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1. DRYING

2. DRYING
• Drying is defined as the removal of small
amounts of water or other liquid from a
material by the application of heat
• Drying is possible when the environment is
unsaturated with water vapor

3. APPLICATIONS
• Prepartion of bulk drugs
Drying is the final stage of processing
e.g. Dried aluminum hydroxide
Spray dried lactose
Powdered extracts
Drying step is essential after filtration and
centrifugation operations

4. APPLICATIONS
• Preservation of drug products
Drying is necessary to avoid deterioration of
drugs
Crude drugs of animal and vegetable
origin – chemical decomposition
Blood products, skin, tissue – microbial
growth

5. APPLICATIONS
• Improved characteristics
Drying produces materials of spherical shape ,
uniform size, free flowing end enhanced
solubility
e.g. granules are dried to improve fluidity
and compression characteristics
Viscous and sticky materials are not free
flowing. Drying modify these characteristics

6. THEORY OF DRYING
• In a wet mass water may be present as bound
water and unbound water
• Bound water :
It is the minimum water held by the material that
exerts an equilibrium vapor pressure less than
pure water at the same temperature
The substances containing bound water –
hygroscopic substances

7. THEORY OF DRYING
• Unbound water
It is the amount of water held by the material
that exerts an equilibrium vapor pressure
equal to that of pure water at the same
temperature

8. EQUILIBRIUM RELATIONSHIPS
• Air of constant temperature and humidity is
passed over the wet solid
• After a long exposure equilibrium is attained
• On attaining equilibrium further exposure will
not alter the moisture content in the solid
• At this stage VP of the wet solid = surrounding
atmosphere .

9. EQUILIBRIUM MOISTURE CONTENT
• it is the amount of water present in the solid
which exerts a vapor pressure equal to the
vapor pressure of the atmosphere
surrounding it.

10. DESORPTION :-
When air of constant temperature and humidity
is continuously passed over the solid
Solid containing moisture > EMC
Then solid loses water continuously till EMC is
reached
This phenomenon is known as desorption

11. SORPTION
• When air is continuously passed over the solid
containing moisture less than the EMC, then
solid absorbs water continuously till EMC is
reached
• This phenomenon is known as sorption

12. Measurement of EMC of materials
• The solid samples are placed in a series of
desiccators
• This samples exposed to several humidity
conditions
• This exposure is continued until the material
attains a constant weight
• The difference in the final and initial weights
gives the moisture content

13. Applications of EMC
• Over drying can be avoided
Drying should be stopped when the moisture
content reaches the level of EMC under
experimental condition

14. FREE MOISTURE CONTENT
It is the amount of water that is free to
evaporate from the solid surface
FMC = Total water content – EMC

15. Rate relationships
• Rate relationship can be studied with a simple
model which mimic the conditions of a dryer.
• a wet slab of material to be dried is placed in the
trays whose bottom and sides are insulated.
• The air having constant temperature, humidity,
pressure and air velocity is blown over the solid.
• The superficial water diffuses through the
surrounding stationary air film

16. Rate relationships
• The air carries this water
• Periodically the slab 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 a wet weight or dry weight basis

17. Rate relationships
• % loss on drying(LOD)
= * 100
• %moisture content (MC)
= *100
• Drying rate =

18. DRYING RATE CURVE.
• From the above experiment we get data for
FMC .
• FMC plotted on x axis and drying rate on y axis
• The curve so obtained is called drying rate
curve.

19. DRYING RATE CURVE

20. INITIAL ADJUSTMENT RATE PERIOD
• Time corresponding to A-B
B- wet bulb temperature of drying air

21. CONSTANT RATE PERIOD
• Time corresponding to BC
• Temperature and rate of drying remains
constant
• Moisture evaporating from the surface is
replaced by the water diffusing from the
interior of the solid
• Critical moisture content

22. FIRST FALLING RATE PERIOD
• Time corresponding to CD – unsaturated
surface drying
• Once the surface water evaporated then there
will not be enough water to maintain
continuous film on the surface

23. FIRST FALLING RATE PERIOD
• Dry spots begins to appear
• Rate of drying fall off
• The point D – second critical point
at this point the film of surface water is
completely evaporated

24. SECOND FALLING RATE PERIOD
• Time corresponding to DE
• Rate of drying falling rapidly than the first
falling rate period
• Rate of drying dependent on the rate of
diffusion of water vapor
• E- Equilibrium moisture content

25. SECOND FALLING RATE PERIOD
• Beyond E, the drying rate is equal to zero.
• i.e. temperature and moisture content remain
constant

26. • Don’t worry about what I am doing. Worry
about why you are worried about what I am
doing