Abstract

Desiccants are classified by four drying mechanisms: chemical reaction, physical absorption with constant relative humidity, physical absorption with variable relative humidity, and physical adsorption. Solid desiccant materials including activated alumina, silica gel, molecular sieves, and lithium chloride are commonly used in both static and dynamic drying. Other solids such as barium oxide, calcium chloride, calcium oxide, calcium sulfate, clays, metal halides, perchlorates, phosphorus pentoxide, sodium and potassium hydroxides, are used mainly in static drying.

Desiccants must be chemically compatible with the material being dried, and, if in doubt, the compatibility should be tested prior to use. Ideally, the desiccant and the material should not react to avoid producing harmful or undesirable by-products. The water capacity of some static desiccants and the efficiency (or effectiveness) of various desiccants in drying air and laboratory solvents vary widely. The dynamics of closed-system drying involves the exponential decay of the water content over time.

Desiccants used in dynamic applications include both liquids and solids. Solid adsorbents are often used in drying systems with multiple beds, and sometimes in rotating wheels. The largest users of solid desiccants are the hydrocarbon processing, insulating glass drying, and packaging industries.