Drying involves the removal of water or other liquids from materials through evaporation. There are two main types: drying and evaporation. Drying removes small amounts of liquid using hot air circulation, while evaporation removes larger amounts through boiling. The key difference is that drying produces a dry solid, while evaporation produces a concentrated solution, suspension or slurry. Common drying methods include tray drying, drum drying, spray drying, and freeze drying. Each method has advantages and disadvantages depending on the material and desired end product. Proper control of factors like temperature, air flow, and humidity is important for achieving efficient drying.

3. : It is a Process of removal of small
amounts of water or other liquid from material
In drying , water is usually removed by
circulating hot air over the material in order to carry
away the water vapour.
1. Mechanically- Filter press(cheap)
2. Thermally- Vaporization
 Product obtained is the solid product
 Humidity in the environment is important for drying of
solids

4. EVAPORATION
• Process of removal of much larger quantities
of liquid.
• Water is removed by the means of boiling a
solution as a pure water vapour mixed with
other gases.
Product obtained is either concentrated solution
or suspension or slurry.

5. DRYING EVAPORATION
Removable of liquids in
small amounts.
Removable of liquids in
large amounts
Product obtained is dry
solid
Product obtained is
concentrated solution or
slurry.
Water is removed by
means of circulating hot
air over the material.
Water is removed by the
means of boiling.
Humidity is most
important for drying.
Humidity may present
during evaporation
It is carried out on mainly
solid materials
It is mainly carried out on
liquid materials

6. APPLICATIONS
Preservation is the principle reason for drying
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. flow ability, 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.

7. THEORY OF DRYING
• Drying involves heat and mass transfer
simultaneously.
Heat transfer takes place from heating medium
to the solid.
Mass transfer involves movement of moisture
to the surface of solid and flows to exterior surface.
o In solid, moisture is present in two forms
oBound moisture
oUnbound moisture

8. BOUND WATER
[hygroscopic materials]
Water in fine capillaries, cell
and fiber walls, physical
interaction
Vapour pressure of wet
solids is < vapour pressure
of pure water
UNBOUND WATER
[non hygroscopic materials]
Water in void
spaces
Vapour pressure of
wet solids is =
vapour pressure of
pure water

9. Explanation
• Bound water is the minimum water [moisture] held by
the material that exerts an equilibrium vapour pressure
less than the pure water at same temperature.
Substance containing bound water are called
hygroscopic substances.
• Un bound water 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
temperature.
Substance containing unbound water are called
Non hygroscopic substances.

10. THEORY
Equilibrium
relations
Equilibrium moisture
content
Free moisture
content
Rate
relations
Dry rate

11. • Air of constant temperature and humidity is
passed over the wet solid.
• After a long time equilibrium is attained and
further exposure will not alter the moisture
content in solid.
• At that stage vapour pressure is equal to
atm.pressure so, there is no driving force.

12. Equilibrium moisture content(EMC)
• It is the amount of water present in solid which exerts
a vapour pressure equal to the vapour pressure of
atmosphere surrounding it.

13. Sorption and Desorption
• When air is continuously passed over the solid containing
moisture less than EMC, then solid absorbs water
continuously till EMC is reached. This phenomenon is called
sorption
• When air is continuously passed over the solid containing
moisture more than EMC ,then solid losses water
continuously till EMC is reached. This is desorption.

14. • It is the amount of water that is free to evaporate
from solid surface.
FMC = Total water content- EMC
Note: FMC and EMC depends on drying
conditions

15. Rate relationships
• Take a material(known wt) containing sufficiently high
moisture and bottom sides are insulated.
• Air is blown over the material. Under constant drying
conditions. The water diffuses through surrounding
stationary air film.
• The material is weighed. The difference in the weights of
two successive periods gives the loss of moisture content.
The moisture present in the solid can be expressed on wet
weight or dry weight basis.

16. Calculations for drying rate
Mass of water in sample
%Loss on drying(LOD) = total mass of wet sample x 100
% moisture content(MC) =
mass of water in sample
mass of the dry sample x 100
Drying rate =
wt of water in sample
time x wt of dry solid

17. EQUIPMENTS
Type of
dryer
Examples Advantages Disadvantages
Static bed
dryer
Tray dryer and
Freeze dryer
Attrition is not
observed
Only a fraction
of solid is
directly exposed
Moving bed
dryer
Drum dryer Entire material
is exposed to
heat source
Attrition is
possible
Fluidised
bed dryer
Fluidised bed
dryer
Uniform drying Attrition takes
place
Pneumatic
dryer
Spray dryer Efficient and
raid drying

18. TRAY DRYER

19. • PRINCIPLE: Tray dryer is a batch dryer where ,
Hot air is continuously circulated and
convection heating takes place to remove
moisture.
• Construction: It has rectangular chamber
whose walls are insulated.
Number of trays are placed in the heating
chamber. Dryers in lab has 3 and industry dryers
has 20 trays.
tray is in rectangular or square shape about
1.2 to 2.4m2 in area.
Distance between the trays must be 40mm.

20. WORKING:
Wet solid is loaded in trays and kept in the chamber.
Fresh air is introduced through the inlet and air is heated up
by the heaters.
The water is picked up by air, as water evaporates it
diffuses from the interior of the solid by capillary action. this
occurs in single pass of air.
The constant air flow and temperature over the
material should be maintained for achieving uniform drying.

21. Uses :
1. Sticky materials , plastic substances , crystalline
and pastes can be dried.
2.Crude drugs, chemicals powders , tablet granules
and parts of equipment are also dried.
Advantages:
1. Handling of material can be done without losses.
2. Operated batch wise.
3.Valuable products can be handled efficiently.
Disadvantages:
Requires more labor for loading and unloading so
cost increases.
It is time consuming.

22. • PRINCIPLE : A heated hallow metal drum
which rotates on its longitudinal axis is
partially dipped in solution to be dried

23. CONSTRUCTION:
It consists of a horizontally mounted hallow steel
drum
Below the drum feed pan is placed in such a way that
the drum dips partially into the feed
On one side of drum a spreader is placed and on the
other side a doctors knife is placed to scrap the
dried material
A storage bin is placed to collect the material

24. WORKING:
• Steam is passed inside the drum heat is
transferred by conduction to the material
• Heat transfer coefficient of drum metal is high
• Drum is rotated speed of 10 revolutions p/m
• The liquid material present in the feed pan
adheres as a thin layer to the external surface
of the drum during its rotation and become
completely dried
• The dried material is scrapped by doctors knife
and collected in the storage bin

25. • USES:
– it is used for drying solutions and suspensions
– The products dry are milk products , ferrous salts ,
starch products etc
• ADVANTAGES:
– Drying time is less.
– Drum dryer occupies less space.
• DISADVANTAGES :
– Maintenance cost is high.
– It is not suitable for solutions of salts with less
solubility.

26. SPRAY DRYER

27. SPRAY DRYER
PRINCIPLE:
• The fluid to be dried is atomized into fine drop let
which are thrown radially into moving stream of hot
gas where the temperature of droplets is increased
and get dried.

28. CONSTRUCTION :
• It consist of large cylindrical drying chamber with a short
conical bottom of stainless steel
• Inlet is placed at the roof of the chamber and another inlet
carry in spray disk atomizer which rotates in the speed of
3000-50,000rpm
• Bottom of the dryer is connected to the cyclone separator.
• WORKING
Drying of material involves three stages
– Atomization of the liquid
– Drying of the liquid droplet
– Recovery of their dried product

29. • ATOMIZATION OF THE LIQUID TO FORM
LIQUID DROPLET
– The feed is introduce to atomizer by gravity to
form droplet
– The rate of feed is adjusted in a way that droplet
should be completely dried
• DRYING OF THE LIQUID DROPLET
– Fine droplet are dried in the drying chamber by supplying
hot air through the inlet
• RECOVERY OF DRIED PRODUCT
– Centrifugal force of atomizer drives the droplet to
follow helical path

30.  All these processes are completed in few second
 Particle size depends on solid content in the feed,
liquid viscosity ,disc speed.
• USES
– They are used if the product is a better form than
that of another dryer
– Quantity of the material to be dried is large
– The product is thermo labile ,hygroscopic

31. ADVANTAGES
• Labour costs are low
• It is a continuous process
• It s suitable for drying of sterile products
• Globules of the emulsion can be dried
DISADVANTAGES
• It is very bulky and expensive
• Not easy to operate
• Thermal efficiency is low

32. FLUIDISED BED DRYER

33. FLUIDISED BED DRYER
PRINCIPLE
Hot air is passed at high pressure into the container by
which the granules are lifted from the bottom and suspended
in the steam of air this condition is called fluidized state .In
this way granules are dried
CONSTRUCTION
It is of mainly two types horizontal and vertical
Vertical fluidized bed dryer :
Made of stainless steel or plastic a detachable bowl is
placed at the bottom which is used for charging and
discharging
A fan is mounted in the upper part, fresh air inlet and
heat exchanger,prefilter are connected serially to heat the
air.
Bag filters are placed for the recovery of fines.

34. • Working:
– The granules to be dried are placed in the bowl , the
fresh air is passed which gets heated.
– The hot air flows through the bottom of the bowl and
the fan is made to rotate and air velocity gets
increased.
– When the velocity of air is > settling velocity of
granules which remain partially suspended
– Granules rise in the container because of high velocity.
This condition is said to be fluidized state.
– The hot stream surrounds every granule to make them
dry. The bags are then shaken to remove entrain
particles.
– Drying is achieved at constant rate. Its time for drying is
40 mins
– At last bowl is taken out for discharging

36. – That stage is said to be fluidized state. The gas
surrounds every granule to completely dry them.
• USES
Used for drying of granules in production of tablets
and modified for coating granules.
It can e used for three operations such as mixing ,
granulation and drying.

39. VACCUM DRYER

40. VACCUM DRYER
• PRINCIPLE
– Material is dried by the application of the vacuum.
– Water evaporates faster due to pressure by the
vacuum and the heat transfer becomes efficient.
• CONSTRUCTION
-It is made of cast iron heavy jacketed vessel
-It should be strong to withstand high vaccum
-The enclosed space is divided in to a number of shelves
which are part of jacket.
-These shelves provide larger surface area for conduction
of heat

42. • Uses:
– Heat sensitive materials hygroscopy and dusty
materials can be dried by vaccum dried.
– Drugs containing toxic solvents can be separated in to
closed containers.
• Advantages:
– Provides large surface area for heat transfer.
– Handling of equipment is easy.
– Hot water of desired temperatures can be supplied.
• Disadvantages:
– Heat transfer coefficients are low.
– Expensive and labour and running costs are also high.

43. FREEZE DRYER

44. FREEZE DRYING
IT IS ALSO CALLED LYOPHILISATION.
System is made solvent loving for removing the
same.
• PRINCIPLE:
– In freeze drying the water is removed from the frozen
state by sublimation.
– The drying is achieved by subjecting the material to
temperature and pressures below the eutectic
temperature.
– Under these conditions any heat transferred is used as
latent heat.
– The vapour is removed from the system by
condensation.

45. • Construction: Freeze dryer consists of
I. Drying chamber in which trays are loaded
II. Heat supply in the form of radiation source, heating
coils.
III. Vapour condensing or adsorption system.
Vaccum pump or steam.
Its chamber is designed for batch operation. It consists of
shelves for keeping material.
The condenser consists of large surface cooled by solid
carbon dioxide slurred with acetone.
The temperature of the condenser must be lower than the
evaporated surface of the frozen substance.

46. • Working
I. Preparation and pretreatment.
II. Prefreezing for solidifying water.
III. Primary drying.
IV. Secondary drying.
V. Packing.
PREPARATION AND PRETREATMENT
The solution is pre concentrated under normal vaccum tray
drying. This reduces the actual drying by 8-10 times . The
final product becomes more porous.

47. PREFREEZING FOR SOLIDIFYING WATER
Bottles in which the aqueous solution is packed are frozen in
cold shelves. During this stage it is maintained at low
temperature and atmospheric pressure.
PRIMARY DRYING.
In this step the material to be dried is spread as much as
larger surface as possible for sublimation.
when a solution of solid is dried the depression of freezing
point of water occurs
the temperature at which the frozen solid vaporizes without
conversion to a liquid is referred as
eutectic point

48. • SECONDARY DRYING:
Removal of residual moisture under high vaccum
The temperature of solid is raised as high as 50-60 deg.
But vaccum is lowered
Rate of drying is very low and it takes 10 to 20 hrs.
• PACKING:
After vaccum is replaced by inert gas the bottles are closed.