This document provides an overview of drying processes. It discusses that drying involves removing moisture from solids, solutions, slurries, and pastes. There are various modes of heat transfer used in drying, including convection, conduction, and radiation. Common industrial drying equipment includes batch dryers like tray and agitated dryers, as well as continuous dryers like tunnel dryers, belt dryers, and turbo-tray tower dryers. Drying rates can be constant, falling, or exponentially decreasing depending on the moisture content and properties of the material being dried.

1. Separation Processes-I
(ChE-206)
Lecture No. 22
Drying

2. Drying
• Drying is the removal of moisture (either water or other volatile
compounds) from solids, solutions, slurries, and pastes to give solid
products.
• In the feed to a dryer, moisture may be:
• embedded in a wet solid
• a liquid on a solid surface
• or a solution in which a solid is dissolved
• The term drying also describes:
• A gas mixture in which a condensable vapor is removed from a non-condensable gas
by cooling
• The removal of moisture from a liquid or gas by sorption
• In this topic of drying, we will deal with drying operations that produce
solid products.

3. Some Applications
• Drying is widely used to remove moisture from:
• (1) crystalline particles of inorganic salts and organic compounds to produce a free-flowing product
• (2) biological materials, including foods, to prevent spoilage and decay from microorganisms that cannot live
without water
• (3) pharmaceuticals
• (4) detergents
• (5) lumber, paper, and fiber products
• (6) dyestuffs
• (7) solid catalysts
• (8) milk
• (9) films and coatings
• (10) products where high water content entails excessive transportation and distribution costs.
• Not all drying processes have been successful; the beer industry, for decades, has been trying to market
dehydrated beer with no success whatsoever.

4. Drying Mechanism
• Because drying involves vaporization of moisture, heat must be transferred
to the material being dried.
• The common modes of heat transfer are:
• (1) convection from a hot gas in contact with the material
• (2) conduction from a hot, solid surface in contact with the material
• (3) radiation from a hot gas or surface
• (4) heat generation within the material by dielectric, radio frequency, or
microwave heating.
• These different modes can sometimes be used symbiotically, depending on
whether the moisture to be removed is on the surface or inside the solid.

5. Industrial Example
• The continuous production of 69,530 lb/day of MgSO47H2O crystalline
solids containing 0.015 lb H2O/lb dry solid is an example of an
industrial drying operation.
• The feed to the dryer in Figure 18.1 consists of a filter cake from a
rotary drum vacuum filter.

6. Categories of Wet Solids
•Granular or crystalline solids that hold moisture in open pores
between particles.
•Inorganic materials (crushed rocks, sand, catalysts, titanium dioxide,
zinc sulfate and sodium phosphate)
•Can be dried very rapidly to lower moisture contents
Wet solids First-
Category
•Fibrous, amorphous and gel-like materials that dissolve or trap
moisture in fibers or very fine pores.
•Organic solids (tree plant, vegetable, and animal materials such as
wood, leather, soap, eggs, glues, cereals, starch, cotton and wool)
•Drying to lower moisture content is possible only when using a gas of
low humidity
Wet Solids
Second-Category

7. Equilibrium-moisture content of solids
• A hypothetical equilibrium isotherm
• Moisture content, X, is expressed as mass of moisture per 100 mass units of
bone-dry solid.
• This is the most common way to express moisture content and is equivalent to
wt% moisture on a dry-solid basis.
• This is analogous to expressions for humidity and is most convenient in drying
calculations where the mass of bone-dry solid and dry gas remain constant
while moisture is transferred from solid to gas. Less common is wt% moisture on
a wet-solid basis, W.
• The two moisture contents are related by the expression:

8. Equilibrium Isotherm
• Equilibrium-moisture
content, X , is plotted for a
second-category solid for a
given temperature and
pressure, against relative
humidity, HR.

9. Some Important terms
• In some cases, humidity, H, is used with a limit of the saturation
humidity, Hs. At HR = 100%, equilibrium-moisture content is called
bound moisture, XB.
• If the wet solid has a total moisture content, XT > XB, the excess, XT-XB,
is unbound moisture.
• At a relative humidity < 100%, the excess of XT over the equilibrium
moisture content, i.e., XT – X* , is the free-moisture content.

10. • Free Moisture: The difference between the total water content of solid and
the equilibrium water content.
• Bound Moisture: Part of the moisture present in a wet solid to such an
extent to prevent it from developing its full vapor pressure and from being
easily removed. Such moisture is described as “bound moisture” and is
more difficult to remove than unbound water.
• Unbound Moisture: If a material contains more water than indicated by
intersection with the 100% RH-line, the water in excess is called unbound
moisture. It is easily lost by evaporation until the equilibrium moisture
content of the solid is reached.
• Total Moisture Content: Total amount of moisture in the solid: bound plus
unbound water or free moisture plus equilibrium moisture content.

11. Removal of moisture
• In the presence of a saturated gas, only unbound moisture can be
removed during drying.
For a partially saturated gas, only free moisture can be removed.
• But if HR = 0, all solids, given enough time, may be dried to a bone-dry
state.
• Bone-dry Solid: A material or the product that contains no liquid
contents.

12. Example of equilibrium-moisture isotherms
• Experimental equilibrium-moisture isotherms
at 25C and 1 atm for second-category
materials
• At low values of HR <10%, moisture is bound
to the solid on its surfaces as an adsorbed
monomolecular layer.
• At intermediate values of HR = 20-60%
multimolecular layers may build up on the
monolayer.
• At large values of of HR > 60% moisture is
held in micropores.

13. Effect of Temperature
• Temperature has a significant effect on
equilibriummoisture content.
• For cotton at 96–302F
• At an HR of 20%, equilibrium moisture
content decreases from 0.037 to 0.012
lb H2O/lb dry cotton.

14. Drying Periods
• The decrease in average moisture content, X, as a function of time, t, for drying
either category of solids in a direct-heat dryer was observed experimentally by
Sherwood.
• The final equilibrium-moisture content is X*.
• Although both plots exhibit four drying periods, the periods are more distinct in
the drying-rate curve.
• A-B: From A to B, the wet solid is being preheated to an exposed-surface
temperature equal to the wet-bulb gas temperature, while moisture is
evaporated at an increasing rate.
• B-C: Constant rate period
• C: Critical moisture content
• C-D: first falling rate drying period
• D-E: second falling rate drying period

16. • From A to B, the wet solid is being preheated to an exposed-surface temperature
equal to the wet-bulb gas temperature, while moisture is evaporated at an
increasing rate.
• At the end of the preheat period, if the wet solid is of the granular, first category,
a cross section has the appearance of Figure 18.29a, where the exposed surface
is still covered by a film of moisture.
• A wet solid of the second category is covered on the exposed surface by free
moisture. The drying rate now becomes constant during the period from B to C,
which prevails as long as free moisture covers the exposed surface.
• This surface moisture may be part of the original moisture that covered the
surface, or it may be moisture brought to the surface by capillary action in the
case of wet solids of the first category or by liquid diffusion in the case of wet
solids of the second category.
• In either case, the rate of drying is controlled by external mass and heat transfer
between the exposed surface of the wet solid and the bulk gas.
• Migration of moisture from the interior of the wet solid to the exposed surface is
not a rate-affecting factor.
• This period, the constant-rate drying period, terminates at point C, the critical
moisture content.

17. • At C, the moisture just barely covers the exposed surface; and then until
point D is reached the surface tends to a dry state because the rate of
liquid travel by diffusion or capillary action to the exposed surface is not
sufficiently fast.
• In this period, the exposed-surface temperature remains at the wet-bulb
temperature if heat conduction is adequate, but the wetted exposed area
for mass transfer decreases. Consequently, the rate of drying decreases
linearly with decreasing average moisture content. This is the first falling-
rate drying period.
• During the period from C to D, the liquid in the pores of wet solids of the
first category begins to recede from the exposed surface. In the final period
from D to E, evaporation occurs from liquid surfaces in the pores, where
the wet-bulb temperature prevails.
• However, the temperature of the exposed surface of the solid rises to
approach the dry-bulb temperature of the gas.
• During this period, the second falling-rate drying period, the rate of drying
may be controlled by vapor diffusion for wet solids of the first category and
by liquid diffusion for wet solids of the second category. The rate falls
exponentially with decreasing moisture content.

19. Drying Equipment
Drying Equipment
Mode of Operation
Batch (when production
rate is less than 500 lb/h of
dried solid)
Continuous (when
production rate is more
than 2000 lb/h)
Mode of Heat Transfer
Direct-heat Dryers
(convective or adiabatic)
contact material with hot
gas
Indirect-heat Dryers (non-
adiabatic) provide heat to
material by conduction or
radiation from a hot surface
Degree to which material is
Agitated
Feed is stationary
Agitation may be necessary,
if the material is sticky. For
example fluidized bed dryer

20. Batch Dryers
• Equipment for drying batches includes:
• (1) tray (also called cabinet, compartment, or shelf) dryers
• (2) agitated dryers.
• Together, these two types cover many of the modes of heat transfer
and agitation discussed above.

21. Tray Dryers
• The oldest and simplest batch dryer
• Useful when low production rates of multiple products are involved and
when drying times vary from hours to days.
• The material to be dried is loaded to a depth of typically 0.5–4 inches in
removable trays that may measure 30 x 30 x 3 inches and are stacked by a
forklift on shelves about 3 inches apart in a cabinet.
• If the wet solids are granular or shaped into briquettes, noodles, or pellets
with appreciable voids, the tray bottom can be perforated so that heating
gas can be passed down through the material (through-circulation) as
shown in Figure 18.2b.
• Otherwise, the tray bottom is solid and the hot gas is passed at velocities of
3–30 ft/s over the open tray surface (cross-circulation), as in Figure 18.2a.

23. Agitated Dryers
• Indirect heat with agitation and, perhaps, under vacuum, is desirable for
batch drying when any of the following conditions exist:
• (1) material oxidizes or becomes explosive or dusty during drying
• (2) moisture is valuable, toxic, flammable, or explosive
• (3) material tends to agglomerate or set up if not agitated
• (4) maximum product temperature is less than about 30°C.
• Heat-transfer rates are controlled mostly by contact resistance at the inner
wall of the jacketed vessel and by conduction into the material being dried.
• A wide variety of heating fluids can be used, including hot liquids, steam,
Dowtherm, hot air, combustion gases, and molten salt.

24. If material tends to agglomerate or set up if
not agitated
• Feed is a liquid, slurry, or paste
• This dryer consists of a shallow (2–3-ft high), jacketed, flat-
bottomed vessel, equipped with a paddle agitator that rotates
at 2–20 rpm and scrapes the inner wall to help prevent cake
buildup.
• Capacity of up to 1,000 gallons
• Heat-transfer surface from 15 to 300 ft2
• The material to be dried occupies about 2/3 of the vessel
volume.
• The degree of agitation can be varied during the drying cycle.
• With a thin liquid feed, agitation may vary from very low
initially to very high if a sticky paste forms, followed by
moderate agitation when the granular solid product begins to
form.
• Several hours are required for drying
• Vacuum may also be applied

25. Double-cone (also called tumbler) vacuum
dryer
• When any or all of the above four conditions
apply, but only mild agitation is required.
• V-shaped tumblers are also available
• The conical shape facilitates discharge of dried
product, but, except for the tumbling, no
means is provided to prevent cake buildup on
the inner walls.
• Heat-transfer surface areas of 1 to 56 m2 .
• Additional heat-transfer surface can be
provided by internal tubes or plates.
• Up to 70% of the volume can be occupied by
feed.

26. Ribbon or paddle-agitated, horizontal-cylinder
dryer
• When any or all of the above four conditions are
relevant
• The cylinder is jacketed and stationary.
• The ribbons or paddles provide agitation and
scrape the inner walls to prevent solids buildup.
• Dimensions range up to diameters of 6 ft and
lengths up to 40 ft.
• The agitator can be rotated from 4 to 140 rpm,
resulting in overall heat-transfer coefficients of 5 to
35 Btu/h-ft2 - F.
• Typically from 20 to 70% of the cylinder volume is
filled with feed.
• Drying times vary from 4 to 16 hours.

27. Continuous Dryers
• A wide variety of industrial drying equipment for continuous
operation is available.
• The following descriptions cover most types, organized by the nature
of the wet feed:
• (1) granular, crystalline, and fibrous solids, cakes, extrusions, and pastes
• (2) liquids and slurries
• (3) sheets and films
• Types include:
• Tunnel dryers
• Belt or band dryers
• Turbo-tray tower dryers

28. Tunnel Dryers
• The simplest, most widely applicable, and perhaps oldest continuous dryers
• Suitable for any material that can be placed into trays and is not subject to dust
formation.
• The trays are stacked onto wheeled trucks, which are conveyed progressively in series
through a tunnel where the material in the trays is contacted by cross-circulation of hot
gases.
• Hot gases can flow countercurrently, cocurrently, or in more complex flow configurations
to the movement of the trucks.
• As a truck of dried material is removed from the discharge end of the tunnel, a truck of
wet material enters at the feed end.
• The overall drying operation is not truly continuous because wet material must be
loaded into the trays and dried material removed from the trays outside the tunnel,
often with dump truck devices.
• A typical tunnel might be 100 ft long and house 15 trucks.

30. Belt or Band Dryers
• Carry the solids as a layer on a belt conveyor, with hot gases passing
over the material.
• The endless belt is constructed of hinged, slotted-metal plates, or,
preferably a thin metal band, which is ideal for slurries, pastes, and
sticky materials.
• More common are screen or perforated-belt or band conveyor dryers.

31. Turbo-Tray Tower Dryers
• When floor space is limited but headroom is available, the turbo-tray or
rotating-shelf dryer is a good choice.
• For rapid drying of free-flowing, non-dusting granular solids.
• Annular shelves, mounted one above the other, are slowly rotated at up to
1 rpm by a central shaft.
• Wet feed enters through the roof onto the top shelf as it rotates under the
feed opening.
• At the end of one revolution, a stationary wiper causes the material to fall
through a radial slot onto the shelf below, where it is spread into a pile of
uniform thickness by a stationary leveler. This action is repeated on each
shelf until the dried material is discharged from the bottom of the unit.
• Fans that provide cross-circulation of hot gases at velocities of 2 to 8 ft/s
across the shelves.
• The bottom shelves can be used as a solids-cooling zone. Because solids are
showered through the hot gases and redistributed from shelf to shelf,
drying time is less than for cross-circulation, stationary-tray dryers.
• Typical turbo-tray dryers are from 2 to 20 m in height and 2 to 11 m in
diameter, with shelf areas to 1,675 m2 .

32. Direct-Heat Rotary Dryers
• For free-flowing granular, crystalline, and flaked solids of relatively small
size, when breakage of solids can be tolerated, is the direct heat rotary
dryer.
• It consists of a rotating, cylindrical shell that is slightly inclined from the
horizontal with a slope of less than 8 cm/m. Wet solids enter through a
chute at the high end and dry solids discharge from the low end.
• Hot gases (heated air, flue gas, or superheated steam) flow counter-
currently to the solids, but co-current flow can be employed for
temperature-sensitive solids.
• Bulk solids occupy 8– 18% of the cylinder volume, with residence times
from 5 minutes to 2 h.

34. Indirect-Heat, Steam-Tube Rotary Dryers
• When materials are:
• (1) free flowing and granular, crystalline, or flaked
• (2) wet with water or organic solvents
• (3) subject to undesirable breakage, dust formation, or contamination by
air or flue gases
• It consists of a rotating cylinder that houses two concentric rows of
longitudinal finned or unfinned tubes that carry condensing steam and
rotate with the cylinder.
• Wet solids are fed into one end of the cylinder through a chute or by a
screw conveyor.
• A gentle solids-lifting action is provided by the tubes.
• Dried product discharges from the other end

36. Fluidized Bed Dryers
• Free-flowing, moist particles can be dried continuously with a residence time of a
few minutes by contact with hot gases in a fluidized-bed dryer.
• This dryer consists of a cylindrical or rectangular fluidizing chamber to which wet
particles are fed from a bin through a star valve or by a screw conveyor, and
fluidized by hot gases blown through a heater and into a plenum chamber below
the bed, from where the particles pass into the fluidizing chamber through a
distributor plat.
• The hot gases pass up through the bed, transferring heat for evaporation of the
moisture, and pass out the top of the fluidizing chamber and through demisters
and cyclones for dust removal.
• The solids are circulated by the action of the hot gases in the bed and by baffles,
and sometimes mixers, but eventually pass out of the chamber through an
overflow duct, which also serves to establish the height of the fluidized bed.

37. Advantages
• Fluidized-bed dryers have become very popular in
recent years because they:
• (1) have no moving parts
• (2) provide rapid heat and mass transfer between
gas and particles
• (3) provide intensive mixing of the particles, leading
to uniform conditions throughout the bed
• (4) provide ease of control
• (5) can be designed for hazardous solids and a wide
range of temperatures (up to 1200C), pressures (up
to 100 psig), residence times, and atmospheres
• (6) can operate on electricity, natural gas, fuel oil,
thermal fluids, steam, hot air, or hot water
• (7) can process very fine and/or low-density
particles
• (8) provide very efficient emissions control

38. Spray Dryers
• When solutions, slurries, or pumpable pastes—containing more than 50 wt% moisture,
at rates greater than 1,000 lb/h— are to be dried.
• The drying chamber has a conical shaped bottom section with a top diameter that may
be nearly equal to the chamber height.
• Feed is pumped to the top center of the chamber, where it is dispersed into droplets or
particles from 2 to 2,000 mm by any of three types of atomizers:
(1) single-fluid pressure nozzles
(2) pneumatic nozzles
(3) centrifugal disks or spray wheels
• Hot gas enters the chamber, causing moisture in the atomized feed to rapidly evaporate.
• Gas flows co-currently to the solids, and dried solids and gas are either partially
separated in the chamber, followed by removal of dust from the gas by a cyclone
separator, or, as shown in Figure 18.13a, are sent together to a cyclone separator, bag
filter, or other gas–solid separator.
• The hot gas can be moved by a fan.

40. Book
• Seader, J. D.; Henley, E. J.; Roper, D. K., Separation Process Principles:
Chemical and Biochemical Operations. 3rd Ed.; John Wiley & Sons,
Inc.: 2011.
• Chapter 18: Drying of Solids