The document discusses the principles of drying in pharmaceutical engineering. It defines drying as the removal of liquid from a material through the application of heat. Thermal methods and freeze drying are common drying techniques. Drying is used to prepare bulk drugs, preserve products, and improve material characteristics. The document outlines the mechanisms, equilibrium relationships, and rate relationships involved in the drying process. It describes concepts like bound water, unbound water, air dry, bone dry, desorption, sorption, and equilibrium moisture content. The different periods of the drying rate curve are also explained.

1. PRINCIPLES OF DRYING –
PHARAMACEUTICAL ENGINEERING
VISHNU.A.S
ASST.PROFESSOR
SCPER,KOPERGAON

2. DRYING -
Principles Of Drying

3. DEFENITION
• Removal of all or most of the liquid from a
material by the application of heat (to cause
thermal vapourization).
• Is a downstream unit operation and commonly
utilized in the last stage of an industrial process.
• Dry solid is the final product.
• Drying is possible when the environment is
unsaturated with water vapour.
• Common methods employed are thermal
methods and freeze drying.

4. APPLICATIONS
• For the preparation of bulk drugs.
• For preservation of drug products.
• For improving the material characteristics.
• For improving the handling conditions

5. PRINCIPLES OF DRYING
• Water can be present as bound water and
unbound water.
Bound water: minimum moisture held by the
material that exerts an equilibrium vapour
pressure less than that of pure water at the same
temperature.
Unbound water: amount of moisture held by the
material that exerts an equilibrium vapour
pressure equal to that of pure water at the same
temperature.

6. Air dry: condition after the removal of moisture
which is normally removable, by usual drying
process, depending on temperature and
humidity conditions.
• In air dry condition, resulting solid is not
completely free from water molecules.
• Unbound water can be easily removed at
normal conditions.
Bone dry: condition after the complete removal
of water.

7. • Unbound water mainly exists in the void spaces
of solids.
• In a non – hygroscopic material, all the liquid is
unbound water.
• In a hygroscopic material, unbound water is the
liquid in excess of equilibrium moisture content,
corresponding to saturation humidity.
• Substances containing bound water are often
called as hygroscopic substances.
• In hygroscopic substances, bound water is found
in fine capillaries, cell and fibre walls and also
found in physical interaction with the material.

8. MECHANISMS OF DRYING PROCESS
• Heat must be transferred to the material in
order to supply the latent heat required for
vaporization of the moisture.
• Heat transfer takes place from heating
medium to the solid material.
• Mass transfer involves the transfer of moisture
to the surface of the solids.
• Subsequent vapourization from the surface to
surroundings.

9. EQUILIBRIUM RELATIONSHIPS OF DRYING
Equilibrium moisture content(EMC): amount of
moisture present in the solid which exerts a
vapour pressure equal to the vapour pressure of
surrounding atmosphere.
• EMC is the moisture which is difficult to remove
in practice.
• On particular conditions of temperature and
humidity, after reaching EMC , further exposure
of air will not alter the moisture content in the
solid.
• At this stage, vapour pressure of wet solids is
equal to that of surrounding atmosphere.

10. Desorption: when air of constant temperature and
humidity is continuously passed over the solid
containing moisture content more than EMC ,
then the solid loses moisture continuously till
EMC is reached.
Sorption: when air of constant temperature and
humidity is continuously passed over the solid
containing moisture content less than EMC , then
the solid adsorbs moisture continuously till EMC
is reached.
 Exposure to air at a definite temperature and
humidity will cause a material to loss or gain
moisture until EMC is attained.

11. Significance of EMC
• EMC curve helps in the selection of
experimental conditions for drying of a
particular material,
• Drying should be stopped when the moisture
content reaches the level of EMC.
• Overdrying can leads to regaining of moisture
from ambient conditions.
• Overdrying also leads to wastage of energy

12. • First step in reducing moisture content is to
reduce the relative humidity of ambient air.
• For this purpose desiccators (on small scale) or
air conditioning systems (on large scale) can
be employed.
• Some materials such as tablet granules, have
superior compaction properties with a small
amount(1-2%) of residual moisture content.

13. Factors affecting EMC
Nature of material
 Non porous insoluble solids – have an EMC of
practically zero.
 Fibrous or colloidal organic substances – EMC
values are high and variable.
 Porous solids: EMC values are much higher and
variable.
Nature of air
 For air of zero humidity, EMC of all materials is
zero
 as the temperature of air increases, EMC of solid
decreases

14. Free moisture content (FMC)
 Is the amount of water, that is free to evaporate
from solid surface.
 Free moisture content = Total moisture content –
Equilibrium moisture content
 The distinction between FMC and EMC depends
on drying conditions.
 Under conditions of saturation humidity, free
moisture is same as the unbound moisture.
 But in other circumstances, free moisture may
consist of both unbound and bound moisture.

15. RATE RELATIONSHIPS OF DRYING
 Loss on drying (LOD) = (mass of water in
sample in kg / total mass of wet sample in
kg)×100
 Moisture content (MC) = (mass of water in
sample in kg / mass of dry sample in kg)×100
 Drying rate = (weight of water in sample in
kg / time in hours × weight of dry solid in kg

16. Parts of drying rate curve
1. Initial adjustment period:
 Solids absorb heat and temperature increases.
 At the same time, moisture begins to evaporate
and thus tends to cool the drying solid.
 After sometime, heating and cooling rate
become equal and stabilizes the temperature.
 This temperature is equal to the wet bulb
temperature of drying air.

17. 2. Constant rate period:
 Temperature and drying rate is constant.
 Moisture evaporating from the surface is
replaced by water diffusing from the interior of
the solid.
 The rate of diffusion is equal to rate of
evaporation.
 The moisture content at the end of constant rate
period is termed as critical moisture content
(CMC)
 CMC is low for non porous materials and high for
colloidal organic substances.

18. 3. First falling rate period:
 Also known as unsaturated surface drying.
 During this period, the surface water is no
longer replaced at a rate fast enough to
maintain a continuous film on the surface,
 Dry spots begin to appear.
 Rate of drying begins to fall
 End point is referred as second critical point.
 At this end point , film of surface water is
completely evaporated.

19. 4. Second falling rate period:
 Rate of drying falls even more rapidly than the
first falling rate period.
 During this period, rate of drying is
dependant on the rate of diffusion.
 End point is equilibrium moisture content
(EMC).

20. 5. Beyond EMC
 Beyond EMC, the drying rate is equal to zero.
 Temperature and moisture content remain
constant.
 Beyond EMC, continuation of drying in given
conditions, is waste of time and energy.

21. REFERENCES
1. Subrahmanyam CVS, Thimma Setty J, Sarasija
Suresh, Kusum Devi V. Pharmaceutical
Engineering – Unit Operations II, Second ed.
Delhi – 110033: M K Jain for Vallabh
Prakashan. 2015. Pages 287 – 296.
2. Edited by Carter SJ. Cooper and Gunn’s
Tutorial Pharmacy, 6th ed. New Delhi –
110002: CBS Publishers and Distributers.
2005. Pages 270 – 275.