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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.