Introduction and Methods of Drying

The discussions of drying in this chapter are concerned with the removaTof water from process materials and other substances. The term drying is also used to refer to removal of other organic liquids, such as benzene or organic solvents, from solids. Many of the types of equipment and calculation methods discussed for removal of water can also be used for removal of organic liquids. 

Drying, in general, usually means removal of relatively small amounts of water from material. Evaporation refers to removal of relatively large amounts of water from material. In evaporation the water is removed as vapor at its boiling point. In drying the water is usually removed as a vapor by air. 

In some cases water may be removed mechanically from solid materials by presses, centrifuging, and other methods. This is cheaper than drying by thermal means for removal of water, which will be discussed here. The moisture content of the final dried product varies depending upon the type of product. Dried salt contains about 0.5% water, coal about 4%, and many food products about 5%. Drying is usually the final processing step before packaging and makes many materials, such as soap powders and dyestuffs, more suitable for handling. 

Some biological materials and pharmaceuticals, which may not be heated for ordinary drying, may be freeze-dried as discussed in Section 9.11. Also, in Section 9.12, sterilization of foods and other biological materials is discussed, which is another method often employed to preserve such materials. 

Drying methods and processes can be classified in several different ways. Drying processes can be classified as batch, where the material is inserted into the drying equipment and drying proceeds for a given period of time, or as continuous, where the material is continuously added to the dryer and dried material continuously removed. 

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