Desiccant

A desiccant is a hygroscopic substance that is used to induce or sustains a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids that absorb water. Desiccants for specialized purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules. They are commonly encountered in foods to retain crispness. Industrially, desiccants are widely used to control the level of water in gas streams.Although some desiccants are chemically inert, others are extremely reactive and require specialized handling techniques. The most common desiccant is silica, an otherwise inert, nontoxic, water-insoluble white solid. Tens of thousands of tons are produced annually for this purpose. Other common desiccants include activated charcoal, calcium sulfate, calcium chloride, and molecular sieves (typically, zeolites).One example of desiccant usage is in the manufacture of insulated windows where zeolite spheroids fill a rectangular spacer tube at the perimeter of the panes of glass. The desiccant helps to prevent the condensation of moisture between the panes. Another use of zeolites is in the dryer component of air conditioning systems to help maintain the efficacy of the refrigerant. Desiccants are also commonly used to protect goods in shipping containers against moisture damage. Hygroscopic cargo, such as cocoa, coffee, and various nuts and grains, are particularly susceptible to mold and rot when exposed to condensation and humidity. Because of this, shippers often take precautionary measures to protect against cargo loss.Desiccants are also used to remove water from solvents, typically required by chemical reactions that do not tolerate water, e.g., the Grignard reaction. The method generally, though not always, involves mixing the solvent with the solid desiccant. Studies show that molecular sieves are superior as desiccants relative to chemical drying reagents such as sodium-benzophenone. Sieves offer the advantages of being safe in air and recyclable.