PRINCIPLES OF DRYING

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

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.

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

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.

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.

• 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.

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.

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.

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.

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

• 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.

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

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.

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

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.

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.

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.

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).

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.

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.

PRINCIPLES OF DRYING

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