credit seminar drying

8/7/2019 Credit Seminar Drying

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CREDIT SEMINAR

ON

DRYING

GURU JAMBESHWARUNIVERSITY

OF

SCIENCE &TECHNOLOGY

SUBMITTED TO SUBMITTED BY

DR.B.S.KHATKAR JYOTI

FOOD DEPTT. 10081008

G.J.U.S&T M.TECH (1ST

YR.)

HISSAR FOOD ENGG.


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AbstractDrying is one of the most common and oldest method used for food preservation. It is used since

antiquity. Benefits include increased shelf life and reduction of bulk. Fundamentals of drying

include the drying curve and moisture content. Most commonly used dryers are rotary dryers,

spray dryers, drum dryers, fluidized dryer, tunnel dryers, hot air dryers etc. selection criteria of

dryers depend on the particular application.


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CONTENTS:-

1. Introduction

2 fundamentals of drying

2.1 The Drying Curve

3.) Classification of dryer

4) Different types of dryers:-

4.1 Rotary Dryers:

4.2 Pneumatic/Flash Dryer:-

4.3 Spray Dryers:

4.4) Drum Dryers:-4.5) Fluidised Bed Dryers

4.6) Tunnel Dryers:-

4.7) Band Dryers:-

4.8) Hot Air Dryer- Stenter

4.9) Infrared Dryers

4.10) Tray Dryers

4.11) Freeze Dryers

4.12) Vacuum Dryers:-

4.13) Microwave (MW) and Radio Frequency (RF) Drying

5.) Selection of dryers

6) Reference


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1. INTROUCTION[7]

Drying is perhaps the oldest, most common and most diverse of chemical engineering unit

operations. Over four hundred types of dryers have been reported in the literature while over

one hundred distinct types are commonly available. Energy consumption in drying ranges

from a low value of under five percent for the chemical process industries to thirty five percent

for the papermaking operations.

Drying occurs by effecting vaporization of the liquid by supplying heat to the wet feedstock.

Heat may be supplied by convection (direct dryers), by conduction (contact or indirect dryers),

radiation or volumetrically by placing the wet material in a microwave or radio frequency

electromagnetic field. Over 85 percent of industrial dryers are of the convective type with hot airor direct combustion gases as the drying medium. Over 99 percent of the applications involve

removal of water.

This is one of the most energy-intensive unit operations due to the high latent heat of

vaporization and the inherent inefficiency of using hot air as the (most common) drying medium.

This manual describes different types of dryers, their industrial applications and energy

conservation opportunities. Although here we will focus only on the dryer, it is very important to

note that in practice one must consider a drying system which includes pre-drying stages (e.g.,

mechanical dewatering, evaporation, pre-conditioning of feed by solids back mixing, dilution or

pelletization and feeding) as well as the post-drying stages of exhaust gas cleaning, product

collection, partial recirculation of exhausts, cooling of product, coating of product,

agglomeration, etc. Energy cost reduction measures are also generally visible in pre and post

drying operations and supporting equipments like blowers and pumps as well.


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2 FUNDAMENTALS OF DRYING:-[9][10]

2.1 The Drying Curve[8]

For each and every product, there is a representative curve that describes the drying

characteristics for that product at specific temperature, velocity and pressure conditions. This

curve is referred to as the drying curve for a specific product. Fig below shows a typical drying

curve. Variations in the curve will occur principally in rate relative to carrier velocity and

temperature.

Drying Curve

Drying occurs in three different periods, or phases, which can be clearly defined.

The first phase, orinitial period, is where sensible heat is transferred to the product and the

contained moisture. This is the heating up of the product from the inlet condition to the process


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Calculation of the quantity of water to be evaporated is explained below with a sample

calculation.

If the throughput of the dryer is 60 kg of wet product per hour, drying it from 55% moisture to

10% moisture, the heat requirement is:

60 kg of wet product contains 60 x 0.55 kg water = 33 kg moisture and 60 x (1 - 0.55) = 27 kg

bone-dry product.

As the final product contains 10% moisture, the moisture in the product is 27/9 = 3 kg and so

moisture removed = (33 - 3) = 30 kg

Latent heat of evaporation = 2257 kJ kg-1(at 100 C so heat necessary to supply = 30 x 2257 =

6.8 x l04 kJ

2.3 Estimation of drying time

The rate of drying is determined for a sample of substance by suspending it in a cabinet or

duct, in a stream of air from a balance. The weight of the drying sample can then be measured

as a function of time from wet product to bone dry product. The curve of moisture content as a

function of time, similar to fig 2.1, can be plotted. While different solids and different conditions

of drying often give rise to curves of very different shapes in the falling rate period, the curve

shown above occurs frequently.

During the above measurements, the following conditions are to be followed.1. The sample should be subjected to similar conditions of radiant heat transfer

2. Air should have the same temperature, humidity & velocity

Electronic moisture balances with online data collection/plotting can be used to establish drying

curves of materials.


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3.) CLASSIFICATION OF DRYERS[1]

There are numerous schemes used to classify dryers (Mujumdar, 1995; van't Land,

1991). Table 1 lists the criteria and typical dryer types. Types marked with an asterisk (*)

are among the most common in practice.

Criterion types

Mode of operation y BatchyContinuous*

Heat input-type yConvection*, conduction, radiation,electromagnetic fields, combination of heat

transfer modes

yIntermittent or continuous*

yAdiabatic or non-adiabatic

State of material in dryer yStationaryyMoving, agitated, dispersed

Operating pressure yVacuum*yAtmospheric

Drying medium (convection) yAir*ySuperheated steam

yFlue gases

Drying temperature yBelow boiling temperature*yAbove boiling temperatureyBelow freezing point

Relative motion betweendrying medium and drying

solids

yCo-current

yCounter-current

yMixed flow

Number of stages ySingle*

yMulti-stage

Residence time yShort (< 1 minute)

yMedium (1 60 minutes)

yLong (> 60 minutes)

Classification of dryers on the basis of the mode of thermal energy input is perhaps the most

useful since it allows one to identify some key features of each class of dryers.


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Direct dryers also known as convective dryers are by far the most common. About 85

percent of industrial dryers are estimated to be of this type despite their relatively low thermal

efficiency caused by the difficulty in recovering the latent heat of vaporization contained in the

dryer exhaust in a cost-effective manner. Hot air produced by indirect heating or direct firing is

the most common drying medium although for some special applications superheated steam has

recently been shown to yield higher efficiency and often higher product quality. Flue gases may

be used when the product is not heatsensitive or affected by the presence of products of

combustion. In direct dryers, the drying medium contacts the material to be dried directly and

supplies the heat required for drying by convection; the evaporated moisture is carried away by

the same drying medium.

Drying gas temperatures may range from 50 C to 400 C depending on the material.

Dehumidified air may be needed when drying highly heat-sensitive materials. An inert gas such

as Nitrogen may be needed when drying explosive or flammable solids or when an organic

solvent is to be removed. Solvents must be recovered from the exhaust by condensation so that

the inert (with some solvent vapor) can be reheated and returned to the dryer. Because of the

need to handle large volumes of gas, gas cleaning and product recovery (for particulate solids)

becomes a major part of the drying plant. Higher gas temperatures yield better thermal

efficiencies subject to product quality constraints.

Indirect dryers involve supplying of heat to the drying material without direct contact with

the heat transfer medium, i.e., heat is transferred from the heat transfer medium (steam, hot gas,

thermal fluids, etc.) to the wet solid by conduction. Since no gas flow is presented on the wet

solid side it is necessary to either apply vacuum or use gentle gas flow to remove the evaporated

moisture so that the dryer chamber is not saturated with vapor. Heat transfer surfaces may range

in temperature from -40 C (as in freeze drying) to about 300 C in the case of indirect dryers

heated by direct combustion products such as waste sludges. In vacuum operation, there is no

danger of fire or explosion. Vacuum operation also eases recovery of solvents by direct

condensation thus alleviating serious environmental problem. Dust recovery is obviously simpler

so that such dryers are especially suited for drying of toxic, dusty products, which must not be

entrained in gases. Furthermore, vacuum operation lowers the boiling point of the liquid being

removed; this allows drying of heat-sensitive solids at relatively fast rates.


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Heat may also be supplied by radiation (using electric or natural gas-fired radiators) or

volumetrically by placing the wet solid in dielectric fields in the microwave or radio frequency

range. Since radiant heat flux can be adjusted locally over a wide range it is possible to obtain

high drying rates for surface-wet materials. Convection (gas flow) or vacuum operation is needed

to remove the evaporated moisture. Radiant dryers have found important applications in some

niche markets, e.g., drying of coated papers or printed sheets. However, the most popular

applications involve use of combined convection and radiation. It is often useful to boost the

drying capacity of an existing convective dryer for sheets such as paper.

Microwave dryers are expensive both in terms of the capital and operating (energy) costs. Only

about 50 percent of line power is converted into the electromagnetic field and only a part of it is

actually absorbed by the drying solid. They have found limited applications to date. However,

they do seem to have special advantages in terms of product quality when handling heat-sensitive

materials. They are worth considering as devices to speed up drying in the tail end of the falling

rate period. Similarly, RF dryers have limited industrial applicability. They have found some

niche markets, e.g., drying of thick lumber and coated papers. Both microwave and RF dryers

must be used in conjunction with convection or under vacuum to remove the evaporated

moisture. Standalone dielectric dryers are unlikely to be cost-effective except for high value

products in the next decade. See Schiffmann (1995) for detailed discussion of dielectric dryers.

It is possible, indeed desirable in some cases, to use combined heat transfer modes, e.g.,

convection and conduction, convection and radiation, convection and dielectric fields, to reduce

the need for increased gas flow which results in lower thermal efficiencies. Use of such

combinations increases the capital costs but these may be offset by reduced energy costs and

enhanced product quality. No generalization can be made a priori without careful tests and

economic evaluation. Finally, the heat input may be steady (continuous) or time varying. Also,

different heat transfer modes may be deployed simultaneously or consecutively depending on

individual application. In view of the significant increase in the number of design and

operational parameters it is desirable to select the optimal operating conditions via a

mathematical model. In batch drying intermittent energy input has great potential for reducing

energy consumption and for improving quality of heat-sensitive products.


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4) Different types of dryers:-[2][3][5]

4.1 Rotary Dryers:

The cascading rotary dryer is a continuously operated direct contact dryer consisting of a slowly

revolving cylindrical shell that is typically inclined to the horizontal a few degrees to aid the

transportation of the wet feedstock which is introduced into the drum at the upper end and the

dried product withdrawn at the lower end . To increase the retention time of very fine and light

materials in the dryer (e.g., cheese granules), in rare cases, it may be advantageous to incline the

cylinder with the product end at a higher elevation.

Figure

Figure A cascading rotary dryer

The drying medium (hot air, combustion gases, flue gases, etc.) flows axially through the drum

either concurrently with the feedstock or countercurrently. The latter mode is preferred when the

material is not heat-sensitive and needs to be dried to very low moisture content levels. The

concurrent mode is preferred for heat-sensitive materials and for higher drying rates in general.

In this type of dryer, a wide assortment of granular products of diverse shapes, sizes and size

distributions can be processed by proper design of the internal flights and lifters. Special

internals are needed for materials that tend to form large lumps that must be broken to avoid

major problems in the later stages of drying. The lifters lift the material to the top of the drum


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where it showers down in the form of cascades. The major heat and mass transfer processes are

accomplished during the flight of the particles from the top to the bottom of the drum by gravity.

The drying medium is in cross-flow with respect to the cascading particles. Clearly, particles

with terminal velocities below the cross-flow gas velocity will be entrained and collected in the

gas cleaning equipment. The cascading action may cause severe attrition of fragile materials,

especially when the drum diameter is large.

Although numerous attempts have been reported which permit calculation of particle residence

times in rotary dryers, the design of commercial units is still based on pilot tests and empirical

rules (often proprietary) based on prior experience with similar material and similar design of

rotary dryer hardware. The drying process is essentially intermittent. It is intense during the

cascading motion under gravity when the particles contact the cross-flowing hot gas stream.

When the particles settle on the drum wall as a bed and carried upward by the revolving shell,

there is a soaking or tempering period when the temperature and moisture content fields in

the particles tend to equalize before the particles are exposed to the convective drying condition

again.

Rotary dryers can be designed for drying time from 10 to 60 minutes. If large retention time is

needed for removing the internal moisture in the falling rate period, it is possible to use a smaller

shell diameter at the wet end for surface moisture removal with low holdup of material in the

drum and then increase the shell diameter at the dry end to allow longer retention time with

larger holdup. In some designs, it is possible to use a pneumatic conveyor to carry the product

out of the dryer.

Thermal efficiencies of rotary dryers vary widely in the range of 30-60%. For good efficiency,

the product holdup (typically 10-15 percent of volume) should be such as to cover the flights or

lifters fully. The lifters should be carefully designed to ensure good cascading action, avoiding

large clusters of material falling from the flights. Length-todiameter ratios of 4 to 10 are common

in industrial practice.

Rotary dryers can be operated at very high temperatures to accomplish various reactions in

addition to or instead of simple drying; these units are referred to as kilns. It is necessary to line

the shell of rotary kilns with suitable refractory materials.

In order to enhance the drying rates in the rotary dryer without raising the gas temperature or gas

flowrate excessively, it is possible to introduce steam-heated tubes or coils within the shell.


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Aside from providing additional energy for drying, such internals can also help with

redistribution or delumping of the material. Of course, it is possible to use internal heaters only if

the material does not stick to the walls of the internals.

A new variant of the classical rotary dryer uses a central axial header for the drying gas that is

injected at discreet intervals along the length of the rotating shell directly into the kilning bed

of particles. This type of flow distribution is more effective for heat and mass transfer and results

in volumetric heat and mass transfer coefficients up to two times larger than those in the

cascading dryer. However, this design is not suited for all types of materials.

Rotary dryers are very flexible, very versatile and are especially suited for high production rate

demands. On the negative side, they are typically less efficient, demand high capital costs and

significant maintenance costs depending on the material being dried. They are not recommended

for fragile materials and for low production rates. Finally, it is useful to note that while most of

the continuous rotary dryers are operated under near atmospheric pressure, the term vacuum

rotary dryer refers to an entirely different class of dryers. It is, in fact, an indirect type batch

dryer because of the difficulty of maintaining vacuum under continuous feeding and discharge

conditions. Here, the horizontal cylindrical shell is stationary while a set of variously designed

agitator blades revolves on a central shaft to agitate the material contained in the dryer shell.

Heat is supplied by heating the shell jacket using condensing steam or a thermal fluid. In larger

units, the central agitator shaft and the blades may also be heated. The agitator may be a single-

or double-spiral. The outer blades are set close to the wall and may have a scraper attached to

keep the material from building up on the walls and deteriorating the thermal performance of the

unit. This type of dryer is useful for handling heat-sensitive materials, which dry at lower

temperatures because of the vacuum conditions.

4.2 Pneumatic/Flash Dryer:-

The pneumatic or flash dryer is used with products that dry rapidly owing to the easy removal

of free moisture or where any required diffusion to the surface occurs readily. Drying takes

place in a matter of seconds. Wet material is mixed with a stream of heated air (or other gas),

which conveys it through a drying duct where high heat and mass transfer rates rapidly dry the

product. Applications include the drying of filter cakes, crystals, granules, pastes, sludges and

Slurries; in fact almost any material where a powdered product is required. Salient features are


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as follows.

y Particulate matter can be dispersed, entrained and pneumatically conveyed in air. If

this air is hot, material is dried.

y Pre-forming or mixing with dried material may be needed feed the moist material

y The dried product is separated in a cyclone. This is followed by separation in further

cyclones, fabric sleeve filters or wet scrubbers.

y This is suitable for rapidly drying heat sensitive materials. Sticky, greasy material or that

which may cause attrition (dust generation) is not suitable.

Figure :Pneumatic/Flash Dryer

4.3 Spray Dryers:

Over 20,000 spray dryers are presently in use commercially to dry products from agro-

chemicals, biotechnological, fine and heavy chemicals, dairy products, dyestuffs, mineral

concentrates to pharmaceuticals in capacities ranging from a few kg per h to 50 tons per h

evaporation capacity. Liquid feedstocks, such as solutions, suspensions or emulsions can be

converted into powder, granular or agglomerate form in one step operation in spray dryer. Figure

7 gives a process schematic for a spray dryer plant.

Atomized feedstock in the form of a spray is contacted with hot gas in a suitably designed drying

chamber. Proper selection and design of the atomizer is vital to the operation of the spray dryer

as it is affected by the type of feed (viscosity), abrasive property of the feed, feed rate, desired


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particle size and size distribution as well as the design of the chamber geometries and mode of

flow, e.g., concurrent, countercurrent or mixed flow .

A process schematic of a spray dryer plant


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Figure Concurrent, countercurrent and mixed flow spray dryer chambers

Here, we will summarize the key aspects of spray dryers in a tabular form. It must be noted that

design of spray dryers depends heavily on pilot scale testing. It is impossible to scale-up quality

criteria for spray dryers. Fortunately, in most cases, it is found that the larger scale dryer

provides better quality product than the one obtained in smaller scale pilot tests. Aside from

drying rate and quality tests, it is also important to check potential of deposits in the drying

chamber as this may lead to fire and explosion hazards. Essentially, three major types of

atomizers are used in practice. They are:

(a)Rotary wheel (or disk) atomizers,

(b) Pressure nozzle and

(c) Two-fluid nozzle.Figure below shows some typical atomizer designs. Ultrasonic and electrostatic atomizers can

also be used for special applications to produce monodisperse sprays but they are very expensive

and low capacity units. Most spray dryers operate at slight negative pressure. New designs may

use low pressure chambers to enhance drying rates at lower temperatures to dry highly heat-

sensitive products.


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Figure

Figure Typical spray dryer atomizer designs

The design of the spray drying chamber depends on the needed residence time (see Table 1) .

The mode of flow, i.e., concurrent, counter-current, mixed flow, depends on the desired

characteristics of the product as summarized in Table 2.


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Table 1Residence time requirements for spray drying of various products

Residence time in chamber Recommended for

Short (10-20 s) Fine, non-heat sensitive products; surfacemoisture removal, non-hygroscopic

Medium (20-35 s) Fine-to-coarse sprays (dmean = 180_m);drying to low final moisture

Long (> 35 s) Large powder (200-300_m); low finalmoisture, low temperature operation for

heat-sensitive products

Table 2Selection of mode of flow in spray drying chamber based on desired powder

Characteristics

Dryer design flow type Characteristics

Concurrent Low product temperature

Mixed flow with integratedfluidized beds

To produce agglomerated powder

Mixed flow (fountain type) For coarse sprays in small chambers; productno heat-sensitive

Counter-current flow Products which withstand high temperatures;

coarse particles; high bulk density powders

Since the choice of the atomizer is very crucial it is important to note the key advantages and

limitations of the wheel and pressure nozzles, which are most common in practice. Although

both types may be used for the same feedstocks, the product properties (bulk density, porosity,

size, etc.) will be different.

a. Rotary wheels (or disk) atomizers

Advantages:

y Handle large feed rates with single wheel


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y Suited for abrasive feeds with proper design

y Negligible clogging tendency

y Change of rpm controls particle size

y More flexible capacity

Limitations:

Higher energy consumption compared to pressure nozzles

More expensive

Broad radical spray requires large drying chamber (cylindrical-conical

type)

b. Pressure nozzles

Advantages:

Simple, compact, cheap

No moving parts

Low energy consumption

Limitations:

Low capacity (flow rates)

High tendency to clog

Erosion can change spray characteristics


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Figure Spray dryer schematics. (a) Wheel atomizer; (b) Single or two-fluid nozzle

Above Figure shows schematics of two spray dryers, one fitted with a wheel atomizer

(cylindrical-conical) and the others with a nozzle atomizer (single or two-fluid), which is a

cylindrical vessel. These figures also show other components of the system, i.e., feed tank, filter,

pump, air heater, fan cyclone, exhaust fan.

Figure below shows the layout of a spray dryer system, which is self-inertizing and used to

handle materials with high risk of fire and explosion. Here, excess air entering the system passesthrough the burner flame and used as combustion air, thus inactivating it.


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Figure Self-inertizing spray dryer system


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Figure A two-stage spray dryer followed by a fluidized bed agglomerator

When the product coming out of the spray dryer is too fine it does not wet readily and so is

harder to reconstitute. To make the product instantly soluble it is agglomerated in a small

fluidized or vibrated fluidized bed, as shown in Figure 12. This two-stage arrangement is used in

the production of instant coffee, milk powder, cocoa, etc. An extension of this basic concept is

the so-called Spray-Fluidizer which dries the material in two stages. The surface moisture from

droplets is removed fully, along with some internal moisture, which takes longer time to come

out, in the first stage (spray dryer).

The final moisture content is achieved in a fluidized bed located at the bottom of the spray

chamber as an integral part of it. This two-stage arrangement makes the drying process very

efficient and economic. The fluidized bed drying unit can be replaced with a through circulation

band dryer at the bottom of the chamber; this concept is the basis of the so-called Filtermat dryer

used for sticky and sugar-rich materials which are hard to dry. The spray chamber in this case ismuch wider at the bottom, unlike the Spray- Fluidizer.

4.4) Drum Dryers:-

In drum dryers, slurries or pasty feedstocks are dried on the surface of a slowly rotating steam-

heated drum. A thin film of the paste is applied on the surface in various ways. The dried film is


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doctored off once it is dry and collected as flakes (rather than powder). Figure 13 shows four

types of commonly used drum dryer arrangements, which are self-explanatory. The design of

applicator rolls is important since the drying performance depends on the thickness and evenness

of the film applied. The paste must stick to the surface of the drum for such a drop to be

applicable.

Figure 13Four types of drum dryers in common use

Four key variables influence the drum dryer performance. They are: (a) steam pressure or

heating medium temperature, (b) Speed of rotation, (c) Thickness of film and (d) Feedproperties, e.g., solids concentration, rheology and temperature. Because it allows good control

of the drying temperature, drum dryers may be used to produce a precise hydrate of a chemical

compound rather than a mixture of hydrates.

Vacuum operation of both single- and double-drum dryers are done commercially to enhance

drying rates for heat-sensitive materials, such as pharmaceutical antibiotics. They are also used


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when a porous structure of product is desired. When recovery of solvents is an issue, once again,

vacuum operation is recommended. When recovering high boiling point solvents such as

ethylene glycol, lowering the pressure depresses the boiling point. For a detailed description and

discussion of the various types of drum dryers, the reader is referred to Moore (1995).

4.5) Fluidised Bed Dryers

Fluid bed dryers are found throughout all industries, from heavy mining through food, fine

chemicals and pharmaceuticals. They provide an effective method of drying relatively

freeflowing particles with a reasonably narrow particle size distribution. In general, fluid bed

dryers operate on a through-the-bed flow pattern with the gas passing through the product

perpendicular to the direction of travel. The dry product is discharged from the same section..y With a certain velocity of gas at the base of a bed of particles, the bed expands and

particles move within the bed.

y High rate of heat transfer is achieved with almost instant evaporation.

y Batch/continuous flow of materials is possible.

y The hot gas stream is introduced at the base of the bed through a dispersion/distribution

plate.

Figure : Fluidised bed dryer


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4.6) Tunnel Dryers:-

In this simple dryer concept, cabinets, trucks or trolleys containing the material to be dried aretransported at an appropriate speed through a long insulated chamber (or tunnel) while hot drying

gas is made to flow in concurrent, countercurrent, cross-flow or mixed flow fashion . In the

concurrent mode, the hottest and driest air meets the wetted material and hence results in high

initial drying rates but with relatively low product temperature (wet-bulb temperature if surface

moisture is present). Higher gas temperatures can be used in concurrent arrangements while in

counter-current dryers the inlet drying gas must be at a lower temperature if the product is heat-

sensitive. If the material to be dried is not heat-sensitive and low residual moisture content is a

requirement, one may employ higher gas temperatures in the countercurrent arrangement as well.

Combination flow or cross-flow arrangements are used less commonly. The latter offer high

drying rates but the tunnels must be designed to fit the trolleys snugly so the drying gas flows

through the material much like a through-circulation packed bed dryer. Total drying times that

can be handled range from 30 minutes to 6 hours.


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Figure A tunnel dryer

4.7) Band Dryers:-

For relatively free-flowing granules and extrudates that may undergo mechanical damage if they

are dispersed, band dryers are a good option. It is essentially a conveyor dryer wherein the band

is a perforated band over which the bed of drying solids rests.

Drying air at rather low velocities flows upwards through the band to accomplish drying.

Clearly, this type of dryer is not a good choice for very wet or very fine solids. If the bed depth is

large (over 10-15 cm) there may be a significant moisture profile in the bed with the solids

resting on the band over dried and overheated. One option to alleviate this problem is to reverse

the gas flow direction alternately over the length of the dryer. This evens out the moisture profile

while increasing the drying rate as well. Another option is to cause mixing of the bed at

appropriate interval of space. In some commercial designs, so-called multi-pass dryers, several

bands are stacked one above the other and the material is made to drop under gravity from the

higher to the next lower band which causes some random mixing of the material before it

undergoes further throughcirculation drying. It is possible to use a temperature profile along the

length of the conveyor so that the drier product can be exposed to lower gas temperatures if that


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is desired. Also, the final section may be a simple cooler so the product is ready for packaging or

storage. Residence times from 10 minutes to 60 minutes are economically feasible. These dryers

are quite versatile and can handle relatively large and arbitraryshaped particles that may be heat-

sensitive and fragile at the same time.

Gas cleaning requirements are minimal as low gas velocities are used. Also, power requirements

for air handling are low due to the low pressure drops needed. In commercial designs of very

large band dryers, it is important to ensure uniform distribution of the product on the band and

also uniform distribution of the air flow within the chamber of the dryer to ensure uniform

product moisture content. A schematic of a single-pass band dryer is shown in Figure.

Figure A single-pass band dryer

4.8) Hot Air Dryer- Stenter

Fabric drying is usually carried out on either drying cylinders (intermediate drying) or on

stenters (final drying). Drying cylinders are basically a series of steam-heated drums over whichthe fabric passes. It has the drawback of pulling the fabric and effectively reducing its width. For

this reason it tends to be used for intermediate drying.

The stenter is a gas fired oven, with the fabric passing through on a chain drive, held in place

by either clips or pins. Air is circulated above and below the fabric, before being exhausted to

atmosphere. As well as for drying processes, the stenter is used for pulling fabric to width,


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chemical finishing and heat setting and curing. It is a very versatile piece of equipment.

Figure 3-5:Schematic of a stenter

Modern stenters are designed with improved air circulation, which helps to improve drying

performance, and with integrated heat recovery and environmental abatement systems. Infrared

drying is used for both curing and drying. It is used as either a stand-alone piece of equipment, or

as a pre-dryer to increase drying rates and hence fabric speed through a stenter.

In the carpet industry there are a number of different types of drying/curing machine used. Wool

wash dryers at the end of scouring machines for drying the loose stock wool; wool drying ranges

for drying wool hanks prior to weaving; and wide 4 and 5-metre latexing or backing machines

used to apply and dry/cure the latex backing on to carpets. Low level VOC emissions are

produced by this process.

4.9) Infrared Dryers

Infrared (IR) dryers may be gas-fired ceramic radiators or electrically heated panels. The IR

wavelength range is from 0.1 Qm to 100 Qm, which generates heat in the exposed physical body.

The wavelength ranges 0.75 - 3.0 Qm; 3.0 - 25 Qm and 25 100 Qm are referred to as near IR,


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middle IR and far IR ranges, respectively. Industrial radiators are of two types: (1) Light

radiators (near IR), e.g., quartz glass with peak radiation intensity at 1.2 Qm and (2) Dark

radiators, e.g., ceramic (3.1 Qm) or metal radiators (2.7 - 4.3 Qm).

While convection can yield heat fluxes of the order of 1-2 kW m-2, radiation can yield much

higher levels of heat flux, i.e., 4-12 kW m-2 (light radiators) or 4-25 kW m-2 (dark radiators).

In many drying operations, the evaporation rates feasible are not high enough to require IR

radiators, however. There are some niche applications for IR dryers in some certain industries,

e.g., drying of coated paper, booster drying of paper in paper machines. They offer the

advantages of compactness, simplicity, ease of local control and lowequipment costs. Also, in

combination with convection, IR dryers offer the potential forsignificant energy savings and

enhancement in drying rates with better product quality.

On the negative side, the high heat flux may scorch product and enhance fire and explosion

hazards. Clearly, IR must be used in conjunction with convection or vacuum.

Good control is essential for the safe operation, i.e., IR power source must be cut off if there is

upset in the process which may lead to overheating of the product.

4.10) Tray Dryers

By far the most common dryer for small tonnage products, a batch tray dryer consists of a stack

of trays or several stacks of trays placed in a large insulated chamber in which hot air is

circulated with appropriately designed fans and guide vanes.

Often, a part of the exhausted air is recirculated with a fan located within or outside the drying

chamber. These dryers require large amount of labor to load and unload the product. Typically,

the drying times are long (10-60 hours). The key to successful operation is the uniform air flow

distribution over the trays as the slowest drying tray decides the residence time required and

hence dryer capacity. Warpage of trays can also cause poor distribution of drying air and hence

poor dryer performance.


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It is possible to convert the batch tray dryer into a continuous unit. Figure 2 shows the so-called

Turbo dryer, which consists of a stack of coaxial circular trays mounted on a single vertical shaft.

The product layer fed onto the first shelf is leveled by a set of stationary blades, which scratch a

series of grooves into the layer surface. The blades are staggered to ensure mixing of the

material. After one rotation, the material is wiped off the shelf by the last blade and falls onto the

next lower shelf. Up to 30 trays or more can be accommodated.

Hot air is supplied to the drying chamber by turbine fans. In the design shown, the air is heated

indirectly by passage over internal heaters. The wet granular material is fed at the top and it falls

under gravity to the next tray through radial slots in each circular shelf. A rotating rake mixes the

solids and thus improves the drying performance. Such dryers can be operated under vacuum for

heat-sensitive materials or when solvents must be recovered from the vapor. In a modified

design, it is possible to heat the trays by conduction and apply vacuum to remove the moisture

evaporated.


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4.11) Freeze Dryers

Highly heat-sensitive solids, such as some certain biotechnological materials, pharmaceuticals

and foods with high flavor content, may be freeze dried at a cost that is at least one order-of-

magnitude higher than that of spray drying itself not an inexpensive drying operation. Here,

drying occurs below the triple point of the liquid by sublimation of the frozen moisture into

vapor, which is then removed from the drying chamber by mechanical vacuum pumps or steam

jet ejectors. Generally, freeze drying yields the highest quality product of any dehydration

techniques. A porous, non-shrunken structure of the product allows rapid rehydration. Flavor

retention is also high due to the low temperature operation (-40o C). Living cells, e.g., bacteria,

yeast's and viruses can be freeze dried and the viability on reconstitution can still be high.


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Figure A paddle-type vacuum dryer

For drying of granular solids or slurries, vacuum dryers of various mechanical designs are

available commercially. They are more expensive than atmospheric pressure dryers but are

suited for heat-sensitive materials or when solvent recovery is required or if there are risks of fire

and/or explosion. Single-cone and double-cone mixers can be adapted to drying by heating the

vessel jackets and applying vacuum to remove moisture.

The paddle dryer is suited for sludge-like materials while the vacuum band dryer is good for thin

pastes or slurries. The material forms a film over the heated band; it may boil and form a highly

foamy, porous structure of very low bulk density.


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A band-type vacuum dryer

4.13) Microwave (MW) and Radio Frequency (RF) DryingUnlike conduction, convection or radiation, dielectric heating heats a material containing a polar

compound volumetrically, i.e., thermal energy supplied at the surface does not have to be

conducted into the interior, as limited by Fourier's law of heat conduction. This type of heating

provides the following advantages:

yEnhanced diffusion of heat and mass

yDevelopment of internal pressure gradients which enhance drying rates

yIncreased drying rates without increasing surface temperatures

yBetter product quality

When an alternating electromagnetic field is applied to a lossy dielectric material, heat is

generated due to friction of the excited molecules with asymmetric charges, e.g., water. This is aresult of ionic conduction or dipole oscillations (Strumillo and Kudra,1986). The radio frequency

range extends from 1-300 MHz while the microwave range is from 300 to 3000 MHz. However,

only specific frequency ranges are permitted for industrial heating applications, i.e., ranges

13.56, 27.12 and 40 MHz for RF and 915 (896 in Europe) and 2450 MHz for MW. Bound and


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free waters have different loss factor. Since loss factors increase with temperature there is a

danger of runaway, i.e., an accelerated heating rate causing a thermal damage to the material.

Table 6 summarizes the basic characteristics of MW and RF techniques. The main limitation of

MW and RF drying is that the technique is highly capital-intensive. It also consumes high-grade

energy, i.e., electricity, and the conversion efficiency to dielectric field is only in the order of

50%. Thus, these techniques are suited only for special applications involving very high value

products, extremely long drying time to remove traces of moisture or to obtain products of

special characteristics not obtained otherwise.

It is therefore not surprising that MW/RF drying is used only in special niche applications.

Further, these techniques are used mainly to boost drying capacity (to remove free water rapidly

without generation of large thermal gradients in the material) or to remove the last few percent of

water which comes out very slowly. Generally, dielectric heating is combined with convection or

vacuum to reduce the energy consumption. Microwave vacuum drying and microwave freeze

drying are among the commercial drying technologies that have so far found some applications.

Microwave freeze drying is typically carried out at temperatures well below the triple point of

water.

Typical conditions are: pressure in the range of 500 Pa and temperature of 40o C. Use of

excessive power as well as maldistribution of power due to nonhomogeneities in the frozen

solids can cause problem in MW drying. The main hurdle to commercialization of MW freeze

drying is the high cost.

Numerous laboratory and pilot scale studies have been reported on MW drying at atmospheric as

well as vacuum conditions. It is also possible to pipe microwave energy in various dryer

configurations, e.g., fluidized bed, spouted bed, vibrated bed or tray dryers, to enhance

convective drying rates. Unfortunately, while all these techniques do provide significant

enhancement of the drying time required, the initial and operating costs are such that the

enhancement obtained does not offset the added cost. Drying of treated grapes in combined

microwave and convection dryer has been shown to be very rapid and energy-efficient.

However, the costs ate prohibitively high.


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5.) SELECTION OF DRYERS:-[1][4]

In view of the enormous choices of dryer types one could possibly deploy for most products,

selection of the best type is a challenging task that should not be taken lightly nor should it be

left entirely to dryer vendors who typically specialize in only a few types of dryers. The user

must take a proactive role and employ vendors' experience and benchscale or pilot-scale facilities

to obtain data, which can be assessed for a comparative evaluation of several options. A wrong

dryer for a given application is still a poor dryer, regardless of how well it is designed. Note that

minor changes in composition or physical properties of a given product can influence its drying

characteristics, handling properties, etc., leading to a different product and in some cases severe

blockages in the dryer itself.Tests should be carried out with the real feed material and not a simulated one where

feasible. Although here we will focus only on the selection of the dryer, it is very important to

note that in practice one must select and specify a drying system which includes predrying stages

(e.g., mechanical dewatering, evaporation, pre-conditioning of feed by solids backmixing,

dilution or pelletization and feeding) as well as the post-drying stages of exhaust gas cleaning,

product collection, partial recirculation of exhausts, cooling of product, coating of product,

agglomeration, etc. The optimal cost-effective choice of dryer will depend, in some cases

significantly, on these stages. For example, a hard pasty feedstock can be diluted to a pumpable

slurry, atomized and dried in a spray dryer to produce a powder, or it may be pelletized and dried

in a fluid bed or in a through circulation dryer, or dried as is in a rotary or fluid bed unit. Also, in

some cases, it may be necessary to examine the entire flowsheet to see if the drying problem can

be simplified or even eliminated. Typically, non-thermal dewatering is an order-of-magnitude

less expensive than evaporation which, in turn, is many-fold energy efficient than thermal

drying. Demands on product quality may not always permit one to select the least expensive

option based solely on heat and mass transfer considerations, however. Often, product quality

requirements have over-riding influence on the selection process.

As a minimum, the following quantitative information is necessary to arrive at a suitable dryer:

y Dryer throughput; mode of feedstock production (batch/continuous)

y Physical, chemical and biochemical properties of the wet feed as well as


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y desired product specifications; expected variability in feed characteristics

y Upstream and downstream processing operations

y Moisture content of the feed and product

y Drying kinetics; moist solid sorption isotherms

y Quality parameters (physical, chemical, biochemical)

y Safety aspects, e.g., fire hazard and explosion hazards, toxicity

y Value of the product

y Need for automatic control

y Toxicological properties of the product

y Turndown ratio, flexibility in capacity requirements

y Type and cost of fuel, cost of electricity

y Environmental regulations

y Space in plant

For high value products like pharmaceuticals, certain foods and advanced materials, quality

considerations override other considerations since the cost of drying is unimportant. Throughputs

of such products are also relatively low, in general. In some cases, the feed may be conditioned

(e.g., size reduction, flaking, pelletizing, extrusion, back-mixing with dry product) prior to

drying which affects the choice of dryers.

As a rule, in the interest of energy savings and reduction of dryer size, it is desirable to reduce

the feed liquid content by less expensive operations such as filtration, centrifugation and

evaporation. It is also desirable to avoid over-drying, which increases the energy consumption as

well as drying time.

Drying of food and biotechnological products require adherence to GMP (Good Manufacturing

Practice) and hygienic equipment design and operation. Such materials are subject to thermal as

well as microbiological degradation during drying as well as in storage.

If the feed rate is low (< 100 kg/h), a batch-type dryer may be suited. Note that there is a limitedchoice of dryers that can operate in the batch mode. In less than one percent of cases the liquid to

be removed is a non-aqueous (organic) solvent or a mixture of water with a solvent. This is not

uncommon in drying of pharmaceutical products, however. Special care is needed to recover the

solvent and to avoid potential danger of fire and explosion.


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REFERENCE:-

1. CLASSIFICATION AND SELECTION OF INDUSTRIAL DRYERS Arun S.

Mujumdar

2. Kudra, T., Mujumdar, A.S., 2000, Advanced Drying Technologies, Marcel Dekker, New

York.

3. DRYERS FOR PARTICULATE SOLIDS, SLURRIES AND SHEET-FORM

MATERIALS Arun S. Mujumdar

4. Fundamental Aspects of drying by Taylor & Francis Group, LLC.

5. Description of Various Dryer Types by Taylor & Francis Group, LLC.

6. Drying Food University of Illinois at Urbana-ChampaignCollege of AgricultureCooperative Extension ServiceCircular 1227

7. www.processheating.com8. Handbook of Drying Technologies- Arum Mujumdar-Marcel Decker Publications9. Industrial Drying- A. Williams Gardner- George Godwin Ltd.

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