Drying and Pharmaceutical Dryer

 

DRYING

·      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.

·        
Drying is accomplished by thermal techniques but non–thermal drying processes such as
squeezing wetted sponge, adsorption by desiccant (desiccation) and extraction are also used.

·      In bio products like food, grains, and
pharmaceuticals like vaccines, the
solvent
to be removed is almost invariably water.

Difference between Drying
and
Evaporation

 

·        
The main objectives of drying include to preserve foods and increase their shelf life by reducing
the water content and water activity; avoid the need for use of refrigeration systems
for transport and storage (expensive);
reduce space requirements
for storage and transport.

·        
In pharmaceutical technology, drying is
carried out for one or more of the fallowing 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.

APPLICATIONS OF DRYING

·        
Preparation of bulk drugs: In the preparation of bulk drugs, drying is the final stage of processing. A few examples are – dried aluminum hydroxide, spray dried lactose and powdered extracts.

·        
Preservation of drug products: Drying is necessary in order to avoid deterioration. For examples protection of blood products, skin, tissues and crude drugs from microbial growth.

·        
Improved characteristics: Drying produces materials of spherical shape, uniform size, free flowing
and enhanced solubility.

·        
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.

·        
Drying
as final step: Drying is the final step in evaporation, filtration, and
crystallization

Mechanism
of drying

• Involves both heat and mass transfer

• Rate of evaporation is related to rate of heat transfer as

             dW/d∅=q/ƛ

• The rate equation for mass transfer

            dW/d∅=KA (Hs-H
g )

Moisture
content

• Bound moisture and free moisture

• % moisture content=  
weight of water in samplex100/ weight of dry sample

• If exactly 5 g of moist sample is brought to a constant dry
weight of 4g, calculate loss on drying and moisture content

• Percent MC is more realistic value in determination of dryer
load capacity

• Equilibrium moisture content depends on

– Temperature and humidity of air

– Properties of the material

Rate of
drying & Time of drying

Classification
and types of dryers

·        
Static Bed
Dryers (Ex. Tray Dryers, Freeze Dryer)

·        
Moving Bed Dryer (Ex. Drum Dryer)

·        
Fluidised Bed Dryer

·        
Vacuum Dryer

·        
Pneumatic Dryer (Ex. Spray Dryer)

TRAY DRYER

Principle

·        
The basic working principle of this incredible machine is the continuous circulation
of hot air.

·  
In the tray dryer,
moisture is removed from the solids that are placed in the tray by
a forced convectional heating.

·        
The moist air is removed is partially but in a simultaneous
fashion.