Table of Contents
Freeze Dryer
Principle of Freeze Dryer
- In freeze dryer, water is removed from the frozen state by sublimation that is direct change of water from solid in vapour without conversion to a liquid phase.
- Heat is supplied which transfer as latent heat and ice sublimed directly into vapour state.
Eutectic point- The pressure and temperature at which the frozen solid vaporizes without conversion to a liquid is referred to s the eutectic point.
- Drying chamber in which trays are loaded.
- Heat supply in the form of radiation source, heating coils.
- Vapour condensing or adsorption system.
- Vacuum pump or steam ejector or both.
Working/process of freeze dryer
Working of freeze dryer include in many steps.
- Preparation and pretreatment.
- Prefreezing and solidifying water.
- Primary drying (sublimation of ice under vacuum).
- Secondary drying (removal of residual moisture under high vacuum).
- Packing.
- Preparation and pretreatment-
- The volume of solution introduced into the container is limited by its capacity. Satisfactory freeze drying beyond a certain limit of depth of liquid is not possible.
- Therefore pre-treatment is essential.
- The solution is pre-concentrated under normal vacuum tray drying. The final products become obtained becomes more porous.
- Prefreezing and solidifying water-
- Vials, ampoules or bottles in which the aqueous solution is packed are frozen in cold shelves about -50°C.
- During this stage cabinet is maintained at low temperature and atmospheric pressure.
- Primary drying-
- In this step, the material to be dried is spread as much large surface as possible for sublimation.
- The temperature and pressure should be below the triple point of water, that is 0.0098°C and 0.533 KPa for the sublimation, when water alone present.
- Primary stage removes easily removable moisture. During this stage about 98% to 99% water is removed.
- Secondary Drying-
- The temperature of the solid is raised to as 50 to 60° C but vacuum is lowered below that is used in primary drying.
- The rate of drying is very low and it takes about 10 to 20 hours.
- Packing-
- After vacuum is replaced by inert gas, the bottles and vials are closed.
Uses/Applications of freeze dryer
- It is commonly used in the production of dosages forms as injections, solutions, and suspensions.
Advantage of freeze dryer
- Moisture level can be kept as low as possible without decomposition.
- Thermolabile materials can be dried
- Sterility can be maintained.
Disadvantage of freeze dryer
- Equipment costs very high.
- The period of drying is very long.