Lecture 6
Drying
Unit Operations II
Department of chemistry
Collage of natural science
In this Lecture…
 Basic principles and terminology in
drying;
 Phase equilibria of drying process;
 Drying curve and drying rate curve
under constant drying conditions;
 Material and heat balances,
calculation of drying rates and drying
time;
 Principles and structures of typical
drying equipment.
Introduction
Drying is a complex operation involving transient transfer of
heat and mass along with several rate processes, such as
physical or chemical transformations, which, in turn, may cause
changes in product quality as well as the mechanisms of heat
and mass transfer.
Physical changes that may occur include: shrinkage, puffing,
crystallization, glass transitions.
 In some cases, desirable or undesirable chemical or biochemical
reactions may occur leading to changes in color, texture, odor or other
properties of the solid product.
Drying occurs by effecting vaporization of the liquid by supplying
heat to the wet feedstock.
Over 85 percent of industrial dryers are of the convective type with
hot air or direct combustion gases as the drying medium and Over 99
percent of the applications involve removal of water
Drying,
Introduction
 the mass and heat transfer operation that converts a solid, semisolid or liquid feedstock into a solid product by evaporation
of the liquid into a vapor phase via application of heat.
 This definition excludes conversion of a liquid phase into a
concentrated liquid phase (evaporation), mechanical dewatering
operations such as filtration, centrifugation, sedimentation,
supercritical extraction of water from gels to produce
extremely high porosity aerogels (extraction) or so-called drying
of liquids and gases by use of molecular sieves (adsorption).
 Phase change and production of a solid phase as end product are
essential features of the drying process.
 An essential operation
in
the
chemical,
agricultural,
biotechnology, food, polymer, ceramics, pharmaceutical,
pulp and paper, mineral processing, and wood processing
industries.
Cont…
 Needed for the purposes of preservation and
storage, reduction in cost of transportation, etc.
 Over four hundred types of dryers have been
reported in the literature while over one hundred
distinct types are commonly available.
 It competes with distillation as the most energyintensive unit operation due to the high latent heat
of vaporization and the inherent inefficiency of using
hot air as the (most common) drying medium.
 National energy consumption for industrial drying
operations
– 10-15% USA, Canada, France, and UK
– 20-25% Denmark and Germany
– ???
Ethiopia (Energy audit data not available)
Why drying?






Need for easy-to-handle free-flowing solids
Less than 10% moisture microorganisms not active
Usually dry to less than 5 % to maintain asls
Preservation ,storage and elimination product deformation with time,
Reduction in cost of transportation,
Achieving desired quality of product, etc
Drying makes materials more convenient in packaging, transporting,
preserving, fabricating, and applying; and improves quality of products.
Drying application
Agriculture, dry Fruit and vegetable,
Chemical industry, Detergent powder
Pharmaceutical powder
Ceramic and polymer
Features of drying
 Product size may range from microns to tens of
centimeters (in thickness or depth)
 Product porosity may range from zero to 99.9 percent
 Drying times range from 0.25 sec (drying of tissue paper)
to five months (for certain hardwood species)
 Production capacities may range from 0.10 kg/h to 100
t/h
 Product speeds range from zero (stationary) to 2000 m/s
(tissue paper)
 Drying temperatures range from below the triple
point to above the critical point of the liquid
 Operating pressure may range from fraction of a milli-bar
to 25 atmospheres
 Heat may be transferred continuously or
intermittently by convection, conduction, radiation or
electromagnetic fields
Classification of Drying
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Drayer types classification Criteria
Mode of operation
Batch
Continuous*
Heat input-type
Convection*,
 Conduction, radiation,
 Electromagnetic fields,
 Combination of heat transfer modes
Intermittent or continuous*
Adiabatic or non-adiabatic
Cont’d
 State of material in dryer
Stationary
Moving, agitated, dispersed
 Operating pressure
– Vacuum*
– Atmospheric
 Drying medium (convection)
– Air*
– Superheated steam
– Flue gases
Drying temperature
Below boiling temperature*
Above boiling temperature
Below freezing point
Relative motion between drying medium and drying solids
Co-current
Counter-current
Mixed flow
Number of stages
Single*
Multi-stage
Residence time
Short (< 1 minute)
Medium (1 – 60 minutes)
Long (> 60 minutes)
Why so many dryer types?
 Over 500 reported in literature studies; over 100
commercially available
 Over 50,000 materials are dried commercially at
rates of a few kg/hr to 30 T/hr or more
 Drying times (residence times within drying
chamber) can range from 1/3 sec. to months
 Temperature and pressure range from below triple
point to super-critical
 Numerous constraints on physical/chemical
properties of feed as well as dried product require
a bewildering array of dryer designs
 Wide range of feeds (liquid, solid, semi-solid,
particulate, pasty; sludge-like; sticky etc); wide
specs on dried product
Cont’d
 Different sources of energy input( conduction,
convection, radiation, MW,RF etc)
 Energy input continuous or intermittent
 Batch, continuous or semi-continuous operation
 Quality is key parameter for many products
 Limited number used in pharma industry
 Need to reduce the cost
 Need to consider drying system rather than
dryer, i.e. Pre- and post- drying stages are
important and often cost more than dryer
 Environmental regulations demand new drying
techniques
Criterion for selection of dryers
Numerous criteria , with different weights
Many dryers can typically meet specs; hence
several dryers can do a given job in general.
 Mode of operation,
 Physical form of feed and dried product
desired; heat sensitivity; quality requirements;
production rate; whether non-aqueous solvents
are present in feed; whether material is
toxic/inflammable or friable etc
 Energy, environment, safety and cost.
 Products -quality is NO 1 criterion!
As a minimum, the following quantitative information is
necessary to arrive at a suitable dryer:
 Dryer throughput; mode of feedstock production
(batch/continuous)
 Physical, chemical and biochemical properties of the wet
feed as well as desired product specifications; expected
variability in feed characteristics
 Upstream and downstream processing operations
 Moisture content of the feed and product
 Drying kinetics; moist solid sorption isotherms
 Quality parameters (physical, chemical, biochemical)
 Safety aspects, e.g., fire hazard and explosion hazards,
toxicity
 Value of the product
Cont…
Need for automatic control
Toxicological properties of the product
Turndown ratio, flexibility in capacity
requirements
Type and cost of fuel, cost of electricity
Environmental regulations
Space in plant
• For high value products like pharmaceuticals,
certain foods and advanced materials, quality
considerations override other considerations.
• In some cases, the feed may be conditioned (e.g.,
size reduction, flaking, pelletizing, extrusion, backmixing 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 batchtype dryer may be suited. Note that there is a
limited choice of dryers that can operate in the
batch mode
Typical check list for selection industrial
dryers
Cont.
Conventional Vs innovative techniques
selection
Why select dryer carefully?
Can affect bottom-line..
Product quality , energy usage affected
by choice
Expert systems exist for selection.
Different expert systems give
different selections
Know product and process well before
choosing drying system; imitation can
cause problems!
Simple decision trees suggested (SPS)
Difference between drying and evaporation
1- In drying processes, the main operation usually carried out on
solid materials, e.g. powders, or products.
2- Drying in most of the cases means the removal of relatively
small amounts of water from solids .Evaporation include the
removal of large amounts of water from solutions.
3- In most cases, drying involves the removal of water at
temperatures below its boiling point, whereas evaporation
means the removal of water by boiling a solution.
4- In drying , water is usually removed by circulating air over the
material in order to carry away the water vapour , while in
evaporation , water is removed from the material as pure water
vapour mixed with other gases.
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Drying equipments
Types of Dryers
 Dryers for dilute solutions, suspensions and slurries
a) Drum dryer
b) Spray dryer
 Dryers for damp solid materials
a) Tray or shelf dryer
b) Tunnel dryer
c) Rotary dryer
d) Fluidized bed dryers (FBD)
e) Vacuum dryer
f) Freeze dryer
PH 101.58
27
Tray dryer
•
•
•
•
•
•
Rectangular chamber containing two truck
that support racks (H) that load material to
dry
Rack contain shallow trays: 750mm square
and 50-150mm deep
Air velocity: 2-5 m/s by fan (C) and motor (D)
Baffles (G) distributes air uniformly over the
stack of trays
Make up air added through exhaust duct (B)
Disadv:
–
–
–
–
•
Used when production rate is small
High labor cost for loading and unloading
Expensive to operate
Drying by circualation of air is slow and drying
cycles – 4-48 h per batch
Applications: drying of valuable products- dyes
and pharmaceuticals
Screen conveyer dryer
•
•
•
•
Layer 25-150 mm thick dried slowly
carried on a traveling metal screen
through a long drying chamber
Chamber contain series of separate
sections each with its own fan and heater
At inlet air is passed upward through
screen and at discharge to prevent loss of
dusty dried material air is passed downward
Typical specifications:
–
•
Adv:
–
–
–
•
2 m long and 4-50m wide, drying time 5120min, minimum screen size = 30 mesh
Handle variety of solid continuously
Reasonable cost
Steam consumption low
Application:
–
When the drying conditions must be
appreciably changed as the moisture content
of solid is reduced.
Screw Conveyor Dryers
• Continuous indirect heat dryer
• Horizontal screw conveyor enclosed in a cylindrical
jacketed shell
• Solid fed in one end is conveyed slowly through the
heated zone and discharges from the other end
• Specification:
– Shell 75-600 mm dia, 6 m long, rate of rotation =
2-30 r/min, heat transfer coefficient = 1560W/m2 oC
• Applications:
– For solids that are too fine and too sticky for
rotary dryer
Screw Conveyor Dryers
Tower dryers
• Series of circular trays mounted one
above the another on central
rotating shaft
• Solid feed dropped on the topmost
tray is exposed to stream of hot air
that passes across the tray
• Solid is scraped off and dropped to
tray below
• Specifications:
– Air velocity: 0.6-2.4 m/s
• Bottom two trays act as cooling
section for dry solids
• functions partially by cross
circulation drying as in tray dryer
and partly by showering the particles
through the hot gas as they tumble
from one tray to another
Rotary Dryer
•
•
•
•
Revolving cylindrical shell,
horizontal or slightly inclined
Wet feed enters one end
of cylinder, dry material
discharges from other
As the shell rotates, internal flights lift the
solid and shower them down. While in contact
with the shell it gets heated by external
jacket, during showering, it gets heated by
direct contact with gas
Typical specification
– Allowable mass velocity = 2000-25000 kg/m2h
– Inlet gas temp = 120-175 oC for steam heated
air and 550-800 oC for flue gas
– Diameter = 1-3 m
– Peripheral speed of the shall = 20-25 m/min
Rotary Dryers have many applications but are most commonly seen
in the mineral industry for drying sands, limestone, stones and soil,
ores, fertilizers, wood chips, coal, iron sulphate, filter cakes,
sewage sludge, etc
Rotary Dryer
.
Fluid bed dryer
• Solid fluidized by drying gas
• Wet feed is admitted to the top
of the bed; dry product is taken
out from the side, near the
bottom
• Velocity should be greater than the
settling velocity of the particles
• Particles are lifted in the gas stream
• Average particle stay: 30-120 s
when only surface is to be
evaporated; 15-30 min if internal
diffusion
• Care must be taken for small
particles as problem of carry away
is severe.
Advantage of FBD
 High rates of moisture removal due to excellent
gas-particle contact which results in high heat and
mass transfer rates.
 High thermal efficiency is usually achieved if part
of the thermal energy for drying is supplied by the
internal heat exchanger
 Lower capital and maintenance cost
 Reduced contact time for drying.
 Ease of control
 The conventional hot air fluidized bed dryer is not
a good choice of dryer when handling toxic or
flammable solids since there is danger of fire or
explosion of flammability limits are exceeded.
Disadvantage FBD
 High pressure drops results as a result of the need
to suspend the entire bed in gas which equally leads
to high energy consumption.
 Requires increased gas handling due to extensive
recirculation of exhausts gas for high thermal
efficiency operation.
 Poor fluidization and low flexibility especially if the
feed is too wet.
 Not the best choice of equipment when organic
solvents need to be removed during drying.
 Non uniform product quality for certain types of
fluidized bed dryer. Entertainment of fine
particles.
FBD
Flash dryer
• Wet pulverized solid is transported
for few seconds in hot gas stream
• Rapid drying with drying time 3-4
sec
• Temperature of gas too high 650 oC
at inlet
• Too small contact time rarely raised
temperature of solid above 50 oC
• Application
To sensitive materials that in other
dryers would have to be dried
indirectly by a much cooler heating
medium
Spray dryer: for solution and slurries
Slurry or liquid solution dispersed
into a stream of hot gas in the
form of mist of fine droplets
• Moisture rapidly vaporized from
droplets leaving residual particles
of dry solid
• Droplets formed by pressure nozzles
• To prevent droplets or wet particles of
solid from striking solid surface,
diameter is made large (2.5-9 m)
• Specification:
• Spray disk = 300mm, rotates at 500010000 r/min
• Gas leaving is passed to cyclone for
removal of entrained solid
• Cylindrical chamber is having conical
bottom to collect the solids
Drum dryers
•
•
•
•
•
•
•
Heated metal rolls on the outside of which
a
thin layer of liquid is evaporated to dryness
Dried solid is scraped off the rolls at they
slowly revolve
Application
Effective with dilute solutions,
concentrated solution of highly soluble
material and moderately heavy slurries
Disadvantage
– Not suitable for solutions of salts with
limited solubility or for slurries of
abrasive solids that settles out and create
excessive pressure between drums
Specifications
0.6—3 m dia, 0.6-4m long, revolving at
1-10 r/min, time for solid contact = 6-15
s, HT coeff= 1200-2000 W/m2oC, drying
capacity = 5-50 kg per square meter
of dryign surface per hour
Freeze Drying
 It is known as sublimation drying
 It is also known as lyophillization as the dried product has
great affinity for water ( lyophillic = water loving) is a low
temperature dehydration process which involves freezing
the product, lowering pressure, then removing the ice by
sublimation.
 Freeze drying results in a high quality product because of
the low temperature used in processing.The original shape
of the product is maintained and quality of the rehydrated
product is excellent.
PH 101.60
42
Cont..
Freeze-drying does not usually cause shrinkage
or toughening of the material being dried.
In addition, flavours, smells, and nutritional
content generally remain unchanged, making the
process popular for preserving food.
 However, water is not the only chemical
capable of sublimation, and the loss of other
volatile compounds such as acetic acid (vinegar)
and alcohols can yield undesirable results.
Components of Freeze dryer
 Chamber for chilling
the sample
 A vacuum source
 A heat source
 A vapor removal system
Fig: 60.1
PH 101.60
44
Application of freeze drying
Pharmaceuticals and
biotechnology
Freeze drying of food
Process
 It is carried out for batch process
 The product is frozen first by putting on a shelf
circulating refrigerants
 After freezing, vacuum is applied to the chamber
 Mild heat is supplied to the product by electric coils
or by circulating hot water
 Process is continued till a dry, spongy solid is obtained
PH 101.60
46
 Products to be freeze dried
are sterilized and aseptically
distributed into final
containers
 Container kept open during the
process and sealed immediately
after drying process
PH 101.60
47
Advantages
• The product is light, porous with good solubility
• No chances of hydrolysis because, drying is at low
temperature
• No chances of oxidation because of no contact with
air
• Freeze dried products are more stable than vacuum
dried product
PH 101.60
48
Disadvantages
• Very expensive
• Time taking process
• Dried product is very hygroscopic and
requires special packing precautions
PH 101.60
49
Uses
• Mostly suitable for highly thermolabile products
• Used for drying biological products such as blood
plasma, vaccines enzymes, microbiological cultures,
hormones and antibiotics and food ( ice cream, etc)
PH 101.60
50
5.2 Terminologies pertaining to drying operations
Moisture content of wet solids
The moisture content of a wet solid is expressed as kilograms of
moisture associated with 1 kg of the moisture – free solid. Thus a
moisture content of 0.4 means that 0.4 kg of removable water is
present per kg of the solid. It is sometimes calculated as
percentage moisture content.
Total moisture content: This is the total amount of liquid
associated with a wet solid. The easily removable water is known
as the free moisture content, and the moisture which is more
difficult to remove is the equilibrium moisture content. The
easily removable water is known as unbound water.
Unbound water: This water exists as a liquid and exerts its fully
vapour pressure, it can be removed readily by evaporation. During
a drying process this water is easily lost but the resulting solid is
not completely free from water molecules.
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Equilibrium moisture content:
• The moisture content present in a solid under steady-state
ambient conditions is termed the eq. moisture content. Its value
changes with temperature, humidity and the nature of the solid.
Bound water :
• Part of the moisture present in a wet solid may be adsorbed on
surfaces of the solid or be adsorbed within its structure to such
an extent to prevent it from developing its full vapour pressure
and from being easily removed by evaporation. Such moisture is
described as “bound” and is more difficult to remove than
unbound water.
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definitions
53
Definitions (cont..)
54
Definitions (cont..)
55
Definitions (cont..)
56
Relative humidity (RH) of air
• Air at a given temperature is capable of taking up water vapour
until it is saturated (at 100% RH ). If the temperature is raised
then the air will be able to take up more moisture and the
relative humidity falls.
• The RH of air is dependent not only on the amount of moisture in
the air , but also on its temperature, as the amount of water
required to saturate air is itself dependent on temperature.
• It should be noted that in convective drying, where warm air is
passed over the surface of a wet solid, the relative humidity
may rise during the drying process as a result of two separate
factors:• 1- Uptake of evaporated water vapour from the wet solid,
• 2- The cooling of the supply air as it transfers heat to the wet
solid (evaporative cooling).
• If the cooling is excessive the temperature of the air may fall
to a value known as the dew point, when liquid water will
condense and be deposited.
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