A desiccant is a substance that promotes drying by absorbing moisture or water vapor from the air. Consumers are probably most familiar with small paper packets of desiccants that are often packed with items so that they will not rust or mold during shipment, especially for imported items that will spend time in the hold of a ship between the time they leave the factory and arrive at the store.
In industrial settings, desiccants are used for the same reason in compressed air systems: to dry the air. The changes in pressure when air is compressed will cause water to condense, and this moisture must be removed. To do this, compressed air is passed over desiccant material to dry it out. There are several types of desiccants:
Silica occurs in nature as Silica dioxide, SiO2. It is processed into gel-like beads for use as a desiccant dryer. Silica gel was first used in World War I to absorb vapor in gas mask canisters. Pure silica gel is one of the few desiccants that is safe for use around food.
Indicating Silica Gel
Indicating silica gel is silica gel that is processed with cobalt chloride. The cobalt chloride will change color from blue to pink as the silica gel absorbs moisture. This 'indicates' when the silica gel is becoming saturated. Because cobalt chloride is a heavy metal salt, it cannot be safely used around food.
Clay that is high in magnesium aluminum silicate, a naturally occurring mineral, is processed into Montmorillonite clay for use in drying. It works best at room temperature or below. Clay is one of the most inexpensive desiccants, which makes it popular for industrial use
Quicklime and Gypsum
Quicklime is a common term for Calcium Oxide. It is alkaline and highly caustic, so special care in handling is necessary. Its chief benefit is that it can be used to gradually drop the humidity to a very low state. Gypsum is Calcium Sulfate.
Molecular Sieve is a synthetic desiccant manufactured from aluminosilicates. Like the name sounds, it has many tiny holes or pores the absorb liquid and gas. It is a highly effective desiccant that can absorb water up to 22% of its own weight.
Most of these various types of desiccant dehumidifiers can be recharged and recycled by using a heating process to dry them for reuse.