This slideshare discuss about drying. Drying is defined as the removal of water or other liquids from a material through the application of heat. It involves three steps: heat transfer to the material, mass transfer of moisture to the surface and evaporation, and transfer of vapor away from the material. There are several theories that describe the drying mechanism, including diffusion, capillarity, and pressure gradient theories. The drying rate curve shows an initial adjustment period, constant rate period, falling rate periods, and an equilibrium moisture content where drying stops. Factors like material properties, air conditions, and particle size influence the drying process and rate. various equipment involved in drying are as discussed.

1. DRYING

2. Introduction
• Drying involves removal of water or another solvent by
evaporation from a solid, semi-solid or liquid by application
of heat and finally a liquid free solid product is obtained
• In bioproducts like food, grains, and pharmaceuticals like
vaccines, the solvent to be removed is almost invariably
water .

3. Objective
• The main objectives of drying include to preserve foods and
increase their shelf life by reducing the water content .
• Reduce space requirements for storage and transport.
• In pharmaceutical technology, drying is carried out for one
or more of the following reasons:
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 like
granules, e.g. Flowability, compressibility

4. APPLICATIONS OF DRYING
1. Preparation of bulk drugs: In the preparation of bulk
drugs, drying is the final stage . examples are dried
aluminium hydroxide, spray dried lactose and powdered
extracts.
2. Preservation of drug products: Drying is necessary in
order to avoid deterioration. For examples protection of
blood and crude drugs from microbial growth.
3. Improved characteristics: Drying produces materials of
spherical shape, uniform size, free flowing.
4. Improved handling: To reduce the cost of transportation of
large volume materials. To make the materials easy or more
stable for handling. Drying reduces moisture content .

5. MECHANISM OF DRYING PROCESS
•Mechanism of drying process involves both heat
transfer and mass transfer processes simultaneously.
• Heat transfer takes place from the heating medium to
the solid material.
• Mass transfer involves the transfer of moisture to the
surface of the solids subsequently vapourisation from
the surface into surroundings.

6. MECHANISM OF DRYING PROCESS
• Various theories are proposed to explain the movement of
moisture. These are given
1. Diffusion theory
2. Capillarity theory
3. Pressure gradient theory
4. Gravity flow theory
5. Vapourisation and condensation mechanism

7. Bound Water: Moisture content of a substance which exerts as
equilibrium vapour pressure less than of the pure liquid at the same
temperature is referred to as bound moisture or bound wated.
Unbound Water: Moisture content of the solid which exerts an
equilibrium vapour pressure equal to that of pure liquid at the given
temperature is the unbound moisture or unbound water.
Free Moisture Content (FMC): The moisture content of solid in
excess of the equilibrium moisture content is referred as free
moisture.
Equilibrium Moisture Content (EMC): The moisture contents of
solid when it is in equilibrium with given partial pressure of vapour in
gas phase is called as equilibrium moisture content.

8. The EMC of a hygroscopic material surrounded at least partially by air
is the moisture content at which the material is neither gaining nor
losing moisture. The value of the EMC depends on the material and the
relative humidity and temperature of the air with which it is in contact.
Critical Moisture Content (CMC): Similarly, the moisture content at
which the constant rate drying period ends and the falling rate drying
period starts is called critical moisture content.
Constant Rate Drying Period: During the constant rate drying period,
the moisture evaporated per unit time per unit area of drying surface
remains constant.
Falling Rate Drying Period: In falling rate drying period the amount
of moisture evaporated per unit time per unit area of drying surface
continuously decreases.

12. DRYING EQUIPMENTS
A. On the basis of mechanism of drying:
1. Static bed dryer. e.g. Tray dryer, freeze dryer
2. Moving bed dryer. e.g. Drum dryer
3. Fluidized bed dryer: e.g. Fluidized bed dryer
4. Pneumatic dryer: e.g. Spray dryer.
B. On the basis of contact with material:
1. Direct dryers (direct contact between wet solid and hot gases)
(a) Batch dryers: e.g. Tray dryer.
(b) Continuous dryers: e.g. Spray dryer, fluidized bed dryer.
2. Indirect dryers:
(a) Batch dryers: e.g. Freeze drying, vacuum tray dryer
(b) Continuous dryers: e.g. Drum dryers.

13. Tray Dryer
• The simplest form of the dryer in this category is a laboratory oven.
• The best type of a tray dryer is that of the directed circulation form,
in which the air is heated and is directed across the material in a
controlled flow. The material to be dried is spread on the tiers of the
trays. The trays used have solid perforated or wire mesh bottoms.
• In a modern tray dryers a uniform temperature and air is maintained
by the use of a well insulated cabinet with strategically placed fans
and heating coils.

14. Principle
•Hot air is circulated over the material.
•Moisture is removed from the material by forced
convection. Simultaneously some moist air of dryer is
continuously getting replaced with fresh air

15. Fig: Tray dryer

16. Construction
• It consists of a double walled rectangular chamber.
• In between walls the insulator material is present.
• The trays are arranged inside the heating chamber. The
number of trays may vary with the size of the dryer.
• Dryers of the laboratory size may contain a minimum of the
three trays, where as dryers of industry size may contain
more than 20 trays

17. Construction
• The distance between the bottom of upper tray and surface of
the substance loaded in the subsequent tray must be 40 mm.
• Electric heaters are provided for heating. Fans are fitted in
heating chamber in order to circulate the hot air over all the
trays.
• In the corner of the chamber direction vanes are placed to
direct air in the expected path

18. Working
• Trays loaded with wet material are placed in the chamber.
• Fresh air is introduced through inlet, which gets heated by
heaters.
• The hot air is circulated by means of fans. The speed of fans is
generally kept between at 2 to 5 meter per second.
• Turbulent flow lowers the partial vapour pressure in the
atmosphere.
• The drying of the material occurs at its surface due to hot air
circulation.

19. Working
• As the water on surface evaporated the remaining moisture of the
material which was inside comes on the due to capillary action.
• These events occur in a single pass of air.
• The hot air cannot picked up a enough air at a single pass as the
time of contact is less. So it is recirculated along with 20% of the
fresh air.
• Moist air is discharged through outlet.
• Thus constant temperature and uniform air flow over the materials
can be maintained for achieving uniform drying.

20. Uses
1.Tray dryer is used in the drying of the sticky material
2.Tray dryers are used in the drying of the granular mass or
crystalline materials.
3.Plastic substances can be dried by the tray dryers.
4.In the tray dryers the crude drugs, chemicals, powders and
tablet granules are also dryed and shows free flowing of the
materials by picking up the water
5.Some types of equipments can also be dried in the tray
dryers.

21. Advantages
1. Tray dryer is operated batch wise. Batch drying allows handling of
material as separate part. So mistakes in the previous batch cannot
continue in next batch
2. .2. Wide variety of materials can be dried.
Disadvantages
1.A tray dryer requires more labour to load and unload. Hence increase
in the cost.
2. The process is time consuming.

22. DRUM DRYER OR ROLLER DRYER or(
Film Drum Dryer)

23. Principle:
•In drum dryer, a heated hallow metal drum rotates on
its longitudinal axis, which is partially dipped in the
solution to be dried. the solution is carried as a film on
the surface of the dryer and dried to form a layer. A
suitable knife scraps the dried material, while the drum
is rotating.

24. Construction
• The construction of a drum dryer is shown in Figure
• It consists of a horizontally mounted hollow steel drum of 0.6
to 3.0 metres diameter and 0.6 to 4.0 metres length, whose
external surface is smoothly polished.
• Below the drum, feed pan is placed in such a way that the
drum dips partially into the feed.
• On one side of the drum a spreader is placed and on the other
side a doctor's knife is placed to scrap the dried material.
• A storage bin (or a conveyor) is placed connecting the knife
to collect the material.

25. Working
• Steam is passed inside the drum.
• Heat transfer coefficient of the drum metal is high.
• Drying capacity is directly proportional to the surface area of the
drum.
• Heat is transferred by conduction to the material.
Simultaneously drum is rotated at a rate of 1-10 revolutions per
minute.
• The liquid material present in the feed pan adheres as a thin layer
to the external surface of the drum during its rotation.

26. Working
• The material is completely dried during its journey in slightly
less than one rotation (i.e., from one side to another side of
the drum).
• The dried material is scrapped by the doctor's knife, which
then falls into a storage bin. The time of contact of the
material with hot metal is 6 to 15 seconds only.
• Therefore, processing conditions such as film thickness,
steam temperature are closely controlled.

27. Uses:
• Drum dryer is used for drying solutions, slurries, suspension
etc.
• The products dried are milk products, starch products, ferrous
salts
• Suspensions of zinc oxide, suspension of kaolin, yeast,
pigments, m extracts, antibiotics, glandular extracts,
insecticides, DDT, calcium barium carbonates

28. Advantages
(1)In drum dryer, drying time is less, only a few seconds.
Therefore, heat sensitive materials can be dried.
(2)Drum dryer occupies less space, as it is compact when
compare to spray dryer.
(3)As a thin film of liquid is formed on the large heating
surface rates of heat transfer and mass transfer are high.
(4)The product obtained is completely dried and is in the final
form.

29. Disadvantages:
(1)Maintenance cost of a drum dryer is higher than spray
dryer.
(2)Skilled operators are essential to control feed rate, film
thick. ness, speed of rotation and temperature.
(3)It is not suitable for solutions of salts with less
solubility.

30. SPRAY DRYER

32. Figure: SPRAY DRYER

36. FLUIDIZED BED DRYER

39. FIG:FLUIDZED BED
DRYER

44. VACCUM DRYER

46. FIG:VACCUM DRYER