Freeze Dryer

Freeze Dryer

Freeze dryer (specialized):

Principle: Freeze dryer is an indirect batch dryer. Since materials are dried by freezing, it is called freeze drying and the equipments by which freeze drying is performed is called freeze dryer.

The material to be dried is first frozen and then subjected under a high vacuum to heat (supplied by conduction or radiation) so that the frozen liquid sublimes leaving only the porous solid.

How to works: The materials to be dried are placed into the chamber:
 

Stages of the freeze-drying process:

1. Freezing: Usually the material is frozen before the application of vacuum to avoid frothing.
2. Vacuum creation: The containers and the frozen material must be connected to a vacuum source sufficient to drop the pressure below the triple point and remove the large volumes of low pressure vapor formed during drying.
3. Heat supply: When a heat of sublimation is supplied the ice slowly sublimes leaving a porous solid which still contains about 0.5% moisture after a primary drying.
4. Vapor removal: The vapor removal system is started either by using vacuum pump, a desiccant (e.g. P₂O₅) or a condenser. The vapor removal is started to remove large volume of water during drying the material.

Applications: The method is applied only to biological products: e.g.

1. Antibiotics (Other than penicillin).
2. Blood products e.g. blood serum, plasma.
3. Vaccines e.g. B.C.G. small pox, yellow fever.
4. Enzyme preparations e.g. hyaluronidase.
5. Microbiological cultures.
6. Hormones e.g. insulin, oxytocin etc.
7. Various types of foodstuffs.

Advantages:

1. Heat sensitive substances like blood products, antibiotics etc can be easily dried without any decomposition.
2. Volatile substance can be dried easily.
3. There is no contact with the air and oxidation is minimized since vacuum is applied.
4. Proteins and protein containing substances are freeze dried to prevent coagulation.
5. The porous form of the product gives ready solubility.

Disadvantages:

1. The porosity, ready solubility and complete dryness yield a very hygroscopic product.
2. The process is very slow and uses complicated plant, which is very expensive.
3. It is not a general method of drying.

It is difficult to adapt the method for solutions containing non-aqueous solvents.