i

Table of Contents Dedication .................................................................................................................................................... iii

Abstract ........................................................................................................................................................ iv

Acknowledgement ........................................................................................................................................ v

Declaration ................................................................................................................................................... vi

Administration chart ................................................................................................................................... vii

CHAPTER ONE ............................................................................................................................................... 1

1.0 Brief History of the Company .............................................................................................................. 1

1.2 Social Responsibility of Corrugated Sheets Limited ............................................................................ 1

1.2.1 Society and the company ............................................................................................................. 1

1.3 Introduction to pickling ....................................................................................................................... 3

1.3.1 Pickling Solutions ......................................................................................................................... 4

1.3.2 Hydrochloric Acid Pickling ............................................................................................................ 5

1.3.3 Continuous-Strip Pickling Lines .................................................................................................... 5

CHAPTER ONE ........................................................................................................................................... 7

1.0 Design of fluidized-bed systems ......................................................................................................... 7

1.1 Fluidization Vessel ........................................................................................................................... 7

1.2 Bed .................................................................................................................................................. 8

1.3 Freeboard and Entrainment ............................................................................................................ 9

1.4 Gas Distributor .............................................................................................................................. 11

1.4 Scale-up ......................................................................................................................................... 15

1.5 Circulating or Fast Fluidized Beds ................................................................................................. 17

1.6 Pneumatic Conveying .................................................................................................................... 18

1.7 Heat Transfer ................................................................................................................................ 18

1.8 Temperature Control .................................................................................................................... 18

1.9 Solids Mixing ................................................................................................................................. 18

1.10 Gas Mixing ................................................................................................................................... 19

1.11 Size Enlargement ......................................................................................................................... 19

1.12 Size Reduction ............................................................................................................................. 20

1.13 Instrumentation .......................................................................................................................... 20

CHAPTER TWO ............................................................................................................................................ 22

2.0 Cyclone Design ...................................................................................................................................... 22

ii

2.1 Flow Pattern .................................................................................................................................. 24

2.2 Collection Efficiency ...................................................................................................................... 25

2.3 Factors Affecting Collection Efficiency .......................................................................................... 26

2.4 Theoretical Collection Efficiency ................................................................................................... 29

2.5 Lapple’s Efficiency Correlation ...................................................................................................... 32

2.6 Leith and Licht efficiency Model ................................................................................................... 34

2.7 Comparison of efficiency model results........................................................................................ 36

Pressure Drop...................................................................................................................................... 36

2.8 Cyclone Design Factors ................................................................................................................. 38

2.9 Cyclone Roughness ....................................................................................................................... 39

2.10 Cyclone Inlets .............................................................................................................................. 41

2.11 Solids Loading.............................................................................................................................. 43

2.12 Cyclone Length ............................................................................................................................ 44

2.13 Saltation ...................................................................................................................................... 45

CHAPTER THREE ...................................................................................................................................... 46

3.0 Design and Application of Wet Scrubbers ........................................................................................ 46

3.1 Collection Mechanisms And Efficiency ......................................................................................... 47

3.2 Collection Mechanisms And Particle Size ..................................................................................... 47

3.3 Selection And Design Of Scrubbers ............................................................................................... 49

3.4 Devices For Wet Scrubbing ........................................................................................................... 49

3.5 A Model For Counter-Current Spray Scrubbers ............................................................................ 52

3.6 A Model For Venturi Scrubbers ..................................................................................................... 54

CHAPTER FOUR ........................................................................................................................................... 59

4.0 Summary of the process ....................................................................................................................... 59

4.1.1 Reactor with cyclone Separator ..................................................................................................... 59

4.1.2 Venturi ........................................................................................................................................... 59

4.1.3 Absorption tower including scrubbing ........................................................................................... 60

4.1.4 Fan .................................................................................................................................................. 60

CHAPTER FIVE ............................................................................................................................................. 61

5.0 Safety ................................................................................................................................................ 61

SITE PLAN AND LOCATION .......................................................................................................................... 65

References .................................................................................................................................................. 66

iii

Dedication It is to my dear mother, my sister Pascalia and my brother Peter and my cousins: Ronald and

Joseph, my supervisor and lecturer Ms. Florence Ajiambo that I dedicate this report for their

priceless support throughout my life and to the staff at corrugated sheets for their cooperation and

hospitality during my attachment at the company.

iv

Abstract In order for the student to acquire enough understanding on his field of study it is mandatory that

he/she be attached to a relevant company. At Moi University the student engineer is supposed to

get attachment at the end of the third, fourth and fifth year of study. At the end of each attachment,

the student is expected to write a comprehensive report that is then submitted to the school.

During the attachment the student becomes part of the company so as to learn the operations of

the company.

v

Acknowledgement I would like to take this opportunity to pass my sincere thanks to Corrugated Sheets Limited

especially my supervisors Mr. Rajneesh and my trainer technician Ngome and to Moi University

School of Engineering Chemical and Process Department for giving me this special opportunity

to attend industrial attachment as part of my learning.

vi

Declaration I declare that this report is my own work. It has never been submitted to any institution for the

purpose of learning or examination. I submit this report to Moi University School of Engineering

being a requirement for completion of the bachelor’s degree in Chemical and Process

Engineering.

Name: Mgoja K. Safari

Reg. No. CPE/41/09

Sign: …………………..

vii

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1

CHAPTER ONE

1.0 Brief History of the Company The Corrugated Sheets Ltd was founded in 1958, started by an entrepreneur trading under the

name of Venus Metal (Africa) Limited. The company has risen to the status of one of the largest

Steel Houses in East Africa. The company serves as one stop shop for all steel products. Be it

Hot Rolled or Cold Rolled, Flat Products or Long Products and Galvanized or Pre-Painted, a

wide range is available under the Group umbrella. The company has an annual capacity of over

100,000 MT of various steel products in mixed ranges. It has been exporting Galvanized

Corrugated sheets, Pre-Painted sheets, nails and other wire products in the neighbouring African

countries. Quality of product has always been its single most priority. And quality checks at

various stages of production, ensures the product quality meeting the Kenyan and other

International Standards. The company believes in building a long term relationship with it’s

customers and its endeavour is to work towards attaining total customer satisfaction.

1.2 Social Responsibility of Corrugated Sheets Limited

1.2.1 Society and the company

1.2.1.1The Company’s Priorities

Corrugated Sheets Limited recognizes social responsibilities as a corporate to the citizens. The

company provides limited financial and material support to projects within those areas it

operates. It is also determined to contribute to the development of manpower needs. It manages a

number of projects in relation to youth development and children of tender age by involving

them in games and tournaments that foster their mental development and embark on generous

donations for education of children by assisting their school fees which keep them away from

drugs and other mischief. Health and safety are also absolute priorities for the company with

implementation of stringent safety policy for its employees and subcontractors. Corrugated

Sheets Limited prides itself in being the safest company in its sector and has achieved significant

reduction in both the frequency and the gravity of work-related accidents due to its strong health

and Safety Management System which defines the minimum safety levels required for all

employees. All Corrugated Sheet limited employees are committed to respecting certain rules to

ensure the greatest levels of health and safety within the company.

To achieve a zero fatal accident level and keep lost time injuries to minimum, Corrugated Sheets

Limited:

Informs its employees and subcontractors about risks related to their activities and provide

appropriate training.

Supervises the systematic application of safety standards.

Implements procedure for reporting incidents and undertakes regular audits to minimize them.

Identifies and communicates best practices and drives their adoption across all work sites.

2

1.2.1.2 Environmental Policy

It is widely acknowledged that tomorrow's world, even more than today's or yesterday's will be

in need of quality products and services, with more and more stringent requirements. The

company’s goodwill is to be increasing recognized as one of the best in this industry not only in

Kenya but in Africa and to prosper from reputation of excellence and efficiency that we will

earn. As part of company’s corporate social responsibility, Corrugated Sheets Ltd is committed

to carrying out her production, services, deliver activities and processes while ensuring it

conserves the environment for the future generations. Corrugated Sheets Ltd environmental

management system is modeled in line with requirements of the international Standard ISO

14001:2004.

Corrugated Sheets limited has a commitment to conduct its activities in a responsible manner in

order to protect and enhance the environment. It is the policy of Corrugated Sheets Limited to:

Ensure that facilities are operated, maintained and wherever necessary modified to ensure

compliance with laws and regulations and with Corrugated Sheets Ltd Environmental standards.

Ensure that its products and their manufacture will not be harmful to people or the environment.

Strive to continuously improve the efficiency of its operations so as to minimize the use of

resources and generation of waste and to ensure that any waste generated is dealt with in an

environmentally responsible manner.

All employees will adhere to the applicable guideline and procedures and take care in carrying

out their duties in a manner that may have a negative impact on the environment.

Managers and engineers will assure compliance with environmental guidelines and procedures

and ensure execution of improvement actions within each area.

Train its employees to achieve high standards of environmental performance. All employees will

share responsibility for implementation of environmental policy.

Corrugated Sheets Limited will demonstrate commitment to this policy by fostering open and

effective communication with all stakeholders and other interested parties and by ranking

environment protection with other key business objectives. Corrugated Sheets Limited is

committed to the prevention of pollution by seeking to achieve minimal adverse impact on air,

water and land through programs which incorporate responsible environment

management.Environmentalprotectionisaprimaryesponsibilityofeveryemployee.

1.2.1.3Continuous Improvement to Environment:

Continuous Compliance with all applicable environmental laws and regulations and any other

applicable requirements. All environmental non compliances will be investigated, assigned root

causes, documented, implemented and verified for effectiveness. Preventive measures are

reviewed for relevance, implemented and evaluated for effectiveness.

3

To train and educate employees to conduct their activities in an environmentally sound manner.

No one has authority to cause or allow an environmental non compliance for the sake of

production.

Environmental impact evaluation of proposed actions, including process and practice changes,

which may have an environmental impact.

Improvement, continually of our environmental performance, including objectives and targets,

through annual reviews with Corrugated Sheets Limited's top management. Appropriate

information regarding environmental performance is periodically provided to top management

and employees and made available to the public. Environmental performance will be regularly

measured and assessed for conformance to the ISO guidelines and compliance to legal

requirements.

As a commitment, the company has formulated and environmental policy for effective

implementation of her environment management system.

1.3 Introduction to pickling Oxide scale must be completely removed from hot-worked or hot-rolled steel before subsequent

processing is initiated, in order to prevent wear on dies and rolls and avoid surface defects in the

final product. This oxide scale originates during the hot working or hot rolling of steel, when the

surface of the metal reacts with oxygen in the air to form oxides of iron, or mill scale. The scale

actually consists of three iron oxides with different proportions of iron and oxygen. Hematite,

𝐹𝑒2𝑂3, which contains 30.1% oxygen, is the outermost oxide in the scale layer, whereas wustite,

𝐹𝑒𝑂, with 22.3% oxygen, is the innermost oxide. Magnetite, 𝐹𝑒3𝑂4, contains 27.6% oxygen;

when all oxides are present, the middle layer in the scale is magnetic. At temperatures above 566

°C (1050 °F), wustite is the predominant oxide, but during cooling below 566 °C (1050 °F), a

portion of it is transformed to iron and magnetite (4𝐹𝑒𝑂 = 𝐹𝑒3𝑂4 + 𝐹𝑒). In cases of rapid

cooling, which can occur with rod and bar, substantial amounts of wustite are retained in the

cooled product. When cooling after hot rolling is relatively slow, as it is with coiled strip,

magnetite is the main oxide constituent of the scale in the cooled product.

Pickling is the most common of several processes used to remove the scale from steel surfaces.

The term pickling refers to the chemical removal of scale by immersion in an aqueous acid

solution. The process originated in the late 1700s, when sheets of steel were descaled by

immersion in vats of vinegar. Wide variations are possible in the type, strength, and temperature

of the acid solutions used, depending on time constraints (batch vs. continuous operations), as

well as the thickness, composition, and physical nature (cracks) of the scale. Pickling is

applicable for many types of forgings and castings, for merchant bar, blooms, billets, sheet, strip,

wire, and for tubing.

4

1.3.1 Pickling Solutions

For carbon steel, sulfuric acid (𝐻2𝑆𝑂4) is used in most batch pickling operations, whereas

hydrochloric acid has become the pickling agent of choice, as of 1994, for continuous operations

with wire and strip. Hydrochloric acid (𝐻𝐶𝑙) is also used for special purposes, such as etching

before galvanizing or tinning. Nitric-hydrofluoric acid mixtures are used to pickle stainless steel.

Hydrofluoric acid is sometimes used when pickling castings to remove sand.

Mixtures of hydrochloric and sulfuric acids have been used in batch pickling, often by adding

rock salt (𝑁𝑎𝐶𝑙) to a sulfuric acid pickling bath. Such practices might be expected to give the

bright, pickled steel surface characteristics associated with hydrochloric acid and to increase

pickling rates, but not without some drawbacks. The proportion of 𝐻𝐶𝑙 to 𝐻2𝑆𝑂4 that is required

to achieve the rapid scale removal rate that is possible with 𝐻𝐶𝑙 alone is too high to be

economical, and the mixed acids cannot be properly handled by many of the spent pickle liquor

disposal methods now in use.

Acids other than 𝐻𝐶𝑙 to 𝐻2𝑆𝑂4 have been used to remove rust and scale from carbon steel. Citric

acid, oxalic acid, formic acid, hydrofluoric acid, fluoboric acid, and phosphoric acid are all

capable of removing mill scale from steel, but the rates of removal are generally not regarded as

useful or veconomical for most commercial applications, especially continuous operations.

The mechanism of scale removal, or pickling, by mineral acids involves the penetration of acid

through cracks in the scale, followed by the reaction of the acid with the innermost scale layer

and base metal. The presence of hydrogen gas, which forms when acid reacts with the base

metal, and the dissolution of 𝐹𝑒𝑂 help detach the outer scale layer from the metal surface. This

classical concept of pickling with 𝐻2𝑆𝑂4 is supported by experimental work and commercial

practices that demonstrate substantial increases in scale removal rates when scale cracking is

initiated by flexing, temper rolling (of strip), or tension leveling.

The reaction of 𝐻2𝑆𝑂4 with 𝐹𝑒𝑂 or with scale that is substantially 𝐹𝑒3𝑂4 mixed with iron will

form ferrous sulfate and water:

𝐹𝑒𝑂 + 𝐻2𝑆𝑂4 = 𝐹𝑒𝑆𝑂4 + 𝐻2𝑂

𝐹𝑒3𝑂4 + 𝐹𝑒 + 4𝐻2𝑆𝑂4 = 4𝐹𝑒𝑆𝑂4 + 4𝐻2𝑂

The reaction of sulfuric acid with base metal forms ferrous sulfate and hydrogen gas:

𝐹𝑒 + 𝐻2𝑆𝑂4 = 𝐹𝑒𝑆𝑂4 + 𝐻2(𝑔)

With hydrochloric acid, descaling primarily involves direct attack on the oxides. However, the

penetration of acid through cracks in the scale does contribute to the scale removal process,

although the magnitude of the effect resulting from enhanced scale cracking is somewhat less

than it is with sulfuric acid. The reaction of 𝐻𝐶𝑙 with 𝐹𝑒𝑂 or with scale that is substantially

𝐹𝑒3𝑂4 mixed with iron will form ferrous chloride and water:

𝐹𝑒𝑂 + 2𝐻𝐶𝑙 = 𝐹𝑒𝐶𝑙2 + 𝐻2𝑂

5

𝐹𝑒3𝑂4 + 𝐹𝑒 + 8𝐻𝐶𝑙 = 4𝐹𝑒𝐶𝑙2 + 4𝐻2𝑂

The reaction of hydrochloric acid with base metal forms ferrous chloride and hydrogen gas:

𝐹𝑒 + 2𝐻𝐶𝑙 = 𝐹𝑒𝐶𝑙2 + 𝐻2(𝑔)

1.3.2 Hydrochloric Acid Pickling

Hydrochloric acid is preferred for the batch pickling of hot-rolled or heat-treated high carbon

steel rod and wire. Continuous pickling operations also use hydrochloric acid to produce the very

uniform surface characteristics required for both low- and high-carbon steel. The possibility of

over pickling is minimized in these short time operations. The acid also dissolves lead oxides

that adhere to steel previously heat treated in molten lead baths.

Operating conditions for batch pickling in hydrochloric acid solutions typically involve acid

concentrations of 8 to 12 g/100 mL, temperatures of 38 to 40 °C (10 to 105 °F), and immersion

times of 5 to 15 min, with a maximum allowable iron concentration of 13 g/100 mL. Operating

conditions for continuous pickling in hydrochloric acid solutions typically involve acid

concentrations of 2 to 20 g/100 mL, temperatures of 66 to 93 °C (150 to 200 °F), and immersion

times of 1 to 20 s.

Hydrochloric acid offers a number of advantages, when compared with sulfuric and other acids.

It consistently produces a uniform light-gray surface on high-carbon steel. The possibility of over

pickling is less than it is with other acids.

Effective pickling can be obtained with iron concentrations as high as 13 g/100 mL. Rinsing is

facilitated because of the high solubility of chlorides. The cost of heating the bath for batch-type

operations is less than it is with sulfuric acid because of lower operating temperatures. The chief

disadvantage of hydrochloric acid is the necessity for a good fume control system.

Emissions from hydrochloric acid pickling include hydrogen chloride gas and must be

adequately vented to prevent localized corrosion of equipment and unsatisfactory working

conditions.

1.3.3 Continuous-Strip Pickling Lines

A few pickling lines make use of vertical towers in which one or two hydrochloric acid spray

columns are used. The acid spray columns are assembled and sealed in sections made of

fiberglass-reinforced polyester, with a tower height of 21.3 to 45.7 m (70 to 150 ft). The tank

sections are made from rubber lined steel. After use, acid flows into a sump and is returned to the

circulating tank. The composition of the acid in the recirculation tank is typically maintained at

11 g/100 mL 𝐻𝐶𝑙 and 13% 𝐹𝑒𝐶𝑙2. It is passed through a carbon-block heat exchanger and

delivered to the sprays at 77 °C (170 °F). Most lines of this type have acid-regenerating facilities.

Entry and exit coil handling are similar to the more common horizontal lines.

Continuous-strip pickling lines with horizontal pickling tanks are capable of handling coils that

are welded head to tail.

6

The entry section comprises a coil conveyer, one or two uncoilers, one or two processors, one or

two shears, and a welder. Processors are integral with the uncoiling equipment and consist of a

mandrel, hold-down roll, and a series of smaller diameter rolls. As the strip is flexed through the

processor, some cracking occurs in the scale layer, although not nearly as much as that imparted

by a temper mill. Proper welding and weld trimming is essential to avoid strip breaks in the line.

The section prior to the pickling tanks uses bridles for tensioning the strip; a strip accumulator,

either in the form of wet looping pits or, for more modern lines, a coil-car accumulator; and, for

many lines, a temper mill to crack the scale on the surface of the strip. A stretch leveler can

replace the temper mill and not only effectively cracks the scale, but also contributes to superior

strip shape.

The pickling section usually contains three or more tanks. So-called "deep tanks" are typically

1.22 m (4 ft) in depth and up to 31.3 m (90 ft) in length. Acid tanks are steel shells with layers of

rubber bonded to the steel. The rubber is protected from abrasion by a lining of silica-base acid-

proof brick. Most lines have a cascade flow of pickling solutions countercurrent to the direction

of strip movement. When fresh acid is added to the last tank, it will contain the highest

concentration of acid. Acid concentrations will decrease from the last tank to the first tank, from

which the spent pickle liquor is discharged. A rinse section follows the pickling section.

An especially effective rinsing method used on many continuous lines is the cascade rinse

system. Several rinse compartments are used, and fresh water is added to the last compartment.

The solution in that compartment cascades over weirs into the preceding compartments. The

excess overflows from the first compartment and is sent to the waste-water treatment plant (a

portion can be used for makeup water in the pickle tanks). Each compartment contains less acid

than the previous compartment. At the exit end of the line, there are usually an exit strip

accumulator, steering rolls, a strip inspection station, dual side trimmers, an oiler, and two

coilers. Pickling lines must have fume scrubbers to capture emissions/spray from the pickle

tanks.

In some modern lines, the pickling solution is contained in shallow tanks with liquid depths of

approximately 0.41 m (16in.) and lengths up to approximately 36 m (118 ft). Although they

involve a cascade system, the solution in each tank is recirculated through a heat exchanger.

During a line stop, the pickling solution can be rapidly drained from shallow tanks into

individual storage tanks and then pumped back when the line starts up. Lines with deep tanks

usually have strip lifters provided to remove the strip from the acid solution during an extended

line stop. Tank covers may be made from fiberglass or polypropylene. Some lines have squeegee

rolls, covered with acid-resistant rubber, located above and below the strip at each tank exit to

minimize acid carryover from one tank to another.

Maximum speeds in modern lines in the pickling section can be as high as 305 to 457 m/min

(1000 to 1500 ft/min).

Although sustained operation at such speeds is limited by other aspects of coil handling, the

selection of pickling tank acid concentrations and temperatures must be such that complete scale

removal is achieved during periods of high-speed operation. The combination of a pickling line

7

and a cold reduction mill in tandem represents a new state of the art in continuous processing

facilities. Another type of strip pickling line suitable for plants with moderate production

requirements is the push-pull type, which has many of the features of the continuous-type lines,

but no welder.

Turbulent-flow, shallow-tank, continuous-strip lines that claim to provide more effective

pickling action than conventional lines have been developed.

CHAPTER ONE

1.0 Design of fluidized-bed systems Fluidized beds are reactors in which small particles (with average size below 0.1 mm) are

fluidized by the reactant gases or liquids. When the linear velocity is above the minimum

required for fluidization, a dense fluidized bed is obtained. As the superficial velocity increases,

the bed expands and becomes increasingly dilute. At a high enough linear velocity, the smallest

particles entrain from the bed and have to be separated from the exhaust gases and recycled.

Advantages of fluidized beds are temperature uniformity, good heat transfer, and the ability to

continuously remove catalyst for regeneration.

Disadvantages are solids back mixing, catalyst attrition, and recovery of fines. Baffles have been

used often to reduce back mixing.

The major parts of a fluidized-bed system can be listed as follows:

Fluidization vessel

Fluidized-bed portion.

Disengaging space or freeboard

Gas distributor

Solids feeder or flow control

Solids discharge

Dust separator for the exit gases

Instrumentation

Gas supply

1.1 Fluidization Vessel

The most common shape is a vertical cylinder. Just as for a vessel designed for boiling a liquid,

space must be provided for vertical expansion of the solids and for disengaging splashed and

entrained material. The volume above the bed is called the disengaging space. The cross-

sectional area is determined by the volumetric flow of gas and the allowable or required

fluidizing velocity of the gas at operating conditions. In some cases the lowest permissible

velocity of gas is used, and in others the greatest permissible velocity is used. The maximum

flow is generally determined by the carry-over or entrainment of solids, and this is related to the

dimensions of the disengaging space (cross-sectional area and height).

8

1.2 Bed

Bed height is determined by a number of factors, either individually or collectively, such as:

Gas-contact time

L/D ratio required to provide staging

Space required for internal heat exchangers

Solids-retention time

Generally, bed heights are not less than 0.3 m (12 in) or more than 16 m(50 ft).

Although the reactor is usually a vertical cylinder, generally there is no real limitation on shape.

The specific design features vary with operating conditions, available space, and use. The lack of

moving parts lends toward simple, clean design.

Many fluidized-bed units operate at elevated temperatures. For this use, refractory-lined steel is

the most economical design.

The refractory serves two main purposes:

it insulates the metal shell from the elevated temperatures, and

it protects the metal shell from abrasion by the bed and particularly the splashing solids at

the top of the bed resulting from bursting bubbles.

Depending on specific conditions, several different refractory linings are used [Van Dyck,Chem.

Eng. Prog., 46–51 (December 1979)]. Generally, for the moderate temperatures encountered in

catalytic cracking of petroleum, a reinforced-gunnite lining has been found to be satisfactory.

This also permits the construction of larger units than would be permissible if self-supporting

ceramic domes were to be used for the roof of the reactor.

When heavier refractories are required because of operating conditions, insulating brick is

installed next to the shell and firebrick is installed to protect the insulating brick. Industrial

experience in many fields of application has demonstrated that such a lining will successfully

withstand the abrasive conditions in the bed for many years without replacement. Most serious

refractory wear occurs with coarse particles at high gas velocities and is usually most

pronounced near the operating level of the fluidized bed.

Gas leakage behind the refractory has plagued a number of units. Care should be taken in the

design and installation of the refractory to reduce the possibility of the formation of “chimneys”

in the refractories. A small flow of solids and gas can quickly erode large passages in soft

insulating brick or even in dense refractory. Gas stops are frequently attached to the shell and

project into the refractory lining. Care in design and installation of openings in shell and lining is

also required.

9

In many cases, cold spots on the reactor shell will result in condensation and high corrosion

rates. Sufficient insulation to maintain the shell and appurtenances above the dew point of the

reaction gases is necessary. Hot spots can occur where refractory cracks allow heat to permeate

to the shell. These can sometimes be repaired by pumping castable refractory into the hot area

from the outside.

The violent motion of a fluidized bed requires an ample foundation and a sturdy supporting

structure for the reactor. Even a relatively small differential movement of the reactor shell with

the lining will materially shorten refractory life. The lining and shell must be designed as a unit.

Structural steel should not be supported from a vessel that is subject to severe vibration.

Fig. 1.1 Non-catalytic fluidized bed system

1.3 Freeboard and Entrainment

The freeboard or disengaging height is the distance between the top of the fluid bed and the gas-

exit nozzle in bubbling- or turbulent-bed units. The distinction between bed and freeboard is

difficult to determine in fast and transport units.

At least two actions can take place in the freeboard: classification of solids and reaction of solids

and gases.

10

As a bubble reaches the upper surface of a fluidized bed, the bubble breaks through the thin

upper envelope composed of solid particles entraining some of these particles. The crater-shaped

void formed is rapidly filled by flowing solids. When these solids meet at the center of the void,

solids are geysered upward. The downward pull of gravity and the upward pull of the drag force

of the upward-flowing gas act on the particles. The larger and denser particles return to the top of

the bed, and the finer and lighter particles are carried upward. The distance above the bed at

which the entrainment becomes constant is the transport disengaging height, TDH. Cyclones and

vessel gas outlets are usually located above TDH. Figure 17-9 graphically estimates TDH as a

function of velocity and bed size.

Fig. 1.2 Estimating transport disengaging height (TDH).

The higher the concentration of an entrainable component in the bed, the greater its rate of

entrainment. Finer particles have a greater rate of entrainment than coarse ones. These principles

are embodied in the method of Geldart (Gas Fluidization Tech., Wiley, 1986, pp. 123–153) via

the equation,

𝐸(𝑖) = 𝐾∗(𝑖)𝑥(𝑖) … … … … … … … … … … 1.1

Where,

11

𝐸(𝑖) = entrainment rate for size 𝑖, 𝑘𝑔/𝑚2 𝑠;

𝐾∗(𝑖) = entrainment rate constant for particle size i; and

𝑥(𝑖) = weight fraction for particle size i.

𝐾∗is a function of operating conditions given by

𝐾∗(𝑖)/(𝑃𝑓 𝑢) = 23.7 𝑒𝑥𝑝 [−5.4𝑈𝑡(𝑖)

𝑈… … … … … … … … 1.2

The composition and the total entrainment are calculated by summing over the entrainable

fractions. An alternative is to use the method of Zenz as reproduced by Pell (Gas Fluidization,

Elsevier 1990, pp. 69–72).

In batch classification, the removal of fines (particles less than any arbitrary size) can be

correlated by treating as a second-order reaction

𝐾 = (𝐹

𝜃) [

1

𝑥(𝑥 − 𝐹)] … … … … … … … … … … … … … … 1.3

where

𝐾 = rate constant,

𝐹 = fines removed in time θ, and

𝑥 = original concentration of fines.

1.4 Gas Distributor

The gas distributor (also often called the grid of a fluidized bed) has a considerable effect on

proper operation of the fluidized bed. For good fluidized-bed operation, it is absolutely necessary

to have a properly designed gas distributor. Gas distributors can be used both when the gas is

clean and when the gas contains solids. The primary purpose of the gas distributor is to cause

uniform gas distribution across the entire bed cross-section. It should operate for years without

plugging or breaking, minimize sifting of solids back into the gas inlet to the distributor, and

minimize the attrition of the bed material. When the gas is clean, the gas distributor is often

designed to prevent backflow of solids during normal operation, and in many cases it is designed

to prevent backflow during shutdown. To provide good gas distribution, it is necessary to have a

sufficient pressure drop across the grid. This pressure drop should be at least one third the

pressure drop across the fluidized bed for gas upflow distributors, and one-tenth to one-fifth the

pressure drop across the fluidized bed for downflow gas distributors. If the pressure drop across

the bed is not sufficient, gas maldistribution can result, with the bed being fluidized in one area

and not fluidized in another. In units with shallow beds such as dryers or where gas distribution

is less crucial, lower gas distributor pressure drops can be used.

12

a) b)

Fig. 1.3 Gas inlets designed to prevent backflow of solids. (a) Insert tuyere; (b) clubhead tuyere.

(Dorr-Oliver, Inc.)

When both solids and gas pass through the distributor, such as in some catalytic cracking units, a

number of different gas distributor designs have been used. Because the inlet gas contains solids,

it is much more erosive than gas alone, and care has to be taken to minimize the erosion of the

grid openings as the solids flow through them. Generally, this is done by decreasing the inlet

gas/solids velocity so that erosion of the grid openings is low.

There are three basic types of clean inlet gas distributors:

a perforated plate distributor,

a bubble cap type of distributor, and

a sparger or pipe-grid type of gas distributor.

The perforated plate distributor is the simplest type of gas distributor and consists of a flat or

curved plate containing a series of vertical holes. The gas flows upward into the bed from a

chamber below the bed called a plenum. This type of distributor is easy and economical to

construct. However, when the gas is shut off, the solids can sift downward into the plenum and

may cause erosion of the holes when the bed is started up again. The bubble cap type of

distributor is designed to prevent backflow of solids into the plenum chamber or inlet line of the

gas distributor on start-up. The cap or tuyere type of distributor generally consists of a vertical

pipe containing several small horizontal holes or holes angled downward from 30° to 45° from

the horizontal. It is difficult for the solids to flow back through such a configuration when the

fluidizing gas is shut off.

13

The pipe distributor (often called a sparger) differs from the other two distributor types because

it consists of pipes with distribution holes in them that are inserted into the bed. This type of

distributor will have solids below it that are not fluidized. If this is not acceptable for a process,

then this type of distributor cannot be used. However, the pipe distributor has certain advantages.

It does not require a large plenum, the holes in the pipe can be positioned at any angle, and it can

be used in cases when multiple gas injections are required in a process.

To generate a sufficient pressure drop for good gas distribution, a high velocity through the grid

openings may be required. It is best to limit this velocity to less than 60 m/s to minimize attrition

of the bed material. The maximum hole velocity allowable may be even lower for very soft

materials that attrite easily. The pressure drop and the gas velocity through the hole in the gas

distributor are related by the equation

∆𝑃 =𝑢2𝜌𝑓

2𝑐2𝑔𝑐 for fps units…………………………………………..1.4

∆𝑃 =𝑢2𝜌𝑓

2𝑐2 for SI units…………………………………………….1.5

where

𝑢 = velocity in hole at inlet conditions

𝜌𝑓= fluid density in hole at conditions in inlet to hole

𝛥𝑃 = pressure drop in consistent units, 𝑘𝑃𝑎 𝑜𝑟 𝑙𝑏/𝑓𝑡2

𝑐 = orifice constant, dimensionless (typically 0.8 for gas distributors)

𝑔𝑐= gravitational conversion constant, 𝑓𝑡 ⋅ 𝑙𝑏𝑚/(𝑠2 ⋅ 𝑙𝑏𝑓)

Due to the pressure drop requirements across the gas distributor for good gas distribution, the

velocity through the grid hole may be higher than desired in order to minimize or limit particle

attrition. Therefore, it is common industrial practice to place a length of pipe (called a shroud)

over the gas distributor hole such that the diameter of the pipe is larger than the diameter of the

distributor hole. This technique effectively allows a smaller hole to give the required pressure

drop, but the larger hole diameter of the shroud reduces the exit gas velocity into the bed so that

particle attrition at the grid will be minimized. This technique is applied to both plate and pipe

spargers.

14

Experience has shown that a concave-downward gas distributor is a better arrangement than a

concave-upward gas distributor, as it tends to increase the flow of gases in the outer portion of

the bed. This counteracts the normal tendency of the gas to flow into the center of the bed after it

exits the gas distributor. In addition, the concave-downward type of gas distributor tends to assist

the general solids flow pattern in the bed, which is up in the center and down near the walls. The

concave-upward gas distributor tends to have a slow-moving region at the bottom near the wall.

If solids are large (or if they are slightly cohesive), they can build up in this region.

Structurally, distributors must withstand the differential pressure across the restriction during

normal and abnormal flow. In addition, during a shutdown, all or a portion of the bed will be

supported by the distributor until sufficient backflow of the solids has occurred into the plenum

to reduce the weight of solids above the distributor and to support some of this remaining weight

by transmitting the force to the walls and bottom of the reactor. During start-up, a considerable

upward thrust can be exerted against the distributor as the settled solids under the distributor are

carried up into the normal reactor bed.

When the feed gas is devoid of or contains only small quantities of fine solids, more

sophisticated designs of gas distributors can be used to effect economies in initial cost and

maintenance. This is most pronounced when the inlet gas is cold and noncorrosive. When this is

the case, the plenum chamber gas distributor and distributor supports can be fabricated of mild

steel by using normal temperature design factors.

The first commercial fluidized-bed ore roaster [Mathews, Trans.Can. Inst. Min. Metall. L11:97

(1949)], supplied by the Dorr Co. (now Dorr-Oliver Inc.) in 1947 to Cochenour-Willans, Red

Lake, Ontario, was designed with a mild-steel constriction plate covered with castable refractory

to insulate the plate from the calcine and to provide cones in which refractory balls were placed

to act as ball checks. The balls eroded unevenly, and the castable cracked. However,when the

unit was shut down by closing the air control valve, the runback of solids was negligible because

of bridging. If, however, the unit were shut down by deenergizing the centrifugal blower motor,

the higher pressure in the reactor would relieve through the blower and fluidizing gas plus solids

would run back through the constriction plate. Figure 17-11 illustrates two designs of gas inlets

which have been successfully used to prevent flowback of solids. For best results, irrespective of

the design, the gas flow should be stopped and the pressure relieved from the bottom upward

through the bed. Some units have been built and successfully operated with simple slot-type

distributors made of heat-resistant steel. This requires a heat-resistant plenum chamber but

eliminates the frequently encountered problem of corrosion caused by condensation of acids and

water vapor on the cold metal of the distributor. When the inlet gas is hot, such as in dryers or in

the upper distributors of multibed units, ceramic arches or heat-resistant metal grates are

generally used. Self-supporting ceramic domes have been in successful use for many years as gas

distributors when temperatures range up to 1100°C. Some of these domes are fitted with alloy-

steel orifices to regulate air distribution. However, the ceramic arch presents the same problem as

the dished head positioned concave upward. Either the holes in the center must be smaller, so

that the sum of the pressure drops through the distributor plus the bed is constant across the

15

entire cross section, or the top of the arch must be flattened so that the bed depths in the center

and outside are equal. This is especially important when shallow beds are used.

It is important to consider thermal effects in the design of the grid to-shell seal. Bypassing of the

grid at the seal point is a common problem caused by situations such as uneven expansion of

metal and ceramic parts, a cold plenum and hot solids in contact with the grid plate at the same

time, and start-up and shutdown scenarios. When the atmosphere in the bed is sufficiently

benign, a sparger-type distributor may be used. In some cases, it is impractical to use a plenum

chamber under the constriction plate. This condition arises when a flammable or explosive

mixture of gases is being introduced to the reactor. One solution is to pipe the gases to a

multitude of individual gas inlets in the floor of the reactor. In this way it may be possible to

maintain the gas velocities in the pipes above the flame velocity or to reduce the volume of gas

in each pipe to the point at which an explosion can be safely contained. Another solution is to

provide separate inlets for the different gases and to rely on the rapid axial mixing of the

fluidized bed. The inlets should be fairly close to one another, as lateral gas mixing in fluidized

beds is poor.

Much attention has been paid to the effect of gas distribution on bubble growth in the bed and the

effect of this on catalyst utilization, space-time yield, etc., in catalytic systems. It would appear

that the best gas distributor would be a porous membrane because of its even distribution.

However, this type of distributor is seldom practical for commercial units because of structural

limitations and the fact that it requires absolutely clean gas. Practically, the limitations on hole

spacing in a gas distributor are dependent on the particle size of the solids, materials of

construction, and type of distributor. If easily worked metals are used, then punching, drilling,

and welding are not expensive operations and permit the use of large numbers of holes. The use

of tuyeres or bubble caps permits horizontal distribution of the gas so that a smaller number of

gas inlet ports can still achieve good gas distribution. If a ceramic arch is used, generally only

one hole per brick is permissible and brick dimensions must be reasonable.

1.4 Scale-up

Bubbling or Turbulent Beds: Scale-up of noncatalytic fluidized beds when the reaction is fast,

as in roasting or calcination, is straightforward and is usually carried out on an area basis. Small-

scale tests are made to determine physical limitations such as sintering, agglomeration, solids-

holdup time required, etc. Slower (k < 1/s) catalytic or more complex reactions in which several

gas interchanges are required are usually scaled up in several steps, from laboratory to

commercial size. The hydrodynamics of gas-solids flow and contacting is quite different in

small-diameter high-L/D fluid beds as compared with large-diameter moderate-L/D beds. In

small-diameter beds, bubbles tend to be small and cannot grow larger than the vessel diameter.

In larger, deeper units, bubbles can grow very large. The large bubbles have less surface for mass

transfer to the solids than the same volume of small bubbles. The large bubbles also rise through

the bed more quickly. Homogeneous noncatalytic reactions are normally carried out in a

fluidized bed to achieve mixing of the gases and temperature control. The solids of the bed act as

a heat sink or source and facilitate heat transfer from or to the gas or from or to heat-exchange

16

surfaces. Reactions of this type include chlorination of hydrocarbons or oxidation of gaseous

fuels.

The size of a bubble as a function of height was given by Darton et al. [Trans. Inst. Chem. Eng.,

55, 274–280 (1977)] as

𝑑𝑏 =0.54(𝑢 − 𝑢𝑚𝑏)0.4(ℎ + 4√𝐴𝑡 𝑁0⁄ )0.8

𝑔0.2… … … … … … … … … … … … 1.6

Where

𝑑𝑏 = bubble diameter, m

ℎ = height above the grid, m

𝐴𝑡 𝑁0⁄ = grid area per hole

Bubble growth in fluidized beds will be limited by the diameter of the containing vessel and

bubble hydrodynamic stability. Solids and gas back mixing are much less in high-L/D beds

(whether they are slugging or bubbling) compared with low-L/D beds. Thus, the conversion or

yield in large, unstaged reactors is sometimes considerably lower than in small high-L /D units.

To overcome some of the problems of scale-up, staged units are used. It is generally concluded

than an unstaged 1-m-(40-in-) diameter unit will achieve about the same conversion as a large

industrial unit. The validity of this conclusion is dependent on many variables, including bed

depth, particle size, size distribution, temperature, and system pressure.

There are several methods available to reduce scale-up loss. These are summarized in Fig. 17-16.

The efficiency of a fluid bed reactor usually decreases as the size of the reactor increases. This

can be minimized by the use of high velocity, fine solids, staging methods, and a high L/D. High

velocity maintains the reactor in the turbulent mode, where bubble breakup is frequent and back

mixing is infrequent. A fine catalyst leads to smaller maximum bubble sizes by promoting

instability of large bubbles. Maintaining high L/D minimizes back mixing, as does the use of

baffles in the reactor. By these techniques, Mobil was able to scale up its methanol to gasoline

technology with little difficulty [Krambeck, Avidan, Lee, and Lo, A.I.Ch.E.J., 1727–1734

(1987)].

Another way to examine scale-up of hydrodynamics is to build a cold or hot scale model of the

commercial design.

17

Fig. 1.4 Reducing scale-up loss. (From Krambeck, Avidan, Lee, and Lo, A.I.Ch.E.J.,

1727–1734, 1987.)

1.5 Circulating or Fast Fluidized Beds

The circulating or fast fluidized bed is actually a misnomer in that it is not an extension of the

turbulent bed, but is actually a part of the transport regime, as discussed above. However, the fast

fluidized bed operates in that part of the transport regime that is dominated by the static head of

solids pressure drop term (the part of the regime where the solids concentration is the highest).

The solids may constitute up to 10 percent of the volume of the system in this regime. There are

no bubbles, mass transfer rates are high, and there is little gas backmixing in the system.

The high velocity in the system results in a high gas throughput which minimizes reactor cost.

Because there are no bubbles, scale-up is also less of a problem than with bubbling beds.

Many circulating systems are characterized by an external cyclone return system that usually has

as large a footprint as the reactor itself. The axial solids density profile is relatively flat.

There is a parabolic radial solids density profile that is termed core annular flow. In the center of

the reactor, the gas velocity and the solids velocity may be double the average. The solids in the

center of the column (often termed a riser) are in dilute flow, traveling at their expected slip

velocity 𝑈𝑔 − 𝑈𝑡. Near the wall in the annulus, the solids are close to their fluidized-bed density.

The solids at the wall can flow either upward or downward. Whether they do so is determined

18

primarily by the velocity used in the system. In circulating fluidized-bed combustor systems, the

gas velocity in the rectangular riser is generally in the range of 4 to 6 m/s, and the solids flow

down at the wall. In fluid catalytic cracking, the velocity in the riser is typically in the range of

12 to 20 m/s, and the solids flow upward at the wall. Engineering methods for evaluating the

hydrodynamics of the circulating bed are given by Kunii and Levenspiel (Fluidization

Engineering, 2nd ed., Butterworth, 1991, pp. 195–209), Werther (Circulating Fluid Bed

Technology IV, 1994), and Avidan, Grace, and Knowlton (eds.), (Circulating Fluidized Beds,

Blackie Academic, New York, 1997).

1.6 Pneumatic Conveying

Pneumatic conveying systems can generally be scaled up on the principles of dilute-phase

transport. Mass and heat transfer can be predicted on both the slip velocity during acceleration

and the slip velocity at full acceleration. The slip velocity increases as the solids concentration is

increased.

1.7 Heat Transfer

Heat-exchange surfaces have been used to provide the means of removing or adding heat to

fluidized beds. Usually, these surfaces are provided in the form of vertical or horizontal tubes

manifolded at the tops and bottom or in a trombone shape manifolded exterior to the vessel.

Horizontal tubes are extremely common as heat-transfer tubes. In any such installation, adequate

provision must be made for abrasion of the exchanger surface by the bed.

1.8 Temperature Control

Because of the rapid equalization of temperatures in fluidized beds, temperature control can be

accomplished in a number of ways.

Adiabatic. Control gas flow and/or solids feed rate so that the heat of reaction is removed as

sensible heat in off gases and solids or heat supplied by gases or solids.

Solids circulation. Remove or add heat by circulating solids.

Gas circulation. Recycle gas through heat exchangers to cool or heat.

Liquid injection. Add volatile liquid so that the latent heat of vaporization equals excess energy.

Cooling or heating surfaces in bed.

1.9 Solids Mixing

Solids are mixed in fluidized beds by means of solids entrained in the lower portion of bubbles,

and the shedding of these solids from the wake of the bubble (Rowe and Patridge, “Particle

Movement Caused by Bubbles in a Fluidized Bed,” Third Congress of European Federation of

Chemical Engineering, London, 1962). Thus, no mixing will occur at incipient fluidization, and

mixing increases as the gas rate is increased. Naturally, particles brought to the top of the bed

must displace particles toward the bottom of the bed. Generally, solids upflow is upward in the

center of the bed and downward at the wall.

19

At high ratios of fluidizing velocity to minimum fluidizing velocity, tremendous solids

circulation from top to bottom of the bed assures rapid mixing of the solids. For all practical

purposes, beds with L/D ratios of from 4 to 0.1 can be considered to be completely mixed

continuous-reaction vessels insofar as the solids are concerned.

Batch mixing using fluidization has been successfully employed in many industries. In this case

there is practically no limitation to vessel dimensions.

All the foregoing pertains to solids of approximately the same physical characteristics. There is

evidence that solids of widely different characteristics will classify one from the other at certain

gas flow rates [Geldart, Baeyens, Pope, and van de Wijer, Powder Technol., 30(2),

195 (1981)]. Two fluidized beds, one on top of the other, may be formed, or a lower static bed

with a fluidized bed above may result. The latter frequently occurs when agglomeration takes

place because of either fusion in the bed or poor dispersion of sticky feed solids.

Increased gas flows sometimes overcome the problem; however, improved feeding techniques or

a change in operating conditions may be required. Another solution is to remove agglomerates

either continuously or periodically from the bottom of the bed.

1.10 Gas Mixing

The mixing of gases as they pass vertically up through the bed has never been considered a

problem. However, horizontal mixing is very poor and requires effective distributors if two gases

are to be mixed in the fluidized bed.

In bubbling beds operated at velocities of less than about 5 to 11 times Umf the gases will flow

upward in both the emulsion and the bubble phases. At velocities greater than about 5 to 11 times

Umf the downward velocity of the emulsion phase is sufficient to carry the contained gas

downward. The back mixing of gases increases as U/Umf is increased until the circulating or fast

regime is reached where the back mixing decreases as the velocity is further increased.

1.11 Size Enlargement

Under proper conditions, solid particles can be caused to increase in size in the bed. This can be

advantageous or disadvantageous. Particle growth is usually associated with the melting or

softening of some portion of the bed material (i.e., addition of soda ash to calcium carbonate feed

in lime reburning, tars in fluidized-bed coking, or lead or zinc roasting causes agglomeration of

dry particles in much the same way as binders act in rotary pelletizers). The motion of the

particles, one against the other, in the bed results in spherical pellets. If the size of these particles

is not controlled, rapid agglomeration and segregation of the large particles from the bed will

occur. Control of agglomeration can be achieved by crushing a portion of the bed product and

recycling it to form nuclei for new growth. Often, liquids or slurries are fed via a spray nozzle

into the bed to cause particles to grow. In drying solutions or slurries of solutions, the location of

the feed injection nozzle (spray nozzle) has a great effect on the size of particle that is formed in

the bed. Also of importance are the operating temperature, relative humidity of the off-gas, and

20

gas velocity in the bed. Particle growth can occur as agglomeration (two or more particles

sticking together) or by the particle growing in layers, often called onion skinning.

1.12 Size Reduction

Attrition is the term describing particle reduction in the fluidized bed. Three major attrition

mechanisms occur in the fluidized bed: particle fragmentation, particle fracture, and particle

thermal decrepitation. Particle fragmentation occurs when the protruding edges on individual

particles are broken off in the bed. These particle fragments are very small—usually on the order

of 2 to 10 μm. Particle fracture occurs when particle interaction is severe enough to cause the

particles to break up into large individual pieces.

Because of the random motion of the solids, some abrasion of the surface occurs in the bed.

However, this abrasion is very small relative to the particle breakup caused by the high-velocity

jets at the distributor. Typically, particle abrasion (fragmentation) will amount to about

0.25 to 1 percent of the solids per day. In the area of high gas velocities at the distributor, greater

rates of attrition will occur because of fracture of the particles by impact. As mentioned above,

particle fracture of the grid is reduced by adding shrouds to the gas distributor.

Generally, particle attrition is unwanted. However, at times controlled attrition is desirable. For

example, in coking units where agglomeration due to wet particles is frequent, jets are used to

attrit particles to control particle size [Dunlop, Griffin, and Moser, J. Chem. Eng. Prog. 54:39–43

(1958)].

Thermal decrepitation occurs frequently when crystals are rearranged because of transition from

one form to another, or when new compounds are formed (i.e., calcination of limestone).

Sometimes the stresses on particles in cases such as this are sufficient to reduce the particle to

the basic crystal size. All these mechanisms will cause completion of fractures that were started

before the introduction of the solids into the fluidized bed.

1.13 Instrumentation

1.13.1 Temperature Measurement

This is usually simple, and standard temperature-sensing elements are adequate for continuous

use. Because of the high abrasion wear on horizontal protection tubes, vertical installations are

frequently used. In highly corrosive atmospheres in which metallic protection tubes cannot be

used, short, heavy ceramic tubes have been used successfully.

1.13.2 Pressure Measurement

Although successful pressure measurement probes or taps have been fabricated by using porous

materials, the most universally accepted pressure tap consists of a purged tube projecting into the

bed. Minimum internal diameters of the tube are 0.6 to 1.2 cm (0.25 to 0.5 in). A purge rate of at

least 1.5 m/s (5 ft/s) is usually required to prevent solids from plugging the signal lines. Bed

density is determined directly from ΔP/L, the pressure drop inside the bed itself (ΔP/L in units of

weight/area × 1/L). The overall bed weight is obtained from ΔP taken between a point just above

the gas distributor and a point in the freeboard. Nominal bed height is determined by dividing the

ΔP across the entire bed and dividing it by the ΔP/L over a section of the bed length. Splashing

21

of the solids by bubbles bursting at the bed surface will eject solids well above the nominal bed

height in most cases. The pressure drop signal from fluidized beds fluctuates due to bubble

effects and the generally statistical nature of fluid-bed flow parameters. A fast Fourier transform

of the pressure drop signal transforms the perturbations to a frequency versus amplitude plot with

a maximum at about 3 to 5 Hz and frequencies generally tailing off above 20 Hz. Changes in

frequency and amplitude are associated with changes in the quality of the fluidization.

Experienced operators of fluidized beds can frequently predict what is happening in the bed from

changes in the ΔP signal.

1.13.3 Flow Measurements

Measurement of flow rates of clean gases presents no problem. Flow measurement of gas

streams containing solids is almost always avoided. The flow of solids is usually controlled but

not measured except solids flows added to or taken from the system.

Solids flows in the system are usually adjusted on an inferential basis (temperature, pressure

level, catalyst activity, gas analysis, heat balance, etc.). In many roasting operations, the color of

the calcine discharge material indicates whether the solids feed rate is too high or too low.

22

CHAPTER TWO

2.0 Cyclone Design Cyclones are very common particulate control devices used in many applications, especially

those where relatively large particles need to be collected. They are not very efficient for

collecting small particles because small particles have little mass that can generate a centrifugal

force. Cyclones are very simple devices that use centrifugal force to separate particles from a gas

stream. They commonly are constructed of sheet metal, although other materials can be used.

They have a low capital cost, small space requirement, and no moving parts. Of course, an

external device, such as a blower or other source of pressure, is required to move the gas stream.

Cyclones are able to handle very heavy dust loading, and they can be used in high temperature

gas streams. Sometimes they are lined with castable refractory material to resist abrasion and to

insulate the metal body from high-temperature gas.

Fig. 2.1 Cyclone-separator proportions.

23

It has a tangential inlet to a cylindrical body, causing the gas stream to be swirled around.

Particles are thrown toward the wall of the cyclone body. As the particles reach the stagnant

boundary layer at the wall, they leave the flowing gas stream and presumably slide down the

wall, although some particles may be re-entrained as they bounce off of the wall back into the

gas stream. As the gas loses energy in the swirling vortex, it starts spinning inside the vortex and

exits at the top.

The vortex finder tube does not create the vortex or the swirling flow. Its function is to prevent

short-circuiting from the inlet directly to the outlet. Cyclones will work without a vortex finder,

although the efficiency will be reduced.

Fig. 2.2 Schematic of standard cyclone.

24

2.1 Flow Pattern

In a cyclone, the gas moves in a double vortex with the gas initially spiraling downward at the

outside after it enters the inlet; then, the gas flows upward in the center of the cyclone before it

exits. When the gas enters the cyclone, its velocity undergoes a redistribution so that the

tangential component of velocity increases with decreasing radius, as expressed by 𝑉𝑐𝑡 − 𝑟−𝑛.

The tangential velocity in a cyclone may reach a value several times the average inlet gas

velocity. Theoretical considerations indicate that n should be equal to 1.0 in the absence of wall

friction. Actual measurements [Shepherd and Lapple, Ind. Eng. Chem. 31: 972 (1939); 32: 1246

(1940)], however, indicate that n may range from 0.5 to 0.7 over a large portion of the cyclone

radius. Ter Linden [Inst. Mech. Eng. J. 160: 235 (1949)] found n to be 0.52 for tangential

velocities measured in the cylindrical portion of the cyclone at positions ranging from the radius

of the gas outlet pipe to the radius of the outer wall. Although the velocity approaches zero at the

wall, the boundary layer is sufficiently thin that pitot-tube measurements show relatively high

tangential velocities there, as shown in Fig. 17-37. The radial velocity 𝑉𝑟 is directed toward the

center throughout most of the cyclone, except at the center, where it is directed outward.

Superimposed on the “double spiral,” there may be a “double eddy” [Van Tongran, Mech. Eng.

57: 753 (1935); and Wellmann, Feuerungstechnik 26: 137 (1938)] similar to that encountered in

pipe coils. Measurements on cyclones of the type shown indicate, however, that such double-

eddy velocities are small compared with the tangential velocity (Shepherd and Lapple, op. cit.).

Recent analyses of flow patterns can be found in Hoffman et al., Powder Technol. 70: 83 (1992);

and Trefz and Muschelknautz, Chem. Eng. Technol. 16: 153 (1993).

The inner vortex (often called the core of the vortex) rotates at a much higher velocity than the

outer vortex. In the absence of solids, the radius of this inner vortex has been measured to be 0.4

to 0.8 r. With axial inlet cyclones, the inner core vortex is aligned with the axis of the gas outlet

tube. With tangential or volute cyclone inlets, however, the vortex is not exactly aligned with the

axis. The nonsymmetric entry of the tangential or volute inlet causes the axis of the vortex to be

slightly eccentric from the axis of the cyclone. This means that the bottom of the vortex is

displaced some distance from the axis and can “pluck off” and re-entrain dust from the solids

sliding down the cyclone cone if the vortex gets too close to the wall of the cyclone cone.

At the bottom of the vortex, there is substantial turbulence as the gas flow reverses and flows up

the middle of the cyclone into the gas outlet tube. As indicated above, if this region is too close

to the wall of the cone, substantial re-entrainment of the separated solids can occur. Therefore, it

is very important that cyclone design take this into account.

The vortex of a cyclone will precess (or wobble) about the center axis of the cyclone. This

motion can bring the vortex into close proximity to the wall of the cone of the cyclone and

“pluck” off and reentrain the collected solids flowing down along the wall of the cone. The

vortex may also cause erosion of the cone if it touches the cone wall. Sometimes an inverted

cone or a similar device is added to the bottom of the cyclone in the vicinity of the cone and

dipleg to stabilize and “fix” the vortex. If it is placed correctly, the vortex will attach to the cone

and the vortex movement will be stabilized, thus minimizing the efficiency loss due to plucking

the solids off the wall and erosion of the cyclone cone.

25

Hugi and Reh [Chem. Eng. Technol. 21(9): 716–719 (1998)] have reported that (at high solids

loadings) enhanced cyclone efficiency occurs when the solids form a coherent, stable strand at

the entrance to a cyclone. The formation of such a strand is dependent upon several factors. They

reported a higher cyclone efficiency for smaller (dp,50 = 40 _m) solids than for larger solids

(dp,50 = 125 _m). This is not what theory would predict. However, they also found that the

smaller particles formed coherent, stable strands more readily than the larger particles, which

explained the reason for the apparent discrepancy.

Fig. 2.3 Variation of tangential velocity and radial velocity at different points in a cyclone. [Ter

Linden, Inst. Mech. Eng. J., 160, 235 (1949).]

2.2 Collection Efficiency

When a particle moves at a constant speed in a circular direction, the velocity vector changes

continuously in direction, although not in magnitude. This creates acceleration resulting from a

change in direction of the velocity, which is just as real and just as much an acceleration as that

arising from the change in the magnitude of velocity. By definition, acceleration is the time rate

of change of velocity, and velocity, being a vector, can change in direction as well as magnitude.

Force, of course, is defined by Newton’s Second Law (F = ma). Centrifugal force is given by:

𝐹 =𝑚𝑉2

𝑟… … … … … … … … … … … … … … … … 2.1

where

F = centrifugal force

26

m = mass of particle

V = velocity of particle, assumed to equal inlet gas velocity

r = radius of cyclone body

Fig. 2.4 Single particle collection efficiency curve. (Courtesy of PSRI, Chicago.)

2.3 Factors Affecting Collection Efficiency

Several factors that affect collection efficiency can be predicted. Increasing the inlet velocity

increases the centrifugal force, and therefore the efficiency, but it also increases the pressure

drop. Decreasing the cyclone diameter also increases centrifugal force, efficiency, and pressure

drop. Increasing the gas flow rate through a given cyclone has the effect of efficiency shown in

Equation 2.2:

𝑃𝑡2

𝑃𝑡1= (

𝑄1

𝑄2)

0.5

… … … … … … … … … … … … … … … … … 2.2

where

27

𝑃𝑡 = penetration (𝑃𝑡 = 1 –)

= particle removal efficiency

Q = volumetric gas flow

Interestingly, decreasing the gas viscosity improves efficiency, because drag force is reduced.

Centrifugal force drives the particle toward the wall of the cyclone, while drag opposes the

centrifugal force. The terminal velocity of the particle toward the wall is the result of the force

balance between the centrifugal and drag forces. Increasing gas to particle density difference

affects penetration as shown in Equation 2.3:

𝑃𝑡2

𝑃𝑡1= (

𝜇2

𝜇1)

0.5

… … … … … … … … … … … … … … … … 2.3

where: μ= gas viscosity. Note that decreasing the gas temperature increases the gas density, but

contrary to intuition, decreases the gas viscosity, which reduces drag force and results in a small

efficiency improvement. However, decreasing the gas temperature also decreases the volumetric

flow rate, which affects efficiency as described above in Equation 2.2.

Finally, particle loading also affects efficiency. High dust loading causes particles to bounce into

each other as they move toward the wall, driving more particles toward the wall and their

removal.

𝑃𝑡2

𝑃𝑡1= (

𝐿1

𝐿2)

0.18

… … … … … … … … … … … … … … … 2.4

where L = inlet particle concentration (loading).

28

Fig. 2.5 Effect of inlet loading on collection efficiency for Geldart group A and group C

particles. (Courtesy of PSRI, Chicago.)

Fig. 2.6 Effect of inlet loading on collection efficiency (Geldart group B and group D) particles.

(Courtesy of PSRI, Chicago.)

29

Relative dimensions are based upon the diameter of the body of the cyclones. High-efficiency

cyclones tend to have long, narrow bodies, while high-throughput cyclones generate less

pressure drop with fat bodies.

Fig. 2.7 Generalized efficiency relationships.

2.4 Theoretical Collection Efficiency

The force balance between centrifugal and drag forces determines the velocity of the particles

toward the wall. Resident time of particles in the cyclone, which allows time for particles to

move toward the wall, is determined by the number of effective turns that the gas path makes

within the cyclone body. An empirical relationship for the number of effective turns is provided

in Equation 2.5:

𝑁𝑒 =1

𝐻(𝐿𝑏 +

𝐿𝑐

2) … … … … … … … … … … … … … 2.5

where

𝑁𝑒= number of effective turns

30

𝐻 = height of the tangential inlet

𝐿𝑏= length of cyclone body

𝐿𝑐= length of cyclone lower cone

Cyclones work by using centrifugal force to increase the gravity field experienced by the solids.

They then move to the wall under the influence of their effectively increased weight. Movement

to the wall is improved as the path the solids traverse under centrifugal flow is increased. This

path is equated with the number of spirals the solids make in the cyclone barrel. Figure 17-38

gives the number of spirals Ns as a function of the maximum velocity in the cyclone. The

maximum velocity may be either the inlet or the outlet velocity depending on the design. The

equation for 𝐷𝑝𝑡ℎ, the theoretical size particle removed by the cyclone at 50 percent collection

efficiency, is

𝐷𝑝𝑡ℎ = √9𝜇𝑔𝐵𝑐

𝜋𝑁𝑠𝜐𝑚𝑎𝑥(𝜌𝑝 − 𝜌𝑔)… … … … … … … … … … … 2.6

Fig. 2.8 Ns versus velocity—where the larger of either the inlet or outlet velocity is used.

31

The theoretical efficiency of a cyclone can be calculated by balancing the terminal velocity with

the residence time resulting from a distance traveled in the cyclone. This force and time balance

results in Equation 2.7:

Fig. 2.9 Cyclone dimensions.

Table 2.1

32

𝐷𝑝𝑥 = [𝑥

100

9𝜇𝑊

𝜋𝑁𝑒𝑉𝑖(𝜌𝑝 − 𝜌𝑔)]

0.5

… … … … … … … … … … 2.7

where

𝑑𝑝𝑥= diameter of a particle with x% removal efficiency

𝜇 = viscosity

𝑊 = inlet width

𝑁𝑒= number of effective turns

𝑉𝑖= inlet velocity

𝜌𝑝= density of particle

𝜌𝑔= density of gas

2.5 Lapple’s Efficiency Correlation

Unfortunately, the theoretical efficiency relationship derived above does not correlate well with

real data. The relationship works reasonably well for determining the 50% cut diameter (the

diameter of the particle that is collected with 50% efficiency). To better match data with

reasonable accuracy, the efficiency of other particle diameters can be determined from Lapple’s

empirical efficiency correlation. This correlation can be set up for automated calculations using

the algebraic fit given by Equation 2.8:

𝜂𝑗 =1

1 + (𝑑𝑝50

𝑑𝑝𝑗)

2 … … … … … … … … … … 2.8

where

𝜂𝑗= collection efficiency of particle with diameter j

𝑑𝑝50= diameter of particles with 50% collection efficiency

𝑑𝑝𝑗= diameter of particle j

33

Fig. 2.10 Lapple’s efficiency curve.

Lapple’s efficiency curve was developed from measured data for cyclones with the “standard”

dimensions shown in Table 2.1. The efficiency curve can be tailored for different industrial

cyclone dimensions by adding a slope parameter, B, to the correlation:

𝜂𝑗 =1

1 + (𝑑𝑝50

𝑑𝑝𝑗)

𝐵 … … … … … … … … … 2.9

where B = slope parameter, typically ranging from 2 to 6.

34

Figure 2.1 below llustrates the effect of the slope parameter, B. Note that the larger value for B

results in a sharper cut. Since more mass is associated with larger particles, the sharper cut

results in higher overall mass removal efficiency.

Fig. 2.11 Effect of slope parameter, B.

2.6 Leith and Licht efficiency Model

Other models have been developed to predict cyclone performance. One is the Leith and Licht

model shown in Equation 2.10:

𝜂 = 1 − exp(−Ψ𝑑𝑝𝑀) … … … … … … … … … … 2.10

𝑀 =1

𝑚 + 1

𝑚 = 1 − [(1 − 0.67𝐷𝑐0.14) (

𝑇

283)

0.3

]

35

Ψ = 2 [𝐾𝑄𝜌𝑝𝐶′(𝑚 + 1)

18𝜇𝐷𝑐3 ]

𝑀2

where

𝑑𝑝= particle diameter in meters

𝐷𝑐= cyclone body diameter in meters

T = gas temperature, °K

K = dimensional geometric configuration parameter

Q = volumetric gas flow

𝜌𝑝= particle density

C′= cunningham slip correction factor

𝜇= gas viscosity

The geometric configuration parameter is estimated based on the cyclone configuration. Table

2.2 shows relative dimensions for three types of cyclones: the standard cyclone, the Stairmand

design, and the Swift design.Note that the Stairmand and the Swift cyclones have smaller inlet

openings than the standard design, which means a higher inlet velocity for the same size body.

This results in more centrifugal force and increased efficiency. In the Leith and Licht model, a

larger geometric configuration parameter results in a higher predicted efficiency.

Table 2.2

36

Fig. 2.12 Cyclone efficiency curves.

2.7 Comparison of efficiency model results

Efficiency models are adequate for getting a fair idea of performance, but there can be a rather

wide variation in model predictions. Part, but not all, of the variation can be explained by

empirical factors for the cyclone configuration. Figure 2.12 shows cyclone efficiency curves as a

function of particle diameter based on several sources. Each curve is based upon the same gas

flow and gas and particle conditions. The lowest efficiency is predicted by Lapple’s curve for a

standard cyclone. Interestingly, the Leith and Licht model for the same standard cyclone predicts

a significantly higher efficiency. The Leith and Licht model for the higher efficiency Stairmand

and Swift cyclone designs shows incremental improvement over the standard design. Vendor

data also were collected for the same set of gas and particle conditions, with significant predicted

performance improvement. Perhaps the vendors were being overoptimistic about their designs,

or perhaps there have been significant improvements in cyclone design over the years. It does

point out that performance guarantees for cyclones must be written with specific information

about the gas and particle properties, including the particle size distribution, to ensure that

vendor guarantees can be measured and substantiated after installation.

Pressure Drop

Pressure drop is first determined by summing five pressure drop components associated with the

cyclone.

1. Inlet contraction

Δ𝑃 = 0.5𝜌𝑔(𝜐𝑖𝑛2 − 𝜐𝑣𝑒𝑠𝑠𝑒𝑙

2 + 𝐾𝜐𝑖𝑛2 ) … … … … … … … 2.11

37

where K is taken from Table 17-3. Using SI units gives the pressure drop in Pa. In English units,

the factor of 32.2 for g must be included. This loss is primarily associated with cyclones located

in the freeboard of a fluidized bed. If the cyclone is located external to a vessel and the high

pressure tap used to measure the cyclone pressure drop is in the inlet pipe before the cyclone, the

measured pressure drop will not include this pressure loss, and this term should not be used to

calculate total cyclone pressure drop. However, if the high-pressure tap to measure the cyclone

pressure drop is located in the freeboard of the bed, this component will be included in the

measured pressure drop, and it should be included in the calculation of the total cyclone pressure

drop.

2. Particle acceleration

Δ𝑃 = 𝐿𝜐𝑖𝑛(𝜐𝑝𝑖𝑛 − 𝜐𝑝𝑣𝑒𝑠𝑠𝑒𝑙) … … … … … … … … … … … … 2.12

For small particles, the velocity is taken as equal to the gas velocity and L is the solids loading,

kg/m3.

3. Barrel friction

The inlet diameter din is taken as 4 × (inlet area)/ inlet perimeter. Then

Δ𝑃 =2𝜌𝑔𝜐𝑖𝑛

2 𝐷𝑐𝑁𝑠

𝑑𝑖𝑛… … … … … … … … … … … … … … … 2.13

where the Reynolds number for determining the friction factor f is based on the inlet area.

4. Gas flow reversal

38

Δ𝑃 =𝜌𝑔𝜐𝑖𝑛

2

2… … … … … … … … … … … … … … 2.14

5. Exit contraction

Δ𝑃 = 0.5𝜌𝑔(𝜐𝑒𝑥𝑖𝑡2 − 𝜐𝑐

2 + 𝐾𝜐𝑒𝑥𝑖𝑡2 ) … … … … … … … … … … … . .2.15

where K is determined from Table 17-3 based on the area ratio of barrel and exit tube of the

cyclone. The total pressure drop is the sum of the five individual pressure drops.

However, the actual pressure drop observed turns out to be a function of the solids loading. The

pressure drop is high when the gas is free of solids and then decreases as the solids loading

increases up to about 3 kg/m3 (0.2 lb/ft3). The cyclone Δ𝑃 then begins to increase with loading.

The cause of the initial decline is that the presence of solids decreases the tangential velocity of

the gas [Yuu, Chem. Eng. Sci., 33, 1573 (1978)].When solids are absent, the observed pressure

drop can be 2.5 times the calculated pressure drop with solids present.

2.8 Cyclone Design Factors

Cyclones are sometimes designed to meet specified pressure drop limitations. For ordinary

installations, operating at approximately atmospheric pressure, fan limitations generally dictate a

maximum allowable pressure drop corresponding to a cyclone inlet velocity in the range of 8 to

30 m/s (25 to 100 ft/s). Consequently, cyclones are usually designed for an inlet velocity of 15 to

20 m/s (50 to 65 ft/s), although this need not be strictly adhered to.

Because of the relatively high gas velocities at the inlet of cyclones, particle attrition in fluidized-

bed systems is generally dominated by the attrition produced in the cyclone. In some catalytic

systems with very expensive catalysts, the economics of the process can be dependent on low

attrition losses. In such cases, reducing the inlet velocity of the cyclone will significantly reduce

the attrition losses in the process. To compensate for the reduction in inlet velocity, the exit gas

velocity will generally be increased (by reducing the diameter of the outlet tube) in order to

maintain high cyclone efficiencies. Reducing the outlet tube diameter increases the outlet gas

velocity and increases the velocity in the vortex of the cyclone-increasing collection efficiency.

However, as the vortex velocity is increased, its length is also increased. Therefore, care must be

taken to ensure that the cyclone is long enough to contain the increased vortex length. If it is not,

the vortex can extend far into the cone and can entrain solids flowing on the sides of the cone as

it comes near them.

39

2.9 Cyclone Roughness

Large weld beads, etc., can also reduce cyclone efficiency. If the solids flow along the wall of a

cyclone encounters a large protuberance such as a weld bead, the weld bead acts as a type of “ski

jump” and causes the solids to be deflected farther into the center of the cyclone, where they can

be thrown into the vortex and carried out of the cyclone. In small pilot or research cyclones, this

is especially common, because the distance between the wall of the cyclone and the vortex tube

is very small. Because of their detrimental effect on cyclone efficiency, weld beads should be

ground off to make the cyclone inner wall smooth.

In high-temperature processes, cyclones are often lined with refractory to both minimize heat

loss and protect the metal surfaces from abrasion. These refractory surfaces are not as smooth as

metal, but after a few days of operation, the refractory becomes smoother because of the abrasive

action of the solids.

With very small laboratory or pilot cyclones, some solids (large polymer beads, spherical

particles, etc.) can sometimes bounce off the cyclone wall immediately across from the cyclone

inlet and be deflected into the vortex. Very large particles can be found in the gas outlet stream

of the cyclone with these very small cyclones and with particles that bounce. To increase cyclone

efficiency with these types of solids, the cyclone barrel diameter can be increased. This increases

the distance between the cyclone vortex and the wall and prevents most of the solids from

bouncing back into the vortex.

Theoretically, a primary design factor that can be utilized to control collection efficiency is the

cyclone diameter. A smaller-diameter unit operating at a fixed pressure drop has a higher

efficiency than a larger diameter cyclone [Anderson, Chem. Metall. 40: 525 (1933); Drijver,

Warme 60: 333 (1937); and Whiton, Power 75: 344 (1932); Chem. Metall. 39: 150 (1932)]. In

addition, smaller-diameter cyclones have a much smaller overall length. Small-diameter

cyclones, however, will require multiple units in parallel to give the same capacity as a large

cyclone. In such cases, the smaller cyclones generally discharge the dust into a common

receiving hopper [Whiton, Trans. Am. Soc. Mech. Eng. 63: 213 (1941)]. However, when

cyclones discharge into a common hopper, there is a tendency of the gas to produce “cross-talk.”

This occurs when the gas exiting from one small cyclone passes up the exit of an adjoining

cyclone, thus reducing efficiency. Various types of mechanical devices are generally added to

the bottom of these small cyclones in parallel to reduce the cross-talk. The final cyclone design

involves a compromise between collection efficiency and the complexity of equipment. It is

customary to design systems for a single cyclone for a given capacity, resorting to multiple

parallel units only if the predicted collection efficiency is inadequate for a single unit or single

units in series.

Reducing the gas outlet diameter should increase both collection efficiency and pressure drop.

To exit the cyclone, gas must enter the cyclonic flow associated with the outlet tube. If the outlet

diameter is reduced, the outlet vortex increases in length to compensate. Therefore, when the

outlet area is less than the inlet area, the length of the cyclone must increase. Too short a cyclone

is associated with erosion of the cone and reentrainment of solids into the exit flow. Table 17-4

40

gives the required increase in cyclone length as a function of outlet-toinlet area. The cyclone

length is measured centrally along a cylinder 10 cm larger than the inner diameter of the outlet

tube to prevent interference with the cone. If the cone interferes with this “extended vortex,” the

barrel must be lengthened.

As discussed above, theoretically a smaller-diameter cyclone should be able to collect smaller

particles because it can develop a higher centrifugal force. However, using smaller cyclones

generally means that many have to be used in parallel to accommodate large gas flows.

The problem with parallel cyclones (as indicated above) is that it is difficult to get even

distribution of solids into all the cyclones. If maldistribution occurs, this can cause inefficiencies

that can negate the natural advantage of the smaller cyclones.

Cyclone diameters can be very large. Perhaps the largest cyclones are those used in circulating

fluidized-bed combustors, where cyclone diameters approach 10 m. Large-diameter cyclones

also result in very long cyclones, and so these large-diameter, long-length cyclones are really not

feasible as internal cyclones in fluidized beds (they make the vessel too tall).

The minimum cone angle of the cyclone should be 60°. It is generally greater, with steeper cone

angles appropriate to materials that are more cohesive. The cyclone inlet is usually rectangular

(more efficient at getting material to the wall), but in some cases has been circular. In either case,

projection of the inlet flow path should never cause interference with the outlet tube. This

generally means that the inlet width of a cyclone should always be less than the distance between

the wall and the outside diameter of the outlet tube. If a very heavy solids loading is anticipated,

the barrel diameter should be increased slightly to minimize interference with the outlet gas tube.

Collection efficiency is normally increased by increasing the gas throughput (Drijver, op. cit.).

However, if the entering dust is agglomerated, high gas velocities may cause breakup of the

agglomerated solids in the cyclone, so that efficiency remains the same or actually decreases.

Also, variations in design proportions that result in increased collection efficiency with dispersed

dusts may be detrimental with agglomerated dusts. Kalen and Zenz [Am. Inst. Chem. Eng. Symp.

Ser. 70(137): 388 (1974)] report that collection efficiency increases with increasing gas inlet

velocity up to a minimum tangential velocity at which dust is either reentrained or not deposited

because of saltation. Koch and Licht [Chem. Eng. 84(24): 80 (1977)] estimate that for typical

cyclones the saltation velocity is consistent with cyclone inlet velocities in the range of 15 to 27

m/s (50 to 90 ft/s). Lapple (private communication) reports that in cyclone tests with talc dust,

collection efficiency increased steadily as the inlet velocity was increased up to a maximum of

52 m/s (170 ft/s). With ilmenite dust, which was much more strongly flocculated, efficiency

decreased over the same inlet velocity range. In later experiments with well-dispersed talc dust,

collection efficiency continued to increase at inlet velocities up to the maximum used, 82 m/s

(270 ft/s).

Another effect of increasing the cyclone inlet gas velocity is that friable materials may

disintegrate (or attrit) as they hit the cyclone wall at high velocity. Thus, the increase in

41

efficiency associated with increased velocity may be more than lost due to generation of fine

attrited material that the cyclone cannot contain.

Internal cyclones have the advantages that they require no inlet piping (their inlets can be open to

the freeboard) and no high-pressure shell, and they have straight cyclone diplegs. Internal

cyclones are generally smaller in diameter than external cyclones because their size is limited by

the headspace available in the freeboard above the fluidized bed. These size limitations result in

using several smaller cyclones in parallel instead of one large cyclone. In addition, it is difficult

to aerate second-stage cyclone diplegs (generally an advantageous technique) when internal

cyclones are used. Aerating secondary cyclone diplegs can improve the operation of the diplegs

significantly.

The advantages of external cyclones are that

(1) they can be much larger than internal cyclones,

(2) they are more accessible than internal cyclones, and

(3) their diplegs can be aerated more easily.

The disadvantages of external cyclones are that

(1) they require a pressure shell and

(2) external cyclone diplegs generally require a section with an angled or a horizontal pipe to

return the solids to the bed. The angled or horizontal dipleg sections can result in poor dipleg

operation, if not designed correctly.

2.10 Cyclone Inlets

The design of the cyclone inlet can greatly affect cyclone performance. It is generally desired to

have the width of the inlet Bc as narrow as possible so that the entering solids will be as close as

possible to the cyclone wall where they can be collected. However, narrow inlet widths require

that the height of the inlet H be very long in order to give an inlet area required for the desired

inlet gas velocities. Therefore, a balance between narrow inlet widths and the length of the inlet

height has to be struck. Typically, low-loading cyclones (cyclones with inlet loadings less than

approximately 2 to 5 kg/m3) have height/width ratios H/Bc of between 2.5 and 3.0. For high-

loading cyclones, this inlet aspect ratio can be increased to as high as 7 or so with the correct

design. Such high inlet aspect ratios require that the cyclone barrel length increase.

42

A common cyclone inlet is a rectangular tangential inlet with a constant area along its length.

This type of inlet is satisfactory for many cyclones, especially those operating at low solids

loadings. However, a better type of inlet is one in which the inner wall of the inlet is angled

toward the outer cyclone wall at the cyclone inlet. This induces solids momentum toward the

outer wall of the cyclone. The bottom wall of the inlet is angled downward so that the area

decrease along the inlet flow path is not too rapid and acceleration is controlled. In addition, the

entire inlet can be angled slightly downward to give enhanced efficiencies. This type of inlet is

superior to the constant-area tangential inlet, especially for higher solids loadings (greater than 2

to 5 kg/m3).

Hugi and Reh [Chem. Eng. Technol. 21(9):716–719 (1998)] report that continuous acceleration

of the solids throughout the inlet is desired for improved efficiency and that the angled inlet

described above achieves this. If the momentum of the solids is sufficient and the solids are

continuously accelerating along the length of the inlet, the stable, coherent strand important for

high collection efficiencies is produced.

The best inlet for high solids loadings is the volute cyclone inlet. At high inlet loadings (above

approximately 2 to 3 kg/m3) in a tangential cyclone inlet, the gas-solids stream expands rapidly

from its minimum width at the point of contact. This rapid expansion disturbs the laminar gas

flow around the gas outlet tube and causes flow separation around the tube. At some loadings,

the inlet stream can expand to such an extent that the solids can impact the gas outlet tube. Both

effects result in lowered cyclone efficiency. However, when a volute inlet is used, the expanding

solids stream is farther from the gas outlet tube and enters at an angle so that the solids do not

induce as much flow separation or asymmetric flow around the gas outlet tube. Therefore,

cyclone efficiency is not affected to as great a degree. If a tangential cyclone is used at high

solids loadings, an extra distance between the gas outlet tube and the cyclone wall should be

designed into the cyclone to prevent the solids from impacting on the gas outlet tube. At low

solid loadings, the impacting on the gas outlet tube does not occur. Because tangential cyclone

inlets are less expensive than volute inlets, the tangential cyclone is typically utilized for low

loadings-and the volute inlet cyclone is used for high loadings.

The nature of the gas solids flow in the inlet ducting to the cyclone can affect cyclone efficiency

significantly. If the solids in the inlet salt out on the bottom and result in dune formation and the

resulting unsteady or pulsing flow, cyclone efficiency is adversely affected. To minimize the

possibility of this occurring, it is recommended that the inlet line to the cyclone operate above

the saltation velocity [Gauthier et al., in Circulating Fluidized Bed Technology III, Basu, Horio,

and Hasatani (eds.), 1990, pp. 639–644], which will prevent the solids from operating in the dune

or pulsing flow regime. If this is not possible, then the inlet line can be angled downward

(approximately 15° to 20°) to let gravity assist in the flow of the solids. Keeping the inlet line as

short as possible can also minimize any pulsing of the solids flow.

43

A cyclone will operate equally well on the suction or pressure side of a fan if the dust receiver is

airtight. Probably the greatest single cause of poor cyclone performance, however, is the leakage

of air into the dust outlet of the cyclone. A slight air leak at this point can result in a tremendous

drop in collection efficiency, particularly with fine dusts. For a cyclone operating under pressure,

air leakage at this point is objectionable primarily because of the local dust nuisance created.

For batch operation, an airtight hopper or receiver may be used. For continuous withdrawal of

collected dust, a rotary star valve, a doublelock valve, or a screw conveyor may be used, the

latter only with fine dusts. A collapsible open-ended rubber tube can be used for cyclones

operating under slight negative pressure. Mechanical trickle and flapper valves at the end of

cyclone diplegs can also be used for continuous withdrawal into fluidized beds or into the

freeboard of fluidized beds. Open diplegs simply immersed in a fluidized bed can be used in

cases where start-up losses are not excessive and are the simplest type of discharge system

returning solids to a fluidized bed (see “Fluidized-Bed Systems: Solids Discharge”). Special

pneumatic unloading devices can also be used with dusts. In any case it is essential that sufficient

unloading and receiving capacity be provided to prevent collected material from accumulating in

the cyclone.

2.11 Solids Loading

Cyclones can collect solids over a wide range of loadings. Traditionally, solids loadings have

been reported as either kilograms of solids per cubic meter of gas (kg/m3), or as kilograms of

solids per kilogram of gas (kgs/kgg). However, loading based on mass is probably not the best

way to report solids loadings for cyclones. This is so because the volume of solids processed by a

cyclone at the same mass loading can vary greatly, depending on the density of the solids. For

example, many polymers have a bulk density of approximately 400 kg/m3, and iron ore has a

bulk density of approximately 2400 kg/m3. This is a factor of 6. Therefore, a cyclone operating

with polymer would have to process 6 times the volume of solids that a cyclone operating with

iron ore would process at the same mass loading. If the cyclone operating with the polymer were

designed to operate at high loadings on a mass basis, it would probably plug. In addition, the

diplegs below the cyclone operating with the polymer may experience operational problems

because of the high volumetric loading.

At ambient conditions, cyclones have been operated at solids loadings as low as 0.02 kg/m3

(0.0125 kg/kg) and as high as 64 kg/m3 (50 kgs/kgg) or more. This is a factor of 3200. In

general, cyclone efficiency increases with increasing solids loading. This is so because at higher

loadings, very fine particles are trapped in the interstices of the larger particles, and this

entrapment increases the collection efficiency of the small particles. Even though collection

efficiencies are increased with increased loading, cyclone loss rates are also increased as loading

is increased. This is so because the cyclone efficiency increase is almost always less than the

increase in the solids loading.

Generally cone-and-disk baffles, helical guide vanes, etc., placed inside a cyclone, will have a

detrimental effect on performance. However, a few of these devices do have some merit under

special circumstances. Although an inlet vane will reduce pressure drop (and may result in

significant erosion), it generally causes a correspondingly greater reduction in collection

44

efficiency. Its use is recommended only when collection efficiency is normally so high as to be a

secondary consideration, and when it is desired to decrease the resistance of an existing cyclone

system for purposes of increased air handling capacity or when floorspace or headroom

requirements are controlling factors. If an inlet vane is used, it is advantageous to increase the

gas exit duct length inside the cyclone chamber.

A disk or cone baffle located beneath the gas outlet duct may be beneficial if air in-leakage at the

dust outlet cannot be avoided. A heavy chain suspended from the gas outlet duct has been found

beneficial to minimize dust buildup on the cyclone walls in certain circumstances.

Such a chain should be suspended from a swivel so that it is free to rotate without twisting.

Substantially all devices that have been reported to reduce pressure drop do so by reducing spiral

velocities in the cyclone chamber and consequently result in reduced collection efficiency.

At low dust loadings, the pressure in the dust receiver of a single cyclone will generally be lower

than in the gas outlet duct. Increased dust loadings will increase the pressure in the dust receiver.

Such devices as cones, disks, and inlet vanes will generally cause the pressure in the dust

receiver to exceed that in the gas outlet duct. A cyclone will operate as well in a horizontal

position as in a vertical position. However, departure from the normal vertical position results in

an increasing tendency to plug the dust outlet. If the dust outlet becomes plugged, collection

efficiency will, of course, be low. If the cyclone exit duct must be reduced to tie in with proposed

duct sizes, the transition should be made at least five diameters downstream from the cyclone

and preferably after a bend. In the event that the transition must be made closer to the cyclone, a

Greek cross should be installed in the transition piece to avoid excessive pressure drop.

2.12 Cyclone Length

As described above, the cyclone length should be great enough to contain the vortex below the

gas outlet tube. It is generally advisable to have the cyclone somewhat longer than required so

that modifications to the gas outlet tube can be made if required. Either the barrel or the cone can

be increased in length to contain the vortex. However, cyclone barrels can be made too long. If

the barrel is too long, the rotating spiral of solids along the wall can lose its momentum. When

this happens, the solids along the wall can be re-entrained into the rotating gas in the barrel, and

cyclone efficiency will be reduced.

Hoffman et al. [AIChE J. 47(11): 2452–2460 (2001)] studied the effect of cyclone length on

cyclone efficiency and showed that the efficiency of a cyclone increases with length. However,

they also found that after a certain length, cyclone efficiency decreased. They reported that

cyclone efficiency suddenly decreased after a certain cyclone length, which in their cyclone was

at a length/diameter ratio of 5.65. (Although many researchers employ this length/diameter ratio

as a correlating parameter to make the length parameter dimensionless, it is likely that it is the

actual length of the cyclone that is important.) Hoffman et al. stated that the probable reason for

the sudden decrease in cyclone efficiency was the central vortex touching and turning on the

cyclone cone. When this occurred, the efficiency collapsed, causing increased solids re-

entrainment.

45

Hoffman et al. also reported that cyclone pressure drop decreased with increasing cyclone length.

This probably occurs for the same reason that cyclone pressure drop decreases with increasing

cyclone loading. For long cyclones, the increased length of the cyclone wall results in a longer

path for the gas to travel. This creates greater resistance to the flow of the gas in the cyclone

(much as a longer pipe produces greater resistance to gas flow than a shorter pipe) that results in

reducing the tangential velocity in the cyclone and, therefore, the cyclone pressure drop.

2.13 Saltation

The concept of “saltation” by Kalen and Zenz indicates that, more than just diminishing return

with increased velocity, collection efficiency actually decreases with excess velocity. At

velocities greater than the saltation velocity, particles are not removed when they reach the

cyclone wall, but are kept in suspension as the high velocity causes the fluid boundary layer to be

very thin. A correlation for the saltation velocity was given by Koch and Licht:

𝑉𝑠 = 2.055𝐷0.067𝑉𝑖0.667 [4𝑔𝜇

(𝜌𝑝 − 𝜌𝑔)

3𝜌𝑔2

]

0.333

{(

𝑊𝐷 )

0.4

[1 − (𝑊𝐷 )]

0.333} … … … … … … … … … … 2.16

where

𝑉𝑠= saltation velocity, ft/s

D = cyclone diameter, ft

𝑉𝑖= inlet Velocity, ft/s

g = acceleration of gravity, 32.2 ft/s2

𝜇= gas viscosity, lbm/ft-sec

𝜌𝑝= particle density, lbm/ft3

𝜌𝑔= gas density, lbm/ft3

W = width of inlet opening, ft

The maximum collection efficiency occurs at Vi = 1.25Vs, which typically is between 50 and

100 ft/s.

46

CHAPTER THREE

3.0 Design and Application of Wet Scrubbers In wet scrubbing, an atomized liquid, usually water, is used to capture particulate dust or to

increase the size of aerosols. Increasing size facilitates separation of the particulate from the

carrier gas. Wet scrubbing can effectively remove fine particles in the range from 0.1μm to 20

μm. The particles may be caught first by the liquid, or first on the scrubber structure, and then

washed off by the liquid. Because most conventional scrubbers depend upon some form of

inertial collection of particulates as the primary mechanism of capture, scrubbers when used in a

conventional way have a limited capacity for controlling fine particulates. Unfortunately inertial

forces become insignificantly small as particle size decreases, and collection efficiency decreases

rapidly as particle size decreases. As a result, it becomes necessary to greatly increase the energy

input to a wet scrubber to significantly improve the efficiency of collection of fine particles.

Even with great energy inputs, wet scrubber collection efficiencies are not high with particles

less than 1.0 μm in size.

Wet scrubbers have some unique characteristics useful for fine particulate control. Since the

captured particles are trapped in a liquid, re-entrainment is avoided, and the trapped particles can

be easily removed from the collection device. Wet scrubbers can be used with high-temperature

gases where cooling of the gas is acceptable and also with potentially explosive gases. Scrubbers

are relatively inexpensive when removal of fine particulates is not critical. Also, scrubbers are

operated more easily than other sophisticated types of particulate removal equipment.

Wet scrubbers can be employed for the dual purpose of absorbing gaseous pollutants while

removing particulates. Both horizontal and vertical spray towers have been used extensively to

control gaseous emissions when particulates are present. Cyclonic spray towers may provide

slightly better particulate collection as well as higher mass transfer coefficients and more transfer

units per tower than other designs. Although there is theoretically no limit to the number of

transfer units that can be built into a vertical countercurrent packed tower or plate column, if it is

made tall enough, there are definite limits to the number of transfer units that can be designed

into a single vertical spray tower. As tower height and gas velocities are increased, more spray

particles are entrained upward from lower levels, resulting in a loss of true counter-currency.

Achievable limits have not been clearly defined in the literature, but some experimental results

have been provided. There have been reports of 5.8 transfer units in a single vertical spray tower

and 3.5 transfer units in horizontal spray chambers. Researchers have attained 7 transfer units in

a single commercial cyclonic spray tower. Theoretical discussion and a design equation for

cyclonic spray towers of the Pease-Anthony type are available. Whenever more transfer units are

required, spray towers can be used in series.

When heavy particulate loads must be handled or are of submicron size, it is common to use wet

particulate collectors that have high particle collection efficiencies along with some capability

for gas absorption. The Venturi scrubber is one of the more versatile of such devices, but it has

absorption limitations because the particles and spray liquid have parallel flow. It has been

indicated that venture scrubbers may be limited to three transfer units for gas absorption.

47

The liquid sprayed wet electrostatic precipitator is another high-efficiency particulate collector

with gas absorption capability. Limited research tests have indicated that the corona discharge

enhances mass-transfer absorption rates, but the mechanism for this has not been established.

The disadvantages of wet scrubbers include the necessity of reheating cooled scrubber effluents

for discharge up a stack. Furthermore, the water solutions may freeze in winter and become

corrosive at other times. In some cases, the resultant liquid sludge discharge may have to be

treated for disposal. It should be noted also that operating costs can become excessive due to the

high energy requirements to achieve high collection efficiencies for removal of fine particulates.

3.1 Collection Mechanisms And Efficiency

In wet scrubbers, collection mechanisms such as inertial impaction, direct interception, Brownian

diffusion, and gravity settling apply in the collection process. Most wet scrubbers will use a

combination of these mechanisms, therefore, it is difficult to classify a scrubber as predominately

using one particular type of collection mechanism. However, inertial impaction and direct

interception play major roles in most wet scrubbers. Thus, in order to capture finer particles

efficiently, greater energy must be expended on the gas. This energy may be expended primarily

in the gas pressure drop or in atomization of large quantities of water. Efficiency of collection

may be unexpectedly enhanced in a wet scrubber through methods that cause particle growth.

Particle growth can be brought about by vapor condensation, high turbulence, or thermal forces

in the confines of the narrow passages in the scrubber structure. Condensation, the most common

growth mechanism, occurs when a hot gas is cooled or compressed. The condensation will occur

preferentially on existing particles rather than producing new nuclei. Thus, the dust particles will

grow larger and will be more easily collected. When hydrophobic dust particles must be

collected, there is evidence that the addition of small quantities of nonfoaming surfactants may

enhance collection. The older literature is contradictory on this point, but careful experiments by

Hesketh and others indicate enhancement can definitely occur.

3.2 Collection Mechanisms And Particle Size

When a gas stream containing particulates flows around a small object such as a water droplet or

a sheet of water, the inertia of the particles causes them to move toward the object where some of

them will be collected. This phenomenon is known as inertial impaction, which customarily

describes the effects of small-scale changes in flow direction. Because inertial impaction is

effective on particles as small as a few tenths of a micrometer in diameter, it is the most

important collection mechanism for wet scrubbers. Since this mechanism depends upon the

inertia of the particles, both their size and density are important in determining the efficiency

with which they will be collected. All important particle properties may be lumped into one

parameter, the aerodynamic impaction diameter which can be calculated from the actual particle

diameter by the following relationship:

𝑑𝑎𝑝 = 𝑑𝑝(𝜌𝑝𝐶′)1

2⁄… … … … … … … … … … … … 2.17

48

where

𝑑𝑎𝑝= aerodynamic impaction diameter in m-gm/cm3

𝑑𝑝= physical diameter in m

𝜌𝑝= density of particle in gms/cm3

𝐶′= Cunningham’s correction factor

By a fortunate circumstance, most methods for measuring particle size determine the

aerodynamic impaction diameter. The Cunningham correction factor is given by the following

formulas:

𝐶′ = 1 +2𝜆

𝑑𝑝[1.257 + 0.400𝑒𝑥𝑝 − (

0.55𝑑𝑝

𝜆)] … … … … … … … … … .2.18

𝜆 =𝜇

0.499𝜌𝑔√8𝑅𝑇/𝜋𝑀𝑊

Where

𝜆= mean free path of the gas in m

𝑑𝑝= diameter of particle in m

𝜇= gas viscosity in N-s/m or kg/m-s

𝑀𝑊= mean molecular weight of the gas

𝜌𝑔= gas density in kg/m3

𝑅= universal gas constant (8.3144 J/kg-mol-K)

𝑇= gas temperature in K

For air at room temperature and pressure, Equation 22-4 is a good approximation of C′:

𝐶′ = 1.0 +0.16

𝑑𝑝

49

Knowing the value of the mean free path of molecules at a given temperature and pressure, the

mean free path at other conditions can be calculated from Equation 22.5:

𝜆 = 𝜆𝜊 (𝜇

𝜇𝜊) (

𝑇

𝑇0)

12⁄

(𝑃𝑜

𝑃)

Where

𝜆𝑜= 0.0653 μm for air at 23°C and 1.0 atm

𝜇𝜊 , 𝑇𝜊 , 𝑃𝜊=viscosity, temperature, and pressure, respectively, at the same conditions for which is

known

3.3 Selection And Design Of Scrubbers

Calvert and co-workers have prepared an extensive report of wet scrubbers from both theoretical

considerations and literature data. In considering the types of scrubbers to use for a particular

application, the designer must have in mind the required collection efficiency for a particular size

emission. The data of Table 3.1 can be used as a rough guide for initial consideration of

adequacy of different devices.

3.4 Devices For Wet Scrubbing

The following material is a compilation of facts and figures for typical wet scrubbers. Table 3.1

serves as a guideline to the general operational characteristics of various types of devices.

50

Fig. 3.1 Packed Tower.

51

Table 3.1

Table 3.2

52

3.5 A Model For Counter-Current Spray Scrubbers

Drops are formed by atomizer nozzles and then sprayed into the gas stream. In the counter-

current tower, drops settle vertically against the rising gas stream which is carrying the particles.

Atomization provides a wide variety of droplet size. It is customary to take the Sauter mean drop

diameter equivalent to the volume/surface area ratio and defined by the following equation to

represent all the droplets.

𝑑𝑝 =58600

𝑉𝑔(

𝜎𝐿

𝜌𝐿)

0.5

+ 597 [𝜇𝐿

(𝜎𝐿𝜌𝐿)0.5]

0.45

[1000𝑄𝐿

𝑄𝐺]

1.5

… … … … … … … … 3.1

Where

𝑑𝑝= Sauter mean droplet diameter, 𝜇𝑚

𝜌𝐿= Density of the liquid, gm/cm3

𝜎𝐿= Liquid surface tension, dyne/cm

𝜇𝐿= Liquid viscosity, poise

𝑉𝑔= Superficial gas velocity, cm/s

𝑄𝐿= Volumetric liquid flow rate, M3/s

𝑄𝐺= Volumetric gas flow rate, M3/s

Inertial impaction is depicted in the figure, primary capture mechanism. In this case of inertial

impaction, a particle is carried along by the gas stream. Approaching the collecting body which

is a water droplet in the case of a spray scrubber, the particles tend to follow the streamlines.

However, for many particles, their inertia will result in the particle separating from the gas

stream and striking the water droplet. The result is for the water droplet to collect the particle.

The separation number in, 𝑁𝑠𝑖 , is the same as the inertial impaction parameter, Kp, defined by

the following equation:

𝑁𝑠𝑖 = 𝐾𝜌 = 𝐾𝑚𝜌𝑝𝐷𝑝2

𝑉𝑝

18𝜇𝑔𝐷𝑏=

𝐶′𝜌𝑝𝑑𝑝2𝑉𝑝

18𝜇𝑔𝑑𝑑=

𝑑𝑎𝑝2 𝑉𝑝

18𝜇𝑔𝑑𝑑… … … … … … … 3.2

Where

𝐶′= Cunningham correction factor

53

𝜌𝑝= particle density, gm/cm3

𝑑𝑝= physical particle diameter, cm

𝑉𝑝= particle velocity, cm/s

𝜇𝑔= gas viscosity, poise

The curves apply for conditions in which Stokes’ Law holds for the motion of the particle.

Equation 3.3 is an approximate equation for a single-droplet target efficiency,

𝑑

= (𝐾𝑝

𝐾𝑝 + 0.35)

2

… … … … … … … … … … 3.3

Fig 3.2 Bubble cap trays

54

3.6 A Model For Venturi Scrubbers

Calvert et al.3 have developed a model similar to Equation 3.4 for several different types of

scrubbers. Following is a model for Venturi scrubbers.

𝑃𝑡 = 𝑒𝑥𝑝 [𝑄𝐿𝑉𝐺𝜌𝑝𝑑𝑝

55𝑄𝐺𝜇𝐺] 𝐵 … … … … … … … … … 3.4

𝐵 = [(−0.7 − 𝐾𝑝𝑓 + 1.4𝐼𝑛 (𝐾𝑝𝑓 + 0.7

0.7) +

0.49

0.7 + 𝐾𝑝𝑓)

1

𝐾𝑝]

Where

𝐾𝑝= inertial impaction parameter at throat entrance

f= empirical factor

(f = 0.25 for hydrophobic particles, f = 0.50 for hydrophilic particles)

This model works well for f = 0.50 for a variety of large-scale venturi scrubbers and other spray

scrubbers where the spray is atomized by the gas.

55

Fig. 3.3 Optimum droplet size for collection by inertial impaction when droplets are moving in

the gravitational field of the earth in a spray tower. (With permission from Stairmand, C. J.,

Transactions of the Institution of Chemical Engineers, 28, 130, 1950.)

56

Fig. 3.4 Venturi scrubber

• Gas accelerated at throat

• Atomized water droplets added at throat as a spray or jet, collect particles

• Can be combined with a cyclonic collector to disengage water droplets from air stream

• Has a large pressure drop

57

Fig. 3.5 Venturi jet scrubber:

Used for fume scrubbing

High pressure water atomized from a jet nozzle into a throat of a Venturi which induces

the flow of the gas to be scrubbed

58

Fig. 3.6 Relative velocities of a water droplet and a particle.

Fig. 3.7 Mass in – Mass out – Mass collected = Accumulation

𝐶𝑄𝐺 − (𝐶 + 𝑑𝐶𝑖)𝑄𝐺 − 𝑀𝑐 = 0

59

CHAPTER FOUR

4.0 Summary of the process

4.1.1 Reactor with cyclone Separator There is a fluidized bed inside the reactor. The fluidized bed consists of particles of iron oxide

which are kept in a turbulent state by the combustion gas. In this fluidized bed, the concentrated

venturi liquid is pumped through a titanium lance. As result of the high temperature of 850°C in

the fluidized bed the 𝐹𝑒𝐶𝑙2which has been brought is split up into 𝐹𝑒2𝑂3 and 𝐻𝐶𝑙. Thus the

reverse of pickling reaction takes place in the reactor.

2𝐹𝑒𝐶𝑙2 + 2𝐻2𝑂 +1

2𝑂2 → 𝐹𝑒2𝑂3 + 4𝐻𝐶𝑙

About 95% of this splitting up takes place on the surface of the oxide particles present, and only

approximately 5% in the gap. While the oxide reacting on the surface of the oxide particles

contributes to the growth of oxide particles, the iron formed in the gas phase generates fine iron

oxide dust which is carried out of the reactor with the combustion gases. In the cyclone, the dust

is again removed from the gas flow and send back more to the fluidized bed by means of dust

recirculation pendulum flaps.

The iron oxide formed in the reactor is constantly drawn off by means of vibration channel and

conveyed into the oxide tank via spiral conveyor.

Constant drawing off the iron oxide keeps the amount of the material in checks and thus the

height of the fluidized bed remains constant.

4.1.2 Venturi The reaction gases drawn out of the reactor at a temperature of 850°C are cooled in the venturi

down to approximately 100°C. This cooling process is achieved by the evaporation of water out

of the venturi liquid. At the same time, the extremely fine particles of dust in the waste gas are

washed out. The venture liquid needed for cooling is taken in from separator by means of venturi

pump and pumped to venturi. The venturi liquid flow through narrow position of the venture

together with the gas flow, thereby cooling the hot waste gases. In separator the venture liquid is

separated from the gas flow while the gas from separator thus goes into absorber. The venturi

liquid is again pumped to the narrow position of the venturi. As a result of the evaporation of

water during the cooling process, the venturi liquid becomes very concentrated. Adding rinse

water sets a density of approximately 1.48kgldm3 in the venturi circulation system. A split

stream is taken out of this venturi circulation system and conveyed to the reactor. The waste acid

stacked in the storage station is pumped by means of pumps into receiver vessel. This receiver

vessel communicates with the separator. Any reduction in the level of the separator as a result of

the consumption of venturi liquid in reactor is thus automatically equalized by the addition of

waste acid. Any excess waste acid conveyed to the receiver vent flows back into the approximate

acid storage tank.

60

4.1.3 Absorption tower including scrubbing The hydrochloric acid present in the waste gas flow is scrubbed out in the absorber in stages. To

accomplish this, the upper part of the absorber is impinged with rinse water which then flows

through the absorber from top to bottom against the flow of the gas. In doing so, the hydrochloric

and carried in the gas flow is scrubbed out. This recycled regenerated acid flows from the

bottommost part of the absorber into the acid stage station.

The waste gases coming out of the absorber are then impinged in further scrubbing stage with

freshwater thus washing out the traces of hydrochloric acid it has been carrying.

4.1.4 Fan The waste gas flow now free of hydrochloric acid is blown through fan via fume stack and from

there via drop separator into the open air. The condensates arising from cooling are separated out

by the built-in drop separator and reprocessed in the regeneration Unit.

Fig. 3.7 Fluidized-bed process for regeneration of hydrochloric acid pickling solutions

a) Separating tank; b) Venturi scrubber; c) Reactor; d) Cyclone; e) Absorber; f) Scrubbing stage;

g) Off-gas blower; h) Stack; i) Mist collector

61

CHAPTER FIVE

5.0 Safety Safety generally means being safe or freedom from danger or risk. It is an area

of safety engineering and public health that deals with the protection of workers' health, through

control of the work environment to reduce or eliminate hazards.

Chemical process safety refers to the application of technology and management practices;

To prevent accidents in plants

To reduce the potential for accidents.

Work place hazards can generally be grouped into:-

Mechanical hazards

Chemical hazards

Physical hazards

Biological hazards

Psycho-Social hazards

Unsafe working conditions and production may lead to industrial accidents and can result in:

Temporary or permanent injuries.

Fatalities.

Loss of future productivity by training new personnel

Loss of valuable work hours

Cost implications due to compensation, medical fees, insurance etc.

The Occupational Safety and Health Act (OSHA, 2007) stipulates the guidelines for ensuring

favorable and bearable working conditions in Kenya. The Act establishes occupational, safety

and health standards to be adhered to in places of work.

Major provisions of this Act include:

Inspection of work places

Maintenance of accurate records of employees

Maintenance of accurate records of any toxic or harmful material whose levels exceed

those prescribed by an applicable standard.

Provides for the rights of employees to be informed of any violations by employers cited

by inspectors of work places.

The provisions of this Act are enforced by inspection officers who carry out inspections for work

places.

Potential hazards in the aloe vera products processing plant include the following:

62

Electrical components malfunction, electrocution and risk of electrical fires

Failure of instruments and process equipment

Risk of fires

Odour

Leakages from equipment causing spills

Slips, trips and falls

Corrosion

Spillages and wastes

Spillages are to be taken note off, contained and collected.

When opening valves, the risk of spillages should be considered.

Any spillage is to be reported to the Health Safety & Environment Department immediately

while possible control and containment of the spill is carried out.

First Aid

All emergency handling team members are trained in first aid.

First Aid boxes are available in all departments

Safety Signs and Instructions

To create the Health, Safety and Environment awareness at all levels of management and to

communicate the specific risk / hazards, at relevant locations Health, Safety and Environment

Signs, Warning Labels, Instruction to be displayed.

Labeling

All equipment and chemicals should be classified according to their risk and labeled accordingly.

Lighting

Illumination sufficient for maintaining safe working conditions are provided where ever

personnel is required to work or pass , including in passageways, stairways and landings.

No work area has illumination of less than 50 lux or otherwise specified.

Chemical Safety

All employees know the hazards of the chemicals they may deal or work with.

All employees make sure that they have a copy of the MSDS (Material Safety and Data Sheets),

read and understand it.

All employees use appropriate personal protective equipment while handling the chemicals.

All chemical containers and bottles are labeled correctly.

Store the chemical as per the incompatibility.

63

Obey warning and danger signs.

Try to stop the spillage, if any, and report the same to the shift in-charge and safety department

simultaneously through your colleagues earliest possible.

Spacing

There is adequate spacing between equipment and pipelines.

Emergency Contact Numbers

List of key personnel is available at emergency control center, main gate.

Safety Relief Vents, Interlocks and trip systems

For pressure vessels, relief vents are installed. Interlocks and trip systems are installed in case of

failure of the instruments.

Control valves

There are remote control valves to isolate equipment and areas of the plant in case of emergency.

Inspection of equipment

There is regular inspection of equipment such as storage vessels and pipelines

Training of workers

Specialized training of workers on chemical safety, personal protection equipment, fire

prevention and protection techniques, accidents prevention and safety management can

contribute significantly to risk management in the plant.

Accident Documentation

All accidents should be reported to the relevant section managers and eventually to the safety

manager for effective investigation.

Laboratory Safety

All chemicals and regent bottles are clearly labeled. They are stored in their appropriate places.

Volatile, combustible, flammable chemicals must are stored away from direct flame and other

sources of heat.

Exhaust fans and blowers must be kept continuously on to drive out any fumes or vapours if

present.

While handling toxic and corrosive chemicals, proper personal protective equipment.

There are energy lines and taps in laboratory. Get them inspected periodically and see that leaks

are detected and rectified quickly.

64

Equipment Safety

All new equipment are procured as per user's safety requirement and designed accordingly.

The equipment should be qualified for installation, operation and performance.

The Persons working on the equipment should undergo the operation training.

Employee Requirements

It shall be a requirement for each worker to have the following PPEs (personal protective

Equipment) within the factory premises.

Masks: for protection against solvent vapour or fumes

Protective clothing: these include overalls and dust coats to be replaced weekly for cleaning

purposes.

Protective shoes: special shoes to prevent any damage due to falling objects and also to prevent

falling or sliding.

Ear plugs: Prevent damage caused by vibration or noise produced by equipment.

Safety helmet: mandatory inside the factory to protect the head from metal objects.

Management and Safety

Management at the forefront in enforcing safe engineering practices.

Organizing safety trainings and safety promotional campaigns.

Enacting rules and policies to be adhered to concerning safety, for which there are repercussions

for violations committed.

Management should ensure that they get a safety report periodically.

There should be an independent inspector doing regular safety audits.

Ensuring there is proper and regular inspection and maintenance of equipment.

65

Key

ADB-Administration Block

WB-Weighbridge

MG-Main Gate

FG-Factory Gate

SITE PLAN AND LOCATION

AC

ID R

EGEN

ERA

TION

PLA

NT

PIC

KLIN

G LIN

E

TENSIO

N LEV

ELING

LINE C

OLO

R C

OA

TING

LINE

CO

NTIN

UO

US G

ALV

AN

IZING

LINE

AD

B

AD

B

WB R

oad

Wall Fen

ce

FG

AC

ID R

EGEN

ERA

TION

PLA

NT

PIC

KLIN

G LIN

E

TENSIO

N LEV

ELING

LINE

CO

NTIN

UO

US G

ALV

AN

IZING

LINE

CO

LOU

R C

OA

TING

LINE

66

References Don W. and Robert H “Perry’s Chemical Engineers’ Handbook, 8th edition.”

Karl B. and Charles A “Pollution Control Technology Handbook”

ASME steel books volume 1-23

Ullmann-'s-Encyclopedia-of-Industrial-Chemistry-(Wiley,-2007)

Robert T. “Mass Transfer”

Company Process Manual

www.steelitems.com