Principle:
The material is frozen in a suitable container connected to a high vacuum system, so that the vapour pressure of water is reduced to less than that of material being dried.
Thus, it reduces the temperature and pressure to values bellow the triple point.
Under these conditions, any heat transfer is used as latent heat and the ice sublimes directly to the vapour state.
The water vapour is removed from the system by condensation in a condenser maintained at a temperature lower than a frozen material.
Components of Freeze dryer:
1 A chamber for vacuum drying
2 A vacuum source
3 A heat source
4 A vapour removal system
Working:
Pre-treatment:
It is done to reduce volume of solution.
The solution is pre-concentrated under normal vacuum tray drying.
This reduces drying time by 8-10 times.
Pre-freezing:
This is done to solidify water.
Sample is frozen at a temp. below -50 °C.
Primary drying:
Material is spread on the surface to increase surface area.
Temp. & pressure is kept below the triple point of water.
Heat is supplied & ice sublimes directly into vapour form.
Secondary drying:
Moisture remained after primary drying is removed by an ordinary vacuum drying.
Vacuum drying is done at a temp. 50-60 °C.
Packing:
Packaging of product is performed carefully to protect it from moisture.
The containers should be closed under aseptic conditions.
Containers are labeled and packed in card-board boxes.
Advantages:
The product obtained is light and porous having excellent solubility.
The chances of hydrolysis are minimized as drying takes place at a very low temperature.
Drying takes place under vacuum; hence oxidation is minimized as there is no contact with air.
The heat-sensitive materials can be dried.
The loss of volatile material is minimum.
The freeze-dried material can be stored at room temperature if it is properly sealed in an inert atmosphere.
The sterility of the product can be maintained.
Disadvantages:
1. The process is very expensive because a complicated plant is used.
2. The product obtained is very hygroscopic, so packaging requires special precautions.
3. The period of drying is quite long.(usually not less than 10 hours)