Foundations and Process Engineering

Freeze drying or lyophilizalion is a drying process, in which the solvent and/or the medium of suspension is crystallized at low temperatures and thereafter sublimated from the solid state directly into the vapor phase. 

Freeze drying is mostly done with water as solvent. Fig. 1.1 sows the phase diagram of water and the area in which this transfer from solid to vapor is possible. This step is difficult, even for pure water. If the product contains two or more components in true solutions or suspensions, the situation can become so complicated that simplified model substances have to be used. Such complex systems occur ubiquitously in biological substances. 

The second step, the drying, transforms the ice or water in an amorphous phase into vapor. 

Due to the low vapor pressure of the ice, the vapor volumes become large, as can be seen in Fig. 1.2. 

The most important goal of freeze-drying is to produce a substance with good shelf stability and which is unchanged after reconstitution with water, though this depends also very much on the third step of the process the packing and conditions of storage. 

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