ARBAMINCH UNIVERSITY

COLLEGE OF NATURAL SCIENCES

DEPARTMENT OF CHEMISTRY

Advanced Chemical Engineering

IChem 4181
Unit-3
Unit operations
3.3 Drying & Humidification
Prepared By Dr.Ballekallu
 Mass Transfer Unit Operations

Drying
&
Humidification

2
Introduction
The separation operation of drying converts a solid, semi-
solid 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.
 Drying is an essential operation in the chemical,
agricultural, biotechnology, food, polymer,
ceramics, pharmaceutical, pulp and paper, mineral
processing, and wood processing industries.
Needed for the purposes of preservation and storage,
reduction in cost of transportation, etc. Most common
and diverse operation with over 100 types of dryers in
industrial use .
Competes with distillation as the most energy-
intensive operation
National energy consumption for industrial drying
operations
10-15% USA, Canada, France, and UK
20-25% Denmark and Germany
Drying generally refers to the removal of moisture from a
substance and is commonly the last stage in a manufacture
process.
Drying is the final removal of water from material (usually
by heat)

Non –thermal drying

1- As Squeezing wetted sponge
2-Adsorption by desiccant (desiccation)

Purposes of drying
- In pharmaceutical technology, drying is carried out for one
or more of the following reasons:

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Cont’d
1-To avoid or eliminate moisture which may lead to
corrosion and decrease the product or drug
stability.
2-To improve or keep the good properties of a
material, e.g. flowability, compressibility.
3-To reduce the cost of transportation of large
volume materials ( liquids)
4-To make the material easy or more suitable for
handling.
5- Preservative.
6- The final step in: Evaporation- Filtration-
Crystallization.
 Why drying?
Need for easy-to-handle free-flowing solids,
Preservation and storage,
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.
 Classification of drying

Freeze-drying is a special form of drying that

removes all moisture and tends to have less of an
effect on a food's taste than normal dehydration
8 does.
 Dryer types

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 Main dryer classification criteria Criterion
Types
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)
 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 baffling array
of dryer designs
Wide range of feeds (liquid, solid, semi-solid,
particulate, pasty; sludge-like; sticky etc); wide specs
on dried product
 Different sources of energy input( conduction,
convection, radiation 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.
 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

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Dry-bulb temperature t and wet-bulb
temperature tw

t Under steady-
tw
state, tw<t

Make-up water

u>5m/s (Turbulent flow)

Const
Humidity H
Air Temperatur e t

Dry-bulb temperature t: the actual gas temperature.

Wet bulb temperature tw: The liquid temperature when steady
state is reached.
Definitions (cont..)

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 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).

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 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.
Relationship between equilibrium moisture content
(EMC) and relative humidity
The EMC of a solid exposed to moist air varies with the
relative humidity. Ordinary atmospheric conditions are
of the order of 20 C and 70-75 RH, so that if exposed to
atmosphere a material such as kaolin will contain about
1% moisture, whereas a starch –based product may have
as much as 30% or more. Materials exposed to humid
conditions will regain moisture, and so there is no
advantage in drying to moisture content lower than that
which the material will have under the conditions of use

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 Loss of water from wet solids
 Unbound water is easily lost by evaporation until the
equilibrium moisture content of the solid is reached,
Once the solid reaches its EMC , extending the time
of drying will not change the moisture content as an
equilibrium situation has been reached. The only way
to reduce the moisture content is to reduce the RH
of the ambient air. This can be done mechanically
with an air-conditioning system.
On small scale, desiccators are used.

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 Different techniques of drying of solids ( Drying methods)
 The following points should be considered before
the selection of the suitable drying method:
1- Heat sensitivity the material being dried.
2- Physical characteristics of the material.
3- Nature of the liquid to be removed.
4- The scale of the operation.
5- Available sources of heat (steam, electrical).

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 The general principles for efficient drying can be
summarized as follows

1- Large surface area for heat transfer.

2- Efficient heat transfer per unit area (to supply
sufficient latent heat of vaporization or heat of
sublimation in case of freeze-drying)
3- Efficient mass transfer of evaporated water
through any surrounding boundary layers, i.e.
sufficient turbulence to minimize boundary layer
thickness.
4- Efficient vapour removal , i.e. low relative
humidity air at adequate velocity.
It is convenient to categorize pharmaceutical driers
according to the heat transfer method they use, i.e.
convective, conductive or radiant.
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 Psychrometric Chart

- Enthalpy
--- drybulb
--- humidity
---dewpoint
---wetbulb
Process: from one state to another
Human comfort zone
 Drying process
I Preheating period
 drying rate is slowly increasing
 only a very slight change in MC
II Constant-rate period
 drying rate is constant in time
 removal of surface water
 grain temperature is constant
 energy used to evaporate water
III Falling-rate period
 drying rate declines over time -> Temperatures for drying paddy
 transfer of internal moisture to • Seeds:
the surface – Maximum of 43 ºC
 drying rate declines • Paddy for milling:
 grain heats up (danger) – During constant-rate period: >
 for paddy grain starts at 18% 100°C
– During falling-rate period: < 55
°C
(depending in drying system)
 Rate of drying
With the passage of time X
T
falls as follow (line A)
Feed is heated to
vaporization temperature
and then the graph is linear,
then curve toward
horizontal and then levels
off
Drying rate is shown in B.
it is horizontal for much
length indication the
constant rate of drying
Drying rate
 Stated in percent moisture removed per hour
 Affected by:
Temperature and relative humidity of the drying air
Seeds: max. 43°C
Fist stage drying: max. 120°C
Second stage drying: max. 55°C
Air velocity
Too low -> air is saturated before leaving the dryer
Too high -> faster drying but waste of energy
Low-temperature drying: 0.1 m/s
Heated air drying: 0.15-0.25 m/s
Fluidized bed drying: 2.3 m/s
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.
Choice depends on 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
Key criterion- dryer must be able to handle the product-
move it from feed to exit! Other criteria follow
For pharma products -quality is NO 1 criterion!
Why select dryer carefully?

 Can affect bottom-line..

 Product quality , energy usage affected by choice
 Choose right drying system-not jut dryer
 Weakest link decides ultimate goodness of system choice
 Survey of 10 largest pharma and chemical companies in
Europe in 1990’s identified dryer selection as main
problem facing industry!
 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)
Drying equipments
 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:
 2 m long and 4-50m wide, drying time 5-120min, minimum screen size = 30
mesh
 Adv:
 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.
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
 Turbodryer 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
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 5000-10000 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