Changing Phase Relationships during Freezing

Solid-liquid phase diagrams can be constructed fairly easily for systems where equilibrium conditions are rapidly established and where assays are simple to perform. The situation becomes complicated when the kinetics of dissolution lie outside the time frame of the experiment. This is unfortunately the case for most substances of relevance to freeze-drying. The problem is further discussed in Section 5.7. 

A corollary of this representation of freeze-concentration is that the process, commonly referred to as freezing damage or freezing 

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Figure 9 shows a load-change cycle which is typical for discontinuous SCF extraction. When the pressure release phase following completion of the extraction is considered, the question arises as to the true temperature course. It is wellknown that the pressure-dependent equilibrium temperature of CO2 falls to -79 C under atmospheric conditions. This relationship leads to short-term thermal stresses within the inner surfaces of the pressure vessel, particularly in the lower part where dry ice may form. There is an additional risk to the process that the charge may freeze within the pressure vessel. When designing equipment for the extraction of natural substances, definition of the non-stationary courses of pressure and temperature during pressure release is therefore of especial importance in the choice of materials and for the geometry of the pressure vessel. 

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