Freezing indirect contact

A definite advantage of freeze crystallization, important in many food industry applications, is that volatile flavour components that are normally lost during conventional evaporation can be retained in a freeze-concentrated product. In fact, at present, freeze crystallization finds its main application in the food industry, for the concentration of fruit juices, etc. Indirect-contact freezing processes are normally used, e.g. the liquid feedstock is crystallized in a scraped-surface heat exchanger (section 8.2.2) and the resulting ice slurry passes to a wash column where the crystals are separated and washed to recover valuable product. The wash column is the key item in the process. Figure 8.56 shows an example of the Grenco system of freeze crystallization. 

Freezing. Ill Vacuum cooling-. The material freezes ilsell by the evaporation of water as it is subjected quickly to high vacuum. (2) Direct contact The material is immersed in a cold liquid or in a slrcatn of cold air or inert gases. (3) indirect ronuift The material is frozen on cold surfaces. 

Drying processes can also be categorized according to the physical conditions used to add heat and remove water vapor (1) in the first category, heat is added by direct contact with heated air at atmospheric pressure, and the water vapor formed is removed by the air (2) in vacuum drying, the evaporation of water proceeds more rapidly at low pressures, and the heat is added indirectly by contact with a metal wall or by radiation (low temperatures can also be used under vacuum for certain materials that may discolor or decompose at higher temperatures) and (3) in freeze drying, water is sublimed from the frozen material. 

The first indirect observation of surface melting comes from Faraday who realized that ice at around 0°C is entirely covered by a water layer. As mentioned in Section 8.1.1, once two ice blocks are brought into contact, and kept thermally isolated from the environment, the water in the contact region quickly soHdifies since it is not any more at the surface and the blocks freeze together. 

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