DRYING
By- SANCHIT DHANKHAR
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CONTENTS
 Definition of drying
 Importance of drying
 Difference between drying and evaporation
 Equipments
 References
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DRYING
 Drying is defined as removal of the liquid from a material by
application of heat & is accomplished by transfer of a liquid
from the surface into an unsaturated vapor phase .
 Drying is the final removal of water from material (usually by
heat)
 Drying is commonly the last stage in a manufacture process
 Non-thermal drying
1- As Squeezing wetted sponge
2- Adsorption by desiccant (desiccation)
3- Extraction.
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Difference between drying and
evaporation
Sl. No. Drying Evaporation
1. In drying processes, the main operation
usually carried out on solid materials, e.g.
powders, or products
In evaporation processes, the main
operation usually carried out on
liquid materials, e.g. solution, 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. 4

Preservation of drug products
Preparation of bulk drugs
Improved handling
Improved characteristics
Applications….
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Preservation of
drug products….
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• Drying is necessary in order
to avoid deterioration. A
few examples are…
• --blood products, tissues…
undergo microbial growth
• --effervescent tablets,
synthetic & semi synthetic
drugs undergo…. chemical
decomposition.

Preparation of
Bulk Drugs….
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• Drying is the final stage
of processing . Eg:
• *dried aluminium
hydroxide
• *spray dried lactose
• *powdered extracts (in
fig..chamomile
powder), e.t.c

Improved characteristics…
Drying produces materials of spherical shape,
uniform size, free flowing & enhanced
solubility. Some specific areas of imp are…
 1. Granules are dried to improve the fluidity
& compression characteristics. These are
essential for production of tablets and
capsules.
 2. Viscous & sticky materials are not free
flowing. Drying modifies these characteristics
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IMPROVED
HANDLING….
 Removal of moisture makes the material light in weight and reduces bulk.
 Thus cost of transportation will be less & storage will be efficient.
 If moisture is present, size reduction of drugs is difficult.
 Drying reduces the moisture content.
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Theories of Drying….
 Drying involves both heat transfer as well as
mass transfer
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It is the amount of water (moisture) held by the material that
exerts an equilibrium vapour pressure equal to that of pure
water at the same pressure.
Bound water (moisture)
It is the minimum water(moisture) held by the material that
exerts an equilibrium vapour pressure less than the pure water
at the same temperature.
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Hygroscopic
materials
Water in:
-fine cpillaries
-Cell and fibre walls
-Physical interaction
Bound
water
Vapour pressure of
wet solids is less
than the v.p of pure
water
Non-Hygroscopic
materials
Water in void spaces
UnBound
water
Vapour pressure of
wet solids is equal to
v.p of pure water
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Equipments
1. Drum dryer (Film drying)
2. Spray dryer
3. Freeze Dryer
4. Tray drier
5. Fluidized Bed Dryer
6. Vacuum Dryer
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Drum Dryer (Film Drying)
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Drum Dryer (Film Drying)
 It consists of a drum of about 0.75-1.5
m in diameter and 2-4 m in length,
heated internally, usually by steam and
rotated on its longitudinal axis.
 Working: The liquid is applied to the
surface and spread to a film, this may be
done in various ways, but the simplest
method is that shown in the diagram,
where the drum dips into a feed pan.
Drying rate is controlled by using a
suitable speed of rotation and the drum
temperature. The product is scraped
from the surface of the drum by means
of a doctor knife.
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Uses
It is used for drying solution, slurries, suspensions etc and also the dry
products like this
• Milk products
• Starch products
• Ferrous salts
• Suspension of zinc oxide & kaolin
• Yeast
• Pigments
• Antibiotics
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Adantages
 Drying time is less, only in few seconds.
 Drum dryer occupies less space
 Product are obtained is completely dried and it’s final form
 Rate of heat & mass transfer is high
Disadvantages
• Maintenance cost is higher
• Skilled operators are essential to control feed rate
• Not suitable for solution of salts with less solubility
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Spray Dryer
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Spray Dryer
Working : Drying of the material in spray dryer involves 3 steps
(1) Atomization of the liquid
(2) Drying of the liquid droplets
(3) Recovery of the dried product
 The spray dryer provides a large surface area for heat and mass transfer by
atomizing the liquid to small droplets.
 These are sprayed into a stream of hot air, so that each droplet dries to a solid
particle.
 The drying chamber resembles the cyclone ensuring good circulation of air, to
facilitate heat and mass transfer, and that dried particles are separated by the
centrifugal action.
 Spray dryer can be operated efficiently at various feed rates.
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Uses
 Product obtained are better than other dryer
 Product quantity to be dried is large
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It is continuous process & dried completely 3 to 30 sec.
Labor cost is low
Product of uniform & controllable size is obtained
Products shows excellent solubility
Suitable for drying of sterile products
Disadvantages
It is bulky and expensive
Huge equipment not always easy to operate
Thermal efficiency is low
Advantages

Tray dryer
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Tray dryer
 Air flows in direction of the arrows over each shelf in turn.
 The wet material is spread on shallow trays resting on the shelves.
 Electrical elements or steam-heated pipes are positioned as shown, so that the air is
periodically reheated after it has cooled by passage over the wet material on one
shelf before it passes on the next.
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Uses
Following substances can be dried
 Sticky material
 Plastic substances
 Granular mass
 precipitates
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Advantages
 Handling of the material can be done without looses
 Tray dryer is operated batch-wise
(a) Each batch of material are separate entity
(b) Equipment is readily adjusted for use in drying a wide
variety of material
Disadvantages
 It require more labour to load and unload
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Fluidized Bed Dryer
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Fluidized Bed Dryer
 ‘Fluidized’ means something that behaves like liquid.
In the fluidized bed dryer, the mixture of solids and
gas behave like a liquid and solid are called fluidized.
 It provides good contact between hot air and particles
to obtain efficient drying.
 The hot air is passed through a mesh, which supports
the conical vessel with a porous base.
 This vessel is filled with powder to be dried.
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Uses: It Is Popularly Used For The Drying of Granules
in the Production of Tablets.
Advantages :
 Require less time for drying i.e., 20 to 40 min.
 It is available in different sizes with the drying capacity 5 to 200 kg
 Labour cost are low
 Also used for mixing the ingredients
 Hot spot are not observed in the dryer
 Higher drying temp. can be used
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Disadantages
 Some powder are organic which develop electrostatic
charges to avoid this, efficient electrical earthing of the
dryer is essential.
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Vacuum Dryer
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Steam or
Water Jacket
Condenser
Connection to
Vaccum Pump
Condensate
Receiver

Vacuum Dryer
 This equipment is a good example of conduction
drier. The vacuum oven consists of a jacketed vessel
to withstand vacuum within the oven.
 There are supports for the shelves giving a larger area
for conduction heat transfer. The oven can be closed
by a door.
 The oven is connected through a condenser and liquid
receiver to a vacuum pump.
 Operating pressure can be as low as 0.03-0.03 bar, at
which pressures water boils at 25-35 0C.
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Advantages
 It provides large surface area for heat transfer
 Handling of the material, trays and equipment is easy
 It is easy for switching over to the next material
 Hot water of desired temp. can be supplied
Disadvantages:
It has a limited capacity
It is more expensive than tray dryer
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Freeze Dryer
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Freeze Dryer
 Freeze drying is a process used to dry extremely heat-
sensitive materials. It allows the drying , without
excessive damage, of proteins, blood products and
even microorganisms, which retain a small but
significant viability.
 In this process the initial liquid solution or suspension
is frozen, the pressure above the frozen state is
reduced and the water removed by sublimation.
 Thus a liquid-to-vapour transition takes place, but
here three states of matter involved: liquid to solid,
then solid to vapour.
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THANKYOU
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