The importance of drying
The differences between drying and evaporation
Definition of drying
Purpose of drying
The names of different dryers used in pharmacy
Why do we dry the clothes?
Why do we dry the green leaves or crude drugs?
Why do we dry the washed vessels?
Why do we dry the tablet granules?
Why do we dry the bottles, glass apparatus after cleaning?
Drying is done to carry out size reduction
Drying is done to avoid deterioration on storage
Drying is done to dry the tablet granules to reduce the moisture
Drying reduces the bulk and weight hence, less transportation charges
Drying is done for certain preparations Ex: spray dried lactose
It is defined as a process in which the liquid is removed from a material by the use of heat
Drying
Removal of less amount moistureof
Drying occurs below boiling
point
Done to get a stable dry product
Emphasize on solid product
Evaporation
Removal of large amount liquidof
Evaporation occurs more boiling pointat
Generally done for concentration
Emphasize on reducing the
Used in drying are depends on:
Physical characteristics of the product
Thermolabile or not
Nature of solvent used
Quantity of the product
Source of heat applied
Dryers for dilute solutions, suspensions and slurries a) Drum dryer
b) Spray dryer
Dryers for damp solid materials a) Tray or shelf dryer
b) Tunnel dryer c) Rotary dryer
d) Fluidized bed dryers (FBD) e) Vacuum dryer
Also known as cabinet or compartment dryer
These are essentially hot air ovens
Material spread in trays in thin layer
In large scale, heating is done by forced circulationFig: 58.1
It is for batch process
Material is over heated
It is long duration process
Loading and unloading, labour costs are more
Fig: 58.2
Trays are loaded on trucks (racks fitted with wheels)
Loaded trucks are pushed and taken out of the drying chamber
Convenient in loading and unloading of the material
Fig: 58.3
Also known as vacuum oven
Consists of jacketed vessel
Has to withstand vacuum in the
oven and steam present in the jacket
Oven and dryer can be loaded with air tight seal
Connected to a vacuum pump through a condenser and receiver
At a vaccum of 0.03 to 0.06 bar water boils at 350 C
Fig: 58.4
Very suitable for heat sensitive products
Porous and friable product is obtained
Valuable solvents can be recovered
Fig: 58.5
Heat transfer may be low and non uniform
Limited capacity
Labour and running costs are high
Finely divided powder may be drawn into the vacuum pump
Fig: 58.6
Construction and working of fluidized bed dryer
Advantages and disadvantages of fluidized bed dryer
Gas is allowed to flow upward through a bed of material
Velocity should be greater than the settling velocity of the particles
Particles are lifted in the gas stream
The mixture of solid and gas is fluidized
Each solid particle is surrounded by drying gas
Drying takes place at a shorter period
Fig: 59.2
Vertical type fluidized bed dryer
Used for batch process
Horizontal type fluidized bed dryer
Used for continuous drying
Material to be dried is placed in Stainless Steel chamber having perforated bottom
Drying chamber fitted with wheel facilitates for loading and unloading
Filtered hot air passes through the bed with the
If the velocity of the air
increases, the bed begins to expand
Further increase in velocity causes rapid expansion
Particles show a turbulent motion and “ fluidized ”
Fig: 59.5
Efficient heat exchange takes place between individual
particles and hot air
Moist air is carried away quickly
5kg to 200kg can be dried in about 20-40 minutes
These dryers are efficient and 15 times faster than conventional Tray dryers
Drying takes place from the individual particle and not from the whole bed
Heating time is short for thermolabile product
Free flowing product is obtained
Due to short time, unit has high output
Drying temperature is uniform
Advantages
Due to turbulent action more fines are produced but, can be avoided by using suitable binders
For too wet granules not suitable, as they stick together
May develop electrostatic charges and can be avoided by earthing the unit
The principle of freeze drying
The working of freeze dryer
Advantages and disadvantages of freeze dryer
It is also known as sublimation drying
It is also known as lyophillization as the dried product has great affinity for water ( lyophillic = water loving)
First the material to be dried is frozen in suitable container
By connecting to a vacuum system, the vapor pressure is greatly reduced
The temperature and pressure reduced to the values below the triple point
Mild heat is supplied and the frozen ice sublimes
Chamber for chilling the sample
A vacuum source
A heat source
A vapor removal system
It is carried out for batch process
The product is frozen first by putting on a shelf circulating refrigerants
After freezing, vacuum is applied to the chamber
Mild heat is supplied to the product by electric coils or by circulating hot water
Process is continued till a dry, spongy solid is obtained
Products to be freeze dried are sterilized and aseptically
distributed into final containers
Container kept open during the process and sealed
immediately after drying
process Fig: 60.3
The product is light, porous with good solubility
No chances of hydrolysis because, drying is at low temperature
No chances of oxidation because of no contact with air
Freeze dried products are more stable than
Very expensive
Time taking process
Dried product is very hygroscopic and requires special packing precautions
Mostly suitable for highly thermolabile products
Used for drying biological products such as blood plasma, vaccines, enzymes,
microbiological cultures, hormones and antibiotics
