiabatic) 2. Single-pass, jacketed tube for increasing heat input, faster drying
(nonadiabatic) 3. Flash dryer tubes of variable cross sections along its length (with
delayed chambers)
dryer tubes to process particles with broader size distribution and cohesive particles prone to form lumps
5. Use of SHS as carrier gas–adiabatic/nonadiabatic designs 6. Use of inert carrier particles in a pneumatic tube to dry slurries
3.4 Trends in Drying Research and Development
about the direction drying technologies will take in the next several
past three or four decades. As the general standard of living around the world rises along with the population of the world, it is obvious that the need for drying technologies will increase. New demands will be
legislative measures, utilization of renewable energy for drying, and better-quality products at lower total costs. Currently, the major driving force for innovative drying techniques is the need to produce better-quality products at higher throughputs. If the price of fossil fuels rises rapidly and the scenarios proposed regarding the impending shortage of oil and the resulting skyrocketing price of oil, then the R&D in dry-
energy-saving measures that are not cost-effective now would become very attractive if the price of oil doubles or triples in the next one or twodecades.
In general, drying techniques designed to enhance quality are very
pharmaceuticals, nutraceuticals, and some foods) can be dried at low temperatures and under vacuum, albeit at higher costs. As noted else-where in this book, two-stage, hybrid heat pump dryers or microwave (MW)-assisted vacuum dryers can compete with freeze-drying (F-D) pro-cesses to produce a high-quality dried product at a lower cost. However, these processes are still very expensive for drying of low-value products.
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4. Multipass, automatic, aerodynamic classification in ring-shaped
It is extremely difficult, if not impossible, to make definitive statements
made on better energy efficiency, lower environmental impact through
ing will again be driven by the need to enhance efficiency. Some of the
product-specific. For example, high-valued, heat-sensitive products (e.g.,
Also, scale-up to very high production rates is difficult at this time.
decades. Most of the developments in this field have occurred in the
© 2009 by Taylor & Francis Group, LLC
Innovation and Trends in Drying Technologies 25
This book focuses on new drying technologies. Where possible, the merits and limitations of various new technologies are proposed in the literature, and novel technologies marketed by vendors around the world are evaluated as objectively as possible. For proprietary reasons, some key details could not be located in some instances. Almost with-out exception, two key pieces of information are not reported by most authors, that is, the cost-effectiveness of their proposed innovations and the objective comparison with competing current technologies. Readers will have to make such judgments carefully if they wish to use this information in practice. Many of the processes may be protected by patents, as well.
The main goals of new drying technologies are to
• Produce better-quality product• Operate at higher capacities, safely, and with good control• Operate at lower total cost by lowering capital as well as running
costs (energy, maintenance, emissions, etc.)
One or more of these objectives can be reached in several possible ways. The following is a short list:
• Use of indirect heating mode, where feasible• Use of heat pumps to save energy• Use of hybrid dryers• Use of multistage dryers• Use of new gas–solids contactors• Use of SHS as a drying medium where possible• Use of enhancement techniques such as application of acoustic or
• Use of better combustion techniques such as pulse combustion
Note that there is a cost associated with any additional complexity in the drying process. It is imperative to make a technoeconomic evalua-tion of conventional but more complex as well as newer (advanced) dry-
depend on the
• Value and production rate of the product• Cost of electricity/fossil fuels (depends on time and geographic
location of the plant)• Risk assessment due to uncertainties of scale-up and life-cycle
cost evaluation
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ultrasonic fields
ing technologies before a final choice is made. The outcome will often
© 2009 by Taylor & Francis Group, LLC
26 Advanced Drying Technologies
• Choice of vendors, delivery times, and performance guarantees• Expected variability in product characteristics as well as produc-
are concerned
References
Foster, R. 1986. Innovation—The Attacker’s Advantage. Summit Books, New York, 316p.
New York, 154p.Lehtinen, J. 1998. Condebelt board and paper drying. Drying Technol., 16(6):
1047–1073.
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Other factors may also need to be considered for specific applications.
Howard, W. G. and Guile, B. R. (Eds). 1992. Profiting from Innovation. Free Press,
tion rates as some techniques are not flexible as far as capacities
© 2009 by Taylor & Francis Group, LLC
Part II
Selected AdvancedDrying Technologies
This part covers a number of relatively new but not commonly used drying techniques and technologies. Most have found commercial application for selected products in some countries. These technologies have demonstrated their potential to compete with conventional technologies and surpass them in performance in many instances, for example, superheated steam drying, drying of suspensions on inert particles, and heat pump drying. With greater awareness and industrial interest, several of these will become common technologies within the next decade or two.
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© 2009 by Taylor & Francis Group, LLC
29
4Drying on Inert Particles
4.1 Introduction
Drying of liquid materials on inert solid carriers is a relatively new commer-cial technology to produce powders from solutions, suspensions, slurries, and pastes. Although this technique was developed in the former Union of Soviet Socialist Republics (USSR) in the 1950s and used for industrial
of biological origin (e.g., Kutsakova et al., 1964; Reger et al., 1967; Minchev et al., 1968; Anonymous, 1992), it was not widespread, mostly because of the language barrier. Over the past two decades, however, drying on inert particles has found a renewed interest mainly because of its ability to pro-duce powders even from the coarsely dispersed liquid feed at evaporation
1983; Adamiec et al., 2007; Kudra and Mujumdar, 2007; Reyes et al., 2008). Extensive studies, carried out in Poland, Brazil, England, New Zealand, and Australia, have resulted in several pilot units and custom-made instal-
dryers with inert particles have recently been marketed by companies such as Carrier Vibrating Equipment Co., United States, and Euro-Vent, England, as well as PROKOP INOVA in the Czech Republic that offer dryers utilizing a swirling bed of inert particles (Kutsakova et al., 1990, 1994). Recently, the idea of drying of liquids sprayed on the surface of inert particles has been extended to drying of highly wet materials such as the granules of pressed yeast (70% moisture content), which were dried as a
4.2 Drying Mechanism and Process Considerations
rials such as swirling stream dryers, impinging stream dryers, or pneu-
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drying of pigments, fine chemicals, pharmaceuticals, and certain materials
rates competitive to spray, drum, and film-rotary dryers (Strumillo et al.,
matic dryers (Figure 4.1). Independent of the hydrodynamic configuration,
lations (e.g., Anonymous, 1986; Grbavcic et al., 1998). In addition, fluid bed
fluidized mixture with inert polyethylene beads (Alsina et al., 2005).
Drying on inert particles is typically performed in a variety of fluid beds (classical fluid bed, spouted bed, spout-fluid bed, jet spouted bed, vibrated fluid bed, cyclone dryer, etc.) as well as in other dryers for dispersed mate-
© 2009 by Taylor & Francis Group, LLC
30 Advanced Drying Technologies
FIGURE 4.1
with inner conveyor screw, (d) spouted (jet spouted) bed, (e) vortex bed, (f) swirling streams,
(a) (b) (c)
(d) (e) (f )
(g) (h) (i)
(j)
Feed
Air
Air + powder
Inert particles
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Basic configurations of dryers with inert carriers: (a) Fluid bed, (b) spout-fluid bed, (c) fluid bed
(g) vibrofluidized bed, (h) rotary dryer, (i) pneumatic dryer, and (j) impinging stream dryer.
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 31
the principle behind this technology lies in dispersing the liquid feed
the sole hydrodynamic impact of the hot air stream or by the combined impact of an air stream and a mechanical device such as a screw con-veyor, a vibrator, or lifters (Flick et al., 1990; Kudra et al., 1989; Pallai et al., 2007; Erdesz and Ormos, 1986; Kudra and Mujumdar, 1989, 2007; Pan et al.,
ferrite (Kovalev et al., 1989).Depending on the hydrodynamic conditions, the liquid coat on the par-
ticle surface dries by convective heat transfer from hot air and by contact heat transfer due to sensible heat stored in the inert particles. When the coat is dry enough to be brittle, it cracks because of particle-to-particle and particle-to-wall collisions and peels off from the surfaces of inert particles. Because of intense attrition, a dry product is discharged from the dryer
produced, especially when drying brittle materials of biological origin.
Figure 4.2 presents the idealized mechanism of drying on inert particles, which boils down to the following sequence of kinetic processes: heating of inert particles, coating with dispersed liquid, drying of the coat, and cracking and peeling-off the dry product. Because of continuous supply
coats at the same time not only the material-free particles but also par-ticles with a dry but not peeled-off material and particles with a partially dry layer. Thus, quasi-equilibrium is established between the individual rates of the component processes. Stable operation of the dryer requires the combined rate of drying/peeling-off to be greater than the rate of coat-ing. Otherwise, the wet coat would build up on the inert particles and the bed would eventually collapse eventually. The bed would also collapse with excessive saturation of exhaust air (Schneider and Bridgwater, 1989).
Another condition for stable operation of the dryer with inert particles stems from the material properties—no elastic shell should be formed on the solid carrier at any stage of drying as impact due to particle collisions
particles made of two bimetallic canopies, which change their shape when subject to temperature changes during drying, could facilitate cracking of a dry shell (Dmitriev et al., 1989). Fibrous materials (e.g., pulp and paper sludge), which could bridge solid particles and therefore immobilize the bed, are also not good candidates for drying on inert particles. The bed can also collapse when drying sticky materials such as meat-rendering sludge with excessive fat content. In such a case, the melted fat acts as a binder,
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magnetic field if they are made of ferromagnetic material such as barium
with the exhaust air as a fine powder of rounded particles. When chip-
of the liquid feed and a definite material residence time, the liquid spray
might not be sufficient to crack the shell. Here, the almond-shaped inert
over the surface of an inert solid carrier. This carrier is fluidized either by
2000; Limaverde et al., 2000). Particles can also be fluidized by an external
ping due to the impact of inert particles prevails attrition, small flakes are
Small flakes can also be obtained when using inert particles with a corru-
grooves on the particle surface (Kutsakova et al., 1985).gated surface. The size of the flakes is then proportional to the size of the
© 2009 by Taylor & Francis Group, LLC
32 Advanced Drying Technologies
which immobilizes particles and traps dry meat powder inside the dryer. This problem can be solved when altering the process by contact-sorption drying (see Chapter 12). In this particular case, the meat-rendering sludge with fat content up to 30% w/w was successfully dried in the jet spouted bed dryer when using either calcium carbonate or wheat bran in the mass ratio of 4.6 and 2.5%, respectively (Amazouz and Benali, 2000). An alter-native solution to the problem of hydrodynamic stability is the use of a hydrocyclone to remove excess fat before thermal drying (Kudra, 2000).
Studies on drying of a single 8 mm ceramic particle coated with a 0.6–0.8 mm layer of the pasty pigment have shown that drying kinetics are typical for convective drying of capillary porous material (Leontieva et al., 2002). The clearly marked plateau of the material temperature cor-responds well with the wet bulb temperature of the air stream during the
FIGURE 4.2Process schematic and idealized mechanism for drying of liquids on inert particles.
Outlet gas + dry powder
Drypowder
Inertparticles
Wet feed(paste, slurry)
Hot gas
Wet coat Inert particle
ConvectionConduction
Drying mechanism—idealized sequence of processes
Heating Coating Drying Peel-off
I II III IV
Heat transfer mode
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© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 33
material temperature during drying lies in-between the wet bulb tem-perature and the exhaust air temperature. This is because not all inert particles in the bed are always fully covered with the wet coat; therefore, heat conduction supplements convective drying and thus the condition for the wet bulb temperature does not hold. Also, the material tempera-ture at the moment of peel-off is higher than the wet bulb temperature because removal of a dry coat starts at a certain moisture content, which
extracted literature data (Leontieva et al., 2002) to combine the curve rep-resenting the kinetics of the peel-off process with the drying and tem-perature curves.
The same conclusion holds for other literature data as well. For exam-ple, Figure 4.4 presents the relationship between the equilibrium material (bed) temperature and inlet and outlet air temperatures for the drying of egg melange in a vortex bed dryer with forced pulsation of inert particles (Kutsakova and Utkin, 1989). Clearly, the material temperature is higher than the respective wet bulb temperature. At the same time, the material temperature is lower than the outlet air temperature, and this difference tends to be larger at lower inlet air temperatures. These data are consis-tent with measurements by Markowski (1992), who found the difference between the particle surface temperature and the wet bulb temperature to
ture is lower by 15°C than the temperature of the outlet air.
Moi
stur
e con
tent
(%
w.b.
)
Drying curve
Peel-off curve
Temperaturecurve
Time (s)
Mat
eria
l tem
pera
ture
(ºC)
Peel
ed-o
ff m
ass (
g)
80
60
40
20
00 200 400 600
20
40
60
800.05
0.04
0.03
0.02
0.01
FIGURE 4.3Drying, material temperature, and peel-off curves for drying R-salt on inert particles. (Adapted from Leontieva, A. I. et al., Drying Technol., 20(4&5), 729–747, 2002.)
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first drying period (Figure 4.3). An analysis of literature data on drying in
is close to the final one. This can be seen in Figure 4.3, in which we have
spouted and fluidized beds of inert particles indicates, however, that the
be at least 20°C. Also, Grbavcic et al. (1998) found that fluid bed tempera-
© 2009 by Taylor & Francis Group, LLC
34 Advanced Drying Technologies
Assuming that drying the relatively thin layer of the wet material on inert carriers is externally controlled, Kutsakova and Utkin (1987, 1989) proposed the following equation for the moisture evaporation rate:
dXdt
1 XR T 273
P T ,X Pi
SS p g
( )( ) ( )
(4.1)
where R = 462 J/(kg K) is the gas constant for water vapor, PS and PgP are thewater vapor pressures at the material surface and in the gas core, respec-
on the surface of inert particles given by the mass of the wet material that adheres to the unit surface area of inert particles. TS represents the average gas temperature in the boundary layer at the surface of the inert particle.
Integration of Equation 4.1 gives the following relationship for drying time:
tR T
XI T P I T P
dX
P T X PS
ip g p g
S p gX
X
i
f
2731
( )( ) ( ) ( ) ( )∫, ,
,(4.2)
65 70 75 80 85 90 9555
60
65
70
75
80
210C
220C
250C
240C
230C
Exhaust gas temperature (C)
Mat
eria
l tem
pera
ture
(C)
FIGURE 4.4Material temperature versus exhaust and inlet air temperatures. (Extracted from Kutsakova, V. E. and Utkin, Yu. V., Trans. VUZOV. Food Technol., 5, 92–93 (in Russian), 1989.)
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tively, and φ is the parameter that quantifies the distribution of the wet coat
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 35
water vapor pressure at the material surface with the average temperature of the inert particles and the material moisture content.
For drying in a vortex bed of inert particles, Kutsakova and Utkin (1987) developed the following experimental equation:
P T X A BX DXT T
FS pC E
p r
, exp( ) ( )
50731
2731273
where the parameters in Equation 4.3 for selected protein-based materials are given in Table 4.1.
The average temperature of the bed (TpTT ) can be determined experimen-tally or can be calculated from the following relationship (Kutsakova, 2004; Kutsakova and Utkin, 1987, 1989):
TT T W c a
r c G Xa
r nh
Wpeq out g g
p p p i
p
g
1
3 1
1
4 2exp( )( )( ) cca
(4.4)
where TeqTT is the equilibrium temperature of evaporation (°C), h is the gas-2 K), G is the dryer throughput
with respect to the dry product (kg/s), and n is the number of inert par-ticles in the dyer.
The mean temperature of inert particles with an accuracy of 3°C can also be determined from the following empirical formula (Kutsakova, 2004):
T T bp out (4.5)
The constant b varies from 10 to 20, depending on the drying material; for protein hydrolyzate b = 15, whereas for casein b = 10.
TABLE 4.1
Parameters in Equation 4.3
Material A B C D E F TrTT (°C)
Protein hydrolyzate
11.53 1.027 −0.8 4.605 −1.8 1.0 120
Skim milk 21.53 0.053 −1.0 0.313 −2.3 0.032 (112 − Tp) 100Whey 11.53 0.00378 −1.7 6.620 −1.7 0.032 (112 − Tp) 100
Source: Extracted from Kutsakova, V. E. and Utkin, Yu. V., J. Appl. Chem. USSR, 60(5),1077–1081, 1987.
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To calculate the drying time, one should first identify the variation of the
to-particle heat-transfer coefficient (W/m
© 2009 by Taylor & Francis Group, LLC
36 Advanced Drying Technologies
The water vapor pressure in the bulk of a drying gas can be calcu-lated based on the exhaust gas humidity. The φ parameter should be
and material characteristics, and operating parameters. For the drying of the materials listed in Table 4.1, the following semiempirical formula can be used (Kutsakova and Utkin, 1987; Kutsakova, 2004):
r c T T
X XX
H c T T
p p p p eq
i f
ig in out
( )( )
3
1
(4.6)
When drying the protein hydrolyzate from Xi = 4 kg/kg to Xf = 0.05 kg/kg at Tin = 300°C, TpTT = 105°C, and TeqTT = 100°C, the parameter φ was foundto be 0.7 kg/m2 at dry coat thickness from 0.2 to 0.3 mm. The calculated drying time varied from 60 to 90 s, whereas the experimentally deter-mined material residence time was in the order of 200–400 s. This indi-cates that several wet coats were dried in this case before the dry material was cracked and peeled off from the inert carrier (Kutsakova and Utkin, 1987). Nomograms that facilitate design calculations can be found in the papers by Kutsakova and Utkin (1987) and Kutsakova (2004) for various proteins and in the book chapter by Kudra (2007) for chicken eggs.
Despite extensive studies on drying on inert particles, there is practi-cally no data on the material residence time except those cited here for a swirling bed and a classical spouted bed of inert particles (Berni and Freitas, 2007). Also, the material residence time from 30 to 85 s has been
®
compared to the residence time of the drying air, such a substantial resi-
ganisms when combined with the material temperature in the range of 70–90°C (Rysin et al., 1981).
From another point of view, the residence time in the order of several minutes in combination with the relatively low material temperatures explains successful drying on inert carriers of some biomaterials such as Zn-bacitracine, animal blood, vegetable extracts, egg products, pea pro-tein, starch, and meat processing sludge (Re and Freire, 1989; Markowski, 1992; Pan et al., 1994, 1995; Amazouz et al., 2000; Amazouz and Benali, 2000). However, the drying of heat-sensitive products of biotechnology such as enzymes or vitamins as well as living microorganisms (e.g., yeast
* Registered trademark of E. I. duPont de Nemours and Company, Inc. (Wilmington, Delaware).
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determined experimentally as it depends on dryer configuration, particle
specified for the drying of animal blood and egg products as a film of
dence time was claimed to be sufficient to suppress undesirable microor-
60–200 µm on a fluidized bed of 3–5 mm Teflon * cubes (Table 4.2). As
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 37
and bacteria) is yet a challenging task because of product degradation. The positive or negative results depend on the following main factors that result from the drying mechanism (Kudra and Strumillo, 1998):
1. Mechanical effect of inert particles due to wall-to-bed and particle-to-particle impacts and attrition owing to circulation of the bed.
erties of inert particles, dryer geometry, as well as dryer con-
spouted bed, etc.). 2. Thermal effect due to convective and contact heat transfers that
depends mainly on the heat-transfer rate, bed hydrodynamics,
3. The effect of material accumulation, which depends on rheological properties of the material being dried and bed loading, and there-fore it can be important only for some materials.
loss risk (QLR) as
QLR HT M A1 (4.7)
where ∆HT,TT ∆M, and ∆A are the heat-transfer, mechanical, and accumu-lation effects, respectively. Detailed information and an example of the QLR analysis for drying in a jet spouted bed can be found in the reference literature (Markowski, 1992, 1993a, 1993b; Kudra and Strumillo, 1998).
TABLE 4.2
Drying of Food Products on Inert Particles
Parameter Egg Melange Egg White Animal Blood
Inlet air temperature (°C) 110–12 120–130 110–135Outlet air temperature (°C) 63–68 67–80 63–75Unit feed rate (kg product-h/kg bed)
1.35 1.35 1.4
Air consumption (kg/kg product)
2.0 2.0 1.7
Layer thickness (mm) 0.06–0.20 0.06–0.15 0.10–0.20Material temperature (°C) 69–77 77–90 73–85Residence time (s) 50 30 85
Source: Extracted from Rysin et al., Method of drying of pasty products, Russian Patent 1024668, 1981.
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figuration (jet spouted bed, pneumatic transport, mechanically
properties of drying material, and dryer configuration.
The risk of product degradation can be quantified in terms of the quality
These two phenomena depend on airflow rate, mass and prop-
© 2009 by Taylor & Francis Group, LLC
38 Advanced Drying Technologies
From Equation 4.5, it follows that success in drying of living microor-ganisms can be achieved when reducing any of the components that con-tribute to quality losses. An interesting option to alleviate thermal effect appears to be heat pump drying (Alves-Filho et al., 1998). With reference to Figure 4.5, the bacterial suspension (Streptococcus thermophilius in a potas-sium phosphate buffer with and without trehalose protectant) was dried
air temperature was kept below 35°C by the use of a classical heat pump circuit (see Chapter 15). The powdery product, which was dried with the addition of 100 mM of trehalose, showed 86% viability at an inlet air tem-perature of 10°C. Thus, it was concluded that this technique could be com-
offers 100% viability but when drying at −20°C for 15 h, then at −10°C for
4.3 Modeling and Scale-Up
Despite numerous published papers on drying suspensions in the bed
design, with the possible exception of a vortex bed dryer with inert particles (Kutsakova and Bogatyriev, 1987). Drying on inert particu-
FIGURE 4.5
Condenser (air heater)
Condenser
Evaporator(air cooler)
Feed
Heat pump
Dryer
Inertparticles
Product
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2 h, at −5°C for 0.5 h, and finally, at 10°C for 0.3 h (Alves-Filho et al., 1998).
of inert particles, there is insufficient data available in the open litera-ture that can be used with confidence for process calculations and dryer
late carriers can be performed in diverse hydrodynamic configurations
Heat pump dryer with a fluidized bed of inert particles.
by spraying over the fluid bed of 3.5 mm polypropylene beads. The inlet
petitive to heat pump–assisted freeze-drying (F-D) in a fluidized bed that
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 39
(see Figure 4.1), which have distinctly different characteristics with respect
distributions, recoating ratio, surface renewal rate, etc. Moreover, the dry-ing mechanism—and hence the corresponding mathematical model for
including adhesion, cohesion, possible case-hardening, and the critical moisture content at which the elastoplastic layer turns into an elastobrittle coat (Strumillo et al., 1983).
Because the overall dying rate depends on the rate of the component subprocesses, that is, on the drying of the wet layer on individual parti-cles and removal of the dry coat, the mathematical model for this process must adequately account for variables that affect both these processes. It is generally recognized that drying on inert carriers is controlled by the external heat and mass transfer conditions. Even for materials with internal resistance to transfer processes, the operating conditions are chosen (thin layer and rapid evaporation at 100°C) so as to minimize
and is indirectly affected by the solids’ circulation rate. The rate of dried coat removal is affected by air velocity, mass of the inert bed, feed rate,
particle adhesion.Since removal of a dry material from the dryer is critical for the
hydrodynamic stability of dryers with inert carriers, the preferential
the particulate phase, which promotes fracturing of the dried coat by interparticle collisions resulting in attrition. Moreover, the conical dry-
the cylindrical one (Kutsakova et al., 1964). Consequently, most stud-ies that aimed at process modeling have been carried out with conical spouted beds.
Pham (1983) has proposed a simple model for the classical spouted bed dryer with clearly separated spout and annulus zones. Although no inter-action between these two zones was assumed, he obtained satisfactory agreement between his model predictions and experimental data. A more realistic approach, which accounts for interactions between gas and par-ticles, was proposed by Barrett and Fane (1989). However, no mathemati-cal formulation for this qualitative mechanistic model was given. Other models for drying of suspensions on inert particles are of the lumped-parameter type where drying rates are given by equations obtained from multiple regressions of experimental data or from heat/mass balances over the entire bed (Markowski, 1992; Oliveira et al., 1994; Reger et al., 1967; Csukas et al., 1976). Such models often have limited applicability only for the given material and equipment operated over the range of parameters chosen by the authors.
CRC_73877_Ch004.indd 39CRC_73877_Ch004.indd 39 12/26/2008 3:00:13 PM12/26/2008 3:00:13 PM
a particular dryer configuration—depends on material characteristics
this resistance (Kutsakova and Utkin, 1987). Hence, the rate of wet film
and the solids’ circulation rate aside from the nature of the dry film-to-
configuration is the conical spouted bed as it offers intense motion of
ing chamber has been shown to result in better energy efficiency than
to bed structure, particle and gas flow patterns, circulation velocities and
drying is directly affected by the inlet air temperature and its flow rate
© 2009 by Taylor & Francis Group, LLC
40 Advanced Drying Technologies
Considering the hydrodynamics of the spouted bed, the most logical description of the drying process appears to be provided by the three-region model (TRM) (Oliveira and Freire, 1996). This model is based on a generalized assumption that each distinct region of the bed (an annu-lus, a spout, and a fountain) contributes to water evaporation in a propor-
residence times within the zones, and local temperature and humidity gradients.
Spitzner and Freire (1998) have critically reviewed various models for drying in spouted beds of inert particles and made a qualitative compari-son between the TRM and the thermodynamic model using literature data for drying of animal blood and aluminum hydroxide in a conical spouted bed. They have shown that the TRM predicts an outlet air temperature and drying rate with deviations from experiments of <15% in contrastto the thermodynamic equilibrium model, where a 50% difference was obtained between the model estimates and experimental data.
The TRM is built on the assumption that the solids’ temperature (inert particle and drying material) is equal to the wet bulb temperature at the outlet from each region. In our opinion, better accuracy from the model could be expected when allowing for material temperature higher than the wet bulb temperature, as shown in several papers reviewed in the preceding paragraphs.
built in sizes from laboratory models to industrial units up to 10 m3 of the dryer volume, the scale-up procedure is still based on know-how
hydrodynamic stability of the spouted beds based on the mechanics of heterogeneous media (Menshutina et al., 2000). Aside from extensive but inaccessible works from the former USSR, the most complete scale-up procedure developed for the jet spouted bed is due to Strumillo et al. (1980). Here, sizing of the dryer for required throughput boils down to maximizing the ratio of the most active zone (VaVV ) to the dryer volume (V)that comprises a cylindrical section and a conical one with the 35° cone angle. The volume of the most active zone is assumed to be a cylinder with a diameter equal to the inlet cone diameter and the height equal to the dynamic bed height:
VD
Hain
d 2
4(4.8)
For the cylindrical–conical dryer, the volume is given as
V D D D D H D Hin in w w c w w 12 4
2 2 2 ( ) (4.9)
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tion determined by their respective heat- and mass-transfer coefficients,
rather than theoretical foundations, except for attempts to define the
Although spouted and fluidized bed dryers of inert particles are
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 41
The inlet diameter is calculated from the following equations:
DvQ
c T T uinH in
4
0 ( ) (4.10)
QwV H
k
(4.11)
where w is the volumetric evaporation rate and k is the heat utilization
heat input.These equations are complemented by the following inequalities, which
identify the conditions for stable operation:
DD
H Dw
inst in 3 2 (4.12)
u u u m smf mf 6 35 / (4.13)
The volumetric evaporation rate, heat utilization factor, and dynamic bed height need to be taken from experimentally obtained correlations. The static bed height (Hst) can be chosen assuming that, for a particulardrying material, its residence time in the small and large dryers is the same:
tmW
(4.14)
where m is the mass of the material in the bed and W is the material feed rate.
Since the material holdup below the clogging point is related to the mass of inert particles, the static bed height can thus be determined.
The aforementioned scale-up procedure was used to design a series of jet spouted bed dryers based on experiments performed on a 0.5 m3
3
in equivalent diameter have shown that the volumetric evaporation rate varied from 55.4 kg/m3 h to 120.6 kg/m3 h against 100 kg/m3 h taken fordesign calculations. Also, the heat utilization factor from 0.192 to 0.335 corresponded well to the assumed value of 0.25 (Strumillo et al., 1980).
Recently, Kutsakova (2007) considered another aspect of the scale-up problem when, for various reasons, the inert particles should be changed.
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factor defined by the ratio of the heat for moisture evaporation to the total
unit. Field trials on a 1.5 m dryer filled with 30 kg of Teflon cubes 4 mm
For example, when replacing steel beads with less-dense Teflon particles,
© 2009 by Taylor & Francis Group, LLC
42 Advanced Drying Technologies
the question arises, how to select the characteristic size of inert par-ticles to not alter the operating conditions such as gas velocity, tempera-ture, or feed rate, which are determined by drying kinetics and product properties? The analysis was performed for protein-based feed dried on 1.25 mm stainless steel beads in a 0.0152 m3
(Figures 4.1f and 4.9). It was shown that to maintain the same hydrody-
productivity, as indicated in Table 4.3.
4.4 Selected Dryers with Inert Particles
The principles of operation, basic design, and process characteristics for most common types of dryers with inert carriers are well described in
dryer (Re and Freire, 1989), the jet spouted bed dryer (Markowski, 1992), the pneumatic dryer (Blasco et al., 1996), the rotary dryer (Limaverde et al., 2000), the impinging stream dryer (Kudra and Mujumdar, 1989), a dryer with a vertical inner conveyor screw (Pallai et al., 2007), etc. Therefore,
research and applications points of view.
1990). The toroidal (doughnut) collector for liquid feed is positioned in a
tor is equipped with an even number of spray nozzles, which face one another, so that the liquid to be dried is dispersed in a countercurrent
TABLE 4.3
Drying Indices for Two Types of Inert Particles
Parameter Steel Particles
Particle diameter (mm) 1.25 3.00Product diameter (μm) 5.0 6.9Final moisture content (% w.b.) 3.5 4.2
(kg product/m2 of inert particles)0.035 0.009
Productivity of a dryer(kg evaporated water/h)
100 118
Source: From Kutsakova, V. E., Drying Technol., 25(4), 617–620, 2007. With permission.
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Specific throughput
modified spouted bed
ever, will alter the product size, its final moisture content, and dryer
here we present the less- common configurations that are interesting from
Teflon Particles
namic condition, the 3 mm Teflon particles should be used. This, how-
easily accessible literature. These are the vibrated fluid bed dryer and the
(Kudra and Mujumdar, 2007; Erdesz and Ormos, 1986), the spouted bed fluid bed dryer in their conventional and two-chamber configurations
Figure 4.6 displays the so-called confined fluid bed (Fedorovich et al.,
drying chamber in the middle of a fluidized bed of particles. The collec-
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 43
collector mounted above the collector for liquid feed provides secondary
motion. Heating of liquid before spraying to a level from 50 to 70°C and heating the inert packing permit processing of heat-sensitive substances such as protein concentrates or saccharose solutions. Evaporation rates of up to 200 kg/h can be achieved at the inlet and outlet air temperatures in the ranges of 120–200°C and 40–50°C, respectively.
tial bed circulation (Pikus et al., 1975). Here, a cylindrical drying chamber
FIGURE 4.6
Spraynozzles
Liquidcollector
Secondaryair
Liquidfeed
Primary air
Discharge
ProductAircollector
Exhaust air
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Confined
is fitted with a twin set of oppositely directed stubs for tangential entry
fluid bed
Confined fluid bed dryer.
mode perpendicularly to the fluidizing air stream. An additional toroidal
air directed against the fluidizing air. The combined action of the liquid
its dynamic bed height without losing advantages of a highly turbulent spray and air jets results in confinement of the fluid bed, thus reducing
Figure 4.7 depicts a schematic of a fluid bed of inert particles with par-
© 2009 by Taylor & Francis Group, LLC
44 Advanced Drying Technologies
of the secondary air. A streamlined insert located in the center of the chamber creates velocity distribution with a maximum at the impinge-ment plane from the two counterrotating streams of secondary air. A fraction of the inert particles (which are generally free of dry material entrained with the exhaust air) is picked up by a peripheral ring-type entry located just below the limiting grid and returned to the near-grid region, where they are recoated with a liquid feed. Such a design elimi-nates possible dead zones in the dryer and permits extension of the par-ticle residence time.
called swirl-jet spouted bed (SJSB) designed for drying of slurries and pastes on the surface of inert particles (Markowski et al., 1977).
FIGURE 4.7Fluid bed dryer with partial recirculation of inert particles.
Hot airHot air
Atomizer
Gas distributor
Profiled insert
Air
Air
Air
Air
Limiting grid
Product
Exhaust air
A-A
B-BA A
B B
Returnduct
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An interesting modification of the classical spouted bed is the so-
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 45
The dryer (Figure 4.8) consists of cylindrical and conical chambers equipped with standard feeding and product discharge devices. In con-
particles, which are loaded to the dryer during the start-up period through
a double spiral at the bottom of the cone, which act as a swirl vane. These ducts are coupled to the cylindrical chamber by two short stubs attached tangentially to the cylinder wall. At each duct–stub junction, an air nozzle is mounted so as to operate as an injector. A standard pneumatic nozzle is used to disperse the liquid or pasty material on the surface of the inert
Swirl vane
Swirlvane
Swirlvane
Cone
Injector
Air nozzle
Inert particles
Material feed
Air inlet
Product
Spray nozzle
Tangentialstub
Tangentialstub
Tangentialstub
Venturitube
W
W
A-A
A A
FIGURE 4.8Swirl-jet spouted bed dryer.
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an air lock. The gas inlet comprises two jointly twisted ducts configuring
Deflector
trast to the classical spouted or fluid bed dryers, no grid supports the inert
© 2009 by Taylor & Francis Group, LLC
46 Advanced Drying Technologies
particles. This special design of the gas inlet produces an inlet gas swirl, which forces the particles to spout in a rotary motion. Relatively high gas-to-particle velocity and intense interparticle and wall-to-particle col-lisions enhance heat and mass transfer rates between the gas and the wet
from the inert particles. Owing to the injectors, a noticeably lower pres-sure exists in the near-wall region of the cylinder. Thus, a fraction of the inert particles is sucked off the chamber, accelerated with an air stream, and redirected to the dryer inlet. During passage through the venturi tubes and air ducts, the inert particles are additionally heated by a hot air stream so that the temperature of recycled inert is much higher than the bed temperature. Hence, evaporation of moisture from the wet coat is not only due to sensible heat of gas phase but also due to the sensible heat stored in the inert particles. The performance characteristics of an SJSB for dye Red-K are given in Table 4.4.
As compared to the jet spouted bed dryer of inert particles used for dry-ing of detergents and dyestuffs, backmixing of inert particles along with swirl-jet spouting allows for the following (Markowski et al., 1978):
Increase in the volumetric evaporation rate by 25–30%•
Figure 4.9 illustrates the operating principle of the spouted bed dryer with counterrotating swirling streams developed by Kutsakova et al. (1994)
TABLE 4.4
Performance Characteristics of the SJSB Dryer for Dye Red-K
Dryer volume (m3) 0.046Feed rate (kg/h) 4.5Moisture content (% w.b.) Initial 80 Final 7Particle size (mm) 0.01–0.025
3 250Air temperature (°C) Inlet 180 Outlet 127
Equivalent diameter (m) 0.0046 Mass of load (kg) 2.5
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film on the inert particle surface and facilitate removal of the dry coating
• Decrease in the final moisture content by 0.03–0.05 kg/kg• Increase in thermal efficiency up to 0.45
Airflow rate (N m /h)
Inert particles (Teflon cubes)
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 47
and Kutsakova (2004). The dryer comprises a vertical cylindrical cham-ber with a primary air distributor located in its bottom part. The dis-
toroidal grid that provides a multiplicity of inclined air jets around the chamber wall. The central part of the grid is closed with a conical solid
dynamic conditions as in a spouted bed although instead of the central air jet the spouting air is a ring-shaped stream adjacent to the chamber wall. As the air ring emerges from the inclined vanes, the spouting air swirls around the conical insert. If the air velocity is set properly, the inert particles picked up by the air stream at the grid zone follow the spi-ral trajectory to the top of the conical insert. Because the feeding nozzle is located in the lateral wall of the dryer at the cone level, the dispersed
FIGURE 4.9Spouted bed dryer with counterrotating swirling streams.
Exhaust air + dry powder
Secondary airdistributor Feed
Vanes
Primary air distributor
Conical insert
Inertparticles
Air inlet I
Air inlet II
Spray cone
Counterrotating gas–solids streams
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tributor is built of profiled vanes sloped to horizontal at 21–34º to form a
insert. This particular configuration of a drying chamber creates hydro-
© 2009 by Taylor & Francis Group, LLC
48 Advanced Drying Technologies
feed coats the inert particles only when they pass through the spray zone. The swirling trajectory of the gas–solid stream in such a spouted bed results in intermittent coating of inert particles. Such conditions give time to dry the wet coat before subsequent coating occurs. When the multilayer coat on the surface of inert particles is dry enough to be brittle, it breaks by the impact with the dryer wall, the conical insert, and particle-to-particle collisions. To intensify the crushing of the dry product, a secondary air distributor is mounted above the primary one at the height of about 1–1.25 of the chamber diameter. The slots in this air distributor are tangential to the dryer wall, and therefore the air jets force inert particles at the distributor zone to swirl in the opposite direction to their ascending counterparts. This coordinated action of two opposite gas–solid streams creates hydrodynamic conditions similar to the ones in counterrotating impinging streams (see Chapter 5), where the dry coat is not only peeled off the inert particles and ground by
to the bottom of the drying chamber are again picked up and transported upward by the swirling air stream from the primary air distributor.
A proper mass ratio of inert particles in the counterrotating streams is necessary for stable operation of the dryer. This mass ratio depends, among others, on the open area of both gas distributors, the angle of vanes in the principal air distributor, and the grid-to-bed pressure drop ratio. For stable operation of the dryer, the open area of the principal-to-second-ary distributor should be within 1.5–2.5, whereas the primary distributor-
through the secondary air distributor varies from 0.35 to 0.5 of the total
parameters that are set for a given material to be dried. The dryer retains
inlet air temperature by 20–40°C (Kutsakova et al., 1994).Figure 4.10 presents the general principle of the so-called Ecal dryer
(Moeller, 1974), which falls into the category of a dryer with an internal conveyor screw (Pallai et al., 2007). The cylindrical drying chamber has a centrally located vertical screw conveyor. When rotated at 10–30 revolu-tions per minute (rpm), this conveyer transports inert particles from a dis-engaging zone at the grid area to the top of the bed, where they are coated with the feed by rolling in the liquid on a collar. Then the coated particles descend slowly as a falling bed through the annular drying zone, which is about 0.8 m high. Drying air is supplied to the falling bed by radial air inlet ducts located above the bed bottom. Hence, the hot air is split into two fractions: the upward stream, which evaporates most of the water and thus cools down almost to the dew point, and the downward stream
conditions are selected in such a way that the peeling-off starts below
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attrition, but also finish-dried in the impingement zone. Completely dry fines are entrained from the drying chamber with exhaust air and sepa-rated in a cyclone and a bag filter. The particles that lose momentum and fall
(∼10% of the total inlet air), which provides finish-drying. The operating
to-bed pressure drop ratio should range from 0.33 to 1.0. The airflow rate
volumetric flow rate, depending on dryer characteristics and operating
its performance even when the airflow rate is reduced by 15–25% and the
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 49
the air inlet, and therefore, the powdery material is entrained only with
along with the mechanical action of the screw conveyor facilitates abra-sion of the dry coat, and therefore the particles transported by the screw conveyor are essentially free of the drying material.
size inert particles, relatively large balls (18–40 mm in diameter) made of plastics are used as inert carriers in Ecal dryers. When drying liquids such as whey or animal blood, the thickness of the coat ranges from 0.1 to 1 mm. Inlet and outlet air temperatures vary from 150 to 65°C and from 50 to 25°C, respectively. To dry highly labile bioproducts such as living yeast cells, a secondary cold air stream can be injected at the dryer bottom to lower the cyclone air temperature. Drying times for the same unit are between 50 and 5000 s, depending on the drying material and operating conditions.
FIGURE 4.10Schematic of the Ecal dryer and temperature distribution along the bed height. (Adapted from Moeller, A. L., Advances in Preconcentration and Dehydration of Foods., Wiley, New York, 1974.)
Temperature
Air
Material
Air + product(to cyclone)
Hot air
Exhaust air
Inertcarrier
Driedmaterial
Collector
Feed
Bed
heig
ht
Air inlevel
Gridlevel
Bedlevel
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the concurrent air stream. The flow of particles over the supporting grid
Unlike spouted and fluidized bed dryers, which require relatively small-
© 2009 by Taylor & Francis Group, LLC
50 Advanced Drying Technologies
The evaporation capacity of the Ecal dryer is up to 2000 kg H2O/hfor food products, and up to 6000 kg H2O/h for sludges, which are not heat-sensitive. Interestingly, quality products were obtained even when drying large-size solid particles such as sliced parsley, onion, leeks, and apples.
Symbols
b Constant, °C
cH Humid heat, J/kg KD Diameter, mG Dryer throughput, kg/s
2 KH Height, m∆H Latent heat of evaporation, J/kgk Heat utilization factorm Mass, kgn Number of inert particlesP Pressure, Pa
r Radius, mR Gas constant, J/kg Kt Time, sT Temperature, °C, (K)
3
V Volume, m3
w Volumetric evaporation rate, kg/m3 sW Material feed rate, kg/sX Moisture content, %, (kg/kg)
Greek Symbols
β φ Mass of the material held on inert particles, kg/m2
ρ Density, kg/m3
Subscripts
a ActiveA Accumulationc Coned Dynamiceq Equilibrium
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c Specific heat, J/kg K
h Heat-transfer coefficient, W/m
u Superficial velocity, m/sv Specific volume, m /kg
Mass-transfer coefficient, m/s
Q Heat flow rate, W
© 2009 by Taylor & Francis Group, LLC
Drying on Inert Particles 51
f Finalg GasHT Heat transferi Initialin Inlet
M Mechanicalout Outletp Inert particler Referencest StaticS SurfaceW Cylinder0 Ambient
Acronyms
QLR Quality loss riskrpm Revolutions per minuteSJSB Swirl-jet spouted bedTRM Three-region modelw.b. Wet basis
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Markowski, A. S. 1992. Drying characteristics in a jet-spouted bed dryer. Can. J. Chem. Eng., 70(10): 938–944.
Markowski, A. S. 1993a. Quality interaction in a jet-spouted bed dryer for bio-products. Drying Technol., 11(2): 369–387.
Markowski, A. S. 1993b. Quality losses risk during drying of bio-products in a jet-spouted bed dryer. Meeting of the EFCE Drying Working Party. Utrecht, The Netherlands.
Markowski, A., Kaminski, W., Strumillo, C. and Kudra, T. 1977. Apparatus for heat and mass transfer. Polish Patent 100376.
Markowski, A., Kaminski, W., Strumillo, C. and Kudra, T. 1978. New design of spouted bed dryer with inert particles. Proc. 3rd Drying Symposium. Lodz, Poland (in Polish).
namic stability for a jet spouted bed of inert particles. Proc. 12th InternationalDrying Symposium (IDS2000). Noordwijkerhout, The Netherlands (CD-ROM, paper 264).
Minchev, A. D., Romankov, P. G. and Rashkovskaya, N. B. 1968. Studies on drying of pastelike materials in the spouted bed. Zhurnal Prikladnoi Khimii, 41(4): 1249–1250 (in Russian).
Moeller, A. L. 1974. The Ecal process—A novel dehydration process. In: Advances in Preconcentration and Dehydration of Foods. A. Spicer (Ed.). Wiley, New York, pp. 499–504.
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Kutsakova, V. E. 2004. Drying of liquid and pasty products in a modified spouted
Kutsakova, V. E. and Bogatyriev, A. N. 1987. Intensification of Heat and Mass Transfer
and mass transfer during drying of a liquid film from the surface of a single
Menshutina, N. V., Tsoukanov, V. A. and Kudra, T. 2000. Definition of hydrody-
relationships of the drying process in fluidized and spouted beds. Zhurnal
emulsions and solutions on fluidized bed of inert particles. J. Appl. Chem.
1985. Inert particle for fluid bed dryer. Russian Patent 1592683.
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Oliveira, W. P. and Freire, J. T. 1996. Analysis of evaporation rate in the spouted bed zones during drying of liquid materials using a three region model. In: Drying’96. C. Strumillo and Z. Pakowski (Eds.). Proc. 10th International Drying Symposium (IDS’96). Krakow, Poland, pp. 504–512.
Oliveira, W. P., Silveira, A. M. and Freire, J. T. 1994. Analysis of the drying of pastes in conical spouted beds. In: Drying’94. V. Rudolph and R. B. Key (Eds.) Proc. 9th International Drying Symposium (IDS’94). Gold Coast, Australia, pp. 495–502.
Pallai, E., Szentmarjay, T. and Mujumdar, A. S. 2007. Spouted bed drying. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). 3rd edition. Taylor & Francis, Boca Raton, FL, pp. 363–384.
Pan, Y. K., Li, J. G., Zhao, L. J., Ye, W. H., Mujumdar, A. S. and Kudra, T. 2000.
Proc. 12th International Drying Symposium (IDS2000). Noordwijkerhout, The Netherlands (CD-ROM, paper 266).
Pan, Y. K., Pang, J. Z., Li, Z. Y., Mujumdar, A. S. and Kudra, T. 1994. Drying of
International Drying Symposium (IDS’94). Gold Coast, Australia, pp. 819–824.Pan, Y. K., Pang, J. Z., Li, Z. Y., Mujumdar, A. S. and Kudra, T. 1995. Drying of pho-
13(1–2): 395–404.Pham, Q. T. 1983. Behaviour of a conical spouted-bed dryer for animal blood.
Can. J. Chem. Eng., 61(6): 426–434.Pikus, I. F., Kuts, P. S., Tutova, E. G. and Bogdanov, V. M. 1975. Apparatus for
drying of solutions and suspensions. Russian Patent 445808.Re, M. I. and Freire, J. T. 1989. Drying of pastelike materials in spouted beds. In:
Drying’89. A. S. Mujumdar and M. Roques (Eds.). Hemisphere PublishingCorporation, New York, pp. 426–432.
Reger, E. O., Romankov, P. G. and Rashkovskaya, N. B. 1967. Drying of paste mate-rials on inert substances in spouted beds. Zhurnal Prikladnoi Khimii, 40(10):2276–2280 (in Russian).
bed of inert particles. Drying Technol., 26(1): 122–131.Rysin, A. P., Zaitsev, V. M., Shumkov, Ye. G., Gusyev, A. A. and Smirnov, O. S. 1981.
Method of drying of pasty products. Russian Patent 1024668.Schneider, T. and Bridgwater, J. 1989. Drying of solutions and suspensions
in spouted beds. In: Drying’89. A. S. Mujumdar and M. Roques (Eds.). Hemisphere Publishing Corporation, New York, pp. 421–425.
Spitzner, P. I. N. and Freire, J. T. 1998. Evaluation of models on the drying of pastes in spouted beds with inert particles. In: Drying’98. C. B. Akritidis, D. Marinos-Kouris and G. D. Saravakos (Eds.). Proc. 11th International Drying Symposium. Halkidiki, Greece, pp. 2009–2016.
Strumillo, C., Kaminski, W. and Markowski, A. 1980. Scaling-up of dryers with a spouted bed of inert bodies. In: Developments in Drying, Drying’80. A. S.Mujumdar (Ed.). Vol. 1, Hemisphere Publishing Corporation, New York, pp. 180–183.
Strumillo, C., Markowski, A. and Kaminski, W. 1983. Modern developments in drying of paste materials. In: Advances in Drying. A. S. Mujumdar (Ed.).Vol. 2. Hemisphere Publishing Corporation, Amsterdam, pp. 193–232.
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Drying of a dilute suspension in a vibrated fluidized bed of inert particles.
heat sensitive bio-products on solid carriers in a vibrated fluid bed. Proc. 9th
tosynthetic bacteria in a vibrated fluid bed of solid carriers. Drying Technol.,
Reyes, A., Herrera, N. and Vega, R. 2008. Drying suspensions in a pulsed fluidized
© 2009 by Taylor & Francis Group, LLC
55
5Impinging Stream Drying
5.1 Basic Features
which moisture evaporation from wet particles or liquid droplets occurs in the impingement zone that develops as a result of the collision of two oppositely directed high-velocity gas streams, at least one of which con-tains the dispersed material to be dried. At the outset, a distinction should be made between the impingement dryers, in which gas jets are directed onto web, sheet-form, or slablike materials (Mujumdar, 2007), and the jet-zone
high-velocity airstreams exiting a number of perpendicularly oriented air nozzles (Kudra and Mujumdar, 2007).
streams at the feed plane, then it accelerates from zero velocity to a certain
5.1). After crossing the impingement plane, the particle penetrates into the opposite stream due to its inertia and decelerates to a full stop at some dis-
nation point). Thereafter, the particle accelerates in the opposite direction and, after crossing the impingement plane again, penetrates the original gas stream up to the opposite stagnation point. Thus, the deceleration and acceleration processes take place repeatedly. After several damped oscil-lations, the particle velocity in the impingement zone drops to the termi-nal velocity so that it is carried away with the outgoing gas stream. (For
Because of such an oscillatory motion with progressively decreasing amplitude, the residence time of a single particle (of large inertia) in the impingement zone is longer than that of the gas stream. This residence time may be reduced for a multiplicity of particles because of interpar-ticle collisions that lead to enhanced energy dissipation. Although it is possible to feed solids to both gas streams, the rate of particle collision and the resulting loss of momentum are much higher than that for a single stream feed, because particles that do experience nonelastic col-lisions greatly lose their momentum and are driven out of the system.
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depending on the configuration of the impingement chamber.)
The term impinging stream dryer (ISD) refers to a class of flash dryers in
dryers, in which a layer of particulates is pseudofluidized by a multiplicity of
If a solid or liquid particle starts to flow with one of the impinging gas
velocity resulting from the hydrodynamics of the gas–solid flow (Figure
tance of penetration within the domain of the opposing jet flow (the stag-
simplicity, Figure 5.1 depicts only the particle trajectory flowing with the upward gas stream, whereas downward or both flows are also possible,
© 2009 by Taylor & Francis Group, LLC
56 Advanced Drying Technologies
single feed, as well, especially for high solid concentration. In our opin-ion, the problem could be alleviated by intermittent material feed with frequency proportional to the material residence time.
The constraint of reduced residence time is practically eliminated in impinging streams with a mobile impingement zone (Elperin and Meltser, 1978). In this arrangement, the impingement plane is made to move between the two locations (I and II in Figure 5.2) by alternate switching
erate with the original gas stream into one of the impingement chambers,
the stagnation point, and then begin to accelerate in the opposite direc-
closed, whereas the outlet of the second impingement chamber is open.
into the original gas stream, and the following acceleration toward the
the particles is self-controlled by decreasing the inertia force due to the reduction of particle size in the course of processing and reducing the par-ticle mass due to moisture evaporation. Alternatively, a suitable limiting
Impingement plane
Impingement zone
Gas stream IGas stream II
Particle trajectory
Gas duct IGas duct II
Gas streamlines
Particle or droplet
Feed plane
The principle of ISDs.
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This might result in such a significant decrease of the residence time
dramatically reduced. The same effect of particle collisions exists in a and penetration depth that the beneficial effects of impingement are
tion. At this moment, the gas outlet of the first impingement chamber is
first impingement zone are repeated. The period of oscillatory motion of
of the gas flows from left to right and then from right to left. The particles
duct that links both impingement chambers. In either case, particles accel-can be fed into the accelerating flow duct or into the central (reverse flow)
where they collide with the counter-flowing gas stream, penetrate it up to
ment chamber, where the processes of jet collision, penetration of particles
This procedure results in the flow of a secondary gas stream with accel-erating particles along the reverse flow duct toward the second impinge-
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 57
grid can be placed at the outlet ducts to restrict the entrainment of the oversized particles.
tains inert spherical particles that oscillate continuously between the two impingement zones throughout the operation of the dryer. This allows drying of slurries or suspensions that are sprayed on the surface of inert particles. Because the inert particles are heated by the hot air, drying of the liquid coat occurs by combined convective and conductive heat trans-fer. Also, drying with simultaneous grinding can be performed if metallic beads are used along with the material being dried.
streams (Figures 5.3 and 5.4). For example, in a system consisting of four
the two primary gas streams mixed in an impingement zone are split into two secondary streams carrying the particles off the impingement zone.
Figure 5.5 shows the principle of semicircular impinging streams in which a primary gas stream is split equally into two secondary streams
thickness of the boundary layer at the solid surface.Another variant of the impinging stream concept can be formed when
one or two gas–solid suspensions are brought into collision inside a
Impingement chamber
Material feed
Impingement zone
Acceleration zone Deceleration zone
Stagnation point
Impingement zone I Impingement zone II
Constraining grid
Gas feed duct
H
u0 u0D
L
H H
FIGURE 5.2Coaxial ISD with a mobile impingement zone.
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Another modification of the ISD with the mobile impingement zone con-
Similar transport phenomena occur in other configurations of impinging
mutually perpendicular ducts of the same diameter (the “X” configuration),
Reverse flow duct
flowing inside two ducts bent to form semicircular channels. In this case,
carrying gas) is affected by centrifugal forces, which might reduce the the flow of the drying material (and, to a smaller extent, the flow of the
© 2009 by Taylor & Francis Group, LLC
58 Advanced Drying Technologies
Air out
Air out
Feed
Feed
Feed
Feed
Feed
Product
Product
Product
Product
Product
Product
Product
Air out
Air out
Air out
Air out
Air out
Feed
Feed
Feed
Feed
Feed
Air inAir in
Air in
Air in
Air in
Air in
Air in
Air in
Air in
Air inAir in Air in
Air in
Air inFeed
A-A
A
A B
B
B-B
(a)
(c)
(e)
(b)
(d)
(f)
FIGURE 5.3
tion), (e) counterrotating countercurrent, and (f) corotating countercurrent.
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tion, (c) curvilinear countercurrent, (d) semicircular impinging streams (chain configura-Basic types of impinging stream flows: (a) Coaxial, (b) mutually perpendicular X configura-
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 59
(a) (c)
(b) (d)
FIGURE 5.4
F FcFcp
FD
x2x1
FIGURE 5.5Particle trajectory for double penetration in a semicircular impinging stream (Fc, centrifu-gal force; Fcp, centripetal force; FD, drag force; F, lift force).DD
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Impinging plane/zone geometries: (a) Planar with radial flow, (b) planar with circumferen-tial flow, (c) tubular, and (d) annular.
© 2009 by Taylor & Francis Group, LLC
60 Advanced Drying Technologies
Grinholtz, 1987). Figure 5.6 presents some of these impinging stream reac-tors that could be used for drying granular or liquid feeds (Tamir, 1994).
Product
Feed
Air
(a) (b) (c)
(d) (e) (f )
(g) (h)
FIGURE 5.6
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confined volume that forms a cyclone, for example (see Figure 5.3c).
ical reactions, it is commonly termed the impinging stream reactorr (Tamir andBecause such a configuration was originally developed to carry out chem-
Some configurations of the impinging stream reactors.
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 61
according to
a. Rectilinear, where streamlines are parallel with either aligned
b. Rotating (swirling), where streamlines do follow a helix c. Curvilinear, where streamlines are represented by circular
arcs 2. Flow direction
in opposite directions
the same direction 3. Streamline direction
a. Counterrotating, where gas (or solid suspension) streams rotate in opposite directions
b. Corotating, where gas (or solid suspension) streams rotate in the same direction
4. Type of impingement zone
a. Stationary, in which the position of the impingement plane does not change with time
b. Mobile, in which the position of the impingement plane changes periodically or continuously
5. Geometry of impingement plane/zone
toward the impingement plane
concentric circles in the impingement plane
c. Tubular, in which impinging streams meet at the cylindrical surface
d. Annular, in which impinging streams meet in a sector of annulus
Single impinging stream units can be combined in series or parallel to form a system, allowing extension of the residence time and develop-ment of different hydrodynamic or temperature regimes. Also, hybrid
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5.4), the following basic variants of impinging streams can be identified
configurations such as the coaxial impinging streams combined with the
Depending on the geometry and flow direction (see Figures 5.3 and
1. Type of flow
axes of the gas jets (coaxial flow) or displaced axes of the gas jets (eccentric flow)
a. Countercurrent, where gas (or solid suspension) streams flow
b. Concurrent, where gas (or solid suspension) streams flow in
a. Planar with radial flow, in which streamlines diverge radially
b. Planar with circumferential flow, in which streamlines form
© 2009 by Taylor & Francis Group, LLC
62 Advanced Drying Technologies
semicircular ones have found certain advantages over the simple systems (Dengying et al., 1999; Huai et al., 2000).
Besides the high intensity of turbulence in the impingement zone, a common feature of any impinging stream is the unsteady particle motion, acceleration, deceleration, and movement of the particles against the gas
enhance the heat- and mass-transfer processes and hence reduce drying times (see Table 5.1).
The unique characteristics of impinging streams also offer special advantages for thermal processing. An example is the intermittent drying that takes place when the opposing streams are supplied at different tem-peratures. Thus, particles are heated and cooled periodically when oscil-
of grains (thermal micronizing), which occurs when wet grain, for exam-ple, is subject to high moisture and temperature gradients.
5.2 Hydrodynamics and Heat Transfer
modeling of the hydrodynamics, and heat/mass transfer as well as scale-up and design recommendations can be found elsewhere (Kudra and Mujumdar, 1989, 2007; Kudra et al., 1990, 1995; Meltser et al., 1992; Tamir, 1994; Strumillo et al., 1996). Here, only the information of a gen-eral nature and the correlations pertinent to coaxial impinging streams are provided to give the reader the background knowledge on imping-ing streams.
TABLE 5.1
Range of Operating Parameters for Coaxial ISDs
Gas velocity (m/s) 10–150Particle diameter (mm) 0.2–3Volumetric concentration of solids (m3/m3) Up to 0.0035Particle residence time (s) 0.5–15Production rate of (kg/h)
Dry material 50–2,000Water evaporated Up to 5,000
Volumetric throughput of wet material (kg/m3 h) 1,500–30,000Evaporative capacity (kg/m3 h) Up to 1,700Pressure drop (kPa) 1–10
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lating between the hot and cold gas streams. Another example is puffing
figurations of impinging streams and resulting correlations allowing
stream for half of its oscillatory flow. All these phenomena significantly
Detailed descriptions of the fluid–particle dynamics in various con-
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 63
5.2.1 Penetration Depth and Oscillation Time
The maximum penetration depth (i.e., the distance the particle covers
(i.e., time spent by a particle in the penetration distance) can be deter-mined from the following relationships (Elperin et al., 1972):
Laminar region (Rer ≤ 1)
x 0.016u d
t 0.036d
maxp2
pmax
p2
p
(5.1)
Transient region I (1 < Rer < 13)
x 0.01415u d
t 0.328d
umax
0.8p1.8
p0.8 max
p1.8
p0.8 0.2
(5.2)
Transient region II (13 ≤ Rer ≤ 800)
x 0.02675u d
t 0.0635d
umax
0.5p1.5
p0.5 max
p1.5
p0.5 0.5
(5.3)
Turbulent region (Rer > 800)
x 0.598d
t 1.54d
umaxp p
maxp p
(5.4)
where the Reynolds number for the accelerating/decelerating particle is r p p
The penetration depth and oscillation time for subsequent periods of oscillatory motion, which are useful for process calculation and dryer design, can be determined from the following empirical correlations valid over 2.5 × 10−4 ≤ dp ≤ 4 × 10−3 m, 10 ≤ u ≤ 150 m/s, 0.2 ≤ x0 ≤ 4.8 m, 3 ≤Rep ≤ 80, and 30 ≤ T ≤ 300°C (Levental et al., 1981):
t 1.42Re L x 0.09Re L*p0.28 0.51 *
p0.21 0.44
with
tt u
xx
xx
* max
0
* max
0
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during first penetration into the opposing stream) and penetration time
defined in terms of the gas–particle relative velocity as Re = d (u ± u )/ν.
© 2009 by Taylor & Francis Group, LLC
64 Advanced Drying Technologies
where
Reud
L2 x
dpp 0
p p
(5.7)
The dimensionless time and distance are related to their maximum val-
For Rep
t 9.23L x 0.37L* 0.51 * 0.44 (5.8)
impingement zone of length H (Soloviev, 1964):
8uH
34d
H9ud
4p2
p
p2
p
(5.9)
ticle diameter are respectively given by
u4 H9d
d9 H4 up
2p
pp
(5.10)
Since the particle velocity decreases during consecutive passage through the impingement plane, during some oscillation, it approaches the gas
stream. This critical gas velocity at which the particle is carried away from the impingement zone can be calculated from the following equa-tion (Soloviev, 1964):
uDd
gdcrp
pp
0 9
1 7
.
/
(5.11)
or
u D dcr0.34
p0.36 p
0.5
0.25 5 6.
(5.12)
where β is the volumetric concentration (m3/m3) of the solid phase.
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ues, which appear in the first period of oscillation.> 800, Equation 5.5 can be simplified to the form
In a given impinging streams configuration, the following inequalities should be satisfied for the oscillatory motion of a single particle about the
Thus, the minimum gas velocity in the flow ducts and the minimum par-
velocity necessary to suspend the particle in a horizontally flowing gas
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 65
5.2.2 Pressure Drop
Because of the high gas velocities, solids’ loading ratios, and momentumloss in the collision zone, the pressure loss in ISDs is much greater than that
bed dryers and varies from 1 to 10 kPa for the range of parameters thatprovide a stable operation (Kudra and Mujumdar, 1989; Tamir, 1994). The
with the classical systems for drying of particulates and pastes (Table 5.2).s
can be calculated from
∆ ∆P Pu2s s
2
(5.13)
s is the fric-
velocity, and the solids’ concentration and can be estimated using the fol-lowing equation (Elperin et al., 1968):
s t 3420 0 18Re . (5.14)
This equation is valid for an ISD with a cylindrical impingement zone for45 ≤ Ret ≤ 1150 and β ≤ 0.0006 m3/m3.
Note that the term geometry of impingement zone refers only to the activefraction of the impingement chamber volume that contributes to the trans-fer processes. On the basis of numerous experiments, the limiting valueof H/D below which the impingement zone can be considered to be activeis 1.2–1.5.
TABLE 5.2
Performance of Selected Dryers for Granules of Aluminum Alloy
Parameter Fluidized Bed ISD
Drying timea (s) 300–480 600 1–20Unit gas consumptionb
(N m3/kg)8 13 1.2–2
Unit energyconsumptionb (kJ/kg)
1380 3240 850
Product qualityc Decreased Decreased Unchanged
Note: Reference productivity = 250 kg/h; dp = 0.65−2 mm; X1 = 0.17 kg/kg.a To reach X2 = 0.01 kg/kg.b Per kilogram of dry solids.c Estimated from oxidation tests.
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impinging stream configurations can, however, compete in various aspects
tion coefficient resulting from the solid phase present in a gas stream.This coefficient is related to the geometry of the impingement zone, gas
in pneumatic dryers, but it is comparable with that of fluidized and spouted
The pressure drop in the two-phase flow (∆P ) in the impingement zone
where ∆P is the pressure drop in the single-phase flow, and ζ
Vibrofluidized Bed
© 2009 by Taylor & Francis Group, LLC
66 Advanced Drying Technologies
calculated from the following correlations derived for impinging streams with cylindrical impingement zones for H/D varying from 0.25 to 1.125 (Elperin et al., 1968):
EuHD
8 31 3
0 25. Re.
.
(5.15)
for 1.2 × 104 ≤ Re ≤ 4 × 104 and
EuHD
0 61 3
..
for Re > 4 × 104.
5.2.3 Heat Transfer
Owing to the inherent hydrodynamics of ISDs, the heat-transfer rates from gas to particle depend on time and space. Reported data indicate, how-
from its local value, which gives acceptable accuracy for design calcula-tions (Meltser and Pisarik, 1980).
are much higher than those in classical dryers that operate under similar
in a coaxial ISD is 850 W/m2 K, the values for pneumatic dryers calcu-lated for the same operating conditions from several available equations ranged from 300 to 520 W/m2 K (Strumillo and Kudra, 1986). For the two-impinging streams (TIS) dryer with coaxial impinging streams, the heat-
similar spouted bed under the same operating conditions (Tamir, 1989).
125,000 W/m3
Mujumdar, 1989). Comparing the geometrically similar systems, one can
spouted bed and cyclone dryers (Tamir, 1989), and about 20 times higher than those for spray dryers (Kudra and Mujumdar, 1989). As the evap-
spray dryers.
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ever, that the average heat-transfer coefficient for ISDs differs by <10%
In general, the average gas-to-particle heat-transfer coefficients for ISDs
hydrodynamic regimes. For example, when the heat-transfer coefficient
transfer coefficient is 1.1–1.8 times higher than that for the geometrically
The volumetric heat-transfer coefficients for ISDs are also higher, reaching K for coaxial, countercurrent configurations (Kudra and
find that the volumetric heat-transfer coefficients for curvilinear, counter-current configurations are three to four times higher than the values for
oration rate during the first drying period is proportional to the heat-
The pressure drop for single-phase flow (∆P in Equation 5.13) can be
transfer rate, ISDs are much smaller than the spouted bed, fluid bed, and
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 67
calculated within ±18% from the following correlations (Meltser andPisarik, 1980):
Nu r0.61 0 173 0 55. Re . (5.17)
for 300 < Rer < 3500 and β ≤ 0.0009 m3/m3 or
Nu r 1 59 0 55. Re . (5.18)
for 300 < Rer < 3500 and 0.0009 < β < 0.0021 m3/m3, where the Reynolds number is based on the relative gas–particle velocity averaged over the length of the impingement zone.
mobile impingement zones can be calculated from the standard equation
Nu Gup 2 1 05 0 5 0 33 0 175. Re Pr. . . (5.19)
and averaged over the total residence time of particles, increases as the gas velocity increases and decreases as the gas temperature increases. Such a
in gas properties but also from an appreciable change in the hydrodynamic conditions; with an increase in the gas temperature, the penetration depth into opposing gas stream increases, which extends the period of unsteady particle motion, thus enhancing the total heat transfer.
5.3 Drying
Because drying in impinging streams (without use of inert media) is purely convective, both constant and falling drying rate periods can be observed if the processing materials have both internal and external
period is equal to the wet bulb temperature or slightly higher when dry-ing on inert particles because of supplementary conductive heat transfer. Generally, the period of a constant drying rate is short because of the high
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The average heat-transfer coefficient in an impingement zone can be
Interestingly, the heat-transfer coefficient in impinging streams with
For any impinging stream system, the heat-transfer coefficient, both local
for gas flowing past a sphere with a particle Reynolds number based on the gas velocity in an accelerating flow duct:
temperature influence on heat transfer results not only from the variation
resistances to mass flow. The material temperature during the first drying
© 2009 by Taylor & Francis Group, LLC
68 Advanced Drying Technologies
heat- and mass-transfer rates and is measured in fractions of a second. The dryer ducts are usually insulated, but it may be possible to heat the ISD walls and provide additional heat by conduction to enhance both the
literature for this case, however.Figure 5.7 shows the time variation of moisture content, temperature,
and location for 1 mm aluminum beads covered with a thin layer of sur-face water that is evaporated in an ISD with mobile impingement zones (Meltser et al., 1985). It can be seen that the heat-transfer rate differs sig-
the different gas–particle relative velocities. Despite the stabilizing effect of oscillatory motion, this difference can be as high as 35–40%. Because of surface water evaporation, the drying rate is controlled by the rate of external heat transfer. Thus, it is higher in the deceleration period than in the acceleration period because of the larger difference between gas and particle velocities. The particles are completely dry within 0.6 s, that is, within less than one cycle of oscillation.
Figure 5.8 presents the drying kinetics of crystalline lysine with an ini-tial moisture content of 15.2% in an ISD with mobile impingement zones.
0 0.5 1.0 1.5 2.0 2.5–2250
–1500
–1750
0
750
1500
2250
0
0.05
0.10
0.15T
X
x Dist
ance
(mm
)
Moi
stur
e con
tent
(% w
.b.)
0
100
200
300
Tem
pera
ture
(ºC
)
Time (s)
FIGURE 5.7Temporal variation of axial position, moisture content, and particle temperature in an ISD with a mobile impingement zone (Tg1TT = 250°C, u0 = 30 m/s, X1 = 0.11 kg/kg).
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drying capacity and thermal efficiency. No reports are available in the
nificantly in the period of particle deceleration and acceleration due to
The air temperature was 120°C, flow velocity 20–23 m/s, and the frequency
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 69
of reversing motion of ∼1.0 to 1.2 Hz (Meltser and Tutova, 1986). Curve 1 represents drying of monodisperse crystals (0.4 mm mean diameter) at mass concentrations of 0.2–0.5 kg of lysine per kilogram of air. In this case, the surface water is removed within one period of motion (2–3 s).
periods of oscillation. Curve 2 shows the drying kinetics for polydisperse lysine with a mean diameter (d = Σxidi) equal to 1 mm with simultane-ous grinding due to the oscillatory motion of the inert material (mixture in 1:1 mass ratio of 2 mm steel and 3 mm aluminum beads) at a mass concentration of 1–1.5 kg of inert particles per kilogram of drying material. Compared to curve 1, the drying rate with simultaneous disintegration is
reduction of the heat-transfer resistance inside the material and increase of the mass-transfer area due to particle disintegration. After a certain number of oscillations the drying rate decreases and follows a similar decrease in the intensity of crystal disintegration (curve 3).
Figure 5.9 presents a schematic of the impinging stream dryer used to dry a crystalline lysine. The dryer consists of air-conveying ducts with two impingement chambers situated at the ends of a central drying duct 1.5 m in length. Both chambers are connected through outlet ducts and
Cycles of treatment
Conc
entr
atio
n (%
mas
s)
Part
icle
dia
met
er (m
m)
0
0.2
0.4
0.6
0.8
1.0
3
2
1
0 1 2 3 4 5 6 70
2
4
6
8
10
12
14
16
Crystalline lysine
FIGURE 5.8Drying and grinding characteristics for lysine: 1—drying kinetics for monodisperse crys-tals, 2—drying kinetics for polydisperse crystals, and 3—grinding curve for polydisperse crystals.
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The desired moisture content level of 1% can be achieved within five
appreciably higher in the first period of reverse motion mainly due to a
cyclones to a flow-switching device. The outlet ducts are constrained by
© 2009 by Taylor & Francis Group, LLC
70 Advanced Drying Technologies
grids with 1.3 mm grid openings. A mixture of steel (2 mm) and alumi-num (3 mm) beads in 1:1 mass ratio is used to disintegrate the crystals and break lumps. The mass concentration of the inert beads in the drying duct is maintained at 1–1.5 kg/kg. The frequency of reverse motion between the impingement chambers is 1–1.2 cycles/s. Hot air at 135°C and 20–23 m/s enters both ends of the impingement chambers. A switching device moves the impingement zone periodically between the two impingement chambers. This results in an oscillatory motion of both the inert beads and wet particles, which are continuously fed to one end of the duct only. After performing 4–5 oscillations, which are equivalent to 10–15 s of drying time, the material is well ground and dried (see Figure 5.8) and is carried away through the constraining grids to the cyclones. The volu-metric evaporative capacity of the dryer is ∼1700 kg H2O/m3 h, whereasthe air and heat consumption (without heat recovery) per kilogram of water evaporated are 200–250 N m3 and 3–3.6 MJ, respectively. These show
(Meltser and Tutova, 1986).
and higher volumetric evaporation rates) was found for ISDs with a rectan-gular impingement chamber than for the cylindrical one that is commonly used in laboratory studies and industrial units (Cao and Liu, 2000).
Air in
Air out
Product
Wetmaterial
Switching device
Impingementchamber
Heater
Accelerating
Reverse
FIGURE 5.9ISD with mobile impingement zone for drying of crystalline lysine.
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Interestingly, better drying performance (lower final moisture contents
flow duct
flow duct
that the performance of the ISD is superior to that of a fluidized bed dryer
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 71
improved heat and mass transfer.More information on other types of industrial dryers with impinging
streams can be found in the open literature (Kudra et al., 1995; Kudra and
developed for thermal processing of grains in superheated steam (SHS)
ical reactions), the vortex spray dryer for suspensions and slurries that operates essentially as a countercurrent counterrotating ISD, the hybrid system based on drying of a sewage sludge in a coaxial ISD followed by
ing segregation in the centrifugal separator, the impinging stream spray dryer for salt solutions and products of biotechnology, as well as the ISD with swirling streams for highly dispersed pharmaceuticals.
5.4 Some Recent Studies on ISD
Mujumdar (1990) proposed a novel two-dimensional SHS ISD to minimize
from the advantages of SHS as a drying medium (discussed elsewhere in this book). Hosseinalipour and Mujumdar (1997) carried out a funda-
include drying of mono-sized particles entrained into one of the opposing jets with SHS as the drying medium. Furthermore, the effects of all key
operating pressure for various size particles entering the system at dif-ferent locations within the jet were examined. It was shown that under certain conditions the particles penetrate deep into the opposite stream before being re-entrained into the impingement zone. This phenomenon may repeat itself several times, resulting in longer residence times for the larger particles. The smaller particles follow the streamlines and exit the chamber with shorter residence times.
There are several advantages to a mathematical simulation of the sys-tem when there are numerous parameters affecting the outcome. ISD provides a particularly challenging transport problem for computa-
Wang and Mujumdar, 2007a, 2007b). In view of the fact that no detailed
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Mujumdar, 2007; Tamir, 1994). These are the semicircular configurations
(e.g., drying, puffing, and carrying out certain thermally induced biochem-
the problems of scale-up when using round ducts in ISDs and to benefit
verification of various geometrical configurations (Wang et al., 2006;
turbulence measurements are available in the literature for a confined
This effect can be attributed to a different flow pattern that results in
flash drying in the pneumatic duct and drying in a swirling stream dur-
mental computational fluid dynamic analysis of the confined opposing jet system for the first time to explore the flow fields in both single- and two-phase particle–gas turbulent flows. A two-phase turbulence model was used to predict the turbulent two-phase flow. This study was extended to
parameters (e.g., geometry, flow rate, and steam temperature) as well as
tional fluid dynamics (CFD) simulations but at the same time allows
© 2009 by Taylor & Francis Group, LLC
72 Advanced Drying Technologies
opposing jet system, the model of Hosseinalipour and Mujumdar remains to be validated with data. The results, however, are physically realistic. For numerical details, the reader is referred to the source work (Hosseinalipour, 1997).
Some of the basic conclusions of the simulation work are summarized as follows: The residence time of particles in the ISD decreases with increasing Reynolds number of the jet (stream) but increases with spac-ing between the opposing nozzles. Particles entering closer to the jet mid-plane tend to have longer residence times. Increasing steam temperature yields better drying performance. The effect of operating pressure is quite complex due in part to the highly nonlinear behavior of the system and the effect of pressure on thermophysical properties of steam. The optimum operating pressure for SHS drying of particles in the surface water removal period in the simple opposing jet system model was found
residence time for the particles. The existence of optimum pressure is,
inherently highly nonlinear nature of the hydrodynamics involved as well as the nonlinearity of temperature dependence of the thermal and physical properties of the carrier medium. This remains a challenging problem for both theoretical and experimental research.
More advanced mathematical models along with careful experimental data are needed to design and analyze ISD systems more reliably. There is a further complication associated with the fact that the entrained particles may agglomerate or disintegrate on collision with other particles. Also, little is known about the collision characteristics of wet particles, which must depend on their moisture content, as well.
Symbols
C Heat capacity, J/kg Kd Particle diameter, mD Duct diameter, mF Force, Ng Acceleration due to gravity, m/s2
h 2
H Distance between accelerating ducts, mk Thermal conductivity, W/m KL Dimensionless distance∆P Pressure drop, Pat Time, sT Temperature, K (°C)u Velocity, m/sx Coordinate, mX Moisture content, kg/kg (% w.b.)
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to be 202 kPa, which gave a higher heat-transfer coefficient and longer
however, very difficult to explain in simple physical terms due to the
Heat-transfer coefficient, W/m K
© 2009 by Taylor & Francis Group, LLC
Impinging Stream Drying 73
Greek Symbols
β Volumetric concentration of solids, m3/m3
μ Dynamic viscosity, kg/m sν Kinematic viscosity, m2/sρ Mass density, kg/m3
ζ Friction factor
Subscripts
cr Criticalg Gas p Particle (droplet) s Solid (suspension)t Terminalmax Maximum01 Inlet2 Outlet
Superscripts
* Dimensionless parameter
Dimensionless Numbers
Eu = ∆P/(u2ρ)ρρ Euler numberGu = (T – TpTT )/T Gukhman numberNu = hdp/k Nusselt numberPr = cμ/k Prandtl numberRe = u0D/ν Reynolds numberRep = u0dp/ν Particle Reynolds numberRer = (u ± up)dp/ν Reynolds number (based on relative velocity)Ret = utdp/ν Reynolds number (based on terminal velocity)
References
Cao, C. W. and Liu, X. D. 2000. Experimental study on impinging stream dry-ing of particulate materials. International Drying Symposium (IDS2000), Noordwijkerhout, the Netherlands (CD-ROM, paper No 76).
Dengying, L., Xiulan, H., Zuyi, L., Qun, M. and Xiaoming, C. 1999. Experimental
cular impinging stream dryer. Proc. Asian-Australian Drying Conference, Bali, Indonesia, pp. 652–659.
CRC_73877_Ch005.indd 73CRC_73877_Ch005.indd 73 12/26/2008 5:38:23 PM12/26/2008 5:38:23 PM
Initial/flow duct
investigation on the flow and drying characteristics of two stage semicir-
© 2009 by Taylor & Francis Group, LLC
74 Advanced Drying Technologies
Elperin, I. T., Enyakin, Yu. P. and Meltser, V. L. 1968. Experimental investigation of hydrodynamics of impinging gas-solid particles streams. In: Heat and MassTransfer. A. V. Luikov and B. M. Smolsky (Eds.). Minsk, Belarus, pp. 454–468(in Russian).
Elperin, I. T. and Meltser, V. L. 1978. Apparatus for thermal treatment of dispersed materials. Russian Patent 596792.
Elperin, I. T., Meltser, V. L., Pavlovskij, L. L. and Enyakin, Yu. P. 1972. Transfer Phenomena in Impinging Streams of Gaseous Suspensions. Minsk, Nauka i Tekhnika, 216p (in Russian).
Hosseinalipour, S. M. 1997. Transport processes in laminar and turbulent oppos-ing jet contactors. Ph.D. Thesis. McGill University, Montreal, Canada.
Hosseinalipour, S. M. and Mujumdar, A. S. 1997. A model for superheated steam drying of particles in an impinging stream dryer. In: Mathematical Modelingand Numerical Techniques in Drying Technology. I. Turner and A. S. Mujumdar (Eds.). Marcel Dekker, New York, pp. 537–574, 679p.
Huai, X. L., Liu, D. Y., Li, C. Z. and Meng, Q. 2000. The investigation of com-bined impinging stream dryer of vertical and sloped semi-circle. InternationalDrying Symposium (IDS2000), Noordwijkerhout, The Netherlands (CD-ROM, paper No 104).
Kudra, T. and Mujumdar, A. S. 1989. Impingement stream dryers for particles and pastes. Drying Technol., 7(2): 219–266.
Kudra, T. and Mujumdar, A. S. 2007. Special drying techniques and novel dryers. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). 3rd edition. Taylor & Francis, Boca Raton, FL, pp. 453–517.
Kudra, T., Mujumdar, A. S. and Meltser, V. L. 1990. Impinging stream dryers: prin-ciples, practice and potential. In: Drying of Solids. A. S. Mujumdar (Ed.). Sarita Prakashan, Meerut, India, pp. 17–32, 361p.
Kudra, T., Mujumdar, A. S. and Meltser, V. L. 1995. Impinging stream dryers. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). Marcel Dekker, New York, pp. 539–566, 1423p.
Levental, L. I., Meltser, V. L., Baida, M. M. and Pisarik, N. K. 1981. Investigation of solid particle motion in reverse gas-solid suspension. In: Investigation of Transport Processes in Dispersed Systems. ITMO AN BSSR, Minsk, Belarus,pp. 69–76 (in Russian).
Meltser, V. L., Gurevich, G. L., Starovoitenko, E. I. and Deryugin, A. I. 1985. The calculation method of thermal processing of wet particles in reversing gas-solid streams. In: Heat and Mass Transfer Investigations in Drying and Thermal Processing of Capillary-Porous Materials. ITMO AN BSSR, Minsk, pp. 131–138 (in Russian).
considerations for impinging stream dryers. Proc. Int. Forum on Heat andMass Transfer—Int. Drying Symposium. Vol. 1. Kiev, Ukraine, pp. 181–184 (in Russian).
Meltser, V. L. and Pisarik, N. K. 1980. Interphase heat transfer in impinging sin-gle and two-phase streams. Proc. XI All Russian Conference on Heat and Mass Transfer, Vol. VI, Part 1, Minsk, pp. 132–135 (in Russian).
Meltser, V. L. and Tutova, E. G. 1986. Drying of crystalline lysine in reversing impinging streams. Biotekhnologia, 2: 70–74 (in Russian).
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Meltser, V. L., Kudra, T. and Mujumdar, A. S. 1992. Classification and design
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Impinging Stream Drying 75
Mujumdar, A. S. 1990. Superheated steam drying: principles and potential for use of electricity. CEA Report No 817 U 671.
Mujumdar, A. S. 2007. Impingement drying. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). 3rd edition. Taylor & Francis, Boca Raton, FL, pp. 385–395.
Strumillo, C. and Kudra, T. 1986. Drying: Principles, Applications and Design. Gordon and Breach Science Publishers, New York, 345p.
Strumillo, C., Kuts, P. S. and Zbicinski, I. 1996. The analysis of swirl-impact dryer hydrodynamics. In: Drying’96. Proc. 10th International Drying Symposium(IDS96). Krakow, Poland, pp. 569–576.
Soloviev, M. I. 1964. Suspending and transportation of granular materials in hori-zontal ducts. Inzhenerno Phyzicheski Zhurnal, 7(10): 62–67 (in Russian).
Tamir, A. 1989. Vertical impinging streams and spouted bed dryers: comparison and performance. Drying Technol., 7(2): 183–204.
Tamir, A. 1994. Impinging-Stream Reactors: Fundamentals and Applications. Elsevier,Amsterdam, 679p.
Tamir, A. and Grinholtz, M. 1987. Performance of continuous solid-liquid two-impinging-stream (TIS) reactor: dissolution of solids, hydrodynamics, mean residence time, and holdup of the particles. Ind. Eng. Chem. Res., 26(4):726–731.
Wang, S. J., Devahastin, S. and Mujumdar, A. S. 2006. Effect of temperature differ-
Appl. Therm. Eng., 26(5–6): 519–529.
mixing characteristics of a novel in-line opposing-jet mixer. Ind. Eng. Chem. Res., 46(2): 632–642.
Wang, S. J. and Mujumdar, A. S. 2007b. Flow and mixing characteristics of multiple and multi-set opposing jets. Chem. Eng. Process., 46(8): 703–712.
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ence on flow and mixing characteristics of laminar confined opposing jets.
Wang, S. J. and Mujumdar, A. S. 2007a. Three-dimensional analysis of flow and
© 2009 by Taylor & Francis Group, LLC
77
6Drying in Pulsed Fluid Beds
Fluidization is one of the most commonly used techniques for drying particulate solids. The technique relies on a high-velocity hot gas stream
A relatively high pressure drop that calls for high-pressure blowers• Particle shape restricted to those approaching sphere or cuboid•Bed height limited to between the minimum height that will •
the pressure drop•
bed instability and reduced transfer characteristics
The negative effects from the last two constraints can be alleviated or
is used in some dryers (e.g., Anonymous, 1990), in most cases gas pulsa-
gas velocity varies across the bed area from zero to the maximum. Better
tional pressure drops due to the on–off operation of the valve as well as by the back-pressure developed that can create problems with direct gas
in a design patented by Sztabert et al. (1978), pulsation of a gas stream is used to vibrate the supporting grid in a special type of the vibrated
by 90°. Such a design greatly reduces the negative effects of a single pulsa-tor as the gas shut-off phase is alternated between two longitudinal seg-ments of the bed.
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Restricted particle size and size distribution; polydisperse mate-rials cause excessive entrainment, whereas both fine and coarse
rotated at a specified angular velocity. Thus, the instantaneous superficial
firing. Vibration of the dryer body is also of important concern, although
that creates a fluid bed with special hydrodynamic and heat/mass-transfer
have the following disadvantages that limit their application:characteristics. Although well entrenched in industry, fluid bed dryers
ensure uniform fluidization and the maximum height allowed by
materials do not fluidize well• Susceptible to aggregative fluidization or channeling resulting in
even eliminated by applying periodic flow of the fluidizing gas, which creates the so-called pulsed fluidized bed (PFB). Although simple shut-off and start-up of a fluidizing gas stream with frequency from 0.5 to 4 Hz
tion is generated by a butterfly valve installed in the gas inlet duct and
hydrodynamics of such a pulsated fluid bed are, however, offset by addi-
fluid bed. Figure 6.1 presents the schematic of a PFB dryer manufactured by Bühler-Miag GmbH (Germany) where pulsation of a fluidizing gas is obtained by the rotation of a dual butterfly valve with wings displaced
© 2009 by Taylor & Francis Group, LLC
78 Advanced Drying Technologies
supplied sequentially to different sections of the bed. In fact, the idea of
particles originated nearly 50 years ago (Belik, 1960), but it was not fol-lowed up. A similar principle of the gas jet relocation has been proposed
jet (Gawrzynski, 1987; Elenkov et al., 1989). With reference to Figure 6.2,
tinuous gas jet is injected into the bed through a rotating disk with one
is relatively low (several revolutions per minute [rpm]), the bed behaves as the rotating-jet spouted bed with a distinct spout moving around an
The basic hydrodynamic parameters of the RPFB can be calculated from the following relationships (Elenkov and Djurkov, 1992; Djurkov, 1998):
Re 0.013Armf0.63 (6.1)
∆PmF
9 36 160. (6.2)
Product
ExhaustWet feed
Pulsator
Hot air
FIGURE 6.1
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relocation of a supplementary gas to eliminate channeling in a FB of fine
or several sector-shaped orifices. When the rotational speed of the disk
essentially fixed bed of particles (see Chapter 34). When the disk rotates with sufficient velocity, which depends on the number of orifices, the bed
cation >2 Hz, the transition from fixed to FB as well as the bed pressure
as additional benefits of the RPFBs (Elenkov and Djurkov, 1992).
PFB dryer with a dual butterfly valve.
The pulsed fluidization can be further altered with some advantage if one uses periodic relocation of the gas stream, that is, the fluidizing gas is
to fluidize particulate material by sequential relocation of the spouting gas
the so-called rotation-pulsed fluidized bed (RPFB) is formed when a con-
starts to be fully fluidized. It was found that for a frequency of gas jet relo-
drop do not depend on the shape or particle size. High fluidization ratio (operating air velocity to minimum fluidizing velocity) at lower volumet-ric flow rates as well as uniform but intensive particle mixing are claimed
© 2009 by Taylor & Francis Group, LLC
Drying in Pulsed Fluid Beds 79
or
∆P K K ( . [ . ])1 0 52 1 2 7 10 4
2
KmgF
g
s
1
and Remffvelocity and equivalent particle diameter.
Feed Exhaust
Valve
Gasdistributor
A
Gas distributorProduct
Bearings
Gas in
A
Supportinggrid
A-A
FIGURE 6.2
swirling inside a chamber, Polish Patent 150,412, 1987.)
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Orifice
where the parameter K (N/m ) is defined as
Rotation-PFB dryer. (From Gawrzynski, Z., Apparatus for generation of a pulsed fluid bed
is the Reynolds number defined by the minimum fluidization
© 2009 by Taylor & Francis Group, LLC
80 Advanced Drying Technologies
As reported by Djurkov (1998), Equation 6.3 was used for the successful designing of two industrial dryers: for sesame seeds and minerals with capacities of 100 kg/h and 3000 kg/h, respectively. More information on hydrodynamics, drying kinetics, and modeling of this dryer type can be found in recent papers by Reyes et al. (2007, 2008).
gas stream is the so-called PFB with relocated gas stream, which combines
Glaser, 1996; Gawrzynski et al., 1989; Nitz and Taranto, 2007a). A PFB dryer
that the plenum chamber below the supporting grid is divided into sev-eral sections by vertical partitions (Figure 6.3). The primary hot air enters the dryer through a rotary valve distributor, which periodically interrupts the air stream and directs it in rapid succession to different gas chambers
that segment of the bed, which is located above the currently active cham-
the air stream is directed to the next chamber. In practice, however, the
as long as the frequency of gas relocation is high enough (typically from6 to 12 Hz). Even a simple on–off valve can be used to direct a gas stream to consecutive sections of the bed. The advantage of the rotary valves is, how-
Exhaust
Active chamber
Wet
feed
Product
Gas distributor
Supportinggrid
Hot air
FIGURE 6.3Principle of the PFB with relocated air stream.
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Another drying technology that is based on relocation of the fluidizing
features of pulsofluidization with gas stream relocation (Gawrzynski and
is essentially similar in design to a conventional fluid bed dryer except
below the supporting grid. An air stream in the active chamber fluidizes
ber. This fluidized segment of the bed becomes almost stagnant when
entire bed is fairly fluidized because of gas compressibility and bed inertia
ever, that the nearly sinusoidal variation of the gas flow makes relocation
© 2009 by Taylor & Francis Group, LLC
Drying in Pulsed Fluid Beds 81
of the active bed smoother. In addition, a special design of a disk-type gas
zation by superposing constant and pulsed air streams (Kudra et al., 1999).
along the trough-type drying chamber, thereby inducing the crosswise pulsation, it is possible to arrange for lengthwise pulsation that is claimed to reduce the dryer size and improve the bed structure (Gawrzynski and
with a rectangular cross-sectional area (Jezowska, 1991, 1993).The hydrodynamic characteristics of a PFB are essentially the same
the pressure–velocity curve; the pressure drop increases gradually
Zgorzalewicz and Glaser, 1989). Although the pressure drop in the PFB
velocity and the same free cross-sectional area of the supporting grid, the PFB technology offers a lower pressure drop because of the follow-ing combined effects:
1. Larger free cross-sectional area (up to 30%) of the supporting grid
0 0.5 1.0 1.5 2.00
1
2
3
4
Gas velocity (m/s)
Pres
sure
dro
p (k
Pa)
upfd
upfd
upfi
upfi
Peasf = 51/sX = 11% w.b.
Hst (m)
0.2 0.4FB
PFB
FIGURE 6.4Pressure drop in an FB dryer and a PFB dryer with relocated air stream. (Adapted from Zgorzalewicz, J. and Glaser, R., Chem. Proc. Eng., 1, 3–21 [in Polish], 1989.)
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Although in a typical configuration of the PFB the gas stream relocates
Pieczaba, 2007). A modification of the PFB with relocated gas stream is a
tributor periodically activates the bed of particles in a tapered chamber spouted bed configuration where the gas stream from a rotary valve dis-
distributor eliminates the back-pressure effect as well as facilitates fluidi-
as those of fluidized or spouted beds. However, there is no peak in
with gas velocity, even with fully developed fluidization (Figure 6.4;
is of the same order as that in a classical fluid bed for the same gas
as compared to several percent in the case of conventional fluid beds otherwise needed for stable fluidization
© 2009 by Taylor & Francis Group, LLC
82 Advanced Drying Technologies
2. Lower operating velocity (on average by 8–25%) because both the
shifted toward lower values
Examples of correlations for pressure drop in a PFB are given in Table 6.1.
zation velocity) and above, the bed performs like the vibrated or vibro-
static bed height, and frequency of the air stream relocation (Gawrzynski and Glaser, 1996; Blacha-Jurkiewicz et al., 1987). With increasing air veloc-ity, more air bubbles and isolated spouts appear in the bed; this velocity
velocity, the bed turns into a classical FB, and the effect of the pulsating
A thorough review of the published papers and our own experiments indicate that the PFBs can handle a variety of particulate materials rang-
through large granules and crystals up to several millimeters (pulp and
size and shape, as well as adhesive and cohesive materials such as sugar-infused berries (Grabowski et al., 2007; Nitz and Taranto, 2007b). The advan-
The typical operating parameters of a PFB dryer are
Bed height• . From 0.1 to 0.4 mGas velocity• . From 0.3 to 3.5 m/s (depending on the particle characteristics)Pressure drop• . From 300 to 1800 PaFrequency of gas pulsation• . From 4 to 16 Hz
Table 6.2 (Glaser, 1991) provides, as an example, the set of design equations for drying sliced vegetables in a PFB dryer with relocated gas stream.
experiments with wheat, barley, beet, onion, rapeseed, and carrot seeds is given in Table 6.3. Detailed information as well as relationships for other products such as sugar or pharmaceuticals can be found in a comprehen-sive paper by Gawrzynski and Glaser (1996).
Drying kinetics in a PFB (Figure 6.5) are similar to those for convec-tive drying in a system with perfect gas–solid contact, which conforms to the thin-layer drying conditions. The following equations are given as examples.
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ing from fine powders of ∼0.05 mm in diameter (recycled polypropylene)
foil, sliced vegetables), and fiber (wood waste) to large strips like wood
Another set of equations for a spouted bed configuration obtained from
minimum and the maximum pulsed fluidization velocities are
It should be noted that the term fluidization does not adequately reflectthe bed behavior as particles in the PFB are vibrated rather than fluidized. Thus, at incipient pulsed fluidization (also called the lower pulsed fluidi-
fluidized bed with frequency and amplitude depending on air velocity,
is called the upper (developed) pulsed fluidization velocity. Above this
airflow, and thus particle vibration, become relatively insignificant.
paper [P&P] primary sludge, potash, beans), flakes (shredded polyethylene
strands. Therefore, the real advantage of the PFB over the classical fluid bed is its ability to process materials that are difficult to fluidize because of
tage over the vibrofluidized bed is that the dryer body does not vibrate.
© 2009 by Taylor & Francis Group, LLC
Drying in Pulsed Fluid Beds
83
TABLE 6.1
Pressure Drop in PFB for Selected Materials
Material Correlation Remarks References
Recycledpolypropylene ∆P gH (Pa)st
de = 0.00005–0.0011 mu = 0.18 – 0.25 m/sf = 5–15 1/sN = 3
Gawrzynski et al., 1999
Acenola
PentaerythritolCalciumgluconicum
∆P 1.52u Re ArHd
2 1.45 0.68
1.01st
e
Hst = 0.05 – 0.2 mf = 5–15 1/sde = 0.0006–0.0024 mN = 3
(Gawrzynski et al., 1989)
Thiohexameb
∆P 7242 u H f0.409st 1.06 0.08
st = 0.055–0.2 mf = 5–15 1/sN = 3de = 0.0015 m
(Gawrzynski et al., 1989)
Peas and bean ∆P 28.73 u H
d
uu
d f21.06
st
e
0.86
mf
0.17e
mf
uu
X0.15
Hst = 0.05–0.4 mf = 2.9 – 10.7 1/sN = 6
(Gawrzynski et al., 1989)
P&P primary sludge
∆ ∆P 0.47 P H f (Pa)st st0.18 0.19
de = 0.0001–0.01 mf = 5 – 10 1/sHst = 0.05 – 0.2 mN = 3u = 0.1 – 1.4 m/s
(Kudra et al., 2002)
a OHC6H4NHCOCH3.b C13H16S2N2.
CR
C_73877_C
h006.indd 83C
RC
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12/18/2008 9:14:04 PM
© 2009 by Taylor & Francis Group, LLC
84 Advanced Drying Technologies
TABLE 6.2
Design Correlations for Drying Sliced Vegetables in a PFB Dryer
Parameter Equation
pfi
e
st 3 0.442
stu 0.486
d Ar
1( )
and ε are not known)pfi
e
0.386u 0.612 d
Ar
velocities ratio
For raw materialpfd
pfi
u
u
1 91 15. %
For productpfd
pfi
u
u1.88 20%
Pressure drop (Pa) (bed of raw material)
pfi
pfdu u 0.7 u
∆Pu
ArHd2
1.33 st
e 2 75 10 5 1 72
1 48
. Re .
.
. −
Pressure drop (Pa) (bed of product, except onion)
pfi
pfdu u u
∆P 1229 u H 0.76st 1.09
Pressure drop (Pa) (at u upfd )a
Bed of dry product ∆P H gst bd
Bed of raw material ∆P H (1 ) g H gst m st st bw
b ∆ ∆P PXX
00
0 61
.
b H HXX
00
0 28
.
: Parsley, carrot, beet, celery, potato, and onion.a ρbd and ρbw are bulk densities of the product and raw material, respectively; ρm is solid (wet
material) density.b Index 0 refers to the initial conditions.Source: From Glaser, R., Proc. of VII Drying Symposium, Lodz, Poland (in Polish), 147–154,
1991.
TABLE 6.3
Equations for Pulsed Spouted Bed Dryer for Grain and Vegetable Seeds
Parameter Equation
Pressure drop at incipient spouting (Pa) 12.08 Hst1.11 ρb
0.99
Pressure drop at fully developed spouting (Pa) 8.63 Hst0.97 ρb
0.99
Incipient spouting velocity (m/s) 72.87 Hst0.52 de
0.62 ρb0.09
Fully developed spouting velocity (m/s) 190.56 Hst0.56 de
0.74 ρb0.08
Volume: Technology, 605, 99–107 (in Polish), 1991.
CRC_73877_Ch006.indd 84CRC_73877_Ch006.indd 84 12/18/2008 9:14:05 PM12/18/2008 9:14:05 PM
Source: From Jezowska, A., Scientific Papers of Wroclaw Academy of Economics,
Incipient pulsed fluidization velocity (m/s)
Incipient pulsed fluidization velocity (m/s) (if φ
Developed/incipient pulsed fluidization
© 2009 by Taylor & Francis Group, LLC
Drying in Pulsed Fluid Beds 85
For sliced vegetables (beet, parsley, carrot, celery, potato, onion) dried in a rectangular PFB with relocated air stream (Gawrzynski and Glaser, 1996),
XX
e kt
0
(6.5)
1993),X X
X Xkteq
eq
n
0
exp( ) (6.6)
where
k B T Cg 273( ) (6.7)
B = −1.94 × 10−12; C = 3.66; n = 0.78 (for beet seeds)B = −1.91 × 10−21; C = 7.28; n = 0.63 (for wheat seeds)
A comparison of the PFB dryer with conventional band dryers for sliced vegetables operated under the same conditions (Table 6.4; Glaser and Gawrzynski, 1990) clearly indicates better performance of the PFB dryer, which results in lower energy consumption and smaller dryer volume.
dehydrated cranberries dried in the PFB over conventional dryers such as
(Grabowski et al., 2002).
Time (min)
Mat
eria
l tem
pera
ture
(˚C)
40
35
30
25
20
15
10
5
00
u = 0.21 m/sTg=70˚CHst= 0.1 m
60
45
30
15
020 40 60 80 100
Moi
stur
e con
tent
(% w
.b.)
FIGURE 6.5
Drying and temperature curves for recycled polypropylene. (Reproduced from Gawrzynski, Z., Glaser, R. and Kudra, T., Drying Technol., 17(7&8), 1523–1532, 1999. With permission.)
CRC_73877_Ch006.indd 85CRC_73877_Ch006.indd 85 12/18/2008 9:14:09 PM12/18/2008 9:14:09 PM
For wheat and beet seeds dried in the spouted bed configuration (Jezowska,
Advantageous energy efficiency of PFB was also found for osmotically
cabinet dryers, vacuum shelf dryers, fluid bed dryers, and freeze-dryers
© 2009 by Taylor & Francis Group, LLC
86 Advanced Drying Technologies
It should be noted that the PFB can also be used to perform other opera-tions such as coating (Nitz and Taranto, 2008), roasting, granulation, and the like.
Symbols
d Particle diameter, mf Frequency of pulsation, 1/sF Bed cross-sectional area, m2
g Acceleration due to gravity, m/s2
H Bed height, mK Parameter in Equation 6.3, Pam Mass, kgN Number of chambers∆P Pressure drop, Pat Time, sT Temperature, °C
X Material moisture content, kg/kgε Bed voidageφ Shape factorμ Viscosity, kg/m sρ Density, kg/m3
Subscripts
b Bulkbd Bulk (dry)bw Bulk (wet)e Equivalenteq Equilibriumg Gasm Material
TABLE 6.4
Comparison of the PFB with Band Dryers for Sliced Parsley and Celery Roots
Index PFBaBand Dryer SP-66
(Yugoslavia)Band Dryer
41A (Bulgaria)
Unit consumption of thermal and electrical energy
1.0 1.16 1.37
Floor area (dryer only) 1.0 4.7 6.25Space volume (dryer only) 1.0 3.04 4.06
Note: Throughput, 150 kg/h of dry product.a The PFB dryer is taken as a basis.Source:
Volume: Technology, 528, 71–87 (in Polish), 1990.
CRC_73877_Ch006.indd 86CRC_73877_Ch006.indd 86 12/18/2008 9:14:10 PM12/18/2008 9:14:10 PM
From Glaser, R. and Gawrzynski, Z., Scientific Papers of Wroclaw Academy of Economics,
u Superficial velocity, m/s
mf Minimum fluidization
© 2009 by Taylor & Francis Group, LLC
Drying in Pulsed Fluid Beds 87
s Solidst Static0 Reference, initial
Superscripts
d Developed (upper)i Incipient (lower)
Dimensionless Numbers
Argde
3s g
( )2
Archimedes number
Re ude
Reynolds number
Acronyms
FB Fluidized bed
P&P Pulp and paperrpm Revolutions per minute
w.b. Wet basis
References
Anonymous. 1990. Drying and Thermal Treatment of Wet Materials. Nauka i Tekhnika, Minsk, Belarus (in Russian).
23(4): 253–257 (in German).Blacha-Jurkiewicz, J., Gawrzynski, Z. and Glaser, R. 1987. Drying of granular mate-
Publishing Corporation, Washington, pp. 109–114.
ized bed dryer. In: Drying’98. C. B. Akritidis, D. Marinos-Kouris and G. D.Saravacos (Eds.). ZITI Editions, Thessaloniki, pp. 160–167.
moisture content bioproducts. In: Drying’92. A. S. Mujumdar (Ed.). Elsevier Science Publishers, Amsterdam, pp. 1636–1641.
Elenkov, V. R., Djurkov, T. G. and Kondov, P. S. 1989. Fluidized bed device. Bulgarian Patent 47977.
inside a chamber. Polish Patent 150,412.
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pf Pulsed fluidization
PFB Pulsed fluidized bed
RPFB Rotation-pulsed fluidized bed
Belik, L. 1960. Drying of granular materials by fluidization. Chemie-Ing.-Technik,
rial in pulsofluidized bed. In: Drying’87. A. S. Mujumdar (Ed.). Hemisphere
Djurkov, T. G. 1998. Modeling of bed pressure drops in rotation-pulsed fluid-
Elenkov, V. R. and Djurkov, T. G. 1992. Rotating pulsed fluidized bed dryer for high
Gawrzynski, Z. 1987. Apparatus for generation of a pulsed fluid bed swirling
© 2009 by Taylor & Francis Group, LLC
88 Advanced Drying Technologies
stream. In: Drying Technology – Special Issue on Progress in Drying Technologies. A. S. Mujumdar and T. Kudra (Eds.). Vol. 14, Number 5, Marcel Dekker, New York, pp. 1121–1172.
cyclically relocated gas stream with lengthwise pulsation. Drying Technol., 24(9): 1133–1137.
Gawrzynski, Z., Glaser, R. and Kudra, T. 1999. Drying of powdery materials in a
Gawrzynski, Z., Glaser, R. and Zgorzalewicz, J. 1989. Drying of granular material
Proc. VII Drying Symposium. Lodz, Poland (in Polish), pp. 147–154.
71–87 (in Polish).Grabowski, S., Marcotte, M., Poirier, M. and Kudra, T. 2002. Drying characteris-
tics of the osmotically pretreated cranberries—energy and quality aspects. Drying Technol., 20(10): 1989–2004.
Grabowski, S., Marcotte, M., Quan, D., Taherian, A. A., Zereifard, M. R., Poirier, M. and Kudra, T. 2007. Kinetics and quality aspects of Canadian blueberries and cranberries dried by osmo-convective method. Drying Technol., 25(3): 367–374.
Jezowska, A. 1991. Studies on hydrodynamics of spouted bed with cyclically relo-
Volume: Technology, 605: 99–107 (in Polish).Jezowska, A. 1993. Kinetics of drying in a cyclically shifted spouted bed. In: Drying
Technology – Special Issue on Spouted Beds. T. Kudra and G. S. V. Raghavan (Eds.). Vol. 11, Number 2, Marcel Dekker, New York, pp. 319–337.
5,918,569.Kudra, T., Gawrzynski, Z., Glaser, R., Stanislawski, J. and Poirier, M. 2002. Drying
917–933.
Conference. G. Chen (Ed.). August 13–15, Hong Kong, China, pp. 185–190.
bed of inert particles. Drying Technol., 26(1): 122–131.
bed. Drying Technol., 25(4): 581–590.
bed of granular materials. Polish Patent 92,255.
coarse materials. Chem. Proc. Eng., 1: 3–21 (in Polish).
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bed. Scientific Papers of Wroclaw Academy of Economics. Volume: Technology, 528:
cated pulsed air stream. Scientific Papers of Wroclaw Academy of Economics.
Gawrzynski, Z. and Glaser, R. 1996. Drying in a pulsed-fluid bed with relocated gas
Gawrzynski, Z. and Pieczaba, B. 2007. Hydrodynamics of the pulsed fluid bed with
pulsed fluid bed dryer. Drying Technol., 17(7&8): 1523–1532.
in pulsofluidized bed. Hung. J. Ind. Chem., 17: 245–255.Glaser, R. 1991. Possibilities for drying of sliced vegetables in a pulsed fluid bed.
Glaser, R. and Gawrzynski, Z. 1990. Drying of sliced vegetables in the pulsed fluid
Kudra, T., Gawrzynski, Z. and Glaser, R. 1999. Pulsed fluidized bed. U.S. Patent
of pulp and paper sludge in a pulsed fluid bed dryer. Drying Technol., 20(4):
Nitz, M. and Taranto, O. P. 2007a. The influence of process vaiables on the drying of a porous material in a pulsed fluid bed. Proc. 5th Asia-Pacific Drying
Nitz, M. and Taranto, O. P. 2007b. Drying of beans in a pulsed fluid bed dryer: Drying kinetics, fluid-dynamic study and comparisons with conventional
Nitz, M. and Taranto, O. P. 2008. Film coating of theophylline pellets in a pulsed-fluidization. J. Food Eng., 80: 249–256.
fluid bed coater. Chem. Eng. Proc., 47(8): 1412–1419.Reyes, A., Herrera, N. and Vega, R. 2008. Drying suspensions in a pulsed fluidized
Reyes, A., Moyano, P. and Paz, J. 2007. Drying of potato slices in a pulsed fluidized
Sztabert, Z., Przybyla, J. and Grzelczyk, Z. 1978. Method for generating a fluidized
Zgorzalewicz, J. and Glaser, R. 1989. Hydrodynamics of a pulsed fluid bed of
© 2009 by Taylor & Francis Group, LLC
89
7Superheated Steam Drying
7.1 Introduction
Although the concept was originally proposed over 100 years ago and the
superheated steam drying (SSD) has emerged as a viable technology with immense potential only in the past decade or so. Essentially, SSD involves the use of superheated steam (SHS) in a direct (convective) dryer in place
for drying and to carry off the evaporated moisture. Any direct or direct/indirect (e.g., combined convection/conduction) dryer can be operated as an SHS dryer, in principle. However, the SSD technology is more com-plex, and hence this conversion is not simple. Also, additional criteria must be considered when selecting a dryer for possible adaptation to SSD conditions.
Currently, fewer than 10 major dryer manufacturers around the world offer SSD technology on a commercial scale. Often, it is necessary to cus-tom design an SHS dryer for a new application, which may involve a new dryer type or an application to a new product. Mujumdar (1990) has pre-sented an extensive review of principles, practices, industrial applications, potential new applications, market penetration potential, and research and development (R&D) needs for SSD technologies. Special consider-ation is given to potential applications for use of electricity and reuse of the exhaust steam from an SSD. Kumar and Mujumdar (1990) have pro-vided an extensive bibliography and discussed the basic principles and applications of SSD.
One of the obvious advantages of SSD is that the dryer exhaust is also
Indeed, at current world prices of energy sources, it is often more expen-sive to recover energy in the exhaust stream than to waste it in the stack gases for most direct dryers with low- to medium-temperature exhaust.
possible to recover all latent heat supplied in the SSD from the exhaust by condensing the exhaust stream or by mechanical or thermocompression
the SSD produces steam equal in amount to the water evaporated in the
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first industrial applications were reported some 70 years ago in Germany,
steam, albeit at lower specific enthalpy. In air drying, the latent heat in the exhausted stream is generally difficult and expensive to recover.
If air infiltration is avoided (or minimized to an acceptable level), it is
to elevate its specific enthalpy for reuse in the mechanical dryer. Since
of hot air, combustion, or flue gases as the drying medium to supply heat
© 2009 by Taylor & Francis Group, LLC
90 Advanced Drying Technologies
dryer, it is necessary to have a useful application for this excess steam in the process plant. If this steam is used elsewhere, the latent heat recovered
1000–1500 kJ/kg water removed for the SSD compared with 4000–6000 kJ/kg water removed in a corresponding hot air dryer. Thus, reduced net energy consumption is a clear advantage of the SSD. Other key advan-tages of SHS dryers are as follows:
No oxidative or combustion reactions are possible in SSDs. This •
product.Higher drying rates are possible in both constant and falling • rate periods, depending on the steam temperature. The higher thermal conductivity and heat capacity of SHS lead to higher drying rates for surface moisture above the so-called inver-sion temperature. Below the inversion temperature, drying in air is faster. Higher product temperature in SSD (over 100°C at1 bar) and no resistance to diffusion of water vapor in SHS are the main reasons for the accelerated drying rate above the inversion temperature. Also, it is known that many products that exhibit the so-called case-hardening in rapid air drying do not form such water-impermeable shells when dried in SHS.For products containing toxic or expensive organic liquids that •
explosion while allowing condensation of the off-streams in rela-tively smaller condensers.SSD permits pasteurization, sterilization, and deodorization of • food products.
Accompanying these aforementioned advantages are several limitations:
The SHS system is more complex. No leaks can be allowed since • noncondensables cause problems with energy recovery by com-pression or condensation. Feeding and discharge devices must
bring in noncondensables. Start-up and shutdown are more com-plex operations for SSD than for an air dryer.Since feed enters the dryer at ambient temperature, there is inev-•itable condensation in the SSD before evaporation begins. This adds ∼10 to 15% to the residence time in the dryer. At 1 bar operat-ing pressure, the drying begins at a product temperature of 100°C in the constant-rate period when surface water is being removed. Alternatively, a preheater is needed for the feedstock.
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is not charged to the SSD, leading to a net energy consumption figure of
means no fire or explosion hazards and often also a better-quality
must be recovered, steam drying avoids the danger of fire or
protect the dryer from infiltration of air. The product itself may
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 91
Products that may melt, undergo glass transitions, or be other-• wise damaged at the saturation temperature of steam at the dryer operating pressure clearly cannot be dried in SHS even if they contain only surface moisture. Operation at reduced pressure, however, is a feasible option that may also enhance the drying rate. Products that may require oxidation reactions (e.g., browning of foods) to develop desired quality parameters cannot be dried in SHS. However, it may be possible to consider a two-stage dry-ing process (e.g., steam drying followed by air drying). For dry-ing of silk cocoons, for example, such a process appears to yield a
If the steam produced in the dryer is not needed elsewhere in the • process, the energy-related advantages of SSD do not exist. Also, steam cleaning may not always be a simple task. The chemical composition of the condensate must be carefully evaluated. The cost of the ancillaries (e.g., feeding systems, product collection systems, and exhaust steam recovery systems) is typically much
ated and very large tonnage systems because of the technoeco-nomics of the ancillary equipment needed.
cantly in the coming decade. In the meantime, more pilot testing is recommended for the SSD application since the quality aspects are generally unpredictable although the drying rates are predict-
7.2 Basic Principles of Superheated Steam Drying
Superheated steam has heat-transfer properties superior to air at the same temperature. As there is no resistance to diffusion of the evaporated water in its own vapor, the drying rate in the constant rate period is dependent
between steam and the solid material surface can be estimated using stan-dard correlations for interphase heat transfer. Neglecting sensible heat effects, heat losses, and other modes of heat transfer, the rate at which surface moisture evaporates into steam is given simply by
wqH
h T TH
ss sD
( )
(7.1)
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higher-quality product, namely, brighter and stronger silk fiber.
more significant than the cost of the steam dryer alone. In most cases, the SSD is a justifiable option only for continuously oper-
• There is currently limited field experience with SSD for a smaller range of products. This database is expected to increase signifi-
able with some confidence in some cases.
only on the heat-transfer rate. The convective heat-transfer coefficient (h)
© 2009 by Taylor & Francis Group, LLC
92 Advanced Drying Technologies
where wD is the evaporation rate (kg water/m2 s), TsTT is the drying sur-face temperature, which corresponds to the saturation temperature at the operating pressure, and TssTT is the temperature of the SHS.
In hot-air drying, the surface temperature is equal to the wet bulb tem-perature (TsTT = TWB), and hence at the same gas temperature, the tempera-
enon of inversion—a temperature at which SSD rate is greater than dry-air
for water as well as several organic solvent systems (superheated vapor dry-ing). It is observed that the inversion temperature is in the order of 160–
for convective drying when surface moisture is being removed. The inver-
not for internal moisture removal. Note that SSD may lower the critical moisture content, as well (Shibata et al., 1988).
Often, it is observed that drying rates in the falling rate period are also higher in SSD relative to air drying. The mechanisms responsible for this phenomenon are, however, quite different. In SSD, the product tempera-ture is higher, and hence moisture mobility is greater. Note that in a steam environment, case-hardening (or skinning) may not occur, which elimi-ggnates a possible impediment to intensive drying. The product is likely to be more porous, as well.
SteamRH = 100%
Dry airRH = 0%
100°CTemperature (°C)
InversiontemperatureD
ryin
g ra
te
P = 100 kPa
RH2>RH1
RH1
FIGURE 7.1Drying rate versus gas temperature for air and steam.
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(h) is lower. It turns out that these counteracting effects lead to the phenom-ture difference (∆∆∆T) is higher in air drying but the heat-transfer coefficient
drying rate (Figure 7.1). This is confirmed experimentally and numerically
sion temperature is thus defined only for surface moisture evaporation and
200°C for evaporation of water in SHS for various flow configurations and flow regimes, that is, laminar/turbulent boundary layer flows, impinging
SHS will yield high drying rates above the corresponding air drying rates jet flows, free convective flow over bodies of complex geometry, etc. Thus,
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 93
7.3 Steam Dryers
ing pressure. The product temperature necessarily exceeds the saturation temperature of steam corresponding to the operating pressure. Hence, for products that may undergo undesirable physical transformations (e.g., melting) or chemical transformations (e.g., hydrolysis) at elevated tem-peratures with steam or with small quantities of oxygen that usually leak into any drying system, a low-pressure operation is desirable. The vapor evolving from the product may be withdrawn from the chamber, con-densed, and the latent heat recovered. Alternatively, the vapor is reheated within the chamber by tubular or plate heat exchangers, and recirculated as a convective drying medium to enhance the drying rate. Such systems are used commercially to dry timber with very attractive results; while the net energy consumption is reduced severalfold, the product quality is enhanced and the environmental problem of emission of the volatile organic compounds (VOCs) produced during drying is also eliminated (Mujumdar, 1990). The organic volatile components (boiling point ranging from room temperature to over 200°C) are condensed out with the steam; since these two are immiscible, simple decantation allows recovery of the condensables, which may be sold separately (e.g., terpenes and essential oils).
Superheated steam dryers
Low pressure High pressureNear-atmosphericpressure
Vacuum steamdryers for wood*
Vacuum steamdryers for silkcocoons**
Fluidized bed dryersfor coal*
Impingement andthrough dryers fortextiles, paper***
Flash dryers forpeat (25 bar)****
Conveyor dryers forbeet pulp (5 bar)****
Fluidized bed dryersfor pulps, sludges*
Extensive commercial applicationsLaboratory-scale testing
Pilot-scale testingAt least one major installation
***
*******
FIGURE 7.2
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C lassification and Selection of Superheated
Figure 7.2 provides a classification of SHS dryers based on their operat-
Classification of SHS dryers.
© 2009 by Taylor & Francis Group, LLC
94 Advanced Drying Technologies
In general, SSD is worth considering as a viable option only if one or more of the following conditions apply:
1. Energy cost is very high; product value low or negligible (e.g., commodities such as coal, peat, newsprint, tissue paper, and waste sludges, which must be dried to meet regulatory requirements).
2. Product quality is superior if dried in steam rather than in air (e.g., newsprint, which yields superior strength properties in steam and permits lower chemical pulp content to attain the same strength and runnability).
(e.g., coal, peat, and pulps). Lower insurance premiums may partially offset the higher investment cost of a steam dryer.
4. The quantity of water to be removed and the production capac-ity required are high. This affords economy of scale. Clearly, such dryers are worth considering only for continuous operation because of the inherent problems associated with start-up and shutdown when water condensation on the product as well as the presence of noncondensables (air) cause problems.
7.4 Quality Considerations
It is impossible to generalize the effect of steam drying on product qual-ity. Table 7.1 gives a summary of tests carried out at McGill University on batch drying of hand sheets in a static apparatus in which the samples
all positive relative to drying in an air environment or, at least, comparable. Similar tests on paper dried on a dynamic pilot-scale apparatus in Finland have yielded comparable results (Douglas, 1994).
Extensive results have been reported in the literature on drying of wood, as well. Again, all quality indicators are positive. Thus, the vacuum-SSD systems have already been commercialized by several vendors world-wide. It is noteworthy that besides enhancement of the product quality,
reduction of inventory cost of the dryer.For drying of silk cocoons, laboratory-scale tests done in China had
proven the enhanced quality parameters (e.g., brightness and strength of
certain food or vegetable products, the yielded porous structure of the product dried in SHS (due to the evolution of steam within the product, which enhances porosity) is a desirable characteristic. This decreases the
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3. Risk of fire, explosion, or other oxidative damage is very high
the drying times are reduced two- to fivefold, which results in a significant
fiber) of the silk produced by steam drying at a temperature ∼45°C. For
were dried under an impingement flow of SHS. Note that the effects are
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 95
TABLE 7.1
Summary of Tests Carried Out at McGill University
PropertyEffect of Steam Drying(Relative to Air Drying)
Bulk Slight increaseSurface roughness Marginal increaseBurst 20% enhancement (typical)Tensile index 20–30% improvementStretchFolding endurance No noticeable differenceScott internal bond strength 10–20% higherWet strength Little effectPermeability Little effectCrystallinity No measurable changeBrightness Above 300°C, a little worseFiber strength (zero span) Little change
bulk density of the product while enhancing rehydration characteristics. The color and texture of the product may also become more desirable. Thus, the increased cost of steam drying may be offset by the additional credit received for the better quality of the dried product.
As noted previously, any direct dryer, in principle, can be converted to
by supplying a part of the heat indirectly (e.g., by conduction or radiation). Note that the inversion temperature is lowered in the presence of an indi-
of appropriate radiant heating, the inversion temperature can be reduced from 250°C to even 170°C.
It is not necessary to always operate the SHS dryer above the inversion
other than higher drying rates or lower energy consumption, such as quality or safe operation, may dominate the selection procedure in most cases. For relatively low-value products (e.g., municipal sludge, coal, peat, and hog fuel) that are also readily combustible in hot air and dried in large tonnages, the reduced net energy consumption in SSD is particu-larly advantageous since this also reduces the environmental emissions of greenhouse gases (e.g., CO2) as well as toxic gases (NOx, SOx, etc.).However, the excess steam produced must have a viable application in or near the process.
Table 7.2 summarizes the key factors contributing to the feasibility of SSD. Both product- and process-related factors must be considered. Laboratory testing is required before proceeding with any decision to consider SSD.
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No significant effect
dryers). Thermal efficiencies can be improved and the unit size reduced
rect heat supply, which is an added benefit. For example, in the presence
temperature to benefit from the advantages of the SSD, however. Aspects
SHS operation (e.g., flash, fluidized bed, spray, impinging jet, and conveyor
© 2009 by Taylor & Francis Group, LLC
96 Advanced Drying Technologies
The following dryer types have been successfully tested at a pilot scale and commercialized for at least some products:
Flash dryers, with or without indirect heating of dryer walls, with • high-pressure steamFluidized bed FB dryers, with or without immersed heat exchang-• ers, operated at low, near-atmospheric, or high (up to 5 bar) pressuresSpray dryers (operated at near-atmospheric pressures, for drying • of whey; pilot scale only)Impinging jets (for newsprint, tissue paper, etc., at small scale; for • textiles at commercial scale)Conveyor dryer, operated at near-atmospheric or high pressures• Agitated bed dryers, operated at near-atmospheric pressures• Packed bed/through circulation dryers•
TABLE 7.2
Factors Contributing to the Feasibility of SSD
Factor Impact
Product-related factors Low temperature sensitivity To avoid higher steam temperature, more
expensive vacuum systems are required High moisture content Since the latent heat of the moisture can
be recovered with SSD, the bigger the portion it represents, the greater will be
High thermal resistance Higher surface temperature with SSDreduces heating and drying times
High sensitivity to oxidation Lack of oxygen with SSD improves product quality
Undesirable taste or aroma SSD strips more of the acids thatcontribute to bitter aromas
High product values Drying time reductions provide biggestinventory cost savings with higher- value products
Process-related factors Other uses of steam available Marginal energy consumption for the
process is small and the capital costs areminimized
Environmental emissions from dryers SSD provides easier recovery of solventsand particulates
Combustion/explosion hazards
Expensive source of thermal energy Thermal energy savings with SSD will offset greater energy costs than withwaste fuels (e.g., hogged wood waste)
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the efficiency improvement
Lack of oxygen significantly reduces fire/explosion hazards
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 97
Impinging stream (opposing jet) dryers•
Rotary dryers (pilot scale, for brown coal)•
Recent papers on SSD can be found in a special issue of Drying Technologyedited by Devahastin (2008).
Because of space limitations, this chapter discusses steam dryers only for a selected range of products. However, the references cited provide detailed information. Also, other chapters in this book include discus-sions of some steam-based drying technologies such as airless drying and solvent drying.
7.5 Superheated Steam Drying of Selected Products
7.5.1 Drying of Sludges
agitated trough-type dryers using SHS. Hirose and Hazama (1983) have
bed dryer. The sludge containing 400% water (d.b.) was mechanically dewatered and then evaporated to 75% before it was fed to the dryer to
used as a heat source in the evaporator. Heat for the dryer was obtained
sludge was in the range of 8.4–19 MJ/kg. In the case of sludge with a
incinerated was required. The incineration process in a cyclone furnace
leached into the soil.
the product has been developed successfully in Japan to dry 15 t/day of
3
entrainment in the exhaust stream, which is cleaned in a cyclone. To avoid odor release, the dryer is held at a pressure of 10–100 mm water column below atmospheric pressure. For start-up, hot air is circulated, and then water is injected into the hot air until the system is full of steam. It should be noted that sludges come in various chemical/biochemical compositions as well as physical characteristics. To save energy, it is also important to dewater (nonthermally) as much of the water as possible before the sludge
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obtain a final moisture level of ∼5%. The steam generated in the dryer was
by incinerating a fraction of the dry product as the calorific value of dry
calorific value of 12.6 MJ/kg, an auxiliary energy of 420 kJ/kg of sludge
vitrifies the sludge (by melting and solidification), thus reducing the vol-ume of waste to be sent to a landfill. Furthermore, the waste product is not
dryer and exits at 150°C, giving a volumetric heat-transfer coefficient of up
• Vibrated fluid bed dryers with immersed heat exchangers
Sludges can be dried continuously in large tonnages in flash, fluid bed, or
reported on a sewage sludge treatment plant that used a steam-fluidized
An agitated multistage steam dryer with a concurrent flow of steam and
dewatered sludge. Steam at 360°C at flow rates up to 3600 kg/h enters the
to ∼100 W/m K. Unlike a fluid bed, the low velocities of steam cause little
© 2009 by Taylor & Francis Group, LLC
98 Advanced Drying Technologies
is fed to a thermal dryer. Not all sludges can be combusted to provide a part or all of the heat required for drying.
There are advantages as well as limitations to each dryer type, and a
where in this book) utilizing a multiple-effect evaporation concept as well as several indirect dryers provides competing technologies for drying of sludges. It should be noted that the use of high-temperature air could lead
tries, steam-drying technologies, which utilize dried sludge as fuel and
chemicals into the soil, which is a major advantage of such a process.Although no details are available, an impinging stream dryer for slud-
ges using SHS as the drying medium has been operated at pilot scale successfully in Russia. Additional information on these special types of dryers can be found elsewhere in this book and in the review papers by Kudra and Mujumdar (1989), Mujumdar (1990), and Kudra et al. (1990).
7.5.2 Drying of Coal
Coal is used as a raw material for many chemical syntheses and also as a fuel. Depending on its initial moisture content, coal is dried to increase
monly required in North America, drying of low-grade coals contain-ing high levels of moisture is necessary in many parts of the world. Coal
preheating and 10–15% in drying if the coal is pre-dried. Direct dryers
are used commonly with hot air or combustion gas at 700–900°C before the dryer and 60–120°C after the dryer. Rotary dryers with indirect heat-
bed dryers are equipped with internal heat exchanger (Figure 7.3). A commercial vibratory dryer for hard and brown coals (manufactured by Escher-Wyss of Switzerland) uses a vibrational frequency of 50–100 Hz and an amplitude of 0.5–3 mm, giving a conveying velocity of 0.01–0.3 m/s with an angle of inclination of 5° to the horizontal. Low gas velocities
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careful technoeconomic evaluation is necessary when a final selection is to be made. The well-known Carver-Greenfield process (discussed else-
to significant fire and explosion hazards in drying of sludges of all types.
lems of special concern, as well. For very large cities in developed coun-Atmospheric emissions of organic volatiles and fine particulates are prob-
yield vitrified, easy-to-handle dry product for landfills, appear to be espe-cially attractive. Vitrified sludge particles do not leach their undesirable
its calorific value; simplify loading, unloading, and transporting; and improve the efficiency of combustion in the boiler. Although not com-
is also dried for briquetting, coking, gasification, carbonization, liquid fuel synthesis, and so on. Coke-oven efficiency can increase 30–50% in
ing are used for hard coals. These dryers have higher energy efficiencies,
(e.g., rotary, pneumatic, fluid bed, vibrating fluid bed, and shaft dryers)
with ∼3100 kJ/kg water evaporated. For air fluidized bed dryers, the cor-responding figure is 3100–4000 kJ/kg water evaporated. Frequently, fluid
are needed since vibration suspends most of the pseudofluidized beds. The efficiency is better than in a conventional fluid bed employing high
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 99
gas velocities. Attrition is reduced and gas-cleaning requirements are minimized in a vibrated bed dryer.
In pilot trials, Potter and Beeby (1986) and Potter and Guang (1988) have
nal heat exchanger tubes immersed within it. Typical processing condi-tions were reported as follows:
Heating tube temperature• . 140–170°CBed temperature• . 110–127°C
Steam temperature• . 130–150°CCoal feed rate• . 40–70 kg/hProduct• . 16–28 kg/h
Using steam exhausted from one dryer stage as carrier steam for another stage, multiple-effect operation (similar to that common to evaporators) can be achieved yielding a steam economy of 1.9 for a triple-effect dryer.
Product
Steam circulationblower
Electrostatic precipitator
SteamFluid bed dryer
Wet coal
Condensate Preheater
Dry coal
Compressor
FIGURE 7.3
(German design).
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shown that extremely favorable heat-transfer rates and drying efficiencies
Schematic of a fluid bed dryer for pulverized coal with immersed heat exchangers
are obtained when drying brown coal in a steam-fluidized bed with inter-
• Minimum fluidization velocity. 0.57 m/s (approximately)
© 2009 by Taylor & Francis Group, LLC
100 Advanced Drying Technologies
high with four bubble caps to distribute steam. The disengaging region
Recent studies on drying brown coal from 61 to 11% w.b. in the pilot-scale SHS rotary dryer (0.37 m diameter and 3 m long) indicated that the
moisture removal (Clayton et al., 2007).
beds of pulverized coal. They found the inversion temperature of ∼180°C,
bed drying exceeds that in (dry) air drying. For a 2000 kg/h steam dryer for alumina, the capital cost was reduced by 20%, whereas the total energy cost was reduced by 15%. No credit was given to the steam produced in the steam dryer.
Faber et al. (1986) also reported on a successful industrial installation using a steam dryer for activated carbon pellets (2000 kg/h d.b.) from aninitial moisture content of 50 to 2% (d.b.). The pellets were dried to 8% d.b.
content of 2% was achieved. The steam entered the dryer at 300°C and left at 150°C. The discharged steam was used to preheat the feed. The authors reported smooth operation of the dryer since 1985. The installed cost of the steam-drying system was 40% lower than that for a conventional air dryer. Besides, the air dryer can operate at a maximum temperature of 125°C to avoid combustion in the dryer. The energy costs (1986 data) were estimated to be about $3.6/t of dry product in South Africa.
Woods et al. (1994) have reviewed steam-drying technologies for coal and presented interesting results on steam drying of 1.0–1.3 mm coal par-ticles and the evolution of volatiles during drying. It is noted that, in steam drying, the drying time (actual residence time in the dryer) does not affect the volatiles’ liberation, unlike air drying. Furthermore, they found that under the conditions of their experiment, the constant-rate drying period is 6–7 times longer in steam drying and the heat-transfer rate is 1.7–2.0 times that in air. They also report favorable industrial experience with
25 t/h. No details are given about the use of steam produced by the dryer.Black coal generally has low water content, whereas brown coal may
have 60–80% (w.b.) moisture. Regardless of whether brown coal is burned,
pulverized coal must be low in moisture content. Coal drying in a steam-
energy wastage in the subsequent combustion step by two-thirds, result-
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feed rate and stem temperature have the most significant effect on the
basis before feeding an evacuated chamber in which the final moisture
gasified, coked, or liquefied, the wet raw coal should be dried to ∼5 to 10%moisture for economic utilization. For some gasifiers, the additional
ing in some 15% increase in the overall powerplant efficiency, a similar
Potter and Guang (1988) used a continuous fluid bed dryer for drying
was 2.5 m. Both horizontal and vertical tube bundles were tried.
Victoria brown coal. The rectangular 0.3 × 0.3 m fluid bed dryer was 3 m
Faber et al. (1986) have compared drying rates in air and steam-fluidized
above which the steam-drying rate in the constant-rate period in fluidized
steam-fluidized bed drying of brown coal with an evaporative capacity of
requirement of free flowability for a well-metered feed rate means that the
fluidized bed of coal containing 65% moisture (w.b.) is estimated to reduce
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 101
2tion for the same thermal output.
Steam drying is claimed in some reports to reduce power consumption for milling since the grindability index is increased due to steam drying. At elevated degrees of superheat and prolonged exposure times, some studies have reported a reduction in the sulfur content of coal. This is not necessarily an advantage since sulfur will then enter the dryer exhaust steam. Additional data are needed to evaluate this aspect in detail.
bed drying of brown coal:
Better energy utilization by condensing steam generated in the •
Ability to couple the dryer with power plants to use latent heat of • condensed low-pressure steam (cogeneration potential).Coal moisture discharged as liquid water rather than as dusty • vapor. No biological or chemical treatment is needed for the con-densate water.Large-capacity dryers (up to 15 t/h) are feasible.•
200 W/m2 K versus 20–50 W/m2 K in steam tube rotary dryers).The operation is safer (reduced insurance costs).• Product is dried to greater uniformity and better briquette strength.•
The coal dryer itself is not the most expensive component of the drying system; coal grinding before drying, milling after drying, and cleaning the
scale dryers for this application are operational in Australia and Germany.
7.5.3 Drying of Beet Pulp
Drying of wet, pressed beet pulp after extraction of sugar in a conventional high-temperature dryer requires 9 kWh/100 kg beet, which amounts to over 33% of the energy consumption in a sugar factory. BMA AG of Germany developed a high-pressure SHS dryer for beet pulp drying that consumes only 2900 kJ/kg water evaporated, compared with nearly 5000 kJ/kg in conventional air drying. The BMA dryer is a horizontal pressure vessel made of mild steel and is 6.5 m diameter and 37 m long (Bosse and Valentin, 1988). Special airlocks are provided at the entry and discharge ports. The feed pulp is preheated in a screw conveyor to 105°C and then deposited on the drying screen—this avoids condensa-tion and corrosion problems. Three traveling screens of stainless steel have a total surface area of 240 m2 for a throughput of 32 t/h of pressed
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dryer; an overall efficiency was increased from 36 (typical for a brown coal–fired power plant) to 42%.
• The dryer is compact (e.g., heat-transfer coefficients in the order of
level of reduction in CO emission, and a 30% reduction in flue gas genera-
Following are some of the key advantages claimed for steam-fluidized
effluents are by far the most expensive steps in the process. Commercial-
© 2009 by Taylor & Francis Group, LLC
102 Advanced Drying Technologies
beet pulp with 30% solids to be dried to 90% solids. The pulp bed height is between 40 and 120 mm, which affects the dryer throughput. The steam is made to circulate through the bed of beet pulp, causing it to dry as the screen moves. Nine parallel fans capable of handling 200,000 m3/h of steam circulate the steam in the dryer. Heat exchangers are used to reheat the recirculating steam. A mean residence time of up to 720 s is needed for a bed of 120 mm with an effective evaporation capacity of 16–20 t/h. The pulp is brighter than air-dried pulp. However, the dryer costs are high, with a payback period of up to 6–7 years.
Recently, Niro A/S of Denmark has successfully commercialized a pres-
materials (Jensen, 1992). The bed has a special cellular or multicompart-r
that is recycled through a heat exchanger by a fan and blown up through
available. Initiated as pilot scale in 1982, the full industrial scale dryer(6 m in diameter) for drying beet pulp came on stream in Denmark in 1985.
Superheater
Flash
Internalcyclone
Ejector
Static turbine(vapor compression)
Static turbine
CompartmentsFeed
High-pressure steam(10–25 bar)
Excess steam ~3 bar
Grid
Product
Turbine Superheated steam
Condensate
Fluidizedbed
FIGURE 7.4Design principle of pressurized SHS multicompartment FB dryer.
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(fines entrainement)
surized steam-fluid bed dryer for particulate and sludgelike or pulpy
ment design and operates at 3 bar. The fluid bed is driven by SHS flow
the fluid bed (Figure 7.4). Capacities of 2–40 t/h water evaporation are
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 103
Compared with conventional rotary dryers, energy savings of up to90% are feasible. The product quality is found to be better in steam drying (i.e., appearance and texture, as well as digestibility by cattle).
Although existing dryers using this concept are currently operational in Europe for drying beet pulp, pilot tests indicate that this dryer can suc-cessfully dry the following products, as well: spent grain from a brewery,
apple pomace, bark, wood chips, bagasse from sugar cane, and municipal sewage sludge.
following:
Closed system drying eliminates odor emission.• Automatic, reliable, and maintenance-free operation is feasible.•
•Lower or no contamination or oxidation of products as in conven-•
No environmental pollution.•
pressurized steam dryer for 32 t/h of water evaporation. If 16 bar-gauge steam is supplied to the steam dryer, it will generate 3 bar-gauge steam, which could have produced 2 MW of thermal energy. Instead, it is avail-able as low-pressure steam although the 2 MW is not used by the dryer. A rotary dryer will consume 26 MW of fuel energy, none of which can be recovered. Of course, all the advantages are gained at increased capital
energy savings exists only when the steam produced by the dryer can be used effectively elsewhere in the process or at a nearby location.
reported to be in the range of 130–190 kWh/t water evaporated compared with 800 kWh/t water evaporated for conventional air drying (Woods et al., 1994).
7.5.4 Drying of Lumber
Drying is an important step in processing lumber. With few exceptions (Pang, 1997; Haque and Sargent, 2008), SHS drying has been studied experimentally for green softwoods and hardwoods. Effects on resin exudation, surface discoloration, loosening of knots, and other quality parameters have been examined to varying extents for various wood species. High-temperature drying of lumber (>100°C) in air–steam mix-tures causes greater defects such as collapse, honeycomb, and checking
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hey, alfalfa, fish to produce fish meal, peels and pulp from citrus fruits,
Among some of the specific advantages claimed of this dryer are the
No danger of fire or explosion.
tional gas-fired dryers.
costs; as the cost of energy rises, the high energy efficiency of steam dry-ing becomes economically more attractive. The potential for significant
It is interesting to compare the energy flows in the rotary dryer versus the
Specific energy consumption of the Niro pressurized fluid bed dryer is
© 2009 by Taylor & Francis Group, LLC
104 Advanced Drying Technologies
in high-temperature-dried lumber than in lumber dried at lower tempera-tures. Rosen et al. (1982) have given pilot-scale data on pressurized steam drying of lumber for 27 mm thick green and yellow poplar and red oak. Rosen also made a technoeconomic study based on a number of plausible assumptions. Essentially, he showed that the capital cost of the steam dryer was about the same as that of a conventional kiln, although the drying times were reduced. The electrical energy usage was only ∼0.82 kWh/kg(2.95 MJ/kg) water evaporated for the electric heater. Because the steam
was ∼40% higher for steam dryers. However, as the dryer system is aclosed one (in which dryer exhaust is recirculated after electrical heating), this energy is also available for evaporation. It is important to note that the electrical energy requirements for drying depend on the type of lumber and the moisture range. Rosen dried yellow poplar from 100% moisture content to 5% (d.b.) in 28–30 h, whereas red oak was dried from 19 to 5% (d.b.) in 21 h. About 40% of the dollar value of the lumber was lost due to degradation associated with drying. The total energy requirements for red oak were estimated to be triple those for yellow poplar.
In the past decade, a low-pressure steam dryer for timber has gained momentum in Southeast Asia and Europe. The so-called Moldrup pro-cess marketed by Iwotech Limited of Denmark (1993) is carried out in an enclosed autoclave in which the sawn timber is stacked on trolleys for easy feeding and discharge. The drying process is initiated by evacua-tion of the autoclave with a low-power vacuum pump, which takes 1–2 h.
drying is supplied by recirculating the steam at velocities up to 20 m/s at 4 mbar pressure using a series of fans within the chamber. The drying steam is heated by steam or hot-water coils located in the autoclave to a temperature above the saturation temperature of steam. At the operating pressure (∼3–5 mbar), the boiling point of water is lowered to <50°C. The process is controlled automatically. Figure 7.5 shows schematics of two vacuum SHS dryers for wood. The main difference is in the location of the condenser; in one design it is within the chamber and in the other it is placed outside.
Among the advantages claimed for the process are the following:
Better product quality• Simple process control•
No staining and mold attack• No oxidative discoloration (e.g., oak and beech, do not discolor)•Minimal risk of corrosion•
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It is then filled with SHS, which is heated in the range of 50–90°C. Heat for
• High drying rates (drying is by two to five times faster)
• No risk of fire and explosion
flow rates were higher than those in conventional kilns, the blower power
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Superheated Steam Drying 105
Minimal removal of extractables from wood• Reduced stresses, cracking, warping, and the like• Low inventory costs due to severalfold faster drying•
The autoclaves (drying chambers) come in diameters up to 4 m and lengths up to 24 m. The autoclave is equipped with a rubber air tube that is pres-surized to hold against the top part of the timber stack with an overpres-sure of 10 kPa to reduce the risk of deformation. The moisture coming out
Vacuum pumpSuperheatingcoils
Evaporator
Lumber
Dryer
Condenser
Condenser
Heat exhanger
Brunner HildebrandHigh Vac
Cooling towerLarge vacuum pump(start-up only)
Superheatingcoils
Dryer CondenserCondensateTurpentine
IWT Moldrup dryer
FIGURE 7.5Schematics of two low-pressure SHS lumber drying systems.
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106 Advanced Drying Technologies
of the timber is condensed. The drying time ranges from 1 to several days, depending on the wood species and the thickness of the wood. At the end
ing of the charge.The higher drying rates (enhanced by spraying hot water [Oliveira
et al., 2001]) result in energy savings of ∼ 50% over conventional kilns. One of the limitations of the process is that the autoclaves are much smaller than the conventional hot air kilns. However, since the drying cycles are severalfold shorter, this is not a major limitation. In fact, the shorter dry-
of wood while reducing the cost of inventory, especially for wood species requiring several weeks of air drying. No oxygen in the dryer may also help to kill microorganisms or insects in wood, although experimental
perature distribution in the wood may result in much lower temperatures
to kill microorganisms. This process appears to be especially attractive for species and timber dimensions that take a very long time (several weeks) to dry in conventional kilns. This technology is likely to become a stan-dard in wood-drying technology in many parts of the world in the next decade. It should be considered seriously when old kilns are ready to be replaced.
Table 7.3 compares two vacuum-SSD applications (for pine and oak) in terms of energy consumption and costs (Woods et al., 1994). Note the dif-
apply for the Canadian situation in 1995, the present numbers should not differ a lot.
7.5.5 Drying of Pulp
the pulp to be dried was installed in 1979 at the Rockhammars in Sweden. Drying is accomplished by indirect heating with steam extracted from a turbine at 10 bar. Steam formed by evaporation within the dryer ducts is circulated in the dryer, where it acts as a convective drying medium. A fraction of the steam generated is used elsewhere in the mill since the dryer operates at 5 bar. Even after allowing for the energy loss in the turbine, it is found that this dryer consumes less than one-half
Drying times are in the order of 10–30 s. This type of dryer has also been used successfully for drying of peat, sawdust, and forest biomass,
hog fuel.
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of the drying cycle, the vessel is filled with atmospheric air before unload-
verification is needed. Since the drying temperatures may be lower, tem-
in some parts of the wood, which in some instances may not be sufficient
cost of drying oak for the case examined. Even though these cost figures
The first industrial pulp dryer utilizing SHS as the carrier for conveying
which, when pulverized, can be used as fuel in existing oil-fired thermal
ing times offer the user more flexibility in drying different species or sizes
ference in initial moisture levels that is reflected in the significantly higher
of the energy used in flue gas drying—the conventional technology.
power plants. Figure 7.6 gives a schematic of the flash dryer for pulp and
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 107
TABLE 7.3
Energy Consumption and Costs for Two Vacuum-SSD Applications
Wood Type Pine Oak Dimension (mm) 200 50 Charge capacity (m3) 45 37
80/16 35/8
Drying Cycle Temperature (°C) 70 70 Drying time (h) 105 148 Total charge time (h) 120 195 Annual capacity (m3) 2875 1424
Energy Consumption Thermal energy (kJ/m3) 762,650 464,310 Electrical energy (kWh/m3) 52.9 105.8
Energy Costs Fuel oil price ($/L) 0.344 0.344 Electricity price ($/kWh) 0.142 0.142 Heating costs ($/m3) 7.38 4.49 Electricity costs ($/m3) 7.50 14.99
Financial costs Price ($) 420,000 420,000 Depreciation period (years) 7 7 Scrap value ($) 70,358 70,358 Interest rate (%) 10 10 Financial costs/year ($) 7,066 7,066 Financial costs per cubic meter ($)
24.57 49.63
Total cost per cubic meter ($) 39.45 69.12
Heating steam
Exhaust
Generatedsteam
PC
Product
Feed
Condensate
FIGURE 7.6
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Initial/final moisture content (%)
Steam flash drying system.
© 2009 by Taylor & Francis Group, LLC
108 Advanced Drying Technologies
pulp can be designed and operated economically at evaporation capacities of the order of 400 t/day. If the steam produced is used elsewhere in the process, then the net energy consumption chargeable to the steam dryer can be very low. Currently, no full-scale industrial installation appears to
dryer similar to that commonly employed in multiple-effect evaporators.
with lower energy consumption result in the cost of drying pulp by air or steam about the same, that is, of the order of U.S.$ 23/t water removed (in Quebec in 1989).
Possible operating problems for such dryers include deposit formation and erosion of the pneumatic tubes due to the high-velocity transport of particles that may contain abrasive impurities such as sand. Erosion, par-ticularly in bends, can be minimized or taken care of by using special materials or replaceable elbows. Deposits may form within the dryer walls if the transporting steam temperature falls to saturation level. Corrosion is also a problem to be considered; hence, stainless steel construction is often required. It is important to disintegrate the solids before feeding in
the transport steam.
7.5.6 Drying of Peat
commissioned in Sweden using SHS as a transporting medium heated indirectly by the back-pressure steam from a turbine.
Briquetting industries in Ireland and the former Soviet Union have used the so-called Peco dryer for peat for over eight decades (Fagernas and Wilen, 1988). The Peco dryer is a two-stage system where the latent heat of evaporated water from the second stage is used as the drying energy in
air–steam mixture in the pipes, whereas the energy for drying is sup-
(second stage). Peat is separated by cyclones after each stage. The energy consumption for the Peco dryer is 1.7–1.8 MJ/kg water evaporated, which
electrical power demand for the compressors used for pneumatic convey-ing is relatively high.
Another dryer for peat (or lignite) is the tubular steam dryer, mostly used in Germany. It is a rotating, inclined cylinder in which the material
of 0.60 MPa. Air is drawn through the pipes to remove evaporated water
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the first stage. The dryer comprises five columns, each containing about
plied by hot water (first stage) and condensing steam outside the pipes
According to Munter (1989, personal communication), flash dryers for
exist, which utilizes vapor-recompression technology for the steam-flash
The higher capital costs associated with steam flash dryer units coupled
the flash dryer since large lumps and the like will not be suspended by
Peat is usually dried at power plants and briquette factories with flue gases (300–600°C) from the boiler. More recently, steam flash dryers have been
500 pipes of ∼70 mm diameter. The milled peat is carried by airflow or
is much lower than that for a flue gas dryer (3–4 MJ/kg water). However,
flows in 100 mm pipes, whereas steam flows in the shells at a pressure
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 109
The energy consumption is reported to be 2.9–3.2 MJ/kg water removed. The exhaust energy is generally not utilized. It is interesting to note
400 kg/m3, such dryers operate well.
University of Technology (Gothenburg, Sweden) in the early 1970s is ideal for drying peat as well as pulp, bark, and so on. This dryer is a closed, pressurized system in which the peat is exposed to indirectly heated SHS. The dryer consists of transport ducts, heat exchangers, a cyclone, and blowers. The SHS recirculates at a pressure of 2–6 bar. The primary heat-ing steam is condensed (usually 8–15 bar) on the shell side.
The dry material is separated from steam in a cyclone, and the basic stream of steam is recirculated, whereas the excess steam is bled off. If the material at initial dryness from 45 to 50% is dried to 85–90% dryness, ∼1 tof steam is generated per ton of material processed. This steam is available as process steam at 2–6 bar. Typical drying time is 10–30 s and the trans-port velocities are 20–40 m/s. If excess steam is used (e.g., for district heat-ing), then the net dryer energy consumption is only 0.5–0.7 MJ/kg water removed. This may be compared with 3.5–4.8 MJ/kg, which is typical for
drying, thus reducing the insurance costs, as well.A recent installation in Sweden uses peat (replacing coal) as the fuel
in a 440 MW thermal power plant. The peat is dried, briquetted (3 million tons/year) at a source ∼400 km from the power plant, and then milled for combustion. Peat is dried from 60 to 10% d.b. moisture for briquetting at an output level of 20 t/h per drying unit. Two dryers are used in parallel. The total heat-transfer area of the tubular heat exchanger is 5500 m2. The electricity demand of the compressors andblowers is 10 MW. About 3.6 MW district heat is produced, giving a net energy demand of the dryer of ∼0.5 MJ/kg water, which is only about
drying systems.At Helsinki University of Technology, successful demonstration of
(100 kg/h of water evaporated). A tubular heat exchanger immersed in the bed provides indirect heat for drying. It uses condensing steam at 0.8 MPa.
from the upper zone by a screw conveyor. The miffed peat is ∼1 mm
100 W/m2
Sand may be used as a bed material to increase the drying rate, but sepa-
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content. If there is low content of fiber and wood with a bulk density of
in average size. The bed-to-tube heat-transfer coefficient is rather low at
that the flow properties of peat depend on the presence of wood and fiber
The pressurized steam flash dryer originally developed at the Chalmers
hot air flash dryers. Dust and explosion hazards are eliminated in steam
one-sixth to one-seventh the energy consumption of typical flue gas
fluidized bed drying of peat has been reported at the pilot-plant level
Steam evaporated from peat is used as the fluidizing medium. Wet peat is fed into the lower zone of the fluid bed, whereas dry peat is withdrawn
K (i.e., of the same order as that in a flash dryer but much lowerthan that for a fluidized bed due to the poor thermal properties of peat).
ration of peat from sand is not very effective. If a fluidized bed combustor
© 2009 by Taylor & Francis Group, LLC
110 Advanced Drying Technologies
with sand is used, then sand separation is not a problem. Fine size magne-
to 350 W/m2 K while allowing easy magnetic separation from dry peat
for use on new, large power plants. The investment costs are high and a demonstration plant is necessary to study the reliability and techno-
same order.
should apply for drying of lignite, biomass, and similar organic materials
sumption should be of the same order for the same production capacity (i.e., in the order of 0.5–0.7 MJ/kg water evaporated), provided the steam generated by the dryer is used elsewhere.
One important consideration in drying of peat is the evolution of organic
is recovered and the condensate sent as a waste stream. Indeed, such a problem exists whenever the material being dried can result in the evolution of organic compounds due to heating or interaction with steam at elevated temperatures. The quality of the condensate is affected by the drying con-ditions (e.g., total time and residence time in dryer), amount of moisture removed, type of peat, and so on. Acetic acid, formic acid, and furfurals are the main organic compounds in the condensate. The average biological oxygen demand (BOD) is 140–150 mg/kg dry peat, the chemical oxygen demand (COD) is 500–850 mg/kg dry peat, and the total organic carbon (TOC) is 90–300 mg/kg dry peat in different dryers. Table 7.4 provides a
yields higher solid content (∼400 mg/L), higher COD (∼2600 mg/L), and
TABLE 7.4
ParameterFluid Bed
Steam DryerPilot Fluid Bed Dryer
IndustrialPeat Dryer
Temperature (°C) 100–140 140–170 170Pressure (bar) — 2–8 5.7pH of condensate 4.1 5.7 3.8Solids (mg/L) 80 220–400 70–170NH3–N (mg/L) 1.2 28.0 11.0Phosphorus (mg/L) 0.04 1.60 0.07BOD (mg/L) 520 130–190 —COD (mg/L) 880 470–630 440–1300TOC (mg/L) 310 90 310–450
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tite as a bed material increases the bed-to-tube heat-transfer coefficient
(Jahkola et al., 1989). This dryer is claimed to be economically justified
economics of the process. The fluid bed installation is likely to be more compact than the flash dryer but with a net energy consumption of the
Aside from differences in the flow characteristics, similar conclusions
in steam-flash or steam-fluid bed dryers. The cost and net energy con-
compounds when flue gases are discharged into the stack or if the energy
summary of the effluents from steam peat dryers based on the Fagernas and Wilen (1988) data. Note that drying of bark in steam-fluidized beds
Effluents from Steam Peat Dryers
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 111
ent levels are comparable.In general, increased bed temperature leads to increased load of organ-
ics in the condensates. Increasing the bed temperature from 110 to 130°C can increase the BOD, COD, and TOC values nearly threefold. Finally, a
fully in Finland for drying of mined peat, which is then fed continuously
ranted for a drying application.
7.5.7 Drying of Paper and Tissue
No commercial dryers exist for direct drying of paper webs with SHS.
rations for direct and direct/indirect drying of paper using SHS. These include
Pure impingement with steam jets with the web supported on a • heated rollImpingement and through drying (the so-called Papridryer con-•
Pure through drying with steam drawn under suction with the •web (assumed permeable at reasonable pressure drop) supported on a perforated roll or honeycomb roll
All of the aforementioned processes have been tested at a pilot scale or are already in commercial operation using hot air or combustion gases. All of these are highly energy-intensive processes requiring 4800–5700 kJ/kg evaporated water. Also, the electrical power consumption is high for the
recirculation ratios (up to 90% in some high-performance impingement dryers). Such dryers are in use for the drying of highly permeable grades such as tissue and toweling. The Papridryer process has been tested at pilot scale for newsprint (Burgess, Chapman et al., 1972; Burgess, Seto et al., 1972; Crotogino and Allenger, 1979). Mujumdar (1992) has discussed the SSD of paper in some detail including its history, current status, and potential.
Aside from the higher drying rates and lower net energy consumption in a SHS dryer, two additional major advantages of interest to paper-
ity as measured by its strength and optical properties. Cui et al. (1986)
tests. Cui and Mujumdar (1984) and Loo and Mujumdar (1984) employed simple models to calculate the drying rates (under several assumptions
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to a high-pressure gasifier. Clearly, such a sophisticated dryer is not war-
Mujumdar (1981) proposed, for the first time, various possible configu-
figuration but using steam rather than combustion gases that pose a serious fire hazard)
makers are elimination of fire hazard and improvement in product qual-
and later David (1987) confirmed this in laboratory-scale static drying
higher TOC (∼450 mg/L) than corresponding figures for peat; other efflu-
novel high-pressure steam flash dryer (25 bar) has been operated success-
fan power needed for gas jets, as well as the through flow and the high
© 2009 by Taylor & Francis Group, LLC
112 Advanced Drying Technologies
later validated experimentally) and energy consumption for different
in dryers is fully utilized elsewhere (without vapor-recompression), the net energy consumption for paper drying can be as low as 1500 kJ/kg water evaporated. This energy requirement is mainly to account for leakage, sensible heating of the web, other losses, and energy for steam recycle. Pilot and mill tests are needed before these preferential results
Bond (1991) and Poirier (1991) have presented valuable data and analy-ses of steam drying of handsheets in static laboratory test rigs. Limited pilot-scale data exist at this time on steam drying of paper or tissue. Application to tissue drying (impingement and through drying) appears to be the more imminent industrial-scale operation. Another potential near-term application is as a booster dryer to increase the productivity of a dryer-limited mill. Douglas (1994) has summarized various studies on steam drying of paper. Despite the obvious and demonstrated advantages of steam drying of lignin-containing paper, many engineering problems remain to be tackled.
7.5.8 Drying of Wood Particles and Wood Wafers
wood particles for particleboard to very low moisture levels (2–3% water).
heated air drying; both processes do not permit economic energy recov-ery from dryer exhaust. Wood particles were dried in a pressurized steam dryer in which the particle–steam mixture is conveyed through a series of vertical heat exchangers where the mixture is heated to maintain the required degree of superheating. Upon exhaust, the particles were sep-arated from steam in cyclones and recirculated, except for an amount equivalent to that produced by the dryer. A 13 m long single-tube dryer was used in the pilot tests, giving a residence time of 12–15 s; a second pass was needed to achieve the required dryness in 25–30 s. The desired dryness of 2% was achieved for both softwood and hardwood (birch) particles. About 40°C superheat was found to be adequate in the dryer. Fouling of heat exchanger surfaces was minimal, although this should be examined closely for industrial-scale operations. For bark drying in a similar dryer, no problems have been reported on heat exchanger surface encrustation even after 6 years of operation.
Salin (1988) compared the quality of particleboard manufactured from steam-dried versus air-dried wood particles; the bending and tensile strength as well as water absorption properties of the former were found to be superior. This results in a 9% reduction in glue consumption in particle-board making. Optimal selection of dryer temperature and heat treatment
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steam-drying configurations. They concluded that if the steam produced
can be confirmed.
The conventional process uses direct contact flue gas drying or indirectly
© 2009 by Taylor & Francis Group, LLC
Salin (1986, 1988) has reported a significant study on steam drying of small
Superheated Steam Drying 113
can further enhance the board quality. No industrial-scale steam dryers appear to be in operation anywhere at this time for this application.
7.5.9 Low-Pressure Drying with Superheated Steam
For products that are heat-sensitive (e.g., may melt or undergo glass tran-sition), one can employ SHS at lower pressures with some limitations caused by increased investment cost and reduced convective heat transfer due to the low density of SHS at lower pressures. This means longer dry-ing times. Despite this limitation, there are potential advantages to low-pressure steam drying (LPSD).
Elustondo et al. (2001, 2002) examined drying of wood slabs, shrimps, bananas, apples, pears, cassavas, strawberries, etc., at steam pressures of 10–20 kPa, temperatures of 60–90°C, and steam velocities of 2–6 m/s. A mov-ing boundary model simulated the drying kinetics well. The quality of the dried product in terms of color and porosity was also excellent.
Devahastin et al. (2004) and Thomkapanich et al. (2007) have carried out extensive experimental and modeling studies of LPSD. They have compared LPSD with competing drying methods for heat-sensitive materials (e.g., vacuum drying, heat pump drying, and freeze-drying (F-D)). Generally, they observed that vacuum drying and heat pump drying give similar quality faster, whereas F-D gives a better-quality product but takes much longer and at much higher cost. LPSD also produces good color and a porous structure that allows rapid rehydration.
7.5.10 Miscellaneous Applications
Akao et al. (1982) reported on a commercial steam drying/deodorization plant to process soy sauce cakes as feedstock. An agitated trough steam dryer (1.5 t/h dry product capacity) was used. Deodorization commenced at moisture levels below 4% d.b. and at a temperature of 135°C. The oils and fats distilling from the cake resulted in deodorization of the prod-uct. Indeed, the fats and oils were used as auxiliary fuels for the process.
silk cocoons were also reported by Akao (1983). In these products, the odor is due to the oxidation of lipids. He also showed that the quality of green tea and vegetables dried in steam (e.g., color and smell) was acceptable.
Among other applications of steam drying developed in Japan, one may cite drying of potato slices in which color and vitamin C are preserved due to no contact with oxygen. Lack of formation of a case-hardened skin on the product in solvent drying was applied to advantage in dry spin-
were obtained in this process.
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Concurrent steam drying and deodorization of rice bran, fish meal, and
ning of synthetic fibers in superheated solvent vapor. Yoshida and Hoydo (1963) reported that stronger and finer fibers without surface wrinkles
© 2009 by Taylor & Francis Group, LLC
114 Advanced Drying Technologies
7.6 Utilization of Exhaust Steam
Assuming no losses, the evaporated moisture in steam dryers becomes
ally holds the key to the success of the steam-drying process. This steam is typically at near-atmospheric pressure and is likely to be contaminated with solid impurities as is the case in paper drying, where steam picks up
drying gases gets incinerated before recycling. However, in SSD, the steam
ters, cyclones, electrostatic precipitators, etc.) depends on how the steam is recycled or utilized in the process. It is important to ensure that no conden-sation occurs in the vapor cleaning systems.
Figure 7.7 shows schematics of three possible ways to utilize the excess steam produced by a steam dryer. Following is a brief discussion of poten-tial systems for steam utilization from dryer exhausts as a detailed dis-
however, cannot be overemphasized.Vapor-recompression technology is widely used in continuous evapora-
tors. Billet (1989) has discussed this subject in considerable detail. In the pulp and paper industry, there are currently two major application areas
cant energy savings. One is the use of compressors in conjunction with the black liquor evaporators, which results in either a decrease in the number
used to compress the vapor leaving the evaporator and, under appropri-ate conditions, the mechanical energy (or electrical power) required by the
vapor. The second major application area is in the thermomechanical pulp (TMP) plants, in which a compressor can be used to recover heat from the TMP vapor normally vented to the atmosphere. In this system, heat exchangers are needed to generate low-pressure steam using the latent heat of the TMP vapor. The steam compressor boosts the clean low-pressure steam to the pressure required in the paper mill. According to van Gogh (1985), either screw or centrifugal compressors can be used. Table 7.5 summarizes key features of these two types of compressors for vapor (steam) compression.
Note that other types of compressors can also be used for steam (e.g., lobe, reciprocating, axial, and turbo blowers). Benstead (1982) has reported on some preliminary studies at the Electricity Council Research Center (Capenhurst-Chester, England) on a pilot steam-spray dryer for ceramics. The dryer exhaust steam was split off and compressed; the high-pressure steam gave up its latent heat to the incoming steam. He modeled two designs
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excess SHS at lower specific enthalpy. Economic use of this steam gener-
fibers and dust. This in not a major concern in high-temperature drying of tissue paper, for example, in which the fibrous debris picked up by the
must be cleaned for reuse. The degree of cleaning required (using bag fil-
cussion is beyond the scope of this chapter. The significance of this aspect,
in which mechanical vapor compressors are employed to achieve signifi-
of evaporators or in increased evaporator efficiency. The compressor is
compressor is significantly lower than the recoverable heat value in the
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 115
for comparison. One used a standard compressor that withstood the super-heat. The other considered a compressor with water injection into the suc-tion vapor to cool the steam during compression. The latter is ∼10% more
Heater
Resuperheatedrecirculatedvapor
Condensate
SSD dryer
Dry product recycle
Heater
SSD dryer
Resuperheatedrecirculatedvapor
Dry productDry product recycle
Condensate
Excess vapor toother process uses
Saturated exhaustvapor
Feed
Electricity
(a)
(b)
(c)
Feed
Resuperheatedrecirculatedvapor
Excess vapor toother process uses
Saturated exhaustvapor
Dry product
Recompression
Electricity
Excess vapor
Dry product
Saturated exhaust vapor
HP vapor
Condensate
SSD dryer
Dry product recycleFeed
Heater
FIGURE 7.7
exhaust: (a) Excess steam with recuperation by high-pressure steam, (b) excess steam recompressed to superheat, and (c) excess steam with recuperation by electricity.
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efficient due to the reduction in steam volume at lower temperature.
Simplified process flow diagram of SHS dryer configurations to recover energy from dryer
© 2009 by Taylor & Francis Group, LLC
116 Advanced Drying Technologies
Anthony (1989) has summarized the virtues of vapor compression evap-oration (VCE) followed by crystallization to treat waste brines from chemi-cal process plants. He compared VCE followed by a crystallizer with other options such as a reverse osmosis/crystallizer, a crystallizer combined
Anthony also compared the process performance, utility requirements, as well as investment and utility costs of treating cooling tower blow-down
note that the capital costs of all combinations are within 10% of each other, although the operating utility costs of steam, electricity, and fossil fuel (natural gas/oil) can vary by a factor as high as 5. A reverse osmosis/VCE/
cal power consumed by a VCE/spray dryer combination. The selection among different systems is thus dictated by the relative costs of fossil fuel versus electricity.
It is impossible to arrive at generalized guidelines at the present time regarding the technoeconomics of steam compression by thermocompres-sion (e.g., steam jet ejectors) versus mechanical recompression for steam-drying plants. The costs of installation of such systems are such that they should be considered only for large-scale continuous steam dryers, that is, those with evaporation capacities of 100 t/day or higher. The steam com-pressors for such services are expected to cost over $500,000 (1990 esti-mate for North America). To this, one must add the costs of spares and maintenance.
It is important to note that existing steam compressor technologies may not meet the requirements of some of the steam-drying systems, espe-cially if very high degrees of superheat are needed. Often, two centrifugal compressors will be needed to boost the steam pressure to a level that is useful (e.g., 3 bar absolute from nearly atmospheric pressure). Increasing the discharge pressure typically involves increasing the rotational speed of the impeller. In some cases, this may exceed the design speeds of com-mercially available compressors. Thus, some new design problems may arise to obtain the desired discharge steam pressures.
TABLE 7.5
Characteristics of Screw versus Centrifugal Compressors
Feature Screw Centrifugal
Suction capacity (m3/h) 500–25,000 2,000–100,000Maximum pressure ratio 6:1 2:1Temperature difference (K) ∼60 ∼20Other Water droplets, dust
may cause damage.No surge line
Dry steam needed to avoidimpeller erosion. Bypass control to avoid compressor surge
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at utility plants for five different methods utilizing VCE. It is interesting to
crystallizer/solar pond combination consumes only one-fifth the electri-
with a centrifuge and air flash dryer, and a steam-heated crystallizer.
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 117
Detailed design and technoeconomic calculations in close association with manufacturers of compressors, steam jet ejectors, and steam dryers as
regarding the optimal steam reuse/recycle systems for steam dryers.
7.7 Closing Remarks
Although touted as the drying technology of the future some two decades ago, the continuing low-to-moderate cost of energy has kept this technol-ogy from developing faster since low net energy consumption is one of the key advantages of this technology aside from quality enhancement
there are both product and process limitations to be overcome.The cost of electricity as well as fossil fuels clearly drives the selection of
pressors for steam will give a boost to vapor-recompression technology for steam dryers. Cleaning of the steam is another obstacle to be overcome. Overall, it appears that there are some niche markets for SHS drying but much laboratory, pilot scale, and engineering design studies are needed before the technology becomes widespread.
This chapter summarizes the essential features of current steam-drying
sions, safer operation, and the like are some of the key advantages of such drying systems. Much R&D is still needed before some of the technological problems are resolved. For the drying of sludge, pulp, timber, brewer’s spent grain, coal, peat, biomass, and so on, the current steam-drying technolo-gies are already cost-effective and deserve to be considered very seriously despite their limited commercial experience to date. For drying timber, low-pressure SSD kilns is already the accepted drying technology around the
using low-rank coals that are characterized by high moisture content, the use of SHS dryers for pulverized coal is expected to increase over the com-ing decades. Also, as the cost of energy spirals upward along with legisla-tive pressures to reduce carbon emission due to climate change concerns, SSD technologies may have a future in certain industries. Although use of low-pressure SHS for drying foods, marine products, fruits, and vegetables is already feasible, few companies appear to be keen in adopting such tech-
information on SSD can be found in source literature (Mujumdar, 1990, 1992, 2007; Woods et al., 1994).
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well as with utility companies are required to make confident conclusions
and safe operation due to no fire and explosion hazards. Furthermore,
the system chosen. Significant capital costs are involved in SHS operation as well as energy recovery. Development in newer and more efficient com-
technologies. Improved energy efficiency, enhanced quality, reduced emis-
world. As there is increasing trend toward the use of coal-fired power plants
nologies due to their significantly different designs and operations. More
© 2009 by Taylor & Francis Group, LLC
118 Advanced Drying Technologies
Symbols
d.b. Dry basis2 s
∆H Latent heat of vaporization, kJ/kgP Pressure, Pa
2
wD Drying (evaporation) rate, kg/m2 sw.b. Wet basis
Subscripts
s Surfacess steam (superheated)WB wet bulb
Acronyms
BOD Biological oxygen demandCOD Chemical oxygen demandLPSD Low-pressure steam dryingRH Relative humiditySSD Superheated steam dryingTMP Thermomechanical pulpingTOC Total organic carbonVCE Vapor compression evaporationVOC Volatile organic compound
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A. S. Mujumdar (Ed.). Part B. Elsevier, Amsterdam, pp. 1593–1601.Kudra, T. and Mujumdar, A. S. 1989. Impingement stream dryers for particles and
pastes. Drying Technol., 7(2): 219–266.Kudra, T., Mujumdar, A. S. and Meltser, V. L. 1990. Impinging stream dryers: prin-
ciples, practice and potential. In: Drying of Solids. A. S. Mujumdar (Ed.). Sarita Prakashan, Meerut, India, pp. 17–32.
Kumar, P. and Mujumdar, A. S. 1990. Superheated steam drying – A state-of-the-art survey. In: Drying of Solids. A. S. Mujumdar (Ed.). Sarita Prakashan, Meerut, India, pp. 33–71.
Loo, E. and Mujumdar, A. S. 1984. A simulation model for combined impingement and through drying using superheated steam as the drying medium. In: Drying’84. A. S. Mujumdar (Ed.). Hemisphere Publishing Co., Washington,pp. 264–280.
Mujumdar, A. S. 1981. Recent developments in drying. Annual Conference of the Indian Institute of Chemical Engineering, Madras.
Mujumdar, A. S. 1990. Superheated steam drying—principles, practice and poten-tial for use of electricity. Canadian Electrical Association Report 817U671, Montreal, Canada.
Mujumdar, A. S. 1992. Superheated steam drying of paper: principles, status and potential. In: Drying of Solids. A. S. Mujumdar (Ed.). International SciencePublisher, New York, pp. 208–220.
Mujumdar, A. S. 2007. Superheated steam drying. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). 3rd edition. Taylor & Francis. Boca Raton, FL,pp. 439–452.
Oliveira, L. C., Wallace, J. W., Wright, D. M. and Mlcoch, V. 2001. Development of superheated steam/vacuum drying schedules for Western Canadian species. Proc. 7th International IUFRO Wood Drying Conference. July 9–13. Forestry andForest Products Research Institute, Tsukuba, Japan, pp. 140–145.
Pang, S. 1997. Some considerations in simulation of superheated steam drying of softwood lumber. Drying Technol., 15(2): 651–670.
Poirier, N. 1991. The effect of superheated steam drying on the properties of paper. Ph.D. Thesis, Chemical Engineering Department, McGill University, Montreal, Canada.
bed steam drying. In: Drying’86. A. S. Mujumdar (Ed.). Vol. II, Hemisphere Publishing Co., Washington, pp. 595–603.
dryers. Proc. 6th International Drying Symposium. Versailles, France, pp. 321–323.
Rosen, H. N., Bodkin, R. E. and Gaddis, K. D. 1982. U.S. Patent 4343095.
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of heating power plants by steam-fluidized bed drying of fuels. Symposium
Jensen, A. S. 1992. Pressurized drying in a fluid bed with steam. In: Drying’92.
Potter, O. E. and Beeby, C. 1986. Modeling tube-to-bed heat transfer in fluidized
Potter, O. E. and Guang, L. X. 1988. Some design aspects of steam-fluidized heated
© 2009 by Taylor & Francis Group, LLC
Superheated Steam Drying 121
Salin, J. G. 1986. Steam drying of wood particles for particleboard. In: Drying’86. A. S. Mujumdar (Ed). Vol. II, Hemisphere Publishing Co., Washington, pp. 572–574.
Salin, J. G. 1988. Steam drying of wood for improved particleboard and lower energy consumption. Paper Timber, 9: 806–810.
Shibata, H., Mada, J. and Shinohara, H. 1988. Drying mechanism of sintered spheres of glass beads in superheated steam. Ind. Eng. Chem. Res., 27(12): 2353–2362.
Thomkapanich, O., Suvarnakuta, P. and Devahastin, S. 2007. Study of intermit-tent low-pressure superheated steam and vacuum drying of a heat-sensitive material. Drying Technol., 25(1): 205–223.
van Gogh, F. 1985. Mechanical vapor compression for the pulp and paper industry. Appita, 38: 372–376.
Woods, B., Husain, H. and Mujumdar, A. S. 1994. Techno-economic assessment of potential superheated steam drying applications. Canadian Electrical Association Report 9138U888, Montreal, Canada.
Yoshida, T. and Hoydo, T. 1963. Superheated vapor as a drying agent in spinning
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fiber. Ind. Eng. Chem. Proc. Des. Dev., 2: 53–56.
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123
8Airless Drying
Airless drying appears to be a technique for energy conservation in drying as it enables the energy lost to the atmosphere with the water vapor in the exhaust gases in conventional air dryers to be recycled. Because ∼85% of the thermal energy in the water vapor produced by drying is the latent energy, only the remaining 15% can be potentially recovered, as the dew point at which the latent energy could be recovered from the vapor–air mixture at the typical dryer exhaust is ∼40°C and therefore not industrially useful. Hence, replacing hot air in convective drying by water vapor as it is in airless drying would offer substantial energy savings, typically from 40 to 90%, depending on process conditions (Anonymous, 1994; Stubbing, 1994a–1994c, 1999). An example of an energy audit for a conventional and an airless dryer is given in Table 8.1 (Heat-Win Ltd, Bitterley, U.K.).
The concept of airless drying (Stubbing, 1987) relies on evaporating the moisture from the wet product to form an atmosphere of almost pure steam. Since steam is slightly superheated, this leads to energy savings by avoiding the requirement to heat large quantities of air. In addition, the water evaporated in the dryer can be used as a source of heat for other processes. This concept is similar to that of superheated steam (SHS) dry-ing (see Chapter 7), however, at a lower degree of superheating, and when steam is generated by evaporation of water from the wet material.
In batch mode of operation, airless drying has three distinct stages. These are described in the following:
1. Warm-up phase. During this stage, the drying chamber initially contains wet solids and air at ambient temperature and at atmo-spheric pressure. At the start of the process, the recirculation fan is turned on and the air is circulated through the drying chamber and the heat exchanger as it is in conventional convective drying. No air is allowed to enter the dryer, but a fraction of air equal to its expansion volume is vented to the atmosphere. As evaporation progresses, a fraction of humid air is vented to the atmosphere before the remainder passes again through the heat exchanger and the recirculation fan. Now the drying medium is a mixture of air and water vapor. As the sequence of venting, heating, and dry-ing continues, the composition of the drying medium becomes
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richer in water vapor and poorer in air until finally it consists of
© 2009 by Taylor & Francis Group, LLC
124 Advanced Drying Technologies
almost pure water vapor. Because of heating in an internal heater, this water vapor is at a temperature of ∼120 to 150°C on entering the drying chamber and at ∼100°C at exit. Its pressure is slightly above atmospheric pressure. Therefore, the vapor at any point of the dryer is superheated (although the degree of superheating drops along the dryer) and thus is capable of transferring heat to the moisture in the solid and carrying away the vapor without any water condensing out.
2. Airless drying phase. This phase begins as soon as the last portion of air has been vented from the system and the drying medium consists essentially of superheated water vapor. The superheated vapor continues to circulate around the system while a continu-ous stream of excess vapor is vented so as to maintain the required pressure in the system. The vented steam has a pressure of 1 atm and is only slightly above saturation. Therefore, it can be used as a source of heat in a number of ways. It can be condensed by mix-ing with cold water to produce a supply of hot water at slightly <100°C, or it can be mechanically compressed to increase its tem-perature before being used as a source of heat for the dryer itself.
TABLE 8.1
Energy Audit for Conventional and Airless Drying of Ceramic Materialsin a Tray Dryer
kJ/kg %
Evaporation energy (from assumed 20°C ambient temperature)
2594 75.32
Energy for dry material heating (50% moisture content; from assumed 20 to 60°C; at 1.25 kJ/kg K)
50 1.45
Structural energy losses (from well-insulated dryer) 100 2.90
20 to 80°C exhaust temperature)700 20.33
Total 3444 100
Energy use with an airless dryer Evaporation energy (from assumed 20°C ambient temperature)
2594 74a
Energy for dry material heating (50% moisture content; from assumed 20 to 100°C; at 1.25 kJ/kg K)
100 4a
Structural energy losses (from well-insulated dryer) 150 4a
20 to 80°C exhaust temperature)Nil Nil
Total 2844 83a
−2170 −63a
Equals 674 20a
a Percent of the conventional dryer’s thermal energy.Source: Courtesy of Heat-Win Ltd., Bitterley, U.K.
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Energy use with an efficient conventional dryer
Air heating losses (to heat air through-flow from
Air heating losses (to heat air through-flow from
© 2009 by Taylor & Francis Group, LLC
Airless Drying 125
Dryingchamber
Compressor
Condensate
VentBurner
FanFluegases
Indirectheater
Heatexchanger
FIGURE 8.1Airless batch dryer with vapor recompression.
TABLE 8.2
Comparison of Energy Consumption
Type of Dryer kJ/kg
An airless dryer producing hot process water
626
An airless dryer with vented steam compression to recycle energy
400
An airless double-dryer reusing energy from its airless section in its air section
1900
3500
Source: Courtesy of Heat-Win Ltd., Bitterley, U.K.
3. Cool-down phase. When all of the liquid in the wet solid has been evaporated, a cool-down phase is initiated in which air is readmit-ted to the drying chamber to drive out the superheated vapor.
Figure 8.1 presents a schematic diagram of an airless batch dryer with vapor recompression. By compressing the vented steam and then condens-ing it in an internal heater of the airless dryer, most of the latent heat can be repeatedly recycled within the drying process. Since energy savings remain at ∼80% (see Table 8.1), the energy cost savings are lower because of the electrical energy needed for driving the compressor. Alternatively, the vented steam can be either condensed and used as hot process water or reused as a heating medium in another convective dryer. The net exter-nal thermal energy requirements to remove 1 kg of water in three possible
hot air dryer are compared in Table 8.2.
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configurations of an airless dryer (see Stubbing, 1994b) and a conventional
A typical efficient conventional hot air dryer
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126 Advanced Drying Technologies
The main problem to be solved when evolving from the batch process to the continuous one is conveying the material through the dryer within the superheated atmosphere without allowing ambient air to enter the drying chamber. Since mechanical seals around the conveyor would be imprac-tical, a nonmechanical sealing method was developed (Stubbing, 1998). With reference to Figure 8.2, this method utilizes the horizontal thermal
tain level across the ducts that extend from the drying chamber and form the entry and exit ports for the conveyor with the material to be dried. According to Stubbing (1994b), the substantial (55%) difference in density between air and steam at 100°C combined with the temperature difference itself give rise to the stability of the layer. As proven in pilot tests, conveyed
Once a part of the wet material is inside the drying chamber, air heat-ing, circulation, and progressive replacement with water vapor take place as in a batchwise operation until the process temperature reaches 100°C.
product inlet and outlet ducts. From that point on, the wet material can be conveyed through the drying chamber while evaporated water is continu-ously heated and recirculated at atmospheric pressure. To avoid possible oxidation of the product by the air contained in the drying chamber dur-
the target temperature is attained.
FIGURE 8.2Continuous airless dryer.
Dryingchamber
Vent
Burner
Fan
Flue gases
Indirectheater
Hopper(preheater)
Conveyor
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and density differential stratification layers. This layer is located at a cer-
materials can move up and down through the stratification layer with no
was detected through the dryer’s condenser (Anonymous, 1994).apparent disturbance—this was confirmed by the fact that no vented air
At this stage, the stratification layer is formed at a certain level across the
ing the warm-up period, water may first be sprayed in the chamber until
Stratification layer
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Airless Drying 127
Besides the typical advantages of drying in a steam/SHS atmosphere
ing rates due to preferential properties of steam against air, such as viscos-
to be energy savings and thus lower processing costs and reduced CO2emissions. For example, the energy savings in high- humidity paper dry-ing with exhaust heat recovery in comparison with conventional paper drying are claimed to be up to 7.75 TOE (tons of oil equivalent) per 100 t of paper produced. This approximates to 10 t of fossil fuel savings and reduction of CO2 emissions by up to 30 t/100 t of dry paper produced.The cost of thermal and compression energy inputs has been estimated at £456 for airless dryers as compared to £1094 for conventional paper dryers (Stubbing, 1990). See the References section for more information about energy consumptions and cost estimates.
References
Anonymous. 1994. Paper sludge—from a disposal problem into a valuable prod-uct. DTI’s Environmental Management Options Scheme. Project Information Brochure No 3.
2,209,383.Stubbing, T. J. 1990. Airless drying process saves energy and reduces emissions.
Paper Technol., 31(June): 36–39.Stubbing, T. J. 1993. Airless drying: Its invention, method and application. Trans.
IChemE. Part A., 71(A5): 488–495.Stubbing, T. J. 1994. Airless drying—a case study from the ceramic industry.
DEMOS Communicate. Heat-Win Ltd, Bitterley, U.K.Stubbing, T. J. 1994a. Sludge Disposal: Moist Incineration or Dry Combustion—a
Preliminary Comparison. Heat-Win Ltd., Bitterley, U.K., P. 8.Stubbing, T. J. 1994b. Airless drying. Drying 94. Proc. 9th Int. Drying Symposium
(IDS’94). Gold Coast, Australia, pp. 559–566.Stubbing, T. J. 1994c. Airless drying for reducing the cost of drying materials,
reducing the cost of sludge processing and reducing the cost of drying paper. The DEMOS Progress Seminar on Paper Pulp Sludge from a Disposal Problem into a Valuable Product, Leicester, February 8.
Stubbing, T. J. 1998. Method and apparatus for continuous drying in superheated steam. US Patent 7,711,086.
Stubbing, T. J. 1999. Airless drying—development since IDS’94. Drying Technol.,17(7&8): 1639–1651.
CRC_73877_Ch008.indd 127CRC_73877_Ch008.indd 127 12/29/2008 11:28:12 AM12/29/2008 11:28:12 AM
such as no oxidation, reduced fire and explosion hazard, and higher dry-
ity and specific heat capacity, the main advantage of airless drying appears
Stubbing, T. J. 1987. Method and apparatus for energy efficient drying. U.K. Patent
© 2009 by Taylor & Francis Group, LLC
129
9Drying in Mobilized Beds
Fluidized bed (FB) dryers have found widespread applications for dry-ing of particulate or granular materials in the chemical, food, ceramic, pharmaceutical, polymer, and similar industries. More recently, they also found special applications in drying of suspensions, sludges, and soft pastes, which are atomized into an FB of already dry material (back-mixing) or the bed of inert particles (Devahastin, 2000; Mujumdar, 2007). Among the advantages of the FB dryers, one may cite
High drying rates due to excellent gas–particle contact•
for drying is supplied by internal heat exchangersLower capital and maintenance costs compared to a rotary dryer • of similar drying capacityEasy to control•
Depending on density, the particle size is limited to 2–5 mm to main-•
Size distribution should be as narrow as possible to avoid exces-• sive carryover.
High power consumption due to the need to suspend the entire • bed in a gas stream leading to a high pressure drop.
wet.
some of these limitations. The centrifugal FB was developed to intensify heat- and mass-transfer rates for rapid drying of surface-wet particles as well as to deal with the nonconventional properties of certain mate-
cessfully applied to dry materials with relatively low bulk density, such as pieces of potato, apple, carrot (Lazar and Farkas, 1971), bell pepper,
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• Higher thermal efficiency, especially if part of the thermal energy
The main limitations of the fluidized bed dryers are as follows:
tain a technically reasonable superficial velocity of fluidizing air.
• Particles should be isometric as flakes, strips, and fibers cannot be fluidized.
• Low flexibility and potential of defluidization if the feed is too
Several techniques were evolved from the fluid bed principle to overcome
rials. Because a larger airflow may be used in such a dryer, it was suc-
© 2009 by Taylor & Francis Group, LLC
130 Advanced Drying Technologies
beet (Hanni et al., 1976), and quick-cooking rice (Roberts et al., 1980). For
Rysin and Ginzburg, 1992; Pakowski et al., 1984) and vibro-spouted bed
idization of large and nonuniform particles such as tea leaves (Shah and Goyel, 1980). Another approach is the spouted bed technique in various
et al., 1990; Passos et al., 1987).
is of great technical interest. One solution to this problem is a novel tech-nique termed the mobilized bed, which relies solely on its hydrodynamicsto achieve bed mobilization. Although this chapter is based on data from a single source (Legros, 1986), the technique and results appear to be of
ing techniques. The main part of the mobilized bed apparatus is the gas distributor, shown schematically in Figure 9.1. Since the drying chamber used in these experiments was square in cross section, the distributor has
The lower section is 5 × 5 cm, whereas the upper section is 15 × 15 cm(small distributor) or 30 × 30 cm (large distributor). The inclined surfaces
row being at the junction of the base and inclined wall. Each row contains
introduced to the dryer as two independently controlled and metered
wall nozzles, which loosen and partially disentangle the bed material and force the tobacco particles toward the central upward moving region, was
promote overall circulation of the bed material by accelerating the par-ticles in the upward direction.
moves away from the inclined walls of the distributor. The material is compressed and a cavity appears in the lower region adjacent to the lower
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modifications, which allows processing of Geldart’s Type D particles that
zation of fibrous entangled materials such as cut lamina tobacco particles
sufficient significance so as to warrant further development.
a square configuration with the four walls inclined at 30° to the vertical.
have a number of orifices positioned in three horizontal rows, the lowest
a series of orifices 4 mm in diameter, as illustrated in Figure 9.1, which also shows details of the spacings of the orifices on one wall of the larger distributor. The base of the mobilizer contains a single orifice (7 or 10 mm
materials that are difficult to fluidize because of their odd shapes, the
(Gawrzynski and Glaser, 1996). Mechanical vibration applied to the par-pulsed fluid bed described elsewhere in this book can be recommended
ticulate material has led to the vibro-fluidized bed drying (Erdesz, 1990;
drying (Finzer and Kieckbusch, 1992). This makes possible the pseudoflu-
are too coarse or too dense to fluidize well without channeling (Kalwar
Aside from numerous techniques for drying of particulates, the fluidi-
The idea of a mobilized bed is derived from the spout-fluid bed con-cept that combines some characteristics of both fluidization and spout-
in diameter) positioned on the central vertical axis. The fluidizing air is
streams. The fluidization flow provided by the high-velocity jets from the
termed the mobilizing flow by Legros (1986). The spouting flow, providedby the central gas jet, was named the accelerating flow as its purpose is to
When the mobilizing flow is started with no accelerating flow, the bed
orifices. As the flow is increased, the upper bed surface becomes domed
© 2009 by Taylor & Francis Group, LLC
Drying in Mobilized Beds 131
and the bed lifts. At a certain instant, the surface of the bed breaks and motion is apparent from the central vertical axis toward the walls. At this point, the entire bed becomes mobile, with a rolling circulating pattern. Gas and solids together move upward in the central core near the bed axis, whereas solids move outward in the freeboard and then downward along
Mobilizing
Plenumchamber
Accelerating flow
Gas distributor
Mobilizing
300
35
35
28
12
50
30
65
58
50
(a)
(b)
FIGURE 9.1
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orifices
Mobilized bed distributor: (a) Design and (b) spacings of the orifices (large distributor).
flow
the inclined walls. With increasing gas flow, the motion becomes more
© 2009 by Taylor & Francis Group, LLC
132 Advanced Drying Technologies
turbulent until eventually the rolling mobilization is disrupted and the charge is transported out of the column. Once mobilization is established,
induce the initial motion without losing circulation. Observation of par-
conditions.The mechanisms involved during the onset of bed mobilization can be
better described with the help of a plot of the bed pressure drop as a func-
from the walls and a cavity appears. The slightly compressed bed has reduced voidage so that the pressure drop continues to rise along the line AB.
3. At point B, the bed is fully suspended by the hydrodynamic action
4. Beyond point B, the relationship between pressure drop and
which is related to the self-cohesion of the bed. It was observed
ticles away and start behaving like free jets, thereby diminish-
tobacco, the particles are entangled, and the jets penetrate only a short distances before losing their identity.
Figure 9.2, the pressure drop falls sharply and the bed becomes
on the bed history and is not fully reproducible. 6. Once the bed is mobilized, the pressure drop remains substan-
lized until point C′, which corresponds to a lower value of mobi-′
point depends on the manner in which the mobilization stops. The
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ticle motion indicated that the fibers are largely disentangled under these
as a standard fixed bed of particulate material with the pressure
that for short-fiber tobacco, the mobilizing gas jets push the par-
ing the interaction between the gas and solids. With long-fiber
sion, without significant effect on the pressure drop.
it is possible to reduce the gas flow below the incipient level required to
tion of the mobilizing gas flow rate (Figure 9.2). The following sequence of events was reported as the mobilizing flow rate is increased (Legros, 1986):
1. For small mobilizing flow rates, the bed of cut tobacco behaves
drop steadily increasing with airflow.2. At a certain airflow, the lower region of the bed starts to move away
of the gas flow and the pressure drop reaches maximum. With
causing the pressure drop to fall.further increase of the gas flow rate, the bed starts to expand,
mobilizing flow is dependent on the fiber length of the tobacco,
5. At a certain mobilizing flow rate, corresponding to point C in
mobile. The value of the mobilizing flow rate is very dependent
tially constant with a further increase of mobilizing flow. The excess flow results in a more turbulent bed and higher bed expan-
7. When the mobilizing flow is decreased, the bed remains mobi-
lizing flow than at point C. This value is called the minimum mobilizing flow rate. The bed pressure drop curve beyond this
© 2009 by Taylor & Francis Group, LLC
Drying in Mobilized Beds 133
bed may collapse and turn into a packed bed with no channel. In this case, the bed pressure drop increases sharply as mobilization is lost and then decreases smoothly with reducing mobilizing
a way that a preferential channel still exists. The pressure drop
(path C′B*A).
7 mm in diameter), the dependence on the magnitude of the accelerat-
Mobilizationbegins
B
B
C CB*
A
Bed
pres
sure
dro
p (P
a)
FIGURE 9.2
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rates at minimum incipient mobilization is also dependent on the orifice size of the accelerating nozzle. As the orifice size is reduced (from 10 to
Mobilizing flow rate (L/min)
Pressure drop versus mobilizing flow rate.
flow (path C′B′′A). More frequently, the bed may collapse in such
then remains at a low value, as most of the air flow is through the channel and decreases slowly with decreasing mobilizing flow
Although it is possible to fluidize particles with the mobilizing flow only, stable fluidization is possible over a wide range of flows when mobilizing and accelerating flows are used in combination. The behavior of the bed is the same as that observed without accelerating flow (Figure 9.3). The mobilizing flow needed to initiate motion is dependent on the accelerat-ing flow rate. The relationship between mobilizing and accelerating flow
ing flow becomes more important. Figure 9.4 shows the range of stable
© 2009 by Taylor & Francis Group, LLC
134 Advanced Drying Technologies
FIGURE 9.3Flow pattern in a mobilized bed.
Upwardmoving
core
Domed surface
Accelerating flow
Mobilizing flowEntrained flow(parallel to wall)
Distributor
FIGURE 9.4Phase diagram for mobilization.
Start-upconditions
Fast mobilization
Minimummobilization
Accelerating flow rate (L/min)
Mob
ilizi
ng fl
ow ra
te (L
/min
)
Stablemobilization
Pneumatic transport
Fixed bed
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© 2009 by Taylor & Francis Group, LLC
Drying in Mobilized Beds 135
mobilization for a bed of tobacco particles. Start-up represents the condi-tions where the rolling motion of the bed starts, as distinct from mini-
conditions to maintain motion. The fast mobilization regime corresponds to excessive carryover of the bed material.
According to Legros (1986), the behavior of cut lamina tobacco in a larger mobilizer is similar to that observed in a smaller one. However, the
rate necessary to initiate mobilization. In general, the operation remains stable on a larger-scale apparatus.
port a bed of cut lamina tobacco at the peak pressure drop (point B in Figure 9.2) can be calculated from the following equations:
F VV sin
C n dV
DF Vy
2
m m2 p( ) ( )
42
2
(9.1)
F V mgy( ) (9.2)
where Fp(V) = ΣFpi(V) is the total pressure force calculated by adding the contributions from individual pressure forces for each zone of the mobi-
mobilizer height (y):
F V AQ y yBQ
d y d y tanpi i i ii
m i m i
( ) ( )tan tan
112
12
12 +
(9.3)
found to be
A B3
254401
127401
2
3
( ) ( )
(9.4)
for ε > 0.8.The bed voidage was determined from the measured bulk density (ρb)
at a given moisture content (X) as
b X 665 1 1( ) ( ) (9.5)
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lizer that are identified by the position of mobilizing orifices along the
On the basis of the packed bed experiments, the coefficients A and B were
mum mobilization conditions, which correspond to the minimum flow
accelerating flow has a noticeably smaller effect on the mobilizing flow
Following Legros (1986), the mobilizing flow rate (V) necessary to sup-
© 2009 by Taylor & Francis Group, LLC
136 Advanced Drying Technologies
To estimate the peak pressure drop across a bed of cut lamina tobacco having a bulk density (ρb) and moisture content (X), Legros (1986) pro-posed the following procedure:
1. Calculate the bed voidage from Equation 9.5 3. Plot the ratio Fy(V)/mg as a function of V with Fy(V) calculated
from Equation 9.1 4. Determine the value of V for which the ratio of Fy(V)/mg = 1 5. Calculate the pressure drop from the following equation:
∆ ∆P PAQ
d y d y
BQ
ii
m i m i
i2
∑ ∑=
21
21
21tan tan tan
661
2
1
213 3tan tan tan d y d ym i m i
( ) ( )
(9.6)
i
Figure 9.5 shows a comparison between the predictions of the bed pressure drop calculated from Equation 9.6 and experimental results
b =95 kg/m3 and X = 0.16 kg/kg were equal to 570 kg/m3 s and 2330 kg/m4, respectively.
Heat-transfer experiments (Legros, 1986) performed at various hydro-dynamic conditions showed that the mobilized bed may be regarded as being perfectly mixed for both gas and solid phases. The heat-transfer rates were high enough to achieve thermal equilibrium in both phases. In the case of drying, a falling rate period was observed for the entire drying
drying rate was found to be controlled by the internal mass-transfer rate. Furthermore, a drying model was proposed which is based on a liquid diffusion mechanism. Because the cut tobacco particles had a width of 1 mm, it was assumed that the moisture migration occurs only in one direction, that is, through the cut faces of the lamina. For such a unidirec-
∂∂
∂ ∂∂
∂Xt
DXx
x
e
(9.7)
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2. Calculate the coefficients A and B from Equation 9.4
obtained from the 30 × 30 cm mobilizer. The coefficients A and B for ρ
curve of cut lamina tobacco; for air superficial velocities above 0.8 m/s, the
It should be noted that the gas flow rate (Equations 9.3 and 9.6) in each zone (Q ) is considered to be the sum of the total gas flow entering below the zone’s lower boundary, where V represents the mobilizing flow rate.
tional flow, the solution of the diffusion equation is
© 2009 by Taylor & Francis Group, LLC
Drying in Mobilized Beds 137
instantaneously the hygroscopic moisture content in equilibrium with the
X XX X n
n DL
te
ee
02 2
228 1
2 12 1
2
( ) ( )
∑ exp (9.8)
where 2L is the thickness of the tobacco lamina, X0 is the initial moisture content, and Xe is the equilibrium moisture content.
The experimental moisture content data were then plotted according to
a function of the bed temperature is shown in Figure 9.6. It is clear that the
0 500 10000
100
200Be
d pr
essu
re d
rop
(Pa)
m = 0.4 kg
Experiment
Model (Equation 9.6)
D = 0.3 m
(A = 570 kg/m3 s, B = 2330 kg/m4 s)
FIGURE 9.5Comparison between model predictions and experimental data. (From Legros, R., Ph.D. Thesis, University of Surrey, U.K., 1986.)
CRC_73877_Ch009.indd 137CRC_73877_Ch009.indd 137 1/8/2009 11:19:26 AM1/8/2009 11:19:26 AM
For a constant effective diffusion coefficient and particle surface reaching
gas phase, the following equations is derived for an infinite slab:
Equation 9.8, and the effective diffusion coefficient was determined from the slope of the plot. The logarithm of the effective diffusion coefficient as
Mobilizing flow rate (L/min)
© 2009 by Taylor & Francis Group, LLC
138 Advanced Drying Technologies
ing to the Arrhenius relationship given by
DE
RT RTea
0 00102 0 00102
39500. exp . exp (9.9)
plify Equation 9.8 to the following form:
X XX X L
D te
ee
0
2
2exp
(9.10)
100 80 60 40 305070901.0E6
0.2
0.4
0.6
0.8
1.0E5
0.2
0.4
0.6
0.8
Temperature (ºC)
21.0E4
FIGURE 9.6
University of Surrey, U.K., 1986.)
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Effec
tive d
iffus
ion
coeffi
cien
t (cm
/s)
effective diffusion coefficient varied with the solid’s temperature accord-
Effect of temperature on the effective diffusion coefficient. (From Legros, R., Ph.D. Thesis,
To reflect the experimental drying curves, Legros (1986) proposed to sim-
© 2009 by Taylor & Francis Group, LLC
Drying in Mobilized Beds 139
Comparison of the actual drying results with predictions from the models given by Equations 9.8 and 9.10 is shown in Figure 9.7. Clearly, the model based on moisture diffusion can be used to predict adequately the drying
Detailed derivations of the equations cited in this chapter as well as
materials, heat transfer, and drying results can be found in the thesis by Legros (1986) and papers by Legros et al. (1994, 1995).
Symbols3
4
FIGURE 9.7Prediction of drying curves for cut tobacco lamina. (From Legros, R., Ph.D. Thesis, University of Surrey, U.K., 1986.)
Drying time (s)
Moi
stur
e con
tent
(kg/
kg)
0 40 80 120 160 2000
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Equation 9.10
Equation 9.8
Tbed = 70.2ºC
De = 8.63 10−6cm2/s
Xe = 0.01 kg/kg
m = 0.3 kg
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kinetics of tobacco fibers in the mobilized bed dryer.
other correlations describing a mobilization technique for drying fibrous
A Coefficient, kg/m sB Coefficient, kg/m
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140 Advanced Drying Technologies
Cm dm Diameter (size) of the mobilizer base, mD Diameter (size) of the mobilizer top, mDe
2
Ea Activation energy, J/kmolg Acceleration due to gravity, m/s2
Fp Force due to pressure gradient, NFy Upward force from momentum balance, NL Half-thickness of the lamina, mm Mass of tobacco, kgn Consecutive number
∆P Pressure drop, PaQi
3
R Gas constant, J/kmol Kt Time, sT Temperature, K
3
X Moisture content, kg/kgX0 Initial moisture content, kg/kgXe Equilibrium moisture content, kg/kgy Upward distance from the base of the mobilizer, mα Angle, °ε Bed voidageρ Gas density, kg/m3
ρb Bulk density, kg/m3
References
Devahastin, S. 2000. Mujumdar’s Practical Guide to Industrial Drying. Exergex Corporation, Montreal, Canada.
Erdesz, K. 1990. Bibliography of literature on fundamentals and applications of vibration in particle processing. In: Drying of Solids. A. S. Mujumdar (Ed.).Sarita Prakashan, Meerut, India, pp. 326–338.
Finzer, J. R. D. and Kieckbusch, T. G. 1992. Performance of an experimental vibro-spouted bed dryer. In: Drying’92. A. S. Mujumdar (Ed.). Elsevier Science Publisher, Amsterdam, pp. 762–772.
gas stream. Drying Technol., 14(5): 1121–1172.Hanni, P. F., Farkas, D. F. and Brown, G. E. 1976. Design and operating param-
1172–1176.
CRC_73877_Ch009.indd 140CRC_73877_Ch009.indd 140 1/8/2009 11:19:28 AM1/8/2009 11:19:28 AM
Mobilizing orifice discharge coefficient
Effective diffusion coefficient, m /s
n Number of mobilizing orifices
Volumetric flow rate in each zone of the mobilizer, m /s
V Mobilizing flow rate, m /s
Gawrzynski, Z. and Glaser, R. 1996. Drying in a pulsed fluid bed with relocated
eters for a continuous centrifugal fluidized bed dryer (CFB). J. Food Sci., 41:
© 2009 by Taylor & Francis Group, LLC
Drying in Mobilized Beds 141
Kalwar, M. I., Raghavan, V. G. S. and Mujumdar, A. S. 1990. Bibliography on spouted bed technology. In: Drying of Solids. A. S. Mujumdar (Ed.). Sarita Prakashan, Meerut, India, pp. 343–352.
studies on piece-form foods. J. Food Sci., 36: 315–319.
University of Surrey, U.K.Legros, R., Clift, R. and Millington, C. A. 1995. Development of a mobilization
Legros, R., Millington, C. A. and Clift, R. 1994. Drying of tobacco particles in a mobilized bed. Drying Technol., 12(3): 517–543.
Mujumdar, A. S. (Ed.). 2007. Handbook of Industrial Drying. Taylor & Francis, Boca Raton, FL.
Pakowski, Z., Mujumdar, A. S. and Strumillo, C. 1984. Theory and application of
Drying. A. S. Mujumdar (Ed.). Vol. 3. Hemisphere, Washington, pp. 245–306.Passos, M. L., Mujumdar, A. S. and Raghavan, G. S. V. 1987. Spouted bed for
drying: Principles and design considerations. In: Advances in Drying.A. S. Mujumdar (Ed.). Vol. 4. Hemisphere, Washington, pp. 359–398.
Roberts, R. L., Carlson, R. A. and Farkas, D. F. 1980. Preparation of a quick-cooking
1080–1081.Rysin, A. P. and Ginzburg, A. S. 1992. Theory and technology of food product
International Science Publisher, New York, pp. 86–124.
bed (Part 1). In: Drying’80. A. S. Mujumdar (Ed.). Vol. 2. Hemisphere PublishingCorporation, Washington, pp. 176–181.
CRC_73877_Ch009.indd 141CRC_73877_Ch009.indd 141 1/8/2009 11:19:29 AM1/8/2009 11:19:29 AM
technique for fibrous materials. Powder Technol., 85: 105–114.
Lazar, M. E. and Farkas, D. F. 1971. The centrifugal fluidized bed (CFB) 2. Drying
Legros, R. 1986. Processing of vegetable materials in fluidized beds. Ph.D. Thesis.
vibrated beds and vibrated fluid beds for drying processes. In: Advances in
brown rice product using a centrifugal fluidized bed dryer. J. Food Sci., 45:
drying in fluidized vibrated bed. In: Drying of Solids. A. S. Mujumdar (Ed.).
Shah, R. M. and Goyel, S. K. 1980. Drying characteristics of tea fluidized on a vibrating
© 2009 by Taylor & Francis Group, LLC
143
10Drying with Shock Waves
One of the innovative technologies for atomizing viscous liquids and thin pasty materials in a gas is the use of shock waves. This method could be used in spray drying to disperse the liquid feed (see Chapter 14) and thus eliminate the conventional atomizer from the system. This will avoid problems with erosion in disk atomizers and clogging of single- or two-stream spray nozzles. Potential applications of this idea in other types
vortex dryers, in which the momentum of the shock waves can enhance the hydrodynamic impact of a conventional gas carrier.
It should be noted that the term shock waves refers to a pressure wave of large amplitude that arises from sharp and violent disturbances when the velocity of wave propagation exceeds the velocity of sound propagation. Characteristically, an abrupt change of the medium properties (e.g., pres-sure, stress, density, particle velocity, and temperature) takes place in a limited space across the shock wave (Schetz and Fuhs, 1996; Shapiro, 1953; Anderson, 1982; Saad, 1993). In the case described in this chapter, the physical phenomenon of a shock wave is restricted to a one-dimensional plane wave propagation, in which the properties of air in the resonant tube of the wave generator vary with respect to only one spatial coordi-nate and time. Although it is technically feasible to generate a true shock wave by moving a piston into a compressible medium, the velocity of wave propagation in the system presented here is lower than the sonic
exiting the generator is similar to a nearly discontinuous shock front
tion for the pressure wave of such characteristics, the term shock wave is used throughout this chapter.
Atomization and drying in shock waves is still at the development stage, for example, laboratory- and pilot-scale experiments. Figure 10.1 presents schematically the experimental shock-wave generator used by Lyulin (1998) to disperse pasty materials in a spray dyer. The generator consists of a stainless steel tube 0.04 m in diameter, which propagates the pressure waves and serves as a quarter-wave (Helmholtz) resonator (see Chapter 14). One end of the resonance tube is open to the atmosphere, whereas the other end is connected to a pulse generator through a trun-cated cone 0.2 m long. The generator comprises a cylindrical chamber 0.08 m in internal diameter and a crankshaft-connecting rod assembly,
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propagating at a supersonic velocity. Because there is no adequate defini-
of dryers for dispersed materials include pneumatic, ring, spin-flash, and
velocity. However, the almost flat pressure profile at the front of the wave
© 2009 by Taylor & Francis Group, LLC
144 Advanced Drying Technologies
which moves the piston in a reciprocating manner. The stroke of the pis-ton is 0.082 m. The pulse generator is driven by a 7 kW DC motor with controllable rotational speed, allowing variable frequency of the strokes from 7 to 28 Hz.
The instantaneous pressure and velocity distributions along the reso-nance tube are monitored by piezoelectric transducers and a hot-wire
recorder. These make possible the tuning of the generator to the acoustic resonance by varying the rotational speed of the crankshaft. Also, there is a provision to attach resonance tubes of various lengths and diameters to obtain required amplitudes of pressure and velocity pulsations.
Figure 10.2 presents a typical temporal pressure oscillation at various
lation near the piston is sinusoidal because of the reciprocating movement of the piston. As the pressure wave travels along the resonant tube, its descending part (the front of the wave) becomes shorter, whereas the
steep pressure front characteristic of a shock wave. The extremely high gra-dients of pressure at the front of the plane wave not only initiate mechani-cal effects such as liquid dispersion but also can alter drying rates due to
cally unchanged along the resonant tube, and distortion of the sinusoidal velocity pulsation occurs only at the tube outlet. As shown in Figure 10.3, the amplitude of velocity pulsation decreases sharply with the distance of wave propagation in open air. To take full advantage of the momentum of the shock waves, liquid to be dispersed should be fed close to the outlet
Variablespeed drive
Oscillograph
Pulsator
Resonator
Amplitude ofvelocity pulsation
Velocitysensor Pressure sensor
rpm Meter
FIGURE 10.1
of drying instant pigments by the use of shock wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
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Schematic of the experimental shock-wave generator. (From Lyulin, N. B., Intensification
anemometer, which are connected to the oscilloscope and photo film
points along the resonance tube. It is clear that the profile of pressure oscil-
ascending one (the tail of the wave) levels off gradually, finally giving a
Unlike pressure, the profile of the velocity pulsation remains practi-enhanced evaporation and capillary flow of the liquid and vapor moisture.
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 145
from the resonant tube, in this case up to ∼20 cm. Also, the strongest impactof shock wave due to air velocity might be expected over a distance of up to 1 m from the resonance tube.
Figure 10.4 presents amplitude–frequency characteristics of two
x = Lt (at piston)
x = 0.25 Lt
x = 0.5 Lt
x = 0 (at tube outlet)
FIGURE 10.2Characteristics of pressure oscillations in the resonance tube.
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different resonant tubes, where the first and the second resonance
© 2009 by Taylor & Francis Group, LLC
146 Advanced Drying Technologies
f=22.5 Hz
0
25
50
75
100
125
0 0.25 0.5 0.75 1.0 1.25 1.5Distance (m)
Velo
city
(m/s
)
f = 22.5 Hz
FIGURE 10.3Variation of the amplitude of the pulsating velocity with distance from the outlet of the resonance tube.
8 10 12 14 16 18 20 2260
50
100 Lt= 6.4 mLt= 5.8 m
Second resonance
First resonance
Frequency (Hz)
Velo
city
(m/s
)
FIGURE 10.4Typical resonance characteristics of the shock-wave generator. (From Lyulin, N. B.,
Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
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Intensification of drying instant pigments by the use of shock wave generator, Ph.D. Thesis,
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 147
respective curves.For a given tube length, the resonance frequency is calculated as
f nvLr
t
2 14
( ) (10.1)
where v is the velocity of sound propagation, Lt is the tube length, and n is the order of the harmonic frequency.
From Equation 10.1, it follows that the product fr ∙ Lt does not depend on the tube diameter but is a constant characteristic of the gaseous environ-
nance is obtained at fr ∙ Lt = 82.5 m/s.The maximum amplitude of the velocity pulsation appears at n = 1
(Figure 10.4). It is quite reasonable to assume that the amplitude of air pulsation at the tube outlet is proportional to the amplitude of the velocity of piston movement:
u Kuc (10.2)
Accounting for the different diameters of the resonant tube (dt) and sub-stituting uc by (π(( fs), the following equation (valid for u <140 m/s) was derived by Lyulin (1998):
u Z f sdd
c
t
(10.3)
where s is the piston stroke length, dc is the piston diameter, and Z is aconstant equal to 11.0 for 0.044 < dt < 0.05 m and equal to 9.1 for 0.033 <dt ≤ 0.044 m. These values were determined experimentally.
Expressing the frequency in terms of the crankshaft revolutions per minute (rpm), the following experimental correlation can be used to cal-culate the length of the resonant tube and the amplitude of the air velocity at the tube outlet:
u 0.0523ZsNdd
(m/s) L15vN
(m)c
tt
(10.4)
The shock wave acts as a liquid atomizer up to the point at which the momentum of the shock wave is able to disperse the liquid stream. By
conditions can be construed as
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frequencies are clearly identified by the consecutive maxima on the
ment. Hence, at the sound velocity in air equal to 330 m/s, the first reso-
analogy with two-phase flows in packed columns, such hydrodynamic floodinggg. Hence, the region of stable
at omization is mapped in terms of a flooding coefficient, defined as the
© 2009 by Taylor & Francis Group, LLC
148 Advanced Drying Technologies
β W WW
d (10.5)
12
Au
udd
g
l
t
l
(10.6)
where A is a constant, dl is the diameter of the liquid jet subject to atomi-zation, and ul and ug are the liquid jet velocity and the amplitude of thegas velocity pulsation, respectively.
Figure 10.5 is
u fu
dlg
l
0 0016137 3
1 3
13
0 25.
..
.
( ) (10.7)
where the liquid jet diameter, dl, is in millimeters; the frequency, f, is in hertz; and the gas velocity, ug, is in meters per second.
The dispersion of pasty pigments with shock waves was studied experi-mentally using a shock-wave generator with a resonance tube of 0.033 m. This allows achieving air velocity equal to 163 m/s at 19.8 Hz of the recip-rocating movement of the piston. The pasty feed was located 0.05 m from the resonance tube outlet at its axis. Such a distance was found to be opti-mal over the range 1.5–5 times the tube diameter.
The following procedure was used to obtain a droplet size distribu-
paper placed perpendicularly to the tube axis 1 m from the pasty feed.
under a microscope, and about 2000 droplets were counted for the size distribution. Knowing the dry mass of the dispersed pigment (m) and the total surface area of the spots (SS), the volume of the droplet having (X, %wet basis [w.b.]) moisture content that leaves the spot with diameter (dS) can be calculated from the following equation:
Vd m
S XpS
m S
2
4100
100 (10.8)
where ρm is a density of the wet material.
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shock-wave field:
ing coefficient:
tion (Muranov et al., 1997). Dispersed pigments were trapped in a filter
Segments of the filter paper with spots of droplets were then analyzed
mass flow ratio of undispersed liquid to the total liquid subject to the
Lyulin (1998) provided the following empirical correlation for the flood-
Thus, the generalized correlation for the flooding velocity shown in
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 149
0 0.2 0.4
0.2
0.4
0.6
0.8
1.00.80.60
1.0
X = 0.00161f1.31 (ug 37.3)1/3
ul dl
X = 0.00161f 1.31 (ug 37.3)1/3
ul dl
β = (1 X)2
β =
(1
X)2
5.7 7.8 4.2 3.7 8.1 7.8 7.8 7.8
19 19 19.2 19.2 19.1 19.2 19.2 19.2
113 113 113 113 113 97 84 37.6
4.7 2.4 5.77 8.37 2.33 7.2 7.28 2.51
1.96 1.16 1.44 1.86 1.26 1.36 1.36 0.84
dl (mm)
f (Hz)
ug (m/s)
ul (m/s)max
ul (m/s)min
7.8 7.8 7.8 7.8 7.8 7.8 7.8
19.2 19.2 19.2 15 16.8 20 22
55.5 67.2 83 83 83 83 83
1.78 2.51 7.28 7.28 7.28 7.28 7.28
0.42 1.05 1.15 0.73 0.94
7.8
17.8
83
7.28
0.94 1.15 1.36
dl (mm)
f (Hz)
ug (m/s)
ul (m/s)max
ul (m/s)min
0.25
FIGURE 10.5
of drying instant pigments by the use of shock wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
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Flooding curve for atomizing liquids with shock waves. (From Lyulin, N. B., Intensification
© 2009 by Taylor & Francis Group, LLC
150 Advanced Drying Technologies
Thus, the droplet diameter is
dV d m
S Xpp S
S m
6 150100
1 3 21 3
( )
(10.9)
paper is
S S dd ii
n
p ii
n
, ( )1
2
1∑ ∑
A typical distribution of droplet size for anticorrosive paint based on zinc phosphate is shown in Figure 10.6. The symbol dMmedian diameter (midpoint of the size distribution), and dsV is the sur-face–volume diameter, that is, the diameter of the sphere having the same ratio of surface area to volume as the droplet and is calculated as
dd nd nsV
i i
i i
ΣΣ
3
2 (10.11)
Figure 10.6 indicates that droplets generated by the shock wave are nearly monodispersed as the mass fraction of the droplets larger than
Because the surface-volume diameter falls into the range of large drop-lets (106 µm), the median diameter appears to be more appropriate for evaluating the atomizing capability of shock waves. The experiments also proved that the size distribution depends on air velocity, and the median diameter decreases with decreasing velocity to reach the range 40.5–54 μm at 36.7 m/s (Lyulin, 1998). The best dispersion was found for air velocities >100 m/s.
The pilot plant experiments were carried out with various pigments (Table 10.1) dried in a spray dryer with the shock-wave atomizer designed according to the aforementioned procedure (Figure 10.7). Each test was run for 8 h to obtain a representative sample of ∼2000 kg of dry product.
dried under optimum conditions determined from laboratory tests, that is, at an inlet air temperature of 170°C and a feed rate of ∼120 kg/h. Anaverage evaporation rate for all tests was ∼28 kg H2O/m3 h, which is mark-edly higher than for currently used spray dryers for which the volumetric evaporation capacity is ∼10 kg/m3 h.
Table 10.2 presents as an example the basic characteristics of zinc phos-phate dried in a pilot spray dryer with reference to industrial standards (Lyulin, 1998). It is evident that pigments dried in a spray dryer with the shock-wave atomizer satisfy all the requirements for the commercial product.
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whereas the total surface area of the dispersed liquid trapped in the filter
identifies here the
27 μm is negligible compared to the mass fraction of the fine droplets.
The filter cake at the initial moisture content from 50 to 60% w.b. was
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 151
0 27 54 81 108 135 162 189
0
0.2
0.4
0.6
0.8
1.0
0 27 54 81 108 135 162 1890
0.2
0.4
0.6
0.8
1.0dM
dM
dsV
(a)
(b) dp (µm)
dp (µm)
FIGURE 10.6Droplet size distribution for zinc phosphate–based anticorrosive paint at 163 m/s: (a) Frequency distribution (histogram) and (b) cumulative distribution. (From Lyulin, N. B.,
Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
In addition, pigments obtained with the shock-wave atomizer have a nar-rower size distribution, and therefore the throughput of the ball mills used in a downstream process of standardizing is increased by ∼2 to 30%.
Although the results of this pioneering Russian study have docu-mented advantages of drying with shock waves, extension of this novel
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Intensification of drying instant pigments by the use of shock wave generator, Ph.D. Thesis,
© 2009 by Taylor & Francis Group, LLC
152 Advanced Drying Technologies
TABLE 10.1
Comparison of Selected Pigments Dried in a Spray Dryer with Shock-WaveAtomizer with a Reference Pigment
Pigment
Inlet Air Temperature
(°C)
Covering Power (g/m2) Grindabilitya (μm)
Reference Product Reference Product
Titanium dioxide R-02 170 40 35 15 10 A-01 180 45 40 15 12Lithopone 250 140 130 15 10Chrome yellow KL-1 200 60 60 15 10 K2H-1 200 50 45 15 10 OS 200 20 17 20 15Zinc phosphate 170 — — 50 30
a After 30 min of grinding.
wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia(in Russian), 1998.
FIGURE 10.7Schematic of the experimental setup for spray drying with shock waves.
Liquid feed
Pump Dryingchamber
Exhaustgas
Resonator
Shock-wavegenerator
Heat exchanger(cooler)
Diffuser Confusor
Air
ProductHeater
drying technology to other products will require additional studies,both theoretical and experimental. To our knowledge, Lyulin’s thesis
pendent tests.
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Source: From Lyulin, N. B., Intensification of drying instant pigments by the use of shock
is the only study of this novel concept. It is therefore difficult to make definitive recommendations about its potential without extensive inde-
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 153
Appendix: Process Calculations and Equipment Design
The design criteria for a spray dryer with a shock-wave atomizer used by Lyulin (1998) are as follows:
Dryer type• . Horizontal, concurrent, material fed to the zone of shock waveDrying agent• . Ambient air at 20°C and 70% relative humidity (RH)Material dried•
pump
80°C
<0.5% w.b.Shock-wave generator•
Piston 0.08 m in diameter, stroke length 0.072 m Motor 4 kW
TABLE 10.2
Quality Indices for Zinc Phosphate for Anticorrosive Paint
Index Reference Pilot Dryer
Density (kg/m3) 3000 3000Mass fraction of phosphorus compounds in terms of ZnO (%)
45.0 – 55.0 50.0
Mass fraction of phosphorus compounds in terms of P2O5 (%)
30 – 40 33
Mass fraction of water-soluble components (%)
<0.3 <0.1
Mass fraction of volatiles (%) <0.3 <0.5pH of aqueous suspension 6.0–8.0 6.7Oversize solids on sieve0.063 mm (%)
<0.5 <0.05
Mass capacity (g/100 g pigment)
<50 <50
Whiteness, arbitrary units >92 >93Degree of dispersion (µm) <30 <15
Note: White loose powder with no tendency to agglomerate.
of shock wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia (in Russian), 1998.
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Source: From Lyulin, N. B., Intensification of drying instant pigments by the use
Pasty filter cake (viscosity of ∼200 P) fed by the metering
Initial material temperature 20°C, final material temperature
Initial moisture content >50% w.b., final moisture content
© 2009 by Taylor & Francis Group, LLC
154 Advanced Drying Technologies
Frequency 22.5 Hz Spray angle 30°
Because the spray angle generated by the shock-wave atomizer is 30°, and the feeding point is at the outlet from the resonance tube, it is clear that the optimal drying chamber should be made as a truncated cone of
at the outlet from the resonance tube located at the cone tip will decelerate while traveling along the cone axis until their terminal velocity reaches
into a steady one, and the drying chamber can be shaped as a cylinder.The following assumptions can be made for design calculations (Lyulin,
1998):
1. Droplets are spherical and do not collide during drying. 2. Droplet size and mass do not change during drying; heat- and
mass-transfer area are equal to the total surface area of dry particles.
the shock wave in the plane of propagation.
5. The distribution of a dispersed material and a drying agent is uniform throughout the dryer volume.
All these assumptions except the third one are usual in engineering cal-culations for spray dryers. Assumption 3 might appear to be too general, but considering the novelty of shock-wave drying, it can be accepted as
Experiments on the atomization of various pasty materials showed that 100 m/s is the minimum amplitude velocity required for the narrow droplet size distribution. Based on assumption 3, the maximum (i.e., initial) velocity of liquid droplets is thus equal to 100 m/s. Furthermore, the spatial varia-tion of droplet velocity will be the same as for the amplitude velocity of the
u u epL 14 11 73 26. . (10.12)
Solution of Equation 10.12 for up unsteady region of the dryer and hence determines the maximum length of the conical drying chamber. In a conventional spray dryer, however,
CRC_73877_Ch010.indd 154CRC_73877_Ch010.indd 154 12/29/2008 12:25:58 PM12/29/2008 12:25:58 PM
included angle 30°. In such a configuration, the liquid droplets atomized
3. The velocity field of a droplet spray follows the velocity field of
6. The heat-transfer coefficient can be calculated using Nu = 2.
the first approximation to more accurate calculations.
shock wave shown in Figure 10.3 and is defined by the following equation:
= 0 gives L = 1.65 m, which identifies the
the velocity of a drying air stream. At this point, the unsteady flow turns
4. The flow pattern in the dryer is a plug flow.
d roplets evaporate in an air stream flowing at a superficial velocity from
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 155
0.2 to 0.5 m/s. It is therefore reasonable to accept that the terminal velocity of droplets will be equal to the air velocity. Hence, for air velocity in a cylin-
equal to 0.5 m/s, the real length of the conical drying chamber is 1.64 m.Substituting the droplet velocity in Equation 10.12 for dL/dt, the follow-
ing equation is obtained for the residence time distribution:
tdLeL
L
14 11 73 260 . .∫ (10.13)
given by
Lvf
mtr
4
3304 22 5
3 66⋅ .
.
where the velocity of sound propagation in air at 20°C is 330 m/s.Taking Z = 11, the maximum diameter of the resonance tube securing
the amplitude velocity at least 100 m/s should be larger than
d Zd f s
umt
c r
11 00 08 22 5 0 072
1000 045.
. . ..
tube and the drying air inlet should be concentric. Hence, the drying chamber is formed as a truncated cone with the inlet diameter (D) and the outlet diameter equal to the diameter of the cylindrical drying chamber (Dch). For an inlet diameter of 0.2 m, the chamber diameter is
D D L tg tg mch 22
0 2 2 1 64302
1 08
. . .
VD
u m schg ch
2 23
41 08
40 5 0 458,
.. . /
velocity of the pasty feed can be calculated from Equation 10.7 as
u fu
dlg
l
0 0016137 3
0 00161 22 5100 37 3
1 3
1 3
0 251 31.
.. .
..
..( ) (
/
))1 3
0 250 0081 26
/
/.
..
m s
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The length of the resonance tube for the first (maximum) resonance is
To obtain a uniform velocity profile in a drying chamber, the resonance
drical part of the drying chamber (stationary region of the air-droplet flow)
Hence, the volumetric flow rate of the drying air is
Assuming the diameter of the material feeding tube to be 8 mm, the flow
© 2009 by Taylor & Francis Group, LLC
156 Advanced Drying Technologies
V ud
m hl ll
2 23
41 26
0 0084
3600 0 228..
. /
order of 1600–1700 kg/m3, the maximum capacity of the shock-wave atom-izer of these characteristics is in the order of 370–390 kg/h.
From the drying kinetics, it is clear that drying of pasty pigments that belong to the class of capillary-porous materials takes place mainly in the constant drying rate period. Hence, the drying time can be calculated from the heat-transfer equation as
tQ
h F Te
(10.14)
where F is the heat-transfer area, which is equal to the total surface area of
hk Nu
dW m K
p
0 0285 22 10
28505
2.
/
where the droplet diameter is taken from Figure 10.6.The evaporation rate of water is
W GX X
XG Gw m
i f
fm m
10050 0 5
100 0 50 497
..
.
Because the dryer throughput is limited by the capacity of the shock-wave 3
GV Y Y
kg smg g f i
( ) ( )
0 4970 458 1 2 0 088 0 011
0 4970 0826
.. . . .
..
/ 297 kg h/
which is <390 kg/h, found as the maximum capacity of the shock-waveatomizer.
The density of drying air was calculated from the humid volume vH as
gH
Yv
kg m 1 1 0 011
0 84361 21 3 .
.. /
where
vY T
Hi g i
22 4129 18 273 15
22 4129
0 01118
293 1.
..
. .,
55273 15
0 8436 3
.. m kg/
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Because the density of the pasty pigments from the filter press is in the
droplets, and h is the heat-transfer coefficient calculated from Nu = 2 as
The maximum volumetric flow rate of the feed is
atomizer (airflow rate is equal to 0.458 m /s), the maximum feed rate is
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 157
The humidity of the exhaust air equal to 0.088 kg/kg was taken as the maximum, yielding a dew point of the exhaust air of 50°C, which is the value needed to avoid condensation of water vapor in cyclones and bag
Heat transferred to the dispersed material is used for sensible heating of the material and for moisture evaporation. Thus,
Q W H G c T T G W c T Tw m m wb m i m w s m f wb
( ) ( )
. ., ,1
0 0826 0 497 23
( )777 0 0826 0 504 53 20 0 0826
1 0 0826 0 18 80 53 99 8
. . ( ) .
( . ) . .
( ) kW
The heat-transfer area per unit time can be calculated as
FG W
dm sm w
s p
6 6 0 0826 0 497 0 0826
6000 2 102 08
52( ) ( )
ρ. . .
. /
Hence, the drying time is
tQ
h F Ts
e
975802850 2 08 70 2
0 235 . .
.
where the temperature difference is calculated as a logarithmic mean temperature from the inlet and outlet temperatures of air and material in
TT T T T
T T
T T
eg i m i g f m f
g i m i
g f m f
, , , ,
, ,
, ,
ln
( ) ( ) ( )250 20 900 80250 2090 80
70 2
( ) °
ln. C
Thus, the minimum length of the cylindrical drying chamber is
L u t mch g 0 5 0 235 0 12. . .
Considering 110°C as the lowest possible inlet air temperature, a similar sequence of calculations gives Lch = 1.1 m. Thus, the total dryer length(excluding the confusor) should be equal to 2.74 m.
The minimum heater duty is calculated from the enthalpy of air stream assuming 10% of heat losses:
Q V I I kWh g g g i g ( ) . . ., ,0 0 458 1 2 290 48 1 1 142 ( )
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filters.
a concurrent flow:
© 2009 by Taylor & Francis Group, LLC
158 Advanced Drying Technologies
Symbols
A Constant (Equation 10.6)
d, D Diameter, mf Frequency, HzF Heat-transfer area, m2
G Dryer throughput (wet material), kg/h2
∆H Latent heat of vaporization, kJ/kgI Enthalpy, kJ/kgk Thermal conductivity, W/m KK Constant (Equation 10.2)L Length, mm Mass, kgn The order of a harmonic frequencyn NumberN Rotational speed, rpmQ Heat rate, WRH Relative humidity, %s Stroke length, mS Surface area, m2
t Time, sT Temperature, °C∆TeTT Equivalent temperature difference, °Cu Velocity, m/sv Sound velocity, m/svH Humid volume, m3/kgV Volume, m3
3
w.b. Wet basis (kg H2O/kg wet material)X Moisture content, % w.b.Y Air humidity, kg/kgZ Constant (Equation 10.3)βφ Heat loss factorγ Cone angle, °γµ Dynamic viscosity, kg/m sρ Density, kg/m3
Nuhdp g
g
Nusselt number
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c Specific heat, kJ/kg K
h Heat-transfer coefficient, W/m K
Flooding coefficient
V Volumetric flow rate, m /sW Mass flow rate, kg/s
© 2009 by Taylor & Francis Group, LLC
Drying with Shock Waves 159
Subscripts
c Piston (cylinder)ch Chamberd Dispersedf Final (outlet)g Gash Heateri Initial (inlet)l Liquidm MaterialM Medianp Particle (droplet)r Resonances SolidsV Surface–volumeS Spott Tubew Waterwb Wet bulb0 Ambient
References
Anderson, J. D., Jr. 1982. Modern Compressible Flow. McGraw-Hill, New York.
wave generator. Ph.D. Thesis. Mendeleyev University of Russia. Moscow, Russia (in Russian).
Muranov, V. A., Lyulin, N. B., Ryzhukhin, O. A., Bykov, A. E. and Tarakanova, E. E. 1997. Spray drying of pasty pigments. Lakokrasochnye Materialy, 12: 18–21 (in Russian).
Saad, M. A. 1993. Compressible Fluid Flow. Prentice Hall, Englewood Cliffs NJ.Schetz, J. A. and Fuhs, A. E. (Eds.). 1996. Handbook of Fluid Dynamics and Fluid
Machinery. Wiley, Inc., New York.Shapiro, A. H. 1953. The Dynamics and Thermodynamics of Compressible Fluid Flow.
Ronald Press, New York.
CRC_73877_Ch010.indd 159CRC_73877_Ch010.indd 159 12/29/2008 12:26:04 PM12/29/2008 12:26:04 PM
Lyulin, N. B. 1998. Intensification of drying instant pigments by the use of shock
© 2009 by Taylor & Francis Group, LLC
161
11Vacu Jet Drying System
One of the major problems with classical vacuum drying is the relatively long retention time required, which sometimes may be as long as 100 h. Since thermal degradation is a time–temperature-related process that dou-bles for every 10°C increment, a long residence time may result in product deterioration, even at low processing temperatures. The Vacu Jet Drying (VJD) system, designed for continuous processing of liquid materials at relatively high temperatures with a short residence time, appears to be advantageous over the vacuum-tray, vacuum-drum, or freeze-dryers. The system uses indirect heating of the liquid feed, sudden adiabatic expan-sion, and vacuum drying with mixing in series to evaporate solvents from the liquid feed and get a powdery product.
The VJD system (known in Japan as the Crux Vacuum Drying System) represents a continuous, closed, single-stage vacuum-drying process. It appears to be suitable for drying pumpable feeds and sludges that are composed of nonvolatile components or solids, and water or organic sol-
to dry in a single step. It has proven to be versatile for such applications
eration and for the direct drying of polymers and other substances in solvent solutions with solvent recovery (Maekawa, 1994a).
tions, shown schematically in Figure 11.2, differ in the method of vapor-ized solvent–dry material separation, which depends greatly on the physical properties of the solid and vapor phases. Independently of the
third section generates vacuum and accomplishes solvent recovery.An integral part of the system is a Vacu Jet Dryer—a serpentine tube-type
evaporator, which is typically 8–80 m long with a diameter of 8–50 mm. The inlet of the evaporator is connected to the feeding system, which supplies the liquid material diluted with a solvent, if necessary. In the kneader-type
ucts are crushed before being discharged to the jacketed ribbon blender.
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vents. The system is particularly suitable for materials that are difficult
as the production of ultrafine particles with minimal secondary agglom-
Figure 11.1 presents the basic configuration of the VJD system used to dry tar-like residues and polymer solutions. Other possible configura-
particular configuration, each system consists of three integral sections.
tion. The second section, operated under vacuum, performs drying and The function of the first section is material feeding and moisture evapora-
product blending as well as fines collection and product discharge. The
configuration, the outlet of the evaporator is connected to the double-screw kneader fitted with a heating jacket, where porous or rigid prod-
© 2009 by Taylor & Francis Group, LLC
162 Advanced Drying Technologies
The kneader and the ribbon blender form a postdrying stage where the residual solvent is removed from the product. The auxiliary vacuum pump and powder valves installed at the kneader and blender outlets allow intermittent product discharge without leakage. Before condensing in the
VJD system (product discharge and exhaust from the vacuum pump) are
solvent and solids collection reaches 99.99% (Anonymous, 1994).The majority of liquid heating and solvent evaporation takes place inside
the evaporator, which is heated indirectly by steam or hot oil. Typically, the wall temperature is kept 20–30°C higher than the solvent boiling point.
material expands adiabatically in the product collection section that is held under reduced pressure.
The liquid feed diluted (if necessary) with an additional amount of sol-vent is metered by a feed pump into the evaporator, where the solvent starts evaporating after a short heating period. Because of vapor generation, the volume of the feed expands, which causes a rapid increase in the veloc-ity of the liquid–vapor mixture (Figure 11.3a). As vaporization progresses
Feeding and evaporation section Product collection section Solvent recovery andVacuuming section
N2
Steam
SteamEvaporatorKneader
Condenser
Recoveredsolventtank
Recoveredsolvent
Exhaust
Vacuumpump
Auxiliaryvacuum pump
Product
Solvent pumpPowdervalve
Powdervalve
Ribbonblender
SteamN2
SolventSolventtank
Feed
Feedtank
Feed pump
M
M
M
M
MM
FIGURE 11.1Schematic of the VJD System for tar-like materials. (From Maekawa, Y., Processing Possibilityby Vacu Jet Drying System, Hosokawa Micron Corporation, Japan, 1994b. With permission.)
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Bag filter
solvent recovery section, the evaporated solvent passes through a bag filter of special design for complete recovery of fines. As only two points of the
open to the atmosphere, the system is pollution-free as the efficiency of
A significant portion of the remaining solvent is evaporated when the
along the evaporator, the flow pattern evolves from an initial bubble flow through a plug flow, a turbulent flow, a ring flow, to a spray flow in most
© 2009 by Taylor & Francis Group, LLC
Vacu Jet Drying System 163
HT
FT
FP
SP
VP VP
PC CD
PV
PV
Solid
Recoveredsolvent
Exhaust
Basic type
RT
PV
PV PV
CY
CY
MT
BL - Ribbon blenderCD - CondenserCY - CycloneFP - Feed pumpFT - Feed tankHT - Heat exchangerKK - Ko-kneaderMT - Material tankPC - Powder collectorPV - Powder valveRT - Recovered solvent tankSC - Spray chamberSP - Solvent pumpST - Solvent tankVP - Vacuum pump
BL
BL
SC
PC
A
A
A
A
B
B
B
B
C C
C
C
PC
BL
KKPV
Advanced type Ko-kneader type
Cyclone type
ST
FIGURE 11.2
Vacu Jet Drying System, Hosokawa Micron Corporation, Japan, 1994b. With permission.)
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Typical configurations of the VJD system. (From Maekawa, Y., Processing Possibility by
© 2009 by Taylor & Francis Group, LLC
164 Advanced Drying Technologies
cases (Figures 11.3b and 11.3c). Finally, the slightly superheated vapor–liquid mixture is discharged from the evaporator to the product collection sec-
velocity. Such conditions, superposed with a reduced pressure in the prod-uct collection section, result in a rapid evaporation of the liquid spray, thus
distribution of titanium oxide before and after drying in the VJD system. Solid particles in the 25% w/w water slurry had a mean diameter of 1.19 µm with a maximum diameter of 10 μm. After drying, the mean diameter was reduced to 0.41 μm, and no particle was larger than 1 μm.
A powdery product is obtained in cases where solid particles in a slurry feed do not dissolve even at the solvent boiling point. In the case of viscous
Feed
Feed
Inlet Outlet
Pres
sure
Atmospheric pressure
Jacket temperature
Tem
pera
ture
Velo
city
Flow velocity
Pressure
Temperature
0
(a)
(b)
(c)
Preheatingzone
Bubbleflow
Slugflow
Turbulentflow
Sprayflow
Ringflow
Ringflow
Pocketflow
FIGURE 11.3
Japan, 1994b. With permission.)
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tion (kneader, bag filter, spray chamber, or cyclone, depending on the sys-tem configuration) at a velocity ranging from 100 m/s to the near-sound
producing a finely dispersed material. Figure 11.4 presents a particle-size
Maekawa, Y., Processing Possibility by Vacu Jet Drying System, Hosokawa Micron Corporation,
Flow characteristics in the VJD: (a) Distribution of temperature, pressure, and flow veloc-ity along the evaporator; (b) spray flow mechanism; and (c) pocket flow mechanism. (From
© 2009 by Taylor & Francis Group, LLC
Vacu Jet Drying System 165
liquids, which contain only a small amount of the solvent and therefore do not vaporize so intensely, the liquid feed is forced to the exit of the evaporator by the feeding pump and vaporized solvent. Here, the pocketsof superheated liquid evaporate rapidly (explode) as a result of an abrupt eepressure drop (see Figure 11.3c). At the same time, the product tempera-ture decreases due to the Joule–Thompson effect and latent heat of vapor-ization. Such drying conditions allow the production of nighly porous or submicron powders, depending on the feed characteristics.
end part of the evaporator as well as the residual vapor in the kneader and blender are superheated. This accelerates the removal of the solvent as super-heated steam (SHS) has preferential thermal properties (see Chapter 7). In addition, the high velocity of the liquid–vapor mixture in the tube results in self-cleaning that prevents fouling of the inner wall of the evaporator. The high liquid–vapor velocity in the evaporator compensates its length, so that the residence time of the feed in the Vacu Jet Dryer varies from 1 to several minutes for small systems and from several minutes to 20 min for larger
feed. Therefore, thermal degradation of the product dried at 120°C in the VJD is lower than that during conventional processing at 80°C for 6 h.
One of the preferable applications of the VJD system is the separation of solids from solvent slurry or solution with simultaneous recovery of the solvent for recycling. In conventional methods such as hybrid-drying technologies that involve mechanical separation followed by thermal
FIGURE 11.4Particle-size distribution for titanium dioxide dried in the VJD system. (From Maekawa, Y., Processing Possibility by Vacu Jet Drying System, Hosokawa Micron Corporation, Japan, 1994b.With permission.)
0.1 1.0 10 1000
20
40
60
80
100
Particle size (10− 6 m)
Cum
ulat
ive d
istrib
utio
n (%
)
Feed slurry
Dry product D50 = 0.4110−6 m
D50 = 1.1910−6 m
TiO2−water
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d rying (see Chapter 29), the filter cake is filled with residual solvent.
It should be noted that the vapor–liquid mixture flowing through the
ones. The distribution of this time is also small due to the piston-flow of the
© 2009 by Taylor & Francis Group, LLC
166 Advanced Drying Technologies
Additionally, a pulverizing process for the dry solid product is required because of the caking that occurs during drying. In the case of solutions or diluted slurries, a spray dryer could be used, but it incurs high operating costs due to its special design and low solvent recovery ratio. Similarly, a rotary vacuum dryer has a relatively low productivity because of the long processing time and the need for postprocessing pulverization. In comparison with the aforementioned methods, the VJD system provides the following merits (Maekawa, 1994a):
Hazardous solvents are claimed to be safely removed with supe-• rior recovery.
thus does not need subsequent pulverizing or milling.The solids are continuously dried from the slurry or solution in a • single process.The system is remarkably smaller as compared with the spray • dryer, which translates into lower capital, installation, and main-tenance costs.
According to Maekawa (1994b), the materials that are generally easily pro-cessed are slurries in which an organic solvent (or water) contains insoluble solids even at the boiling point. Solid components of ceramics, organic/inorganic materials, metal powders, etc., can be listed as examples. Organic and thermoplastic resins and similar materials that are dissolved in organic solvents may be processed under certain conditions. The products obtained from such a solution would be in the form of powder, porous solid matter,
ter, if the softening point of the solid component is not 50°C higher than the boiling point of the solvent, but sometimes a form of paste can be obtained.
organic materials are not recommended unless the desired product qual-
particles, etc. These are, for example, salts of organic acids; solutions of ther-mosetting resin; and emulsions of polymer, rubber, and similar materials.
Typical operating parameters of the Vacu Jet Dryer for representative materials are given in Table 11.1 (Anonymous, 1995). More information about the industrial applications of the VJD system can be found elsewhere (Maekawa, 1994a).
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blesome for safety reasons, and the recovery of organic solvent is difficult.
• In most cases, the final product is a fine or very fine powder and
or paste, so the proper configuration of the system as shown in Figure 11.2 is required. In general, it is difficult to obtain a powder or porous solid mat-
If the paste can easily be solidified by cooling, the kneader-type system is recommended to continuously extract the solidified lumps. Alternatively, the paste can be pumped out of the system and solidified separately in a
ity cannot be obtained in other ways due to the strong coagulation of fine
As organic solvents are usually flammable, the selection of a dryer is trou-
flaker, for example. Aqueous solutions and water slurries of inorganic/
© 2009 by Taylor & Francis Group, LLC
Vacu Jet Drying System 167
TABLE 11.1
Typical Operating Parameters of the VJD System for Drying Applications
ProductSolvent (Boiling
Point) Feed
Temperature in Evaporator
(°C)
Solvent Content (%)
Feed Product
Organicchemicalsa
Hexane + water,69°C, 100°C
Slurry 140 80 1
Agricultural materialsb
Benzene + water,80.1°C, 100°C
Slurry 130 62 0.5
Polymer intermediatesc
Various solvents, 160°C
Solution 200 60 0.5
Dyeintermediatesd
Benzene, 80.1°C Jelly 135 70 0.2
Dyese Aniline, 184.7°C Viscous tar 210 55 1Organicpigmentsf
Trichlorobenzene,212.8°C
Slurry 210 70 2
Dyesg Water, 100°C Slurry 158 85 1
a Impossible to dry by any other method.b Instant drying including removal of crystalline water is possible. Improved quality.c
d Better solvent recovery ratio.e Remarkable rationalization of the process. No pollution.f Same as above.g
Source: From Anonymous, Crux Vacuum Drying System, Bulletin C1, Orient Chemical Ind. Ltd., Hosokawa Micron Corporation, Osaka, Japan, 1995. With permission.
References
Anonymous, 1994. Crux Vacuum Drying System—Technical Information. Micron Powder Systems-Hosokawa Micron Corporation, Osaka, Japan.
Anonymous, 1995. Crux Vacuum Drying System. Bulletin C1. Orient Chemical Ind.Ltd. Hosokawa Micron Corporation, Osaka, Japan.
Maekawa, Y. 1994a. Instantaneous vacuum drying system. In: Drying 94—Proc. International Drying Symposium IDS94. Gold Coast, Australia, pp. 359–366.
Maekawa, Y. 1994b. Processing Possibility by Vacu Jet Drying System. Hosokawa Micron Corporation, Osaka, Japan.
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Significant decrease in utilities as compared with conventional steam distillation.
Simplified process.
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169
12Contact-Sorption Drying
12.1 General Characteristics
Contact-sorption drying integrates two drying techniques: (a) classical contact drying, where the moisture is evaporated using heat conducted from a heated solid surface, and (b) sorption drying (also termed desiccant drying or adsorption drying), where moisture transfer is driven by a massggconcentration gradient between the material being dried and the adsor-bent material it is contacted with.
A very general scheme of contact-sorption drying comprises mixing a solid sorbent with the material being dried, followed by separation of these two media once the desired mass transfer has taken place. The solid sorbent is then regenerated and returned to the process. Clearly, the tech-
bent can easily be regenerated and recycled.The sorbent may be passive (inert) or active, depending on whether it is
used only for moisture removal and then separated from the dried mate-rial or becomes an integral part of the dry product. Typical inert sorbents are molecular sieves, zeolite, chabazite, activated carbon, bentonite, or silica gel. The group of active sorbents comprises starch, peat, bran, straw, cellulose, wheat bran, corn meal, potato starch, sugar beet pulp, fruits and oil plants, cut green parts of plants such as hey, and many others. Often the already dry product is regarded as an active sorbent since it can be mixed with the wet feed before drying (backmixing). An example of such a process is simultaneous drying and granulating of biomaterials in
suspensions that are either spread over the surface of inert particles or absorbed within the porous structure of the solid material (Kudra and Strumillo, 1998; Tutova and Kuts, 1987).
In contrast with drying on inert sorbents, which has found application in drying leather and agricultural products (Piper and Alimpic, 1993; Lapczynska and Zaremba, 1987; Ciborowski and Kopec, 1979; Anonymous, 1980; Ghate and Chhinnan, 1983; Alikhani, 1990; Sotocinal et al., 1997; Li et al., 2002; Osorio-Revilla et al., 2006; Ye et al., 2007; Kudra and Mujumdar, 2007), contact-sorption drying on active sorbents has been proven to be especially suitable for drying biomaterials and products of biosynthesis
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nical justification for contact-sorption drying depends on whether the sor-
sorbent is a carrier or fillerrr, as the materials dried are usually solutions ora fluidized bed (Dencs and Ormos, 1989). The alternative term for active
© 2009 by Taylor & Francis Group, LLC
170 Advanced Drying Technologies
as it turns the process of drying of liquid droplets into drying of capil-lary-porous materials wetted with the same liquid. This makes it possible to take certain advantages of convective drying of solids such as drying at the wet-bulb temperature. In such a case, it is possible to maintain the biological activity of heat-sensitive products at a much higher level than with most other drying techniques (Kudra and Strumillo, 1998; Tutova and Kuts, 1987; Adamiec et al., 1990; Pan et al., 1994, 1995). Also, the end product can be a ready-to-use mixture of a sorbent carrier with other
pesticides, and bacteria concentrates. This eliminates otherwise necessary downstream operations such as grinding, sieving, or blending. Because the majority of applied research was done using a sorbent carrier, the commonly used term contact-sorption drying is synonymous with dryingon active sorbents.
A typical fermentation culture is an aqueous mixture of microorganisms or biopolymers, unreacted residues of nutrients, by-products, process-controlling additives, and other components, with solid content amount-ing to several percent. Thus, the solid sorbent used in contact-sorption drying performs the following two basic functions:
1. It absorbs a great fraction of moisture from the highly diluted sus-pension of biomaterials subject to drying. This alters the heat- and mass-transfer characteristics as the moisture is to be evaporated from a capillary-porous solid instead of from a liquid spray.
2. It changes the physical structure of the material fed to the dryer from a diluted suspension to the multicomponent mixture of a particulate material.
suspension to be dried is contacted with the solid sorbent simultaneously with the process of moisture sorption and partial evaporation. A typical
ized bed of the sorbent particles or atomized within the spray cone of a dispersed sorbent as it is in the spray dryer with a solid–liquid feed. The
and the sorbent get into contact before drying in a separate device, which is installed outside the drying chamber. This is the case of drying of a pre-formed feed in a rotary, band, tunnel, or compartment dryer as well as in
in the mixer with subsequent granulation that precedes drying is an exam-ple of such a variant of contact-sorption drying (Kudra and Strumillo, 1998; Tutova and Kuts, 1987). An added feature of mixing the sorbent with the wet material before drying is the preservation of living cells. As reported by Mille et al. (2004), the wet pellets of lactic acid bacteria mixed with a
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Although both functions are inseparable, the first one dominates when the
second function of the sorbent filler plays the key role when the suspension
moisture content. Mixing of the suspension to be dried with a sorbent filler
additives such as vitamins, antibiotics, amino acids, flavor enhancers,
example is drying of the fermentation broth that is sprayed onto a fluid-
a classical fluid bed dryer, all of which accept particulate materials of high
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 171
temperature of 35°C, exhibited 100% survival of Lactobacillus plantarum in a dry product suitable for long-term storage (aw < 0.2). On the contrary, thesurvival of L. bulgaricus desiccated under identical conditions was <10%, which indicates that either the sorbent is not suitable for this species (see Section 12.3) or the processing parameters should be optimized.
12.2 Mechanism of Contact-Sorption Drying
The mechanism of contact-sorption drying is very complex because mois-ture transfer takes place in heterogeneous and multicomponent systems and is accompanied by thermal effects. The idealized scheme of contact-
Stage 1 Stage 2 Stage 3
Driv
ing
forc
e
Mass transfer betweenmaterial particle and
sorbent particle
Mass transfer insidesorbent particle and
inside material particle
Mass transfer betweensorbent−sorbent particles and
between material−material particles
Microscale Microscale
Macroscale
Moisture contentdifference
Gradient of moisture concentration
Moisture contentdifference
Penetration+ diffusion Diffusion Penetration
+ diffusion
Mec
hani
smM
ass
tran
sfer
Scal
eCo
nfigu
ratio
n Material
Sorbent
Material−sorbentcontact
Material−materialcontact
Sorbent−sorbentcontact
FIGURE 12.1General mechanism of contact-sorption drying in a dynamic particulate system.
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dry casein powder in 1:1.2 mass ratio, and dried in a fluidized bed at a
sorption drying shown in Figure 12.1 reflects the phenomena taking place in a dynamic system (e.g., mixing or fluidization), where the interaction
© 2009 by Taylor & Francis Group, LLC
172 Advanced Drying Technologies
between the sorbent and material, the material and material, and the sorbent and sorbent is likely to occur.
When the wet material is brought into contact with a capillary-porous dry sorbent, the surface layer of the sorbent starts to adsorb liquid mois-
depends greatly on the contact area. Because the amount of liquid mois-ture at the material surface decreases gradually, the suction potential at the material surface equals, at a certain point, the suction potential at the sorbent surface. This stops the moisture transfer through the interface between both media. However, the concentration gradient that developed in the sorbent and material particles causes moisture from the surface to diffuse throughout the sorbent volume as well as the moisture from the material core to migrate to the contact surface. When contact time is suf-
and the material particle can be expected. Because at this moment the mass exchange is maximal, the sorbent could be separated and regener-ated, whereas the material being dried could be contacted again with a dry (or regenerated) sorbent for deeper drying. In practice, the contact time is much shorter than that required to attain equilibrium conditions, and therefore the diffusion of moisture proceeds when sorbent and mate-rial particles are no longer in contact. In case of random motion, which
there is a possibility of sorbent–sorbent and material–material contact that may lead to further moisture transfer at the microscopic level. Then the sorbent particle can again contact temporarily more wet material, and as the process continues, a dynamic equilibrium between the material being dried and the wet–dry sorbent mixture is attained at a lower level from the previous one.
Although contact-sorption drying was a subject of numerous stud-ies, mostly in Russia, the mechanism of the process and the respective mathematical models are not well established. According to Tutova (1988), mass transfer during contact-sorption drying is determined by a dynamic nature of the sorption process. Therefore, to analyze the pro-
Before physical contact, the initial moisture concentration in the material being dried (material moisture content) is Cm
i , and the initial concentra-tion of moisture in dry sorbent is Cs
i = 0. At the contact time (t = 0), mois-ture from the material surface is assumed to be transferred instantly to the sorbent surface. Thus, the sorption front with the moisture content being in equilibrium with the material moisture content is established at the sorbent surface. At the same instant, moisture from the sorbent sur-
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ficiently long, equilibrium between the moisture content in the sorbent
cess of mass transfer, she proposed a model configuration in which a semi-infinite plate of a bone dry sorbent is suddenly contacted with a semi-infinite slab of the wet material of uniform moisture distribution.
face begins to diffuse into the sorbent core. Assuming definite velocity of
ture by ordinary capillary flow, and therefore the rate of mass transfer
is the characteristic of fluid beds, vibrated fluid beds, and spouted beds,
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 173
a progressing sorption front u, the time needed for the sorption front to travel a distance x is given by
txu
d (12.1)
cess of moisture sorption starts at the k-th layer of a sorbent located at the distance x. The net time of sorption (tk) at the k-th layer is then equal to
t t t txuk k
d k (12.2)
lished in the sorbent volume. In due time, the moisture concentration at any point of a sorbent attains the equilibrium value Cs
f, which is consid-sff
Accounting for the time-delay effect, the equation for moisture concen-tration in the sorbent can be written as
C t x HC t t x for t ts mk k( , ) ( , ) (12.3)
C t x for t tsk,( ) 0 (12.4)
where H is the Henry’s constant (H = Csf Cm
i ).The function Cm (t – tk, x) can be expanded into a Taylor series with the
argument t as
C t t x C t xC
ttm
k
m
tk( , ) ( , )
0
… (12.5)
Then, at tk = 0,
dCdt
HdC
dt
s m (12.6)
Finally, the following expression can be obtained by combining Equations 12.3, 12.5, and 12.6:
dCdt t
HC Cs
k
m s 1 ( ) (12.7)
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and it quantifies the time delay (relaxation time) after which the pro-
Because of the time delay, a certain moisture concentration profile is estab-
ered as the final moisture content of the sorbent.
© 2009 by Taylor & Francis Group, LLC
174 Advanced Drying Technologies
kto the ordinary equation of sorption kinetics as
C t x HC t xs m( , ) ( , ) (12.8)
Analyzing the preceding mathematical model for the dynamics of the contact-sorption drying, one can conclude that
An increase in the velocity of a sorption front and a decrease in •the delay time accelerate the moisture transfer rate.Larger contact areas create favorable conditions for adsorption •rather than for much slower sorption, which is limited by diffu-sion in the sorbent volume.Because of opposite processes of mass transfer in the sorbent • and the material being dried, the maximum transfer rate can be attained at an optimum contact time.
time and the variation of dimensionless sorption capacity with time in contact-sorption freeze-drying (F-D) of water-saturated ceramics by a
1
0 40 80 120 160 2000
0.4
0.8
1.2
1.6
2.0 1.0
0.8
0.6
0.4
0.2
0
Time (min)
Redu
ced
dryi
ng effi
cien
cy
Sorp
tion
capa
city
3´
1´
3
2´
2
FIGURE 12.2Drying and sorption curves for contact-sorption F-D for different thicknesses of the zeolite layer: 1, 1′—10 mm; 2, 2′—20 mm; 3, 3′—30 mm.
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Figure 12.2 illustrates the variation of reduced drying efficiency with
granulated zeolite-type CaA (Tutova, 1988). The reduced drying efficiency η is defined here as the ratio of the moisture content drop during F-D
When the delay time is very short (t >> t ), the general equation simplifies
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 175
with the assistance of a sorbent to that in pure F-D taken as a reference process:
XX0
(12.9)
whereas the dimensionless sorption capacity a* relates the current sorp-tion capacity of the sorbent to its initial sorption capacity:
aaai
* (12.10)
maximum at the moment of contact with a solid sorbent and drops with time, reaching η = 1 when the sorption capacity approaches zero. Becausethe maximum sorption capacity is at the beginning of the process when the sorbent is dry, and it reduces dramatically when the sorbent is close to saturation, it is reasonable to interrupt the contact of the material with the sorbent after a certain time. Based on numerous experiments, the follow-ing relationship was proposed for determining the optimum contact time (Tutova, 1988; Tutova and Kuts, 1987):
t tc s 0 3 0 5. .( ) (12.11)
where ts is the time for sorbent saturation according to the kinetic sorption curve.
Once the contact time is chosen, the partially saturated sorbent may be removed and replaced with the new charge of a dry sorbent; renewal of the sorbent can then increase the rate of contact-sorption drying even by severalfold (Tutova and Kuts, 1987). Having the contact time established, the rate of sorbent renewal can be expressed by the number of renewals per batch (run) and determined from the ratio of the total amount of mois-ture to be removed to the amount of moisture absorbed during the contact time as follows:
nm
mH O
H O tc
2
2 ,
(12.12)
In practice, renewal of the partially wet sorbent can be accomplished by complete separation of the sorbent from the material being dried, by
various technologies can be found elsewhere (Kudra and Strumillo, 1998; Tutova and Kuts, 1987; Tutova, 1988).
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It is clear that the efficiency of contact-sorption drying attains its
a counterflow of sorbent–material layers, or by continuous replacement of a fraction of the sorbent as it is in contact-sorption drying in a fluid-ized bed. These methods are briefly described in Section 12.4. Details of
© 2009 by Taylor & Francis Group, LLC
176 Advanced Drying Technologies
Another approach to mathematical modeling of contact-sorption drying that was used to simulate drying of corn by mixing with zeolite particles
differential equations for heat and mass transfer (Luikov, 1966; Luikov and Mikhailov, 1961):
Ct
D C T 2 2 (12.13)
Tt
TH
cCT
α2 (12.14)
Considering grain as a spherical particle surrounded by a layer of powdery sorbent (Figure 12.3) and neglecting the second term in Equation 12.13
particular equations for corn and sorbent, respectively:
Ct
DR
CR
CR
CC
C C2 2
2
(12.15)
Tt R
TR
TR
Hc
Ct
CC
C C C
C
C2 0 52
2
. (12.16)
Corn
R1
R2
TS(R,t)
TC(R,t)
CS(R,t)
CC(R,t)
Sorbent(zeolite)
Nomenclature for the mathematical model of contact-sorption drying.
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(Alikhani, 1990) is based on the following simplified system of Luikov’s
as insignificant, Equations 12.13 and 12.14 turn into the following set of
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 177
for t > 0 and 0 ≤ R ≤ R1, and
Ct
DR
CR
CR
SS
S S2 2
2
(12.17)
Tt R
TR
TR
HC
Ct
SS
S S S
S
S2 1 02
2
.(12.18)
for t > 0 and R1 ≤ R ≤ R2.The parameter R in Equations 12.15 through 12.18 is the radial coordi-
nate, ∆H is the heat of sorption, and the subscripts C and S refer to corn
assumed a priori to be 0.5 for corn and 1.0 for zeolite, and these were then
The initial conditions at t = 0 for the system shown in Figure 12.3 are as follows:
C C T T for R R
C C T T for R R R
C Ci C Ci
S Si S Si
0 1
1 2
(12.19a)
(12.19b)
lowing equations:
RCR
TR
C C 0 0 0
(12.20a)
R RCR
TR
S S 2 0 0
(12.20b)
R R C C kTR
h T TC S CC
C S 1
( ) (12.20c)
particulate sorbent.Equations 12.15 through 12.18, together with the initial and boundary
conditions, were solved numerically using differential systems simulator DSS/2 and source code for the computer programs written in FORTRAN 77 (Alikhani, 1990). Figures 12.4 and 12.5 present typical variations in tem-perature and moisture content of the corn–zeolite mixture as a function of time and the spatial coordinate. Figure 12.6 shows the temporal variation
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and sorbent, respectively. The values of the phase change coefficient were
confirmed during process simulation (Alikhani, 1990).
The boundary conditions for an adiabatic process are defined by the fol-
where h is the heat-transfer coefficient between a granular material and a
© 2009 by Taylor & Francis Group, LLC
178 Advanced Drying Technologies
Tem
pera
ture
(ºC
)
100
20
40
60
80
120
140
160
180
R (grid points)1 6 11 16 21 26 31 36 41 46 51 56 61
50 s
50 s
20 s
20 s
0 s
0 s
10 s
10 s
30 s
30 s
200 s
200 s
Corn−zeolite
FIGURE 12.4
ticulate medium for contact heating and drying of corn, Ph.D. Thesis, McGill University, Montreal, 1990.)
Moi
stur
e con
tent
(% d
.b.)
20
0
5
10
15
25
30
35
40
R (grid points)1 6 11 16 21 26 31 36 41 46 51 56 61
50 s
50 s
20 s
20 s
0 s
0 s
10 s
10 s
30 s
30 s
200 s
200 s
Corn−zeoliteTSi = 180ºC
FIGURE 12.5
ticulate medium for contact heating and drying of corn, Ph.D. Thesis, McGill University, Montreal, 1990.)
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Temperature profile of corn–zeolite mixture. (Extracted from Alikhani, Z., Zeolite as par-
Moisture profile of corn–zeolite mixture. (Extracted from Alikhani, Z., Zeolite as par-
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 179
of an experimentally determined average moisture content of corn kernels and the one calculated from the mathematical model. The relatively small
quacy of the mathematical model for the contact-sorption drying of corn kernels. Details of the experiments, model solution, and validation can be found in the source literature (Alikhani, 1990; Alikhani and Raghavan, 1991; Alikhani et al., 1992).
12.3 Characteristics of Sorbents/Carriers
When selecting a sorbent for contact-sorption drying, the following aspects should be taken into account:
Type of dry product and its end-use•
material being driedSorption characteristics of the sorbent•
16
18
20
22
24
26
28
Time (s)0 200 400 600 800 1000
Moi
stur
e con
tent
(%
w.b.
)
140ºC
180ºC
220ºC
Corn−zeolite
FIGURE 12.6Temporal variation of an average corn moisture content. (Extracted from Alikhani, Z., Zeolite as particulate medium for contact heating and drying of corn, Ph.D. Thesis, McGill University, Montreal, 1990.)
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difference between experimental and simulated data confirms the ade-
• Physical and biochemical affinity of the sorbent/carrier and the
© 2009 by Taylor & Francis Group, LLC
180 Advanced Drying Technologies
Commercial availability of the sorbent and economic aspects• Possibility of performing upstream and downstream processes•
The type of dry product and its end-use are one of the main criteria for selection of a sorbent. Because of separation, the end-use of a product is usually not critical in contact-sorption drying on inert sorbents unless con-tamination of the product by traces of sorbent is undesirable, as in the case of products for human consumption. On the other hand, the presence of residual sorbent in a product may be an additional feature of contact-sorp-tion drying. This is the case of drying grains for animal feed using natural zeolite where certain minerals or salt should otherwise be added. The main feature of contact-sorption drying on active sorbents is, however, the option
sorbent/carrier not only cannot pose any limitations in further use of the product, but also should improve its quality, extend its shelf-life, or broaden the range of application. For example, a dry product that is to be used as an additive to animal feed (e.g., vitamins, antibiotics, and protein concentrates) should incorporate such a sorbent/carrier that is not only easily digestible
a dry product is to be used as an additive to fertilizers, then the properly prepared calcium or other industrial by-products, which contain compo-nents that improve soil structure or composition, such as peat, chaff, silicon, and magnesium compounds, are best as the sorbent/carrier. In the case of pesticides, sorbents such as kaolin, silicones, and activated carbon with aro-matic compounds, which have adhesive properties, are recommended as they allow the pesticide to stick to the plant or compose the soil.
being dried means no negative effect of the sorbent on the product qual-ity. For example, activated carbon, despite its high sorption activity, may not be used in dewatering of bacterial biomass as it kills microorganisms.
these otherwise good sorbents are substrates for the growth of lactic acid bacteria and other microorganisms.
Sorption characteristics, which comprise sorption kinetics, sorption iso-
drying and the selection of the drying method and dryer design.Figure 12.7 shows a set of isotherms for the sorption of water vapor on
most frequently used sorbents. A variety of sorption isotherms, which frequently change the slope at certain moisture concentrations, compli-
sorption capacity over the entire range of moisture concentrations might not be a representative of that over the narrow range of moisture concen-trations to be used in a particular case. To evaluate the sorption capacity
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of having the sorbent to be an integral part of the final product. Thus, the
Physical or biochemical affinity of the sorbent/carrier to a material
therms, and sorption capacity, determine the efficiency of contact-sorption
cate the direct comparison of different sorbents. Specifically, the average
of various sorbents under comparable conditions, Tutova (1988) defined
by animals but can also enhance the taste and flavor of the end product. If
The use of wheat flour or corn meal is also not recommended because
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 181
1
2
3
4
78
5
Relative humidity (%)
Moi
stur
e co
nten
t (%
w.b
.)
200 40 60 80 1000
10
20
40
30
6
9
FIGURE 12.7Sorption isotherms for selected sorbents: 1—zeolite CaA, 2—activated carbon, 3—peat,
(After Tutova, E. G., Drying Technol., 6, 1, 1–20, 1988. With permission.)
sorption capacity of the sorbent to that of the reference material;
Kaa
XX
XXr r r
0 01. ,max
adsorbed by the unit mass of a sorbent at a given relative air humidity, the relative sorption capacity can also be given in terms of the moisture
whole range of air humidity was taken as a reference material. Hence, the parameter Xr in Equation 12.21 is replaced with 0.01 Xr, max, where Xr, max
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4—sawdust, 5—kaolin, 6—potato starch, 7—silica gel, 8—wheat bran, and 9—filter paper.
a property called the relative sorption capacityyy, which relates the specific
Because the specific sorption capacity is defined as the mass of water
content. A filter paper that features constant sorption capacity over the
is the maximum moisture content of the filter paper at 25°C. The constant 0.01 results from the definition of the mass-transfer potential, where
© 2009 by Taylor & Francis Group, LLC
182 Advanced Drying Technologies
equal to 100º of mass and so the scale extends from 0 to 100.Figure 12.8 shows a variation in the relative sorption capacity of several
sorbents with the relative humidity of the environment. Using absolute values of the relative sorption capacity and comparing the shapes of the curves in Figure 12.8, it is possible to rank and further select the best sor-
with K < 1 as low-capacity sorbents in contrast to high-capacity sorbents,
certain materials (e.g., kaolin or potato starch) exhibit no or little variation of the relative sorption capacity with sorbate concentration. Most sorbents
either fall (e.g., zeolite) or rise (e.g., silica gel) with the relative humidity of the ambient. Therefore, it is advisable to categorize sorbents into low,
tions (Kudra and Strumillo, 1998). Thus, wheat bran, silica gel, and ground rapeseed could be categorized as high-capacity sorbents for air humidity >80% (Figure 12.8).
Obviously, the best option is to use a sorbent with a maximum sorp-tion capacity over the required range of moisture concentrations unless other selection criteria prevail. Zeolites and activated carbon, for example, should be used in processes where a low water-vapor pressure exists such as freeze- and vacuum drying. Wheat bran or ground rapeseed are
0
1
2
3
4
5
Relative humidity (%)
Rela
tive s
orpt
ion
capa
city
0 20 40 60 80 100
8
76
5
2
1
4
3
= kX
Xr
FIGURE 12.8Relative sorption capacity for some sorbents: 1—peat, 2—sawdust, 3—wheat bran, 4—ground rapeseed, 5—potato starch, 6—silica gel, 7—activated carbon, and 8—zeolite. (Calculated from data by Nikitina, L. M., Thermophysical Parameters and Mass Transfer
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Coefficients in Wet Materials, Energia, Moscow, Russia [in Russian], 1968.)
the specific sorption capacity of the reference material was assumed to be
bent for a specified application. Tutova (1988) proposed to regard materials
for which K > 3. However, such a classification is not explicit because only
are characterized by a significant change in the sorption capacity that may
moderate, and high capacity over specified ranges of sorbate concentra-
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 183
characterized by a high sorption capacity at ambient air humidity ranging from 80 to 90%. The amount of sorbent to be used to extract a given mass of moisture can be calculated from the mass balance over the material–
necessary to solve the balance equation can be determined from sorption isotherms, as shown in Figure 12.9.
Commercial availability and cost of a sorbent/carrier have a basic practi-
bent should also be easily available irrespective of season. If it is produced seasonally, it should be possible to store it for a long time without special storage conditions. The cost of a sorbent/carrier is particularly important when drying materials of relatively low market value, such as animal blood, pulp and paper secondary sludge, or meat processing sludge.
The possibility of performing upstream and downstream processes is important when considering the operations accompanying contact- sorption drying such as screening, sterilization of sorbents, disintegration of agglomerates, cleaning of the exhaust air, packaging, transportation,
active sorbents does not require certain operations such as homogeniza-tion and mixing because all components of the product may be integrated either before or during contact-sorption drying. Of prime importance are the upstream operations such as disintegration, screening, and steriliza-tion. It is important that, during sterilization, the physical and structural–mechanical properties of sorbents should be preserved. For example, the
and explosion. In the case of many agricultural and food products, it is not possible to carry out direct sterilization with SHS because this might
Sorbent
Material
Air humidity
Moi
stur
e con
tent
Xfs
Xfm
FIGURE 12.9
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Determination of the final moisture content of the sorbent from sorption isotherms.
sorbent system in equilibrium. The final moisture content of the sorbent
cal importance, specifically for technologies with high mass yield. The sor-
and storage of the final product. In return, contact-sorption drying on
recommended for spray drying or drying in a fluidized bed as they are
sterilization of flour or bran with hot air brings about a risk of self-ignition
© 2009 by Taylor & Francis Group, LLC
184 Advanced Drying Technologies
change the material structure and reduce sorption capability. Therefore, a suitable method for sterilizing such materials is indirect heating of the sorbent (Tutova and Slizhuk, 1990). Cold sterilization or gamma irradia-tion of sorbents as well as the addition of antibiotics may also be used where appropriate and permitted (Tutova and Kuts, 1987).
12.4 Technology of Contact-Sorption Drying
12.4.1 Drying of Particulate Materials on an Inert Sorbent
A continuous process apparatus for drying of particulate materials by contact with sorbent particles is depicted schematically in Figure 12.10 (Raghavan and Pannu, 1986). Although originally developed for drying corn with sand as the heat-transfer medium, it also can be used for contact-sorption drying as it integrates three operations in one unit: contact drying, sorption drying, and sorbent regeneration. The unit consists of three coaxi-ally located conical chambers with a central heating chamber in the form of a conical drum supported axially by a drive and stationary shafts. The stationary shaft is made as a hollow cylinder, thus permitting fuel feed to the burner situated on the longitudinal axis of the heating chamber. The gas burner, fed additionally by compressed air for complete combustion, gener-
water from the sorbent particles while they traverse the heating chamber.
Wet feed
Feed chute
Drive shaft
Product
Propane burner
Flights
Conveying auger
Inert particlesScreen
FIGURE 12.10Continuous contact-sorption dryer for particulate materials.
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ates a flame that extends into the heating chamber and directly evaporates
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Contact-Sorption Drying 185
To accelerate heating and moisture evaporation, the sorbent particles are
Wet particulates fed into the dryer are mixed with hot sorbent particles discharged from the outlet end of the heating chamber and conveyed con-tinuously by the auger disposed between the heating chamber and the outer conical drum. This auger is thus rotated with the heating chamber at 100–160 rpm, which causes the material–sorbent mixture to be thoroughly mixed and transported along the drying section. As the granular mixture is conveyed, the moisture is removed by contact-sorption drying, which is enhanced by evaporation due to sensible heat stored in the hot sorbent particles. The tail end of the outer drum is screened to permit the sorbent particles to pass through and to be collected in an annular cone-shaped collector. The end of the collector is a peripheral annular chute equipped with buckets to transport the particles back to the feeding chute.
Although the residence times of the sorbent particles in the heating chamber are relatively short (in the order of 15–20 s), direct contact with
∼100 to 200°C (Raghavan and Pannu, 1986; Sotocinal, 1998). This is suf-
heat the sorbent to the required temperature. The sorbent renewal rate can be controlled by the sorbent-to-particulate mass ratio, which (aside from the geometry of the dryer) depends on the wet feed rate, the mass of the sorbent in the system, and the rotational speed of the dryer.
12.4.2 Drying in a Fluid Bed of Inert Sorbent
A novel technique for contact-sorption processing of large sheet-type sol-
(Ciborowski and Kopec, 1979; Anonymous, 1980). In this technique, the
particles, which extract moisture from the wet material and release it to
bed of sorbent particles. Heat required for moisture release from the sor-
idized beds of active particles not only enhances the convective but also the material–sorbent, sorbent–sorbent, and sorbent–heater contact heat/
with the cylinder dryer is given in Table 12.1.
gel for drying 1.5–2.5 mm thick cowhides from 60 to 20% at 60°C allows reduction of the drying time from 7 h to 15–20 min, reduction of the steam consumption by 30%, and lowering of the capital cost by a factor of 3. The dryer is of modular type with up to 20 modules, which form a compact assembly 2.1 m long, 2.2 m wide, and 2.0 m high (Anonymous, 1980).
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ficient not only to regenerate sorbent by moisture evaporation but also to
showered by scope-like flights staggered at the chamber wall.
flame and flue gases allows for an increase of particle temperature by
ids such as textiles, leather, or veneer is drying in an active fluidized bed
material to be dried is immersed in a fluidized bed of highly hygroscopic
the fluidizing air (Figure 12.11). In a continuous system, the band of dry-ing material is transported in a serpentine fashion through the fluidized
bent to the fluidizing air is supplied by immersed heaters. The use of flu-
mass-transfer rates. Comparison of the fluid bed dryer with inert sorbent
As compared to convective drying, the application of fluidized silica
© 2009 by Taylor & Francis Group, LLC
186 Advanced Drying Technologies
Air
inlet
Exhaust air
Drying material
Heating elements
Sorbent
Air distributor
Air
inlet
Exhaust air
(a) (b)
FIGURE 12.11
TABLE 12.1
Comparison of Fluid Bed Dryer with Inert Sorbent with Cylinder Dryer
Parameter Fluid Bed Dryer Cylinder Dryer
Drying medium Air SteamTemperature (°C) 100 160Drying time (min) 10 37
2 4,200 22,300Path length (m) 12 56
lation of highly adhesive materials. Fine particles of a suitable sorbent are
(Figure 12.12). Wet granules formed in a disk-type granulator are coated
the high rates of contact heat and mass transfer, the surface layers of the granules are desiccated rapidly. The resulting porous (and thus permeable to vapor) dry coating facilitates further moisture diffusion and does not allow the lumps and granules to aggregate. Drying of the coated loose
1979; Tutova, 1988). The same principle of reducing surface moisture con-tent by coating wet granules with dry powder recovered from an exhaust
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Specific heat consumption (kJ/kg H O)
granules to the final moisture content is then carried out in a standard
Contact-sorption drying in a fluid bed: (a) Batch and (b) continuous.
A fluidized bed of sorbent particles can also be used for drying-granu-
fluidized by hot air in the upper chamber of a two-stage dryer-granulator
by the hot, pulverized sorbent as it falls through the fluid bed. Owing to
fluid bed, which is formed in the lower chamber of the dryer (Tutova et al.,
gas was applied in a fluid bed dryer for casein (Marchevski, 1998).
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 187
als that dry in the prevailing falling rate period. Thus, a two-stage process
lower air velocity could reduce energy consumption and product deterio-ration (Ye et al., 2007).
12.4.3 Sorption Drying Using Filler Materials
With respect to the product, the solid sorbent can be neutral (placebo)
sensitive and otherwise hard-to-dry materials. Typical examples here are
Pasty feed
Exhaustgas II
Exhaustgas I
Product
Sorbentcarrier
Hot gas II
Hot gas I
Sorbent
Wet granules
Granulator
FIGURE 12.12
Technol., 6(1), 1–20, 1988. With permission.)
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or active. In the first case, the sorbent is used only as a carrier for heat-
Contact-sorption fluid bed dryer for pasty materials. (Extracted from Tutova, E. G., Drying
Although fluidization provides high heat- and mass-transfer rates, dry-ing in fluidized beds is not always energy-efficient, especially for materi-
comprising fluidized bed drying (FBD) followed by fixed-bed drying at a
© 2009 by Taylor & Francis Group, LLC
188 Advanced Drying Technologies
liquid bioproducts such as antibiotics, enzymes, yeast, and amino acids, which, when conventionally dried, can lose up to 70% of their biological activity. Sawdust, activated carbon, ground straw, and hay are typical solid carriers. After drying, the solid sorbent is either separated from the product or left as is for further utilization, as in the case of fodder anti-biotics. Figure 12.13 shows the concurrent and countercurrent modes of
ration (Tutova and Kuts, 1987).An interesting option for contact-sorption drying is the use of a sor-
bent, which can be incorporated into the drying material, as an inte-
industries (Adamiec et al., 1990; Tutova and Kuts, 1987; Tutova et al., 1979, 1985, 1987). As applied to the drying of lysine, for example, the use of
Reduces the relative hygroscopicity of the product by a factor of •2 and thus lowers three- to fourfold the rate of moisture sorption during storageShifts the equilibrium humidity from 50 to 70% in the tempera-• ture range of 20–50°CEliminates otherwise necessary downstream operations such as • mixing and temperingStabilizes moisture evaporation during drying, which in fact • takes place from a porous material instead of from a highly vis-cous dropletEliminates product buildup on dryer walls• Lowers material temperature during drying, thus preserving up • to 98% of its biological activity in the liquid state
Figure 12.14 presents a schematic of the spray-drying system for con-tact-sorption drying of bacterial preparations, whereas the design of a three-stream nozzle used to disperse solid sorbent within the spray of these preparations is shown in Figure 12.15. The characteristics of
Table 12.2.An example of an industrial spray-drying system where the combined
sorption-spray drying is applied for manufacturing forage lysine-based concentrates is shown in Figure 12.16. As reported by Tutova and Kuts (1987), the introduction of wheat bran directly into the spray cone of dispersed lysine (with the mass ratio of 0.7–1.0 kg of bran per kilogram of
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contacting solid carriers with liquid biomaterials in a spray dryer configu-
gral part of the final product. Wheat bran, starch, powdered skim milk,
liquid bioproducts in fodder, fermentation, pharmaceutical, and similar
wheat bran as an active filler gives the following advantages:
contact-sorption drying for selected bacterial cultures are specified in
casein, peat, corn flour, etc., are reported to be good fillers for drying
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 189
Collector
Nozzle
Nozzle
Collector
Liquid feed
Atomizer
Exhaust air+ product
Sorbent
Hot air
Raw materialSorbentSorbent
Dryingair
Exhaust
air
Product(a)
(b)
FIGURE 12.13Contact-sorption spray drying: (a) Concurrent and (b) countercurrent.
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© 2009 by Taylor & Francis Group, LLC
190 Advanced Drying Technologies
dry lysine) markedly improved the product quality. In particular, the fol-lowing features were claimed:
tion rate for pure lysine at 80% relative humidity is 10.5% per day, whereas that of lysine with wheat bran is ∼2.5 to 4% per day.Lowering the relative hygroscopicity by 50–300% as compared to •lysine without sorbent.Stabilizing of the liquid moisture content; for pure lysine an •increase in air humidity from 50 to 60% increases by 1.5 times the equilibrium moisture content. If the relative humidity approaches 80%, the equilibrium moisture content is increased by 4–4.5 times. Moreover, for lysine with sorbent, the equilibrium moisture con-tent does not depend practically on temperature and the relative humidity of the ambient air.
Compressedair
Raw material
Three-streamnozzle
Sweeper
Sorbent
Product
Air outlet
Air inlet
Filter
Sterilizer
Drive
FIGURE 12.14Contact-sorption spray dryer for bacterial preparations. (Extracted from Tutova, E. G. and
cultures, Trans. ITMO, Minsk, Belarus, Paper UDK 663.15.047 [in Russian], 1990.)
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Slizhuk, D. S., Technology of contact-sorption dewatering of lactic acid and nitrogen fixing
• Significant reduction of the water vapor sorption rate; the sorp-
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 191
It may be noted that the sorption capacity of organic sorbents can be
mixture with salt or lactose (Tutova and Kuts, 1987). Another method of improving sorption capacity is drying with internal steam generation. Figure 12.17 depicts a sterilizer for granular peat in which the sterilization
Liquid feed
Sorbent
Compressedair
Blow-off air
Sorbent/feedspray
Mixingchamber
Swirlingdevice
FIGURE 12.15Three-stream nozzle for dispersing particulate sorbent within the liquid spray.
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improved significantly (up to 40%) when in a relatively small (5–10% w/w)
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192 Advanced Drying Technologies
TABLE 12.2
Contact-Sorption Drying of Bacterial Preparations on Wheat Bran
Culture
Moisture Content of Biomass (% w.b.)
FinalMoisture
Content of Sorbent(% w.b.)
Sorbent-to-Biomass MassRatio (kg/kg)Initial Optimum
Lactobacillus plantarum 95.0 40.0 35.9 1.66Lactobacilluspentoaceticum
94.0 47.6 21.8 3.50
Streptococcus lactis diastaticus
96.0 35.0 18.7 4.30
Source: From Tutova, E. G. and Slizhuk, D. S., Technology of contact-sorption dewatering of
663.15.047 [in Russian], 1990.
Liquid L- lysine
To wet scrubberHot air
Product
Sorbent
Atomizer
Dryer
Contact-sorption spray drying of lysine. (Adapted from Tutova, E. G. and Kuts, P. S., Drying of Microbiological Products, Agropromizdat, Moscow, 303p [in Russian], 1987.)
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lactic acid and nitrogen fixing cultures, Trans. ITMO, Minsk, Belarus, Paper UDK
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 193
process takes place by water vapor evaporated from the peat and super-heated due to heat supplied by indirect steam. In addition to the steam-heated jacket of a cylindrical sterilizer, a system of vertical steam tubes supplies heat to the moving bed of peat that is vibrated to facilitate the
granular peat with an initial moisture content from 35 to 55%, the tem-perature of ∼50 to 160°C maintained for 90–60 min was found to be opti-mum. Such conditions guarantee not only the required moisture content but also a greater porosity of the dry peat due to the internal generation of SHS (Tutova and Slizhuk, 1990).
Feed Excess steam
Steam
Steam
Steamjacket
Vibrator
A A
Internalheater
Excesssteam
A–A
Condensate
Condensate
Product
FIGURE 12.17Sterilizer for particulate materials. (After Tutova, E. G. and Slizhuk, D. S., Technology
Minsk, Belarus, Paper UDK 663.15.047 [in Russian], 1990.)
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of contact-sorption dewatering of lactic acid and nitrogen fixing cultures, Trans. ITMO,
flow of the particulate material and eliminate channeling. In the case of
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194 Advanced Drying Technologies
12.4.4 Contact-Sorption Freeze-Drying
The principle of contact-sorption drying on inert sorbents can also be applied under conditions of reduced pressure. Since the evaporation rate in F-D depends on, among other factors, the partial pressure of water vapor at the material surface and at the condenser surface, one of pos-sible methods to reduce mass-transfer resistance is to shorten the distance between the sublimation zone and the region of vapor condensation. This can be done when using an inert sorbent in direct contact with the mate-rial surface, which not only minimizes the diffusion path for water vapor but also increases the heat- and mass-transfer area per unit volume. In this case, zeolites of group A are preferable over other sorbents as they possess a high sorption capacity at reduced pressures (see Figure 12.8), which practically does not vary with temperature.
1
23
0 80 160Time (min)
Moi
stur
e con
tent
(% w
.b.)
8
16
24
0
4
FIGURE 12.18
material surface, 2—two-layer structure of material and sorbent, 3—three-layer structure of sorbent and material and sorbent, and 4—multilayer structure. (Extracted from Tutova, E. G., Drying Technol., 6, 1, 1–20, 1988.)
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Drying curves for F-D in layered sorbent–material configurations: 1—sublimation from free
© 2009 by Taylor & Francis Group, LLC
Contact-Sorption Drying 195
The intensity of F-D depends greatly on the partial pressure of the water vapor in the vapor–air mixture at the material surface and on the intensity of vapor removal. One of the methods to reduce the partial pressure of water vapor is to shorten the distance between the sublimation zone and the region of vapor condensation. Solid sorbents that are in direct contact with the sur-face of freeze-dried material could play the role of a desublimator. Figure 12.18 illustrates the effect of a sorbent layer on F-D curves. It is clear that the layered structure allows F-D to be enhanced up to fourfold.
In general, the release of heat of sorption has a negative effect on the pro-cess kinetics as the thermal resistance of the sorbent increases and its sorp-tion capacity decreases. Even in the case of zeolite CaA, an increase of sorbent temperature from 20 to 80°C at a pressure of 133.3 Pa causes a decrease in sorption capacity by ∼35 to 40%. However, the heat of sorption can be used to intensify the process of contact-sorption F-D with no additional energy
late the heat of sorption released during the period of moisture absorption and next to transfer it to a wet material directly during F-D with moisture sorption (Tutova et al., 1985). This method is based on a comb struc-ture made of high-thermal conductivity metals such as copper and aluminum that are placed in between the material to be dried and the
tact of the material and the sorbent (zeolite), the sorption capacity of zeolite increased by 9%, the heat demand decreased by 7.9–8.5 kJ/kg of evaporated water, and the dewatering rate increased by almost 50% as compared to contact-sorption the dewatering under vacuum when no elements for accumulation and transfer of the heat of sorption were applied (Tutova and Kuts, 1987).
FIGURE 12.19Basic structures for contact-sorption drying with heat-transferring elements in vertical (a)
(Extracted from Tutova, E. G. et al., Method of contact dewatering of capillary porous mate-rials, Russian Patent 1,170,245, 1985.)
1
3
3
1
2
(b)(a)
1
2
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consumption. For this purpose, a method has been devised first to accumu-
sorbent layers (Figure 12.19). Experimental verification of this method for F-D of granulated baker’s yeast confirmed that at double-sided con-
and horizontal (b) configurations: 1—sorbent, 2—material, and 3—solid (metal) structure.
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196 Advanced Drying Technologies
Symbols
a Sorption capacity, kg/kga* Dimensionless sorption capacityc Heat capacity at constant pressure, J/kg KC Concentration, kg/kg (kg/m3)
2
h 2
H Henry’s constant∆H Heat of sorption/desorption, J/kgk Thermal conductivity, J/kg KK Relative sorption capacitym Mass, kgn Sorbent renewal rate, 1/batchR Radius, mt Time, sT Temperature, Ku Velocity, m/sx Distance, mX Moisture content, kg/kgα Thermal diffusivity, m2/sδ εηd.b. Dry basisw.b. Wet basis
Subscripts
C Cornc Contactf Finali Initialk Elementary k-th layer0 Reference stater Reference materials SorptionS Sorbentmax Maximum
Superscripts
d Delaym Materials Sorbent* Dimensionless
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D Diffusion coefficient, m /sParticle heat-transfer coefficient, W/m K
Thermogradient coefficient, 1/KPhase change coefficientReduced drying efficiency
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Contact-Sorption Drying 197
References
Adamiec, J., Kudra, T. and Strumillo, C. 1990. Conservation of bio-active materials by dehydration. In: Drying of Solids, A. S. Mujumdar (Ed.). Sarita Prakashan,Meerut, pp. 1–16.
Alikhani, Z. 1990. Zeolite as particulate medium for contact heating and drying of corn. Ph.D. Thesis. McGill University, Montreal.
Alikhani, Z. and Raghavan, G. S. V. 1991. Simulation of heat and mass transfer in corn-zeolite mixtures. Int. Comm. Heat Mass Transfer, 18: 791–804.
Alikhani, Z., Raghavan, G. S. V. and Mujumdar, A. S. 1992. Adsorption drying of corn in zeolite granules using a rotary drum. Drying Technol., 10(3): 783–797.
Anonymous, 1980. Technical Information. Institute of Chemical Engineering,Warsaw Technical University, Poland (in Polish).
thickness and apparatus for drying such materials. Polish Patent 126,843.Dencs, B. and Ormos, Z. 1989. Recovery of solid content from ferment liquor con-
Ghate, S. R. and Chhinnan, M. S. 1983. Pecan drying with silica gel. Energy Agri., 2: 11–20.
Kudra, T. and Mujumdar, A. S. 2007. Special drying techniques and novel dryers. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). 3rd edition. Taylor & Francis, Boca Raton, FL, pp. 453–517.
Kudra, T. and Strumillo, C. (Eds). 1998. Thermal Processing of Biomaterials. Gordonand Breach Science Publishers, Amsterdam, The Netherlands, 669p.
Lapczynska, B. and Zaremba, A. 1987. Adsorption drying of wheat and oat. Proc. VI Drying Symposium. September 21–25, Lodz, Poland, pp. 103–109(in Polish).
Li, Z. Y., Kobayashi, N., Watanabe, F. and Hasatani, M. 2002. Sorption drying of soybean seeds with silica gel. Drying Technol., 20(1): 223–233.
Luikov, A. V. 1966. Heat and Mass Transfer in Capillary Porous Bodies. TransliteratedA. V. Lykov. Pergamon Press, Oxford, 523p.
Luikov, A. V. and Mikhailov, U. A. 1961. Theory of Energy and Mass Transfer. Transliterated A. V. Lykov. Prentice-Hall Inc., NY, 324p.
Marchevski, V. N. 1998. Technology for continuous drying of casein. Bulletin of Kiev Technical Institut, Kiev, Ukraine (in Russian).
Mille, Y., Obert, J.-P., Beney, L. and Gervais, P. 2004. New drying process for lactic bacteria based on their dehydration behaviour in liquid medium. Published online in Wiley InterScience, www.interscience.wiley.com.
Materials. Energia, Moscow, Russia (in Russian).Osorio-Revilla, G., Gallardo-Velazquez, T., Lopez-Cortes, S. and Arellano-
Cardenas, S. 2006. Immersion drying of wheat using Al-PILC, zeolite, clay and sand as particulate media. Drying Technol., 24(8): 1033–1038.
Pan, Y. K., Pang, J. Z., Li, Z. Y., Mujumdar, A. S. and Kudra, T. 1994. Drying of
International Drying Symposium. Gold Coast, Australia, pp. 819–824.
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Unit Operations and Processes. Balatonfired, Hungary.
Nikitina, L. M. 1968. Thermophysical Parameters and Mass Transfer Coefficients in Wet
Ciborowski, J. and Kopec, J. 1979. Method of drying flat materials with small
centrates in fluidized bed spray granulator. Proc. V Conf. on Applied Chemical
heat-sensitive bio-products on solid carriers in vibrated fluid bed. Proc. 9th
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Pan, Y. K., Pang, J. Z., Li, Z. Y., Mujumdar, A. S. and Kudra, T. 1995. Drying of
Technol., 13(1–2): 395–404.Piper, P. M. and Alimpic, M. 1993. Possibilities of maize seed drying by adsorption
method. CHISA’93, Praha, Czech Republic, Paper 13.38.Raghavan, G. S. V. and Pannu, K. S. 1986. Method and apparatus for drying and
heat treating granular materials. U.S. Patent 4,597,737.Sotocinal, S. 1998. Design and testing of a particulate medium thermal processor.
Ph.D. Thesis. McGill University, Montreal.Sotocinal, S. A., Alikhani, Z. and Raghavan, G. S. V. 1997. Heating/drying using
particulate medium: A review (part I and II). Drying Technol., 15(2): 441–475.Tutova, E. G. 1988. Fundamentals of contact-sorption dehydration of labile materi-
als. Drying Technol., 6(1): 1–20.Tutova, E. G., Feldman, R. I. and Meltser, A. V. 1985. Method of contact dewatering
of capillary porous materials. Russian Patent 1,170,245.Tutova, E. G. and Kuts, P. S. 1987. Drying of Microbiological Products. Agropromizdat,
Moscow, 303p (in Russian).Tutova, E. G., Kuts, P. S., Dushin, V. V., Bogdanov, V. M., Kruglov, N. A., Rusnak, M. N.
and Korolev, V. A. 1979. Method of treatment of fodder micro-biological prepa-rations. Russian Patent 685,886.
Tutova, E. G., Kuts, P. S., Slizhuk, D. S., Temlyak, F. Ch., Nikiforova, L. A., Kuznetsov, E. V. and Vasilyev, V. M. 1987. Method of obtaining the lactic acid bacterial preparates. Russian Patent 1,325,070.
Tutova, E. G. and Slizhuk, D. S. 1990. Technology of contact-sorption dewater-
Paper UDK 663.15.047 (in Russian).Ye, J. S., Li, X. L., Hou, H. Y. and Li, Z. Y. 2007. Sorption drying of soybean seeds
pp. 608–613.
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ing of lactic acid and nitrogen fixing cultures. Trans. ITMO, Minsk, Belarus.
G. H. Chen (Ed.). World Scientific Publishing Co. Pte. Ltd., Singapore,
photosynthetic bacteria in a vibrated fluid bed of solid carriers. Drying
with silica gel in a fluidized bed dryer. Proc. 5th Asia-Pacific Drying Conference,
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199
13Sonic Drying
13.1 Basic Characteristics of Sound
Although known for many decades, sound- and ultrasound-assisted dry-
progress in nonthermal dewatering (Mujumdar, 1991; Muralidhara and Lockhart, 1988). Sound is a special form of energy transmitted through
ment of a particle of this elastic medium from its mean position results in an instantaneous increase in pressure. When leveling, this pressure peak not only restores the particle to its original position but also passes on the disturbance to the next particle. The cycles of pressure increase (compression) and decrease (rarefaction) propagate through the medium as a sound wave.
On a microscale, sound is characterized by pressure and particle veloc-ity. The product of these two parameters is called sound intensity—a vec-
area normal to the direction of sound propagation:
Soundintensity pressure particle velocityforcearea
distan
cce
timeenergy
area timepowerarea
(13.1)
As seen in Figure 13.1, sound generated by a point source with power W propagates as a spherical wave, so the sound intensity is inversely propor-tional to the square of the distance from the sound source.
The variations of both pressure and velocity follow a sinusoid; if they are in phase, the peak pressure occurs at the same time as the peak in the particle velocity, and the product of these two gives the intensity, which is not only the maximum instantaneous intensity but also the maximum time-averaged intensity (Figure 13.2). At the other extreme, when pressure and velocity are out of phase, the time-averaged intensity is zero. Phase compatibility is extremely important in drying because processes that affect the drying rate (e.g., cavitation) depend on the sound intensity.
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pressure fluctuations in air, water, or other elastic media. Any displace-
tor quantity that describes the rate of energy flow through a unit surface
ing has recently found renewed interest mostly because of the significant
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200 Advanced Drying Technologies
r
3r
W36πR2I2 =
FIGURE 13.1Sound intensity around the point source of acoustic energy.
Pressure
Particlevelocity
=
Instantaneousintensity
Pressure
Particlevelocity
=
Instantaneousintensity
Phase shift 0°
Phase shift 90°
Time-averagedintensity
Time-averagedintensity (zero)
FIGURE 13.2Time-averaged sound intensity versus phase shift.
On a macroscale, sound is primarily characterized by the frequency (f), which relates the speed of wave propagation (u) (sound velocity) to the wavelength (λ):λλ
fu
(13.2)
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© 2009 by Taylor & Francis Group, LLC
Sonic Drying 201
The second main quantity used to characterize sound on a macroscale
sure level (SPL) on the decibel (dB) scale with 20 μPa as the reference level.Because the sound pressure depends on the distance from the source generating given sound power (energy per unit time) and on the acous-
the sound intensity. In view of the large range of sound intensity, it is also given on a decibel scale but with 1 pW/m2 as the reference level. Therelations between sound power, sound intensity, and sound pressure are presented in Figure 13.3. The decibel scale is given in Table 13.1.
ber, the pressure and intensity levels in the direction of propagation are
many times that it travels in all directions with equal magnitude and probability (reverberation chamber), the pressure and intensity levels are different, and this difference is known as the pressure-intensity index (phase index or reactivity index).
Soundpressure
Soundintensity
Soundpower
Lp = 10 log10
LI = 10 log10
Lw = 10 log10
= 20 Pa
P2
P02
P0 = 2.10−5 N/m2
II0
I0 = 1 pW/m2
WW0
W0 = 1 pW
FIGURE 13.3Decibel scale and the relation between the sound power level (LWLL ), sound intensity level (LI), and sound pressure level (LP).
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tic environment (sound field), it is then frequently quantified in terms of
In a free acoustic field such as that in an open air or anechoic cham-
is the amplitude of the pressure fluctuations expressed as the sound pres-
numerically the same. In a diffuse field in which sound is reflected so
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202 Advanced Drying Technologies
TABLE 13.1
Decibel Scale
Units Pressure (N/m2) Decibels Sound Source
1 0.00002 0 Threshold of hearing10 0.0002 20 Forest100 0.002 40 Library1,000 0.02 60 Normal conversation10,000 0.2 80 Workshop100,000 2 100 Pneumatic chipper1,000,000 20 120 Jet takeoff10,000,000 200 140 Threshold of pain
Aside from the frequency and sound intensity, the key point in the design of sound-assisted dryers is the mode of energy propagation. Sound energy can be propagated as longitudinal waves (also called compression waves) or transverse waves for which vibration of the particle in the mate-rial occurs perpendicularly to the direction of wave motion. Since the lat-ter cannot be propagated in gases and liquids except for highly viscous liquids over very short distances (fraction of a millimeter), sound- assisted dryers are designed to accommodate longitudinal waves.
In a free space, the sound source can be considered as a point source (see Figure 13.1). In practical industrial applications, however, sound is either
shapes such as horn, paraboloid, and ellipsoid. In both cases, such sound radiation can be regarded as coming from a plane source. This results in
the sound intensity is constant (the Fresnel zone), whereas outside this zone (the Fraunhofer zone), the sound intensity decreases inversely with the square of the distance from the plane source, that is, in the same way as for a point source (Figure 13.4).
intensity distribution may become important. For example, the Fresnel zone for the plane source 10 cm in diameter is negligible (a couple of mil-limeters) for sound at 100 Hz (cf., frequency of pulse combustion) but extends for 15.6 cm in the range of ultrasound at 20 kHz and 31.2 cm at 40 kHz.
According to the frequency of pressure pulsation, sound can be classi-
ultrasound (f > 20 kHz). In the past, the R&D was targeted at frequencies over 15 kHz. The newly developed pulse combustion technique has, how-ever, shifted the frequency range toward lower frequencies (Kudra and Mujumdar, 2007), in the order of 50–200 Hz (see Chapter 14). Also, inter-esting laboratory results were reported in the range of infrasound (Kudra, 1998; Woods, 1991; Woods, 1992).
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radiated from the source of definite size (e.g., loudspeaker membrane) or,
a specific pattern of sound intensity; in the zone near the sound source,
In some configurations of dryers, the length of zones and thus the sound
fied as infrasound (f < 20 Hz), sound (audible) (20 Hz < f < 20 kHz), and
more frequently, reflected from the point source by surfaces of different
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 203
applications. Low-intensity applications are made typically in the mega-hertz frequencies and acoustic power up to tens of milliwatts and usually do not alter material properties under operation. In contrast, high-intensity ultrasound is generally used for changing the properties of the material through which it is passed or altering the physical–chemical processes. High-intensity applications are made at low frequencies, ∼20 to 40 kHz,and these are used in drying and dewatering.
13.2 Sound Generation
In industrial applications, sound waves are generated by a transducer, which converts the original form of energy to the energy of oscillatory
1. Piezoelectric. The periodic changes in the physical size of cer-tain crystals (such as quartz, tourmaline, and zinc oxide) due to applied electric potential generate mechanical vibrations that are propagated as sound waves. Used in the range from 20 kHz to 10 GHz.
2. Magnetostrictive. Mechanical vibrations are caused by changes in the physical size of certain metals such as nickel, cobalt, and iron, or certain nonmetals known as ferrites due to an external mag-
Fresnel zone Fraunhofer zone
d
I = constant
L = d2 / 4 λSound sourceDirection of sound wave
Propagation pattern from a plane sound source.
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Ultrasonic applications are rigidly classified into low- and high- intensity
motion. Such transducers are classified into six main groups:
netic field. Used in the range of 40–100 kHz.
© 2009 by Taylor & Francis Group, LLC
204 Advanced Drying Technologies
3. Electromagnetic. The vibration of a solid armature (e.g., membrane in loudspeakers and microphones) is due to coupled electric and
4. Electrostatic. The periodic variation of charges in an electrical capacitor of a special design induces mechanical vibration. Used at f < 100 kHz.
5. Mechanical. Sound waves are generated due to the action of a truly mechanical device such as rotating counterbalanced weights or a mechanical device energized by the kinetic
f < 50 kHz. 6. Miscellaneous. Thermal, chemical, optical, and other phenomena
are exploited.
For high-intensity generation and propagation of sound in gases at frequencies up to ∼25 kHz, mechanical generators are used almostexclusively because of their design and operational simplicity, energy capability, and low cost as compared to the other types of transducers. These generators that produce the so-called airborne sound are used in sound and sound-assisted convective drying. Figure 13.5 presents the most frequently used types of mechanical generators from the group of cavity resonators (Galton and Hartman whistles), wedge resonators, and
tioned along the axis of the gas jet. Such a design allows for a more com-
permits the generator to be used in processing materials for which the contact with air is not acceptable. One of the possible designs of industrial sound generators is shown in Figure 13.6. When rotated at 4000–9000 rpm and fed with 0.138 m3/s of compressed air at 0.3–0.5 MPa, this dynamicsiren (d = 0.2 m) emits sound up to 180 dB and 8 kW of acoustic power (Fridman, 1967).
In processing of liquid systems such as dewatering of slurries and pasty materials, piezoelectric or magnetostrictive generators are used. The active part of such a generator (called a driver) generates and transmits
(a horn). This horn serves as an amplitude transformer to amplify the dis-placement of the driver and to match the transducer impedance with the impedance of the material to be processed (Figure 13.7). The mechanical vibrations from the horn are further coupled to the processed material either directly (the horn is inserted into a liquid or pasty material) or indi-rectly through a membrane.
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magnetic fields. Used at f < 50 kHz.
sirens. Usually, in a modified Hartman whistle, a rod is centrally posi-
only results in higher sound intensity (no air turbulence effect) but also
pact generator of increased efficiency. In the Branson pneumatic sound generator, the exhaust air jet is separated from the sound field, which not
mechanical vibrations through a solid rod (a booster) to the metal profile
energy of the working fluid (sirens and whistles). Used at
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 205
NozzleNozzleCavity
Cavity
PistonJet
Nozzle
Compressedair
Orifice
Rotor
Sound
(a) (b)
(c) (d)
(f )(e)
Annular slit Piston
Rectangularslit
Wedge
Nodal supports
Fluidflow
Fluidflow
Fluidflow
Reflector
StemNozzle
ResonatorBody
Resonator
NozzleHorn
Released air
Air
FIGURE 13.5Principles of sound generation in mechanical sound generators: (a) Galton whistle,
and (f) Branson sound generator.
13.3 Mechanism of Sonic Drying
Since only exploratory studies have been reported so far on sonic dry-ing, the following discussion of the drying mechanism focuses on the ultrasonic applications, which have been of research interest for years.
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(b) Hartman whistle, (c) wedge resonator, (d) dynamic siren, (e) modified Hartman whistle,
Definitely, some of these mechanisms may also contribute to sound and
© 2009 by Taylor & Francis Group, LLC
206 Advanced Drying Technologies
infrasound drying. In these cases, postulation of a particular mechanism requires, however, further studies.
An analysis of available literature data indicates the following possible
Alternating compression and expansion due to high-frequency • pressure pulsation create surface cavitation that breaks the bound-ary layer and allows liquid to evaporate under partial vacuum (Boucher, 1959).
•the drying surface promote surface evaporation (Borisov and Gynkina, 1962).
Motor
Compressedair chamber
ReflectorRotorAir nozzle
Stator
FIGURE 13.6Design of a pneumatic sound generator.
Driver Booster Horn
Power supply
50/60 Hz20/40 kHz
FIGURE 13.7Schematic of the ultrasonic generator for operation in liquid and pasty materials.
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mechanisms of heat- and mass-transfer alterations in an ultrasonic field:
Intensive circulation flows (induced by sound pressure) on
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 207
Pressure pulses from the sound waves increase the turbulence •
which reduces the thickness of the laminar sublayer (Zayas and Pento, 1975).An increase in moisture diffusivity and a decrease in viscosity • (Bartolome et al., 1969; Greguss, 1961).A pulsating partial vacuum transmitted into the material affects • water vapor transport, possibly by decreasing or overcom-ing the attraction forces between the water and solid molecules (Muralidhara and Ensminger, 1986).Expansion of the vapor bubbles inside capillaries yields a migra-•
1963).
(Muralidhara and Ensminger, 1986).
surface (Otsuka et al., 1977).•
tion (Muralidhara and Ensminger, 1986; Muralidhara et al., 1986).
The review of literature data indicates that cavitation enhanced turbu-lence of the gas phase at the solid (or liquid droplet) surface and mechani-cal effects due to internal stresses appear to be the major contributors to enhanced moisture removal during drying. It is also reasonable to expect
diffusion of moisture toward the evaporation surface.The term cavitation refers, in general, to the formation and subsequent
dynamic behavior of vapor bubbles in liquids. Acoustic cavitation occurs when high-intensity sound waves are coupled to the liquid surface, which results in the propagation of alternating regions of compression and expansion, and thus in the formation of micron-size vapor bubbles. If the bubbles are of a critical size (determined by the frequency of the sound wave), they may implode violently, releasing energy in the form of impulses (t < 0.1 s) with local point temperature and pressure in the order of 5000K and 1000 atm, respectively (Suslick, 1988). Since the effec-
lapsing bubble, the bulk of the liquid remains practically at the same temperature. However, as the bubbles at or near the surface implode, micron-size liquid droplets can be released into the surrounding air and evaporated instantaneously. Because of the cavitation threshold, larger acoustic pressure amplitudes at a given frequency are required for highly viscous liquids.
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tion of the water filament (sonic diffusion current) (Greguss, 1961,
• Shear reduction in boundary film surrounding the material
Rectified diffusion due to alternating compression and rarefac-
that the liquid viscosity be lowered in ultrasonic fields, which promotes
tive temperature zone is confined to ∼0.2 m from the surface of the col-
of the gas stream flowing over (or through) the drying material,
• Plug flow of liquid through the capillaries toward the material
© 2009 by Taylor & Francis Group, LLC
208 Advanced Drying Technologies
waves disrupts the boundary layer at the solid or liquid surface, so that they should affect the interphase transfer rates and therefore accelerate drying rates. In fact, a substantial decrease (up to 15%) in the boundary layer thickness has been found when conventional spray drying of blood plasma was complemented by high-intensity sound at f = 15 kHz and I =155 dB (Zayas and Pento, 1975).
Aside from cavitation, the enhanced mass-transfer rates in acoustic
the enhanced dispersion of the liquid and vapor moisture due to alternat-ing compression and expansion cycles, which result in reduced viscosity of the liquid–vapor mixture. In fact, a substantial increase in the amount of liquid diffusing through porous solids has been noted in the presence of ultrasound (Fairbanks and Chen, 1969; Woodford and Morrison, 1969; Kuznetsov and Subbotina, 1965). The enhanced diffusion appears to be of the directional type as mass transfer was hindered when ultrasound
Subbotina, 1965).According to Kardashev (1990), the mechanism of acoustic drying of
capillary-porous materials depends on the moisture content level. When the material is very wet (X′ = 200–500% w.b.), the effect of an ultrasound
sion of liquid water, especially at the antinodes of a standing wave. When the material moisture content is much lower (10–70% w.b.), but drying takes place during the constant-rate period, the sound waves reduce the thickness of the boundary layer, which alters moisture evaporation. In the falling rate period, the sound waves enhance only the moisture diffusiv-ity due to temperature rise because the sonic energy is dissipated as heat. In the case of dispersed materials, the positive effect of sound waves on a drying rate appears for SPLs above a certain threshold value. For spherical particles with a diameter smaller than the wavelength, the critical pres-sure level can be determined from the following equation:
P vgd C C
crg
2 5. ( )s (13.3)
where ρ is the gas density, v is the sound velocity in the gas phase, d is the particle diameter, and Cs and Cg are the water vapor concentrations at the particle surface and in the gas core, respectively.
Because SPL is proportional to the square root of the particle diameter,
of powdery materials. The same conclusion can be drawn when consider-ing the evaporation of free water from a spherical particle. Relating the mass-transfer rate due to air stream velocity resulting from a sound wave
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field is truly mechanical, and moisture is removed due to better disper-
It is reasonable to assume that pressure fluctuation caused by sound
fields can be attributed to the plug flow of the capillary liquid as well as to
irradiation was opposed to the direction of diffusive flow (Kuznetsov and
the best result of sonic irradiation can be expected in fluidized bed drying
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 209
velocity, the enhancement ratio can be expressed as (Kardashev, 1990)
EAd
2 (13.4)
where A is the amplitude of the sound wave and d is the particle diameter.
Directional diffusion as well as the acoustic resonance that appears only
experimental results and controversial conclusions on the effect of sound energy on the drying rate (Bartolome et al., 1969; Carlson et al., 1972). In most cases, however, sound energy appears to have a positive effect on the mass transfer, which translates into shorter drying times. For example, application of 160 dB of sound energy in the frequency range from 5 to
as lactic sugar dried from 17 to 0.25 % d.b. by air at 50°C and 1.4 m/s reduced the drying time from 80 to 30 min (Makarov et al., 1972).
The aforementioned phenomena, which contribute to the overall mech-
generally accepted that the increase in the bulk temperature due to energy dissipation is too small for moisture evaporation (Muralidhara and Lockhart, 1988; Zayas and Pento, 1975; Strumillo and Kudra, 1986; Borisov and Gynkina, 1973). In the case of a plane progressive wave penetrating the material being dried, the sound energy is attenuated due to various mechanisms including relaxation processes, viscous shearing effects, and molecular absorption. Neglecting the scattering of sound energy, the amplitude of acoustic pressure at a distance x from the material surface at which the incident sound pressure is P0 may be expressed as
P x P x0( ) exp ( ) (13.5)
properties and sound frequency (in most cases, α should be determinedexperimentally).
Equation 13.5 can be written conveniently in terms of sound intensity,
IPv
2
(13.6)
where ρ is the material density and v the sound velocity. Hence,
I x I x0( ) exp 2( ) (13.7)
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under specific conditions may explain, in our opinion, some contradictory
anism of drying in a sound field, do not include thermal effects as it is
where α (1/s) is the absorption coefficient, which is a function of material
flowing past a sphere to the convective mass transfer due to the same air
15 kHz to the fluidized bed of a highly heat-sensitive, sticky product such
which reflects the energy flux per unit surface area:
© 2009 by Taylor & Francis Group, LLC
210 Advanced Drying Technologies
Equation 13.7 indicates that the energy lost by absorption from the sound of intensity I crossing a unit distance (x = 1) is equal to the product (2αI). Assuming no heat loss from the absorbing volume, the rate of tempera-ture rise is given by
dTdt
Ic
2
(13.8)
Under real drying conditions, heat generated within the material vol-
temperature tends to equilibrium determined by the heat balance. The equilibrium temperature for the bulk of the material is in the order of a few degrees (Zayas and Pento, 1975; Strumillo and Kudra, 1986; Brun
effects of sound irradiation to be neglected. This obviously favors sonic drying as a method for processing heat-sensitive materials.
Considering heat generation on the microscale, thermal effects may become important as the localized temperature increase is likely to affect
viscosity). According to Moy and DiMarco (1970), for example, 7% of the sound-enhanced freeze-drying (F-D) rate of liquid food can be attributed to the thermally induced mechanical effects on the gas phase, resulting in friction and adiabatic compression.
Detailed discussions of these and other phenomena activated by the sound energy can be found elsewhere (Muralidhara et al., 1986; Suslick, 1988; Rosenberg, 1973; Ensminger, 1986, 1988).
13.4 Drying Kinetics
An analysis of the literature data indicates that drying by airborne sound, which in many cases becomes in fact sound-assisted convective drying, is essentially similar in drying kinetics to conventional convective drying;
are several singularities noted for a given class of the material. For exam-ple, sound-assisted drying of capillary-porous materials is characterized by an extended constant drying rate period toward the lower-moisture contents. In contrast, the constant-rate period in sound-assisted convective drying of colloidal solutions such as blood plasma or protein hydrolysate is reduced by 25–30% (Zayas and Pento, 1975). Moreover, the evaporation
velocity and the air velocity vectors (Marziniak, 1972).
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where c is the material specific heat.
ume is transported away by conduction and convection; hence, the final
and Boucher, 1957), which justifies moisture evaporation due to thermal
that is, there are initial, first, and second drying periods. However, there
rate in the first drying period is proportional to the sum of the local sound
fluid properties and solid–fluid interactions (e.g., lower surface tension or
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 211
Acoustic drying of colloidal, capillary-porous materials such as potato starch (370 kHz, 147 dB) with the maximum hygroscopic moisture con-tent of 68% takes place at a much lower constant drying rate, and two periods of the falling drying rate are usually observed. In contrast, the drying kinetics of swollen bentonite, which also falls into the category of colloidal, capillary-porous material, is characterized by the presence of two constant-rate periods (Stadnik and Kazanskii, 1966). Shifting of the critical moisture content can be attributed to the fact that the acous-tic vibrations intensify moisture evaporation, which not only increases the critical moisture content but also alters the process of diffusion, which reduces the critical value. Thus, the balance between the mois-ture transported to and removed from the material surface determines the relative length of the constant-to-falling rate period (Borisov and Gynkina, 1973).
The drying rate depends on the sound wave pattern in a drying cham-ber, which may be attributed to both the Fraunhofer zone and the reso-nance phenomenon, which results in nodes and antinodes in the sound
of sound pressure waves. In the falling drying rate period, these differ-
0 1 2 3 4 5Moisture content (kg/kg d.b.)
Dry
ing
rate
(kg
H2O
/min
kg
d.m
.)
Potato cylinders
D = 9.5 mm; L = 31.7 mmX1 = 80.2 %
Velocity node
Velocity antinode
No sound
f = 8.1 kHz
0
1
2
3
4
5
FIGURE 13.8Drying rate for potato cubes placed in nodes and antinodes. (Adapted from Bartolome,L. G., Hoff, J. E. and Purdy, K. R., Food Technol., 23, 47–50, 1969.)
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pressure/intensity. A significant difference (up to 250%) was observed in the first drying period for potato samples placed in nodes and antinodes
ences were insignificant (Figure 13.8).
© 2009 by Taylor & Francis Group, LLC
212 Advanced Drying Technologies
node), the wavelength of infrasound is long enough for most materials to be exposed to the useful region of the standing wave. Also, infrasound is less attenuated within the material being dried. Moreover, infrasound is not audible, and therefore, at similar pressure levels, it is less of a nuisance in a plant environment than audible sound. The drawbacks of infrasound
large-amplitude vibrations of mechanical parts of the equipment if not of the resonance.
Although there is no clear picture to explain the effect of frequency and sound intensity on the moisture transfer rate, the enhancement of the dry-
little effect of the frequency unless the sound wave attenuation is critical.The effect of acoustic energy on enhanced drying rates is especially
advantageous at low temperatures. For example, in combined acoustic–convective drying, the effective increase in drying rate due to acoustic energy decreases with temperature, and under certain conditions, the
ture and air velocity (Fairbanks, 1970).Aside from low-temperature drying, the sound energy may also be
advantageous in high-temperature drying. The experiments on dry-ing several albumin-based biomaterials in a conventional spray dryer equipped with air-driven whistles located at the disk atomizer level have shown reduced material temperature in the dryer by 10–22°C (Figure 13.9), which results in better product quality. In particular, sound-assisted spray drying of blood plasma carried out at inlet and outlet air temperatures
moisture contents by 0.3–0.5%, increased concentration of soluble protein by 2–3%, and reduced concentration of (-SH) groups by 5–20% as com-pared with the control sample dried without the sound energy (Zayas and Pento, 1975).
Especially attractive is the synergistic effect on drying rate when com-bining sound energy with other mass-transfer driving forces produced
example of such an effect is shown in Figure 13.10, which compiles the results of acoustic drying, microwave drying, and acoustic–(MW) drying of silica gel taken as a model material (Shatalov, 1976). In these studies, MW energy at 2450 MHz was applied in a pulsed mode to maintain the material temperature at 80°C. It was found that the addition of ultrasound
Our detailed analysis of the original data indicates, however, that the experimental points for sound-assisted MW drying are located above the
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From a truly practical point of view, the infrasound field appears to have
accelerated transfer processes occurs at the velocity node (pressure anti-advantages over audible sound or ultrasound fields. Provided the effect of
are the difficulty of acoustic isolation, if required, and the possibility for
ing rate becomes significant at sound intensities in excess of 150 dB with
contribution of the sound energy is as significant as the effect of tempera-
from 115 to 145°C and from 65 to 80°C, respectively, resulted in lower final
by other energy fields such as magnetic, electric, or electromagnetic. An
increases the rate of MW drying in the first drying period by 30–35%.
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 213
40 60 80 100 120 140 160 180Temperature (°C)
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2D
ryer
hei
ght (
m)
Spraylevel
Air inletlevel
Cyclonelevel
Control
MaterialAir
Protein concentrate
155 dB15 kHz
FIGURE 13.9
Pento, V., Myasnaya Industriya, 6, 31–33, 1975.)
0 0.05 0.10 0.15 0.20 0.25 0.30 0.350
0.2
0.4
0.6
0.8
1.0
1.2
f = 20 kHzI = 155 dB
Moisture content (kg/kg)
Dry
ing
rate
(104 kg
H2O
/kg
s)
Ultrasound
Microwaves
Ultrasound and microwaves
FIGURE 13.10Drying rates for sound-assisted MW drying. (From Shatalov, A. L, Elektronnaya Obrabotka Materialov, 3, 37–38, 1976.)v
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Temperature profile in a sound-assisted spray dryer. (Adapted from Zayas, Yu. and
© 2009 by Taylor & Francis Group, LLC
214 Advanced Drying Technologies
curve obtained by superposition of the isolated acoustic and MW drying data. This synergistic effect is more pronounced during the initial and the falling rate periods, which implies alteration of the internal mass-trans-fer mechanism rather than evaporation from the free water surface. The same conclusion can be drawn from the experimental data on vacuum
Muralidhara et al., 1988).The aforementioned mechanical and physical phenomena induced by
ultrasound can be exploited to enhance the rate of transfer processes where diffusion controls the mass transfer, as in the case of food dewater-ing (Mulet et al., 2002; Sun, 2005). For instance, Simal et al. (1998) found that applying 40 kHz ultrasound during osmotic dehydration of apple cubes in 70° Brix sucrose solution held at 40–70°C increased the water loss and sucrose gain in the range from 14 to 17% and from 11 to 23%, respectively. Higher water loss and solute gain were also noted by Carcel et al. (2002) when dewatering apple slices in 30° Brix sucrose solution at 30°C exposed to 20 kHz ultrasound when its intensity was above a cer-tain threshold value. Since ultrasound-assisted osmotic dehydration can
heat-sensitive components to be preserved.
0 2 4 6 8 10 12 1410
15
20
25
30
35
40
45
50
Time (min)
Solid
s con
tent
afte
r dew
ater
ing
(% w
/w)
Vacuum only
Vacuum + electric
Vacuum + electric + ultrasonic
Corn gluten slurry (13.1 % w/w in feed)
FIGURE 13.11
Chauhan, S. P., Senapati, N., Beard, R., Jirjis, B. and Kim, B. C., Sep. Sci. Technol., 23(12 and 13), 2143–2158, 1988.)
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drying with the assistance of electric and ultrasonic fields (Figure 13.11;
Effect of sound and electric field on vacuum drying rates. (From Muralidhara, H. S.,
be performed at lower temperatures, it allows the natural flavor, color, and
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 215
Another application of sound energy is the ultrasonic pretreatment before hot-air drying of fruits and similar materials in which mass trans-fer is controlled by diffusion. Fernandes et al. (2007) documented higher effective diffusivity of water when 25 kHz ultrasound at the intensity of 4.87 kW/m2 was applied to papaya, leading to a 13.8% reduction of sugar in 30 min and shortening the drying time by 16%.
13.5 Sound-Assisted Dryers
The basic constraint in sound-assisted drying is the maximum penetra-tion depth, which restricts the material thickness to ∼30 to 50 mm. Thematerial thickness could be higher in the case of infrasound; however, no data on infrasound drying have been reported to date, although a positive effect on heat transfer has been documented (Woods, 1991, 1992).
(tubes or chambers); hence, the resulting standing waves cause Rayleigh acoustic streaming. This streaming has a closed vortex character, and the vortex scale in the direction of the radiation is equal to a quarter- wavelength (Borisov and Gynkina, 1973). At acoustic pressure levels above 145–150 dB, the streaming velocity can be in the order of several meters per second (Borisov and Statnikov, 1965). Such a high velocity not only
hydrodynamic effect on a particulate material. It was found, for example, that sound waves of frequency from 50 to 500 Hz and intensity above
from 2 to 1000 μm if the density is within the range of 1200–2700 kg/m3.
idized bed was analyzed by Kardashev and coworkers (1972). Considering
to be cylinders of radius r and length L, the following equations were devel-
w k kr P
LD 1 2
3
2 1
4
(13.9)
w k kr P L
rL L
rDa
2 2
2
2 2 2 2 2
2 1 2
0 25
12 16
92
.
(13.10)
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In the case of acoustic dryers, sound propagates in confined volumes
alters the heat- and mass-transfer processes but also can have a definite
111 dB can sustain stable fluidization of particles with diameters ranging
The effect of sonic irradiation on drying of particulate materials in a flu-
a quasi-stationary model of a fluidized bed in which the voids are assumed
oped for a drying rate in the classical fluid bed of particulate material as
and in the fluidized bed with acoustic irradiation as
© 2009 by Taylor & Francis Group, LLC
216 Advanced Drying Technologies
where ∆P and ∆Pa are the pressure drops in a classical (hydrodynamic)
1 2 depend on material properties
and water concentration gradient. For example, for the bed of silica gel with equivalent diameter de = 0.22 mm, k1 = 5.72 × 107 and k2 = 0.23g/cm2 s, whereas k1 = 1.1 × 103 and k2 = 0.23 g/cm2 s in the case of the styrene/α-methylstyrene copolymer with de = 0.12 mm.
Accounting for the penetration depth, surface phenomena, and hydr-odynamic effects, the industrial sonic dryers are therefore mainly of the
dryers (Kudra, 1998; Zayas and Pento, 1975; Kardashev, 1990; Borisov and Gynkina, 1967, 1973; Soloff, 1964; Huxsoll and Hall, 1970; Borisov and Ginin, 1967; Tutova and Kuts, 1987; Mashkova, 1960; Bezzubov et al., 1968; Simonyan, 1965).
Figure 13.12 presents the design principle of a sound-assisted rotary dryer, which was a subject of studies by Soloff (1964), Huxsoll and Hall (1970), Borisov and Gynkina (1967), and Borisov and Ginin (1967). The drum of the dryer used by Soloff (other dryers were similar in design and operation) was 0.2 m in diameter and 1.2 m in length. The drum had eight
controlled the drum rotation up to 80 rpm. The maximum feed rate was 45 kg/h at the inlet air temperature up to 150°C. A pneumatic sound gen-erator (Branson Ultrasonic Corporation) was mounted at the discharge end of the drum. The sound was propagated along the drum, and a stand-
Wet feed
Product
Humid air
Compressed
air
Baffle
DriveDrum
Soundgenerator
FIGURE 13.12Sound-assisted rotary dryer.
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The experimental coefficients k and kand bed characteristics such as particle diameter, diffusion coefficient,
fluidized bed and in a fluidized bed with sonic irradiation, respectively.
dispersed-flow type, for example, spray, throughflow, rotary, or fluid bed
lifting baffles to tumble the particulate material. A variable-speed motor
ing wave was established because of reflection from the feed end of the
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 217
drum. An SPL of 169 dB at 10.9 kHz was attained at the pressure antinodes in the drum. The results of several drying tests are given in Table 13.2.
The major limitation of rotary dryers is the relatively low volumetric load ratio of the drum, which typically ranges from 3 to 16%. Because sound
better energy utilization is in the case of spray dryers as sonic irradiation
zation (see Figure 13.9).An interesting design of the industrial sound-assisted spray dryer in
which the sound energy is used to homogenize the mixture of the liquid bio-material and the solid sorbent-carrier is shown in Figure 13.13. A standard spray dryer with a tangential air inlet is equipped with two sirens situated at the spray cone level. The combined action of the sound energy and the air streams from the sirens activated by compressed air not only improves the dispersion of the solid–liquid spray but also enhances heat- and mass-
(Tutova and Kuts, 1987). Unfortunately, no technical information about the dryer and operating parameters are given in the source literature.
three pneumatic sirens (Mashkova, 1960; Bezzubov, 1968). Fluidization of a particulate material takes place due to the combined action of the sound energy from the static siren located below the supporting grid and the air stream that excites the siren. An interesting feature of this design is the use of two additional sirens located at the sidewall and at the top of the drying
TABLE 13.2
Drying of Selected Materials in Sonic Rotary Dryer
Material
Moisture Content(% w.b.) Residence
Time (min)
Throughput (kg/h)
Initial FinalWith
SoundWithoutSound
5.5 1.53 3.0 40.8 16.8Grated cheese 16.8 5.9 16.2 15.9 11.3Antacid powder 15.1 6.0 15.0 12.2 6.8Gelatin beads 12.9 3.7 20.0 10.0 5.4Enzyme crystals 9.8 6.4 120.0 2.3 0.9Carbon black 48.7 1.0 25.0 8.2 5.4Rubber crumbs 44.0 6.0 90.0 3.2 1.8Polystyrene powder 0.5 0.1 30.0 6.4 2.7Aluminum oxide 0.5 0.2 5.0 25.4 14.5Rice grains 27.6 14.5 11.0 18.1 8.2
Source: After Soloff, R. S., J. Acoust. Soc. Am., 36(5), 961–965, 1964. With permission.
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energy is absorbed in the bulk of particulates, it is difficult to secure the efficient use of high-intensity sound energy in large rotary dryers. Much
not only intensifies heat and mass transfer but also enhances liquid atomi-
transfer rates, which finally allows for the product of required quality
chamber to reduce carryover of fines by acoustic coagulation of aerosols.
Wood flour
Figure 13.14 presents the design of a fluid bed dryer equipped with
© 2009 by Taylor & Francis Group, LLC
218 Advanced Drying Technologies
Air inlet
Compressedair
Compressedair Compressed
air
Product
SorbentSorbentWet feed
Two-streamnozzle
Siren
FIGURE 13.13Sound-assisted spray dryer for drying liquid biomaterials on active sorbents. (Adapted from Tutova, E. G. and Kuts, P. S., Drying of Products of Biosynthesis, Agropromizdat, Moscow, 1987.)
FIGURE 13.14
Fridman, V. M., Ultrasound and its Application in Food Industry, Light Industry and FoodyyPublishing House, Warsaw, 1968 [in Polish].)
Siren
Siren
Siren
Supportinggrid
Air inlet
Airoutlet
Product
Wet feedWire mesh
Wire mesh
Partitions
Spent air
Compressedair
Compressedair
Compressedair
CompressedairCompressed
air
Membrane
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Sound-assisted fluid bed dryer. (Adapted from Bezzubov, A. D., Garlinskaya, E. I. and
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 219
To prevent spent compressed air from affecting bed hydrodynamics, the side sirens are covered with membranes. Because the membranes and the supporting grid absorb ∼50% of the incident sound energy, this dryer hasfound only a limited application.
bed dryer is equipped with a gas-jet sound generator (Galton or Hartman whistle) located in the axis of the upper part of a drying chamber appears
power the air consumption is ∼160 m3/h (Fridman, 1967), this additional air stream exiting the sound generator facilitates the transportation of
mance of the dryer with and without sonic irradiation.The review of available literature indicates that acoustic drying has
unquestionable advantages over other drying technologies as far as dry-ing rate (drying time) is concerned. An analysis of energy requirements done by Borisov and Gynkina (1973) has proved, however, that consump-tion of sonic energy per kilogram of water removed is roughly the same
FIGURE 13.15
VNIINSM, 2(10), 34, 1965.)M
Product
Feed
Fluid bed
Soundsource
CompressedairExhaust
Air
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The design by All-Union Scientific-Research Institute of New Structural
to be more energy efficient (Figure 13.15). Because at 1.5 kW of acoustic
fine particles to the cyclone. In the case of larger particles, the dryer can
as in convective drying, assuming that the efficiency of sound generators
Materials (VNIINSM) (Simonyan, 1965) in which the conventional fluid
be equipped with an overflow discharge. Table 13.3 compares the perfor-
Design principle of sound-assisted fluid bed dryer. (Adapted from Simonyan, S. G., Trudy
© 2009 by Taylor & Francis Group, LLC
220 Advanced Drying Technologies
gas-jet generators ∼25%, the cost of acoustic drying is higher than that of convective drying. This limits sonic drying to expensive and otherwise hard-to-dry materials and to products for which quality issues or safety aspects offset the higher drying cost.
Symbols
A Amplitude, m
C Concentration, kg/m3
d Diameter, m2
E Enhancement ratio, -f Frequency, Hzg Acceleration due to gravity, m/s2
I Sound intensity, W/m2
L Length, mL Relative level, dBP Pressure, N/m2
∆P Pressure drop, N/m2
r Radius, mS Surface area, m2
t Time, sT Temperature, °C, Ku Gas velocity, m/sv Sound velocity, m/swD Drying rate, kg/m2 sW Power, W
TABLE 13.3
Performance of Low-Temperature Drying in an Acoustic Fluid Bed DryerDesigned by VNIINSM
Material
Moisture Content (kg/kg) Temperature
(°C)Throughput
(kg/h)Initial Final
Quartz sand 0.037 0.0080 23 11.5Potassium chloride 0.033 0.0032 23 7.5Potassium chloride 0.030 0.0033 30a 21.0Potassium chloride 0.019 0.0030 23 25.0Potassium chloride 0.003 0 23 24.0
a With hot air stream.Source: After Simonyan, S. G., Trudy VNIINSM, 2(10), 34, 1965.
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is 100%. Since the energy efficiency of dynamic sirens is ∼30 to 35% and
c Specific heat, kJ/kg K
D Diffusion coefficient, m /s
© 2009 by Taylor & Francis Group, LLC
Sonic Drying 221
x Distance, mX′ Moisture content (wet basis), kg water/kg wet materialα λ Wavelength, mν Viscosity, m2/sρ Density, kg/m3
ω Angular frequency, rad/sw.b. Wet basis
Subscripts
a Acousticcr Criticale Equivalentg Gass Surface0 Incident
References
Bartolome, L. G., Hoff, J. E. and Purdy, K. R. 1969. Effect of resonant acoustic vibrations on drying rates of potato cylinders. Food Technol., 23: 47–50.
Bezzubov, A. D., Garlinskaya, E. I. and Fridman, V. M. 1968. Ultrasound and its Application in Food Industry. Light Industry and Food Publishing House,Warsaw, (in Polish).
Borisov, Yu. Ya. and Ginin, V. N. 1967. An acoustic drum dryer. USSR Patent. 184651.Borisov, Yu. Ya. and Gynkina, N. M. 1962. On acoustic drying in a standing sound
wave. Sov. Phys.-Acoust., 8(1): 129–131.Borisov, Yu. Ya. and Gynkina, N. M. 1967. Experiments on acoustic drying.
Ultrazvukovaiya Tekhnika, 5(3): 38 (in Russian).Borisov, Yu. Ya. and Gynkina, N. M. 1973. Acoustic drying. In: Physical Principles of
Ultrasonic Technology. L. D. Rosenberg (Ed.). Vol. 2. Plenum Press, New York,pp. 381–473, 554p.
Borisov, Yu. Ya. and Statnikov, Yu. G. 1965. Flow streams generated in an acoustic standing waves. Akust. Zhurnal., 11(1); 35–38 (in Russian).
Boucher, R. M. G. 1959. Drying by airborne ultrasonics. Ultrasonic News, Second Quarter: 8–16.
Brun, E. and Boucher, R. M. G. 1957. Research on the acoustic air-jet generator. J. Acoust. Soc. Am., 29: 573–583.
sound in osmotic dehydration of apple slices. Proc. IDS 2002, Aug. 27–30,Beijing, China, pp. 936–942.
Carlson, R. A., Farkas, D. F. and Curtis, R. M. 1972. Effect of sonic energy on the air drying of apple and sweet potato cubes. J. Food Sci., 27: 793–794.
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Absorption coefficient, 1/s
Carcel, J. A., Golas, Y., Benedito, J. and Mulet, A. 2002. Influence of power ultra-
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Ensminger, D. E. 1986. Acoustic dewatering. In: Advances in Solid-liquid Separation. H. S. Muralidhara (Ed.). Battelle Memorial Institute, Columbus, pp. 321–334, 485p.
Ensminger, D. E. 1988. Ultrasonics: Fundamentals, Technology, Applications. Marcel Dekker, Inc, New York and Basel, 580p.
49–50.
through porous media. Ultrasonics, 7(3): 195–196.Fernandes, F. A. N., Oliveira, F. I. P. and Rodrigues, S. 2007. Use of ultrasound for
dehydration of papayas. Food Bioprocess Technology, Springer Science, Berlin,Germany (electronic edition).
Fridman, V. M. 1967. Ultrasonic Chemical Equipment. Mashinostroyennie, Moscow, 207p. (in Russian).
Greguss, P. 1961. Drying by airborne ultrasonics. Ultrasonic News, 5: 7–11.Greguss, P. 1963. The mechanism and possible applications of drying by ultrasonic
irradiation. Ultrasonics, 1: 83–86.Huxsoll, C. C. and Hall, C. W. 1970. Effects of sonic irradiation on drying rates of
wheat and shelled corn. Trans. ASAE., 13(1): 21–24.
Technologies. Khimiya, Moscow, 208p (in Russian).
Tekhnologii, 4(4): 633–636 (in Russian).Kudra, T. 1998. Sound-assisted drying. In: Novel Drying Technologies. Y. K. Pan and
X. Z. Wang (Eds). China Chemical Engineering Publishing House, Beijing (in Chinese), pp. 653–660.
Kudra, T. and Mujumdar, A. S. 2007. Special drying techniques and novel dryers. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). 3rd edition. Taylor & Francis, Boca Raton, FL, pp. 453–517.
Kuznetsov, V. V. and Subbotina, N. I. 1965. An effect of ultrasound on the diffu-sion of electrolytic hydrogen through the ion membrane. Elektrokhimia, 1(9): 1096–1098 (in Russian).
Makarov, A. P., Polyanskii, K. K. and Karnaukh, V. I. 1972. Studies on acoustic
Tekhnologiya., 4: 181–183 (in Russian).Marziniak, R. 1972. Kinetic of acoustic drying process. Acustica, 27:122–131 (in
German).Mashkova, T. L. 1960. Acoustic drying. In: Application of Ultrasonics in Processes
of Chemical Engineering. TINTIEPP Publishing House, Moscow, pp. 196–201 (in Russian).
Moy, J. H. and DiMarco, G. R. 1970. Exploring airborne sound in a nonvacuum freeze drying process. J. Food Sci., 35: 811–817.
Mujumdar, A. S. 1991. Drying technologies of the future. Drying Technol., 9(2): 325–347.
Mulet, A., Carcel, J., Benedito, I., Rossello, C. and Simal, S. 2002. Ultrasonic mass transfer enhancement in food processing. In: Transport Phenomena in Food Processing. J. Welti-Chanes, J. F. Velez-Ruiz and G. V. Barbosa-Canovas (Eds).CRC Press, Boca Raton, FL, pp. 265–278.
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Fairbanks, H. V. 1970. Acoustic drying of ultrafine coal. Mining Eng., 22(12):
Kardashev, G. A. 1990. Physical Methods of Process Intensification in Chemical
Kardashev, G. A., Mikhailov, P. E. and Yurchenko, Yu. B. 1972. Increasing efficiency
vibrations for intensification of lactic sugar drying. Izvestiya Vuzov. Pishchevaya
Fairbanks, H. V. and Chen, W. I. 1969. Influence of ultrasonics upon liquid flow
of fluidization by acoustic treatment. Teoreticheskie Osnovy Khimicheskoi
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Muralidhara, H. S., Chauhan, S. P., Senapati, N., Beard, R., Jirjis, B. and Kim, B. C. 1988. Electro-acoustic dewatering (EAD): A novel approach for food process-ing, and recovery. Sep. Sci. Technol., 23(12 and 13): 2143–2158.
Muralidhara, H. S. and Ensminger, D. 1986. Acoustic drying of green rice. Drying Technol., 4(1): 137–143.
techniques for dewatering. Drying Technol.- Theme Issue, 6(3): 361–569.Muralidhara, H. S., Senapati, N. and Beard, R. B. 1986. A novel electroacoustic
Separation. H. S. Muralidhara (Ed.). Battelle Memorial Institute, Columbus, 485p.
Otsuka, T., Purdum, H. and Fairbanks, H. 1977. Drying of temperature sensitive materials with a heated ultrasonically vibrated chute. Proc. Ultrasonic Int.Conf. Brighton, UK, pp. 91–95.
Rosenberg, L. D. (Ed.) 1973. Physical Principles of Ultrasonic Technology. PlenumPress, New York, Vol. 1, 515p, Vol. 2, 544p.
Shatalov, A. L. 1976. Sound-assisted microwave drying. Elektronnaya ObrabotkaMaterialov, 3: 37–38 (in Russian).
Simal, S., Benedito, J., Sanches, E. S. and Rossello, C. 1998. Use of ultrasound to increase mass transport rates during osmotic dehydration. J. Food Eng., 36(3): 323–336.
VNIINSM, 2(10): 34 (in Russian).Soloff, R. S. 1964. Sonic drying. J. Acoust. Soc Am., 36(5): 961–965.Stadnik, B. N. and Kazanskii, M. F. 1966. Effect of sound on the intensity of
drying capillary-porous materials. In: Investigation of Heat and Mass Transferin Technological Processes and Equipment. Luikov, A. V. (Ed.). Nauka i Tekhnika, Minsk, 240p (in Russian).
Strumillo, C. and Kudra, T. 1986. Drying: Principles, Applications and Design. Gordon and Breach Science Publishers, New York, 448p.
Sun, D. -W. 2005. Emerging Technologies for Food Processing. Elsevier, Amsterdam.Suslick, K. S. (Ed.) 1988. Ultrasound: Its Chemical, Physical and Biological Effects.
VCH Publishers, New York, 336p.Tutova, E. G. and Kuts, P. S. 1987. Drying of Products of Biosynthesis. Agropromizdat,
Moscow, 304p (in Russian).Woodford, R. and Morrison, J. C. 1969. The diffusion of sennoside A through a
cellulose membrane. J. Pharm. Pharmacol., 21(9): 602–606.Woods, B. G. 1991. Electrical load reduction via infrasonic enhancement of process
heat transfer and drying. Ontario Hydro Research Division, Report 91-68-K.
and its impact on process power consumption. Int. J. Heat Mass Transfer, 35(10): 2367–2376.
Myasnaya Industriya, 6: 31–33 (in Russian).
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Muralidhara, H. S. and Lockhart, N. C. (Eds) 1988. Combined field separation
separation process for fine particle suspensions. In: Advances in Solid-Liquid
Zayas, Yu. and Pento, V. 1975. Drying of thermally labile solutions in acoustic field.
Simonyan, S. G. 1965. Low-temperature drying of free-flowing materials. Trudy
Woods, B. G. 1992. Sonically enhanced heat transfer from a cylinder in cross flow
© 2009 by Taylor & Francis Group, LLC
225
14Pulse Combustion Drying
14.1 Principles of Pulse Combustion
potential since it is claimed to increase productivity, maximize utiliza-tion of the input fuel, enhance heat transfer, and reduce pollutant emis-sions (Muralidhara and Lockhart, 1985; Mujumdar, 1991; Zbicinski et al., 2002; Kudra et al., 2003; Mujumdar and Wu, 2004; Kudra and Mujumdar, 2007).
The term pulse combustion originates from the intermittent (pulse) com-bustion of solid, liquid, or gaseous fuel in contrast to continuous com-bustion in conventional burners. Such periodic combustion generates intensive pressure, velocity, and, to a certain extent, temperature waves propagated from the combustion chamber through a tailpipe (a diffuser) to the process volume (an applicator) such as a dryer, calciner, or incin-erator. Because of the oscillatory nature of the momentum transfer, pulse
The mechanism behind the operation of a pulse combustor is a complex interaction between an oscillatory combustion process and acoustic waves that are propagated from the combustor (Figure 14.1). The process of pulse combustion is initiated when air required for combustion and fuel in the form of a gas jet or a liquid spray are drawn into the combustion chamber, mixed to form an explosive gas, ignited by a spark plug, and combust instantly in an explosion-like manner. At this moment, the air and fuel inlet ports are closed either by a valve-operating mechanism or due to the hydrodynamic action of the rapidly rising pressure. The combustion-
pressure in a combustion chamber to decrease (Figure 14.2). When the pressure reaches its minimum, the inlet ports open and admit fresh fuel and air into the combustion chamber. This new charge ignites itself due
ing cycle, which reenter the combustion chamber during the minimum
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Pulse combustion drying is an emerging technology of significant
combustion intensifies the rates of heat and mass transfer.
generated pressure forces the combustion products to flow out through
as long as the pressure rise in the combustion chamber is larger thanthe tailpipe to the process volume. The exhaust of flue gases continues
the pressure drop due to the outflow of combustion products. As the hot flue gases discharge, the resulting outward momentum causes the
to contact with remnants of hot flue gases in the tailpipe from the preced-
© 2009 by Taylor & Francis Group, LLC
226 Advanced Drying Technologies
frequency, which depends on the design of the pulse combustor and the characteristics of the tailpipe. Ignition of the air–fuel mixture by a spark plug is needed only during the start-up cycle because the mixture is
Intake of air and fuelto form an explosivemixture. Valves open.
Initiation of combustionby a spark plug.Valves start to close.
Combustion completed.Flow in tailpipe.Valves closed.
Outflow of flue gas from tailpipe.Intake of new charge of air and fuel.Valves open.
Reignition of air−fuel mixtureby residual flue gas.
Backflow of residual flue gas.Valves start to close.
Fuel
Air
Fuel
Air
Fuel
Air
Fuel
Air
Fuel
Air
Fuel
Air
Tailpipe
Combustionchamber
IgniterInlet ports
FIGURE 14.1Principle of pulse combustion.
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pressure period. These combustion cycles repeat themselves at a definite
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 227
reignited automatically under a stable operation. In addition, the start-up fan is normally shut off once self-aspiration is established.
It should be noted that thorough mixing of the combustion air with fuel
bustor. In the design of the Novadyne pulse combustor (Novadyne Ltd., Hastings, ON, Canada), for example, a fuel distributor in the combustion chamber injects the fuel in streaks in the combustion air stream, forming a striated charge (Kitchen, 1987). For liquid fuels, striated charges have been
improve the combustion during the start-up period. Furthermore, striated charges produce pressure and sound waves that are of larger amplitude,
of producing excessive noise.
FIGURE 14.2(a) Theoretical and (b) experimental pressure traces in a pulse combustor: A—air and fuel enter the combustion chamber; B—fresh charge ignited, pressure rises as combustion gases
gases are vented; and D—momentum of exhausting gases creates negative pressure in the combustion chamber.
Valve open
Valve closedA
B C
D
Neg
ativ
epr
essu
rePo
sitiv
epr
essu
re
Time
0
(a)
(b)Time (s)
Pres
sure
ampl
itude
(atm
)
0 0.01 0.02 0.03 0.04−0.3
−0.2
−0.1
0.0
0.1
0.2
0.3
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is not a sine qua non condition for the efficient operation of a pulse com-
shown to be more efficient than thoroughly mixed charges in that they
which is beneficial in some applications but may also have the drawback
heat up, stop of air and fuel inflow; C—combustion complete, pressure decreases as flue
© 2009 by Taylor & Francis Group, LLC
228 Advanced Drying Technologies
Typically, pulse combustors operate at frequencies from 20 to 250 Hz. Pressure oscillation in the combustion chamber of ±10 kPa produces velocity oscillation in the tailpipe of approximately ±100 m/s, and there-fore the instantaneous velocity of a gas jet at the tailpipe exit varies from
gases at higher pressure than the inlet air pressure, the resulting increase in stagnation pressure is relatively small. This restricts practical applica-tions of pulse combustors to systems where a pressure drop is not critical. The amplitude of the pressure rise may vary from 10 (domestic heating applications) to 100% as for heavy-duty pulse combustors for industrial
combustors ranges from 70 to 1000 kW.Numerous studies on various pulse combustor designs have demon-
strated that pulse combustion can offer the following advantages over conventional (i.e., continuous) combustion (Keller et al., 1992; Ozer, 1993; Stewart et al., 1991):
Increased heat- and mass-transfer rates (by a factor of 2–5)• Increased combustion intensity (by a factor of up to 10)•
Reduced pollutant emissions (especially NO• x, CO, and soot) by a factor of up to 3
Reduced space requirements for the combustion equipment•
The major disadvantage of pulse combustion systems is the noise gener-ated in normal operation, which may be a deterrent to wide industrial applications. The sound pressure level (SPL) emitted by the burner that resonates in the applicator depends on the operating pressure and fre-quency; the higher the operating pressure, the higher the SPL. From a technical viewpoint, large pressure amplitudes are desirable in processes such as drying, as they tend to increase the rate of heat and mass transfer and thus reduce the size of the burner required to deliver a given amount of heat. However, from a practical point of view, noise emission may be a much more important design criterion than minimizing the burner size. Resonant frequency may also be critical in the design of pulse combustion systems because high-frequency noise can be far more easily attenuated than low-frequency sound.
A solution to the sound problem is a rotary air-valved combustor that is encased in a steel shell (see Figure 14.3). Because in spray drying applica-tions such a combustor runs out of its resonant frequency, the sound level is <85 dB at 1 m distance from the dryer (Rehkopf, 2008).
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use (Kentfield, 1993). The output power for commercially available pulse
• Higher combustion efficiency with low excess air values
• Improved thermal efficiency by up to 40%
0 to 100 m/s (Keller et al., 1992). Although pulse combustors deliver flue
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 229
14.2 Pulse Combustors: Design and Operation
14.2.1 Basic Types of Pulse Combustors
Pulse combustors may be categorized into three distinct classes accord-
These are the quarter-wave (or Schmidt) combustor, r the Helmholtz com-bustor, r and the Rijke-type combustor.
14.2.1.1 Quarter-Wave (Schmidt) Pulse Combustor
The operation of the Schmidt pulse combustor is based on the principle of the quarter-wave sound resonator comprising a tube closed at one end. The acoustic pressure oscillation excited within this tube attains its maxi-mum value at the closed end (at the lid) and its minimum value (which is approximately zero) at the open end. In the Schmidt combustor, a mechan-ical or an aerodynamic valve replaces the lid that closes the quarter-wave resonator. Thus, the combustor may be divided into three distinct sec-tions: the inlet, t the combustion chamber, and the tailpipe. Basically, thecombustion process takes place in the vicinity of the mechanical valves, where the pressure oscillation is maximized and the fresh charges of fuel
Combustion air
Flue gases
to process
Fuel Combustionchamber
IgniterRotaryvalve
Annularair diode
Tailpipe
Motor
Mixingchamber
Cooling air
FIGURE 14.3Design principle of pulse combustor with rotary valve. (After Lockwood, Jr., H. N., Pulse combustion energy system, U.S. Patent 4,708,159, 1987.)
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ing to the specific acoustic system on which their operation depends.
© 2009 by Taylor & Francis Group, LLC
230 Advanced Drying Technologies
and air are admitted into the combustor when the pressure has reached its minimum value. Therefore, to sustain pulsation, it is essential to balance
a greater portion of the combustion heat is released when the combustor pressure is near its maximum. This implies that both the mixing and the
The combustor will cease to function if the duration of either the pulsation
14.2.1.2 Helmholtz Pulse Combustor
This combustor operates under the principle of the standard acoustic Helmholtz resonator in which a short, small-diameter stub (tailpipe) is attached to one of the walls of a large cavity (combustion chamber), and valves are placed at the wall opposite to the tailpipe. A Helmholtz resona-tor operates at a frequency determined by both the volume of the combus-tion chamber and the length and cross-sectional area of the tailpipe. It is important to note that the pressure within the Helmholtz combustion chamber is considered to be uniform in space while the pressure oscilla-tions become space-dependent once within the tailpipe.
14.2.1.3 Rijke-Type Pulse Combustor
This class of pulse combustors has its design based on the operating prin-ciple of the Rijke tube. The Rijke tube consists of an open channel where air and fuel are supplied through one open end and products of combus-tion exit the system through the other. To ensure stable operation, it is
L/4 from the entrance plane of the combustor, where L represents the length of the tube. Furthermore, it has been shown that the length of the tube equals half the wavelength of the oscillation.
14.2.2 Valve and Valveless Pulse Combustors
The oscillatory pressure variation inside the combustion chamber allows one to use mechanical valves actuated by over- and underpressure, which is especially advantageous when low output power or high turndown ratios are required (Buchkowski and Kitchen, 1995a). An alternative design of a pulse combustor is the so-called valveless combustor, in which mechanical valves are replaced with an aerodynamic diode in the form of
through the inlet section.
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correctly the duration of the mixing and the firing phases, and therefore
firing processes must be completed within half the period of pulsation.
or the mixing/firing process is too short.
necessary to complete the firing phase within the first half of the combus-tor length. Even more specifically, the process should occur at a distance
a profiled orifice in the inlet pipe, contoured diffuser, or a shrouding duct. This aerodynamic diode admits the inflow of air and fuel to the combustion chamber but prevents the backflow of flue gasses out of the combustor
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 231
Based on the manner in which the fuel and air charge the combustion chamber, pulse combustors are divided into two general categories: those with mechanical valves and those with aerodynamic valves (also called valveless combustors). Mechanical valves can be further divided into
bustion products out of the pulse combustor through the combustor inlet during the positive pressure phase of the pulse combustion cycle, the
tors. There are, however, some problems associated with the design of mechanical inlet valves. Owing to the high frequencies attained in pulse combustors, it becomes necessary to have valve inertia minimized. The need to protect the valves from thermal stress, which may be incurred due to the combustion process as well as from corrosion when in contact
14.2.2.1 Flapper and Reed-Type Valves
In general, the type of valve incorporated in heavy-duty pulse combustors is reed valves made from thin-sheet spring steel (Figure 14.4a). The spring action of reed valves is such that the valves normally shut are sprung
area of the inlet reed valves is considerably less than the cross- sectional area of the combustion zone. At present, one major problem often encoun-tered with reed-type mechanical valves is fatigue failure.
Flapper
Backstop
Inletport
Baseplate
(a)
Backstop
Inletport
Baseplate
Reed
(b)Closed position
FIGURE 14.4
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problems are amplified when considering heavy-duty pulse combustors operated at large pressure amplitudes (Kentfield, 1993).
three types: flapper valves, reed valves, and rotary valves.Since mechanical valves provide a physical barrier to the flow of com-
unidirectional flow is the fundamental feature of valved pulse combus-
with hot flue gases, is another design consideration. These specific design
lightly. To ensure vigorous mixing of the fuel and air, the fully open flow
Mechanical valves: (a) Reed type and (b) flapper type.
© 2009 by Taylor & Francis Group, LLC
232 Advanced Drying Technologies
Valves that are not controlled through spring action but instead move
the valve must be able to withstand the maximum pressure differences imposed and yet open with only a small pressure difference. By opening
required to burn the charge.Generally, in gas-fueled pulse combustors, separate gas and air valves
are situated in their respective inlet pipes, thus set far away from the com-bustion chamber. In the case of pulse combustors that burn liquid fuels, fuel valves are generally not present; the liquid fuel is admitted down-stream of the air valve. In some systems, however, the fuel is premixed with air, and in such instances a thermal-protection device is required between the valve and the combustion chamber.
14.2.2.2 Rotary Valves
designed to seal the inlet section during the part of the combustion cycle when the pressure within the chamber is greater than the atmo-spheric pressure (i.e., during the positive pressure phase). The rotation of the valve determines the running frequency of the pulse combustor, and, therefore, an adequate feedback system is required to synchronizethe rotation of the valve with the resonant frequency of the burner. The
mined through the sensing and processing of pressure pulses occurring within the burner.
There are many advantages offered by rotary valves. Since the design of these robust valves is independent of the combustor design, one rotary valve is able to accommodate a wide range of combustor frequencies and
The general design of a rotary valve is composed of two plates: a motor-
slots that are 180° apart from each other. The air for combustion passes through the valve, perpendicular to the direction of rotation, and enters the burner. The height and width of the two slots on the stationary plate
of the rotary valve is based on three coaxially disposed sleeves with a plurality of slots in their lateral surface (Figure 14.3). When rotated by a motor with controllable speed, the slots periodically admit the intake air at the rate required for optimum combustion.
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firing rates. Other important features of rotary valves include durability,
freely are called flapper valves (Figure 14.4b). The basic design consid-erations are that the flappers must move rapidly enough to open and close the flow passages at the burner operating frequency. In addition,
in response to pressure differentials, flapper valves, in effect, control the amount of air flowing into the combustor during each cycle and do so in such a manner that the airflow is sufficient to meet the air/fuel ratio
Rotary valves are closely related to flapper valves in that they are both
speed of the valve, which influences the resonant frequency, is deter-
flexibility of use, and resistance to oil and dirt accumulation.
driven butterfly-shaped rotating plate and a stationary plate possessing two
determine the area for airflow through the rotary valve. The other design
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 233
While discussing the types of valves used in pulse combustors, a distinc-tion should be made between the words pulsed and pulse, which are often eused synonymously to describe combustors operated in a pulse mode.
device with a self-aspirating feature, and this effect is achieved as a con-
bustor is a device with cyclic but nonresonant combustion as dictated by wave events. Pulsed combustors usually operate at a frequency much lower than the natural frequency, often controlled by an ignition, fuel injection,
into a category of pulse combustors, whereas mechanically driven valves
or both should be categorized as pulsed combustors, unless the operation of a mechanical valve is controlled by resonant phenomena in a feedback mode. Such a design is known as a frequency-tunable pulse combustor.
One of the possible designs of a frequency-tunable pulse combustor takes advantage of the natural nonlongitudinal acoustic modes of the processing chamber to which the combustor is coupled (Zinn and Daniel,
the process chamber, the pulse combustor is tuned to one (or more) of
to regulate the exciting combustion gases from the process chamber also assists in tuning the combustor.
The frequency-tunable pulse combustor consists of a tube comprising a combustion zone and an exhaust zone. It is within the combustion zone that the reaction between fuel and air occurs, causing heat to be released and thereby exciting an acoustic wave in the combustor. Separate air and fuel inlets are present to supply the combustion zone with the necessary reactants. Rotary valves are used in the pulse combustion dryer to control
resulting hot combustion gases are freely exhausted from the combustor.The acoustics of the combustor may be altered in such a manner as to
provide a selectively variable (or frequency-tunable) pulsating combus-tion. There exist several methods by which the acoustic characteristics of the combustor may be altered to match the optimum frequency of the
and by moving it, the length of the combustor tube can be increased or decreased. Thus, the length of the wave within the combustor may be changed, thereby allowing for tuning. The length of the combustor can also be changed by means of the axially translatable back wall of the combus-tor. The location of the fuel injector may be adjusted to reposition the com-bustion zone to a more preferred location within the pulse combustor.
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According to Kentfield (1993), a pulse combustor is a combustion-driven
1988). To obtain the maximum benefit from the acoustic oscillations of
processing chamber. An axially translatable interfitting sleeve is present,
sequence of the internal unsteady flow events. In contrast, a pulsed com-
or a valve sequence. Therefore, valveless or flapper valve combustors fall
(e.g., rotary valves) used to control either air or fuel inflow, flue gas outflow,
these acoustic modes by modulating the flow of fuel to the combustion
acoustic resonance within the fuel line or by periodic interruption of the chamber. This modulation of fuel flow may be accomplished by exciting
fuel flow by use of a rotary valve. Placement of a valve in the exhaust flow
the flow of fuel and air into the combustion chamber (Lockwood, 1987). The
© 2009 by Taylor & Francis Group, LLC
234 Advanced Drying Technologies
14.2.2.3 Aerodynamic Valves
tion products out of the combustor through the inlet section. The main
parts, and therefore the risk of mechanical breakdown or failure is elimi-nated. This is a key consideration for heavy-duty pulse combustion burn-ers where the inlet section undergoes severe operating conditions.
The most common design concept allows the lowest possible resis-
be completely prevented. One mechanism known to limit the amount of
inlet that diverges gradually toward the combustion chamber initially accelerates the entering stream of air and subsequently diffuses it before it enters the combustion chamber; in such a manner, minimum losses are
and thus may serve to minimize it. Figure 14.5 presents, as an example,
designed at the University of Calgary. Details of the design of valves and
(Wu, 2007).
FIGURE 14.5
Fuel
Compressed air
Flue gas
Combustion chamber
Restrictor plate
Secondary flow ductIgniter plug
Air inlet plenum
Ejector
Compressor—outlet diffuser
Return bend
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advantage of aerodynamic configurations is lack of valves and moving
pulse combustors can be found elsewhere (Kentfield, 1993; Speirs, 1989; Olorunmaie and Kentfield, 1989). Table 14.1 presents the performance
Configuration of pressure-gain valveless pulse combustor designed at University of Calgary. (Courtesy of Kentfield J.A.C. With permission.)
Aerodynamic valves employ the fluid mechanical properties of specially designed inlets to act as a physical barrier to the backflow of combus-
tance to inflow while simultaneously maintaining the greatest possible resistance to outflow. An inlet arrangement such as this is often referred to as a fluid diode. Fluid diodes are, of course, inferior in performance when compared to mechanical one-way valves because backflow cannot
backflow is the presence of a nonuniform cross-sectional area. A tapered
incurred. Such a tapered inlet acts effectively as a nozzle during backflow
the schematic of a reverse-flow, pressure-gain, valveless pulse combustor
characteristics of the flapper valve pulse combustor fed with various fuels
© 2009 by Taylor & Francis Group, LLC
Pulse Com
bustion Drying
23
5
TABLE 14.1
Operational Characteristics of the Pulse Combustor Fed with Various Fuels
Items Propane Butane Methanol Ethanol Fuel Oil
Frequency (Hz) 116 113.8 114.1 120 112Peak-to-peak (negative/positive) pressure amplitude (Pa)
−5082/+6035 −5676/+6998 −5195/+6176 −6351/+6800 −5283/+6404
2094 2090 2075 2106 207016.55 17.15 21.30 15.69 17.67
0.1929 0.1983 0.4789 0.3077 0.2162Combustion heat (MJ/kg) 46.361 45.752 21.102 28.079 40.531Power input (kW) 8.95 9.07 10.11 8.64 8.76Average thrust (N) 0.4244 0.4453 0.5754 0.4018 0.4595
2250 1200 1310 2130Thrust output/power input (N/kW) 0.0474 0.0491 0.0569 0.0465 0.0524
CR
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Specific impulse (N s/kg) 2200
Average flue gas velocity (m/s)Average temperature in chamber (K)
Fuel flow rate (g/s)
© 2009 by Taylor & Francis Group, LLC
236 Advanced Drying Technologies
14.3 Pulse Combustion in Drying
In general, pulse combustion drying can be performed either within the gas jet exiting the combustion chamber or in a separate cavity (also called an applicator or a processing volume) that serves as a sound resonator. In
combustor and a drying chamber where the hydrodynamic action of pres-sure and velocity waves enhances drying rates (Buchkowski and Kitchen, 1993; Kudra et al., 1994, 2003; Mujumdar and Wu, 2004). Under certain con-
resonance. Because the primary function of a pulse combustor in this con-
pulse combustor may supply only a fraction (10–30%) of the heat required
spray, and other dryers. To excite enhanced pulsation in a drying cham-ber, the operating frequency of the pulse combustor must match the fre-quency of one of the natural acoustic modes of the process volume. When
cess is called resonance-driven (Zinn et al., 1991; Bramlette and Keller, 1989). The energy input from the pulse combustor to the drying chamber in the resonance-driven processes could be relatively low. It should, however, exceed a certain threshold value because of energy dissipation.
In practice, the downstream volumes do not affect the acoustic frequency of the pulse combustor provided that the tailpipe is long enough to secure
frequencies of the applicator depend on the chamber geometry and material load, and they may vary in time and space with the operating conditions (e.g., temperature, moisture content, and air humidity). Excluding a trial- and-error method in a pilot-scale apparatus, the best method to achieve resonance-driven processing is to use a frequency-tunable burner, which can actively follow the variations in the amplitude of pressure pulsation in the applicator (Technical Bulletin, 1992; Zinn and Daniel, 1988). Such aprocess known as a Sonotech’s pulse combustion technology based on the
and spray dryers and pilot-tested for water evaporation, limestone calci-nation, and waste incineration (Figure 14.6) (Ozer, 1993; Richards, 1994).
An alternative mode of pulse combustion drying is to disperse liquid
geous when drying biomaterials. Figure 14.7 presents a block diagram of the pulse combustion spray dryer designed by Pulse Combustion Systems (PCS) (Rehkopf, 2008). Here, the sound energy generated in a pulse com-
accelerates convective drying due to the extended surface area of the liquid spray (Keller et al., 1992; Ozer, 1993; Stewart et al., 1991; Ozer et al., 1993;
CRC_73877_Ch014.indd 236CRC_73877_Ch014.indd 236 12/24/2008 11:06:06 AM12/24/2008 11:06:06 AM
the later configuration, the typical pulse combustion dryer consists of a
ditions, these pressure waves can be favorably amplified by an acoustic
for evaporation, which opens retrofit applications to existing rotary, band,
this condition is satisfied, both pulsations are in resonance, and the pro-
frequency-tunable pulse combustor has been retrofitted to rotary kilns
bustor is used primarily for atomization of the liquid feed, which finally
figuration is to excite large-amplitude velocity/pressure fluctuations, the
the backflow of the combustion products. Conversely, the natural acoustic
material within a pulsating jet of flue gases, which is especially advanta-
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 237
Air
Naturalgas
Naturalgas
Air
Air
Naturalgas
Pulse-combustionburner
Main burner
Solidsfeeder
Afterburner
Rotary kiln
Transferduct
Ash bin
Secondaryburner
To air pollutioncontrol system
FIGURE 14.6Rotary kiln with frequency-tunable pulse combustor. (From Richards, M. K., Demonstration of the Sonotech, Inc., frequency-tunable pulse combustion system (CELLO pulse burner), SITE Program Fact Sheet, United States Environmental Protection Agency, Cincinnati, OH, April 1–4, 1994.)
Combustion Exhaust airair
Silencer
Bag filter
Fuel
Pulse combustor
Diluent air
Cooling air
Feed
Spraydryer
Product
Cyclone
Fines
FIGURE 14.7Pulse combustion spray drying system. (From Ozer, R. W., Proc. Powder and Bulk Solids Conference/Exhibition, Reed Exhibition Companies, Des Plaines, IL, 1993.)
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© 2009 by Taylor & Francis Group, LLC
238 Advanced Drying Technologies
Zbicinski et al., 1999). The liquid or pasty feed material is introduced into a gas stream from a pulse combustor in the feed chamber situated at the top of the drying chamber. The dryer and product collection system is held under a slight negative pressure by an exhaust fan to eliminate possible leakage. The gas temperature at the dryer inlet is controlled by the fuel feed rate to the combustor, whereas the outlet temperature is
the evaporative load. Normal operating temperatures for the pulse com-bustor are in the range of 400–1000°C. The frequency range extends from 60 to 200 Hz (typically 125–150 Hz), and the SPL is up to 180 dB inside the combustor. The maximum power output of one of the current units is 900 kW, which allows evaporation of up to 900 kg H2O/h with drying air sup-plied at 1800 kg/h (Rehkopf, 2008). This dryer can handle slurries (with solids up to 2 mm in diameter) with viscosity up to 100,000 cP as well as solutions with viscosity up to 50,000 cP. If desired, nozzles can be built in to preatomize highly viscous liquids. The particle-size distribution is
nozzles and varies from 100 nm to 100 µm. The residence time of the dis-
isms, for which the mortality is greatly reduced (Rehkopf, 2008).The pulse combustion spray dryer is better in handling corrosive and
abrasive materials because of no wear parts in the feed and atomization systems, unlike spray nozzles and rotary disks in conventional spray dry-ers. Narrower particle-size distribution is of interest to industries deal-ing with nanomaterials, metals, minerals, and ceramics, whereas gentle gas dynamic atomization allows drying of bioproducts and encapsulated materials (Rehkopf, 2008). Thus, some materials dried in the PCS pilot-scale pulse combustion dryer compare well against the products from a conventional spray dryer on product quality and unit cost bases, includ-
colorings, caramel, and acrylic latex (Rehkopf, 2008; Keller et al., 1992;
selected products are given in Table 14.2 (PCS), whereas Table 14.3 (PCS) compares the pulse combustion spray dryer designed by the PCS with the conventional one.
The advantage of a strong oscillating hot-gas jet from a tailpipe pro-motes the dispersion of pasty and particulate materials (Buchkowski and Kitchen, 1995b; Buchkowski, 1996, 1999; Buchkowski et al., 2001; Gray,
duct that extends from the tailpipe of the pulse combustor, and devices for material feed and discharge. The pulse combustor uses a self-actuated
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finer than that normally obtained in a spray dryer using conventional
drying of even thermally labile products such as beneficial microorgan-
ing antibiotics, vitamins, yeast, spices, vegetable protein, fibers, eggs, food
Kentfield, 1993). Performance data of the pulse combustor spray dryer for
dryer configuration, the unit consists of a pulse combustor, a pneumatic
adjusted by varying the material feed, which generally floats according to
persed material in the hot zone of flue gases is <5 ms, which allows the
1987; Duncan, 2008). Figure 14.8 shows flow diagrams of two prototype pulse combustion dryers designed by Novadyne Ltd. (Canada). In a flash
flapper valve, and its operating frequency is ∼70 Hz. The rated capacity of
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 239
the combustor equal to 300 kW with a turndown ratio of 3:1 offers an evap-orative capacity of ∼275 kg/h of water. Wet particulate materials such as
and dried intensively due to the high turbulence of the hot combustion gases and the large temperature difference. Finish-drying is accomplished
the motors tended to be smaller and some material handling equipment
TABLE 14.2
Comparison of Pulse Combustion Spray Dryer with Conventional SprayDryer (PCS) for Selected Materials
Material Conditions Spray Dryer PCS Pulse-Dryer
Antibiotics Inlet temperature (°C) 400 704Outlet temperature (°C) 127 104Product moisture content Base SameAtomization Rotary By pulseAir consumption Base 20% lessThroughput Base Up to 11% higher
Vegetables Inlet temperature (°C) 232 704Outlet temperature (°C) 82 88Product moisture content Base SameAtomization Nozzle By pulseAir consumption Base 30% lessFlavor Base Two to three times
Protein Inlet temperature (°C) 300 840Outlet temperature (°C) 82 90Product moisture content Base SameAtomization Nozzle By pulseAir consumption Base 25% lessProtein level Base Same
Acrylic latex Inlet temperature (°C) 204 760Outlet temperature (°C) 71 77Product moisture content Base SameAtomization Nozzle By pulseAir consumption Base 33% less
Whole eggs Inlet air temperature (°C) 260 871Outlet air temperature (°C) 79 82Product moisture content Base SameAtomization Nozzle By pulseAir consumption Base 40% lessProtein level Base Same
Source: Courtesy of Pulse Combustion Systems, San Rafael, CA, www.pulsedry.com. With permission.
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sawdust, hog fuel, corn fibers, or spent coffee grounds are injected directly into the tailpipe of the pulse combustor, where they are finely dispersed
in a pneumatic duct. A series of drying tests proved that the thermal effi-
more flavorful
ciency of a pulse combustion flash dryer is similar to that of conventional flash dryers, but electrical power requirements are lower by 40–50% as
© 2009 by Taylor & Francis Group, LLC
240 Advanced Drying Technologies
(e.g., slinger) was eliminated (Buchkowski, 1999). Table 14.4 provides a sample characteristic of a pulse combustion dryer for green hardwood sawdust and pulverized hog fuel.
drying chamber connected directly to the tailpipe of the pulse combustor
the thrust of the pulse combustor into the drying chamber, where most of the water evaporates. This makes it possible to take advantage of the
of the exhaust thrust and extreme turbulence and gas pulsation in the exhaust zone, which intensify heat- and mass-transfer processes. The gas–solid suspension from the drying chamber is then drawn to the cyclone
content takes place. In case of drying sludge or extremely wet particulates, a fraction of the dry product collected in a cyclone can be backmixed with
the material entering the dryer. For a higher degree of drying, the product from the cyclone can be returned to the drying column.
TABLE 14.3
Basic Differences between Classical and Pulse Combustion Spray Dryers
Dryer Characteristics PCS Pulse-DryerSpray Dryer
(Nozzle Distributor)Spray Dryer
(Disk Distributor)
Atomization Gas dynamic—no shear High-pressure Moderate shear
Feed pump Low pressure (∼7 kPa) High pressure Low pressureFeed deliveryto dryer
Open pipe Variouswheel types
Evaporative heat At pointof atomization
Remote, ducted todrying chamber
Remote, ducted todrying chamber
Mixing of hot air and droplets
Rapid and complete Slow Slow
Heat-transfer rate High Relatively low Relatively lowParts that may wear Feed pump, air valve Feed pump, nozzles Feed pump, rotary
drive unitParts that maycorrode
Feed pipe Pump, feed line, nozzles
Pump, feed line, wheel
Cost to replaceworn parts
Very low Higha Higha
Time to replaceworn parts
Hours Daysb Daysb
a Cost varies with product.b Usually require spare pumps or spare atomizer assembly.Source: Courtesy of Pulse Combustion Systems, San Rafael, CA, www.pulsedry.com. With
permission.
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orifice—high shear
Precision orifice
further modified by replacing the pneumatic duct with a pressurized
(Figure 14.8b). In this configuration, the material is forced vertically by
most beneficial aspects of the pulse combustor, that is, a high momentum
through a conveying duct, where water evaporation to the final moisture
As some materials require longer residence times, the flash dryer was
a feed to reduce the initial moisture content and improve the flowability of
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 241
TABLE 14.4
Test Data for Wood Waste Dried in a Pulse Combustion Flash Dryer
Throughput (wet material) 590 kg/hInitial moisture content 45–55% w.b.Final moisture content 10–20% w.b.Temperature in the tailpipe 1040°CExhaust gas temperature 104°C
306 m3/h1275 m3/h
Heat input 237 kWEvaporation rate 204 kg/hUnit heat consumption 4187 kJ/kg H2OElectrical energy consumption 0.019 kW/kg H2OBackmixing fraction 40%
Source: Courtesy of Novadyne Ltd., Hastings, ON, Canada. With permission.
Pneumaticduct
Product Product
Feed Feed
Air
Natural Naturalgas
Exhaust Exhaust
Pulse combustor
(a)
Pneumaticduct
Air
gas
Dryingchamber
Pulse combustor
(b)
Novadyne Ltd., Hastings, ON, Canada. With permission.)
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Flowsheets of the (a) pulse combustion flash dryer and (b) pulse combustion dryer. (From
Combustion airflow rateFlue gases airflow rate
© 2009 by Taylor & Francis Group, LLC
242 Advanced Drying Technologies
The dryer has provisions to recuperate heat by redirecting exhaust gases from the cyclone to the pulse combustor for cooling the tailpipe,
reduce drying gas temperature.
the case of sawmill residues, larger pieces of wood and bark (up to 50 mm
dryer can also handle sticky materials such as primary sludge no. 2, but mixing with sawdust or backmixing with dry product may be required for smooth operation. As seen in Figure 14.9, the unit heat consumption of
similar in operation. The heat consumption may be reduced by ∼10% as up to half of the fresh air could be replaced with hot recycled gases.
Figure 14.10 (Pulse Drying Systems Inc.) presents another design of a pulse combustion dryer (Pulse-Dryer®), which combines dispersion/drying in a
the system through an air column and is drawn into the combustor room
TABLE 14.5
Characteristics of Selected Feed Materials Tested in Novadyne Pulse Combustion Dryer
MaterialMoisture Content
(% w.b.)MaximumSize (mm) Description
1. Paper mill primarywood sludge no. 1
80 3 Fine, nonsticky granules
2. Paper mill primarywood sludge no. 2
68 20–25 Fine sticky particles forming clumps easily
3. Mixture of Liquid sludge mixed with sawdust in the ratio of 1:0.36 by weight. Appearance isthat of very wet sawdust
Paper mill secondary sludge and sawdust
85 0.560 5
4. P&P deinking sludge no. 1
52 25dewatered by a screw press.
break apart by hand5. P&P deinking sludge
no. 242 25 Similar to No. 4
6. Sawdust 44 5 Coarse mill sawdust with pieces of wood and bark upto 50 mm in length
7. Spent coffee grounds 60 0.05 Fine, nonsticky particles
Source: From Buchkowski, A. G., Pulse combustion dryer development for drying wood waste, EXFOR’99, January 21, Montreal, QC, Canada, 1999. With permission.
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Mixture of clay and fine fibers
Forms clumps difficult to
geneous feeds but also mixtures of fine and coarse particles (Table 14.5). For example, the 25 mm clay fiber lumps that are difficult to break apart by
in length) were carried through the dryer along with fine sawdust. The
tailpipe of the combustor and finish-drying/separation in a primary cyclone.
to preheat the combustion air, and to quench the primary flue gases to
The pulse combustion dryer is flexible enough to handle not only homo-
hand were largely disintegrated by the pulsating action of the flue gases. In
the pulse combustion dryer is in the same order as flash dryers, which are
Air for combustion (and optionally quenching flue gasses) is introduced into
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 243
and around the pulse combustor. The room can accommodate either one or more valveless pulse combustors operated at 250 Hz. Raw material is fed directly to the drying cone formed from a Venturi-shaped tailpipe of the combustor. Dry product and water vapor are blown from the drying
2.0
0
1.0
1.2
1.4
1.6
1.8
0.2
0.4
0.6
0.8
1 2 3 4 5 6 7Material (refer to Table 14.5)
Uni
t hea
t con
sum
ptio
n (k
Wh/
kg w
ater
)
Unit heat consumption by a pulse combustion dryer (Figure 14.8b). (From Buchkowski, A. G., Pulse combustion dryer development for drying wood waste, EXFOR’99, January 21, Montreal,9QC, Canada, 1999. With permission.)
Exhaust
Feed
Air
Product
Economizer
Pulse combustorPrimarycyclone
SpraytowerAir column
Fuel
Combustorroom
Dustcollector
FIGURE 14.10
www.pulsedrying.com. With permission.)
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Basic configuration of the Pulse-Dryer. (From Pulse Drying Systems Inc., Portland, OR,
© 2009 by Taylor & Francis Group, LLC
244 Advanced Drying Technologies
cone into the primary collector, where the heavier particles are separated
to the cyclone. Dry product falls out of the system through airlock valves, whereas vapors are drawn through the spray tower before being vented to the atmosphere. The required moisture content is maintained by the air vent temperature, which controls the material feed rate. The Pulse-Dryer
manure, sewage sludge, fruit and vegetable wastes, kaolin clay, and plating wastes. Particle size may vary from submicron to ∼6 mm, and moisture con-tent may be as high as 99% w.b. Regardless of the feed rate of the raw mate-rial, the system removes a nearly constant amount of water per hour from the feedstock, equal to ∼1225 kg/h per combustor. The single-module dryer shown in Figure 14.10 can accommodate up to two pulse combustors. The
with required capacity. Table 14.6 (Pulse Drying Systems Inc.) summarizes
TABLE 14.6
Characteristics of Pulse-Dryer for Industrial Plating Waste
Parameter Description
Composition Aluminum hydroxide 23% Iron hydroxide 19% Calcium hydroxide 25% Calcium ferrite 12% Othersa 21%Raw material feed 13,600–32,000 kg/day at 10–15% solids as
delivered to the dryer from two belt pressesDry product 1640–3900 kg/day at 10% moistureWater removed 12,000–28,800 kg/dayUnit heat consumption 3373 kJ/kgSystem size Two pulse combustion dryersOperators required One man per shiftFuel required Natural gasHeat consumption 7,380,000 kJ/hOperating hours 12–24 h/daySound level 70 dB when the complete system is housed in
a sound enclosureSpace requirements 4.8 m wide, 11 m long, 5.5 m highMaterials of construction 6.5–25-mm mild steel plateElectrical requirements 440 V, 3-phase, 60 Hz, 50 kWMaterial temperature (in primary cyclone) 52°CProduct temperature (mixture of materialfrom primary cyclone and dust collector)
38°C
Temperature of exhaust gases 74°C
a Nickel, copper, chromium, selenium, and zinc hydroxides plus insolubles made up of sili-cates, aluminates, and phosphates of iron and calcium.
Source: Courtesy of Pulse Drying Systems Inc., Portland, OR, www.pulsedrying.com. With permission.
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can handle products as diverse as fish meal, brewer’s yeast, whey, various
number of dryer modules can be increased to configure a drying system
by gravity. Vapor and fine particles continue to flow through the system
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 245
the typical operating characteristics of the Pulse-Dryer when used for indus-trial plating waste treatment.
The advantageous drying rates result from the impact of the sound pres-sure waves, which separate the surface moisture from the feed material by breaking the cohesion between the water molecules and the solid particles. This greatly increases the surface area of the disaggregated particles as well as causes rapid evaporation of water that was scrapped from the solid
sumption is claimed to be ∼3370 kJ/kg of water evaporated, but it may be
water, such as kaolin clay, metal oxides, or calcium carbonate (Duncan, 2008). Depending on the material to be dried, gas temperatures in the pri-mary cyclone are maintained ∼80 to 110°C or just above the dew point of the gas–vapor mixture passing out of the dryer. This thermal energy can be recovered from exhaust gasses in a gas-to-gas heat exchanger or in a direct gas-to-water heat exchanger (scrubber), providing hot water for wash-down or for use in other parts of the plant. The high temperatures on the exterior of the combustor chamber offer an optional radiant heat recovery. When the pulse combustor is equipped with a steam jacket, the unit is capable of producing live steam of ∼130 kg/h.
Because the residence time in the drying cone is measured in thou-sandths of a second and the exit temperature of most products averages 52°C, the risk of thermal degradation is practically eliminated. At the same time, however, much longer residence time in combination with relatively high temperatures in the primary cyclone ranging from 80 to 120°C creates conditions to reduce the content of pathogen microorgan-isms (Table 14.7). Also, the oxygen concentration in the dryer is <1%,
mizes product oxidation.
the one patented by Lockwood (1983). To avoid attenuation of the pulsat-
of a pulse combustor. Each ducting blade is inclined in the direction of
pass it along the top of the blades over the trailing edge. Streams of pul-
ized bed, each ducting blade includes a tapered skirt descending from
CRC_73877_Ch014.indd 245CRC_73877_Ch014.indd 245 12/24/2008 11:06:09 AM12/24/2008 11:06:09 AM
surface and atomized into many fine droplets. The average energy con-
as low as 2950 kJ/kg for finely dispersed materials having mostly surface
which practically eliminates the explosion and fire hazard and mini-
The radially spaced blades are fixed to the central hub at one end and
The only design of the pulse combustion fluid bed dryer appears to be
ing gas stream by a perforated gas distributor, the flue gases from a pulse combustor enter the bed of particulate material just above the solid floor, which rotates under a plurality of ducting blades adjacent to the floor.
to the inner annular baffle at the other (Figure 14.11). The space between the baffle and dryer wall forms a gas manifold connected to the tailpipe
floor rotation to provide a leading edge and a raised trailing edge located further above the floor that is the leading edge. The leading edge is suf-ficiently close to the floor to sweep the material off the rotating floor and
sating flue gases that flow upward from under the trailing edge fluidize the bed of particles. To improve the gas distribution through the fluid-
© 2009 by Taylor & Francis Group, LLC
246 Advanced Drying Technologies
more even distribution along the blade in the radial direction.
rotation due to the sloping of the blades and the tangential connection of the tailpipe with the gas manifold. Hence, the gas streams passing through the bed have both radial and tangential velocity components. This results in a waving and spiraling of the bed, which greatly reduces channeling and dead zones. To enhance the swirling motion, an auxiliary gas inlet is
fold for controlling the strength of the swirl. In addition, a pair of cyclone
TABLE 14.7
Characteristics of Pulse-Dryer for Sewage Sludge Processing
Parameter
Sludge A Sludge B
Before Drying After Drying Before Drying After Drying
Solids content(% mass)
12.4 90.6 3.3 88.8
Volatile solids(% mass)
41.4 53.4 47.1 55.9
TKN (mg/kg) 32,540 29,540 57,060 29,260NH3–N 8,559 2,117 21,100 2,530T–PO4–P 53,760 34,450 53,650 27,760Pathogens Salmonella Negative Negative Negative Negative Shigella Negative Negative Negative Negative Pseudomonas species (number/mL)
24,000 750 240 1,100
Total coliforms (number/mL)
>240,000 >30 9300 36
Escherichia coli(number/mL)
>240,000 >30 930 36
Unit heat consumption (kJ/kg of dry solids)
19,000 19,000 66,500 66,500
Design capacity Feed rate 160 t/day digested sludge dewatered by belt press to
20% solids Throughput 40 t/day at 80% mass solids Plant size 302,000 m3/day of activated sludge
Notes: NH3–N = ammonia–nitrogen; T–PO4–P = total phosphate phosphorus; TKN = total Kjeldahl nitrogen.
Source: Courtesy of Pulse Drying System Inc., Portland, OR, www.pulsedrying.com. With permission.
CRC_73877_Ch014.indd 246CRC_73877_Ch014.indd 246 12/24/2008 11:06:09 AM12/24/2008 11:06:09 AM
the trailing edge at an angle toward the floor. The skirt is widest near the baffle, tapers along the blade, and narrows to a point at the hub. The skirt restricts the dissipation of flue gases near the baffle and thus provides a
Although the incoming flue gases flow essentially from the peripheryof the floor toward its center, they also swirl concurrently with the floor
tangentially located at the dryer wall above the fluidized bed. An adjust-able bypass diverts the flow of gas from the auxiliary inlet to the gas mani-
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 247
Trailing edge
Skirt
Leading edge
Rotatingdeck
Hub
Inlet gas
Direction ofrotation
Material
Particleoutlet
Gas outletCyclonesupport plates
Gas manifold
Secondarygas inlet
Diverter
Hub
Paddle stirrer
ProductDrive
Rotatingfloor
Ductingblades
Dryerwall
Flexibleseal
Overflowdischarge
(a)
(b)
Flue
gas
FIGURE 14.11
4,395,830, 1983.)
CRC_73877_Ch014.indd 247CRC_73877_Ch014.indd 247 12/24/2008 11:06:09 AM12/24/2008 11:06:09 AM
Pulse combustion fluid bed dryer: (a) Design principle and (b) partial perspective of theducting blade. (From Lockwood, R. M., Pulse combustion fluidizing dryer, U.S. Patent
© 2009 by Taylor & Francis Group, LLC
248 Advanced Drying Technologies
su outlet enhances the swirling motion of the gases in the drying chamber.
Wet material is fed to the dryer through an inlet in the sidewall, and therefore the wet particles drop into an outer portion of the bed. Several stirring arms with inclined paddles spread the material uniformly around
To our knowledge, the design presented here is not yet commercialized.An analysis of the reported data shows that the productivity of industrial
because the drying time depends on the rates at which heat and moisture are transferred to and from the wet material, respectively. Since pulsation of
the application of pulse combustion in drying increases the productivity of such dryers by decreasing the required residence time in a drying chamber and thus allowing for more material to be dried in a given time.
Recently, the potential to enhance the impingement paper drying by a
jet was emitted from a valveless pulse combustor generating the pressure amplitude from 5.6 to 7.3 kPa at the frequency range from 150 to 160 Hz. The temperature levels at the tailpipe exit varied from 710 to 960K, depending on impingement conditions. Circular paper sheets with an average weight of 275 g/m2 and a diameter of 7.62 cm were water-soaked to ∼30% solidcontent. The wet sheet was placed on the 0.4 × 0.6 m impingement plate directly under the tailpipe and exposed to the impinging jet for various periods of time. Four impingement geometries were studied with respect to the nozzle-to-plate distance (H) and tailpipe diameter (D), that is, H/D =
ment. To quantify the effect of the pulsation jet, a steady jet was used at the
drying rates than the steady jet at H/D = 1, for which the enhancementratio was as high as 2.
Aside from decreased drying time, there are a few other advantages offered by pulse combustion drying systems. Such dryers are thermally
rials including low- and high-viscosity liquids and pastes (Figure 14.12). Since the residence time of the product in the drying section is extremely short, little chemical alteration of the product may be expected, and this fact enables drying of biomaterials and temperature-sensitive chemicals. Aside from enhanced heat- and mass-transfer rates due to velocity pulsation, the
gases in pulse combustion drying due to higher heat-transfer rates. This
CRC_73877_Ch014.indd 248CRC_73877_Ch014.indd 248 12/24/2008 11:06:10 AM12/24/2008 11:06:10 AM
exhaust gases carry fine particles through an outlet in the top of the dryer.
pulsating jet was identified by Liewkongsataporn et al. (2006). The pulsating
1 for unconfined impingement and H/D = 1, 2, and 3 for confined impinge-
tailpipe. Results showed that the pulsating jet yields significantly greater
efficient and seem to be effective in the drying of a wide range of mate-
results in an increased thermal efficiency, which translates into lower fuel
pport disks located above the overflow discharge and below the gas
the bed and facilitate fluidization by mechanical mixing. The dry prod-uct is removed from an overflow discharge through the floor. Swirling
dryers is increased if the drying is performed in a pulsating flow. This is true
flue gases tends to increase these rates of heat and mass (moisture) transfer,
same time-averaged mass flow rate and temperature as at the exit of the
larger fraction of input energy is transferred to the wet particles from flue
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 249
consumption. An overview of the available data indicates that, as compared to classical (continuous) drying, pulse combustion drying enables one to
Increase the drying rate by a factor of 2–3•Reduce unit air consumption by 30–40%• Eliminate property (e.g., temperature, concentration, and mois-• ture content) distribution within the dryer, which improves the product qualityLower gas and product temperatures during processing• Handle sticky materials that form lumps or aggregates without • mechanical mixing or disintegrationDisperse liquids, slurries, and suspensions without the need for a • disk atomizer or high-pressure nozzleLower air volumes discharged to the atmosphere• Eliminate the air blower from the system•
bustion drying is its contribution to environment protection. The rapid combustion allowing very little time for the formation of nitrogen oxides, and lower peak temperatures as compared to alternative methods of com-bustion, result in extremely low NOx emissions from the pulse combus-tors (Figure 14.13). This is especially advantageous in drying foods and
Filter cakes(cohesive)
Filter cakes(noncohesive)
Granulesand
powdersPastes(hard)
High-viscosityliquids
Low-viscosityliquids
Sludgesand softpastes
Fluid bedDrum
Rotary
SprayBandPneumatic
Pulsea
Spin flashAgitated
Recommended Applicable Special designa Based on the Pulse-Dryer performance data.
FIGURE 14.12Application chart for selected dryers.
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Besides the aforementioned advantages, an added benefit of pulse com-
© 2009 by Taylor & Francis Group, LLC
250 Advanced Drying Technologies
bioproducts, provided the pulse combustor is fed with no sulfur-contain-ing fuels such as propane or natural gas (Kudra, 1998, 1999).
Recent studies on pulse combustion indicate that the design of the com-bustion chamber and the geometry of a combustor–dryer system may affect the level of noxious emission. Noticeably, lower concentrations of CO, NO, and NOx were obtained for those combinations of the combustor volume and lengths of the tailpipe that provide smoother and sinusoidal
Concerning other gaseous emissions, the high temperatures of combus-tion gases when in contact with the wet material fed into the tailpipe may result in the destruction of odor, toxic contaminants, and other volatile organic compounds (VOC) by thermal dissociation or free radical oxi-dation reaction (Rafson, 1998; Guy et al., 1997). On the other hand, odor concerns are lessened in pulse combustion dryers, where wet material is injected directly into the tailpipe. This is because of an extremely short residence time (fractions of a second) as well as the effect of evaporative
to release volatile substances.
14.4 Numerical Simulation of Pulse Combustion Drying
Although pulse combustion dryers are in the market offer of several companies, the dryer design is largely based on experience. Scarce stud-ies on mathematical modeling of pulse combustion dryers focus on (a)
0
5
10
15
20
25
30
35
40
200 225 250 275 300175 325500
550
600
650
700
750
700
725
750
775
800
825
850
Heat rate (kW)
Flow
rate
(m3 /h
)
Emiss
ions
(% o
r ppm
)
Tem
pera
ture
(°C
)
Excess air (%) NOx (ppm)
CO (ppm)CO2 (%)
O2 (%)
Exhaust gasflow rate
T
FIGURE 14.13Typical emissions from pulse combustor. (From Novadyne Ltd., Hastings, ON, Canada. With permission.)
CRC_73877_Ch014.indd 250CRC_73877_Ch014.indd 250 12/24/2008 11:06:11 AM12/24/2008 11:06:11 AM
cooling, which keeps the product temperatures sufficiently low so as not
pressure fluctuations (Zbicinski et al., 1999).
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 251
processes occurring in a pulse combustor and (b) processes taking place in a drying chamber, mostly in the spray dryer. Examples of the former are numerical simulation of a valveless pulse combustor (Olorunmaie and
(Fureby and Lundgren, 1993). Another example is the study by Kuts et al. (2002), who developed a mathematical model for gas dynamics in a valve-less pulse combustor. Numerical solution of this model for various operat-ing parameters and combustor geometries documented that the temporal variations of the gas pressure and velocity represent sinusoidal plots with a phase shift. The calculated gas velocity and temperature at the outlet from the combustor can be used as input parameters for drying process calculations.
ulation of the combustion process in a gas/air premixed, mechanically valved, Helmholtz-type pulse combustor has been carried out to investi-
the resulting pulsed combustion (Wu, 2007; Wu and Mujumdar, 2008a).
the valve inertia, valve gap, and other design/operating characteristics, thus to reveal how the valve dynamics affects the combustor performance (Wu, 2007). Because the modeling of the pulse combustor is beyond the scope of this chapter, interested readers are referred to the study by Wu (2007).
With respect to modeling and simulation of transport processes in a dry-ing chamber coupled to the pulse combustor, the results of the pioneering studies can be found in the papers by Strumillo et al. (1999), Zbicinski et al. (1999, 2000, 2001), Zbicinski (2002), Wu and Liu (2002a, 2000b), and Mujumdar and Wu (2004).
tion has been examined both numerically and experimentally in view of potential application to paper drying (Wu and Mujumdar, 2005; Wu et al., 2006; Wu, 2007). The tailpipe of the small-scale pulse combustor (110 mm long chamber having 44 mm diameter) was 22 mm in diam-
leum gas (LPG) at a rate of 0.055–0.12 m3/h. The pulse combustor (PC) exhaust gas temperature varied from 540 to 600°C, and its mean veloc-ity ranged from 4.33 to 5.12 m/s. First, a PC pulsating jet impinging on
characteristics, and then the drying of a paper sheet was carried out in a specially designed facility to determine the drying kinetics of this novel drying process.
The transport phenomena inside the pulse combustion impingement zone were simulated using a CFD model at an ambient air temperature of 27°C. Figures 14.14 and 14.15 present the respective transient temperature
CRC_73877_Ch014.indd 251CRC_73877_Ch014.indd 251 12/24/2008 11:06:12 AM12/24/2008 11:06:12 AM
Kentfield, 1989) and one-dimensional models for pulsation combustion
The results confirmed that CFD modeling with the use of a dynamic
Recently, the pulse combustion drying in the impinging jet configura-
eter and 460 mm in length. The combustor burned the liquefied petro-
Using computational fluid dynamics (CFD) modeling, an extensive sim-
gate the dynamics of the flame structure, flow-chemistry interaction, and
mesh is suitable for a parametric study of the flapper valve by modifying
a flat ceramic plate was used to evaluate the convective heat-transfer
© 2009 by Taylor & Francis Group, LLC
252 Advanced Drying Technologies
2
1
1
1
1
1
9
9
2
2
3
2
2
3
3
3
3
0/4 Cycle
1/4 Cycle
3/4 Cycle
2/4 Cycle
4/4 Cycle
1355
2410
3465
4520
5575
6630
7685
8740
9795T (K)
No
Axis of the pulsecombustion gas jet
Impingement plane
FIGURE 14.14
H/D = 1.27. (From Wu, Z. H., Mathematical modeling of pulse combustion and its applica-tions to innovative thermal drying techniques, Ph.D. Thesis, Dept. Mech. Eng., National University of Singapore, Singapore, 2007. With permission.)
CRC_73877_Ch014.indd 252CRC_73877_Ch014.indd 252 12/24/2008 11:06:12 AM12/24/2008 11:06:12 AM
Transient temperature field in the free-impinging zone of a pulse combustion gas jet gas at
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 253
pulsating gas jet is a circle with a diameter of about three times the tail-
(impingement plane), the gas jet is intensively mixed with the ambi-ent air. The entrainment of ambient air causes a toroidal vortex to be formed.
0/4 Cycle
1/4 Cycle
2/4 Cycle
3/4 Cycle
4/4 Cycle
FIGURE 14.15The velocity vectors in the free-impingement zone of a pulse combustion gas jet at H/D = 1.27. (From Wu, Z. H., Mathematical modeling of pulse combustion and its applica-tions to innovative thermal drying techniques, Ph.D. Thesis, Dept. Mech. Eng., National University of Singapore, Singapore, 2007. With permission.)
CRC_73877_Ch014.indd 253CRC_73877_Ch014.indd 253 12/24/2008 11:06:12 AM12/24/2008 11:06:12 AM
The simulation results were confirmed by experimentally determined heat-transfer coefficients at various nozzle-to-plate spacing (defined by
and velocity vectors (Wu, 2007). Clearly, the region of influence of the
pipe diameter. Also, as the pulsating jet propagates toward the flat plate
© 2009 by Taylor & Francis Group, LLC
254 Advanced Drying Technologies
the H/D ratio) against the nondimensional radial distance (r/D), where
from a maximum of 126 W/m2 K at the stagnation point down to neg-ligible values at r/D = 3. In the region of r/D from 0 to 1, convection
Figure 14.16. In the region of r/D from 1 to 3, the ambient air entrain-ment and the impinging toroidal vortex are the dominant factors affect-ing heat transfer. The entrainment of ambient air enhances with the plate
observed in this region.Spray drying of a concentrated common salt (NaCl) solution in the
pulse combustor has been simulated using a CFD model, and parametric
ters on the drying characteristics (Wu, 2007). Numerical results provided
ity distribution as well as heat- and mass-transfer characteristics in the drying chamber, which are otherwise hard to determine experimentally.
toward the outlet along the central axis. The large recirculation zone is formed near the inlet of the drying chamber, and its size and position vary
3
Z. H., Mathematical modeling of pulse combustion and its applications to innovative ther-mal drying techniques, Ph.D. Thesis, Dept. Mech. Eng., National University of Singapore, Singapore, 2007. With permission.)
Nondimensional radial distance
Hea
t-tra
nsfe
r coe
ffici
ent (
W/m
2 K)
0 1 2 3 40
20
40
60
80
100
120
140
160
H/D = 1.27H/D = 2.96H/D = 4.68H/D = 6.36
CRC_73877_Ch014.indd 254CRC_73877_Ch014.indd 254 12/24/2008 11:06:14 AM12/24/2008 11:06:14 AM
FIGURE 14.16
r = 0 denotes the stagnation point. The heat-transfer coefficient varied
thus a high convective heat-transfer coefficient is obtained, as shown in
exit plane distance. Thus, a low convective heat-transfer coefficient is
within the spray drying chamber varies significantly during one pulse
Influence of H/D on convective heat-transfer coefficients when pulse combustion exhaust impinging on a flat ceramic plate with fuel flow rate of 0.1 m /h LPG (Re = 1407). (From Wu,
near the stagnation point is governed by the impinging jet flow, and
intense oscillating high-temperature turbulent flow generated by the
studies were carried out to examine the influence of operating parame-
detailed information on the flow pattern, gas temperature, and humid-
Figure 14.17 shows that the predicted cyclically stable gas flow pattern
cycle from 0.254 to 0.264 s. The main gas flow can be seen to be moving
© 2009 by Taylor & Francis Group, LLC
Pulse Com
bustion Drying
25
5
0.254 s 0.262 s
0.263 s
0.264 s
0.265 s
0.266 s
0.255 s
0.256 s
0.258 s
0.261 s
xNumber
(kg/s)1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0.001 0.003 0.005 0.007 0.009 0.011 0.013 0.015 0.017 0.019 0.022 0.024 0.026 0.028 0.030 0.032 0.034 0.036 0.038 0.040
12
9
3
9
2
7
3
8 7
5 412
15
16 15
13
13
12
12 11 1010
7 6
98
10
7 7
23
5
5
11 108 8 6 4
8
14
14
18
3
19
15
17 17
13 12
16 14 1312 10 912
73
g X
5
11 10 9 84
14
13 16
8
4
4
4
13
5
9
1618
11 10 617
1615 15
1513 17 18 1516 14 13 1311 9
6 7
3
16
171820 13 12 16 17 16 14 13 1210 8
5 72
12
43
1315
14 13
9
14 11 12 13 1210
9 8 6 5 4 32
1
10
13
14 14 13
1211 10 9 7
4 53
2
1516
18
11 10 11 10 6 8
7 6
5 4 3 21
R R
14
FIGURE 14.17
Mathematical modeling of pulse combustion and its applications to innovative thermal drying techniques, Ph.D. Thesis, Dept. Mech. Eng., National University of Singapore, Singapore, 2007. With permission.)
CR
C_73877_C
h014.indd 255C
RC
_73877_Ch014.indd 255
12/24/2008 11:06:14 AM
12/24/2008 11:06:14 AM
Gas flow patterns (streamline) in the spray drying chamber at various times during a pulse combustion cycle (0.254–0.266 s, 83 Hz). (From Wu, Z. H.,
© 2009 by Taylor & Francis Group, LLC
256 Advanced Drying Technologies
mass transfer between the dispersed and continuous phases. Also, it was found that (a) high drying rates and short drying times characterize the pulse combustion spray drying process; (b) gas temperature is the most important parameter governing the drying rate; (c) both pulse amplitude and frequency affect drying performance, mainly through changing the
amplitudes.A new concept on pulse combustion–based spouted bed drying of par-
ticulates proposed by Mujumdar was further examined by Wu (2007). Here, the high-temperature pulsating jet emerging from a pulse com-
(Wu and Mujumdar, 2007, 2008b). Parametric studies were carried out for different operating conditions and for the special case of the pulsat-ing spouting jet. Some typical phenomena observed during experiments
Characteristically, the shape and size of a large bubble and an internal void change during the pulse cycle. The bubble generation can be explained by the oscillation of the spouting gas. The large bubble may intensify the
mixing of the gas and particles. Numerical results also show that (a) high
contributes to better mixing of gas and particles and (b) the bubble size decreases with an increase in the pulse frequency. When the pulse fre-quency exceeds a certain value (10 Hz in this case), the bubble size is neg-
highly computing-intensive. Although it is possible to make such predic-
tool for design or scale-up of spouted beds.In summary, it should be stated that the CFD models of the pulse com-
bustion process and the pulse combustion dryers are indeed valuable tools that can be used to design and analyze the pulse combustion sys-tems. Current efforts are directed at examining a wide variety of combus-tion chamber designs to determine the optimal shape for reduced noise,
Comprehensive information on pulse combustion dryers at various stages of development including industrial dryers, applications, R&D challenges, and similar issues can be found in the recent compilation by Kudra (2008).
CRC_73877_Ch014.indd 256CRC_73877_Ch014.indd 256 12/24/2008 11:06:15 AM12/24/2008 11:06:15 AM
near the inlet part, and this phenomenon is to be intensified at high pulse
others were confirmed through simulation. Figure 14.18 illustrates how
tions at a reasonable level of confidence, it is still not a useful engineering
increased combustion efficiency, and reduced emissions.
within the pulse combustion cycle (0.012 s, 83 Hz). Oscillations of the flow
ing frequency (83 Hz) promote intensive mixing and augment heat and pattern that reflect the natural frequency of the pulse combustor operat-
flow patterns in a drying chamber; and (d) droplet deposits can be formed
bustor acts as the spouting gas. The flow of gas and particles in such a spouted bed dryer was predicted using the two-fluid modeling approach
such as the bubble formation, surface disturbance, flow instabilities, and
the predicted flow patterns in the spouted bed vary during a pulse cycle.
gas–particle flow field in the spouted bed and may contribute to better
pulse amplitude intensifies the unstable flow field in spouted beds and
ligible. The prediction of a detailed flow pattern in the spouted bed is
© 2009 by Taylor & Francis Group, LLC
Pulse Combustion Drying 257
FIGURE 14.18
ms + 0.5 ums ×sin (2π × 2 × t), H = 0.325 m, ds = 1.41 mm, ρs = 2500 kg/m3, ums = 0.54 m/s). (From Wu, Z. H., Mathematical modeling of pulse combustion and its applications to innovative ther-mal drying techniques, Ph.D. Thesis, Dept. Mech. Eng., National University of Singapore, Singapore, 2007. With permission.)
Particle mass fraction (kg/kg)
2/10 Cycle1/10 Cycle
6/10 Cycle5/10 Cycle4/10 Cycle
10/10 Cycle9/10 Cycle8/10 Cycle7/10 Cycle
3/10 Cycle
0/10 Cycle0.010.050.090.130.180.220.260.300.340.380.420.470.510.550.59
CRC_73877_Ch014.indd 257CRC_73877_Ch014.indd 257 12/24/2008 11:06:15 AM12/24/2008 11:06:15 AM
Transient flow patterns in the spouted bed under a pulsating jet (u = 1.1 u
© 2009 by Taylor & Francis Group, LLC
258 Advanced Drying Technologies
Symbols
ds Particle diameter, mmdB DecibelD Tailpipe diameter, mg Gravity, NH Distance (height), mL Length, mr Radial distance, mR Radius, mt Time, sT Temperature, Ku Velocity, m/sums Minimum spouting velocity, m/sw.b. Wet basisX Coordinateν Viscosity, m2/sρs Particle density, kg/m3
Re Reynolds number (uD/ν)ν
Acronyms
CFD Computational Fluid Dynamics
P&P Pulp and paperSPL Sound pressure levelVOC Volatile organic compounds
References
Bramlette, T. T. and Keller, J. O. 1989. Acoustically enhanced heat exchange and drying apparatus. U.S. Patent 4,805,318.
Combustion’96. Ottawa, ON, Canada.Buchkowski, A. G. 1999. Pulse combustion dryer development for drying wood
waste. EXFOR’99. January 21, Montreal, QC, Canada.Buchkowski, A. G. and Kitchen, J. A. 1993. Drying wood waste with a pulse com-
bustion drying. Proc. First Biomass Conference of the Americas. Vol. 1. NationalRenewable Energy Laboratory. Golden, CO, Burlington, VT, pp. 730–735.
Report. Novadyne Ltd., Hastings, ON, Canada.Buchkowski, A. G. and Kitchen, J. A. 1995b. Drying and burning wood waste
using pulse combustion. The Second Biomass Conference of the Americas. Portland, OR.
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LPG Liquefied petroleum gas
Buchkowski, A. G. 1996. The development of a pulse combustion flash dryer.
Buchkowski, A. G. and Kitchen, J. A. 1995a. Pulse combustion flash dryer. Technical
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Pulse Combustion Drying 259
Buchkowski, A., Nalouin, L. -M. and Gosselin, D. 2001. Disposal of sludge in bark boiler using a pulse combustion atomizer. Proc. 87th PAPTAC Annual Meeting. January 29–February 1, Vol. C. Pulp and Paper Technical Association of Canada, Montreal, QC, Canada, pp. C97–C100.
Duncan, S. 2008. Technical Information. Pulse Drying System, Inc., Portland, OR.Fureby, C. and Lundgren, E. 1993. One-dimensional models for pulsating combus-
tion. Comb. Sci. Technol., 94: 337–351.Gray, R. R. 1987. Pulse combustion drying apparatus for particulate materials.
U.S. Patent 4,695,248.Guy, C., Benali, M. and Ostigny, E. 1997. Free radical oxidation process and instal-
Patent 5,641,412.Keller, J. O., German, R. S. and Ozer, R. W. 1992. Fundamentals of enhanced
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York, pp. 191–235.Kitchen, J. A. 1987. Pulse combustion apparatus. U.S. Patent 4,697,358.Kudra, T. 1998. Natural gas—fuel of choice in industrial drying. Proc. 11th
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nant driving to increase the productivity and thermal efficiency of industrial
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263
15Heat Pump Drying
15.1 Introduction
recovery. Their ability to convert the latent heat of vapor condensation into sensible heat of an air stream passing through the condenser makes them attractive in drying applications especially when combined with the abil-ity to produce well-controlled drying conditions. For these reasons, heat pump drying has been used for decades in wood kilns to dehumidify air and control lumber quality (Chou and Chua, 2007). Following the gen-eral trend to improve product quality and reduce energy consumption,
which has resulted in the rapid growth of both theoretical and applied research on heat pump drying (Table 15.1).
15.2 Principle of Heat Pump Operation
The principle of the heat pump is based on the same thermodynamic cycle as that used in the refrigeration technique. The term heat pump originates from the operation of a standard heat pump, where the low-temperature heat from the heat source is upgraded by bringing it (pumping) to the higher temperature in a heat sink. Such energy valorization is done at
in a closed loop of the thermodynamic cycle. An alternative design to the compressor-driven heat pump is the absorption heat pump, in which the function of the mechanical compressor is performed by the so-called chemical compressor driven by the sorption phenomena (Furutera et al., 1996; Labidi et al., 1996). Hybrid technologies such as radiofrequency- assisted heat pumps are presented elsewhere in this book.
Another variant of the classical heat pump is the chemical heat pump (CHP), in which upgrading of the thermal energy is due to the heat of a chemical reaction. This special type of heat pump–drying technology
heat pump comprises four major components: a compressor, a condenser,
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For years, heat pumps have been known as an efficient method of energy
many researchers have acknowledged the specific features of heat pumps,
will be presented later in this chapter. The general configuration of a
the cost of input energy used to compress and drive the working fluid
© 2009 by Taylor & Francis Group, LLC
264 Advanced Drying Technologies
an evaporator, and an expansion (throttle) valve. These elements form
evaporator and condenses in the condenser, thus bringing heat that passes through the evaporator and condenser to a higher temperature level. In
dryer forms a part of the heat recovery system. Following mechanical compression, this vapor is condensed in a heat exchanger, giving up heat for further evaporation of water while the condensate is taken out of the heat pump system.
The operation of the heat pump is illustrated in Figure 15.1a, which pres-
recover the latent heat from dryer exhaust air. Here, the fraction of humid air (A) leaving the dryer is cooled in an evaporator to a temperature TB (B) close to Tev, which is below the dew point. Consequently, part of the moisture from this air stream is condensed, and heat recovered in this
moisture-reduced air is mixed with the remaining fraction of humid air from the dryer (C). With the addition of the external work provided by
TABLE 15.1
Recent Studies in Heat Pump Drying
Source Location Application(s)
Chou et al. (1998),Chua et al. (1998)
Singapore Agricultural and marineproducts (mushrooms, fruits,sea-cucumber, and oysters)
Carrington, Barneveld et al. (1996), Carrington, Bannister et al. (1996), Sun et al. (1996)
New Zealand Timber and wood drying
Prasertsan et al. (1997),Prasertsan and Saen-Saby(1998)
Thailand Agricultural food drying (bananas)
Mason and Blarcom (1993) Australia Macadamia nutsMeyer and Greyvenstein (1992) South Africa GrainsRossi et al. (1992) Brazil Vegetable (onion)Nassikas et al. (1992) Greece PaperStrømmen and Kramer (1994) NorwayHawlader et al. (2006a, 2006b) Singapore Ginger, guava, papayaAlves-Filho and Ross (2006) Norway and Ireland General studySieniutycz (2006) Poland General studySakar et al. (2006a, 2006b) India General studyClaussen et al. (2007) Norway Potato proteinSunthonvit et al. (2007) Australia Nectarinesvan der Pal et al. (2007) New Zealand WoodZhang et al. (2007) China Wood
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Marine products (fish)
ents a schematic diagram of the heat pump dryer-dehumidifier used to
the open-cycle heat pump, the process fluid such as water vapor from a
way causes the working fluid to boil in the evaporator. The cooled and
the compressor, the vaporized working fluid increases its pressure and
a closed cycle for the process fluid (e.g., freon), which evaporates in the
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 265
temperature. It is then condensed in the condenser, thus transferring heat to the air mixture (C) and raising its temperature to TD (D), which is below
Cair is directed to the drying chamber. The thermodynamic cycle for the air stream in a Mollier chart is shown in Figure 15.1b.
To dryer
Fromdryer
Compressor
Condenser
Expansion valve
Liquidtank
Evaporator
B
C
D
A
A
D
A
C
B
B
Tev
TB
TC
TA
TD
TC
RH = 10%
RH = 50%
RH = 100%
YB YCD YA
Absolute humidity (kg/kg)
Enth
alpy
(kJ/k
g) I = constant
DA—DryingAB′—CoolingB′B—CondensationBC + AC—MixingCD—Heating
Condensate(a)
(b)
FIGURE 15.1
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the condensation temperature T . Finally, the heated and dehumidified
Schematic of the (a) heat pump dryer-dehumidifier and (b) air cycle on the Mollier chart.
© 2009 by Taylor & Francis Group, LLC
266 Advanced Drying Technologies
mance (COP) as
COPuseful heat output
power input (15.1)
COP
TT TCarnot
condenser
condenser evaporator
(15.2)
If the condenser temperature is 350K and the evaporator temperature is 300 K, for example, then the COP is 7. This means that the input of 1 kW of mechanical energy to the heat pump recovers 7 kW of thermal energy.
SMER
amount of water evaporatedenergy used
(kg kWh) / (15.3)
Alternatively, the reciprocal of the SMER is reported as the heat pump dryer
The SMER depends on the maximum air temperature in the dryer, the air’s relative humidity, the evaporation and condensation tempera-
some SMER values for a typical range of temperature differences in heat pump dryers plotted against the air inlet temperature (Song, 1990; Strømmen and Kramer, 1994). Characteristically, the relatively high amount of removed water per unit of input energy at elevated tempera-tures decreases dramatically with temperature drop, mostly because of the low COP of the refrigerator. Therefore, higher drying temperatures should be used unless temperatures below the freezing point are required for quality reasons.
The energy consumed by fans (EF) used to force air through the drying
fraction of the total energy consumption. In such a case, the following
COPQ
E Eo
F
*
(15.4)
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The efficiency of a heat pump is defined by the coefficient of perfor-
The maximum theoretical heat pump efficiency is determined by the Carnot efficiency as
Heat pump efficiency that is in the range of 40–50% of the theoretical Carnot efficiency can be achieved in practice.
For drying, a more appropriate efficiency parameter is the specific mois-ture evaporation (extraction) rate (SMER) defined by
efficiency (HPDE), which is the energy required to remove 1 kg of water.
tures, and the efficiency of a refrigeration system. Figure 15.2 presents
system is not included in the classical definition of the SMER. In low-tem-perature drying, however, the energy for driving fans can be a significant
definition of the COP is recommended (Kudra and Strumillo, 1998):
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 267
Thus, energy required to remove the unit mass of water is given by the
HPDEH YCOP*
1SMER*
/( )
The theoretical maximum SMER for conventional thermal drying is1.55 kg/kWh (based on the latent heat of water vaporization at 100°C).A typical SMER value achieved by a heat pump is 3 kg/kWh, which com-pares very favorably with conventional convective drying, for which val-ues ranging from 0.5 to 1 kg/kWh are standard. A summary of moisture
that heat pump dryers are about 10 times as effective as traditional drying systems, such as vapor-recompression dryers and hot-air dryers.
Saensabai and Prasertsan, 2003.*
humidity control in the heat pump dryer, a more advanced heat pump system can be incorporated for drying. Figure 15.5 shows an example of
* In other papers the name of this author is spelled as Saen-Sabay.
Drying temperature (°C)
SMER
(kg
wat
er/k
Wh)
–15 −5 5 15 25 35 450
0.5
1.0
1.5
2.0
2.5
3.0
Kramer, K., Drying Technol., 12(4), 889–901, 1994. With permission.)
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Specific moisture extraction rate versus inlet air temperature. (From Strømmen, I. and
HPDE defined as
removal efficiency for selected dryers is shown in Figure 15.3. It is clear
The heat pump and the dryer can be assembled in various configura-
five basic configurations of HPD (Figure 15.4) can be found in the paper by
To further improve energy efficiency and refine temperature and
a two-stage heat pump in a parallel configuration where the evaporator of the first heat pump operates at a higher pressure level than that of the second heat pump. In terms of energy efficiency, the two-stage heat pump
tions depending on air and energy flow. The performance evaluation of
© 2009 by Taylor & Francis Group, LLC
268 Advanced Drying Technologies
10.0—heat pump dehumidificationat 40°C and 90% RH (Geeraert, 1975)
8.8—heat pump dehumidification at40°C and 80% RH (Geeraert, 1975)
4.5—steam compression dryer (Lopez-Cacicedo, 1980)
0
2
4
6
8
10SM
ER (k
g/kW
h)
0.3—conventional drying of oak (Houpert, 1978)0.3—conventional drying of malt (Guyon, 1980)
FIGURE 15.3Effectiveness of various drying systems.
E
C D
E
C D
E
C D
Ex.Cool.
Ex.Cool.
E
C D
E
C D
Ex.C.
Air
Air
FIGURE 15.4
external condenser, and Ex.Cool.—external cooler.
allows the recovery of much more of the latent energy from the humid air than the single-stage heat pump (Figure 15.6).
One of the unique features of the heat pump dryer is its ability to oper-ate in a closed cycle with precise control over the thermal environment of the system. With advanced proportional-integral-derivative (PID) control-lers, it is possible to regulate the air temperatures at different stages of the
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Basic configurations of heat pump dryers. C—condenser, D—dryer, E—evaporator, Ex.C.—
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 269
Drying chamber
Vapor-mixingchamber
High-pressureevaporator
Low-pressureevaporator
Condenser
Air path Refrigerant path
Compressorwork
Expansionvalves
CondensateCondensate
Schematic of a closed-loop, two-stage heat pump dryer.
Single-stage, electric-motor-drivenheat pump dehumidifier
Single-stage, electric-motor-driven heatpump dehumidifier with subcooling
Single-stage, gas-engine-drivenheat pump dehumidifier
Single-stage, gas-engine-drivenheat pump dehumidifier with subcooling
Two-stage, gas-engine-drivenheat pump dehumidifier with subcooling
0 2 4 6 8Coefficient of performance
Hea
t pum
p co
nfigu
ratio
n
FIGURE 15.6Feasibility study of heat pump drying of barley at 66°C and 5% RH. (Adapted from Perry, E. J., Drying by cascaded heat pumps, Institute of Refrigeration Management, pp. 1–8, 1981.)
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© 2009 by Taylor & Francis Group, LLC
270 Advanced Drying Technologies
drying process to follow the optimum operating conditions. In the case of a two-stage heat pump dryer, two control circuits have been proposed
rator for optimal recovery of both the sensible and the latent heats. With respect to the ambient air, heat pump dryers are able to produce both cold
the drying process. Thus, it is possible to set different combinations of dry-ing conditions to maximize the performance of the system and improve
can be set in a two-stage heat pump dryer (Chua et al., 1998).It should be noted that the mechanical work to run the compressor is the
cost of the electric energy needed to drive the heat pump might be higher
depends largely on the relative costs of electricity and thermal energy.
0 50 100 150 200 250 300 350Drying time (min)
0
10
20
30
40
50
30°C
25°C
Cosine temperature waveformReversed cosine temperature waveformSquare temperature waveform at ~25°C
Square temperature waveform at ~30°C
Tem
pera
ture
(°C
)
FIGURE 15.7
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regulates the mean coil surface temperature of the high-pressure evapo-
(Chua et al., 1998). The first one uses the air-bypass through the evapora-tors to control the degree of dehumidification, whereas the second one
and hot drying conditions with specified humidity at different stages of
product quality. Figure 15.7 shows some of the temperature profiles that
only energy supplied to the heat pump. Therefore, the thermal efficiency of the HP system defined by the COP is higher than 100%. However, the
than the value of the heat recovered. Therefore, the profitability of the device
Temperature profiles in a two-stage heat pump dryer.
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 271
The economics of the heat pump systems can be improved when combined with other heat sources such as natural gas or fuel oil. Figure 15.8 shows a coupled heat pump–gas engine used for heating of a malt house and for dry-ing of malt (Newbert, 1985). The application of this system reduced energy consumption by 40%. In general, the premium value of a higher-quality prod-uct that a heat pump may produce should also be taken into consideration when evaluating the economics and payback times for a heat pump dryer.
Because of economic and technical limitations, heat pumps have so far been used in special applications only, for example, in drying of timber and ceramics that are easily damaged if subjected to high drying rates and for which the more gentle drying obtained at lower temperature with
ever, growing interest is noted for heat pump drying of foods and bioma-terials where low-temperature heat and well-controlled drying conditions
Exhaust air toatmosphere
Evaporator
Compressor
Condenser
Expansionvalve
Pump
Gas engine
Engine exhaust gasheat exchanger
Exhaustgas
P P
P
Fresh air
Water/glycol Refrigerant
Exhaust air heatexchanger
Gas in
Gas engineradiator
Run-around coilheat exchangerCondenser
coil
Hot airto kiln
Air frommalt kiln
FIGURE 15.8Schematics of the coupled heat pump–gas engine system.
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dehumidified air is advantageous (van der Pal et al., 2007). Recently, how-
© 2009 by Taylor & Francis Group, LLC
272 Advanced Drying Technologies
are required to stabilize or enhance product quality (Hawlader et al.,2006a, 2006b; Sunthonvit et al., 2007; Claussen et al., 2007). The higher cost of heat pump drying may be more than offset by the premium price the consumer is willing to pay for the higher quality. The key advantages and limitations of heat pump dryers are as follows:
Advantages
ulting in lower energy consumed for each unit of water removed.
product requirements.A wide range of drying conditions typically from –20 to 100°C • (with auxiliary heating) is feasible.Consistent output of the product.• Excellent control of the environment for high-value products and • reduced electrical consumption for low-value products.Suitable for both high- and low-value products.• Aseptic processing is possible.•
Limitations
Auxiliary heating may be required for high-temperature drying •due to the critical pressure level of some refrigerant, for example, R22—approximately 99.6°C.Initial capital cost may be high due to many refrigerating • components.Requires a steady-state period for the system to attain the desired • drying conditions.Requires regular maintenance of components (compressor, refrig-•
Leakage of refrigerant to the environment if cracking of pipes • occurs due to pressurized system.Three-phase power supply may be required for higher refrigerat-• ing systems.
15.3 Low-Temperature Heat Pump Drying
heat pump and the corresponding processes on a Mollier chart. Drying temperatures are typically in the range of 0–70°C. However, for special
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• Higher energy efficiency with improved heat recovery method, res-
• Better product quality with controlled temperature profile to meet
erant filters, etc). Regular charging of refrigerant is also required.
Figure 15.9 shows a tunnel dryer for heat-sensitive products fitted with a
• Chlorofluorocarbons (CFCs) are used in the refrigerant cycle.
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 273
A
BC
F
Fan
Condenser 2Compressor
Evaporator
Electricalenergy
Condenser 1
Trolleys
A
F
Enth
alpy
/ te
mpe
ratu
re
ϕ = 1.0
B
∆Η =
Q 0/GA
C
Absolute air humidity
Maximum
temperature
temperature
Evaporation
FIGURE 15.9Tunnel dryer with a heat pump and representation of the drying process on a Mollier dia-
Institute of Technology, Division of Refrigeration Engineering, Trondheim, Norway[in Norwegian], 1980.)
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gram. (From Strømmen, I., Drying of heavily salted codfish, Ph.D. Thesis, The Norwegian
© 2009 by Taylor & Francis Group, LLC
274 Advanced Drying Technologies
purposes, the lower temperature limit can be shifted down to −10°C. Inlet relative air humidities are usually kept ∼30 to 40% (Magnussen and Strømmen, 1981; Strømmen, 1980).
For some products, aroma compounds or other volatiles might be released from the material being dried. Since all drying air is recirculated, the partial pressure of these components increases until either saturation or a dynamic equilibrium between the evaporation and condensation/sorption processes in the evaporator is reached. In general, the problem of deodorizing or degassing in a heat pump dryer is much easier to handle because only a liquid phase exits from the drying system. Further recov-ery of the aroma components or removal of absorbed pollutants from a
volume of the exhaust air.The closed system of a heat pump tunnel dryer makes it possible to use 2 2
sion hazards and also to eliminate degradation of products sensitive to oxygen. In this case, however, a special design of the feed and product
tion of the ambient air.
tures required for drying heat-sensitive materials justify the application
Technology (NTNU) and thoroughly tested for various products (Jonassen et al., 1994; Strømmen and Jonassen, 1996; Alves-Filho and Strømmen, 1996) is shown in Figure 15.10. The operating temperature in the dryer is set at the desired level by adjusting the condenser capacity, and the required air humidity is maintained by varying the compressor capacity through frequency control of the motor speed. This setup permits drying at temperatures from −20 to 60°C and air humidities from 20 to 90%. The
0.3 to 5.0 m/s is adjusted by controlling the fan rotation speed.With these features, heat-sensitive materials can be dried under con-
ventional or freeze-drying (F-D) conditions using either air or inert gases (CO2 or N2) as the drying medium. The possibility of sequencing these two operations is especially advantageous when drying food and bio-products because drying under frozen conditions causes minimal shrink-age, although at lower drying rates. Furthermore, splitting of the drying process into F-D at temperatures around −5°C, for example, followed by
ity parameters such as porosity, rehydration rates, strength, texture, color, and taste. Tests performed at NTNU on various heat- sensitive materials of biological origin as well as pharmaceuticals, medicinal and biotech prod-ucts, bacteria, fruits and vegetables, spices, etc., have proved that without
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relatively small amount of water is not as difficult as it is from a large
N , CO , or other inert gases as drying agents to reduce fire and explo-
discharge is required to prevent leakage of the drying agent and infiltra-
Efficient heat and mass transfer in dispersed systems and low tempera-
much faster finish drying at 20–30°C makes it possible to control qual-
of a heat pump system in conjunction with pneumatic, fluid bed, vibrated bed, conveyor, spin-flash, and similar convection dryers. A simplified lay-out of a heat pump fluid bed dryer developed at the Norwegian Institute of
flow rate of the drying agent (air) permitting a superficial velocity from
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 275
product, albeit at a higher cost of drying. This technology is thus recom-mended for premium-grade products (Alves-Filho and Strømmen, 1996).
ing chambers. Each heat pump can run at completely different conditions
kept below the product’s freezing point. The drying conditions are con-
such as to attain the critical moisture content. Then the semidry product
as that of the single-stage unit, but sequential drying conditions enable reductions of moisture content at low temperatures to maintain product
Compressor
Receiver
Fan
Throttlevalve
EvaporatorCondenser 1
Condenser 2
FIGURE 15.10
Drying’96, Lodz Technical University, Lodz, pp. 405–416, 1996. With permission.)
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trolled by adjusting the heat pump’s components according to the specific enthalpy curves (Alves-Filho, 1996). The residence time in the first dryer is
tures for finish drying. The advantage of the multistage dryer is the same
quality and finish drying at an elevated temperature to increase the over-all heat pump efficiency (Alves-Filho et al., 1998).
The laboratory heat pump fluid bed dryer. (From Alves-Filho, O. and Strømmen, I.,
exception heat pump fluid bed drying offers much better quality of the
Figure 15.11 shows a multistage fluid bed heat pump dryer consisting
nected in parallel to each other that provide conditioned air to the dry-of two fluidized bed dryers arranged in series and two heat pumps con-
with different refrigerants and secondary fluids. The wet product fed to the first dryer is fluidized by an air stream at a temperature that is always
is transferred to the second dryer, where it is fluidized at higher tempera-
© 2009 by Taylor & Francis Group, LLC
276 Advanced Drying Technologies
FIGURE 15.11
Strømmen, I., Drying’98, Zitti Editions, Thessaloniki, pp. 94–101, 1998.)
Evaporator
Low-temperaturefluid bed
Low-temperaturefluid bed
Throttle valve
Throttle valve Compressor
Compressor
Condenser
Condenser
Condenser
Condenser
Three-wayvalve
Three-wayvalve
T-head
T-head
Evaporator
Feed
Product
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A multistage fluid bed dryer. (Adapted from Alves-Filho, O., Thorbergsen, E. and
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 277
ate at temperatures from 60 to 110°C (Jonassen et al., 1994; Strømmen and Jonassen, 1996). This dryer has the following features:
Increased drying temperature; a new heat pump with ammonia as •the refrigerant and with a new compressor designed for pressures of 6–7 MPa allows condensing temperatures of above 100°C.If an isothermal drying process can be achieved by supplying •
increase in water content can be reached. Because of the large dry-ing potential, an isothermal process provides severalfold larger water uptake compared to the adiabatic process in the humidity range from 30 to 90%.
between the two beds in the experimental dryer is used as shown schematically in Figure 15.12. The reheating system in the dry-ing beds has two purposes: (a) to reheat the air from the dry bed
Fluid beddryer
Condenser
Cyclone FeedWet fluid
bed
Superheater
Product
CondenserEvaporator
Water
Fan
Electric heater
FIGURE 15.12
Jonassen, O., Drying’96. Lodz Technical University, Lodz, pp. 563–568, 1996. With permission.)
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A schematic of the nonadiabatic heat pump fluidized bed dryer. (From Strømmen, I. and
Figure 15.12 presents a nonadiabatic fluid bed dryer designed to oper-
extra heat to the fluidized bed of a dried material, a substantial
• The use of a multistage fluid bed where air and the product to be dried flow in a countercurrent direction. A superheat exchanger
© 2009 by Taylor & Francis Group, LLC
278 Advanced Drying Technologies
to increase a drying potential in the wet bed and (b) to allow for higher drying temperatures; the products in the upper bed are wet, and even higher temperatures than those in the lower bed can be used without spoiling the product.
Electrically driven heat pumps are potentially attractive as most of the main components were directly transferred from refrigeration technol-ogy. However, this limited the early applications to the temperature range
possible to extend the range of heat pumps to temperatures up to 120°C, thereby reducing energy consumption and allowing the use of this drying method for a wider range of materials. Tests on batch timber drying com-paring modern conventional kilns and a heat pump system using R-114 showed that, with the high-temperature heat pump, an energy cost saving of 40% for drying hardwoods and 65% for softwoods could be achieved with U.K. energy price ratios (Lopez-Cacicedo, 1986). The actual savings depend on the relative costs of electricity and fossil fuels as well as the operating conditions in the dryer. A comparison of energy consumption
of wood can be found in the paper by Zhang and coworkers (2007).The wider application of the vapor-compression heat pump in the
future may be jeopardized by the increasing concern on the ozone-deplet-
temperature heat pumps (e.g., R-114). Extension of heat pump technology to large-scale continuous operations and temperatures above 120°C points
steam compression system. In such a system, water evaporated from the material being dried is compressed and used for heating the incoming material. The principle of operation is similar to the well-known mechani-
process will fall, however, if noncondensable gases such as air are present within the system.
15.4 Chemical Heat Pump Drying
Interesting alternatives to compression heat pumps with ozone-depleting
of chemical energy through an endothermic reaction in a suitably designed reactor. This energy is then released at various temperature levels during the heat demand period by exo/endothermic reactions (Ogura et al., 1991, 1997; Mbaye et al., 1997; Goetz et al., 1991). The CHPs operate using only
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and drying time during conventional drying, dehumidification (heat pump) drying, and combined conventional and dehumidification drying
cal vapor-recompression systems (see Chapter 24). The efficiency of the
of 60–65°C. By choosing working fluids other than R-12 and R-22, it was
ing potential of the working fluids (freons) that were developed for high-
to the possible use of water vapor as a working fluid, if there is an efficient
fluids are the so-called CHPs. A CHP can store thermal energy in the form
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 279
thermal energy and do not release any contaminant gases, which is a dis-tinct feature from the mechanical heat pumps. From numerous possible chemical reactions, the CaO/Ca(OH)2 hydration/dehydration reaction for heat storage and cold/hot heat generation has been validated as a good candidate reaction for CHP by experiments and simulations carried out at Kyushu Institute Technology and Nagoya University in Japan (Ogura et al., 1991, 1997). On the basis of these promising results, an original concept of the chemical heat pump dryer (CHPD) with heat recovery/generation for ecologically friendly and effective utilization of energy in thermal drying has been proposed (Ogura and Mujumdar, 2000). The following descrip-tion is based on their proposal.
a closed system of coupled low- and high-temperature reactors operated according to the reactions given by the following equations:
A s C g AC s QH( ) ( ) ( ) ⇔ (15.6)
B s C g BC s QL( ) ( ) ( ) ⇔ (15.7)
AC(s) + QH
A(s) + C(g)
B(s) + C(g)
BC(s) + QL
C(g)
A(s) + C(g)
AC(s) + QH
BC(s) + QL
B(s) + C(g)
C(g)
H L
H L
Heat-storing step
Heat-releasing step
FIGURE 15.13Operation principle of the CHP (H, high-temperature side reactor; L, low-temperature side reactor).
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Figure 15.13 shows a typical gas–solid CHP configuration. The CHP is
© 2009 by Taylor & Francis Group, LLC
280 Advanced Drying Technologies
The heat-storage and heat-release reactions occur at different pressure lev-els. The low-temperature side reactor has a higher reaction equilibrium pressure. The CHP, as studied earlier, operates as a batch system with a heat-storing step followed by a heat-releasing step. For example, hydration and carbonation reactions can be used on the high-temperature (600–1200 K) side reactor. Evaporation and condensation of the reactant media are often used on the low-temperature (273–523 K) side reactor.
In the heat-storing step, the heat QH is stored in the form of thermochemi-cal energy by decomposition of the reactant AC(s) in the high-temperature
reactor due to a pressure difference maintained between the two reactors. The gas C(g) reacts with the reactant B(s), releasing low-temperature heat QL. As long as the reactant A(s) is separated from the gas C(g), the reaction heat can be stored for any period in the form of chemical energy.
side reactor to the high-temperature side reactor by opening a valve (Figure 15.13) due to the pressure difference between the reactors. The exothermic reaction of the reactant A(s) at a high-temperature level with the gas C(g) takes place in the high-temperature side reactor. The low-temperature side reactor stores the low-temperature heat QL or is cooled down by releasing its decomposition heat when it is insulated.
The CHP has four possible operating modes according to the operating pressure levels: heat-storage mode, heat-enhancement mode, refrigeration mode, and temperature-upgrade mode. Figure 15.14 shows the relation-ships between the reaction equilibrium pressures and temperature for each mode. The actual operating pressures and temperatures of CHPs depend on the reaction equilibrium values—they are governed by the thermodynamics of the system chosen.
In the heat-storage mode, heat QM is stored at TM in the heat-storing step, and this heat is released at the same temperature in the heat-releasing step. In the heat-enhancement mode, the low-temperature heat (e.g., room temperature heat) is used as QL in the heat-releasing step. Hence, it is possible to use QM two times by storing unit QH as a result of the CHP operation. The principle of a CHP operated in the refrigeration mode is similar to that operated in the heat-enhancement mode. Here, the medium-temperature heat QM and cold heat QL are obtained simultaneously in the same heat-releasing step. The cold heat QL is generated by releasing its decomposition heat under adiabatic conditions. In the temperature- upgrade mode, heat QH at the higher-temperature level can be obtained by using QM two times.
Figure 15.15 presents a schematic diagram of the essential concept of the proposed CHPD. To apply CHPs to dryers, thermal energy from some
the thermal energy in an exhaust air from the dryer is stored in the CHP1.
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source should first be stored in the CHP. In this particular configuration,
If the exhaust heat is not sufficient to complete the heat-storing reaction
side reactor. The released gas C(g) flows into the low-temperature side
In the heat-releasing step, the gas C(g) flows from the low-temperature
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 281
taking place in CHP1, additional heat might be required. If the exhaust
released from CHP2 as “hot” heat for drying. These two steps run simul-taneously as a batch cycle, and the continuous supply of heat to the dryer is secured by altering the heat-storing/releasing steps. An air stream
PH PL
TH TM TL 1/T
ln P
QM
QL
QH
QM
BC(s)
A(s)+C(g)
1 2
34
Heat-enhancement mode
PH PL
TH TM TL 1/T
ln P
QM
QL
QH
QM
BC(s)
A(s)+C(g)
1 2
34
Heat-storage mode
PH PL
TH TM TL 1/T
ln P
QM
QL
QH
QM
BC(s)
A(s)+C(g)
1 2
34
Refrigeration mode
PH PL
TH TM TL 1/T
ln P
QM
QL
QH
QM
B(s)+C(g) BC(s)
AC(s)A(s)+C(g)
1 2
34
Temperature-upgrade mode
AC(s)
B(s)+C(g)
AC(s)
B(s)+C(g)
AC(s)
B(s)+C(g)
FIGURE 15.14Basic operating modes of CHPs.
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air needs to be dehumidified, CHP1 can operate as a cooling dehumidi-fier in the refrigeration mode of operation. The stored energy in CHP1 is
changes its flow direction according to the current mode of operation.
© 2009 by Taylor & Francis Group, LLC
282 Advanced Drying Technologies
When the CHPs work in the heat-enhancement mode (Ogura and Mujumdar, 2000), the released energy from the CHP2 can be >200% of the stored heat in CHP1, depending on the chemical system selected. On the contrary, when the temperature of the released heat from CHP2 is required to be much higher than that of the stored heat in CHP1, the temperature-upgrade mode is especially effective.
In addition to the choice of CHP modes, one must select the candidate reactions considering the reaction temperature, the pressure level, reac-tivity, reaction reversibility, safety, corrosion, costs, etc. This means that the features of the reaction system determinate the features of the CHPD system. Some feasible reaction systems for the CHPD can be found in the study by Ogura and Mujumdar (2000).
From the aforementioned considerations, more than two CHPs need to be used for the continuous operation of a CHPD. The reaction system and the operation mode need to be arranged for the special purpose of energy savings in drying.
Ogura and Mujumdar (2000) illustrated the CHPD idea by selecting the calcium oxide/calcium hydroxide reaction from the viewpoints of their safety, reactivity, reaction enthalpy, and reaction equilibrium temperature–pressure level, and material cost. The CaO/H2O/Ca(OH)2 reaction system is expressed by the following reactions:
CaO(s) H O(g) Ca(OH) (s) 104.2 kJ mol2 2 ⇔ / (15.8)
H O g H O l kJ mol2 2 41 7( ) ( ) .⇔ / (15.9)
Figure 15.16a depicts the operating line for this CHPD, showing the rela-tionships between reaction equilibrium pressure and temperature. This system works in the heat-enhancement mode, and both CHPs operate
Dryer
Heat generation Heat recovery /dehumidification
CHP2 CHP1
Humid air
Additionalheat
Hot dry air
Dry air
FIGURE 15.15Concept of a CHP dryer.
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© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 283
FIGURE 15.16Principle of the CHPD with CaO/H2O/Ca(OH)2 operated in the heat-enhanced mode:
PH PL
PS
PR
THS TMR TMS TLR
CHP1
CHP2
1/T
ln P
QMS
QLR
QHS
QMR
H2O(g) H2O(l)CaO Ca(OH)2
1 2
34
1
2 3
4
Ca(OH)2(s) + QMR
CaO(s) + H2O(g)
H2O(l) + QLR
H2O(g)
Ca(OH)2(s) + QHS
CaO(s) + H2O(g)
H2O(l) + QMS
H2O(g)
Dryer
-QD
TMS
TMR
TLR
THS
WA1
TA1 TA2
Y1 Y2
WA2
PS PR H2O(g)H2O(g)
TA0
CHP2CHP1Qin
(a)
(b)
heat-storing step and CHP2 is in the heat-releasing step. These CHPs switch steps with each other every hour or so; the switching time is decided during previous experiments such as those reported in studies by Ogura and coworkers (Ogura et al., 1991, 1997, 1999).
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concurrently. In the configuration shown in Figure 15.16, CHP1 is in the
(a) Operating line and (b) flowsheet.
© 2009 by Taylor & Francis Group, LLC
284 Advanced Drying Technologies
high-temperature heat QHS supplied by a gas burner or a heater with Qin at THS > 594°C is stored in the CaO reactor. Water vapor in the reac-tor is released from the reactor and condensed in the condenser. The condenser releases medium-temperature heat QMS around TMS = 150°C by controlling the heat-transfer rate. On the contrary, CHP2 also releases medium-temperature heat QMR around TMR = 360°C in the CaO reactor by controlling the evaporator temperature at around TLR = 20°C with room-temperature heat. In summary, the CHP system stores the 600°C
CHP steps.To supply dry hot air at TA1 = 130°C, the mass of 100 kg of dry air/h is
recirculated, as shown in Figure 15.16. The amounts of heat QMS and QMR needed to heat up the exhaust air at TA2 = 100°C to the dryer inlet tem-perature TA1 = 130°C can be calculated using the following equation:
Q Q W c T TMS MR A A A1 A2 ( ) (15.10)
From the reactions expressed by Equations 15.6 and 15.7, the heats of reac-tion are produced in the following ratio:
Q QMS MR: . : . : . 41 7 104 2 1 2 5 (15.11)
From Equations 15.10 and 15.11, the following results are obtained: QHS = QMR = 595 W = 2.14 MJ/h, QMS = QLR = 238 W = 0.86 MJ/h. The mass
A1 = 28.6 kg of dry air/h, WA2 = 71.4 kg of dry air/h. The amounts of reactants needed are CaO = 1.15 kg, H2O = 0.37 kg.
In the dryer, heat calculated from Equation 15.10 (QD = QMS + QMR) is used for heating the material and evaporating the moisture. Hence,
Q W c T W H YD B B B A (15.12)
where ∆Y is the mass-transfer driving force equal to Y2 – Y1.From this, the rate of moisture evaporation can be calculated. Neglecting
the sensible heat of the solid material, the evaporation rate is Y2 − Y1 = 0.0125 kg water vapor/kg dry air or Wev = 1.25 kg water vapor/h. In this case, the consumed energy for evaporation in this CHPD is calculated from the following equation:
QQWev
HS
ev
(15.13)
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Figure 15.16b shows a flowsheet of the corresponding CHPD. In CHP1,
heat and continuously releases heat at 150 and 360°C by flipping the
flow rates are calculated to be W
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 285
The calculated value of Qev equal to 1.71 MJ/kg of water evaporated is much lower than the evaporation heat of water, which is ∼2.4 MJ/kg. If the material’s sensible heat is assumed to be 30% of the drying energy QD , the evaporation rate Wev is 0.875 kg water/h, and Qev = 2.45 MJ/kg. Although the rate of evaporation is not high, this amount of consumed energy is a good value for industrial drying.
In the chemical heat pump system, the circulating air gains humidity in the course of drying. In the case under analysis, the relationship between the absolute air humidity and the number of cycles can be written as
Y Y n Y n(1,n) ( , ) ( ) . ( ) .1 0 1 0 01 1 0 0125 (15.14)
Hence, the absolute air humidity Y(1,11) = 0.135 kg water vapor/kg dry air is attained after 10 cycles (n = 11). The relative humidity is, however, <2% because air circulating in the system is kept over 100°C. The above analy-sis was done under the assumption that the drying rate does not change over the period of operation, which might not always be true.
In the case of a convective dryer of the same performance as the CHP dryer, the air should be heated up to TA1 = 130°C from room temperature of TA0 = 25°C. The consumed heat calculated as
Q W c T Tin c A A A A, ( ) 1 0 (15.15)
is then equal to 2.92 kW or 10.5 MJ/h. The energy consumption ratio R Q
RQ
Q00Q
in
in c
,
1 (15.16)
is 20.4%. This means that this particular CHP consumes only ∼20% of the energy needed with a conventional convective dryer of the same capacity.
Mujumdar (2000) to be much greater than that of the conventional con-vective dryer. The features of the CHPD depend on the chemical reaction system chosen as well as on the mode of operation. Further detailed stud-ies, both theoretical and experimental, are needed to examine the poten-tial of CHPDs. Cost and reliability studies are also needed in view of the increased complexity of the new conceptual designs suggested. New reac-tor designs of CHP may be needed for continuous operation at the right temperature levels. Note that the results given here are based on a highly
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defined as
The energy efficiency of the CHPD was estimated by Ogura and
simplified analysis and are subject to refinement.
© 2009 by Taylor & Francis Group, LLC
286 Advanced Drying Technologies
15.5 New Developments and Trends in Heat Pump Drying
The most important considerations concerning heat pumps are summa-rized as follows (Strumillo et al., 2007):
Compressors of heat pump dryers should be able to operate con-• tinuously for extended periods without maintenance.The heat capacity and working temperature of the heat pump • should suit the drying process.The heat pump should not be the only heating device for the • dryer; it should be treated as a moisture-extracting device.For optimum performance, the heat pump should operate at the • same heat load (i.e., with the same amount of moisture condensed and at a constant temperature). Compensation for any changes in dryer operating conditions should be by other methods.The temperature difference between an evaporator and a condenser •for a single-stage vapor-compression heat pump should not exceed 40°C. For such a temperature difference, the COP ranges from 3 to 4.
Recent studies in heat pump drying focus on process control as well as on the individual refrigerating component, in particular, on compressors. The
and size of the compressor have made them technically attractive to be integrated into a heat pump drying system. Also, advanced heat exchang-
better heat transfer and reduce the size of heat exchangers (Matsuo et al., 1984). These features make heat pump dryers more compact and more
mentation of better air-control strategies.As many processes require both high- and low-temperature drying,
frost on the evaporator surface builds up rapidly in low-temperature appli-cations. Once frosting occurs, heat transfer between the evaporator and the drying air decreases dramatically due to the additional resistance to conductive heat transfer. Exploratory research has been initiated to study
objective of eventually coming up with better mechanisms to reduce the buildup rates (Sanders, 1985).
presented in Chapter 33.The basic concept of coupling a CHP to dryers is new and hence not
explored in any depth. It appears that CHPs will need to be designed for
tion in industrial drying. In fact, new reaction systems and new reactor
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latest developments in scroll compressors in terms of energy efficiency
ers with the internal tube’s ribbed surface and external wavy fins promote
energy-efficient through better heat recovery and also allow the imple-
continuous (rather than batch) operation if they are to find wide applica-
the ice buildup process and its influence on air dehumidification with the
Some emerging hybrid technologies based on heat pumps are briefly
© 2009 by Taylor & Francis Group, LLC
Heat Pump Drying 287
designs may be required for drying applications. It is too early to predict the potential of this novel idea at this time.
Symbols
c Heat capacity, kJ/kg KE Energy, WEF Energy for fans, W
g Acceleration due to gravity, m/s2
∆H Latent heat of vaporization, kJ/kgn Consecutive numberP PressureQ Heat rate, kJ/kmolQo Refrigeration capacity, kWR Energy consumption ratioT Temperature, °C (K)
Y Absolute humidity, kg water/kg dry air∆Y Absolute air humidity difference, kg H∆∆ 2O/kg dry airΦ Relative humidity
Subscripts
A AirB MaterialC ConvectiveD DryingEv EvaporationG GasH High-temperature sideIn InletL Low-temperature sideM Medium-temperature sideR Releasing stepS SolidS Storing step0 Initial (ambient)1, 2 Consecutive stage
Acronyms
CHP Chemical heat pumpCHPD Chemical heat pump dryer
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G Mass flow, kg/s
W Mass flow rate, kg/s
CFC Chlorofluorocarbon (Freon)
© 2009 by Taylor & Francis Group, LLC
288 Advanced Drying Technologies
HPD Heat-pump dryerHPDE PID Proportional-integral-derivativeRH Relative humidity
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16Fry-Drying
immersing the raw material into hot oil. It is a well-described technology in applications to a variety of foods such as potatoes, nuts, bakery prod-
ing of foods, comprising the particularities of heat and mass transfer, the properties of fried products and frying media, the process conditions and control, as well as descriptions of industrial frying systems, is given by Oreopoulou et al. (2007). Also, basic information on fry-drying of wood, used to a limited extent for high-temperature processing of wood, can be found in the papers by Rapp and Sailer (2001) and Grenier et al. (2007). Therefore, this chapter addresses only the dewatering of sludge-type materials as a novel application of the frying process.
Based on the frying principles, an interesting concept of simultane-ously managing the sewage sludge and spent oil problems has been put forward by a group of researchers from Ecole des mines d’Albi in France (Peregrina et al., 2004a, 2004b, 2005; Peregrina 2005; Peregrina, Arlabosse et al., 2006). A pioneering study on fry-drying performed with munici-pal sewage sludge and peanut oil showed that very low moisture con-tent can be reached in a few minutes and that oil is adsorbed by the dry solid matrix. The fry-dried sludge is easy to handle and has a higher
be used as a supplementary fuel (Pires da Silva, 2003). To eliminate the need for high-quality oil and thus improve the process econom-ics, further studies were carried out with recycled cooking oil (RCO),
et al., 2004a, 2004b).
tion of spent oil and greasy wastewater collected in the grease traps of restaurants and food industry outlets. Its use for fry-drying of sewage sludge would not only provide an economically attractive option for the disposal of these wastes, but also valorize both the sludge and the RCO by formulating a derived fuel.
The studies at Ecole des mines d’Albi were performed with the munici-pal sewage sludge from a wastewater treatment plant (WWTP). Digested and mechanically dewatered sludge to the moisture content between 4 and
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Frying is defined as a process of simultaneous cooking and drying by
ucts, meat, fish and the like (Fellows, 2002). A capsule knowledge of fry-
calorific value, similar to hot air–dried sludge, and therefore it could
especially because the preliminary experiments showed no significant modification to the drying kinetics as compared to pure oil (Peregrina
RCO is a generic name for the oil obtained after the multistage purifica-
© 2009 by Taylor & Francis Group, LLC
294 Advanced Drying Technologies
6 kg/kg d.b. was then extruded into 145 mm cylinders with diameters ranging from 15 to 25 mm and placed in a grid-type holder. The RCO contained ∼65% water and 35% of other materials, 50% of which was
at 180°C to evaporate water. Tests were performed at oil temperatures ranging from 120 to 180°C with 20°C intervals. The frying time to get an almost completely dry sample varied from 520 to 3000 min, depend-ing on the operating parameters and sample diameter (Peregrina, 2005; Peregrina, Arlabosse et al., 2006). Figures 16.1 and 16.2 present the fry-drying kinetics for various levels of oil temperature. Although the evo-lution of moisture content with time is similar to the one in convective drying, no classical constant drying rate period exists because of the particularities of the fry-drying mechanisms. Peregrina, Arlabosse,
fry-drying that coincide with the four similar periods of heat transfer in extruded sewage sludge: 1. Initial heating period. In the case of a 15 mm sample fried at 160°C
evaporation of surface water occurs almost instantaneously after
is attained. The wet surface of the sample heats up rapidly by convection from the oil to the boiling point of water (102–104°C). Then, the heat is transferred by conduction from the sample
0 500 1000 1500 2000 2500 30000
1
2
3
4
5
6
Time (s)
Moi
stur
e con
tent
(kg/
kg d
.b.) 120ºC
160ºC
140ºC
180ºC
FIGURE 16.1Effect of oil temperature on the drying curves. ( From Peregrina, Arlabosse, et al., Drying Technol., 24, 797–818, 2006. With permission.)
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fat. Before its first use for fry-drying tests, the RCO was heated for 1 h
et al. (2006) identified the following four well-differentiated phases of
taken as an example, it covers the first 150 s, over which the
immersion and the drying rate increases until a first maximum
© 2009 by Taylor & Francis Group, LLC
Fry-Drying 295
surface to its core. Thus, a fraction of the convectively supplied heat from oil is used for boiling while the remaining heat is accu-mulated inside the sample for future evaporation of moisture.
2. Fry-drying as a boiling front process. Over a time period of 150–400 s, two regions start to develop within the sample, that is, the crust and the core (Figure 16.3). These regions are separated by the liquid–vapor interface created due to water evaporation. In the course of drying, the crust builds up as the evaporation front recedes. Consequently, the drying is controlled by heat transfer—
then by conduction through the crust and the sample core. 3. Oil impregnation. In contrast to the frying of foods, where the fall-
ing drying rate period starts once the crust is formed, the sharp
for such an unusual behavior is credibly explained in the publi-cation by Peregrina, Arlabosse, et al. [2006]). Characteristically, the impregnation of the crust with oil takes place, and the ratio-nale for this process is also provided in the aforementioned paper.
4. Fry-drying as a boiling in a porous medium. When drying times are longer than 720 s, the partially dry sludge is formed as a porous
Moisture content (kg/kg d.b.)
120ºC
140ºC
160ºC
180ºC
Dry
ing
rate
× 1
03 (kg/
kg s)
0 1 2 3 4 5 60
2
4
6
8
10
12
14
Period IIIPeriod IV Period II Period I
FIGURE 16.2Effect of oil temperature on the drying rate curves. (From Peregrina, Arlabosse, et al., Drying Technol., 24, 797–818, 2006. With permission.)
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first by convection from hot oil to the oil/sample interface and
increase in the heat-transfer coefficient is noted for frying of sew-age sludge, which leads to a stabilized drying rate (justification
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296 Advanced Drying Technologies
water, vapor, and oil. At this stage, the residual water is removed at a decreasing drying rate and uniform material temperature, albeit some tens of degrees Celsius higher than the boiling point (Figure 16.4).
Core
Crust
FIGURE 16.3Cross section of a partially fry-dried sample. ( From Peregrina, Arlabosse, et al., Drying Technol., 24, 797–818, 2006. With permission.)
FIGURE 16.4
sewage sludge: From the physical mechanisms to the process eco-design. Ph.D. Thesis, National Institute of Applied Sciences in Lyon, France, 2005. With permission.)
0 1 2 3 4 5 6 0
20
40
60
80
100
120
140
160
180
Tem
pera
ture
(ºC
)
Moisture content (kg/kg)
Dry
ing
rate
× 1
03 (kg/
kg s)
0
2
4
6
8 Oil temperature
Period IPeriod IIPeriod IIIPeriod IV
r = 12.5 mm
r = 6.3 mm
r = 0 mm
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Drying rate curve and transient temperature profiles. (From Peregrina, C., Fry-drying of
solid matrix filled with the residual capillary and interstitial
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Fry-Drying 297
Regarding the effect of external conditions, the type and quality of oil as well as the stirring speed have negligible effects on drying kinetics in contrast to oil temperature, for which the range 140–160°C was found opti-mal. Interestingly, the initial moisture content did not affect the total dry-
three periods were different. This means that a higher degree of mechani-cal dewatering of the sludge might not be an issue when fry-drying is used to produce a solid fuel with a low moisture content.
Since the lower heating value (LHV) of the sludge increases with water removal and oil uptake, fry-drying can be seen as a process of formula-tion. Consequently, if the fry-dried sludge is to be disposed by incinera-tion, it is possible to arrange for an autothermal sludge that would offer both economic and environmental advantages (Peregrina, Lecomte, et al., 2006). In contrast, maximum dryness and oil impregnation are desirable when the sludge is dried for solid fuel. Depending on the frying condi-tions, it is feasible to get a solid fuel having an LHV from 18 to 24 MJ/kg of fry-dried sludge, that is, 50–70% higher than the LHV for the air-dried sludge (Peregrina, Arlabosse, et al., 2006).
The economic assessment for fry-drying of 1000 kg/h of dry solids was
including a condenser, a heat pump in a closed system, and with mechani-cal vapor-recompression (Peregrina et al., 2008). This process was shown to have much lower capital costs and comparable operating costs than for cur-rent sludge dryers. Attempts to further cost reduction could focus on cheaper oil sources, for example, oil from grease traps, oil and grease skimmed from the tanks in a WWTP, grease from meat processing plants, and the like. These conclusions cannot, however, be generalized at this time.
Symbols and Acronyms
r Radius, mLHV Lower heating valueRCO Recycled cooking oild.b. Dry basisWWTP Wastewater treatment plant
References
Fellows, P. 2002. Food Processing Technology: Principles and Practice. 2nd edition. CRC Press, Boca Raton, FL.
Grenier, D., Bohuon, P., Meot, J. M., Lecomte, D. and Bailleres, H. 2007. Heat and mass transfer in fry drying of wood. Drying Technol., 25(3): 511–518.
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ing time, although the duration and intensity of drying during the first
performed for various process configurations with heat recovery systems
© 2009 by Taylor & Francis Group, LLC
298 Advanced Drying Technologies
Oreopoulou, V., Krokida, M. and Marinos-Kouris, D. 2007. Frying of foods.In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). 3rd edition.CRC Press, Boca Raton, FL, pp. 1225–1246.
Peregrina, C. 2005. Fry-drying of sewage sludge: From the physical mechanisms to the process eco-design. Ph.D. Thesis. National Institute of Applied Sciences in Lyon, France.
Peregrina, C., Arlabosse, P. and Lecomte, D. 2004a. Fry-drying of sewage sludge: an alternative for disposal of recycled food oils. Proc. 9th International Congress on Engineering and Food—ICEF09. March 7–11, Montpellier, France, pp. 154–159.
sludge fry-drying. Proc. 14th International Drying Symposium. August 22–25, State University of Campinas, Sao Paulo, Brazil, pp. 972–978.
Peregrina, C., Arlabosse, P., Lecomte, D. and Rudolph, V. 2005. Fry-drying: An intermediate sustainable operation for the co-disposal of sewage sludge and waste oil. Proc. 7th World Congress of Chemical Engineering. July 10–14,Glasgow, Scotland (CD-ROM).
Peregrina, C., Arlabosse, P., Lecomte, D. and Rudolph, V. 2006. Heat and mass transfer during fry-drying of sewage sludge. Drying Technol., 24(7): 797–818.
Peregrina, C., Lecomte, D., Arlabosse, P. and Rudolph, V. 2006. Life cycle assess-ment (LCA) applied to the design of an innovative drying process for sewage sludge. Process Safety Environ. Prot., 84(B4): 270–279.
Peregrina, C., Rudolph, V., Lecomte, D. and Arlabosse, P. 2008. Immersion fry-ing for the thermal drying of sewage sludge: An economic assessment.J. Environ. Manage., 86(1): 246–261.
Pires da Silva, D. 2003. Study of sewage sludge drying using immersion fry-ing. Ph.D. Thesis. State University of Campinas, Campinas, Brazil (in Portuguese).
Rapp, A. O. and Sailer, M. 2001. Oil heat treatment of wood in Germany—state of the art. In: Review on Heat Treatments of Wood. A. O. Rapp (Ed.). Proc. Special Seminar of European Commission Research Directorate. February 9, Antibes, France, pp. 47–64.
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Peregrina, C., Arlabosse, P. and Lecomte, D. 2004b. Thermal efficiency in sewage
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Part III
Selected Techniques forDrying and Dewatering
This part describes some new drying techniques that have been dem-onstrated at laboratory scale but need considerable further develop-ment effort. It is expected that they will attract the attention of academic researchers and industries so that more research and development effort can be devoted to testing their eventual viability. In some cases, these ideas could be coupled with some of the conventional drying concepts to increase their chance of commercial success.
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© 2009 by Taylor & Francis Group, LLC
301
17Mechanical Thermal Expression
Mechanical thermal expression (MTE) is a novel drying technique that evolved from the relatively old technique called hydrothermal dewatering.
heated to ∼300°C using water, steam, oil, or some other heating medium.
expelled from the solid matrix and then separated once cooling and pres-sure release are completed. The energy requirements for hydrothermal
order of 1600–1800 kJ/kg of removed water. However, the plant cost, batch nature of the process, and impurities that could be extracted from some materials into process water at such high temperatures have prevented hydrothermal dewatering from wide industrial application.
Dewatering by mechanical separation of water from solids is an attrac-tive technology as energy consumption is only a small fraction of that required for thermal drying and varies from 0.01 to 3%. However, this technology suitable to slurries is impractical for biomaterials having a solid matrix such as lignite, for example, because very high pressures(>16 MPa) are needed to attain the required moisture content in an accept-able processing time.
The limitations of both methods can be overcome by combining mechanical expression with hydrothermal dewatering into MTE (Clayton et al., 2006). Here, the wet material is mechanically pressed to create an overpressure of ∼1–10 MPa. Thereafter, the material is heated above the corresponding boiling point of water, typically from 150 to 200°C, so that hot water is driven off the solid matrix by compressive forces (Figure 17.1).
mechanical force at much lower energy consumption than by evapora-
residual water after releasing the back pressure to atmospheric conditions. The elevated temperature decreases liquid viscosity so that the required pressure and the residence time are brought to manageable levels. Hot water expelled from the solid matrix can be used for the heating of subse-
eral in-depth studies over the past decade. For example, Bergins et al. (1999)
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Because sufficient pressure is maintained to prevent boiling, liquid water is
dewatering are significantly lower than those for evaporation, being in the
Sufficient back pressure prevents boiling; hence, water is removed by
quent batches of the material, thus improving the energy efficiency. The
improve the energy efficiency of the MTE process.Because of its definite energy-saving potential, MTE was the subject of sev-
During hydrothermal dewatering, solid material of a definite structure is
tion. Finish drying of the matrix is achieved by flash evaporation of the
steam from flash evaporation can also be collected and utilized to further
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302 Advanced Drying Technologies
found that for incompressible inorganic materials (mineral particulate systems), the only improvement was due to the reduction in the viscosity
permeability and therefore accelerates the rate of dewatering. However, for compressible biomaterials that have an inherent porous structure such as lignite, sewage sludge, and paper pulp waste (Bergins, 2003; Clayton
ference in dewatering behavior for inorganics and biomaterials relates to the state of water and water-to-material bonding. Most of the water in incompressible inorganic materials exists as bulk water, either funicu-lar or pendular, which can easily be removed using relatively low forces. By contrast, biomaterials have a porous solid structure in which water can be present in different forms. In lignite, for instance, water can exist as (a) bulk water that is contained in macropores and interstices, (b) cap-illary water that is contained in micropores, (c) monolayer water that is hydrogen-bonded to the oxygen-containing functional groups on the lig-nite surface, and (d) multilayer water that is weakly hydrogen-bonded to the monolayer water (Durie, 1991). Although the bulk and most of the cap-illary water can easily be removed using mechanical forces, more severe conditions are required to remove the residual capillary water, multilayer water, and monolayer water adsorbed at the capillary walls. The elevated
Filtration of thermally expelled water
Consolidation
Flashevaporation
Time
Moi
stur
e con
tent
FIGURE 17.1Process stages in mechanical thermal dewatering. (Reproduced from Clayton, S. A., Scholes, O. N., Hoadley, A. F. A., Wheeler, R. A., McIntosh, M. J. and Huynh, D. Q., Drying Technol., 24(7), 819–834, 2006. With permission.)
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of water with increasing temperature, which augments the coefficient of
et al., 2006), a significant benefit of increasing temperature over and above its effect on viscosity justifies the use of MTE for such materials. The dif-
temperatures and pressures used in the MTE process are sufficient to remove a significant fraction of the water, which cannot be removed using
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Mechanical Thermal Expression 303
dewatering technologies suitable for incompressible inorganic materials. Thus, MTE has the potential to be viable for a range of biomaterials, for
destruction (collapse) of the solid matrix is not critical. Examples are the solid residue left after sugar extraction from sugarcane (bagasse) and spent grain from distilleries and breweries. Another example of MTE application is the dewatering of sewage biosolids. However, the high moisture content of the freshly digested sludge in the order of 96–98% w/w would involve
MTE application appears to be for dewatering of the sludge preconcen-trated by mechanical dewatering, centrifugation, or even evaporation.
For MTE to be viable at the industrial scale, it will have to be capable of dewatering the given biomaterial to a desired moisture content and at a
optimization of MTE systems and also from a fundamental perspective,
of dewatering and the achievable moisture content. The particularities of MTE were extensively studied at Monash University in Australia to iden-tify the process mechanism and crucial operating parameters (Clayton et al., 2006, 2007).
The experiments at Monash University were carried out with both inherently porous and compressible materials such as lignite, bagasse, and
dioxide (titania). A bulk of lignite from the Loy Yang mine was milled to a maximum particle size of 3 mm and thoroughly mixed to produce a homogeneous sample of soil-like texture with an initial moisture con-tent of 1.4 kg/kg d.b. (58% w.b.). Bagasse from a sugarcane mill consisted
tent was equal to 1.2 kg/kg d.b. (54% w.b.). Biosolids were derived from the treated sewage sludge that was left for air-drying by evaporation in open pans for more than 3 years. The moisture content of the pan-driedbiosolids, visually similar to soil, was equal to 1.1 kg/kg d.b. (52% w.b.). The sample of titanium dioxide was made from a batch of particles having narrow size distribution (d10 = 0.8 μm, d50 = 1.25 μm, d90 = 1.8 μm).
The material permeability as a function of pressure and temperature
which the directions of both the applied pressure and water removal are in the axial direction. The second is radial, for which the direction of the water removal is perpendicular to the direction of the applied force (Fig-ure 17. 2). Because of no detectable difference in the dewatering perfor-mance, either geometry could be implemented in an industrial system.
The performance of MTE was assessed against temperature and pressure because both the parameters affect the material permeability through the effective and noneffective porosities (Clayton et al., 2007), which greatly
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which severe conditions are required to achieve significant dewatering but
high-energy demand to heat the material. Hence, the real benefit from the
sufficient rate to meet throughput demands. Therefore, for the design and
it is necessary to understand the factors that significantly affect the rate
biosolids, and with noncompressible materials exemplified by titanium
of fibers of different lengths and diameters, and its initial moisture con-
was studied for two system geometries. The first is classified as axial, for
influence the rate of dewatering and the achievable moisture content.
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304 Advanced Drying Technologies
This effect is clearly shown in Figure 17.3, which compares the void ratios (voids-to-solids volumes) for lignite and titanium dioxide at various pres-sure and temperature levels. For lignite, an increase in temperature from 20 to 150°C decreases the void ratio by approximately 0.5 over the entire
23 to 58% at 12 MPa). In contrast, the effect of temperature for titanium dioxide is much smaller than that for lignite.
As shown in Figure 17.1, MTE consists of three distinct phases that
moisture content for increases in pressure up to ∼6 MPa is primarily due to the reduced water viscosity, which enhances the permeability coef-
mechanical force, but the combined effect of temperature and pressure results in pore destruction and thus in the consolidation of the solid matrix (Figure 17.4). In the third phase, the back pressure is released
which results in cooling at the same time the material from the processing temperature to 100°C (Clayton et al., 2006).
The technology concept has proven to be successful with a 25 t/h dem-onstration plant, constructed and commissioned at the Niederaussem power station in Germany. For a stand-alone MTE plant with heat recovery
a
b
i
h
i
h
c
d
e
f
g
FIGURE 17.2
S. A., Scholes, O. N., Hoadley, A. F. A., Wheeler, R. A., McIntosh, M. J. and Huynh, D. Q., Drying Technol., 24(7), 819–834, 2006. With permission.)
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Principle of batch MTE in axial and radial configurations: a, direction of applied force;
line; h, back pressure regulating valve; i, pressure relief valve. (Reproduced from Clayton, b, piston; c, cell body; d, electrical heater; e, sample; f, sintered metal filter; g, water removal
pressure range, which significantly enhances the removal of water (from
result from different dewatering mechanisms. In the first phase, the expelled water is filtered out at early press times. A rapid reduction in
ficient. In the consolidation phase, the water is further expelled due to
under atmos pheric conditions, and the hot water is flash-evaporated,
© 2009 by Taylor & Francis Group, LLC
Mechanical Thermal Expression 305
0 5 10 15 20 25 30Applied pressure (MPa)
0.5
1.0
1.5
2.0
2.5Titania 20ºC
Lignite 20ºC
Titania 150ºC
Lignite 150ºC
Titania 80ºC
Lignite 90ºC
Void
ratio
FIGURE 17.3Void ratios versus applied pressures for compressible and noncompressible materials. (Reproduced from Clayton, S. A., Wheeler, R. A. and Hoadley, A. F. A. Drying Technol., 25(4), 533–546, 2007. With permission.)
FIGURE 17.4The mechanism of the consolidation process. (Reproduced from Clayton, S. A., Scholes, O. N., Hoadley, A. F. A., Wheeler, R. A., McIntosh, M. J. and Huynh, D. Q., Drying Technol., 24(7), 819–834, 2006. With permission.)
Pressure andtemperature
Temperature
Pressure
Effectiveporosity
Noneffectiveporosity== =
Solidlignite
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© 2009 by Taylor & Francis Group, LLC
306 Advanced Drying Technologies
and operating at 200°C, the energy needed to reduce the moisture content of brown coal (lignite) from 60 to 28% w.b. was estimated at 203 kJ/kg of raw coal. This number comprises the mechanical and thermal energy inputs being equal to 8 and 195 kJ/kg of raw coal, respectively. Because the energy needed to achieve the same moisture reduction through evapora-tion amounts to 1059 kJ/kg of raw coal, the use of the MTE process reduces the energy consumption by 80%. This percentage can be higher if less-effective convective drying is applied or if the material is dried in the fall-ing rate period (Clayton et al., 2006). For industrial applications, one must carry out a full technoeconomic study under local conditions.
Symbols
d.b. Dry basisMTE Mechanical thermal expressionw.b. Wet basisw/w Weight-by-weight
References
Bergins, C. 2003. Kinetics and mechanism during mechanical/thermal dewatering of lignite. Fuel, 82 (4): 355–364.
Bergins, C., Berger, S. and Strauss, K. 1999. Dewatering of fossil fuels and suspen-
Technol., 22(11): 923–927.Clayton, S. A., Scholes, O. N., Hoadley, A. F. A., Wheeler, R. A., McIntosh, M. J.
and Huynh, D. Q. 2006. Dewatering of biomaterials by mechanical thermal expression. Drying Technol., 24(7): 819–834.
Clayton, S. A., Wheeler, R. A. and Hoadley, A. F. A. 2007. Pore destruction resulting from mechanical thermal expression. Drying Technol., 25(4): 533–546.
Durie, R. A. (Ed.). 1991. The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization. Butterworth-Heinemann Ltd, Oxford.
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sions of ultrafine particles by mechanical/thermal dewatering. Chem. Eng.
© 2009 by Taylor & Francis Group, LLC
307
18Displacement Drying
One of the nonthermal methods of drying and dewatering is displace-ment drying, where water-immiscible solvent having a higher density than ggwater is used to displace liquid moisture from the wet material (Brandreth and Johnson, 1980; Devine, 1980). Such a displacement results from the combined action of buoyancy and surface forces when the wet material is immersed in a drying solvent. The mixture of both liquids is then sep-arated by gravity, centrifugal forces, or evaporation and recycled to the drying system. The process depends greatly on the ability of the displace-ment liquid to detach water droplets from the solid surface. Since the sur-face tension at the water–solvent and water–solid interfaces is a key factor
active agents. Advantages of displacement drying include
Reduced energy consumption compared to hot-air drying• Higher drying rates and therefore shorter drying times•Lower investment and operating costs• Capability of handling heavier water loads (up to 10% by volume) •
Additionally, this is a spot-free drying technique that leaves no trace of moisture on the material surface, which is especially important when dry-
circuit boards, relays, semiconductors, TV tubes), optics (spectacle frames
and mechanics (tools, cutlery, medical instruments). According to Devine (1980), there were over 600 drying systems in operation in Europe in 1980, ranging in capacity from 40 L to several cubic meters.
Two alternative mechanisms have been proposed to explain displace-ment drying (Brandreth and Johnson, 1980):
1. Roll-up mechanism (Figure 18.1a). When the surface-active agent dis-persed in a displacement liquid adsorbs at the solid–water interface, the wettability of the solid surface for water decreases and the area of contact becomes so small that buoyant forces overbalance the
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in water release, the process can be intensified by the addition of surface-
with no loss in drying efficiency
ing films, plastics, polished metals, ceramics, and so forth. So far, displace-ment drying has found applications in the fields of electronics (printed
and lenses, films, cameras), electromechanics (copiers, electric motors),
adhesion forces and finally cause water droplets to be released.
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308 Advanced Drying Technologies
2. Water–solvent interfacial tension lowering (Figure 18.1b). When the adsorption of a proper surfactant at the liquid–solvent interface results in a marked reduction of the water-displacement liquid tension. In effect, the skin on the water droplet is weakened, the
the surface of the solvent batch due to buoyant forces.
In most cases, both mechanisms contribute to the overall displacement effect, although to different extents. Because displacement drying is based on the surface phenomena, it is clear that internally bound water at the molecu-lar level, osmotically bound water, or crystal water cannot be removed by this technique. It is possible, however, to remove water held in small pores.
Larger parts may be sprayed with a solvent, whereas solid structures such
A perfect solvent to displace water should have high density and low
cible with water, and chemically inert. As a displacement liquid, a variety of organic solvents can be used. Of these, Freon F-113 is the most frequently selected because it is nontoxic (time-weighted average [TWA], 1000 ppm), its density is 50% greater than the density of water, its boiling point is only 48°C, and itd latent heat of evaporation at the boiling point is 146.5 kJ/kg. Unfortunately, Freons are not environment friendly and therefore their use
(a)
(b)
Mechanisms of displacement drying: (a) Roll-up of water droplet (water contact angle increases due to adsorption of a hydrophobic surfactant), and (b) water–solvent interfacial tension lowering (water breakup by lowering interfacial tension between liquid phases).
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droplet necks down in the presence of agitation, and a significant
In a typical configuration of a drying system, small size solids (e.g., fine
as textile fibers and particulate materials can be washed with a drying
fraction of primary water droplets is broken away and floated to
metal or plastic parts) are sunk into the displacement fluid (Figure 18.2).
fluid forced through the solid matrix.
interfacial tension for water. It should also be nontoxic, inflammable, immis-
© 2009 by Taylor & Francis Group, LLC
Displacement Drying 309
is restricted in various countries. In either case, the system should be per-fectly tight to avoid gas release.
The theoretical energy requirements in displacement drying are very small when compared to thermal drying because the external energy is used only for breaking the adhesion and sorption forces. Figure 18.3 pres-ents energy consumption against water loading for hot-air drying and dis-placement drying in a system used to dry small metal parts as shown in
a certain amount of energy because the solvent evaporates even under idle conditions. Under operation, energy consumption remains at the same level regardless of water load because of no water evaporation. On the contrary, energy consumption for hot-air drying is lower at no water load (although not equal to zero because energy is used to heat dry sol-ids) but rises sharply with moisture content. Starting from a water load of ∼37.5 kg/m3 of drying parts for which the energy consumed in each pro-cess is equal, displacement drying always offers energy savings.
Another variant of displacement drying is solvent drying, where theggsolid material to be dried is immersed in a miscible organic solvent that is heated up to its boiling point. During this extraction phase, water con-tained in the porous wet material is displaced by the solvent because of binary diffusion. Thus, the water concentration in the water/solvent mixture decreases gradually inside the material (solid phase), whereas it
FREON T-DF FREON TF WATER
Vapor zone
Separator
Heater
Rinsing sump
Drying sumpWater
drain
Dryingmaterial
Weir
Separator
Filter
Condenser
Solvent
Typical displacement drying system.
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Figure 18.2. At the no-load condition, the fluorocarbon system consumes
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310 Advanced Drying Technologies
increases in the surrounding liquid until equilibrium is established with equal concentration in both phases (Salin, 1990). Then, the material wetted with the solvent-reach mixture is dried thermally to remove the residual solvent. The evaporated solvent is recovered from the gas phase in a con-denser while the solvent from the solvent-lean postprocessing mixture is regenerated by simple distillation. As in the case of displacement drying in water-immiscible liquids, drying in an organic solvent is an energy-saving technology since the latent heat of evaporation of volatile solvents is much lower than that of water. Thus, the energy consumption may
drying and solvent regeneration.This method of drying was studied at the Forest Products Laboratory of
the Technical Research Center, Finland (Pajunen et al., 1988; Salin, 1990). The small-scale laboratory tests were carried out with pine sapwood and heart-wood (Pinus silvestris(( ) as well as with birch (Betula verrucosa) and spruce (Picea abies(( ) veneer. The wood samples were 25 × 25 × 80 mm, and theveneer samples were 1.5 × 90 × 95 mm. The solvents used were methanol,
0 50 100 150 200 2500
25
50
75
100
Convectivedrying
Displacementdrying
Water load (kg/m3)
Ener
gy co
nsum
ptio
n (k
Wh)
Comparison of energy consumption for hot-air and displacement drying.
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be reduced considerably, although thermal energy is needed for finish-
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Displacement Drying 311
ethanol, and 2-propanol at their boiling points. The tests were performed with green wood using various combinations of treatment times from 0.5 to 12 h and solvent-to-wood mass ratios ranging from 4 to 12.
The experimental data were interpreted in terms of the equilibrium between water content in the water–solvent mixture held in the wood pores and that in the surrounding solvent at the end of the extraction phase. A comparison of these two concentrations based on a simple mass balance indicates how far the displacement process proceeds during the treatment
treated for different periods from 0.5 to 3 h at a solvent-to-wood mass ratio of 4. It was found that equilibrium between two phases was attained during the shortest extraction time (30 min) and that further treatment did not have a substantial effect on water concentration. Such a relatively short time to reach equilibrium can likely be attributed to the relatively
Water content in surrounding liquid (%)
Wat
er co
nten
t in
liqui
d he
ld in
woo
d po
res (
%)
Birch–methanol; t = 0.5–3 h
Spruce–methanol; t = 0.5–2 h
Spruce–methanol; t = 0.5 h
Solvent-to-woodmass ratio—4:1
5 10 1500
5
10
15
20
FIGURE 18.4Concentration of water in the water–solvent mixture at the end of the extraction phase of green veneer. (Extracted from Pajunen, J., Salin, J. -G. and Vihavainen, T., Chemical drying of wood, VTT Research Report 523 [in Finnish], 1988.)
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time used. Figure 18.4 presents representative data for veneer samples
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312 Advanced Drying Technologies
small sample thickness (1.5 mm), as for the much thicker wood samples (25 mm), these time periods were much longer. For example, when solid pine sapwood was treated with methanol for different time periods from 2 to 12 h, the concentrations of water close to equilibrium were attained in ∼2 h; thus, the concentration did not substantially change when lon-
range of 8–17% w.b. When propanol was used instead of methanol, equi-librium was reached in ∼6 h. Because the processing temperature in the case of propanol was higher, the lower displacement rates must be due to a slower diffusion of propanol compared with methanol. Thus, the diffu-sion of methanol was estimated to be from three to six times faster than that for propanol, probably due to the lower molecular mass.
In the case of solid pine heartwood treated with methanol for periods of
5 h of extraction. The smaller displacement rates (by one order of magni-tude than those in sapwood) seem to result from the less-porous structure of heartwood, although other factors might contribute to the major differ-ences in the observed displacement rates (Salin, 1990).
To quantify the effect of wood anisotropy on displacement rates, some experiments were performed with solid pine sapwood where the sample ends were sealed to permit mass transfer only in the tangential and radial directions. These tests proved that after 2 h of treatment, the water con-centrations were very far from equilibrium, thus showing that the dis-placement rate is much faster in the longitudinal direction. The difference in these rates is about two orders of magnitude.
Concerning quality issues, no checking or other drying defects were noticed, but the sample dimensions used were small. The surface of the wood samples was light-colored compared with that in conventional dry-ing. The amount of extractives in the solvent phase after treatment was ∼0.2 to 0.3% of the wood’s dry weight for birch veneer and 0.4–0.7% for spruce veneer.
From the equilibrium data, it is clear that single-stage extraction requires
Thus, to reduce energy consumption, the process has to be realized in
extracted with almost pure solvent.To evaluate the potential application of solvent drying to the wood
industry, Salin (1990) made the following process design for veneer and ethanol as the most suitable solvent. He assumed that the water concen-tration in the liquid absorbed by the veneer is 20% higher than in the sur-rounding liquid after each extraction stage. Thus, for a given number of extraction stages, it was possible to calculate the moisture displacement in the veneer samples for different veneer–ethanol mass ratios and different initial water contents in the extraction liquid. The ethanol regeneration
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ger times were used. The final water content in the wood was within the
1–5 h, the final water concentrations were far from equilibrium even after
a multistage countercurrent configuration where residual moisture is
low water concentrations in the solvent phase and high solvent flow rates.
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Displacement Drying 313
TABLE 18.1
Energy Consumption versus the Number of Extraction Stages for Displacement Drying of Veneer in Ethanol
Number of Extraction Stages Energy Consumption (MJ/m3 of Veneer)
0a 12001 11103 9105 82010 730
a Conventional drying.Source: Extracted from Salin, J. G., Solvent drying of wood—some results, 7th
International Drying Symposium, Prague, Czechoslovakia, 1990.
stage was assumed to be a normal distillation unit in which the energy
the extraction stage, the corresponding energy consumption in the regen-eration stage was then calculated. Finally, the energy consumption in the thermal drying stage was estimated from the mass of ethanol and water held in the veneer after the extraction stage. The heat of evaporation for ethanol is only 29% of the corresponding value for water if the compari-son is based on equal liquid volumes. In Salin’s calculations, the theoreti-cal heat consumption was increased by 25% to account for heat losses.
Taking the minimum energy consumption as the design criterion, which translates into a minimum in operating costs, the following results were obtained:
There is an optimal ethanol concentration level of • ∼90% in the regenerated liquid, which is almost independent of other conditions.As the number of extraction stages increases, the optimal amount • of moisture displaced also increases.The total energy consumption decreases as the number of extrac-• tion stages increases (Table 18.1).
This preliminary process design for solvent drying of veneer with ethanol indicates that an energy consumption about one-third lower than that for a conventional process may be achieved when using 5–10 extraction stages to displace ∼70 to 90% of the water in the wet material. Such substan-tial energy savings could make displacement drying of wood products technically attractive, especially if other issues such as product quality and recovery of extractives can offset more complex equipment, handling
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consumption is primarily fixed by initial and final concentrations and by the total flow of liquid. For each combination of the process variables in
inflammable solvents, and so on.
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314 Advanced Drying Technologies
Symbols and Acronyms
t TimeTWA Time-weighted averagew.b. Wet basis
References
Brandreth, D. A. and Johnson, R. E., Jr. 1980. Displacement drying. In: Drying’80. A. S. Mujumdar (Ed.). Vol. 1. Hemisphere Publishing Corporation, Washington, DC, pp. 304–307.
report. In: Drying’80. A. S. Mujumdar (Ed.). Vol. 2. Hemisphere PublishingCorporation, Washington, DC, pp. 467–471.
Pajunen, J., Salin, J.-G. and Vihavainen, T. 1988. Chemical drying of wood. VTT Research Report 523 (in Finnish).
Salin, J. G. 1990. Solvent drying of wood—some results. 7th International DryingSymposium, Prague, Czechoslovakia.
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Devine, T. 1980. Displacement drying with fluorocarbon solvents: A status
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315
19Vapor Drying
The term vapor drying refers to a process of convective drying where the vapor of an organic solvent is used as the medium for heating the material to be dried and removal of the evaporated moisture. Because condensation
tion of air, the process heat is transferred to the drying material more rapidly and uniformly, so that drying times are considerably reduced. Moreover, the use of a partial vacuum in the system increases the rate of moisture removal. A distinction should be made at this point from super-heated steam (SHS) drying (Chapter 7) and airless drying (Chapter 8), where steam is used as the convective drying medium. Also, vapor dry-ing differs from solvent (displacement) drying (Chapter 18), where mois-ture is displaced by the solvent in the liquid phase due to surface tension forces or diffusion in the water–solvent mixture, and evaporation is used only to regenerate the spent solvent.
Credit for the early research on vapor drying belongs to Besemfelder (1910) and Hudson (1942, 1948, 1950), who proved that predrying of large-section lumber for subsequent pressure preservation by the use of sol-vent vapor is advantageous over conventional kiln drying. In addition to
drying degradation, Hudson found much better penetration of the pre-servatives into vapor-dried wood stocks. Such effects were attributed to the greater number of small internal checks that were formed in the outer layers of impervious wood species during vapor drying, compared with a smaller number of much bigger checks in air drying. This promising dry-
Industrial Research Organization (CSIRO), Division of Forest Products, by Ellwood and coworkers (1953) to determine whether the process of vapor drying could have commercial application to Australian native timber. The research was performed with impervious, collapse-susceptible spe-cies such as “ash” eucalyptus and myrtle beech, pervious timbers such as various pines, and miscellaneous impervious and moderately pervious timbers including subtropical rain forest species. Drying was carried out from the “green” and partly air-dried conditions down to moisture con-tents characteristic of seasoned lumber as well as the ones that required treatment with preservatives. The following solvents were examined: Stanvac K9 with an average boiling point of 196°C, mineral turpentine (160°C), perchloroethylene (121°C), and commercial toluene (110°C).
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of vapors offers much higher heat-transfer coefficients than forced convec-
a considerable reduction in drying time, higher final strength, and less
ing technology was further explored at the Commonwealth Scientific and
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316 Advanced Drying Technologies
The working pressure was varied from vacuum of ∼7 kPa through atmo-spheric pressure to an overpressure of ∼55 kPa. The wood temperature was kept within the range of 55–82°C. In general, the usefulness of each
lower volatility gave higher drying rates at a given working pressure but
pressures, which led to considerable losses. Perchlorethylene was useful
low boiling point was ranked as the best for the vapor drying process. Overall, these studies demonstrated a reduction in drying time with equivalent or better product quality by ∼30% in the drying of refractoryspecies and by ∼5–10% in drying pervious timbers as that required for kiln drying.
ties need to be solved for successful industrial application of the vapor drying technique. These are as follows:
Arrange for an even vapor distribution within the drying chamber •to ensure uniform temperature and drying conditions through-out the drying load
•ous timbers when high vacuum is used in the process
•
Vapor-phase heating has found another application in the drying of coils and insulation for shell and core-form power transformers before oil impregnation (Anonymous, 1965; Feather, 1965; Strumillo et al., 1971). These two processes in sequence greatly affect the cost, processing time, and reliability of air and moisture removal from the paper and paper-
for drying consists of hot air heating of the insulation followed by vacuum drying. As residual moisture in a transformer winding can cause failure, the drying conditions are very stringent on temperature uniformity and vacuum control. Aside from nonuniform drying, the major drawback of convective drying is the temperature constraint; at higher temperatures used to improve drying effectiveness, the surface layer of the insulation might be overheated, resulting in the discoloration and embrittlement of some or all of the insulation in windings because of oxidation by the hot air. Also, oxidation of the oil held on the surface of the insulation is a prob-lem if reprocessing of a transformer is required when part of its insulation has already been oil-impregnated.
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became inefficient at low pressures and temperatures. Toluene, the most volatile solvent, was found to be difficult to condense at low working
Despite the attractive features, some engineering and process difficul-
board insulation of oil-filled transformers. The conventional procedure
of these fluids was found to depend on its boiling point. The fluids of
over the widest range of drying conditions and as a fluid with a relatively
Prevent working fluid retention by pervious or moderately pervi-
Identify an inexpensive, nonflammable, and nontoxic working fluid
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Vapor Drying 317
(Feather, 1965). The entire process of heating, drying, and oil impregna-tion is carried out in the same drying chamber and consists of the follow-ing stages:
1. Preliminary evacuation of air from the chamber 2. Heating of the windings under partial vacuum using the vapor of
a boiling organic liquid 3. Final drying under high vacuum 4. Vacuum impregnation with oil
The vapor for heating windings is generated by boiling a shallow pool of the organic liquid in the bottom of the drying chamber. The heat
tion of the vapor in the drying chamber and allows water vapor released from the insulation to be taken away from the chamber with the excess vapor of the organic solvent. The mixture of both vapors is directed to the
Vacuum
Solventtank
Oiltank
Spent solvent
Drying chamber
Transformer
CondenserWallheaters
Boiling pan
Water
Separator
Pumps
FIGURE 19.1Schematic diagram of vapor drying of power transformers.
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input is sufficient to produce excess vapor. This ensures good circula-
A simplified flow diagram for vapor drying is shown in Figure 19.1
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318 Advanced Drying Technologies
condenser, and, after separation, the solvent is reclaimed for reuse. Once the desired temperature of the windings is reached, the liquid solvent is removed from the boiling pan, and the pressure in the drying chamber is further reduced. Such a vacuum operation removes residual water and condensed solvent from the insulation. Wall heaters provide additional heat for evaporation, if needed. At the completion of the vacuum drying,
Figure 19.2 compares the heating and drying characteristics for vapor drying followed by vacuum drying, forced convective drying, and vac-uum drying of the insulation of power transformers (Feather, 1965). The much faster drying and more complete moisture removal during vapor drying than for the other processes can be attributed to the higher heat-transfer rates and more uniform heating of the entire structure of the windings.
the operating temperature and pressure in the drying chamber are such that the organic vapor is saturated or only slightly superheated.
Time (h)
Tem
pera
ture
(ºC
)
Moi
stur
e rem
oved
(%)
1
2
4
5
3
5
4
3
1 2
0 2 4 6 8 10 12 14 16 180
1
2
3
4
5
6
7
20
40
60
80
100
120
140
160
FIGURE 19.2Heating and drying characteristics of the insulation structure of power transformers. 1—vapor drying: 150°C, 6 h; vacuum drying: 150°C. 2—vapor drying: 130°C, 6 h; vacuum drying: 130°C. 3—convective drying: 150°C, 8 h; vacuum drying: 150°C. 4—convective dry-ing: 130°C, 8 h; vacuum drying: 130°C. 5—vacuum drying: 130°C. (Adapted from Feather, L. E., Drying and oil impregnation of power transformer insulation, Proc. 6th ElectricalInsulation Conference, New York, September 13–16, Paper 32C3-8, 1965, pp. 65–69.)
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the chamber is filled with transformer oil to impregnate the windings.
To take advantage of the high condensing heat-transfer coefficients,
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Vapor Drying 319
The solvent to be used in vapor drying should have the following characteristics:
Its boiling temperature should match the desired operating tem-• perature at an absolute pressure that is well below the water vapor pressure at that temperature.The vapor pressure should be high enough to completely remove •the solvent that is trapped in the insulation at the end of the dry-ing stage.It should be compatible with the insulating oil, both electrically • and chemically, so that traces of solvent will not damage the windings and the insulating oils.
The petroleum solvent was the organic liquid that meets all these criteria and was used in the vapor drying of power transformers.
as follows:
Because the heat of condensation combines with the sensible heat • of the vapor, the heating rate is much higher, particularly within
Higher temperatures can be used without deterioration of the •insulation because no oxygen is present in the vapor of organic liquid.All subsequent stages can be performed in a single unit.• The presence of oil-impregnated insulation during reprocess-•ing creates no problems because oil oxidation during the heating stage is essentially eliminated. Because the organic liquid is com-patible with the impregnating oil, cross-contamination of the two liquids is eliminated.
In view of its evident advantages, this old but unpopular drying method is worth further exploration.
References
Anonymous, 1965. Transformer windings dried quickly and uniformly. Westing-house Eng., 25(4): 126.
Besemfelder, E. R. 1910. German Patent 261, 240.
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The key benefits of vapor drying of power transformer insulation are
the insulation structure, where it is difficult to maintain an ade-quate flow of hot air stream during convective heating.
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320 Advanced Drying Technologies
Ellwood, E. L., Gottstein, J. W., Kauman, W. G. and Wright, G. W. 1953. Vapour drying: A summary of research studies. Aust. For., 13(1): 4–16.
Feather, L. E. 1965. Drying and oil impregnation of power transformer insulation. Proc. 6th Electrical Insulation Conference, New York, September 13–16. Paper32C3-8, pp. 65–69.
Hudson, M. S. 1942. U.S. Patent 2,273,039.Hudson, M. S. 1948. U.S. Patent 2,435,218.Hudson, M. S. 1950. Drying lumber by the vapor process. Proc. Am. Wood Pres.
Ass., 46: 209–243.Strumillo, C., Michalowski, S., Grochowski, J. and Adamiec, J. 1971. Evaporation
drying. Inzynieria i Aparatura Chemiczna, 10(4): 16–17 (in Polish).
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321
20Slush Drying
Fresh fruit juices are complex aqueous slurries (80–90% by weight of water) containing numerous organic compounds. Most juices are very heat-sensitive because of the potential for enzymatic reactions and the
Conventional air-drying techniques such as spray drying, for example, require low-temperature operation, which prolongs the drying time and may even lead to undesirable microbiological reactions. Vacuum drying and microwave (MW)-vacuum drying reduce the temperature of evapora-
drying, however, allows product quality to be maintained, but the process
most fruit juices.To handle the dehydration of fruit juices, a technology called slush drying
was proposed and tested with apple juice for the potential loss of volatile
King, 1974). The principle of this method stems from the dependence of the freezing point on the concentration of dissolved solids (Figure 20.1). It boils down to the fact that drying takes place from an ice–liquid mixture (slush) in which 20–70% of the water present in the fruit juice is frozen (with the remainder still being liquid). The higher the dissolved solids content in the unfrozen liquid juice, the greater the retention of the vola-tiles. At the same time, there is no need for maintaining the ultralow tem-peratures required in freeze-drying (F-D), which allows favorable drying rates to be achieved.
Slush drying is somewhat analogous to freeze concentration (Kudra and Strumillo, 1998), except the stage in which separating the ice from the supernatant liquid in slush drying is accomplished by sublimation of the ice along with evaporation of some of the water from the liquid concen-trate. Similar to conventional thermal drying, slush drying is limited by the phase equilibrium (Figure 20.2), which indicates the maximum (equi-librium) percent of ice present at a given temperature and the dissolved solids’ concentration.
For any concentration–temperature conditions in the area below the equilibrium curve, the percent of water present as ice can be calculated from the lever-rule mass balance.
Essentially slush drying is a batch process, which starts with cooling the liquid to be concentrated to a temperature intermediate between the
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is difficult to carry out at a rate fast enough to be economically justified for
volatilization of flavor and aroma components.
tion, but the loss of volatile flavor species can still be significant. Freeze-
flavor and aroma substances (Chandrasekaran and King, 1971; Lowe and
© 2009 by Taylor & Francis Group, LLC
322 Advanced Drying Technologies
freezing point and the eutectic point of the slurry (see Figure 20.1). The semisolid material is then placed in a vacuum chamber and air is evacu-ated to reduce the pressure to ∼133 Pa. The ice crystals are sublimated by the heat supplied by an internal heater. The water vapor and volatile com-pounds are sucked away by a vacuum pump and condensed in a vapor trap (Figure 20.3).
As reported by Chandrasekaran and King (1971), the slush drying of apple juice takes place at a nearly constant drying rate with respect to time
A
C
Mass fraction Mass fraction
Tem
pera
ture
Enth
alpy
T = Constant
II EE
IV V
D
III
I
B
FIGURE 20.1Phase equilibrium of a binary system. I—unsaturated solution, II—saturated solution,
mixture + solute, AEB—saturated solution, and CED—three phases.
0 0.1 0.2 0.3 0.4 0.5–20
–16
–12
–8
–4
0
Solids’ concentration, (kg/kg)
Tem
pera
ture
(ºC)
Coffeeextract
Apple juice
Black currant juiceWine
FIGURE 20.2Freezing equilibriums for selected liquids.
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III—saturated solution + solute, IV—eutectic mixture + solidified solvent, V—eutectic
© 2009 by Taylor & Francis Group, LLC
Slush Drying 323
and the solids’ content, especially if an allowance is made for the lower amount of water contained in a given volume of juice at the higher solids’ concentrations.
Figure 20.4 indicates that the surface temperature during slush drying (and also during F-D) is well below 0°C. In contrast to F-D, where the surface temperature at the end of drying is always below the estimated eutectic temperature of –23°C, in slush drying the surface temperature
Sample
Cooler
Heater
Vacuum pump
Condenser
Drying chamber
FIGURE 20.3Block diagram for slush drying.
0 0.5 1.0 1.5 2.0 2.5 3.0–60
–50
–40
–30
–20
–10
0
Slush drying
F-D
17% w/w
44% w/w
Time (h)
Tem
pera
ture
(°C
)
FIGURE 20.4History of the surface temperature for slush drying and F-D of apple juice.
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© 2009 by Taylor & Francis Group, LLC
324 Advanced Drying Technologies
does not drop below –18°C. Experiments on liquid pool drying per-formed at a chamber pressure of 100 kPa have proved that the surface temperature in pool drying should be at 50–60°C to maintain the same order of drying rate as the combined evaporation and sublimation rates in slush drying (Chandrasekaran and King, 1971).
An interesting phenomenon observed in slush drying of apple juices was the formation of a stable and coarse foam over the surface of the slush bed. The amount and durability of the foam increased with increasing initial content of the dissolved solids. This foam formation did not occur in drying from a free liquid, but occurred in F-D to a minor extent near the end of dry-ing, when traces of the liquid began to appear on the surface. The formation of sugar-rich foam may explain the selective retention of aroma in slush
tion was found to be advantageous in aroma retention over air or vacuum
centration and temperature. Under the evaporation or drying conditions leading to substantial aroma retention, the loss of volatiles is governed by a diffusion mechanism. It is generally accepted (see Chandrasekaran and King, 1972a, 1972b) that aroma retention increases markedly in most drying processes with increasing the initial dissolved solids’ concentration. This
rapidly than those for water as the dissolved solids’ content increases. In slush evaporation, the separation of some of the water as ice leaves a liquid concentrate of the higher dissolved solids’ content. Evaporation from this liquid should then occur with improved volatile retention. Furthermore, any water removed by sublimation should not entail much volatile loss. Volatile compounds can be expected to remain with the concentrate upon partial freezing, rather than being incorporated into the ice crystals—a behavior that accounts for the high degree of volatile retention such as in freeze concentration. At the temperatures prevailing for slush evaporation, diffusion within the concentrate should keep the volatiles from accumulat-ing ice crystal–concentrate interfaces, unless the solubility limit is passed for the formation of a volatiles-rich oil phase. The low temperatures of slush evaporation should also help in aroma retention, as diffusivities of volatile solutes decrease more rapidly with decreasing temperature than does that of water (Chandrasekaran and King, 1972b; Thijssen, 1971).
Although slush drying has apparently yet to be commercialized, there
rates in slush drying, the F-D rates for fruit juices and other sugar-con-taining substances are lower because of the need for holding very low
ing (Bellows and King, 1972). At much higher temperatures of slush evap-oration, the mass-transfer driving forces are higher because of the much higher vapor pressure of water at the operating temperature, and hence the drying rates are much higher. Another advantage in comparison to
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drying from the liquid state because of the beneficial effects of solute con-
is a result of the diffusion coefficients for volatile solutes decreasing more
are several benefits over F-D. In contrast to the relatively high drying
frozen-zone temperatures to achieve adequate solidification during dry-
drying, as sugars are known to trap volatile flavors. In fact, slush evapora-
© 2009 by Taylor & Francis Group, LLC
Slush Drying 325
F-D is the lower refrigeration cost because of a higher condenser temper-ature. In comparison to freeze concentration, slush evaporation has the
product degradation and loss of aroma). In addition, there is no loss of the concentrate entrained with the ice during a solid–liquid separation as there is in freeze concentration.
References
Bellows, R. J. and King, C. J. 1972. Freeze-drying of aqueous solutions: Maximum allowable operating temperature. Cryobiology, 9: 559–561.
during drying of fruit juices. Chem. Eng. Prog. Symp. Ser., 67(108): 122–130.Chandrasekaran, S. K. and King, C. J. 1972a. Volatiles retention during drying of
food liquids. AIChE J., 18(3): 520–526.Chandrasekaran, S. K. and King, C. J. 1972b. Multicomponent diffusion and vapor-
liquid equilibria of dilute organic components in aqueous sugar solutions. AIChE J., 18(3): 513–520.
Kudra, T. and Strumillo, C. (Eds). 1998. Thermal Processing of Bio-Materials. Gordon and Breach Science Publishers, Amsterdam, The Netherlands.
Lowe, C. M. and King, C. J. 1974. Slush evaporation: A new method for concentra-tion of liquid foods. J. Food Sci., 39: 248–253.
Thijssen, H. A. C. 1971. Flavor retention in drying preconcentrated liquid foods. J. Appl. Chem. Biotechnol., 21: 372–377.
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same benefits resulting from lower operating temperatures (e.g., reduced
Chandrasekaran, S. K. and King, C. J. 1971. Retention of volatile flavor components
© 2009 by Taylor & Francis Group, LLC
327
21Atmospheric Freeze-Drying
Vacuum freeze-drying (VF-D) is an expensive operation—up to an order of magnitude more expensive than the next expensive drying operation, namely, spray drying. The high capital and energy costs stem from the need to generate and maintain very low temperatures and very high
high-value materials and high-quality/special properties products such as pharmaceuticals, nutraceuticals, enzymes, and certain products of biotechnology.
Avoiding the need for a condenser for the sublimated vapor (which must be held at temperatures well below 0°C), enhancing contact between the heat- and mass-transfer phases, and operating the process at near- atmospheric pressure are the methods tested to reduce the cost of freeze- drying (F-D). If the frozen product is in contact with a suitable solid adsorbent, the sublimated vapor is adsorbed by the solid, which performs the role of the external condenser in the conventional VF-D
sive layers, the adsorbent captures the water vapor. The heat of sorption transferred to the gas stream is then used for subsequent sublimation. Since the solid–solid contact is poor, the heat- and mass-transfer rates
a heat-transfer medium and a water vapor vehicle due to random colli-sions of the product with the absorbing particles (Wolff and Gibert, 1991). If the heat required for sublimation is nearly balanced by the exothermic heat of adsorption of the water vapor, then the adsorbent acts as a heat
mixed, the bulk of solid adsorbent is far from saturation as mass trans-fer takes place also between the adsorbent particles of different satura-tions. Moreover, a continuous F-D process can be more readily developed with atmospheric conditions rather than the conventional high-vacuum process. The main limitation of such a system is the relatively long pro-
freeze-dried product, and therefore adsorbents that are compatible with
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levels of vacuum. Therefore, this technique is justified mainly for very
ticles have been used in various process configurations. King and Clark (1969), for example, proposed alternating fixed beds of frozen material
figuration is more efficient, however, because the absorbent acts as both
cessing time. In addition, it is difficult to separate the adsorbent from a
process. Both fixed packed beds and fluidized beds of adsorbent par-
with beds of adsorbents. As low-pressure gas flows through the succes-
are low and hence the drying times are very long. The fluidized bed con-
carrier and mass-transfer agent as well. Because the fluidized bed is well
© 2009 by Taylor & Francis Group, LLC
328 Advanced Drying Technologies
atmospheric freeze-drying (AF-D) is depicted in Figure 21.1.As reported by Wolff and Gibert (1990a, 1990b, 1991), the drying
kinetics of AF-D depends not only on the bed temperature and material thickness but also on the operating pressure. Experiments were per-
nized cornstarch (0.16 mm in diameter) that was found to be compatible
Heating Heating
F-D
Separation
Regeneration
Cooling
Cooling
Freezing
Fresh material
Adsorbent
Freeze-driedproduct
Tr
Tfd
Tfd
Tfd Tfd
Tf
T0
T0
T0
T0T0
Tr
Tr—regeneration temperature Tf—freezing temperatureT0—inlet (ambient) temperature Tfd—F-D temperature
FIGURE 21.1
8(2), 385–404, 1990a. With permission.)
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cooling liquid (6 kW at –25°C). The column was filled with pregelati-
Process configuration for continuous AF-D. (After Wolff, E. and Gibert, H., Drying Technol.,
the materials to be dried are necessary. The flow diagram for continuous
formed in a fluidized bed F-D column fitted with a double jacket for
© 2009 by Taylor & Francis Group, LLC
Atmospheric Freeze-Drying 329
with frozen (–25°C) potato parallelepipeds (10 mm square base and 2, 3, and 5 mm thickness). In the course of drying, a potato sample of 0.3 kg
ture was kept at 50, 57, 65, 72, and 85°C, and the bed temperature was maintained at –5, –10, and –15°C, to avoid melting of the product. As expected, an increase in bed temperature and a decrease in potato thickness resulted in a higher drying rate (Figure 21.2). The regenera-tion temperature of the starch studied over the range of 50–85°C did not affect the drying kinetics for the material/adsorbent mass ratio used in these experiments and in the studies by Wolff and Gibert (1990a). The
to permit its recycling.Because the shape of the drying curves indicates that internal resis-
tance to mass transfer is the limiting factor in AF-D, the higher drying rates were obtained at lower levels of operating pressure. According to Wolff and Gibert (1991), a pressure reduction by 500 hPa shortens by a factor of 2 the total F-D time needed to decrease the moisture content of potato samples from 4 to 2 kg/kg. Optimum pressure was found to be ∼10 kPa.
Potato 10 × 10 × 3 mm
0 1 2 3 4 5 6 7 8 9
Moi
stur
e con
tent
(kg/
kg)
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (h)
T = –5ºC
T = –15ºC
T = –10ºC
Model
ized bed of cornstarch. (After Wolff, E. and Gibert, H., Drying Technol., 8(2), 405–428, 1990b. With permission.)
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with superficial velocity equal to 0.04 m/s. The regeneration tempera-
regeneration temperature of 65°C for starch was found to be sufficient
was immersed into a 5 kg bed of cornstarch fluidized by air flowing
The effect of bed temperature on F-D kinetics for potato particles 10 × 10 × 3 mm in a fluid-
© 2009 by Taylor & Francis Group, LLC
330 Advanced Drying Technologies
Assuming a uniformly retreating ice front and using sorption isotherms for the solid adsorbent, Wolff and Gibert (1990b) proposed the following
A p pR bR
M
Pa
dadX
Baa
dpdt
M A
c
e i
a
w
w
ar
wa
w
c
a r
( )
1
log
exp
B X
H MR
Ta
dadX
Baa
dTdt
r av
w
w
ar
wa
w
( )
2 10
log
(21.1)
where Re and Ri are the external and internal resistances to the combined
Rk h
H pMRTe
e e
s1 1 2
2
(21.2)
RRT
MDH pMRTi
w
s 1 2
2
Derivations of Equations 21.1 through 21.3 along with simulation results for different process conditions are given by Wolff and Gibert (1990b).
To compare AF-D with conventional F-D, a series of experiments was conducted with potato particles spread in a monolayer in a conventional freeze-dryer (Wolff and Gibert, 1990a). The heating plate temperature was set at 25°C while the condensing plate temperature was set at –65°C. Drying kinetics were obtained at the operating pressure of 1 Pa, which is equivalent to the product core temperature of about –50°C, as well as at 300 Pa, which corresponds to a core temperature of –8°C, which is close to the bed temperature in AF-D. As seen in Figure 21.3, the drying kinetics for AF-D does not exhibit a constant-rate period as it does in VF-D. The average drying rate in AF-D was estimated at 0.09 kg of water per kilo-gram of fresh product and per hour against 0.15 kg/kg h and 0.42 kg/kg h for VF-D at 1 and 300 Pa, respectively.
Because of the internally controlled mass transfer, drying times for AF-D are longer than that for the VF-D. The difference in drying times increases with particle size (thickness). Thus, for 5 mm thick particles, ∼2 h are needed to reduce the moisture content from 4 to 2 kg/kg in the vacuum process, whereas 6 h are required for AF-D.
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heat and mass transfers, respectively, defined as
model for AF-D in a fluidized bed of particulate absorbent:
© 2009 by Taylor & Francis Group, LLC
Atmospheric Freeze-Drying 331
For 3 mm thick particles, these drying times are shortened to 1.5 and 2 h, respectively.
Although the color and taste of potato dried with these two methods were not different, the edges and corners of the potato particles were
(Wolff and Gibert, 1990a). An average thickness of the product from AF-D was 12% smaller when compared to 5% from the vacuum process. Also, up to 2% w/w of cornstarch remained at the surface of the product from AF-D, which should be taken into account when selecting the adsorbent for the particular product.
Similar results regarding drying kinetics were reported by Di Matteo et al. (2003), who studied AF-D of 10 mm long potato cylinders of 6, 8,
520 W/m2
nal heat- and mass-transfer rates.Considering process economics, Wolff and Gibert (1990a) compared the
energy consumption to remove 1 kg of water contained in the product at 3 kg/kg of the initial moisture content. They calculated the cooling and heating energy requirements for VF-D as 3550 and 3780 kJ, respectively.
0 1 2 3 4 5 6 7 8 9Time (h)
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Moi
stur
e con
tent
(kg/
kg)
Vacuum F-D
AtmosphericF-D
300 Pa, 5 mm1 Pa, 5 mm1 Pa, 3 mm
–5ºC, 5 mm
–5ºC, 3 mm
FIGURE 21.3Comparison between F-D and AF-D for potato particles. (After Wolff, E. and Gibert, H., Drying Technol., 8(2), 385–404, 1990a. With permission.)
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nal heat- and mass-transfer coefficients were relatively high (from 400 to
a key parameter to shortening the drying time over enhancing the exter-K and from 0.073 to 0.143 m/s), the sample size was identified as
found to be rounded after AF-D because of erosion due to fluidization
and 11 mm diameter immersed in a fluid bed of various adsorbents (bran, corn flour, starch, zeolite, and bentonite). Even though the exter-
© 2009 by Taylor & Francis Group, LLC
332 Advanced Drying Technologies
In AF-D, the energy requirement for cooling was calculated as 2250 kJ, whereas that for heating (regeneration) was 3440 kJ. The authors point out that these theoretical energy requirements are underestimated as they did not account for energy losses and savings due to batch (vacuum) and continuous (atmospheric) processings. After the calculation procedure was adjusted for real process conditions, the energy costs were estimated to be lower by 38% on cold requirements and 34% on heat requirements
tion rate (SMER) values for AF-D have been reported to be in the range of 4.6–1.5 kg of water per kWh as opposed to a vacuum freeze-dryer, where the SMER may be as low as 0.4 or less (Claussen et al., 2007; Strømmen et al., 2005).
The feasibility of AF-D for shrimp was demonstrated by Donsi et al. (2000), who showed that the drying time is indeed an order of magnitude longer than in the vacuum process. To shorten the drying time, the part of the water was removed through an osmotic dehydration process. F-D
(zeolite particles, 88 μm mean diameter) or organic adsorbents (wheat bran). The dried shrimp had rehydration properties that are similar to those obtained in conventional F-D. The economics of the process were not reported, however.
freeze-dryer at atmospheric pressure in a two-stage heat pump system
components to keep the air temperature below the drying products’ freez-
was selected so as to reduce the moisture content to the critical value.
bed to be dried at higher temperatures. The advantages of the two-stage system are that the low-temperature drying reduces the moisture content while maintaining product quality and the higher-temperature drying increases the overall heat pump dryer capacity. The authors reported excellent quality of dried shrimp, apple pieces, carrot slices, etc., albeit at a relatively high cost.
Rogers et al. (2003) found that an atmospheric freeze-dried pharmaceu-tical product (danazol micronized by atomizing into liquid nitrogen) had a lower surface area than powders produced by VF-D, but a much higher area than powder obtained by micronizing the bulk drug. However, the solubility of the atmospheric freeze-dried danazol was similar to the dis-
was assessed as a commercially viable method to enhance the dissolution
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when compared to the VF-D. The energy consumption can be signifi-cantly reduced when using a heat pump; typical specific moisture extrac-
but with no adsorbent (see Chapter 15). Drying conditions at the first-
ing point. This control is based on the specific enthalpy curves developed by Alves-Filho (1996). The product residence time in the first-stage dryer
solution profile of the vacuum freeze-dried powder. Therefore, the AF-D
of the hydrophobic drugs in place of costly and difficult-to-scale VF-D.
was carried out in a fluidized bed apparatus using mineral adsorbents
Alves-Filho et al. (1998) have also used a fluidized bed as the first-stage
stage fluid bed dryer were controlled by adjusting the heat pump dryer
Afterward, the semidry product was transferred to the second-stage fluid
© 2009 by Taylor & Francis Group, LLC
Atmospheric Freeze-Drying 333
Comparative studies on AF-D, VF-D, and convective drying of chitosan membrane and granules (a kind of chitin derivative) indicated that the volume and surface area of the mesopores and micropores in the AF-D product were about two times smaller than that in the VF-D product and
(Stawczyk et al., 2006).It is clear that AF-D is affected by parameters such as drying tempera-
ture, material thickness, operating pressure, and dry matter content. To maintain a highly porous structure, AF-D must be operated fully in the solid state without allowing the ice to melt at any time during the process. Drying conditions can affect the product quality. Drying of milk, orange juice, and coffee extract present different problems due to freezing of the lipids in milk, a highly hygroscopic product, and the need for high qual-ity, respectively.
Very few research groups appear to be devoting effort to experimental and modeling studies of AF-D despite the potential for such technology being more cost-effective than VF-D and providing a better-quality prod-uct than vacuum drying and possibly low-pressure superheated steam (SHS) drying. Claussen et al. (2007) have presented a comprehensive review of the basics, applications, and modeling studies on AF-D. They considered technological aspects, product characteristics, drying kinetics, mathematical modeling, product quality, etc. However, no detailed study has appeared so far that presents reliable data on energy consumption and cost of drying per unit mass of a dry product.
Lombrana and Villaran (1997) examined the adsorption F-D of 2 mm of
idized bed of 0.7 mm zeolite. A mathematical model was developed for F-D at atmospheric pressure and under vacuum, which assumes a uniformly retreating ice front in a spherical particle. This model allows simulation of the drying kinetics and determination of the temporal variation of pres-sure and temperature at the sublimation front with the product moisture content. Several operational strategies built on various temperature and
time without causing serious damage to the product. The most favorable strategy consisted of drying at atmospheric pressure and at a tempera-ture of −10°C, followed by a period of reduced pressure and temperature below +15°C. Because of the alternating periods of atmospheric pressure and vacuum, it was possible to reduce the drying time by up to 50%.
Menshutina et al. (2004, 2005, 2006) developed a mathematical model for AF-D in a spouted bed of frozen droplets and validated the model in experiments with a 15% w/w mannitol solution. The model comprises a set of equations for mass, impulse, and energy that is based on the het-erogeneous media mechanics and equations for drying kinetics. Good
period.
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five times larger than that in the product obtained by convective drying
fit of model and experimental data were reported over the entire F-D
commercial cereal food paste wetted to 63.5% w.b. and immersed in a flu-
pressure regimes in the fluidized bed were analyzed to reduce the drying
© 2009 by Taylor & Francis Group, LLC
334 Advanced Drying Technologies
2007) to simulate the phase change and water vapor diffusion within porous media. The model was validated for the AF-D of 10 mm apple
vapor diffusion dominates the AF-D at low temperatures (below 0°C). It was found that water migration was enhanced by absorption and desorp-tion processes taking place in a porous apple tissue. The results of these studies also indicated that the true values of tortuosity and the internal
Alves-Filho et al. (2007) reported on experimental and numerical studies
well with their experimental kinetic data. Tests performed in a heat pump AF-D showed that the color and rehydration ability of the pepper cubes were as desired. Interestingly, the authors noted that the product quality can be adjusted by selecting the appropriate operating temperature and pressure, which are highly product-dependent.
A heat pump AF-D of 10 mm apple cubes was also studied by Stawczyk et al. (2005) to determine the effect of drying kinetics on product quality (dehydration rate, shrinkage, color, and antioxidant properties). The exper-imental results were compared with the results of convective and VF-D. It was found that the AF-D process that runs at temperatures around −10°C led to a highly porous structure of the product, whereas drying at around 0°C resulted in reduced product quality. The apple cubes dried at lower temperatures had similar rehydration kinetics and hygroscopic proper-ties as the VF-D cubes. All the AF-D apple cubes had a higher antioxidant activity than the hot air–dried cubes. The optimal drying trajectory was found for ascending temperature drying (–(( 16/–4/4/22°C), where the mid-dle melting region and constant drying rate occur (Stawczyk et al., 2005).
Symbols
aw Water activityawa Water activity at reference temperatureA Transfer area, m2
Ar Constantb Thickness of the dried layer, mBr ConstantDw Diffusivity of water vapor in the dried layer, m2/she
2
keM Molar mass, kg/molMa Molar mass of the adsorbent, kg/mol
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A film sublimation–based CF-D model was developed by Li et al. (2006,
resistance coefficient are critical for process simulation, and therefore the values assumed for the model should be confirmed experimentally.
on the AF-D of red pepper cubes. A modified diffusion equation fitted
External heat-transfer coefficient, W/m KExternal mass-transfer coefficient, m/s
cubes. A higher mass flux was identified on the leading edge, whereas the flux within the porous zone was relatively uniform, which indicated that
© 2009 by Taylor & Francis Group, LLC
Atmospheric Freeze-Drying 335
p Partial pressure of water vapor at saturation, Papc P Operating pressure, PaR Gas constant, J/K molt Time, sT Temperature, KX Adsorbent moisture content, kg/kg∆Hs Latent heat of sublimation, J/kg∆HvHH Latent heat of vaporization, J/kgλ Thermal conductivity of the dried layer, W/m Kw.b. Wet basisw/w Weight by weight
Acronyms
AF-D Atmospheric freeze-drying
VF-D Vacuum freeze-drying
References
heat and mass transfer on dryer characteristics. Ph.D. Thesis. Norwegian University of Science and Technology. Trondheim, Norway.
Alves-Filho, O., Eikevik, T., Mulet, A., Garau, C. and Rossello, C. 2007. Kinetics and mass transfer during atmospheric freeze drying of red pepper. Drying Technol., 25(7–8): 1155–1161.
Alves-Filho, O., Thorbergsen, E. and Strømmen, I. 1998. A component model for
IDS98. C. B. Akritidis, D. Marinos-Kouris and G. D. Saravakos (Eds.). ZitiEditions, Thessaloniki, pp. 94–101.
Claussen, I. C., Ustad, T. S., Strømmen, I. and Walde, P. M. 2007. Atmospheric freeze drying: A review. Drying Technol., 25(6): 947–957.
Di Matteo, P., Donsi, G. and Ferrari, G. 2003. The role of heat and mass transfer
Eng., 59(2–3): 267–275.Donsi, G., Ferrari, G. and Di Matteo, P. 2000. Atmospheric and vacuum freeze-
drying kinetics of shrimps. Proc. IDS’2000. Amsterdam, The Netherlands.King, C. J. and Clark, J. P. 1969. System for freeze-drying. U.S. Patent 3,453,741.Li, S., Stawczyk, J. and Zbicinski, I. 2007. CFD model of apple atmospheric freeze
drying at low temperature. Drying Technol., 25(7–8): 1331–1339.
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SMER Specific moisture extraction rate, kg/kWh
Partial pressure of water vapor in the fluidized bed, Pa
CFD Computational fluid dynamics
Alves-Filho, O. 1996. Heat pump drying of fruits and roots: The influence of
simulation of multiple fluidized bed heat pump dryers. In: Drying’98. Proc.
phenomena in atmospheric freeze-drying of foods in a fluidised bed. J. Food
© 2009 by Taylor & Francis Group, LLC
336 Advanced Drying Technologies
Li, S., Stawczyk, J., Zbicinski, I. and Zylla, R. 2006. CFD model of apple atmo-spheric freeze drying at low temperature. Proc. 15th International Drying Symposium (IDS 2006), August 20–23, Budapest, Hungary. Szent Istvan University Publisher; Godollo, Hungary, pp. 477–484.
on freeze-drying in an adsorbent medium and establishment of drying strat-egies. Food Res. Int., 30(3–4): 213–222.
Menshutina, N. V., Korneeva, A. E., Goncharova, S. V. and Leuenberger, H. 2004.
CHISA 2004. August 22–26, Prague, Czech Republic.Menshutina, N. V., Korneeva, A. E. and Gordienko, M. G. 2006. Design of dry-
Drying Symposium (IDS 2006), August 20–23, Budapest, Hungary. Szent Istvan University Publisher; Godollo, Hungary, pp. 1180–1183.
Menshutina, N. V., Korneeva, A. E., Leuenberger, H. and Plitzko, M. 2005. Modell-ing and design of atmospheric freeze-drying in spouted bed. 7th WorldCongress of Chemical Engineering. July 10–14, Glasgow, Scotland.
Rogers, T. L., Nelson, A. C., Sarkari, M., Young, T. J., Johnston, K. P., Williams, R. O. 2003. Enhanced aqueous dissolution of a poorly water soluble drug by novel particle engineering technology: Spray-freezing into liquid with atmospheric freeze-drying. Pharm. Res., 20: 485–493
Stawczyk, J., Li, S. and Modrzejewska, Z. 2006. Chitosan stuffs atmospheric freeze-drying kinetics. Proc. 15th International Drying Symposium (IDS 2006), August20–23, Budapest, Hungary. Szent Istvan University Publisher; Godollo, Hungary, pp. 1204–1208.
Stawczyk, J., Li, S., Witrowa-Rajchert, D. and Fabisiak, A. 2005. Kinetics of apple atmospheric freeze-drying. Proc. 11th Polish Drying Symposium. September 13–16, Poznan, Poland.
Strømmen, I., Alves-Filho, O. and Eikevik, T. M. 2005. Atmospheric freeze drying with heat pumps—a new alternative for high quality dried food products. Proc. 3rd Nordic Drying Conference. June 15–17, Karlstad, Sweden.
Wolff, E. and Gibert, H. 1990a. Atmospheric freeze-drying. Part 1: Design, experi-mental investigation and energy-saving advantages. Drying Technol., 8(2): 385–404.
Wolff, E. and Gibert, H. 1990b. Atmospheric freeze-drying. Part 2: Modelling dry-ing kinetics using adsorption isotherms. Drying Technol., 8(2): 405–428.
Wolff, E. and Gibert, H. 1991. Freeze-drying under vacuum and in an adsorb-
Drying’91. A. S. Mujumdar and I. Filkova (Eds.). Elsevier Science Publishers, Amsterdam, The Netherlands, pp. 237–240.
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Lombrana, J. and Villaran, M. C. 1997. The influence of pressure and temperature
Model development of atmospheric freeze drying in fluidized bed. Proc.
ing process flowsheet of pharmaceutical substances. Proc. 15th International
ing fluidized bed: Influence of operation pressure on drying kinetics. In:
© 2009 by Taylor & Francis Group, LLC
337
22Spray-Freeze-Drying
to classical freeze-drying (F-D) such as sublimation under atmospheric
bents (Boeh-Ocansey, 1988; Lombrana and Villaran, 1996; Lombrana, 1997). A step forward in producing materials of unique properties is spray-
particles with high surface area, porosity, and enhanced solubility. Thus, SF-D has a great potential for producing drugs, nutraceuticals, functional foods, advanced materials, and other high-value products.
SF-D can broadly be categorized as SF-D into liquids (SFL), SF-D into
et al., 2007). Each technology can be carried out at atmospheric pressure or under vacuum.
In SFL, a solution is atomized into a cryogenic medium such as liquid nitrogen to freeze the droplets (see Constantino et al., 2000; Rogers et al., 2003). The dispersion of frozen droplets in the cryogenic medium is then separated and dried in a freeze-dryer at atmospheric pressure or under vacuum (Figure 22.1). This mode of SF-D makes it possible to obtain nano-structured particles with a large surface area. Because of F-D, this technol-ogy is better suited for high-value products of low tonnages.
In SFG, the solution is atomized into refrigerated air (gas), so that the droplets freeze instantly (Lebedev and Bihovsky, 2001). The frozen micro-spheres are then freeze-dried in a classical freeze-dryer or in atmospheric freeze-dryers as described in Chapter 21.
In SFG-FB, the particles frozen in cooled gas are then transported to
et al., 2006).Although it is used in the technical literature, the aforementioned ter-
minology is not precise since each process starts with freezing the dis-persed solution by spraying it into either liquid or gas and then F-D the
tial processing by freezing and drying, where the F-D time is relatively long, SF-D is usually carried out in batches.
To avoid transportation of previously frozen particles to the freeze-dryer, we propose to exploit the advantages of the spray dryer with an inter-
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An increasing interest has been noted recently on various modifications
frozen droplets in various process configurations. Because of the sequen-
pressure (see Chapter 21) and sublimation into a fluid bed of solid adsor-
freeze-drying (SF-D), which offers free-flowing, ultrafine, and uniform
gases (SFG), and SF-D into gases over a fluidized bed (SFG-FB) (Claussen
the drying chamber and freeze-dried in a fluidized bed (see Leuenberger
nal fluid bed (see Chapter 1) and configure the system into atmospheric spray-fluidized-bed freeze-drying (ASFBF-D). Although a continuous
© 2009 by Taylor & Francis Group, LLC
338 Advanced Drying Technologies
vacuum SF-D system, in which the frozen droplets are freeze-dried in a
operated in a batch mode, the liquid feed is atomized by a pressure nozzle located at the top of a spray dryer. The droplets are frozen instantly by a stream of air or nitrogen that is cooled to approximately –90°C and
cold gas could be supplied to the top of the spray dryer, and this would
frozen by the cold gas stream from cooler I and fed at the top of the spray
design and operating conditions of such a dryer need to be established by experiments. Huang (2008) is in the process of developing a two-stage
Table 22.1 compares the basic features of the three technologies involv-ing F-D against conventional spray drying (SD).
The following paragraphs outline the key research on SF-D to indicate its potential in producing materials of special properties.
Sonner et al. (2002) investigated the use of SFL technology to produce protein-loaded particles that could serve as a carrier for drug delivery and found it a viable alternative to conventional F-D. The internal and external morphologies of powdery particles (20–80 μm) made from pure excipients
Feed tank
Freeze-dryer
Separator
Pressurenozzle
Cryogenicliquid (N2)
Liquid N2
FIGURE 22.1Principle of SFL.
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beds offer preferential transfer coefficients. Thus, in this new arrangement
reduce or even eliminate the carryover of fine frozen droplets that oth-erwise should be separated in an internal filter. Once the spray-freezing
basically fixed bed, was designed and tested (see Chapter 33), the fluidized
blown to the fluidized-bed dryer. Alternatively, all or a fraction of the
process is completed, the temperature and flow rate of a gas stream fed
product (Figure 22.2). In a continuous operation, the liquid spray would be to the fluidized bed dryer are adjusted to meet the F-D conditions for the
dryer, whereas the bed of fluidized frozen droplets would be dried by another gas stream supplied to the fluidized-bed dryer from cooler II. The
spray-fluid bed freeze-drying system in China.
© 2009 by Taylor & Francis Group, LLC
Spray-Freeze-Drying 339
TABLE 22.1
Comparison among Four Drying Operations
SD F-D SF-D ASFBF-D
Drying time Short Long Long MediumProduct form Agglomerated
or irregularCake Spherical and
porous, monosized
Spherical and porous
Product quality Medium Good Good Good Product yield High Medium Low MediumEnergy consumption Low High Very high MediumOperation Continuous Batch Batch Semicontinuous,
or continuousInvestment cost Low High High High
Feed tank
Air / N2 tank
Cooler I
Cooler II
Pressurenozzle
IntegratedFB dryer
Spraydryer
FIGURE 22.2Flowchart for ASFBF-D.
such as mannitol, trehalose, dextran, and polysorbate in various composi-tions were examined. A trehalose/mannitol/dextran (0.4:0.4:0.2) formula-
during the F-D step exhibited homogeneous external and internal mor-phologies. The model protein trypsinogen lost 15% activity during SF-D.However, it was found that trypsinogen inactivation during SF-D could be completely prevented by the presence of trehalose in the formulation.
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tion was identified, for which the SF-D particles prepared using annealing
Of particular interest is the finding that the atomization of an aqueous
© 2009 by Taylor & Francis Group, LLC
340 Advanced Drying Technologies
trypsinogen solution into liquid nitrogen reverses protein inactivation shown to occur on atomization into air (Sonner et al., 2002).
Maa et al. (1999) compared SF-D protein powders with those obtained by SD. Classical SD generated small (3μm) and dense particles as opposed to SF-D, which produced porous and larger (8–10 μm) particles but with
erties, powders obtained by the SF-D process offered superior aerosol per-formance during inhalation.
Mumenthaler and Leuenberger (1991) developed an apparatus and a technique for spray-freezing aqueous solutions in situ at very low temper-atures (–(( 90°C) and for the subsequent dehydration of the resulting frozen
and mass transfer between the circulating drying medium and the frozen sample; (b) high and homogeneous quality properties of the dry prod-uct with an increased retention of volatile aromatic compounds in foods;
vides good instant properties, that is, wetting and solubility.It is noteworthy that SF-D was also used to produce inorganic mate-
rials of desired properties. For example, Yokota et al. (2001) obtained magnesium sulfate powders of high porosity in the order of 87–90% with open pores on the particle surface. The pore size was <100 nm. Finepowders of several other materials, for example, calcium phosphate and tetragonal zirconia polycrystals, were produced by SF-D, as well (Itatani et al., 2000). Nonetheless, the disadvantages of F-D are still present in the
process. However, the small size of the particles to be freeze-dried con-siderably shortens the drying time.
Because of novelty, the majority of research on SF-D focuses on experi-mental validation of this technology. Apparently, we came across only one paper dealing with process modeling/simulation where the SF-D of dex-tran and mannitol was modeled and experimentally validated, assum-ing the same mass-transfer kinetics as in the high-temperature drying (Leuenberger et al., 2004). No details have been given, however.
Acronyms
AF-D Atmospheric freeze-drying
F-D Freeze-dryingSD Spray dryingSF-D Spray-freeze-dryingSFG Spray-freeze-drying into gasesSFG-FB SFL Spray-freeze-drying into liquid
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preferential fine particle fraction. Because of the better aerodynamic prop-
particles in a stream of cold and dehumidified air. Compared to the classi-cal F-D process, the following differences were identified: (a) improved heat
SF-D process since vacuum freeze-drying (VF-D) is used as the final drying
and (c) free-flowing powder with a large internal surface area that pro-
ASFBF-D Atmospheric spray-fluid-bed freeze-drying
Spray-freeze-drying into gases over the fluid bed
© 2009 by Taylor & Francis Group, LLC
Spray-Freeze-Drying 341
References
7(2): 127–146.Claussen, I. C., Ustad, T. S., Strømmen, I. and Walde, P. M. 2007. Atmospheric
freeze-drying: a review. Drying Technol., 25(6): 947–957.Constantino, H. R., Firouzabadian, L., Hogeland, K., Wu, C., Beganski, C.,
Carrasquillo, K. G., Cordova, M., Griebenow, K., Zale, S. E. and Tracy, M. A. 2000. Protein spray-freeze drying. Effect of atomization conditions on parti-cle size and stability. Pharm. Res., 17(11): 1375–1383. Huang, L. 2008. Personalcommunication, Chinese Academy of Agriculture, Nanjing, China.
Itatani, K., Iwafune, K., Howell, F. S. and Aizawa, M. 2000. Preparation of various calcium-phosphate powders by ultrasonic spray freeze-drying technique. Mat. Res. Bull., 35: 575–585.
freeze-drying and development of a new apparatus for continuous opera-tion. J. Ind. Heat Eng., 23(1&2): 67–75.
Leuenberger, H., Plitzko, M. and Puchkov, M. 2004. Modelling of the spray-freeze drying process. Proc. International Meeting on Pharmaceuties, Biopharmaceuties and Pharmaceutical Technology. March 15–18, Nuremberg, Germany, pp. 871–872.
ized bed at normal and low pressure. Drying Technol., 24(6): 711–719.
in an adsorbent medium and establishment of drying strategies. Food Res.Intl, 30(3–4): 213–222.
Lombrana, J. I. and Villaran, M. C. 1996. Interaction of kinetic and quality aspects during freeze-drying in an adsorbent medium. Ind. Eng. Chem. Res., 35(6):1967–1975.
Maa, Y. -F., Nguyen, P.-A., Sweeney, T., Shire, S. J. and Hsu, C. 1999. Protein inha-lation powders: Spray drying vs. spray freeze drying. Pharm. Res., 16(2):249–254.
Mumenthaler, M. and Leuenberger, H. 1991. Atmospheric spray-freeze drying: A suitable alternative in freeze-drying technology. Int. J. Pharm., 72(2): 97–110.
Rogers, T. L., Nelsen, A. C., Sarkari, M., Young, T. J., Johnston, K. P. and Williams, R. O. 2003. Enhanced aqueous dissolution of a poorly water soluble drug by novel particle engineering technology: Spray-freezing into liquid with atmo-spheric freeze-drying. Pharm. Res., 20(3): 485–493.
Sonner, C., Maa, Y.-F. and Lee, G. 2002. Spray-freeze-drying for protein powder preparation: Particle characterization and a case study with trypsinogen sta-bility. J. Pharm. Sci., 91(10): 2122–2139.
Yokota, T., Takahata, Y., Katsuyama, T. and Matsuda, Y. 2001. A new technique for preparing ceramics for catalyst support exhibiting high porosity and high heat resistance. Catalysis Today, 69(1–4): 11–15.
CRC_73877_Ch022.indd 341CRC_73877_Ch022.indd 341 12/29/2008 11:34:44 AM12/29/2008 11:34:44 AM
in a vacuum dryer: Evaluation of external transfer coefficients. J. Food Eng.,
Lebedev, D. P. and Bihovsky, B. N. 2001. Method of intensification of vacuum
Boeh-Ocansey, O. 1988. Freeze-drying in a fluidized bed atmospheric dryer and
Leuenberger, H., Plitzko, M. and Puchkov, M. 2006. Spray freeze drying in a fluid-
Lombrana, J. I. 1997. The influence of pressure and temperature on freeze-drying
© 2009 by Taylor & Francis Group, LLC
343
23Refractance Window
oped by MCD Technologies for drying liquid and pasty materials and con-
According to the developers of this concept for drying, the term RefractanceWindow relates to the design principles. The dryer, shown schematically in Figure 23.1, is built as an endless strip made of infrared-transparent polyester such as Mylar® to form a conveyor belt carrying the product thatis deposited on the belt as a layer that is 0.2–1 mm thick. The relatively
trough. Thus, the sensible heat of water is transferred through the belt to the drying material, which is transported along the dryer with a belt velocity ranging from 0.6 to 3 m/min. When the material on the belt is dry, heat is transferred from hot water only by conduction. However, when the material is wet, heat is transferred by conduction and radiation because the liquid water held in the drying material absorbs the electro-magnetic energy in the wavelength range spanning from 3.0 to 15.3 μm. Thus, when the water-laden material is placed on the belt conveyor, a win-dow for the passage of infrared energy is created at all the surface areas where the material covers the belt, because refraction at the belt–material interface is minimized. As the moisture content falls in the course of drying, the infrared window is gradually closed, leaving conduction as the predominant heat-transfer mode. Since plastic is a poor conductor of heat, little heat is transferred to the nearly dry material, so the risk of over-heating is greatly reduced.
In general, the material temperature is relatively low because of evaporative cooling and convective heat transfer to the ambient air above the drying material. At the temperature of the circulating water just below the boiling point (95–97°C), the product temperature is usually below 70°C (Nindo and Tang, 2007; Anonymous, 2008). The cooling section at the discharge end of the dryer reduces the product temperature below the glass transition temperature to avoid product stickiness (cohesion and adhesion) and therefore facilitates discharge.
Since the absorption of infrared energy is essential for the RW tech-nique, this technology is suitable for processing (concentrate and dehy-drate) solutions, suspensions, and pastes; although high-moisture solids can also be dried (Anonymous, 2008). Because of the low product tempera-ture and relatively short drying time being in the order of several minutes,
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The Refractance Window (RW™) is an innovative thin-film method devel-
verting them into flakes or granules (Magoon, 1986; Anonymous, 2008).
thin belt (<0.2 mm) floats on the surface of hot water held in a shallow
© 2009 by Taylor & Francis Group, LLC
344 Advanced Drying Technologies
this dryer is recommended by the developers for heat-sensitive products such as fruit and vegetable pieces and purees to be dried at atmospheric pressure, as it permits the inhibition of enzymatic browning; retains color,
nents such as antioxidants, vitamin C (ascorbic acid), and alpha- and beta-carotenes (Abonyi et al., 1999, 2002; Nindo et al., 2003; Clarke, 2004; Nindo and Tang, 2007). Product quality is reportedly close to that achieved by freeze-drying, but at a much lower energy cost. The cost of the RW drying equipment is approximately one-third to half of that of a freeze-dryer to dry a similar mass of product, whereas the energy costs to operate RW dryers are less than half of the freeze-dryers (Nindo and Tang, 2007). The current and potential applications of the RW technology cover prod-ucts such as scrambled egg mix; avocado fruits; high carotenoid–containing algae; herbal extracts; nutritional supplements for human use; food ingre-dients such as herbs, spices, and vegetables; shrimps; chopped nectarines; and wine concentrates as well as chemicals, pharmaceuticals, and nutra-ceuticals (Anonymous, 2008). For effective RW drying, the materials should be well-chopped or fed in the form of suspensions and pastes of the right consistency for easy and uniform spreading on the conveyor belt.
Because the material temperature can be as low as 30°C, it is possible
their microbiological activity. Also, the dried granular product reconsti-tutes quickly. From the other viewpoint, the possibility of drying materi-als at higher material temperature with a short heating-up period that extends the holding time (see Figure 23.2) allows control over undesirable microbial contamination. Experiments with drying pumpkin puree pre-inoculated with coliforms, Escherichia coli, and Listeria innocua indicated
FIGURE 23.1Schematics of the RW dryer (Reprinted from Nindo, C. I. and Tang, J., Drying Technol.,25(1), 37–48, 2007. With permission.)
Coolingwater
Hood
Feed
Exhaust
Product
Water flumesVapor + air
Hot waterpump
Water tankand heater
Belt conveyor
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to dry cultured dairy products such as yogurt and kefir without losing
flavor, aroma, and enzymes; and minimize losses of nutritional compo-
© 2009 by Taylor & Francis Group, LLC
Refractance Window 345
that the microbial counts after RW drying were greatly reduced for all the microorganisms. For the inoculated samples, the test microorganisms were reduced to the minimum detection limit of <5 CFU/mL, which cor-responds to a microbial reduction of at least 6.1 log CFU/mL, 6.0 log CFU/mL, and 5.5 log CFU/mL for coliforms, E. coli, and L. innocua, respectively(Nindo et al., 2003; Nindo and Tang, 2007). The reduction of inoculum populations (106 CFU/mL) of coliforms and E. coli to an undetectable level
foods.Drying kinetics were studied to a limited extent, but it is clear that
the thin layer of the highly wet material dries initially in a constant-rate period, followed by a much longer falling rate period (Figure 23.2) (Nindo et al., 2003). Similar drying kinetics and temperature trajectories were reported by Abonyi et al. (2002) though completely different conclusions, based on incorrect data interpretation, were reported in an earlier study for the same materials (Abonyi et al., 1999).
As in other thermal dryers, the evaporation of water from the product constitutes a major part of the energy consumption in RW drying. Because during the circulating process water is reheated, the dryer demonstrates
rotary dryers. Based on the latent heat of vaporization of water, which is
0 50 100 150 200 250 300 3500
10
20
30
40
50
60
70
80
Drying time (s)
Mat
eria
l tem
pera
ture
(ºC
)
0
1
2
3
4
5
6
7M
oist
ure c
onte
nt (k
g/kg
d.b
.)
Water temperature: 95ºC
Layer thickness: 0.65 mm
Belt velocity: 0.27 m/min
Air temperature: 20.3ºC
Pumpkin puree + 11% maltodextrin
FIGURE 23.2Moisture content and material temperature curves for RW drying. (Compiled from Nindo, C. I., Feng, H., Shen, G. Q., Tang, J., and Kang, D. H., J. Food Process. Pres., 27(2),117–136, 2003.)
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confirms the suitability of the RW dryer to produce microbiologically safe
the same or higher thermal efficiency when compared to drum, spray, and
© 2009 by Taylor & Francis Group, LLC
346 Advanced Drying Technologies
from 52 to 77%, is comparable to drum drying and higher than that for hot-air dryers (Nindo et al., 2003; Nindo and Tang, 2007).
Built mainly of stainless steel (Grade 316), RW dryers are available in
each having an air-handling hood and a circulating water tank. The dryer
and discharge sections. The pilot dryer, which is 4.42 m long, 1.52 m wide, and 1.28 m high, is available for small-scale tests (Anonymous, 2008).
This is a relatively new drying technology with limited industrial instal-
for drying heat-sensitive materials. More research and development is needed along with a detailed mathematical model to advance this tech-nology further. Much of the available literature on RW originates from its developers.
References
Abonyi, B. I., Feng, H., Tang, J., Edwards, C. G., Mattinson, D. S. and Fellman, J. K. 2002. Quality retention in strawberry and carrot purees dried with Refractance Window system. J. Food Sci., 67: 1051–1056.
and quality retention for the Refractance Window™ drying system. Research Report. Washington State University, Pullman, WA.
Anonymous. 2008. Refractance Window®. MCD Technologies, Inc. Tacoma, WA (www.mcdtechnologiesinc.com).
Clarke, P. T. 2004. Refractance Window TM—“Down under”. Proc. 14th InternationalDrying Symposium (IDS2004). State University of Campinas, Sao Paulo, Brazil, August 22–25, pp. 813–820.
Magoon, R. E. 1986. Method and apparatus for drying fruit pulp and the like. U.S. patent 4,631,837.
Nindo, C. I., Feng, H., Shen, G. Q., Tang, J. and Kang, D. H. 2003. Energy utilization
27(2): 117–136.Nindo, C. I. and Tang, J. 2007. Refractance Window dehydration technology: A
novel contact drying method. Drying Technol., 25(1): 37–48.
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taken as 2334 kJ/kg at 95°C, the thermal efficiency of RW drying ranging
modular configurations based on a single dryer module 6.1 × 2.4 × 2.6 m,
can be assembled from up to five modules, with the common product feed
lations so far. However, it apparently displays several significant benefits
Abonyi, B. I., Tang, J. and Edwards, C. G. 1999. Evaluation of energy efficiency
and microbial reduction in a new film drying system. J. Food Process. Pres.,
© 2009 by Taylor & Francis Group, LLC
347
24
®
nique involving the use of a nonvolatile liquid medium as a carrier for dispersed wet solids to be dried in multiple-effect evaporators (Pluenneke and Crumm, 1986). The technology of multiple-effect evaporation is well established in the distillery, sugar, corn, pulp and paper, and other indus-tries dealing with the progressive concentration of aqueous solutions.
is supplied only to the heating element of the last unit from a series of multiple-effect evaporators. If the steam temperature is above the boiling point of the solution to be concentrated, then 1 kg of delivered steam will
can serve as a heating medium in the preceding evaporator. Here almost another kilogram of water is evaporated, and therefore the water vapor may be further used as a heating medium in another evaporator or condensed, if the system features a double-effect evaporation (Perry et al., 1998). Such a reuse of vapor from adjacent evaporators is repeated through any number of effects, and therefore the theoretical steam economy is proportional to the number of effects. In practice, the unit heat consumption is higher than the theoretical one but is still impressive. For example, the energy con-
including steam and electricity (Pluenneke and Crumm, 1986).The economic advantages of a multiple-effect evaporation resulting from
better steam utilization are reduced to a certain extent by additional costs due to (a) pressure gradient over the series of evaporators, which is neces-sary to maintain the reasonable temperature difference between the steam and boiling solution in a sequence of evaporators, and (b) a larger heat-
tion rate. Therefore, the design of a particular evaporation system should be based on economic balance calculations.
So far, the major constraint in application of multiple-effect evapora-
material in the course of water evaporation. The use of oil as a carrier medium, which is the essence of the C-G process, solved this problem,
process stream to virtually zero-moisture content.
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Carver-Greenfield Process
The Carver-Greenfield (C-G) process is an innovative dehydration tech-
Apparently the high thermal efficiency of the multiple-effect evaporation results from a process configuration where steam from an external source
evaporate almost 1 kg of water. Because the final evaporator operates at the highest boiling temperature, the water vapor from the final evaporator
sumption in a five-stage C-G unit averages 163 kJ/kg of water evaporated,
transfer surface area necessary to maintain a technically justified evapora-
allowing this energy-efficient method to be used to dewater solids in the
tion to industrial drying has been the decreasing fluidity of the processed
© 2009 by Taylor & Francis Group, LLC
348 Advanced Drying Technologies
The early C-G process commercialized in 1961 utilized the indigenous carrier oil concept developed for vitamin oil recovery. In this case, tal-low obtained from slaughterhouse waste was used as the oil carrier for processing meat sludge. In the C-G rendering system, the meat waste was
granules. The slurry was then boiled in a two-stage, multiple-effect evap-oration system. The mixture of tallow and dry solids discharged from the second evaporator was separated by centrifuging. High-pressure presses were used to further reduce the tallow content in the meal to ~15% mass. A fraction of tallow from the separator and presses was recycled to the process to maintain the oil-to-product mass ratio of ~8:1. Excess tallow
were used for animal feed. Some 60 plants of this type still operate in the United States.
The critical point of the C-G rendering process was the mechanical separation of the tallow rendered from the centrifuge. The cost of carrier oil lost with dry product greatly reduced the economic advantages of an
recovered by evaporation rather than by mechanical pressing (Pluenneke and Crumm, 1986). Besides minimizing carrier oil losses, light oils also extract indigenous oils present in the feed material, such as greases and fats from sewage sludge, oils from peat and lignite drying, and corn oil from the spent mash in whiskey manufacture. One disadvantage of the light-oil process is the evaporation of some of the oil along with the water. The presence of oil in the water vapor used as a heating medium reduces
utilization includes mechanical vapor recompression (MVR). Here, the exhaust water/oil vapor is mechanically compressed to a higher pressure and temperature and used as a heating medium in the same evapora-tor. This makes it possible to reduce energy consumption to ~50 kJ/kg of
process utilizing MVR to concentrate feeds from solids’ content ranging
TABLE 24.1
Physical Characteristics of Light Carrier Oils
Amsco® 140 Isopar L
Initial boiling point (°C) 185 185 (min)Dry point (°C) 204–210 204–211Flash point (°C) 64 60 (min)Density at 15.6°C (kg/m3) 784 767
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was drawn off, filtered, and sold as a commercial product. Dry solids
energy-efficient system. Therefore, the original heavy-oil process was fur-ther modified to use a more volatile carrier oil (Table 24.1), which can be
the overall heat-transfer coefficient by 25–30% as compared to pure water
point, which requires a more stringent electrical code classification.An advanced modification to the C-G process allowing better energy
water evaporated. Figure 24.1 presents the basic configuration of the C-G
first ground into a rough meal and mixed with tallow to liquefy the solid
vapor. Another concern when using more volatile oil is its lower flash
© 2009 by Taylor & Francis Group, LLC
349
from 2 to 50% mass. Because the particle size must not exceed 7.5 mm along any axis, some feed material may require grinding before mixing with carrier oil. Drying of such sludge is accomplished in a two-effect system comprising the MVR and conventional steam-heated evaporators. The evaporators in all drying stages are typically equipped with forced-circulation heat exchangers. Alternatively, heat exchangers can be of the
convection is required because of increased solids’ content and a higher boiling point.
In the case of the light-oil system, the press-type deoiler is replaced with
and the troughs are steam-heated. The system is operated under vacuum, so that the carrier oil is rapidly drawn off the solids as the process stream
injected directly into the solids.Process condensate from the evaporators is treated in a two-stage system
The second stage is a contact-type unit that polishes the water by forcing itthrough a barrier system, where the oil coalesces.
The system to separate carrier oil from indigenous oils that have
vaporizer followed by a “cook-pot,” both of which are steam-heated.
FIGURE 24.1C-G process with MVR.
Waste waterand oil vaporFeed
By-product oilCarrier oil
Oils
Condensate
Steam
Product
Vent
Wastewater
OilWaste water + oil
SeparatorSeparatorCentrifuge
Final evaporator
CondenserMVR stage
Fluidizingtank
Deoiler
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Carver-Greenfield Process
falling film tubular type except for the final evaporator, in which forced
been extracted from the feed material usually consists of a falling film
a devolatizer. The unit resembles a screw feeder in which both the flights
moves down the flight. To further enhance vaporization, steam may be
to remove most of the oil. The first stage consists of the baffled unit that relies on residence time and low velocity to separate the oil by flotation.
© 2009 by Taylor & Francis Group, LLC
350 Advanced Drying Technologies
orator, where ∼90% of the light oil is vaporized. This hot vapor is used
reduce energy consumption. The concentrate that has not been vaporized is transferred into the pot, which is equipped with a mixer. Vapor from
ized indigenous oil is removed from the pot continuously and pumped to the storage tank.
Current applications of the C-G process include the drying of activated sludge from a wood pulp plant, wool-processing waste from a textile manu-facturing facility, instant coffee processing waste, dairy product waste, and sewage sludge. Products such as lignite and peat have been dried success-fully at the pilot level with energy consumption in the order of 135 kJ/kgwater evaporated. The process is particularly attractive for drying of peat.
tions around the world as engineers and managers concerned with decision- making become familiar with the technology and as their experience with its industrial operations accumulates. Several major installations that use the C-G process for drying municipal waste sludge exist in Japan.
References
Pluenneke, K. A. and Crumm, C. J. 1986. An innovative drying process with diverse applications. In: Drying’86. A. S. Mujumdar (Ed.). Vol. 1. Hemisphere Publishing Corporation, Washington, DC, pp. 617–624.
Perry, R. H., Green, D. W. and Maloney. 1998. J.O. Perry’s Chemical Engineers’ Handbook. Section 11, 7th edition. McGraw-Hill Book Company, New York, 2336p.
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The concentrated sludge from the centrifuge is fed to the falling film evap-
instead of steam in the final evaporator of the multiple-effect system to
One may expect this process and its variants to find new and major applica-
the pot flows directly to the first multiple-effect evaporator. The nonvapor-
© 2009 by Taylor & Francis Group, LLC
Part IV
Hybrid Drying Technologies
In our treatise, the term hybrid drying technologies is used in a rather gen-eral way. As can be seen from the following schematic, it includes drying techniques that use multiple modes of heat transfer as well as those that use two or more stages of dryers of the same or different type. A broad range of successful new drying technologies fall in this category because they are based on intelligent combinations of well-known conventional know-how.
Hybrid drying systems
Combined modesof heat transfer
Multistagedryers
Multiprocessingdryers
Convection + conductionConvection + MW/IR /RF
SimultaneousSequential
ContinuousIntermittent
Drying coolingDrying granulation/ agglomerationDrying coatingDrying filtration
Each stage samedryer type
Each stage adifferent dryer
Two-stage fluid bedTwo-stage vibrofluid bed
Flash / fluid bedSpray / fluid bedFluid bed / packed bed
Superheated steam dryerfollowed by air drying
Different dryingtechnologies per stage
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© 2009 by Taylor & Francis Group, LLC
353
25Microwave–Convective Drying with Cogeneration
Microwave (MW) and radio-frequency (RF) heating are not only highly intensive but also cost-incurred methods of thermal treatment (Sanio, 1989; Sanio and Schmidt, 1989). For these reasons, hybrid technologies involving combinations of various driving forces for heat and mass trans-fer are preferable. Typical examples are RF- and MW-assisted convective drying, where the high-frequency electromagnetic energy may be applied at different drying stages to supplement thermal energy supplied with drying air. By adjusting the balance among component technologies, it is possible to optimize the hybrid systems in terms of the process effective-ness and cost. Of the various hybrid systems based on MW irradiation, MW vacuum drying and MW freeze-drying (F-D) have found the widest applications in dehydrating fruits, foods, and thermolabile biomaterials such as microbial strains, plasmas, serums, enzymes, and others (Kudra, 1990, 1999; Kudra and Strumillo, 1998). Therefore, this chapter focuses on two prospective but less-common MW-assisted drying technologies.
Although drying kinetics for convective and dielectric (MW and RF) drying are similar except the period of internal pressure generation, sev-eral advantages of MW drying stem from volumetric rather than surface
tion. The main advantage is an enhanced drying rate resulting from such phenomena as a relatively large temperature gradient that coincides with
improvement in drying rates can be observed for materials having poor thermal conductivity since the energy is dissipated directly in the wet regions of the material being dried. The other advantage of dielectric heat-ing is that it yields much more uniform drying and improved product quality. Because of the exponential dependence of the diffusivity on tem-
higher in the material core than in the surface region. As a result, the
makes it possible to eliminate such disadvantages of convective drying as case-hardening, surface cracking, and local overheating.
Since the electromagnetic energy is dissipated directly in the dried material, heat losses are considerably reduced. Thus, the performance
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heating and from different temperature profiles due to power penetra-
moisture profile levels off to compensate for increased diffusion, and this
the gradient of moisture concentration or mass flow of vapor and liquid water that adds to capillary and molecular flows. The most dramatic
perature, the molecular flow rate for a given moisture gradient will be
© 2009 by Taylor & Francis Group, LLC
354 Advanced Drying Technologies
of MW drying is better than that of convective drying (Figure 25.1). The capital and operating costs of dielectric drying are higher; hence, MW heating may not be economical as the sole energy source, especially for drying high-moisture-content materials with long constant-rate periods. However, it is well suited for drying low-moisture-content products where
0 50 100 150 200Time (min)
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
Time (min)0 50 100 150 200
0
10
20
30
40
50
60
70
0
0.2
0.4
0.6
1.0
1.2
0.8
MicrowaveVacuumConvection
MicrowaveVacuumConvection Te
mpe
ratu
re (º
C)
Dry
ing
rate
(kg/
m2
h)Re
duce
d m
oist
ure c
onte
nt
FIGURE 25.1Convective, vacuum, and MW drying of chlorpropamide: T = 60°C, P = 0.1 atm, u = 5.4m/s, P′ = 385 W/kg. (From Kardum, J. P., Sander, A. and Skansi, D., Drying Technol., 19(1), 167–183, 2001. With permission.)
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the convective heat transfer is less efficient.
© 2009 by Taylor & Francis Group, LLC
Microwave–Convective Drying with Cogeneration 355
To overcome the economical constraints, the MW and RF drying tech-niques are frequently combined with convective drying. In such a com-bination, the air stream carries away moisture evaporated during MW irradiation. Figure 25.2 presents two basic modes of combining convec-tive and MW drying. In booster drying, electromagnetic energy is added gg
Time
Moi
stur
e con
tent
Final moisturecontent
Final moisturecontent
Time saved
Convective drying
MW-assisted drying
MW+
(a)
Time
Moi
stur
e con
tent
Time saved
Convective drying
MW dryingMW+
(b)
FIGURE 25.2
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Typical applications of MW energy: (a) Booster drying and (b) finish drying.
© 2009 by Taylor & Francis Group, LLC
356 Advanced Drying Technologies
to convective drying when the moisture content reaches its critical value and the evaporation front starts to recede into the solid. The drying rate is increased sharply with a leverage of up to 8 to 1 in terms of increased dry-ing capacity for each unit of electromagnetic energy added (Schiffmann,
the material before drying. It is rarely used due to the high capital costs of
One of the drying technologies that take advantage of combined MW and convective drying is microGas™ drying (Anonymous, 1992). This
thermal and electric energy from a single fuel source. Cogeneration is rec-
provides a cost-effective alternative to traditional methods of generating electricity and heat alone. Aside from producing electricity, cogeneration provides an option to utilize thermal energy, which is otherwise wasted mostly with exhaust gases, engine coolant, and the lubrication oil cooling system. The waste heat recovered as hot water or steam, for example, canbe further used in low-temperature installations such as central heating or air conditioning or for process heating including industrial drying.
a typical cogeneration system approaches 80% versus 35–40% for a utility power plant. The key components of a typical cogeneration plant are
A prime mover (a steam turbine, a combustion gas turbine, or an • internal combustion engine) that, through fuel combustion, pro-duces mechanical energyA generator that converts the mechanical energy into electrical • energyA heat recovery system (usually a heat exchanger) where waste • heat is converted to a useful form
As applied to drying, heat from a cogeneration system can be taken from an engine cooling liquid or engine exhaust gases. Direct use of the
and others is technically feasible. However, the relatively constant energy output from the engine exhaust cannot usually match the hydrodynamic and thermal requirements of drying systems unless the exhaust gases are used as a secondary heat-transfer medium. The latter option is preferred since the temperature of the exhaust gases at the turbine outlet is ∼500°C, and for a majority of drying applications, it must be lowered by mixing with ambient air. Indirect use of the gas engine exhaust is also possible.
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2007). In finish dryinggg, electromagnetic energy is supplied to the material
The third possible mode of combined drying uses MW energy to preheat when its moisture content is so low that convective drying is inefficient.
the MW equipment, which might be difficult to offset.
technology involves cogeneration, defined as the combined production of
ognized as one of the most efficient methods in energy transformation that
Because of such heat recovery, the resulting efficiency of fuel utilization in
exhaust gases (specifically those from gas turbines) as a heat and momen-tum carrier in convective dryers such as fluid bed, pneumatic, spin-flash,
© 2009 by Taylor & Francis Group, LLC
Microwave–Convective Drying with Cogeneration 357
carried out at lower temperatures, whereas the low gas-to-gas overall 2 K) demand heat
exchangers with extended heat-transfer areas. The relatively low temper-ature of the engine coolant (typically 90–95°C) limits industrial applica-tions to low-temperature drying because the drying air temperature in a typical heat exchanger heated by the engine coolant may be raised only up to 85°C. As a general rule, each case of a cogeneration-driven drying system will need to be assessed individually.
Essentially, the microGas™ technology combines MW and convectiveheating in an integrated drying system powered by natural gas. In its
drives a generator that provides electricity to power the MW genera-tor and all ancillary equipment such as feeders, conveyor belts, blowers, and pumps. Waste heat from the engine exhaust and cooling jacket is recovered as hot water at ∼93°C and used to preheat air entering a con-vective dryer. If higher convective heat loads are required, an auxiliary burner may be used to raise the drying air temperature. For solid mate-rials, the dryer is typically a staged, conveyor type with a continuous feed and product discharge. Specially designed inlet and outlet ports
Burner
Gas engineprime mover / generator
Magnetron
Hot air
Primary gas
Auxiliarygas
Air
Heat exchanger
Electricenergy
Convective stage MW + convective stage
Hot water
Exhaust
Product
Wet feed
Belt conveyor
Schematic of the microGas dryer for solid materials. (From Anonymous, The microGas Dryer and the Food Industry, Energy International, Inc., WA, USA, 1992. With permission.)yy
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However, it may not be fully justified because contact drying is usually
heat-transfer coefficients (in the order of 30–60 W/m
basic configuration (Figures 25.3 and 25.4), the gas-fired prime mover
© 2009 by Taylor & Francis Group, LLC
358 Advanced Drying Technologies
prevent leakage of MW radiation that is routed through a waveguide to the convective/MW stages of the microGas dryer. Of the natural gas energy fed to the prime mover, ∼30% is converted to electricity, and 50%
the microGas system is claimed to be over 80%. This seems to be justi-
typically only ∼50%. Even so, the microGas technology appears to becompetitive with conventional drying systems (Tables 25.1 and 25.2). Moreover, the microGas technology offers the process advantages of volumetric heating at lower expense than all-electric systems. In effect, the form value of natural gas is raised, making gas the energy source of choice in more industrial processes.
1. Shorter drying time. Volumetric heat generation results in acceler-ated transfer processes, which allow drying times to be in the order of one-fourth the time required for convective drying.
2. Product quality improvement. Faster and uniform drying reduces the possibility of surface overheating, case-hardening, and stress
cessed foods. The shorter drying time also gives bacteria less time to grow, potentially eliminating the need for fumigation or chem-ical treatment.
Heat exchanger/auxiliary burner
Gasinput
Prime movermodule
MW controlpanel
Microwavegenerators
Dryer
Beltconveyor
FIGURE 25.4
Institute, Chicago, IL, 1995. With permission.)
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A conceptual design of the microGas dryer. (From Anonymous, Tech Profilee, Gas Research
is recovered and utilized as convective heat. The resulting efficiency of
fied for processes where the contribution of MW heat is relatively small (finish drying) because the efficiency of electricity-to-MW conversion is
The principal benefits of microGas drying are claimed to be as follows:
cracking. It also allows better retention of color and flavor in pro-
© 2009 by Taylor & Francis Group, LLC
Microwave–Convective Drying with Cogeneration 359
TABLE 25.1
MicroGas versus Conventional Dryer for Vegetables (Throughput 13,500 kg/h)
microGas Conventional
Heating system NG prime mover, auxiliary
Energy source Natural gas Natural gas + electricityConveyor length (m) 23 68Drying time (min) 15 45Energy consumption Natural gas (MJ/kg) 2.6 4.2 Electricity (kW) 0 270
Source: From Anonymous, Technical Information, Energy International, Inc., WA, 1994. With permission.
TABLE 25.2
MicroGas versus Conventional Dryer: Processing Time and Energy Comparison
Product Process
TimeReduction
(%)
Energy Savings
(%) Other Results
Onions Drying 55 38 Matched existingquality
Frozen French fried potatoes
Drying 83 36
Pepperoni Drying 25 6No-fat potato
chipsCooking/texturing/drying
— — Unique product
Spices Sterilizing/drying — — Eliminates use of ethyleneoxide
Ceramic insulators
Drying 90 45
Source: Adapted from Bernstein, S., Tridball, R. and Groten, B., Powder and Bulk Solids Conference and Exhibition, Chicago, IL, May 3–6, 1993. With Permission.
3. Equipment size reduction. An integrated system is more compact
2 2
tion rate. 4. Reduced operating costs. The MW energy is generated within a gas-
lized for the convective drying section. Both reduced dryer size and shorter processing time make the system easier to stop and restart for shift adjustment, product changes, cleaning, and main-
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Direct, gas-fired burnersgas-fired burner
fired cogeneration system, and all recoverable waste heat is uti-
tenance, which finally reduce the labor costs.
and requires less floor area. In a typical configuration, a microGassystem occupying 42 m of floor space can replace the 400 mfloor space of the conventional dryer for an equivalent produc-
© 2009 by Taylor & Francis Group, LLC
360 Advanced Drying Technologies
fruits and vegetables (apples, grapes, carrots, onions, potatoes), cereals (rice, breakfast foods, pet foods), pasta, candies, and ceramics (Anonymous, 1995).Table 25.3 highlights the basic features of the microGas dryer for sliced vegetables.
The microGas versus conventional dryer performance comparison for the production of 1800 kg/h of short cut pasta is as follows (Anonymous, 1992): the conventional conveyor dryer heated by steam coils supplied
total processing time is ∼8 h, and the unit requires 24 man-hours of labor for each cleaning cycle. A microGas dryer of the same width is only 7.5 m long and the processing time is reduced to ∼75 min. Owingto its smaller size, such a dryer requires only about 6 man-hours of labor for cleaning. The prime mover and auxiliary burner of the micro-Gas system consume natural gas in the energy equivalent of 0.21 kWhper kilogram of product and require no external electricity, whereas the conventional dryer consumes 0.36 kWh of gas equivalent and 0.03 kWh of electricity per kilogram of product. On the basis of an estimated installed cost of $4,300 per square meter of conveyor area, the conven-tional dryer costs between $300,000 and $400,000, whereas the microGassystem costs about $225,000 (including $70,000 for the dryer component, $35,000 for the prime mover/generator combination, and $120,000 for the MW equipment).
TABLE 25.3
Results of Field Experiments on Drying Sliced Vegetables
Maximum Product Throughput 330 kg/h Sliced Vegetables
Moisture contentInitial 80% w.b.Final 5% w.b.
Drying temperature 60°CMaximum belt speed 0.15 m/minResidence time 2 h 5 minRated gas input to prime mover 116.2 kWGenerator power 30 kW
25.7 %Recovered waste heat 64.1 kWRated gas burner output 468 kW (40:1 turndown)Unit energy consumption (53% from gas burners, 44% from recoveredengine heat, 3% from MWs)
2.25 MJ
Energy cost of dry product (based ongas cost of 0.33 cent US/MJ)
8.8 cent US/kg
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Electric generating efficiency
Source: From Anonymous, Tech Profile, Gas Research Institute, Chicago, IL, 1995.
The microGas technology has been laboratory- and field-tested for drying
by an oil- or gas-fired boiler is ∼33 to 48 m long and 1.8 m wide. The
© 2009 by Taylor & Francis Group, LLC
Microwave–Convective Drying with Cogeneration 361
According to Bernstein et al. (1993), the use of microGas drying for ceram-ics is projected to yield the following results:
Thermal and electrical energy costs are reduced by over 90% •
amount of electricity used elsewhere in the plant.Automating the drying process reduces labor costs associated • with drying by ∼50%.The 90% reduction in drying time translates almost directly into • a 90% reduction in factory space requirements.The simple payback on a turnkey installation for this ceramics • drying application is estimated at approximately 2.2 years.
These results are shown in graphic form in Figure 25.5.Considering the economic advantages and the substantial productivity
ing applications (Bernstein et al., 1993):
process time
Conventional
microGas
0 200,000 400,000 600,000 800,000Cost ($)
Thermal energy
Factory labor
Inventory
Microwave O&M
Electrical energy
Factory space
Cracking losses
Power Pkg.O&M
FIGURE 25.5
Groten, B., Powder and Bulk Solids Conference and Exhibition, Chicago, IL, May 3–6, 1993.)
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by the efficient use of natural gas and the production of a small
and quality benefits, the microGas system is recommended in the follow-
• Where there is appreciable value in significantly decreasing the
Cost–benefit analysis for ceramic products. (Adapted from Bernstein, S., Tridball, R. and
© 2009 by Taylor & Francis Group, LLC
362 Advanced Drying Technologies
Where a 20–50% reduction in energy consumption is needed to • obtain cost savingsWhere product unit value is high• Where a large decrease in equipment size is crucial• Where the transformation of a batch process into a continuous •
tact heat transfer through MW radiation can offer the product of unique properties
Symbols
P Pressure, Pa
T Temperature, °Cu
References
Anonymous. 1992. The Microgas Dryer and the Food Industry. Energy International, Inc.WA.
Anonymous. 1994. Technical Information. Energy International, Inc. WA.
Research Institute, Chicago, IL.Bernstein, S., Tridball, R. and Groten, B. 1993. microGas: A combined microwave and
convection dryer for industrial processes. Powder and Bulk Solids Conference and Exhibition. Chicago, IL, May 3–6.
Kardum, J. P., Sander, A. and Skansi, D. 2001. Comparison of convective, vacuum and microwave drying of chlorpropamide. Drying Technol., 19(1): 167–183.
Kudra, T. (Guest Ed.). 1990. Drying Technol.—Special Issue on Dielectric Drying., 8(5): 907–1205.
Kudra, T. (Guest Ed.). 1999. Drying Technol.—Special Issue on Drying and Dewateringin Energy Fields, 17(3): 379–654.
Kudra, T. and Strumillo, C. 1998. Thermal Processing of Bio-Materials. Gordon andBreach Science Publishers, Amsterdam, 669p.
Sanio, M. R. 1989. Economic considerations for dielectrically enhanced heating. In: Electromagnetic Heating—Short Course Workbook. IMPI, Clifton, VA, pp. 57–68.
Sanio, M. R. and Schmidt, P. S. 1989. A procedure for estimating capital and oper-ating costs of dielectric heating equipment. Report TSDD-89-011. Ontario Hydro of Canada.
Schiffmann, R. F. 2007. Microwave and dielectric drying. In: Handbook of IndustrialDrying. A. S. Mujumdar (Ed.). 3rd edition. Taylor & Francis, Boca Raton, FL,pp. 285–305.
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process results in significant productivity increases or noncon-
P′ Specific microwave (MW) power, W/kg dry matter
Superficial velocity, m/s
Anonymous. 1995. Gas-fired convective-microwave industrial dryer. Tech Profile. Gas
© 2009 by Taylor & Francis Group, LLC
363
26Microwave–Vacuum Drying
Microwave–vacuum drying (MVD) offers an attractive, cost-effective alternative to freeze-drying (F-D) in the production of high-quality heat-sensitive products. In most freeze-dryers, heat is transferred to the surface of the frozen material by conduction from heating plates. In the course of drying, heat is transferred from the material surface to the sublimation front by conduction through the layer of already dry material. To perform
ence should be maintained between the material surface and its core. Thus, the surface layers can easily be overheated, especially for materials of low thermal conductivity. A solution to this problem could be microwave (MW) heating, where the material core is usually at a higher temperature because of internal heat generation. In fact, MW F-D has been extensively studied in the past 20 years and a number of laboratory and pilot tests have documented the technical feasibility and substantial advantages over con-
to control because of a much higher loss factor for liquid water than that for ice. Thus, any localized melting would result in runaway overheating(Liapis and Bruttini, 1995). MW drying under vacuum appears to be the solution of choice as it offers all the advantages of dielectric heating but at a reduced processing temperature, which is a function of the operating pressure (Figure 26.1). In addition, a volumetric heat-transfer mechanism coupled with drying in the absence of oxygen (vacuum) provides an ideal low-temperature drying technique.
Figure 26.2 presents drying curves for 3 mm thick carrot slices dried by hot air F-D and MVD (details of the material and procedure can be found in Lin et al., 1998, and Durance, 1999). Characteristically, the MVD curves are relatively linear down to the moisture content of ∼9% w.b.that allows ambient temperature storage of the product. In this experi-ment, F-D required about 3 days to reach the required moisture content (9% w.b.), air drying lasted for 8 h, whereas MVD took only 33 min. For shrimps dried from 88 to 20% w.b., the respective drying times were 72 h for F-D, 4 h for air drying, and 15 min for MVD at an operating pressure of 100 mmHg (Lin et al., 1999). Experimenting with MVD of sliced carrot,
(X = 7.68 kg/kg d.b.) without removal of moisture when the product tem-perature increased linearly with time until the saturation temperature was reached, a constant-rate/temperature period (2 ≤ X < 7.68 kg/kg d.b.), and
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the process at reasonably high drying rates, a sufficient temperature differ-
ventional F-D (Kudra and Strumillo, 1998). MW F-D is, however, difficult
Cui et al. (2005) identified three distinct periods: a warming-up period
© 2009 by Taylor & Francis Group, LLC
364 Advanced Drying Technologies
−40
60
160
260
360
460
560
660
760
40 20 0 20 40 60 80 100Temperature (ºC)
Pres
sure
(Tor
r)
Watervapor
Liquidwater
Ice
F-D (qualityand high cost)
MV(quality and
low cost)
Air drying(low cost and low quality)
FIGURE 26.1MVD versus air and F-D. (From Durance, T., Preservation Technologies for Food, Feed and Fibre, Seminar, Winnipeg, Canada, 1999.)rr
0 2 4 6 80
20
40
60
80
100
Time (h)
Moi
stur
e con
tent
(% w
.b.)
F-D
Air dryingMV drying
Drying rates for carrot slices dehydrated by hot air at 70°C, F-D and MVD. (From Durance, T., Preservation Technologies for Food, Feed and Fibre, Seminar, Winnipeg, Canada, 1999.)rr
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© 2009 by Taylor & Francis Group, LLC
Microwave-Vacuum Drying 365
a heating-up period (X < 2 kg/kg d.b.) in which the drying rate decreasedand the sample temperature increased rapidly. Mathematical models for predicting the carrot temperature were also developed.
Similar drying kinetics were found for MVD of wood, where the rela-tively short warming-up period was followed by a long constant-rate period with constant wood temperature, and completed with the falling-
saturation point (Li et al., 2007). Similar to radio frequency–vacuum dry-ing of lumber (see Chapter 32), MVD offers higher drying rates at lower drying temperatures and yields better wood quality (Seyfarth et al., 2003; Leiker et al., 2005).
The exceptions from the three-period drying kinetics are the results on deep-bed MVD of edamame (Hu et al., 2007), where the warming-up period was immediately followed by the falling-rate period, which indi-cates that this technology can be material- or process-dependent.
Besides offering drying times in the order of minutes rather than hours, MVD provides quality parameters that compare favorably with those of conventional drying techniques. As expected, color, nutrient content, tex-ture, rehydration characteristics, and so on, are found to be superior to those found by air drying and comparable to those obtained by F-D for products such as cranberries (Yongsawatdiguul and Gunasekaran, 1996; Sunjka et al., 2004), ginseng (Popovich et al., 2005), berries (Kwok et al., 2004), grapes (Petrucci and Clary, 1989), carrots (Lin et al., 1998), wild cab-bage (Xu et al., 2004), various fruits (Pappas et al., 1999), and herbs (Yousif et al., 1999). Lin et al. (1998) compared the density, rehydration potential,
sensory properties of carrot slices that were freeze-dried, air-dried, and MV dried. Table 26.1 compares selected quality indices for MVD with air drying and F-D. It is clear that the retention of active ingredients in materials dried by MVD is comparable with the freeze-dried products and much better than hot air–dried products.
TABLE 26.1
Quality Indices for Selected Products
Quality Index Freeze-Dried Air Dried MV Dried
Vitamins in drycarrots (µg/g d.m.) α-Carotene 0.41 0.33 0.40 β-Carotene 1.15 0.93 1.11 Vitamin C 0.46 0.17 0.36Biochemicals in herbs (mg/g d.m.) Alkamides in echinacea roots 3.3 2.8 3.1 Hypericin in St. John’s wort 0.48 0.35 0.45
Source: Extracted from Durance, T. Preservation Technologies for Food, Feed and Fibre, Seminar, Winnipeg, Canada, 1999.
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rate period that extends to the mean moisture content below the fiber
texture, color, flavor, carotene content, vitamin C content, as well as the
© 2009 by Taylor & Francis Group, LLC
366 Advanced Drying Technologies
One limitation of this technique is that it uses electricity, which is gen-
the past decade. Change of operation pressures may permit changes in the quality of the product, such as its density and porosity to some extent. Hybrid systems, which use hot air convection for removal of the easily removable moisture, followed by MVD for the harder-to-remove moisture toward the end of the drying cycle, may be more cost-effective than pure MVD. Intermittent operation (see Chapter 34) was also demonstrated as
2005; Regier et al., 2005). More detailed technoeconomic studies for vari-ous products and local costs of energy sources are needed as these may
of the right system depends on the quality of the desired product and the premium a high quality fetches in the marketplace.
Figure 26.3 presents the schematic of the MV dryer with revolving trays (Anonymous, 1995). The system comprises a horizontal vacuum chamber (2 m in diameter and 2.3 m long), a revolving rack with 12 trays, an MW power supply, a vacuum pump, and a control panel. To secure a uniform distribution of MW energy within the chamber, 16 symmetrically spaced MW feedports are connected through waveguides to separate 1.5 kW 2450 MHz magnetrons. The material to be dried is held in trays made of glass
®
oration rate is ∼23 kg/h when the moisture content is reduced from 90 to20% w.b.
for an even drying of the material held in the container. A family of batch dryers extends from the model µWaveVacxx50 suited for laboratory stud-µies to the industrial dryer µWaveVac0650 delivering 6 kW at 2450 MHz µ(Püschner and Hoon, 2007).
An interesting variant of MVD is MV–convective drying. In this
drying chamber kept under pressure reduced to the level of 10–40 kPa (Figure 26.4). A standard system comprises the 2450 MHz generator with
TABLE 26.2
Energy Cost Estimates for MVD and Air Drying
Method Unit Cost ($) per Kilogram of Water Removeda
100% air 0.290.240.200.17
100% MVD 0.17
a Natural gas: $ 4.71/GJ; electricity: $18.03/GJ (6.5¢/kWh).Source: Extracted from Durance, T. Preservation Technologies for Food, Feed and Fibre, Seminar,
Winnipeg, Canada, 1999.
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95% air / 5% finish MVD85% / 15% finish MVD70% / 30% finish MVD
erally expensive. The cost of magnetrons has declined significantly over
more efficient than continuous MVD (Gunasekaran, 1999; Yang et al.,
affect the selection of the drying method (Table 26.2). The final selection
Another configuration of an MV dryer features a turntable to arrange
novel configuration a conditioned drying agent circulates through the
fiber–reinforced Teflon . The rack with trays is rotated at 4 rpm. The evap-
© 2009 by Taylor & Francis Group, LLC
Microwave-Vacuum Drying 367
Intakemanifold
Exhaustmanifold
Microwavegenerator
Waveguide
Revolvingrack
Cylindricalvacuumchamber
Tray
MW MW
FIGURE 26.3Cross-view of the MW-vacuum dryer with a revolving rack. (From Anonymous, TechnicalInformation, Toshiba Electronics U.K. Ltd, Cumberley, Surrey, U.K., 1995.)
Dryer
HeaterCondenser
Vacuum pump
BlowerSeparator
Exhaust
Air
Water
MW
FIGURE 26.4Schematic of MW–vacuum–convection drying. (From Heindl, A., Microwave assisted vacuum convection drying, Ph.D. Thesis, Technical University of Munchen, Germany, 1993.)
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© 2009 by Taylor & Francis Group, LLC
368 Advanced Drying Technologies
a nominal power of 1.2 kW, the circulator, a coupler, a tuner, and a horn antenna that is used to feed MW energy to the drying chamber (Figure 26.5; Anonymous, 1990). Experiments performed in a pilot-scale dryer with particulate foodstuffs such as meat balls, champignons, and valerian plants revealed much higher drying rates than in air drying and F-D.
Magnetron
Coolingwater
Circulator
Waveguide
Diodedetector
Coupler
Waterload
Diodedetector
Tuner Teflon window
Dryingchamber
Hornantenna
FIGURE 26.5Microwave assembly for MW–vacuum–convection dryer. (From Anonymous, ILV, AnnualReport, Fraunhofer Institute for Food Process Engineering and Packaging , Germany, 1990.)
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© 2009 by Taylor & Francis Group, LLC
Microwave-Vacuum Drying 369
It was also found that such a unique combination of these three drying techniques offers products with a quality equal or superior to the quality of freeze-dried products and also allows optimized and economic energy use (Heindl et al., 1993; Heindl, 1993).
Although MVD provides numerous advantages, only a few companies offer such units at this time (www.by-wb.com; www.enwave.net; www.inap-gmbh.de).
Symbols
X Moisture content, kg/kg d.b.d.b. Dry basisd.m. Dry matterrpm Revolutions per minutew.b. Wet basis
Acronyms
MVD Microwave–vacuum dryingMW MicrowaveMV Microwave-vacuum
References
Anonymous. 1990. Microwave Vacuum Drying of Particulate Foodstuffs. ILV. Annual Report (in German). Fraunhofer Institute for Food Process Engineering and Packaging, Germany, pp. 151–153.
Anonymous. 1995. Technical Information. Toshiba Electronics U.K. Ltd., Cumberley, Surrey, U.K.
Cui, Z-W., Xu, S-Y. and Sun, D-W. 2005. Temperature changes during microwave-vacuum drying of sliced carrots. Drying Technol., 23(5): 1057–1074.
Durance, T. 1999. Vacuum microwave drying for functional and medicinal foods. Preservation Technologies for Food, Feed and Fibre. Seminar. November 21–23,Winnipeg, Canada.
Gunasekaran, S. 1999. Pulsed microwave-vacuum drying of food materials. Drying Technol., 17(3): 395–412.
Heindl, A. 1993. Microwave assisted vacuum convection drying. Ph.D. Thesis. Technical University of Munchen, Germany (in German).
Heindl, A. Holley, W. and Rehmann, D. 1993. Microwave vacuum convection drying of foodstuffs. Int. J. Food Technol. Mark. Packag. Anal., 44(6): 320–327 (in German).
Hu, Q-G., Zhang, M., Mujumdar, A. S., Xiao, G-N. and Sun, J-C. 2007. Performance evaluation of vacuum microwave drying of edamame in deep-bed drying.
Kudra, T. and Strumillo, C. (Eds.). 1998. Thermal Processing of Bio-Materials. Gordonand Breach Science Publications, Amsterdam, 669p.
CRC_73877_Ch026.indd 369CRC_73877_Ch026.indd 369 12/24/2008 3:15:10 PM12/24/2008 3:15:10 PM
In: Proc. 5th Asia-Pacific Drying Conference. G. Chen (Ed.). August 13–15, World Scientific Publishing Co. Pte. Ltd., Singapore, pp. 484–489.
© 2009 by Taylor & Francis Group, LLC
370 Advanced Drying Technologies
Kwok, B. H. L., Hu, C., Durance, T. and Kitts, D. D. 2004. Dehydration techniques affect phytochemical contents and free radical scavenging activities of Saskatoon berries (Amelanchier alnifolia Nutt). J. Food Sci., 69(3): S122–S126.
Leiker, M., Aurich, K. and Adamska, M. A. 2005. Accelerated drying of single hardwood boards by combined vacuum-microwave application. Proc. 9th International IUFRO Wood Drying Conference. August 21–26, Nanjing ForestryUniversity, Nanjing, China, pp. 185–190.
Li, X-J., Zhang, B-G. and Li, W-J. 2007. Microwave vacuum drying characteristics
Liapis, A. I. and Bruttini, R. 1995. Freeze drying. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). Marcel Dekker Inc., New York, pp. 309–343.
Lin, T. M., Durance, T. D. and Scaman, C. H. 1998. Characterization of vacuum microwave, air and freeze dried carrot slices. Food Res. Intl., 31(2): 111–117.
Lin, T. M., Durance, T. D. and Scaman, C. H. 1999. Physical and sensory properties of vacuum microwave dehydrated shrimp. J. Aquatic Food Prod. Technol., 8(4): 41–53.
Pappas, C., Tsami, E. and Marinos-Kouris, D. 1999. The effect of process conditions on the drying kinetics and rehydration characteristics of some MV-vacuum dehydrated fruits. Drying Technol., 17(1&2): 157–174.
Petrucci, V. E. and Clary, C. D. 1989. Vacuum microwave drying of food products. Report No. 2897-3. California State University, Fresno, CA.
Popovich, D. G., Hu, C., Durance, T. D. and Kitts, D. D. 2005. Retention of ginse-nosides in dried ginseng root: Comparison of drying methods. J. Food Sci., 70(4): 447–479.
Püschner, P. and Hoon, L. L. S. 2007. Microwave vacuum drying of fruits &
Regier, M., Mayer-Miebach, E., Behsnilian, D., Neff, E. and Schuchmann, H. P.
noid content. Drying Technol., 23(4): 989–998.Seyfarth, R., Leiker, M. and Mollekopf, N. 2003. Continuous drying of lumber
in a microwave vacuum kiln. Proc. 8th International IUFRO Wood DryingConference. August 24–29, Transilvania University of Brasov, Brasov,Romania, pp. 159–163.
Sunjka, P. S., Rennie, T. J., Beaudry, C. and Raghavan, G. S. V. 2004. Microwave/convective and microwave/vacuum drying of cranberries: A comparative study. Drying Technol., 22(5): 1217–1231.
Xu, Y.Y., Min, Z., Mujumdar, A. S., Zhou, L-G. and Sun, J-C. 2004. Studies on hot air and microwave vacuum drying of wild cabbage. Drying Technol., 22(9):2201–2209.
Yang, W., Changrue, V. and Raghavan, G. S. V. 2005. Microwave vacuum drying of carrots under varying power modes. Proc. Inter-American Drying Conference,IADC 2005. August, Montreal, Canada. Paper A-5.
Yongsawatdiguul, J. and Gunasekaran, S. 1996. Microwave vacuum-drying of cranberries. J. Food Process. Preserv., 20: 145–156.
Yousif, A. N., Scaman, C. H., Durance, T. D. and Girard, B. 1999. Flavor volatiles and physical properties of vacuum-microwave- and air-dried sweet basil (Ocimum basilicum L.). J. Agric. Food Chem., 47: 4777–4781.
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of wood. In: Proc. 5th Asia-Pacific Drying Conference. G. Chen (Ed.). August 13–15, World Scientific Publishing Co. Pte. Ltd., Singapore , pp. 355–360.
vegetables. In: Proc. 5th Asia-Pacific Drying Conference. G. Chen (Ed.). August 13–15, World Scientific Publishing Co. Pte. Ltd., Singapore, pp. 1347–1355.
2005. Influences of drying and storage of lycopene rich carrots on the carote-
© 2009 by Taylor & Francis Group, LLC
371
27Filtermat Drying
Filtermat is the trade name for a multistage dryer that combines two well-established drying technologies: nozzle spray drying and continuous
chamber of a uniquely designed (Figure 27.1) dryer and contacted with air at a high inlet temperature. This provides for rapid evaporation of water, short contact time, and low material temperature, which translate into a favorable quality product and better process economics. The Filtermat spray dryer is designed to operate with two inlet air temperatures. High-temperature air enters the spray chamber around each spray nozzle, while low- temperature air enters the dryer through the out-of-the nozzle area. Such a system of air supply ensures better mixing of the drying and atom-izing air with the wet feed. Even when predrying feed conditions are not
ciency can be maintained by controlling process variables.The belt dryer used as the second-stage dryer offers longer exposure
time to much lower air temperatures needed to complete the drying pro-cess. In this primary drying chamber, the semidried product with internal moisture collects on a continuously moving belt made of a woven polyes-
Because low-temperature drying air is continuously drawn through the powdery material as it deposits on the belt, the product forms a porous mat of agglomerated particles. Although the product from a spray dryer
dryer, the inherent porosity of the bed on the moving belt facilitates fur-ther drying. Thus, by controlling the residence time and drying air tem-peratures, it is possible to successfully dry the product through its falling rate period.
The gas plenum in the belt dryer can be segmented to arrange for dif-ferent temperature regimes. Thus, special powder characteristics can be
drying air, for example, permits crystallization or other desirable trans-formations to occur. For products that require additional drying, warm air can be ducted into the retention zone.
cold and low-humidity air can be blown through to lower the product’s
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first stage of drying, the high-moisture feed is atomized into a spray
optimum, the manufacturer claims that product quality and process effi-
ter filament and is situated at the bottom of the primary spray chamber.
is still wet and thus sticky and difficult to dry in a conventional belt
In the third drying stage, the final moisture content is achieved with warm air. When the dry product reaches the final stage of processing,
flow-through belt drying (Rheinlander, 1982; Anonymous, 2000). In the
obtained in the so-called retention zone (Figure 27.1). A minimal flow of
© 2009 by Taylor & Francis Group, LLC
372 Advanced Drying Technologies
temperature in preparation for postdrying treatment such as milling,
The dryer is optionally equipped with a powder injection system
and controlled contact between the powder and the liquid spray, which facilitates the formulation of stable agglomerates. This results in a new wet surface and builds larger particles before contact with the hot drying air. Thus, by controlling the agglomeration process, the Filtermat dryer
sities. It is claimed to be especially suited for drying of sticky materials(for example, whey, proteins, oranges, and other fruit juices) and which tend to agglomerate and adhere to the dryer wall due to their sugar content.
Commercial Filtermat dryers are available for evaporation capacities from 230 to 5500 kg/h. Approximate building dimensions (length/width/height) to accommodate such units range from 12 × 7.5 × 14 to 27 × 16.5 × 18 m(Anonymous, 2000).
1
1
12
12
2
2
5
4
7 8 96 III
II
I
3
2
12
161
12
1113
10
14
15
FIGURE 27.1
pressure pump, 4—nozzle system, 5—air distributor, 6—primary drying chamber,
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A three-stage spray drying system (Filtermat). 1—air filter, 2—heater–cooler, 3—high-
7— retention zone, 8—finish drying zone, 9—cooling zone, 10—belt conveyor, 11—cyclones, 12—fan, 13—fines recovery system, 14—discharge, 15—screening system, 16—heat recov-ery system, I—first drying stage, II—second drying stage, and III—third drying stage.
product sizing, classification, and final packaging.
around the spray nozzles. The cloud of fine particles provides for close
can produce free-flowing, easily dispersible products with high bulk den-
© 2009 by Taylor & Francis Group, LLC
Filtermat Drying 373
References
Anonymous. 2000. Filtermat Spray Drying System. Technical Information. Niro Filtermat, Fond du Lac, WI.
Rheinlander, P. M. 1982. Filtermat—The 3-stage spray dryer from DEC. In: Proc. 3rd International Drying Symposium. J. C. Ashworth (Ed.). Vol. 1. Drying Research Ltd., Wolverhampton, England, pp. 528–534.
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375
28Spray-Fluid Bed–VibratedFluid Bed Dryer
Figure 28.1 presents the schematic of a hybrid dryer where a spray dryer
the air distributor of a concurrent SD. The homogenized dairy feed with 50% solid content is pumped at 15 MPa into the pressure nozzle located axially at the top of a drying chamber. The semidry product of ~8% mois-
to the atmosphere. Up to this point, such a classical SD with a volumetric evaporation rate of ~5.5 kg/m3
To arrange for FB drying (the second stage of the process), the perforated conical grid is attached to the conical bottom of the SD. Here, the semi-dry granules are dried by 120°C air supplied to the gas plenum formed by the perforated grid and the dryer cone. The product at ~4% moisture
complete drying at 90°C. Cooling of the product before packaging is per-
Drying and cooling in the VFB at a bed height from 200 to 300 mm take ~2 to 3 min.
The capital cost of a three-stage system is higher than that of a one-stage SD, but operating costs are lower because of energy savings (Table 28.1). Besides, the quality of the product from a three-stage dryer is much better because of lower drying temperatures. In addi-tion, granules obtained in a VFB dissolve faster than powder obtained in a classical SD.
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Filtered ambient heated to 150°C in a finned heat exchanger is drawn to
ture content falls onto the dryer bottom, whereas spent air at 90°C is fil-tered off in a bag filter surrounding the drying chamber and discharged
h is considered as the first drying stage.
formed at the last section of the VFB by blowing dehumidified air at 50°C.
(SD) is integrated with a fluidized bed (FB) dryer, and such a unit is com-bined with a vibrated fluid bed (VFB) dryer/cooler (Tang et al., 1999).
content falls through the central overflow to the VFB to agglomerate and
© 2009 by Taylor & Francis Group, LLC
376 Advanced Drying Technologies
50% feed
Exhaust
Air
Air
Air
Air
Product
Spray dryer(first stage)
Fluid bed dryer(second stage)
Vibro-fluid beddryer / cooler(third stage)
90ºC 50ºC
65ºC
90ºC
150ºC
120ºC
FIGURE 28.1Three-stage drying system for dairy products.
TABLE 28.1
Energy Savings in a Three-Stage Drying System
DryerEnergy Consumption
(kJ/kg H2O)Energy Savings
(%)
Single stage (SD) 5265.1 0Two stage (SD + FB) 4454.4 15.4Three stage (SD + FB + VFB) 3985.9 24.3
Source: Based on Tang, J. X., Wang, Z. G. and Huang, L. X. Drying Technol., 17, 1747–1757,1999. With permission.
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© 2009 by Taylor & Francis Group, LLC
Spray-Fluid Bed–Vibrated Fluid Bed Dryer 377
Acronyms
FB Fluid bedSD Spray dryer
Reference
Tang, J. X., Wang, Z. G. and Huang, L. X. 1999. Recent progress of spray drying in China. Drying Technol., 17(9): 1747–1757.
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VFB Vibrated fluid bed
© 2009 by Taylor & Francis Group, LLC
379
29Combined Filtration and Drying
29.1 Introduction
typically from 0.5 to 10 kWh/m3 of the suspension to be separated (Rippenger, 1998). The lower values apply to coarse particles and incom-
as biosolids that form compressible cakes, where water is removed by compaction and expression. Regarding latent heat of water evaporation, mechanical solid–liquid separation is 100–1000 times less energy-inten-sive than thermal drying. Thus, an intelligent combination of mechanical
the dryers and therefore lower production costs.
choice of an appropriate separator according to material characteristics
content after mechanical separation. As pointed out by Rippenger (1998), the often-mentioned rule, “The lower residual moisture content, the lower the operating costs,” is not always valid. Each case should be considered individually because for some materials the total cost can be lower when
A further reduction of costs can be obtained when combining MDW with thermal drying in a single-process vessel than performing these two
(or centrifugation) integrated with thermal drying can offer the following
Improved product quality due to a combination of various •operations in one apparatus (high purity and no product contamination)
transport, no product loss, lower consumption of wash liquid, and shorter batch time)
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The energy consumption in conventional filters and centrifuges ranges
pressible cakes where water is removed by bulk and film drainage. The higher values are expected for fine particles and pasty materials, such
dewatering (MDW) and finish drying could reduce the thermal load on
The key parameter for an optimum process configuration is not only the
(see Figure 29.1; Shafick, 1981) but also the selection of residual moisture
selecting a process configuration with higher residual moisture content
filtration of pigments followed by spray drying, which was less costly than a press filter and a paddle dryer (Rippenger, 1998).
operations in separate units (Figure 29.2). In addition, pressure filtration
benefits:
• Improved economics and production efficiency (no wet material
and thus higher energy consumption. A case has been cited for cross-flow
© 2009 by Taylor & Francis Group, LLC
380 Advanced Drying Technologies
Particle size (µm)
Wat
er co
nten
t (%
w/w
)
1
2
5
10
30
6080
100
1 2 5 10 50 100 200 50020
20
Thermal drying
Static thickeners / filters Classifiers/filters
HydrocyclonesCentrifuges(solid bowl)
Centrifuges
Vacuum filters
(screen bowl, etc.)
Screens
FIGURE 29.1
in Dewatering of Fine Particle Conference, University of Alabama, Tuscaloosa, Alabama,pp. 1–32, 1981.)
Total installation cost
Conventional filter
Dryer
Filter dryer
12
6
Filte
r dry
er ar
ea (m
2 )
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0Normalized cost
FIGURE 29.2
dryer. (Courtesy of De Dietrich Process Systems, Inc., Charlotte, North Carolina. With permission.)
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Liquid–solid separation of wettable minerals. (Adapted from Shafick, H., Proc. Progress
Capital and total installation costs: filter dryer unit versus conventional filter and
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 381
or reslurry washing; operating under vacuum or overpressure)Reduced environmental pollution and minimized personnel • exposure to hazardous media the (closed system also allows for operation with toxic products and organic solvents)
•wash tank, etc.)Compact design (reduced space requirement, easy integration into •existing building, and lower installation and construction costs)Flexibility (cold air or hot air blowing, pressure or vacuum dry-•ing, and drying in inert atmosphere)
Filter dryers are especially appropriate for sterile batch processes where solids have to be separated from the mother liquor mechanically and ther-mally in an enclosed system. They are particularly suitable for process plants in which frequent product changes are needed, for example, when
chemicals, foodstuffs, and so on.
29.2 Pressure Filtration and Drying
Nutsch Dryer (TND), has been on the market for years, and therefore only
(Charlotte, North Carolina) is a heated paddle agitator (Figure 29.3) designed as a rotating three-arm structure with aerofoil blades that pro-vide for increased stability at high torque levels and an extended heat-transfer area. This unique arrangement promotes axial and radial mixing as well as uniform drying by continuous exposure of each particle to the
the individual particles themselves.
with a cold blow-through step to reduce the initial moisture content by liq-uid entrainment. Convective drying with hot gas supplies thermal energy to the cake for evaporation and continues the mechanical liquid entrain-
with intense agitation combined with contact heating from the agitator,
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Reduced investment/running costs (no separate filter, centrifuge,
producing pharmaceuticals, dyes and pigments, fine chemicals, agro-
There are two basic configurations in industrial use for combined filtering and drying: a vertical filter with a heated agitator and a horizontal filter with a revolving housing. The horizontal filter dryer, known as the Titus-
the vertical filter dryers are presented here.The key to the vertical filter dryer developed by Rosenmund Inc.
heated surface of the interior vessel walls, filter plate, paddle agitator, and
Generally, after filtration and washing, most drying processes begin
ment. Finally, vacuum applied from both above and below the filter cake
vessel walls, and filter plate bring the product moisture to its required
• Increased process flexibility and control (normal, countercurrent,
© 2009 by Taylor & Francis Group, LLC
382 Advanced Drying Technologies
value, which is frequently below 0.5%. The representative results for an 8 2
1. The use of a heated agitator increases the heating rate by up to
larger than that from the vessel walls. 3. The estimated heat input contributions by the components for the
25%, respectively.
2
a cake depth of ∼0.4 m and an agitator speed of 6.5 rpm. Clearly, these depend on the feedstock.
FiltrateProduct
Fluidizing gas
Purge gas
Dust filter
Paddle agitator
Slurry
Wash liquid
Pressure gas
Cold/hot water
FIGURE 29.3
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Vertical filter dryer. (Courtesy of De Dietrich Process Systems, Inc., Charlotte, NC.)
m filter dryer are the following (Anonymous, 1990):
70% over that provided by a filter dryer with no agitator. 2. The heat-transfer coefficient from the heated paddles is 3 to 4
times larger than that from the filter plate and up to 1.5 times
heated paddle agitator, vessel walls, and filter plate are 45, 30, and
Typical throughput of an 8 m vertical filter dryer is 1250 kg/batch at
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 383
Figure 29.4 (Kudra and Mujumdar, 2007) presents a schematic of a vertical
it passes through the cake into the helix interior. Alternatively, a heating medium can be passed through the internal tubing of the helix while a vacuum is applied to the vessel. The discharged solids are loosened from
veyor to transport dry and loose cake to the bottom of the pressure ves-sel. A scraper attached to the helix then discharges the cake through the
Drying gasWash liquor
DischargeRemnant filtrate
Exhaust gas +solvent vapor
Helicalelement
Scrapper
Filtercake
Resuspendedsolids
FIGURE 29.4
Industrial Drying, Taylor & Francis, Boca Raton, FL, pp. 453–517, 2007. With permission)gg
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Filtroba helical filter dryer. (Redrawn from Kudra, T. and Mujumdar, A. S., Handbook of
filter dryer with a helix-shaped filter element. During the filtration and
cake formed on the upper surface of the hollow helix that serves as the cake washing stages, the unit operates as a conventional filter with a filter
filter grid. Then, steam or heated gas is introduced into the vessel in which
the filter element by spinning the helix, which further acts as a screw con-
outlet into a closed receptacle. The helix filter dryer finds applications in pharmaceutical and fine chemical manufacturing processes—from coarse salts to fine antibiotics. Typical fed slurries may have solids’ concentrations
have practical applications with solids’ concentrations up to 10% or more. of <1% by weight. Owing to the ease of automatic discharge, these filters
A choice of sintered metal or cloth filter media allows for particle capture
© 2009 by Taylor & Francis Group, LLC
384 Advanced Drying Technologies
down to 0.5 µm. The ability to agitate rapidly settling slurries by rotating the helix makes it possible to also process coarse particles (Anonymous, 1992).
29.3 Centrifuge Dryer
Although classical centrifugation reduces the water content to a great extent, thermal drying is necessary in most cases to obtain a product of desired moisture content. The centrifuge dryer is an innovative technol-
drying (Schilp et al., 2000; Peeters et al., 2008). This two-step process is car-ried out in a continuously operated single unit that is built as a solid bowl or a screen bowl centrifuge placed in a stationary casing, which serves as a drying chamber. The slurry fed to the centrifuge is mechanically dewa-tered due to a combination of the G-force and pressing. Rapid settling of
cal section of the bowl. Also, interstitial water is pressed out as the settled solids are transported by the screw. Final release of water occurs in the conical section of the bowl, where the solids are compressed by press-ing in the axial direction and centrifugal compaction in the radial direc-tion. Because the centrifuge discharge serves as a fast-rotating atomizer, the cake leaving the rotor with a circumferential velocity of 60–80 m/s is
upon collision with a stationary impact cone mounted inside the station-ary casing that holds the centrifuge bowl. Dispersed particles, picked up
the rotating centrifuge bowl and the stationary dryer casing. Dry solids
A large fraction of the exhaust gas is recirculated to reduce energy requirements and the remaining fraction is sent to the scrubber for vapor condensation and sanitary cleaning (Figure 29.5).
This continuous centrifugation drying process produces far more dry product (80–95% DS from the initial 1.5–4% DS) compared to the cake from
plant (WWTP) (Peeters et al., 2008). The production capacity depends on the sludge type and the size of the centrifuge dryer. As an example, for a mixed sludge 65/35 (digested/raw) at 29% DS, the system allows removal of 330 to 3300 kg water/h, depending on its size (Schilp et al., 2000). The
3
throughput at 4% DS is approximately half of that of a conventional cen-trifuge (Anonymous, 1999). Because the footprint of the centrifuge dryer
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the solids and filtrate removal due to the G-force take place in the cylindri-
disintegrated into fine particles (e.g., from 0.30 to 0.65 mm at 2700 rpm)
(DS) are then separated from the gas stream in a cyclone or a hot-gas filter.
the filter press (35–40% DS) used in parallel on the waste water treatment
specific cost per ton of DS (tds) from a centrifuge dryer having a 20 m /h
ogy that combines conventional centrifugal dewatering with hot-gas flash
by a hot-gas stream blown through the drying chamber, are flash-dried during swirling flow of a gas–particles stream in the annulus between
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 385
is about the same as that of a centrifuge, the combination of MDW and
and lowers the capital costs for the plant and ancillary equipment.
29.4 Vapor Pressure Dewatering
the Technical University of Karlsruhe, Germany. Here, the differential
then from superheated steam (SHS) due to intense contact heating of one
and evaporates rapidly, causing a pressure buildup, which drives the bulk
cloth, membrane, etc.). As differential steam pressure needs to be built
convective heat transfer from the SHS and conductive heat transfer from the heating plate (Ruf et al., 1996; Ruf and Stahl, 1997). The mechanism of the VPD technique is illustrated in Figure 29.6, in which the white circles
FIGURE 29.5Centrifuge dryer system with gas recycling.
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Conventional filtration drying represents sequential filtering and dry-ing, that is, drying starts when filtering is completed. Another combined filtration–thermal drying technique where drying is performed during filtration is the so-called vapor pressure dewatering (VPD) developed at
pressure needed to displace bulk liquid comes first from saturated and
side of a water-saturated filter cake. On this side, the bulk liquid heats up
filtrate progressively through the filter cake and the filter medium (filter
up to displace the liquid, total saturation of the filter cake is necessary for successful dewatering. Subsequent drying of the filter cake is due to
thermal drying in a single, compact unit reduces the floor/building space
Impact cone
Feed
Dryer Solid bowl centrifuge
Particle trajectoryEffluent
Hot gas Gas–solidsuspension
© 2009 by Taylor & Francis Group, LLC
386 Advanced Drying Technologies
zones with various saturation levels.The cake is enclosed in a casing with the heating plate at the bottom and
the temperature of the cake layer adjacent to the heating plate crosses the boiling point with respect to the capillary pressure, water starts to evaporate
separates the liquid zone (dark gray area in Figure 29.6) from the saturated steam zone (light gray area in Figure 29.6) detaches from the heating plate
a second front (the drying front) appears due to the continuous heat sup-ply to the saturated steam zone, and therefore the zone with SHS develops. Figure 29.6 shows the instant when the drying front has detached from the
solid particles in the SHS zone are dried by convection from SHS and by conduction from the heating plate. When the dewatering front reaches the
is leveled to ambient conditions.Contrary to dewatering by differential gas pressure, VPD does not pro-
saturation pressure precedes the dewatering front as a result of lower
Filtrate Filter plate
Casing
Dewateringfront
Filter cloth
Waterlayer
Saturatedsteam layer
Superheatedsteam layer
Dryingfront
SolidsHeating plate
FIGURE 29.6Mechanism of VPD.
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represent the solid particles and filled areas of different grayness indicate
a perforated plate over the filter cloth at the top. At the beginning of dewa-tering, the voids in a filter cake are completely filled with liquid. As soon as
rapidly. This causes the pressure to build up, which starts to displace the bulk of the liquid toward the filter medium. Thus, a dewatering front that
and moves to the filtering surface as water evaporation continues. Further,
heating plate but the dewatering front has not reached the filter cloth. The
filter medium, a vapor breakthrough occurs, and thus the internal pressure
duce the fingering effect caused by the different capillary radii. If the steam
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 387
capillary pressure in larger capillaries, the vapor reaches colder layers of
front levels off and a premature vapor breakthrough is avoided. However,
development of cracks by compressing the cake during dewatering. A
slurry inlet is closed, and the membrane is exposed to pressure for pre-
heating plate is heated with electricity, thermal oil, or other means, and the vapor pressure–induced dewatering begins.
As reported by Ruf and Stahl (1997), the theoretical minimum energy needed to displace all of the capillary liquid can be determined on the basis of the void volume, and energy reduction by as much as 95% when compared with thermal drying is feasible. Experiments with model mate-rials performed by these authors have shown that up to 80% of the liquid held in large capillaries can be displaced mechanically, that is, through differential pressure dewatering. This translates into an actual reduction in energy consumption by 80% when compared to thermal drying for the same level of residual moisture.
PressHeating plate Filter cake
Slurry
Filter plateFiltrate
Insulation
FIGURE 29.7
und Mechanik, University of Karlsruhe, Germany. With permission.)
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the filter cake and condenses (Ruf and Stahl, 1997). Thus, the dewatering
this effect fails if a fracture forms in the filter cake. To eliminate this draw-back, the hot filter press technique was further developed to suppress the
schematic diagram of a hot filter press built based on the conventional membrane filter is shown in Figure 29.7. The press feed channel is located in the corner of the filter assembly, and drainage of the filtrate occurs locally at the membrane filter plate. The press is charged while the heat-ing plate is unheated. When a cake of sufficient thickness is formed, the
filtering to achieve the desired consolidation of the filter cake. Then the
Another variant of the hot filter press technology is the so-called thermal dewatering (TDW), where the sludge is fed into a filter chamber comprising
Schematic of the hot filter press. (Courtesy of the Institut für Mechanische Verfahrenstechnik
© 2009 by Taylor & Francis Group, LLC
388 Advanced Drying Technologies
ing liquid such as hot water. The polypropylene (PP) plate with an internal channel to accommodate the squeezing medium is called the membrane plate because it is equipped with an elastic PP membrane. When hot water
the sludge supply is cut off and the squeezing medium at elevated pressure (e.g., water at 1.5 MPa) is run through the membrane plate, and therefore the hydraulic pressure applied to the membrane expels water from the cake. When the feeding and squeezing stages are completed, the hot water supply is shut off and the cake is washed and blown with air. Thereafter,
Comparative studies on MDW and TDW, performed under the same con-
waterworks sludge, indicated that TDW can be an alternative technology
for waterworks sludge and 48% for wastewater sludge, which lowers the handling costs; and (b) a doubled dewatering rate, which compensates for the additional operating costs of TDW (Lee, 2006).
the ROLLFIT® technology (Reiser, 2008). Figure 29.8 presents a typical ROLLFIT system for sludges, which needs polymer-conditioned sludge for better separation, whereas Figure 29.9 illustrates the variant of the
by hot water or another heating medium. For the majority of industrial
Product
Sludge
Polymer
Vacuum system
Conveyor
Heating system
Dry sludge
Vapor
Filtrate
FIGURE 29.8
Switzerland. With permission.)
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two plates covered with a filter cloth (Lee, 2006). The metal plate with an
pumped into the filter chamber until the filter cake is built up. Thereafter,
the plates are set apart, and the cake is removed from the filter chamber.
ditions with difficult-to-dewater wastewater sludge and easy-to-dewater
for sludge dewatering because of (a) reduced final water content by 23%
At present, the concept of the hot filter press is commercialized as
process where the filter cake is squeezed by cold water and then heated
Basic configuration of ROLLFIT technology. (Courtesy of Reisser, Eilers & Partner AG, Zug,
internal flat channel is called the heating plate because it is heated by a flow-
flowing through the heating plate reaches the set temperature, the sludge is
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 389
sludges, the membrane plate pack made of PP is the best technical and economic solution. However, drying of municipal sludge is only possible with the specially designed Ultra-Dry® plate pack. Table 29.1 presents the characteristics of the ROLLFIT technology for three types of municipal and industrial sludges.
TABLE 29.1
Performance of ROLLFIT Technology with Ultra-Dry Plates
Material
AluminumHydroxide
Sludge
Sewage Sludge,(Digested, Polymer
Conditioned)
Sludge from Flue Gas Desulfurization
in Waste Incineration Plant
Quantity (tds/d) 0.2 3.5 0.9Solids’ content after drying (% w.b.)
>70 >90–92 >70–72
Number of cycles (h) 2/24 2/24 3/24Filter press Volume (m3) 0.4 6 2 Dimensions (m) 0.8 × 0.8 1.5 × 1.5 1.2 × 1.2 Cake thickness (mm) 14 25 32 Dewatering/squeezing None Cold water, 15 bar Cold water, 15 bar Heating medium/ temperature
Steam, 140°C Water, 85/90°C Water, 95°C
Vacuum (mbar) 60 50 75
Note: tds/d = ton of dry solids per day.Source: Courtesy of Reisser, Eilers & Partner AG, Zug, Switzerland. With permission.
Filtrate
Suspension/wash liquid
Plate (PP)
Diaphragm
Filtercloth (PP)
Filtercake
Squeezemedium
Heating mediumSqueeze medium
(a) (b) (c)
FIGURE 29.9
blowing, and (c) drying. (Courtesy of Reisser, Eilers & Partner AG, Zug, Switzerland.)
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Process steps in ROLLFIT technology: (a) filtration/split wash, (b) through wash/squeezing/
© 2009 by Taylor & Francis Group, LLC
390 Advanced Drying Technologies
29.5 Steam Pressure Filtration
2000). Moreover, after steam breakthrough, the heated cake becomes
drying and reduces steam consumption (Figure 29.10; Stahl, 1998). SPF
separating the steam room from the interior of the pressure vessel, which
up to the point of steam breakthrough (see Figure 29.10), and the cake is
moisture content by compressed air.
drying. Table 29.2 presents a comparison of energy costs for compressed
0
1.0
Time
Satu
ratio
n
Air drying
Pressurefiltration
Steamdrying
Steambreakthrough
Mechanicaldisplacement
Superheatedsteam drying
FIGURE 29.10
Technologies, May 18/19, Mainz, Germany, 1998.)
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A modification of VPD is the so-called Hi-Bar steam pressure filtration
cake, coming from an external source, allows continuous operation (Bott, (SPF) (Anonymous, 2005), where pure steam for dewatering of the filter
is carried out in a rotary filter (disk or drum) where all sequential opera-tions (mechanical filtration, washing, further dewatering by a dynamic action of steam with simultaneous heating by steam, finish drying by
occur during one rotation of the filtering element. As seen in Figure 29.11, the steam area of the rotary filter is covered and sealed by a steam cabin
is filled with compressed air. Generally, the steam cabin covers the cake
washed, dewatered, and heated in this first stage with minimum steam
SPF can lead to significant cost savings because of integrated filtration and
air filtration with thermal drying and for SPF for two coal concentrates.
Steam pressure filtration. (Redrawn from Stahl, W. H., IEA Workshop on Separation
dewatered by compressed air flowing through the cake, which accelerates
blowing with compressed air, and back-flushing with compressed air)
consumption. After leaving the steam cabin, the cake is dried to its final
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 391
Steam dewatering zone(steam hood)
Compressed airdewatering zone
Compressed air
Discharge zone
Slurry
Pressurevessel
Steam
Air
Compressed airback flush
Filtrate
Cake formation zone
Agitator
Slurrythrough
Solids
FIGURE 29.11
With permission.)
TABLE 29.2
Comparison of Dewatering Technologies for Coal Fines (<63 µm)
Parameter
Coal Concentrate 1 Coal Concentrate 2
Pressure DiskFilter Hi-Bar SPF
Pressure Disk Filter Hi-Bar SPF
Technical CharacteristicsFilter area (m2) 120 146 120 85Pressure difference (bar) 2.5 2.5 2.0 2.5Final moisture content (% w.b.)
19.5a 8.5 18.0a 9.5
Solids’ throughput (t/h) 100 100 70 70Air throughput (m3/h) 4500 1600 3500 2100Steam consumption (t/h) 0 9.2 0 7.0Steam/kg of waterremoved (kg/kg)
— 0.62 — 0.67
Energy CostsAir compression (€/a) 106,400 41,400 71,100 51,640Steam production (€/a) 0 493,900 0 375,800Thermal dryer (€/a) 1,189,800 0 627,900 0Total energy costs (€/a) 1,296,200 535,300 699,000 427,440Energy costs ratio 1.0 0.413 1.0 0.612
a
Source: Courtesy of Bokela, Karlsruhe, Germany. With permission.
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Design principle of Hi-Bar disk filter. (Courtesy of Bokela GmbH, Karlsruhe, Germany.
Because target moisture content is 10% w.b., the filter cake must be thermally dried.
© 2009 by Taylor & Francis Group, LLC
392 Advanced Drying Technologies
29.6 Jet Stream Centrifugation
An alternative technique to SPF is the jet stream centrifugation concept developed by Heinkel AG, Bietigheim-Bissingen, Germany (Anonymous,
compressed gas (e.g., nitrogen) is heated and fed into the drum to expel liquid and evaporate residual moisture to much lower moisture levels than in a conventional centrifuge (Figure 29.12 and Table 29.3). When organic solvents are to be removed, the drying gas circulates in a closed
charge phases of a standard centrifugation process, the gas recirculation system is disconnected from the centrifuge. During the drying step, the gas leaving the centrifuge at normal pressure through a small buffer is
TABLE 29.3
Residual Moisture Content in Conventional and Jet StreamCentrifugation
Product
Residual Moisture Content (% w.b.)
ConventionalCentrifugation
Jet Stream Centrifugation
Sodium ascorbate 4.5 <0.01Mebhydrolin salt 15.0 <0.1Acetylsalicylsalicylic acid 7.0 <0.02Food coloring agent 7.8 0.2Surfactants 39.4 <0.4Penicillin 13.7 <0.5Vitamins 5.0 0.1
Source: Courtesy of Heinkel AG, Bietigheim-Bissingen, Germany.With permission.
Conventional centrifugation
Jet stream centrifugation
Time
Resid
ual m
oist
ure
FIGURE 29.12Comparison of conventional centrifugation with jet stream centrifugation. (Courtesy of Heinkel AG, Bietigheim-Bissingen, Germany. With permission.)
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2000). Here, the filter centrifuge is fitted with a gas-tight drum. Thus,
system as shown in Figure 29.13. During the filling, washing, and dis-
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 393
recompressed in a liquid piston compressor, cooled in a tubular heat exchanger and a washing tower, reheated, and recycled to the centrifuge. The compression heat is transferred to the circulation sealing liquid and released into a tubular heat exchanger with cooling water. Gas cooling and condensation of the vaporized solvent are carried out in a brine-cooled heat exchanger and in a washing column fed with the recircu-
stream centrifugation with the gas recirculation system are the reduction in the fresh gas use of 90–95% and a substantial reduction of environ-mental pollution.
29.7 Microwave-Assisted Filter Drying
McConvey and Shering (2000) have presented results on drying phar-
energy was supplied. Based on the experiments with MW-assisted pres-2
that the drying period could be reduced by at least 50% when compared
Heinkel HF
Solids
Slurry
Filtrate
Gas with solvent Recycled gas
FIGURE 29.13Gas recirculation for jet stream centrifugation. (Courtesy of Heinkel AG, Bietigheim-Bissingen, Germany. With permission.)
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lating brine-cooled washing liquid. The main benefits of combining jet
maceutical compounds in a pressure filter, into which microwave (MW)
sure filter dryers having filtration areas of 0.05 and 0.5 m , they have shown
with drying in hot nitrogen in the same pressure filter. The average MW
© 2009 by Taylor & Francis Group, LLC
394 Advanced Drying Technologies
power input was in the range of 2 to 4 kW/m2
solvent (acetone) had to be removed from the initial moisture level of 20%
tion. In terms of cost, the authors report that MW-assisted drying was 2.5 times more effective than the installation of an additional pressure
nitrogen was as effective as MW-assisted vacuum drying. The latter is, of course, more expensive because of the need for a vacuum system. If the solvent is predominantly water and the material is not very heat-sensitive, then it is preferable not to use the vacuum system. It is worth noticing that the authors could reproduce laboratory-scale results on a pilot scale with a scale-up factor of 150. The optimum conditions for drying of a bulk drug wetted with acetone were found to be intermittent agitation, operation under vacuum, and MW power density of 2 kW/m2.
Recently, Leonelli et al. (2008) have published results on numerical simu-
MW input ports has been simulated for three loading conditions, namely,
inserting, among others, a waveguide transition, modifying the position and orientation of MW inlet ports, and altering the nozzle’s geometry and distribution. The results from simulations were positively validated by experiments on a 3 m2
input.
Acronyms
DS Dry solidsMDW Mechanical dewateringPP Polypropyleneppm Parts per millionrpm Revolutions per minuteSPFTDW Thermal dewateringTND Titus-Nutsch DryerVPD Vapor Pressure Dewateringtds Tons of dry solidsw.b. Wet basis
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of the filter area. The organic
to the final one of 100 ppm. It was found that intermittent agitation of the
tion had a negative effect. Indeed, continuous agitation formed large balls filter cake improved the drying performance, whereas continuous agita-
filter to double the production rate. Interestingly, MW-assisted drying in
lations of an industrial-scale MW-assisted filter dryer. The performance of the Nutsch-type filter dryers (see Figure 29.3) equipped with one or several
an empty filter (applicator), a filter partially filled with a high-lossy slurry at the beginning of filtration, and a filter partially filled with a relatively low-lossy powder obtained in the filtration drying process. The results from simulation allowed the original design of the filter to be improved by
filter dryer equipped with a three-port MW power
Steam pressure filtration
instead of a free-flowing powder that was obtained by intermittent agita-
© 2009 by Taylor & Francis Group, LLC
Combined Filtration and Drying 395
References
Anonymous. 1990. Filtration and drying systems. Tech Update, Rosenmund Inc., 2(3): 1–2.
Anonymous. 1992. Technical Brochure. KETEMA Process Equipment, El Cajon, CA.Anonymous. 1999. Centridry® combined dewatering and drying of wastewater
sludge. KHD Humboldt Wedag AG Technical brochure, # 7-404e, Cologne, Germany.
Anonymous. 2000. Technical Information. Heinkel AG, Bietigheim-Bissingen,Germany.
Anonymous. 2005. Technical brochure 8/2005e: Hi-Bar Filtration. ContinuousPressure & Steam Pressure Filtration. Bokela, Karlsruhe, Germany.
Bott, R. 2000. Hi-Bar Filtration: The Continuous Pressure and Steam Pressure Filtration. Bokela, Karlsruhe, Germany.
Kudra, T. and Mujumdar, A. S. 2007. Special drying techniques and novel dryers. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). Taylor & Francis, Boca Raton, FL, pp. 453–517.
Lee, J. E. 2006. Thermal dewatering (TDW) to reduce the water content of sludge. Dry. Technol., 24(2): 225–232.
Leonelli, C., Veronesi, P. and Grisoni, F. 2008. Numerical simulation of an indus-
J. Microw. Power Electromagn. Energy, 41(3): 4–13.
pharmaceutical compounds. Trans. IChemE, 78(C2): 97–100.Peeters, B., Vernimmen, L. and Meeusen, W. 2008. Effect of activated sludge char-
acteristics on the fouling frequency of a single stage mechanical dewater-ing and thermal drying process. Proc. American Filtration & Separation SocietyAnnual Conference. May 19–22, Valley Forge, PA (CD-ROM).
Reisser, A. 2008. Technical Information. Reisser, Eilers & Partner AG, Zug, Switzerland
Rippenger, S. 1998. Advanced concentration of suspensions and sludge. IEA Workshop on Separation Technologies. May 18/19, Mainz, Germany, pp. 71–82.
Ruf, J., Korger, V. and Stahl, W. 1996. Proc. 7th World Filtration Congress. Budapest,Hungary.
sure dewatering. In: Advances in Filtration and Separation Technology. E. R. Baumann and L. Weisert (Eds.). Vol. 11. American Filtration & Separation
Schilp, R., Leung, W., Hegarty, S., Ismar, M. and Kluge, R. 2000. Continuous slurry dewatering and drying – all in one machine. Fluid/Part. Sep. J., 13(1): 85–96.
of Fine Particle Conference. University of Alabama, Alabama, pp. 1–32.Stahl, W. H. 1998. Synergetic effects by mechanical hybrid processes and process
combinations. IEA Workshop on Separation Technologies. May 18/19, Mainz, Germany, pp. 95–105.
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trial microwave assisted filter dryer: Criticality assessment and optimisation.
McConvey. I. F. and Shering, P. 2000. Microwave assisted pressure filter drying of
(www.reisser-rollfit.com).
Ruf, J. and Stahl, W. 1997. The hot filter-press: The new technology of vapor pres-
Society, Richfield, MN, pp. 558–562.
Shafick, H. 1981. Solid-liquid separation of fine particles. Proc. Progress in Dewatering
© 2009 by Taylor & Francis Group, LLC
397
30Radio-Frequency Drying with 50 Ω Technology
In contrast to the conventional thermal processes that rely on surface heat-ing by convection, conduction, and radiation with a subsequent diffusion of heat into the material interior, dielectric heating is based on volumetric heat generation where high-frequency electromagnetic energy is trans-formed into heat practically throughout the whole volume of the mate-rial. This heating method is particularly advantageous in thermal drying because the favorable dielectric properties of water and other polar liq-uids cause heat to be generated in the wet parts of the material. This greatly eliminates thermal lag caused due to heat diffusion and provides some unique features such as (Strumillo and Kudra, 1986; Metaxas and Meredith, 1983; Metaxas, 1996; Sanga et al., 2000)
• Enhanced diffusion of liquid and vapor moisture• Coincidental temperature and mass concentration gradients• Internal pressure gradient as an additional mass-transfer driving
force• Stabilized material temperature at or below the liquid boiling point
Electromagnetic energy is one of the forms of energy that is stored and
in space as an electromagnetic wave, which is characterized by the
special category of materials called lossy dielectrics, the energy is trans-formed into heat (dielectric heating) due to ionic conduction and dipole
orientation. Materials with no dipolar electric charge do not heat because
which water is by far the most common (Figure 30.1).The power (P) absorbed in a unit volume of a dielectric material and dis-
sipated as heat depends on the dielectric properties of the material (mainly
P = 2πfε0ε''E2 (30.1)
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transmitted by alternating electric and magnetic fields. It is propagated
wavelength λ, frequency f, magnetic field strength H, and electric fieldstrength E. When an alternating electromagnetic field is applied to the
oscillations, which attempt to follow the rapid changes in the electric field
the electric field affects only the asymmetrically charged molecules, of
the loss factor [ε"]) and the electromagnetic field parameters, that is, the fre-quency (f) and the electric field strength (E) within the drying material:
© 2009 by Taylor & Francis Group, LLC
398 Advanced Drying Technologies
where ε0 is the permittivity of free space equal to 8.854 × 10−12 F/m.The effective loss factor (ε") is a frequency-dependent parameter deter-
mined by the material properties and it characterizes the ability of the
the dielectric constant (ε') and the loss tangent (tan δ [δ is the phase angle between circuit currents]):
ε" = ε'tanδ (30.2)
Dielectric heating in applications involving drying, pasteurizing, sterilizing, and similar thermal processes is carried out in two frequency regimes: radio frequency (RF) and microwave (MW). The RF range extends from ∼1 to 300 MHz, whereas the MWs cover the frequency range
These are 13.56, 27.12, and 40 MHz in the RF region and 915 (896 MHz in Europe) and 2450 MHz in the MW region.
be quite complex, even for homogeneous materials with apparently sim-ple geometries. For example, in the RF range the processed objects are usually much smaller than the wavelength and a spatial power distribu-tion can be made using a low-frequency approach, such as an impedance analysis. Such an analysis indicates that a block of a dielectric material placed between the plate electrodes (see Figure 30.2) will distort an origi-
ners and thus producing nonuniform heating rates. At MW frequencies, the processed objects are generally larger than the wavelength, and the
40 MHzalternating
electricfield
Electrode
Electrode+
−
FIGURE 30.1
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Heat generation in an alternating electric field.
material to absorb electromagnetic energy. It is defined as a product of
from 300 to 3000 MHz. Specific frequencies have been designated within the RF and MW bands for industrial, medical, and scientific applications.
Because both the electromagnetic field and the processed material are allocated in a three-dimensional space, the field–material interaction can
nally uniform electric field, causing higher field strength at the block cor-
useful in determining the field pattern. Nonetheless, the electric and mag-netic fields obey few fundamental rules that allow us to present dielectric
reflection and refraction phenomena at the material–air interface are more
© 2009 by Taylor & Francis Group, LLC
Radio-Frequency Drying with 50 Ω Technology 399
in-depth analysis of various problems concerned with the processing in
Metaxas and Meredith, 1983; Schiffmann, 2007; Strumillo and Kudra, 1986; Stuchly and Stuchly, 1983).
RF power for industrial applications is normally generated by a standard oscillator circuit using triode valves, usually with ceramic envelopes and air- or water-cooled anodes. The applicator consists of an electrode system that together with the processed material (load) constitutes a tuned circuit coupled inductively to the generator output. The three most commonly
1.metal plates between which the processed material is located.
Electrodes
Electrodes
Electrodes
Load
Load
Load
Parallel platethrough-field
Staggeredthrough-field
Stray-field
Electric field distribution without load
Typical RF applicators.
CRC_73877_Ch030.indd 399CRC_73877_Ch030.indd 399 12/24/2008 4:08:09 PM12/24/2008 4:08:09 PM
heating and drying within the limits defined by the scope of this book. An
electromagnetic fields may be found in specialized literature (Jones, 1981;
used electrode configurations, shown in Figure 30.2, are as follows:
The through-field plate electrode system is configured as two flat
© 2009 by Taylor & Francis Group, LLC
400 Advanced Drying Technologies
The electrodes can be in contact with the load (sometimes under pressure or vacuum), or an air gap exists, which is the case in a conveyor-type apparatus.
an array of electrodes, but located alternately below and above the
materials.
trodes, usually in the form of rods, strips, or tubes situated in the
material, and it is used to process plate-type materials.
When a wet material is dried, a variation in the loss factor due to water removal and temperature elevation produces a respective variation in the
RF drying, especially in combination with convective heating, is well established in industry for drying paper, paper board, wood, plywood, as well as for combined drying and curing and gluing. The main limitations
inability to control the system during the process, and RF leaks. A con-ventional RF system includes a self-oscillating generator linked with the applicator (dryer) by a matching circuit. Often, the operating frequency determined by a resonant circuit varies with the permissible frequency band. The matching of the impedance is done for the entire system by
a dryer. Such a system does not tolerate variations in the local character-istics of the load, which are typical for a drying process. Furthermore, all
done by an RF specialist.An advanced technique in the RF region, which provides continuous
tuning of the generator to the load, is the so-called 50 Ω technology(Anonymous, 1997, 1998). In contrast to the conventional RF generators with variable impedance, the output impedance of the quartz-driven RF
strained to the location of the power source. In addition, precise control
ance of the load can be easily measured using, for example, a spectrum
matching unit with variable vacuum capacitors. In addition, the use of a phase and amplitude discriminator allows the system to be automated. Quartz-driven generators guarantee frequency stability and hence con-form to the authorized frequency-band regulations.
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2. The staggered through-field electrode system is also arranged as
processed material. Such a configuration is suitable for sheet-type
3. The stray-field electrode system is arranged as an array of elec-
same plane as the material to be processed. Such a configuration of electrodes produces a planar but nonuniform field through the
absorbed power. To compensate for this effect, graduated field configura-tions or pulsed field systems can be used (Strumillo and Kudra, 1986).
of the conventional systems are difficulty in carrying out adjustments,
adjusting the characteristics of the RF to fit a particular characteristic of
the modifications to the process will need a readjustment of the matching
generator used in a 50 Ω technology is fixed at 50 Ω. It permits the use
analyzer. The exact impedance then permits the calculation of a fitted
over the incident and reflected power is possible because the imped-
of flexible coaxial cables (50 Ω) so that the dryer is not physically con-
© 2009 by Taylor & Francis Group, LLC
Radio-Frequency Drying with 50 Ω Technology 401
A typical RF system operated with the 50 Ω technology consists of aRF generator with 50 Ω output impedance, a matching unit, and an appli-cator (a dryer), all linked with 50 Ω coaxial cables (Figure 30.3). Power
being heated and a permissible frequency of 13.56 or 27.12 MHz. The output power of the generator can be adjusted automatically or manually
the RF generator, the system is equipped with a limiter that adjusts the
power.The matching unit permits the maximum of the RF energy from the
generator to be transferred to the product. Such an optimum power transfer
applicator is equal to the output impedance of the generator. To match the impedance, two parameters have to be adjusted: one is the adjustment of the phase and the other is the adjustment of the amplitude. The matching unit can be automatic or operated manually. In most of the cases, the char-acteristics of the material change during the process, hence continuous
E
M1 M2
C1 C250 Ω
27.12 MHz
ApplicatorLoad
50 Ω Coaxial line Power coupler(forward/reflected)
Phase and amplitudediscriminator
Matchingdevice
Control unitRF generator(quartz-driven)
FIGURE 30.3
Neyron, France, www.sairem.com. With permission.)
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Basic configuration of the RF 50 Ω technology for paper drying. (From SAIREM S.A.S.
specifications for the RF generator are defined according to the product
by a potentiometer with the incident and reflected powers displayed on the front panel of the generator. Because the reflected energy can destroy
incident power to restrict the reflected power at 10% of the maximum
(reflected power is zero) is possible only when the input impedance of the
© 2009 by Taylor & Francis Group, LLC
402 Advanced Drying Technologies
adjustments on the matching device are required; in such a case, the man-ual mode is not advised. To have an automatic matching unit, it is neces-sary to measure the phase and the amplitude of the system by means of a discriminator. The matching unit has to be as close as possible to the applicator to minimize the losses.
ticulate materials, sheets or bands, bulk solids), the process to be used (e.g., drying and gluing), the treatment to be made (curing, surface heating, hardening), the complementary energy source (hot air, infrared, heating plates), or special applications such as vapor extraction.
Essentially, the drying characteristics of the RF 50 Ω technology are the same as those for conventional RF drying. The main feature, however, is better utilization of the electromagnetic energy, which alters the drying rate and allows more uniform drying to be obtained.
The energy and drying performance of the 50 Ω technology weretested with a 190 × 190 × 14.3 mm board made of mineral and cellulose
network with pore size ranging from 1 to 100 μm (Dostie and Navarri, 1994). The board, whose initial moisture content was ∼1.6 kg/kg, was placed between two vertical 200 × 200 mm electrodes spaced 40 (or 60) mm apart in a 477 × 350 × 340 mm drying cavity. The electromagnetic energyfrom the 1.2 kW RF solid-state generator operated at 27.12 MHz was transmitted to the applicator by an automatic matching device yielding a constant 50 Ω impedance load. Drying experiments were performed for
0.05 m3/s of ambient air was blown through the cavity to remove the evaporated water.
Three different methods were used to quantify the RF energy dissipated in the product, namely, the thermal energy balance based on the product mass and temperature measurements, electrical losses in the matching device (insertion losses), and the inductance losses calculated from the
Figure 30.4 presents a comparison of the energy transferred to the prod-uct determined with these three methods (for the case of worst discrep-ancy). All these estimates have almost the same shape, which indicates that any one of these methods can be used for the determination of trans-mitted power. Figures 30.5 and 30.6 present the normalized RF power and the drying rate plotted against the product moisture content for several values of the electrode voltage. The normalized values here are the ratios of the actual value to the maximum value obtained in the experiment. Characteristically, these curves are of the same shape, which indicates a close relationship between the RF power and the drying rate and there-
far as transmission and matching issues are concerned.
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The applicator is defined according to the material to be treated (par-
components that form the structure of foamed particles stuck in a fibrous
five levels of electrode voltage (0.8, 1.2, 1.6, 2.0, and 2.4 kV). Approximately
fore confirms the favorable energy performance of the 50 Ω technology, as
flow rate and temperature rise of the cooling water.
© 2009 by Taylor & Francis Group, LLC
Radio-Frequency Drying with 50 Ω Technology 403
Water content (kg/kg)
Pow
er d
ensit
y (kW
/m2 )
2.4 kV−40 mm
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80
5.0
10.0
15.0
20.0
2.5
7.5
12.5
17.5
RF generator output
Insertion losses
Inductance losses
Thermal balance
FIGURE 30.4RF heating as a function of the moisture content. (From Dostie, M. and Navarri, P., Drying’94, 1994.)
0 0.2 0.4 0.6 0.8 1.0 1.2 1.40
0.2
0.4
0.6
0.8
1.0
0.8 kV
1.2 kV
1.6 kV
2.4 kV
Water content (kg/kg)
Nor
mal
ized
RF
pow
er
FIGURE 30.5Normalized RF power as a function of the moisture content. (From Dostie, M. and Navarri, P., Drying’94, 1994.)
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© 2009 by Taylor & Francis Group, LLC
404 Advanced Drying Technologies
The main advantages of this technology are claimed to be as follows (Anonymous, 1998):
• High-frequency stability by the use of quartz-driven generators• Reduction of RF radiation emitted by the applicator due to perma-
nent tuning•• Reduced power consumption as compared to traditional RF
equipment•
complete automation, versatile and remote command of the sys-tem, etc.)
The RF technology appears to be a well-suited heating source for indus-trial applications that require high power and short process times, such as paper drying. Flexibility of the 50 Ω technology enables a single generatorto be used in sequential mode with two or three applicators. In terms of the costs of the RF drying systems, the investments are clearly very high, but the payback period is relatively short.
0 0.2 0.4 0.6 0.8 1.0 1.2 1.40
0.2
0.4
0.6
0.8
1.0
0.8 kV
1.2 kV
1.6 kV
2.4 kV
Water content (kg/kg)
Nor
mal
ized
dry
ing
rate
FIGURE 30.6Normalized drying rate as a function of the moisture content. (From Dostie, M. and Navarri, P., Drying’94, 1994.)
CRC_73877_Ch030.indd 404CRC_73877_Ch030.indd 404 12/24/2008 4:08:11 PM12/24/2008 4:08:11 PM
Improved operation (simplified control, reduced adjusting time,
Limitation of flash risk
© 2009 by Taylor & Francis Group, LLC
Radio-Frequency Drying with 50 Ω Technology 405
Symbols
f Frequency, Hz
P Electromagnetic power, Wtanδ Loss tangentλ Wavelength, mε’ Dielectric constantε’’ Loss factorε0 Permittivity of free space, F/m
Acronyms
MW MicrowaveRF Radio frequency
References
Anonymous. 1997. Radio Frequency Dryer 50Ω Technology for Paper Converting Industry. Technical Information. SAIREM, Vaulx-en-Velin, France.
Anonymous. 1998. RF 50Ω Technology Applied to the Wood Industry. Technical Information. SAIREM, Vaulx-en-Velin, France.
Dostie, M. and Navarri, P. 1994. Preliminary study on drying rate effects in radio frequency drying. In: Drying’94. V. Rudolph and R. B. Keey (Eds.). pp.607–614.
Jones, P. L. 1981. Heat and mass transfer in a radio-frequency dryer. Ph.D. Thesis. Loughborough University of Technology, Great Britain.
488 p.Metaxas, A. C. and Meredith, R. J. 1983. Industrial Microwave Heating. Peter
Peregrinus Ltd, Stevenage, United Kingdom, 357p.SAIREM S. A. S. Neyron, France, www.sairem.com.Sanga, E., Mujumdar, A. S. and Raghavan, G. S. V. 2000. Microwave drying:
Principles and applications. In: Developments in Drying, Vol. I. Food Dehydration. A. S. Mujumdar and S. Suvachittanont (Eds.). Kasetsart University Press, Bangkok, Thailand, pp. 112–141.
Schiffmann, R. F. 2007. Microwave and dielectric drying. In: Handbook of Industrial Drying. 3rd edition. A. S. Mujumdar (Ed.). Taylor & Francis, Boca Raton, FL,pp. 285–305.
Strumillo, C. and Kudra, T. 1986. Drying: Principles, Applications and Design. Gordon and Breach Science Publishers. New York, 448p.
Stuchly, S. S. and Stuchly, M. A. 1983. Microwave drying: Potential and limitations. In: Advances in Drying. A. S. Mujumdar (Ed.). Vol. 2. Hemisphere Publishing Corporation, New York, pp. 53–71.
CRC_73877_Ch030.indd 405CRC_73877_Ch030.indd 405 12/24/2008 4:08:11 PM12/24/2008 4:08:11 PM
H Magnetic field strength, A/m
E Electric field strength, V/m
Metaxas, A. C. 1996. Foundations of Electroheat: A Unified Approach. Wiley, Chichester,
© 2009 by Taylor & Francis Group, LLC
407
31Radio Frequency–AssistedHeat Pump Drying
The limitation of relatively low heat-transfer rates in conventional dry-ing with hot air, particularly toward the falling rate period, can be over-come by taking advantage of volumetric heat generation during dielectric (microwave [MW] and radio frequency [RF]) heating. This can be done by combining RF heating with conventional heat pump drying (Marshall
volumetrically within the material wetted with polar molecules such as water by the combined mechanisms of dipole rotation and ionic conduc-tion. Such internal heat generation speeds up the drying process mainly because of unidirectional temperature and moisture concentration gradi-ents, internal pressure buildup, and short time delays. Nonpolar materials such as fat or oil, and dry solids that usually form a skeleton of the mate-
therefore they are not directly heated by RF energy (Clark, 1997).Figure 31.1 presents the electrical equivalent of an RF dryer where the
applicator is in fact a drying chamber and the load is the material being dried. The electromagnetic power is generated at a frequency determined by the capacitance and inductance of the tank circuit. This power is fed to the applicator circuit through inductive coupling and an electrical trans-mission line. The applicator circuit basically comprises an inductance and
combination of the generator and applicator circuits forms a resonant sys-
resonant frequency of the applicator must be the same as that of the tank circuit. In addition, the inductive coupling between the circuits should be at some critical value determined by the characteristics of the generator and dielectric properties of the drying material. To avoid cumbersome power transfer and tuning of the system, the 50 Ω technology, which is more popular in drying applications, could be used. Details about internal
reference literature (Strumillo and Kudra, 1986; Kudra and Strumillo, 1998; Kudra, 1990; Kudra et al., 1990; Metaxas and Meredith, 1983; Metaxas, 1996), and basic information on heat pump drying can be found in Chapter 15.
The RF-assisted heat pump dryer comprises a vapor compression heat
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and Metaxas, 1998). Similar to MW drying, the RF field generates heat
rial being dried, do not react essentially to the electromagnetic field, and
a capacitor, which induces an electric field in the material to be dried. The
tem. To generate power and transfer it efficiently to the wet material, the
heat generation and drying in electromagnetic fields can be found in a the
pump system retrofitted with an RF-generating system that is capable of
© 2009 by Taylor & Francis Group, LLC
408 Advanced Drying Technologies
imparting RF energy to the drying material at various stages of the drying
ated in the tank circuit of a self-excited power source is coupled through a matching circuit to the parallel-plate–type applicator located in the dry-ing chamber (Marshall and Metaxas, 1999). The bottom electrode is made from a perforated metallic plate, whereas the upper electrode is a solid
heat pump system. The bottom electrode is the negative one and is con-nected to the casing of the drying chamber. The material to be dried and the electrodes form the load circuit of the RF system.
good candidates for RF-assisted drying. Materials with poor heat-transfer
problem materials when it comes to heating and drying. RF makes it pos-sible to heat all parts of the product mass simultaneously and evaporates the water in situ at relatively low temperatures usually not exceeding 82°C
FIGURE 31.1Layout of the RF system.
RF Generator
TransformerRectifierOscillator
Tank circuit Load circuit(dryer)
Electrodes
Dryingmaterial
Inductancetuning
Coupler Applicator
+
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process. In the configuration shown in Figure 31.2, the RF energy gener-
Materials that are difficult to dry with convection heating are likely to be
characteristics, such as ceramics and glass fibers, have traditionally been
plate but of a smaller size to allow the flow of air circulating in the dryer–
© 2009 by Taylor & Francis Group, LLC
Radio Frequency–Assisted Heat Pump Drying 409
(Thomas, 1996). Since water moves through the product in the form of
the porous matrix is avoided. Warping, surface discoloration, and cracking associated with conventional drying methods are also avoided (Thomas, 1996).
The most important advantages and disadvantages of RF-assisted heat pump drying both from the system and the drying technology points of view are compared in Table 31.1.
As RF-assisted heat pump drying is a relatively new technology, there is not much published data concerning energy, performance, quality issues, etc. Table 31.2 compares, for example, the dryer performance for crushed brick and clay agglomerates. It is clear that the combined
moisture extraction rate (SMER) of the system due to reduced electric energy because of RF heating (Marshall and Metaxas, 1998). The pulsed
performance (COP) for the R12 RF-assisted heat pump dryer. This effect was probably due to the pulsed mode of RF application, which forces a pulsed mode of material heating. Interestingly, the pulsed supply of the RF energy generally increases the COP. In the case of clay drying, the
FIGURE 31.2RF-assisted heat pump dryer.
Load
Heat pump dryer RF Generator
Compressor
Condenser
Expansionvalve
Evaporator
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use of pulsed RF heating with heat pump drying improves the specific
drying configuration has shown a slight reduction in the coefficient of
vapor rather than by capillary liquid flow, migration of fine solids through
© 2009 by Taylor & Francis Group, LLC
410 Advanced Drying Technologies
improvement in both COP and SMER can be attained with pulsed RF heating.
With respect to product quality, it appears that RF-assisted heat pump drying reduces discoloring of the dried products, especially those that are highly susceptible to surface color change. This feature results from an inherent feature of RF drying, that is, volumetric heat dissipation, which boils down to the higher temperatures in the material core than at the sur-
overheating of the material surface. Also, cracking, caused by the stresses due to uneven shrinkage during drying, can be eliminated by RF-assisted
TABLE 31.1
Advantages and Limitations of RF-Assisted Heat Pump Drying System
Advantages Limitations
Accelerated drying due to volumetric heat generation by the combinedmechanisms of dipole rotation and ionicconduction effects
RF may not be as effective for dryingmaterials that have poor dielectric properties
Even drying throughout the material volume and thus more precise control of the moisture content and product temperature
An additional power source is required for the RF-generating system
Easy incorporation into a heat pumpdrying system
Additional capital cost should be added to the cost of the heat pump drying system
Can operate as a stand-alone RF dryer when heat pump cycle is temporarily off
Regular maintenance of RF-generatingcomponents besides heat pump dryer isneeded
RF-assisted heat pump with pulsed RF heating improves the SMER of the dryerand the COP of the heat pump system (Marshall and Metaxas, 1999)
Additional control of the RF power density toobtain quality product at desired moisture content is needed
Enhanced drying rate with no need for anauxiliary heating system for the heat pump dryer
to the stray capacitance between the variousparts of the applicator and earth andbetween the applicator and the measuringequipment (Neophytou and Metaxas, 1997)
RF-assisted drying system can operate inboth continuous and pulsed heating modes, resulting in versatility of a drying operation
For a larger RF heat pump–assisted system,an additional system to cool the RF generator is required
Owing to instantaneous heat generationthroughout the product, the dwell timein an RF dryer is far less than in aconventional dryer. This translates into
inside the dryer and to eliminate electricdischarge and material breakthrough
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RF measurements are difficult to perform due
Precautions are required to confine the high-voltage and electromagnetic fields
significant savings in floor space
face. When combined with the outward mass flux, this feature minimizes
© 2009 by Taylor & Francis Group, LLC
Radio Frequency–Assisted Heat Pump Drying 411
drying. This is achieved by even heating of the product and maintaining a uniform moisture distribution from the center to the surface during the drying process.
The potentials for RF-assisted heat pump drying can be summarized as follows:
•the drying time to reach the desired moisture content. The potential for improving the throughput of a product is good. For example, in the bakery industry, the throughput for crackers and cookies can improve by as much as 30 and 40%, respectively (Clark, 1997).By greatly reducing the moisture variation throughout the thick-• ness of the product, differential shrinkage can be minimized. This promotes using an RF-assisted heat pump dryer for drying of materials with high shrinkage.Closer tolerance of the dielectric heating frequency—(a) 13.56 •MHz ± 0.05%, (b) 27.12 MHz ± 0.60%, and (c) 40.68 MHz ± 0.05%
nal drying and thus has a potential in industries with products that require precision moisture removal.The moisture-leveling phenomenon of RF drying ensures a uni-•form level of dryness throughout the product. Industries that have products requiring uniform drying, such as ceramics, can consider RF drying as a good alternative.
TABLE 31.2
Drying Performance for Crushed Brick and Clay Agglomerates
COP
SMER (kg/kWh)R12 R22
Crushed Brick Heat pump only 2.8 2.9 1.16 Heat pump with RF(0.8A) 2.8 3.7 0.89 Heat pump with RF(1A) 2.9 3.7 0.98 Heat pump and pulsed RF(0.8A) 2.5 3.5 1.69
Clay Agglomerates Heat pump only 2.7 2.9 1.16 Heat pump with RF(1A) 2.7 2.9 0.76 Heat pump (R12) and pulsed RF(1A) 3.9 — 1.69
Source: Reprinted from Marshall, M. G., and Metaxas, A. C., J. Microw. Power Electromagn. Energy, 33(3), 167–177, 1998. With permission of the International Microwave Power Institute.
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Simultaneous external and internal drying significantly reduces
(Clark, 1997)—significantly improves the level of control for inter-
© 2009 by Taylor & Francis Group, LLC
412 Advanced Drying Technologies
Acronyms
COPMW MicrowaveRF Radio frequencySMER
References
Clark, T. D. 1997. The current status of radio frequency post-baking dryingtechnology. 72th Annual Technical Conference of the Biscuit and Cracker Manufacturers’ Association, Forth Worth, TX.
Kudra, T. 1990. Drying Technol.—Special Issue on Dielectric Drying, 8(5): 907–1205.Kudra, T., Shivhare, U. S. and Raghavan, G. S. V. 1990. Dielectric drying—a bibli-
ography. Drying Technol., 8(5): 1147–1160.Kudra, T. and Strumillo, C. 1998. Thermal Processing of Bio-Materials. Gordon and
Breach Science Publishers, Amsterdam, 669p.Marshall, M. G. and Metaxas, A. C. 1998. Modeling the radio frequency electric
ticulates. J. Microw. Power Electromagn. Energy, 33(3): 167–177.Marshall, M. G. and Metaxas, A. C. 1999. Radio frequency assisted heat pump dry-
ing of crushed brick. Appl. Thermal Eng., 19: 375–388.
Sons, Chichester, U.K., 488p.Metaxas, A. C. and Meredith, R. J. 1983. Industrial Microwave Heating. Peter
Peregrinus Ltd., Stevenage, U.K., 357p.Strumillo, C. and Kudra, T. 1986. Drying: Principles, Applications and Design.
Gordon and Breach Science Publishers, New York, 448p.Thomas, W. J. 1996. RF drying provides process savings: New systems optimize
Mag., April, 30–34.
CRC_73877_Ch031.indd 412CRC_73877_Ch031.indd 412 12/24/2008 5:21:25 PM12/24/2008 5:21:25 PM
Coefficient of performance
Specific moisture extraction rate
field strength developed during the RF assisted heat pump drying of par-
Metaxas, A. C. 1996. Foundations of Electroheat; A Unified Approach. John Wiley &
radio frequency drying for the ceramic and glass fibre industries. Ceram. Ind.
© 2009 by Taylor & Francis Group, LLC
413
32Radio Frequency–Vacuum Drying
ing or hot-air drying are energy-intensive and time-consuming, mostly because of the low permeability of solid wood. High-temperature dry-ing can shorten the drying time, but it may result in drying defects such as discoloration, degradation, or checking. Aside from a pretreatment before drying such as ponding (Unligil, 1972) or explosion (Hayashi et al., 1994; Noji et al., 2001), one of the methods to accelerate drying is dielec-tric (radio frequency [RF]) drying in combination with hot-air drying (Miller, 1971; Kobayashi et al., 1999; Resch and Gautsch, 2001; Cai and Hayashi, 2001).
Although the early reports on RF drying of wood were published over 60 years ago (Miller, 1948), this technology was not widely used until the recent progress in process control and RF technology. In particular, a vari-able capacitor and other electrical components allow tuning of the RF gen-erator to the varying load of the applicator to compensate for the dielectric characteristics of wood, which change substantially in the course of dry-ing. This makes it possible to take full advantage of the internal pressure that builds up in the wood log due to volumetric heat generation. The pressure gradient drives both liquid and vapor moisture toward the wood surface, which alters evaporation rates. Therefore, the combination of RF heating and hot-air drying shortens the drying time (Figure 32.1) and could thus reduce energy consumption and drying costs, depending on the process characteristics and energy prices.
Another option for drying solid wood is the combination of RF and vac-uum drying (Saito and Sulaiman, 1999; Rasev, 1999; Rozsa, 1994; Smith et al., 1994). At lower drying temperatures, resulting from a reduced boil-ing point, wood retains its mechanical strength, and therefore it is less susceptible to drying defects such as checking. Because moisture is usu-ally removed from the kiln as vapor, there is a potential for increased
An added feature of RF heating for the vacuum drying of wood is the possibility to stack lumber in a solid pile (without stickers). This not only reduces the load volume but also allows the use of the load-restraining system (Anonymous, 1999), which prevents the wood from any movement during drying and thus greatly reduces twist, bow, cap, crook, warp, and other defects.
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energy efficiency as heat is not lost with the vented hot and humid air.
Conventional methods of lumber drying such as dehumidification dry-
© 2009 by Taylor & Francis Group, LLC
414 Advanced Drying Technologies
Figure 32.2 presents typical characteristics for RFV drying of freshly cut northern red oak (Quercus sp.)—57 mm squares, solid packed in bun-dles, and placed between three 2.4 × 7.5 m aluminum electrodes. The center electrode was positive, connected directly to the 3 MHz RF gen-erator, whereas the top and bottom plates were negative and grounded to the drying chamber. The schedule of drying began with 120 kW of RF energy and vacuum of 20 mmHg. The power was then increased at
∼30 to 35% w.b., its ability to absorb electromagnetic energy decreased, and therefore the chamber pressure was increased to ∼90 mmHg to prevent
from the vacuum condenser. At this point, the RF generator was turned off and the vacuum pump was run for an extra hour to complete drying. As seen from the dying curve in Figure 32.2, the drying of wood from ∼90% w.b. takes place at a constant rate (excluding an initial warming
optic probes inserted into four specimens located on the top, middle, and bottom rows of the upper packs and at the top row of the lower packs
kiln, where the capability of the wood load to absorb electromagnetic power drops dramatically. From then on, the wood’s temperature can be raised to maintain a reasonable drying rate without risk of wood damage (Smith et al., 1994).
0 120 240 360 4800
20
40
60
80
100
120
DehumidificationHot air High temperatureCombined
Drying time (h)
Moi
stur
e con
tent
(%)
FIGURE 32.1Comparison of drying curves for sugi (Cryptomeria japonica) lumber 120 × 120 × 1000 mm. (After Kobayashi, Y., Miura, I. and Kawai, Y., High performance drying using combination of HF and hot air under atmospheric pressure, Proc. 6th IUFRO Int. Wood Drying Conference, University of Stellenbosh, Stellenbosch, South Africa, 1999.)
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∼3 kW/h, up to 240 kW. As the wood reached the fiber saturation point at
up period) down to ∼6% w.b. Wood temperature measured by three fiber
indicates the boiling point for the first 35 h of the drying run. This period coincides with the fiber saturation point and the “power reversal” of the
arcing. The end drying point was indicated by a slower water flow rate
© 2009 by Taylor & Francis Group, LLC
Radio Frequency–Vacuum Drying 415
Similar drying characteristics were obtained in a laboratory micro-wave (MW)–vacuum oven for drying of eucalyptus with core tempera-ture maintained at 30 and 40°C (Rozsa, 1994). After initial drying down to 100%, the drying curve was then nearly linear with the drying rate at ∼5 and 7.5% moisture content loss per hour at 30 and 40°C, respectively. During conventional predrying at 35°C and 85% RH, it took about 8 weeks
of ∼1 to 2% per day.Because the penetration of electromagnetic waves at RF frequencies
into the wood stack is much deeper than that of MW, RF heating yields much more even drying of the entire cross section of the wood load than MW heating and obviously much more than conventional hot-air
Wood temperature
top−top top−middle bottom−toptop−bottom
0 10 20 30 40 50 600
20
40
60
80
100 250
225
200
175
150
125
100
Time (h)
Pow
er (k
W)
Moisture content Pressure PowerBoiling point
Tem
pera
ture
(ºC
) M
oist
ure c
onte
nt (%
)Pr
essu
re (m
mH
g)
FIGURE 32.2RF–vacuum drying characteristics for 8/4 in. northern red oak squares. (After Smith, W. B.,Smith. A. and Neauhauser, E. F., Radio-frequency/vacuum drying of red oak: Energy, quality, value, Proc. 4th IUFRO Int. Wood Drying Conference, New Zealand Forest ResearchInstitute, Rotorua, New Zealand, 1994.)
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to reach the fiber saturation point, which gives a moisture removal rate
© 2009 by Taylor & Francis Group, LLC
416 Advanced Drying Technologies
ensures that the entire cross-sectional area of the wood load is dried at the same rate, which results in much more uniform drying when compared with conventional drying.
The end-point detection system integrated in the control circuit allows the average moisture content and the time to complete drying to be moni-tored over the entire drying cycle. Provided with an access to the Internet or an Intranet, the control system secures remote supervising of kiln oper-ation. Once the target moisture content is reached, the kiln automatically stops and unloads the kiln charge.
As applied to the drying of solid wood, RFV technology has the follow-ing production advantages (Anonymous, 1999):
The drying rate is 20 times higher than that in a conventional kiln. •
Accelerated drying for smaller kiln charges. The ability to dry dif-•ferent species and dimensions at the same time allows for greater
Improved product quality; the RFV-dried wood retains its nat-•ural color with the appearance freshly sawn wood. In addition, there is no heat discoloration or brown staining during drying. Chemical oxidation due to contact with drying air is practically eliminated.Stress-free drying at low temperatures substantially reduces •shrinkage, and this allows a reduction in green target sizes and improved lumber yields.
Table 32.1 (Forest Grove Lumber Co., Inc.) compares drying times and electricity consumption for several wood species dried in an RFV wood kiln (Anonymous, 1999).
TABLE 32.1
Performance Characteristics of the RFV Kiln
RFV Kiln Performance
Conventional KilnRedries (22–12%)
2 in. Hem-Fir(55–10%)
4 × 4 in.White Pine
(40–10%)
1 × 2 in. Birch
(50–8%)
12 × 12 in. Douglas-Fir
(45–18%)
Drying time (h) 5 36 36 60 92Total electricity(kWh/Mfbm)a
64 546 356 863 403
a Mfbm = thousand feet board measure; 1 Mfbm = 2.3597 m3.Source: Courtesy of Forest Grove Lumber Co., Inc., McMinnville, OR. With permission.
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drying. Even penetration of the RF field throughout the lumber stack
An added benefit is no stickering of the lumber packages.
used for pasteurization and fixation of preservatives.production and scheduling flexibility. The RFV kilns can also be
© 2009 by Taylor & Francis Group, LLC
Radio Frequency–Vacuum Drying 417
References
Anonymous. 1999. Technical Information. Heat Wave Drying Systems Ltd., CrescentValley, BC, Canada.
Cai, Y. and Hayashi, K. 2001. Pressure and temperature distribution in wood during radio-frequency/vacuum drying. Proc. 7th Int. IUFRO Wood DryingConference. July 9–13, Forestry and Forest Products Research Institute,Tsukuba, Japan, pp. 386–391.
Forest Grove Lumber Co., Inc., McMinnville, ORHayashi, K., Nakamura, K., Kanagawa, Y. and Yasujima, M. 1994. Improvement
of dryability and its distribution in square timber by local steam explosion. Proc. 4th IUFRO Int. Wood Drying Conference, New Zealand Forest ResearchInstitute, Rotorua, New Zealand, 472p.
Kobayashi, Y., Miura, I. and Kawai, Y. 1999. High performance drying using com-bination of HF and hot air under atmospheric pressure. Proc. 6th IUFROInt. Wood Drying Conference, University of Stellenbosh, Stellenbosch, South Africa, pp. 18–21.
Miller, D. G. 1948. Application of dielectric heating to the seasoning of wood. Proc. Forest Prod. Res. Soc. Natl. Annu. Meet., 2: 235–241.
Miller, D. G. 1971. Combining radio-frequency heating with kiln-drying to provide fast drying without degrade. Forest Prod. J., 21(12): 17–21.
Noji, K., Masaoka, H. and Kanagawa, Y. 2001. Wood drying with high tempera-ture heating by radio-frequency under vacuum after treating of local steam explosion. Proc. 7th Int. IUFRO Wood Drying Conference. July 9–13, Forestryand Forest Products Research Institute, Tsukuba, Japan, pp. 134–139.
Rasev, A. I. 1999. Particular features of dielectric and vacuum wood drying. Proc. 6thIUFRO Int. Wood Drying Conference, University of Stellenbosh, Stellenbosch,South Africa, pp. 37–41.
Resch, H. and Gautsch, E. 2001. High-frequency current/vacuum lumber drying. Proc. 7th Int. IUFRO Wood Drying Conference. July 9–13, Forestry and ForestProducts Research Institute, Tsukuba, Japan, pp. 128–133.
Rozsa, A. N. 1994. Dielectric vacuum drying of hardwoods. Proc. 4th IUFRO Int. Wood Drying Conference, New Zealand Forest Research Institute, Rotorua,New Zealand, pp. 271–278.
Saito, S. and Sulaiman, I. B. 1999. Radiofrequency vacuum (RF/V) drying of small diameter Keruing. Proc. 6th Int. Wood Drying Conference, University of Stellenbosh, Stellenbosch, South Africa, 234p.
Smith, W. B., Smith. A. and Neauhauser, E. F. 1994. Radio-frequency/vacuum drying of red oak: Energy, quality, value. Proc. 4th IUFRO Int. Wood Drying Conference, New Zealand Forest Research Institute, Rotorua, New Zealand,pp. 263–270.
Unligil, H. H. 1972. Penetrability and strength of white spruce after ponding. Forest Prod. J., 22(9): 92–99.
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© 2009 by Taylor & Francis Group, LLC
419
33Miscellaneous Hybrid Technologies
This chapter provides concise overviews of selected drying technologies that represent essentially combinations of well-known drying technolo-gies. Often, some synergy is generated when such technologies are com-bined. The discussion on such hybrid technologies is by no means complete.New ideas appear in the literature on a regular basis, and interested read-
33.1 Combined Microwave and Superheated Steam Drying
Microwave (MW) and superheated steam (SHS) are well-known and well-established, if not commonly used, drying technologies. Each has unique advantages and disadvantages, described in Chapters 7 and 25 of this book. In general, both are more expensive than the traditional
tematic study of a combined MW and SHS drying technique using a model material, that is, a thin bed of sintered glass. Their experiments as well as a mathematical model show that the combined MW/SHS drying system yields higher drying rates than either one acting alone. Furthermore, they also showed that the MW/SHS drying was faster than the MW-nitrogen drying under otherwise similar conditions. Interestingly, it was also found that the critical moisture content was lower for the hybrid system, which implies considerably shorter drying times for the MW/SHS system.
Caution must be exercised when extrapolating these results to real materials that may deform, shrink, puff, or undergo thermally driven physical or chemical changes. Such a system is likely to produce a lower
desirable product attribute. Finally, it certainly will be a more expensive process applicable where product quality is the major concern.
A further variant of this technique could be the use of MW energy supplied periodically rather than continuously—this will save electrical energy cost but not the initial investment cost, however.
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drying technologies, and hence they are justified only for some niche applications. Recently, Shibata et al. (2000) made perhaps the first sys-
apparent density product due to puffing, which may or may not be a
ers may find them in archival journals such as Drying Technology.
© 2009 by Taylor & Francis Group, LLC
420 Advanced Drying Technologies
33.2 Combined Infrared and Convection Drying
Paakkonen et al. (1998) reported on an extensive study of combined con-vection and infrared (IR) drying of herbs such as peppermint, parsley, and garden angelica. They dried beds of leaves of these herbs in a con-veyer dryer with three zones through which air was blown. IR lamp panels were installed in each zone. The IR wavelength ranged from 2 to 5 μm. An interesting feature of this study is that IR radiation was applied intermittently with periods of tempering with pure convection to keep the product temperature from reaching values that would be too high. The cooling effect of convection allows the use of higher IR power to speed drying without thermal degradation. As expected, at the highest drying temperature (peppermint), the drying time was the shortest and the energy consumption the lowest. Interestingly, the quality of the IR-dried product was assessed to be better than that dried in an electric oven. The total oil content of IR-dried angelica leaves, for example, was higher than in electric oven–dried samples. This means that the product retains more aromas. In a related study, the authors reported that there was no change in the micro-bial quality when IR drying with intermittent convection was used.
33.3 Infrared Heat Pump Drying
banana, guava, and potato under cyclically varying air-temperature conditions in a batch heat pump dryer; they showed that this process minimizes degradation of the original color of the feed. This is impor-
degradation in conventional air drying. The layer of water on the drying surface that is maintained in lower-temperature cyclical drying seems to protect the surface from undesirable browning reactions—enzymatic and nonenzymatic. The intermittent heat supply allows one to maintain mod-erate drying rates without decreasing product quality.
A highly sophisticated multistage heat pump with proportional-integral-derivative (PID) control was used by Chua et al. (2000) to achieve sinusoidal and square-wave–type temperature variation of the drying air. However,
mittent supply of energy using IR lamps that go on and off at a desired
sources, this method of energy supply is limited to low-frequency varia-tions of heat supply. An alternative option for intermittent heating is volu-metric heat generation by using MW or radio frequency (RF) irradiation.
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Chua et al. (2000) have demonstrated the benefit of drying products like
tant especially for sugar-rich fruits, which tend to suffer significant color
it may be possible to achieve similar benefits in a batch dryer by an inter-
frequency and amplitude of heat flux. Because of the thermal inertia of IR
© 2009 by Taylor & Francis Group, LLC
Miscellaneous Hybrid Technologies 421
This mode of heating allows higher frequencies of intermittent energy sup-ply to be attained. However, the capital costs of the MW and RF equipment
only for high-value products for the high-end market.To improve the productivity with a simple heat pump dryer operating in
the batch mode, it should be feasible to alternate the drying air supply to two or more shelf dryers. This will increase the dryer output (at the cost of a slight increase in batch processing time) without increasing the size and complexity of the heat pump system. No prior work has been reported on this idea, however.
33.4 Heat Pump Superheated Steam Drying
Although superheated steam drying (SSD) has numerous advantages (see Chapters 7 and 8), the use of higher steam temperature for an extended period of time may cause thermal damage to heat-sensitive materials, such as structural degradation, discoloration, and protein denaturation. These disadvantages can be alleviated by combining SSD with heat pump drying in a two-stage drying system (Namsanguan et al., 2004,
dehydrated material with a highly porous structure at a higher drying rate, which is an inherent feature of drying in SHS. Final drying at a low temperature, carried out in a heat pump system, would yield a product of high quality in terms of shape, color, texture, taste, and aroma (Sosle et al., 2003; Sunthonvit et al., 2007). In addition, energy consumption can
(see Chapter 15).In the recent studies conducted by Namsanguan et al. (2007), raw
at inlet steam temperatures of 160 and 180°C and at a steam velocity of ∼2 m/s. Under such conditions, the initial moisture content of 80% w.b. was reduced to the intermediate range between 30 and 40% w.b. The sub-sequent heat pump drying was carried out at 50°C and at an air velocity
moisture content targeted at 20% w.b. was reached in 135 min (30% w.b.) and 185 min (40% w.b.) in the case of steam temperature set at 160°C. The respective drying time was 127 and 180 min for steam temperature of 180°C. Comparative experiments with purely SSD performed at the same
drying times of 60 min at 160°C and 45 min at 180°C. However, shrimps dried in a two-stage system had a much lower degree of shrinkage, better color, and were softer and less tough than that dried in SHS only.
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are generally higher than that for IR, so this technique may be justified
2007). Thus, SSD used as the first drying stage would produce a partially
be reduced due to the high coefficient of performance of the heat pump
shrimps pretreated with 2% NaCl solution were first dried in an SSD
of 0.84 m/s. Depending on the intermediate moisture content, the final
steam temperature and velocity as used in the first-stage drying allowed
© 2009 by Taylor & Francis Group, LLC
422 Advanced Drying Technologies
33.5 Infrared–Microwave Freeze-Drying
Figure 33.1 presents a schematic diagram of a continuous vacuum freeze-dryer used for drying liquid feeds such as black currant juice (Lebedev, Private communication, 2002). In contrast to the conventional batch freeze-dryers with heating plates and IR radiators, a combined mode of heat supply
Vacuum
Condenser I Condenser II
Spray chamber
IR radiator
Meteringpump
Feed tank
Inert gas
Heater
MW generator
Spraynozzle
DischargeGasdistributor
Freeze-dryingchamber
P (Pa)
X (% w.b.)2 100
FIGURE 33.1Continuous IR–MW–convective freeze-dryer. (Adapted from Lebedev, Private communica-tion, 2002. With permission.)
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© 2009 by Taylor & Francis Group, LLC
Miscellaneous Hybrid Technologies 423
is used here, namely, IR irradiation in the freezing and partial sublimation
chamber). The liquid feed from a storage tank is pumped to the upper cham-ber and dispersed by an ultrasound nozzle. Because of an abrupt pressure drop to ∼70 Pa, the droplets freeze rapidly and then agglomerate due to con-tact with a cloud of the dispersed liquid. At the same time, the granules are partially dried because of the IR irradiation (Lebedev and Bihovsky, 2001).
Frozen and surface-dry granules fall through a conical collector into the lower chamber in which a falling bed of particles is maintained through-
frozen water proceeds as the bed moves down by gravity (see pressure and moisture content distribution along the dryer height in Figure 33.1). A screw conveyor is used to transport the dry particles into a vacuum lock, allowing periodical discharge of the product from a continuously operated dryer.
To facilitate the removal of water vapor, a small amount of inert gas (e.g., nitrogen, argon, or helium) heated electrically to 20–30°C is injected into the bed of dried granules through a porous metal-ceramic ring embedded in the chamber wall just above the screw conveyor. The evaporated mois-ture is removed from the inert gas in two alternately operated condens-ers. Although the basic model of the dryer is designed for liquid feeds, semisolid materials such as whole or sliced berries can also be dried if the spray nozzle is replaced with a screw feeder (Lebedev, 2002).
Acronyms
IR InfraredMW MicrowavePID Proportional-integral-derivativeRF Radio frequencySHS Superheated steamSSD Superheated steam dryingw.b. Wet basis
References
Chua, K. J., Mujumdar, A. S., Chou, S. K., Hawlader, M. N. A. and Ho, J. C. 2000. Heat pump drying of banana, guava and potato pieces: Effect of cyclical vari-ations of air temperature on convective drying kinetics and color change. Drying Technol., 18(5): 907–936.
Lebedev, D. P. 2002. Private Communication.
freeze-drying and development of a new apparatus for continuous opera-tion. Ind. Heat Eng., 23(1): 67–75.
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zones (the upper chamber), and MW heating for finish drying (the lower
out the process. Because of MW heating, final drying by sublimation of the
Lebedev, D. P. and Bihovsky, B. N. 2001. Method of intensification of vacuum
© 2009 by Taylor & Francis Group, LLC
424 Advanced Drying Technologies
Namsanguan, Y., Tia, W., Devahastin, S. and Soponronnarit, S. 2004. Drying kinet-ics and quality of shrimp undergoing different two-stage drying techniques. Drying Technol., 22(4): 759–778.
Namsanguan, Y., Tia, W., Devahastin, S. and Soponronnarit, S. 2007. Experimental investigation of two-stage superheated steam and heat pump drying of
pp. 150–157.Paakkonen, K., Havento, J., Bertalan, G. and Pyykkonen, M. 1998. Infrared drying
of herbs. Agr. Food Sci. Finland, 8: 19–27.Shibata, H., Iwao, Y. and Ide, M. 2000. Combined superheated steam and micro-
wave drying of sintered glass beads. Proc. 12th International Drying Symposium(IDS’2000). W. J. Coumans and P. J. A. M. Kerkhof (Eds.). Noordwijkerhout,The Netherlands (CD-ROM).
Sosle, V., Raghavan, G. S. V. and Kittler, R. 2003. Low-temperature drying using a
Sunthonvit, N., Srzednicki, G. and Craske, J. 2007. Effect of drying treatments on the composition of volatile compounds in dried nectarines. Drying Technol., 25(95): 877–881.
CRC_73877_Ch033.indd 424CRC_73877_Ch033.indd 424 12/24/2008 5:10:39 PM12/24/2008 5:10:39 PM
shrimp. In: Proc. 5th Asia-Pacific Drying Conference. G. Chen (Ed.). Hong Kong,China, August 13–15. World ScientificPublishing Co. Pte. Ltd., Singapore,
versatile heat pump dehumidifier. Drying Technol., 21(3): 539–554.
© 2009 by Taylor & Francis Group, LLC
Part V
Other Techniques
This part includes some of the new dryer types and drying concepts that have appeared in the literature in recent years. Most are in their infancy,
and innovative at least for some special applications. They will be of spe-cial interest to readers who wish to identify new ideas for basic research and development.
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presented here are not categorized according to any rigid classification scheme or criteria. Nevertheless, the reader will find the ideas interesting
and so no definite evaluation is feasible at this time. Also, the techniques
© 2009 by Taylor & Francis Group, LLC
427
34Special Drying Technologies
34.1 Drying by Alternating High- andLow-Pressure Operations
Quite recently, an interesting concept was proposed and positively veri-
ing heat-sensitive materials such as yeast, biotech products, and enzymes (Rakotozafy et al., 2000; Maache-Rezzoug et al., 2002). This novel tech-nology, called dehydration by successive decompression and abbreviated as DDS (Déshydration par Détentes Successives), is based on a sequence of compression and decompression cycles during which the material is subjected to pressures of 200–1000 kPa for a certain duration followed by evacuation to low pressures of the order of several kilopascals (Figure 34.1). The product is maintained at the low-pressure level for a certain time interval. This pressure–vacuum cycling may continue over a period of time that is generally much shorter than that needed for freeze-drying (F-D), vacuum drying, and convective drying of the same mate-rial at comparable temperatures (Figure 34.2). The studies focused on the effects of the various relevant parameters including the pressure levels, cycle duration, vacuum level, etc. Examination of the quality of baker’s yeast dried using this method compared with that of freeze-dried showed a better performance of the pressure cycling process (Rakotozafy et al., 2000). Clearly, F-D is a much more expensive and slower process. It is worth noting that the quality of the product (measured in terms of the viability of the yeast) is reported to be better in this process. This is likely due to the shortened duration of the total dehydration process. The cell survival rate for this process was found to be 10 times higher than that in F-D. However, this number is a function of the operating parameters tested and hence should not be generalized. Similarly, the shorter drying time and quality advantages were found for collagen gel dried by DDS over hot-air drying and vacuum drying. For example, the drying time necessary to obtain
initial 6 mm layer using DDS was 270 min with the collagen temperature
moisture content was obtained after 770 min. Further optimization of the process is needed to enhance its performance even better, for example, by the use of variable pressure ranges and variable cycle frequencies, which
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fied by researchers from the University of La Rochelle in France for dry-
a 0.13 mm collagen film at a moisture content of 0.8 kg/kg d.b. from an
below 22°C. Regarding air drying at 27°C, a collagen film of the same final
© 2009 by Taylor & Francis Group, LLC
428 Advanced Drying Technologies
0 20 40 60 80 100 120 140 1600
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8Pr
essu
re (M
Pa)
Time (s)
Duration ofvacuum phase
Duration ofpressure phase Pressure
level
Vacuumlevel
Instantaneouspressure drop
FIGURE 34.1Pressure variation during dehydration by successive pressure drop cycles. (After Rakotozafy, H., Louka, N., Therisod, M., Therisod, H. and Allaf, K., Drying Technol., 18(10), 2253–2271, 2000. With permission.)
0 200 400 600 800 10000
25
50
75
100
125
150
Drying time (min)
Moi
stur
e con
tent
(kg/
kg) D.D.S.
Vacuum: 1.5 kPa; 23.4 ± 1.69ºC
Air: 27ºC
600 kPa/5 s5 kPa/18 s
22.0 ± 2.9ºC
FIGURE 34.2Drying curves for DDS, vacuum, and air drying of collagen gels. (After Maache-Rezzoug, Z., Rezzoug, S. A. and Allaf, K., Drying Technol., 20(1): 109–129, 2000. With permission.)
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© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 429
is easy to achieve in a fully automated system. Because of the low material temperature, this process seems to have potential for applications involv-ing highly heat-sensitive materials. The constraint could be the relatively high power needed to create a vacuum and pressurized air; hence, this new technology would better suited to high-value products.
34.2 Through-Air Rotary Dryer
Rotary dryers belong to the group of the most common dryers for particu-
large throughput, and the ability to handle various feeds ranging from crushed minerals to cut plants such as alfalfa. The convective heat and mass transfer between the drying gas and the wet solids take place mostly during showering of the lifted particles as the contribution of the rolling bed g(dead zone) to the transfer processes is only in the order of 5%. In addi-tion, the material hold-up is relatively small and varies from 10 to 20%.
heavy, relatively expensive, and energy-consuming. Rotary dryers of spe-
rolling bed of particulate material (see Perry and Green, 1997), have better drying performance, but the capital and maintenance costs for such dry-
To offset some of the basic disadvantages of conventional rotary dryers, such as large size for a given throughput, short residence time, heavy attri-tion, and particle carryover, a novel rotary dryer with drying air blown through the rolling bed of wet particles was developed and termed the Yamato Rotary Dryer or Yamato TACO (Through Air Combination Rotary Dryer) (Yamato, 1996, 1987). With reference to Figure 34.3, the drying air in the TACO dryer is injected into the bed of the material being carried in a rotating cylindrical shell, through a multiplicity of pipes branching off from a central tube. The intensive particle motion created by the air jets issuing out of the branching pipes at velocities ranging from 20 to 25 m/s (maximum 45 m/s) secures intimate gas-to-particle contact and provides favorable conditions for heat and mass transfer (Table 34.1; Yamato, 1998). Consequently, the drying rates are nearly doubled compared to the con-ventional rotary dryers; also, particle attrition is greatly reduced. Higher drying rates translate not only into a smaller shell volume (Figure 34.3) but also into more compact ancillary equipment such as dust collectors and fans. A comparison between conventional and through-air rotary dryers is presented in Table 34.2 (Yamato, 1998).
rotary dryer used as a dryer, predryer, or cooler are similar to those for
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ers are significantly higher.
The basic design as well as the system configuration for the through-air
conventional rotary dryers. Figure 34.4 presents a typical configuration
late solids mainly because of their simple design, versatility, flexibility,
Therefore, conventional axial gas-flow rotary dryers are usually large,
cial design such as the louvre type, where drying gas flows through the
© 2009 by Taylor & Francis Group, LLC
430 Advanced Drying Technologies
Dead zone Dead zone
Dry product
Wet feed Exhaust air
Hotair
Lifters
Conventionalrotary dryer
Active zone Active zone
Wet feed
Dry product
Exhaust air
Hotair
Bars
Central tube
Branch pipes
Yamato TACOrotary dryer
FIGURE 34.3Comparison of the size of the conventional rotary dryer with the Yamato TACO rotary dryer for the same capacity. (From Yamato Sanko Mfg. Co. Ltd., Japan. With permission.)
TABLE 34.1
Sample Data for Yamato TACO and Conventional Rotary Dryers
Material
Moisture Content (% w.b.)
Inlet AirTemperature
(°C)Drying Rate
(kg/m3 h)
Heat-Transfer
(W/m3 K)In Out
Yamato TACOCoal (crushed) 12 1 200–300 50–60 160–190Ores 40 10 300–400 70–90 210–270Sand 15 0 300–400 50–80 175–325Stone (crushed) 6 0 300–400 30–40 175–290Silica gel 70 1 250–500 50–90 210–270Clay 30 0 300–400 50–65 175–210Asphalt 16 2 60–80 30–70 270–325Wet ash 30 1 300–400 50–80 160–210Fish meal 60 20 200–250 50–60 115–175
ConventionalCoal (crushed) 13 1 300–500 20–30 95–115Coal (pebbles) 20 10 300–400 20–30 95–115Ores 23 13 350–500 25–30 115–140Fertilizer 13 1 100–400 5–20 60–70
Source: Courtesy of Yamato Sanko Mfg. Co. Ltd., Japan.
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Coefficient
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Special Drying Technologies 431
for drying of particulate materials with partial heat recovery through
as in the case of drying and incineration, there is no need for a burner or the heater. Because of particle mixing by air jets, it is not essential to have
TABLE 34.2
Characteristics of the Yamato TACO and Conventional Rotary Dryers
Parameter Conventional Yamato TACO
Parallel to the bed Through the bedMaterial hold-up (%) 10–20 15–25Drying time Reference ReducedDrying ratio 1 (reference) 2
3 K) 175–230 350–1750Shell rotation ratio 1 (reference) ½Shell volume ratio (for the same capacity) 1 (reference) ½Capacity ratio (for the same shell size) 1 (reference) 2Attrition Reference ReducedDusting Reference ReducedBed zone in the shell Passive (dead) Active
Source: Courtesy of Yamato Sanko Mfg. Co. Ltd., Japan.
Feed
Dry product
Exhaust air
Fines Freshair
Burner(heater)
Fluegases
Yamato TACO dryer
FIGURE 34.4
Japan. With permission.)
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Volumetric heat-transfer coefficient (kW/m
Air flow
General flow diagram for Yamato TACO rotary dryer. (From Yamato Sanko Mfg. Co. Ltd.,
recycling of the exhaust air. When flue gasses are used as drying agents,
internal flights as material lifters. If needed, small bars may be attached
© 2009 by Taylor & Francis Group, LLC
432 Advanced Drying Technologies
to the shell to enhance the agitation of the bed. Since no showering of par-ticles is required for proper drying, the rotational speed of the shell can be
of the trunnions and gears.Another industrially tested application of the Yamato TACO design
is successive drying and incinerating semisolid sludges that contain
as a heat carrier for drying, much better fuel economy can be achieved as compared to direct incineration of the wet sludge. When processing sludge with an initial moisture content of ∼80% w.b. that does not require deodorizing equipment, the combined system based on the TACO dryer
Time (arbitrary units)
Tem
pera
ture
(ºC)
Moi
stur
e con
tent
(% w
.b.)
0
100
200
300
400
500
600
700
800
900
0
33
50
60
67
71
75
78
80
82
Drying Incineration
Sludge temperature
Ignition
Combustiontemperature
Airtemperature
Moisture content
Principle of combined drying and incineration. (From Yamato, Y., Application of a Novel Rotary Dryer (TACO), 1998. Yamato Sanko Mfg. Co. Ltd., Japan. With permission.)
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reduced by half, which simplifies the dryer design and reduces the costs
organic matter (Figure 34.5). By using flue gasses from the incinerator
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 433
and the vortex incinerator is claimed to be operated with no additional
the sludge, reduced fuel consumption results from much better combus-tion in the incinerator when burning sludge predried from 80 to 20% w.b. In such a case, only 25 kg of water per 100 kg of dry combustible matter has to be evaporated in a combined system, instead of 400 kg of water to be evaporated in the case of wet-sludge incineration.
The vortex incinerator used in a preferred combined system with the Yamato TACO dryer can also be used as a standalone dryer-incinerator (Figure 34.6). The wet material is fed to the periphery of the incinerator, which is equipped with a tangentially oriented gas burner mounted on the sidewall opposite the feed point. Owing to the combined action of blades and nozzles attached to the rotating branch pipes, which supply combustion/drying air, the wet material is raked and transported along a spiral path toward the chamber center (Yamato et al., 1974). On this path,
Exhaust gas
Rotating arm
Auxiliaryburner
Ash
Sludge
Combustion fan
FIGURE 34.6Vortex incinerator. (From Yamato Sanko Mfg. Co. Ltd., Japan. With permission.)
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fuel (Yamato, 1998). Besides the calorific value of the organic matter in
© 2009 by Taylor & Francis Group, LLC
434 Advanced Drying Technologies
the material temperature reaches the ignition point and a violent com-bustion takes place, aided with fresh air supplied through the nozzles. The dry material or ash is discharged from the central port through a chute to the take-up device.
The TACO dryer is a simple but innovative dryer design. Pilot testing is recommended, as it may not be suited for the whole range of products handled by the conventional rotary dryer.
34.3 Drying with Bed Mixing
A rather unique drying application involving a bubbling or circulating
sludges has been reported by Imatran Voima Oy, Finland (Hulkkonen et al., 1995; Hulkkonen, 1997). The term bed mixing originates from the
bustor are mixed with the wet fuel to extract a sensible heat of solid particles and take advantage of more effective contact heat transfer. In addition, the use of a steam atmosphere makes it relatively easy to recover the latent heat of evaporation back to the process at a higher tem-
especially in combined heat and power (CHP) applications. This concept
ducing 6 MW electricity and 20 MW district heat. This type of drying appears suitable for sawdust, wood bark, and possibly paper mill slud-ges, as well.
Figures 34.7 and 34.8 show a schematic and general view of the bed
reported trials since only a few seconds are needed for drying the fuel due to
1. The high temperature at the dryer inlet 2. Direct contact between the bed material from the combustor and
the wet fuel (40–60% w.b.) 3. The injection of high-velocity superheated steam (SHS) into the
The SHS at atmospheric pressure carries the dried fuel together with the hot bed material to a cyclone where the solids are separated from the
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the water is first evaporated to the critical moisture content. Soon after,
perature level. Thus, the thermal efficiency of a power plant is enhanced
has been tested successfully in peat- and wood-fired power plants pro-
peat- and wood-fired power plant utilizing this novel drying concept.
fluidized bed combustor using peat, brown coal, wood biomass, and waste
fact that the hot solid particles (the bed) taken out of the fluid bed com-
mixing dryerrr, respectively. Figure 34.9 presents a flow diagram of a
The dryer can be of any type, but a simple flash dryer was used in the
bottom section of the flash dryer
steam and fed into the combustor (fluidized bed [FB] boiler). The pressure
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 435
in the dryer is kept slightly above the boiler pressure. The main parameter to control, however, is the steam temperature after the cyclone, which is maintained at a set value by controlling the feed rate of the bed material to the dryer. Part of the steam from the cyclone is recycled to the dryer. The excess steam can be condensed and used to produce district heat, as the initial Finnish application does, or it could be used for any other purpose. If the wet fuel comes in at 50% w.b., Hulkkonen (1997) reports a 10–15%
the fuel does not go through the boiler. This reduces the size and invest-ment costs of the new boiler or increases the capacity of the plant already in operation. The process characteristics for the power plant shown in Figure 34.9 are given in Table 34.3 (Hulkkonen, 1997). The case study that illustrates the possible capacity increase due to bed mixing is depicted in Table 34.4 (Hulkkonen, 1997).
Among other advantages claimed for this process are simple construc-
It should be noted that bed-mixing drying is somewhat similar to the so-called particulate immersion drying technique that was studied exten-sively by Raghavan and coworkers at McGill University (Pannu and Raghavan, 1987; Tessier and Raghavan, 1984). Using various heated solids such as sand, salt, molecular sieves, and natural zeolite particles as the heat- transfer medium, they developed a rotary processor for drying grains
Fueltank
Dryer
Fluidbedboiler
Fan
Condensate
Exhaust
Noncondensing gases
Steam
Steam
Fuel +bed
material
Hot bed material
FIGURE 34.7Drying with bed mixing. (From IVO Technology Centre, Vantaa, Finland. With permission.)
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improvement in the thermal efficiency of the power plant. Moreover, the
tion, no odor emission, no fire or explosion hazard, and ease of control.
flue gas channels can be smaller than usual because the moisture from
© 2009 by Taylor & Francis Group, LLC
436 Advanced Drying Technologies
Cyclone
Circulatingsteam
Flash dryer
HeatexchangerWet fuel
Dried fuel
Reject
Fans
Bed material
FIGURE 34.8The bed mixing dryer. (From IVO Technology Centre, Vantaa, Finland. With permission.)
and other granular materials of biological origin (see Chapter 12). The dryer essentially mixes the heated particulate media with wet grain, then allows a certain residence time for the necessary heat and mass transfer to
medium. Natural zeolites as the capillary-porous drying medium have
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occur, and finally separates the dried grains from the heat-transfer
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 437
Feed waterBack pressureheat exchanger
Dryingsteamcondenser
Steam turbine
Stack
Fluidizedbed
boilerFuelsilo
Bedmixingdryer
Electrostaticprecipitator
FIGURE 34.9
Technology Centre, Vantaa, Finland. With permission.)
TABLE 34.3
Some Characteristics of the Power Plant with Bed Mixing Dryer
Fuel Peat, wood biomassBoiler
and 510°C)Steam turbine Back pressure steam cycle with district heat produced at one
pressure level
Dryer Off Dryer On
Fuel input (MW) 26.3 26.0Fuel moisture content (%) 55 ∼15Power output (MW) 6.1 6.1District heating output (MW) 17.6 21.2
90 (LHV) 105 (LHV)73 (HHV) 84 (HHV)
Note: LHV, lower heating value; HHV, higher heating value.Source: Courtesy of IVO Technology Centre, Vantaa, Finland.
advantages over sand or other nonhygroscopic particles since such materials also enhance the potential for mass transfer by adsorbing the evolved vapor. Because mineral ingredients are required for animal feed, some retention of zeolite in the dried product is permissible. The fact that particle-to-particle heat-transfer rates are much higher than those
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Flow diagram of the bed mixing dryer in the peat- and wood-fired power plant. (From IVO
Thermal efficiency (%)
Bubbling fluidized bed (8 kg/s of steam at 6.1 MPa pressure
© 2009 by Taylor & Francis Group, LLC
438 Advanced Drying Technologies
for the gas-particle systems helps accelerate the drying rate. The inter-
drying technique as well as on contact-sorption drying in Chapter 12.
34.4 Drying at Ambient Air Temperature
Because drying is the simultaneous heat- and mass-transfer process, one of the options for low-cost drying of waste materials is drying at ambient air temperature, where the moisture concentration gradient is the major driving force. Following this concept, the DRY-REX system was devel-oped in which drying takes place when unsaturated ambient air is drawn through the moving bed of the wet particulate material or preformed pasty sludges such as those from pulp and paper mills (Barre, 1998; Barre and Masini, 2000). With reference to Figure 34.10, drying is carried out in two
sugarcane is disintegrated to enlarge the heat- and mass-transfer areas and reduce internal resistance to moisture diffusion. Pasty or semisolid feeds are mechanically pressed to form pellets (4–6 mm in diameter and 5–15 mm long) to increase the interfacial area and agglomerate the sludge to prevent the formation of dust during drying. In the second stage, dry-ing occurs by passing ambient air through the bed of preformed material fed to the multideck perforated belt conveyor.
The ambient air dryer appears to be competitive to other dryer types because
Its simple design reduces investment and operation costs.•No thermal energy is required for drying.•
TABLE 34.4
Capacity Increase in Power and Steam Output Due to the Bed Mixing Dryer
Parameter Without Dryer With Dryer Change
Fuel Biomass BiomassFuel input (MW) 92.1 110.8 +20%Fuel moisture (% w.b.) 55 ∼20Live steam (kg/s) 32.7 40.8 +25%
3 51 51Stack gas temperature (°C) 130 118Power output (MW) 20.5 26.3 +28%Process steam (MW) 61 90.5 +48%
88.5 105 +19%
Source: Courtesy of IVO Technology Center, Vantaa, Finland.
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Thermal efficiency (%)
ested reader can find additional information on the particulate immersion
stages. In the first stage, the solid material such as bark, wood chips, and
Flue gas flow (N m /s)
Note: For the same flue gas volume.
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 439
Drying at low temperatures (e.g., 5°C) prevents the formation of • bad odor.Pelletizing of the sludge prevents dust formation.• The risk of explosion is practically eliminated (no dust and low • temperature).
These advantages are slightly offset by the need for more powerful fans to be installed to provide air velocity high enough to compensate for the reduced drying rates when using low air temperatures. The ambient air dryer as shown in Figure 34.11 was tested for various waste materials
ash, sugarcane residues, de-inking sludge, and the mixture of primary and secondary (biological) sludges in any mass ratio. Table 34.5 presents repre-sentative data on ambient air drying of various wastes from paper mills. Clearly, air consumption is higher, and the process is slower than hot-air
To boiler forenergy generation
Landfill cover
AgricultureDryproduct
Dryer
Option A
Option B
PretzelsPelletizer
Sludge anddewateredproduct in
Sludger at >25%bark, wood chips,sugar cane
Fans
Grinder
With permission.)
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Basic configuration of the DRY-REX dryer. (From Solutions Mabarex, St. Laurent, Canada.
drying. The air should be sufficiently dry for this method to be efficient.
including water-saturated bark and wood chips, wood pulp, ash and fly
© 2009 by Taylor & Francis Group, LLC
44
0A
dvanced Drying Technologies
FansAir
Air
AirAir
Air
Airoutlet
Vacuum
A
A
Air
Air
i
i
r
r
Air
Wet solids in
Vacuum
Airinlet
Dry solids out
FIGURE 34.11General view of the DRY-REX dryer. (From Solutions Mabarex, St. Laurent, Canada. With permission.)
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Special Drying Technologies 441
34.5 Vacuum-Superheated Steam Drying and Granulation
Unlike the particle-size enlargement by agglomeration when mixing the
end-use aids, the granulation of pasty materials is based on progressive concentration during drying under carefully selected hydrodynamic con-ditions to obtain granules of the required size and distribution. Following the common terminology (Perry and Green, 1997), agglomeration by any
to as granulation, in contrast to pressure agglomeration (compaction), which is also known as extrusion, tableting, briquetting, or pelletizing. This relatively new and unconventional technology, which allows gran-ules to be obtained in a single processor directly from a pasty feed, is the vacuum hot steam process (VHSP) (Dürr, 1996, 1998). Simultaneous drying and granulation are carried out in a vacuum-tight mixer having a rotating mixing pan that is slightly inclined against the vertical line. Rotation of the mixing pan conveys the processing material into the zone of an eccentric high-speed rotor. Because both the rotor and the pan are rotated in opposite directions by independent drives with controllable speeds, the material streams are characterized by high differential veloci-
mechanically due to the rotary movement of the rotor and the structural
tank wall combined with the inclined arrangement of the mixing pro-
TABLE 34.5
Ambient Air Drying of Residues from Paper Mills
Material
AirTemperature
(°C)
Air Relative Humidity
(%)
Moisture Content (% w.b.)
Initial Final
Primary–secondary sludge (% mass) 10–90 17 60 87.4 30.7a
66–33 17 60 85.4 32.0a
64–36 5.5 30 70.2 26.8a
35–65 18 62 84.4 22.0a
30–70 15 40 76.0 17.1 60–40 17 62 81.0 22.0a
De-inking sludge—50% clay 16 92 61.7 7.9Frozen bark 20 62 60.0 30.0a
Bark-sludge (50–50%) 20 29 60.0 20.5a
a Lower values could have been obtained, but these were determined by the end users as
Source: Courtesy of Solutions Mabarex, St. Laurent, Canada.
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sufficient for firing in the boiler.
key components with binders, surfactants, lubricants, fillers, and other
means of agitation (e.g., tumbling, fluidization, and spouting) is referred
ties and shear forces. A pseudofluidized circulating bed is also created
design of the knife-like mixing blades. A stationary deflector plate on the
cessor aids fluidization as it generates strong vertical currents, resulting
© 2009 by Taylor & Francis Group, LLC
442 Advanced Drying Technologies
sticking to the pan wall. Material to be processed is fed to the processor from above and discharged through the central bottom port.
or soft paste in a VHSP (Figure 34.13) is based on progressive mois-
takes place in SHS that circulates in a closed system maintained under reduced pressure to lower the material temperature, even down to 40°C (Dürr, 1996). When granulating dry and multicomponent materials such as those in the production of ferrite granules (Dürr, 1997), the powders are thoroughly mixed and wetted to obtain a homogeneous pasty feed. Once homogenization of the feed material is completed, the system is set at the desired level of vacuum; thus, practically all air inside the proces-sor is removed and replaced by steam. A closed-circuit blower conveys the process steam through a heat exchanger (a superheater) to the mix-ing pan through a specially designed steam distributor arranged later-
of the drying cycle, the material is rapidly heated to the drying temper-ature by complete condensation of the steam (shock heating(( ). As soon as ggthe steady-state bed temperature is reached, the exhaust steam leaves the
closed-circuit blower to the steam superheater and then back to the process volume. The energy for drying is supplied to the system through a heat exchanger as well as by the circulation blower in the vapor compression arrangement. The excess steam due to moisture evaporation is condensed and bled out of the SHS loop. As drying proceeds, the cutting effect of the projecting star-shaped mixing blades of the rotor disintegrates the tough, crust-forming cake into smaller pieces that gradually lose their tendency
FIGURE 34.12Flow pattern in the EVACTHERM® processor. (From EIRICH, Hardheim, Germany. With permission.)
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Granulation of kneadable raw materials such as a filter cake, slime,
processor volume through the heated vapor filter and is redirected by the
in an intensive three-dimensional flow pattern of the material inside the processor (Figure 34.12). The deflector plate also prevents material from
ture evaporation from the pseudofluidized bed of wet clumps. Drying
ally to the deflector plate to enhance pseudofluidization. At the beginning
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 443
to stick together because of surface moisture evaporation. The granula-tion process begins as the removal of liquid from the disintegrated pieces
material in a circulating bed as shown in Figure 34.12 promotes the forma-tion of rounded granules.
Although the EVACTHERM dryer is essentially a batch processor, it is possible to perform so-called “more or less continuous drying.” It is nor-mal that for high-moisture feeds, the load of the dryer decreases as liquid vaporizes. Consequently, at the end of drying, the smaller load of the dryer would diminish the process economics aside from the fact that hydrody-namics and thus the heat- and mass-transfer conditions could be heavily impaired. Industrial practice has shown, however, that the fresh material
dryer at a rate equivalent to water evaporation (Dürr, 1998). In this way, the drying process is continued for a longer period under optimum con-ditions until the material holdup has reached its maximum, and then the processor must be discharged.
During the drying process, the material has a constant temperature according to the preset pressure of the system. A gradual increase of the
Cooling water Exhaust air
Heatingmedium
Condensate
Feed
Heatingmedium
Filter
Mixingreactor
Condensatetank
Blower
Heatexchanger
Condenser
FIGURE 34.13Flow diagram for SHS vacuum drying and granulation. (Courtesy of EIRICH, Hardheim, Germany. With permission.)
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advances. The pseudofluidization along with the rolling movement of the
can be continuously added into the fluidized bed in the EVACTHERM
© 2009 by Taylor & Francis Group, LLC
444 Advanced Drying Technologies
material temperature along with a reduction in the amount of collected condensate indicates the end of drying. At this point, it is recommended that the supply of external energy be cut off and only vacuum be applied to remove the residual moisture using the sensible heat of the load. Besides cooling of the material, the use of a vacuum allows any toxic volatiles to be collected in the condensate.
A distinct feature of the SHS drying-granulation process is that larger
ther drying and by impact of the rotor. Unlike conventional granulation, where either one shell envelops another or several smaller particles con-solidate around the nucleation center, the granules from the SHS process-ing are uniform and thus have higher density and mechanical strength (Table 34.6). Because the process of granulation takes place during a dry-
such a uniform distribution of the residual moisture that downstream
the mixing processor with the rotating mixing pan can also be used for other drying processes such as vacuum drying, SHS drying, vacuum-SHS drying, or superheated steam-granulation in addition to from other operations such as mixing, blending, and homogenizing. Typical con-
oil burners, electric heaters, or steam–oil-heated heat exchangers; (b) convection drying in a closed circuit under atmospheric conditions; and (c) vacuum-convection drying in a closed circuit with inert gases (e.g., N2 and CO2). In closed systems, the air is heated by electric heaters or steam/oil-heated heat exchangers, and the vapor is removed in a water-cooled condenser. The pressure is controlled by a bypass valve on the vacuum pump. The evaporation capacities of the 3 and 7 m3 mixing pro-cessors are ∼1000 and 2200 kg of water/h, respectively. The system appears to be scaled-up easily as the design criteria for a 3 m3 processor can be accurately derived from the experiments with a 0.075 m3 trialprocessor (Dürr, 1996).
TABLE 34.6
Properties of Granulated Ferrites
Parameter VHSP Conventional
Bulk density of raw material (kg/m3) 800 800Bulk density of granules (kg/m3) 2000 1500Solid density of granules (kg/m3) 3000–3500 2600Moisture content of granules (% w.b.) <10 16
Source: Courtesy of EIRICH, Hardheim, Germany.
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granules are generated first and smaller granules are produced by fur-
ing cycle, the granulate has the final moisture content as required; hence, no finish-drying is needed. Moreover, SHS drying-granulation provides
Depending on the system configuration and operating parameters,
figurations include (a) convection drying under atmospheric conditions
maturing can be eliminated from the process flow.
in an open system with heat supplied by flue gases from natural gas or
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 445
34.6 Drying in Rotating-Jet Spouted Bed
Conventional spouted bed (CSB) designs have some important constraints
(Mathur and Epstein, 1974; Passos et al., 1987). Among these constraints, one may cite high pressure drop before onset of spouting, low aeration of the falling bed in the annulus, slow solids turnover, low capacity per unit
tions are concerned with the chamber geometry, spouting origin, air sup-ply, operational mode, etc. (Devahastin et al., 1996). Not all designs have
have, however, found industrial applications and are described elsewhere in this book (e.g., spouted bed of inert particles). The others have not been
rotating-jet spouted bed (RJSB) suitable for drying of Geldart-type D par-ticles (Geldart, 1973). Here, the gas jet causing spouting of the particulate
bed center.
forated grid (Figure 34.14). Below the grid, a slowly rotating air distributor with one or several radially located spouting air nozzles is located in a small clearance from the grid. The RJSB is formed when the air jet (or jets) moves circumferentially in the annular region between the chamber wall and the central spout. Spouting characteristics of the rotating jet are essentially similar to those in CSBs. However, sequential relocation of the air jet greatly affects the onset of spouting—the peak pressure drop is lower by ∼30% compared to the stationary spouting case. This can be explained by the pulsating action of the rotating gas stream, which loos-ens the packed structure and hence facilitates spout evolution (Jumah,
cated air streams (see Chapter 6) where not only the pressure drop is low-
rate required to maintain minimum spouting conditions is increased. It is caused likely by the higher-velocity component in the angular direction
more kinetic energy is required to maintain the spouting state.As seen in Figure 34.14, the RJSB has a central air jet, which does not
relocate as the peripheral one and thus does not provide intermittent dry-ing. Another design for drying materials in the falling rate period that is also based on relocation of the spout is the rotating-jet annular spouted bed (RJASB) (Devahastin, 1997). The RJASB consists of a single rotating jet
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of CSBs, numerous modified configurations have been developed (Passos et al., 1987; Devahastin et al., 1996; Mujumdar, 1984, 1989). These modifica-
potential industrial significance. Some of the spouted bed configurations
commercialized yet. One of these original modifications of the CSB is the
material does not enter the bed in a fixed position but turns around the
that limit the commercial use of spouted beds compared to fluidized beds
floor space, and difficulty of scale-up. To overcome some of the limitations
The RJSB dryer is built as a vertical cylindrical chamber with a flat per-
1995). The same phenomenon is observed in pulsed fluid beds with relo-
ered at the onset of fluidization but also the peak pressure drop does not appear. For higher rotational speeds of the gas jet, the value of the airflow
and hence larger deviation from the vertical flow of the jet. As a result,
© 2009 by Taylor & Francis Group, LLC
446 Advanced Drying Technologies
in the annular region between the dryer wall and a central tube, which
sible over the entire area of the bed annular region (Figure 34.15). A par-ticular portion of the bed is thus spouted only periodically, and in one rotation of the nozzle, the bed is spouted once, which allows better control
thus allowing a deeper bed of the material to be processed. The slight reduction in bed capacity due to the centrally located tube estimated as ∼20% at the same bed height can be compensated by an 8% increase of the chamber diameter.
spouting due to continuous relocation of the gas jet. This periodic spout-ing, which boils down to intermittent drying due to periodic heat supply,
2019
17
17
9
2521
23
24 15
18 2
3 1
416
5 6
7 812 14
14 13
22
10 11
FIGURE 34.14Experimental RJSB with central and peripheral air jets: 1—air distributor, 2—screen, 3—distributor cover, 4—ball bearings, 5—v-belt, 6—pulley, 7—drive, 8—motor controller, 9—glass vessel, 10—air blower, 11—electric heater, 12—PID controller, 13—Pitot tube, 14—gate valve, 15—pressure transducers, 16—pressure taps, 17—thermocouples, 18—mul-tiplexer, 19—IR hygrometer, 20—dew point sensor, 21—dew point hygrometer, 22—humid-
characteristics of a rotating jet spouted bed. Ph.D. Thesis, McGill University, Canada, 1995.)
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ity meter, 23—computer, 24—terminal panel, 25—filter. (After Jumah, R. Y., Flow and drying
for onset of spouting at a fixed value of the distributor rotational speed (as
One clear advantage of such a rotating jet configuration is intermittent
does not allow air and material flow; hence, an on–off operation is pos-
of the drying/tempering time. The RJASB also requires less airflow rate
the higher value of jet velocity is achieved at the same air mass flow rate),
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 447
results in energy savings while drying materials in the falling rate period (Bon and Kudra, 2007). For such materials, the tempering period with-out spouting and therefore without heating permits moisture to diffuse
during the spouting period. This particular advantage occurs only if the rotational speed of the air jet is relatively low (in the order of several rota-tions per minute) as well as the jet area is only a small fraction of the bed’s cross-sectional area. Otherwise, the entire bed would turn into a special
The other feature of both the RJSB and RJASB is the precise control over the tempering-to-spouting period ratio, simply by varying the rotational speed of the air nozzle. In CSBs and spouted beds with a draft channel, this ratio can be controlled only to a small extent by varying the bed
Central tube
Spouting zone
Air distributor
Air distributor
Central tube
Spoutingair
Drive
Exhaust air
Spout
Nozzle
FIGURE 34.15Schematic diagram of the rotating jet annular spouted bed. (After Devahastin, S., Flow and drying characteristics of a novel rotation jet annular spouted bed. M.Sc. Thesis, McGill University, Montreal, Canada, 1997.)
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from the material core to its surface, which intensifies further evaporation
type of pulsed fluid bed as described in Chapter 6.
© 2009 by Taylor & Francis Group, LLC
448 Advanced Drying Technologies
height. Note that it is possible to use multiple spouting nozzles in dryers of larger diameter as well as mechanical agitators to enhance mixing in
construction because of the rotating air distributor and small nozzle-to-grid clearance to be maintained to reduce air bypass.
dynamics, drying characteristics, and modeling of the rotating-jet spouted bed in the literature (Jumah, 1995; Gong et al., 1997; Jumah et al., 1997; Devahastin 1997; Devahastin et al., 1999).
34.7 Drying with Sol–Gel Transformations
Because of the noncontact transfer of electromagnetic energy, microwave (MW) heating can be advantageous in the technology of high-purity sub-stances. In fact, MW processing of metalsiloxane sols allowed higher-purity homogeneous products to be obtained than by using traditional methods of heating. Moreover, syneresis of the resulting gels, which greatly reduces product quality, was practically eliminated (Bessarabov et al., 1994; Gluhan et al., 1983). In addition, water–alcohol-based multicom-ponent sols belong to the class of lossy dielectrics (dielectric loss factor in the order of 30–50) in contrast to the preferred materials of construction
the heating characteristics of the product. Here, we present the original design of an MW rotary processor (Bessarabov et al., 1999) that allows several sequential batch operations to be performed for the continuous production of high-purity multicomponent materials.
As reported by Bessarabov et al. (1999), multicomponent sols can be pro-
aqueous solutions of salts (nitrates of potassium, sodium, aluminum, and lead) and boric acid. The hydrolyzates represent complex multicomponent
of ethoxy groups, dissociation products of salts of alloying components that did not react with the intermediate hydrolysis products of Si(O-Et)4and Ti(O-Bu)4, ethyl alcohol, butyl alcohol, and water. Experiments car-ried out at temperatures ranging from 30 to 300°C have revealed that after 8–10 min of MW heating, the sol started to boil and then passed into a gel-like state. Further moisture evaporation proceeded as a series of vapor blasts of various intensities dispersed and mixed the gel. When drying is nearly complete, a highly porous structure with practically
rial temperature starts to increase, and the mass of the material tends to
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the RJSB. The shortcoming of the rotating-jet configuration is its complex
Interested readers can find more information about the design, hydro-
of the MW apparatus with significant wall thickness without affecting
duced by hydrolysis of ultrapure silica and titanium ethers by acidified
liquid-phase systems containing the products of incomplete saponification
such as quartz glass, fluoroplastics, and most ceramics. It allows design
no resistance to vapor flow is established. At the same time, the mate-
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 449
∼7 to 10% w.b.On the basis of the physical phenomena observed during drying, the
1. Heating up and evaporation of moisture from the sol of initial moisture content X0 down to the moisture content X1, at which gelation of sol starts
2. Gelation of sol with a further increase of material temperature to the boiling point and a decrease of the material moisture content to X2, at which the entire volume of sol is transformed into a gel of porous structure caused by local eruptions of vaporized moisture
0
0.2
0.4
0.6
0.8
1.0
Moi
stur
e con
tent
(kg/
kg)
0 5 10 15 20 25 30 35 40
Time (min)
12
34
100
200
300
0
123
4
321
4Tem
pera
ture
(ºC
)
1, 4—2.5 kg
2—4.2 kg
3—6.3 kg
P = 25 kW/kg
FIGURE 34.16Drying and temperature curves for selected sols; 1–3: SiO2, 4: SiO2-B2O3-Na2O-Al2O3. (After Bessarabov, A., Shimichev, V. and Menshutina, N., Drying Technol., 17(3): 379–394, 1999.With permission.)
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stabilize. The material is considered dry when the final moisture content is
following stages were identified (see Figure 34.16) (Bessarabov, 1999):
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450 Advanced Drying Technologies
3. Drying of a porous gel structure at a constant temperature and constant drying rate—evaporation of free moisture
4. Drying of a porous gel structure at a falling rate period with a continuous increase of material temperature (evaporation of structural moisture)
The product in the process of thermal treatment of polycomponent sols transforms from a nonviscous liquid through a high-viscosity liquid to
eration, it is desirable to separate the processes of sols’ boiling, gelation, and gel–xerogel drying. This can be done when combining the features of batch and continuous processing in a single apparatus, allowing simulta-neous mixing and MW heating.
The pilot-scale MW dryer designed by Bessarabov et al. (1999) (Figure 34.17) is a cylindrical MW resonator equipped with a coaxial quartz drum with four internal spiral inserts. The resonator is slightly inclined toward one end; the slope of inclination can be adjusted. The upper end of the inclined chamber is connected to the feeder of initial sol, the sys-tem for vapor condensation, and discharge. The water load prevents
energy from the generator, a variable speed drive to control and reverse ® container for dry product screened from
10
13 14 15 16
11
129
8765432
1
Water
Water
NitrogenVacuum
FIGURE 34.17Schematic of the experimental MW dryer: 1—motor, 2—MW generator, 3—resonancetube, 4—spiral insert (IV section), 5—viewport, 6—quartz drum, 7—spiral insert (I sec-tion), 8—water load, 9—condenser, 10—product tank, 11—swivel, 12—slope adjuster,13—sol tank, 14—distilled water, 15—condensate tank, 16—condensate discharge. (AfterBessarabov, A., Shimichev, V. and Menshutina, N., Drying Technol., 17(3): 379–394, 1999. Withpermission.)
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a gel and finally to a xerogel. To avoid clinkering, caking, and agglom-
lower end of the chamber is fitted with a waveguide that supplies MW unabsorbed MW energy to be reflected back to the magnetron. The
drum rotation, and a Teflon
© 2009 by Taylor & Francis Group, LLC
Special Drying Technologies 451
ral insert attached to the drum wall, which transports the drying mate-rial along the drum during its rotation. Each of spiral inserts has an inverse angle of the spiral winding with respect to the preceding one. This makes it possible to run the dryer continuously while performing batch processes in each section of the chamber by simply periodically reversing the rotation of the drum.
The procedure for MW processing of sols is as follows: a metered amount of sol is fed into the rotating drum with MW energy set to the
ing is performed in the following sequence of processes: sol is heated to the boiling point so that the moisture is evaporated practically at a con-stant rate. Because of progressive moisture removal, viscosity increases
reversed so that the viscous sol is transported to the second section of
sol to maintain continuous operation of the dryer. Further evaporation of moisture in the second section of the drum results in such an increase of sol viscosity that the sol starts to adhere to the dryer wall and to the spiral insert. Then it moves inside this section of the drum much more like a thin rotating cylinder of gel. Since the evaporation rate is maintained at
generated blasts the gel. Finally, the original solid gel is changed into a bed of still wet but transportable lumps. This completes the second step of processing.
In the third step of processing, the rotation of the drum is reversed again so that the granular gel is transported by the spiral insert to the fourth section of the drum, where it is dried at a constant temperature and at a constant drying rate. Further reversing of the drum rotation transports the essentially loose granules of xerogel to the fourth section of the drum, where bound moisture is removed. Because the temperature of the xerogel at the end of the falling rate period may increase to 570–650K, the spiral insert in this section of the drum is made of quartz ceramics.
The pilot-scale rotary MW processor (0.08 m in diameter and 1.4 m long) was designed and tested for various power levels up to 2 kW. The maxi-mum evaporation rate was 1.5 kg of water/h, and the product moisture content was kept within the required limit from 0.12 to 0.15 kg/kg.
ics, dielectric properties, and design calculation for this novel rotary MW processor in the paper by Bessarabov et al. (1999).
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the chamber-resonator by an MW filter. The drum has a sectioned spi-
required power. Because winding of the first segment of the spiral insert
first section of the drum so that the sol hold-up increases in the course of
When the feeding is over, the sol fills practically the bottom part of the first section of the drum. In this most stretched section, batch MW dry-
the drum, whereas the first section is again fed with a new portion of the
Interested readers can find detailed information about the drying kinet-
is opposite the liquid flow, it hinders transportation of the sol along the
feeding. After a certain period, the sol starts to flow over the spiral insert.
and the sol loses its fluidity. At this moment, the rotation of the drum is
the same level but the gel has a higher resistance to vapor flow, the steam
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452 Advanced Drying Technologies
34.8 Supercritical Fluid Technology to Produce Dry Particles
growing applications in electronics, superselective catalysis, controlled drug delivery, and others (Pakowski, 2004; Menshutina, 2007; Kim et al., 2008). Conventional methods of powder production such as spray and F-D have certain limitations. F-D produces a broad particle-size distribu-tion requiring downstream milling and sieving operations. Spray drying can produce particles of controlled size but may involve degradation for highly heat-sensitive products as used in the pharmaceutical industry.
to produce high-purity, micron- and nano-size particles of desired mor-phological structure in one step.
have been reported in the literature, although their commercial use is not well documented. Among the well-known processes are rapid expansion of supercritical solutions (RESS) (Phillips and Stella, 1993), the gas anti-solvent process (GAS) (Yeo et al., 1993), aerosol solvent extraction system (ASES) (Bleich and Muller, 1996), a precipitation with compressed antisol-vent process (PCA) (Brennecke, 1996), and solution-enhanced dispersion
solvent. Biomolecules such as proteins and nucleic acids cannot be dis-solved, and for such processes the SEDS technique has been shown to work technically, but it results in a loss of activity. Sarup et al. (2000) have described the process and presented results for two antibody frag-ments using isopropanol as a cosolvent in supercritical carbon dioxide. Unfortunately, their results showed a loss of activity due to the presence of the organic solvent. It appears that the choice of temperature and anti-solvent is critical in reducing the magnitude of damage during SEDS pro-cessing of the antibody fragment. More work is needed before this process can be understood better for such special applications.
Symbols and Acronyms
X Moisture content, kg/kgd.b. Dry basisw.b. Wet basisASES Aerosol solvent extraction systemCHP Combined heat and powerDDS Déshydration par Détentes SuccessivesGAS Gas antisolvent processMW Microwave
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Micron, submicron, and ultrafine particles (nanomaterials) find rapidly
Supercritical fluid technology offers an efficient but expensive approach
Several processes utilizing supercritical fluids for materials processing
are for products that are soluble in the supercritical fluid or in an organic by supercritical fluids (SEDS) (Sarup et al., 2000). The first four processes
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Special Drying Technologies 453
PCA Precipitation with compressed antisolventRESS Rapid expansion of supercritical solutionRJASB Rotating-jet annular spouted bedRJSB Rotating-jet spouted bedSEDS TACO Through-air combination
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