desiccant

A desiccant, or drying agent, is a substance used to keep environments or materials dry by removing moisture from the air or other gases. Desiccants work mainly through adsorption, in which water molecules cling to the surface and into the pores of a solid, lowering humidity. Some desiccants also react chemically with water to form hydrates or hydroxides. The choice of desiccant depends on the desired level of dryness, temperature, and compatibility with the product being dried.

Common desiccants include silica gel, a porous form of silicon dioxide, and molecular sieves (zeolites) with uniform pore sizes for selective adsorption. Activated alumina is another high-surface-area adsorbent. Calcium sulfate (Drierite) and various clay-based desiccants are used for economical, general packaging. Calcium chloride is a highly hygroscopic salt used in larger desiccant packs, though it can form brines in very humid conditions and may cause leakage if the pack is compromised.

Applications of desiccants are broad. They are widely used in packaging to protect foods, pharmaceuticals, electronics, optics, and textiles from moisture. They also find use in laboratories, desiccators, and industrial processes such as natural gas dehydration, air drying, and solvent drying.

Regeneration and disposal vary by material. Many physical desiccants can be regenerated by heating or vacuum drying to drive off bound water, with temperatures specific to the material (for example, silica gel is commonly dried around 120–200 °C). Some desiccants are single-use and must be replaced. Spent desiccants should be disposed of according to local regulations, and chemical desiccants may require special handling. Safety notes include not ingesting desiccant beads and avoiding inhalation of dust.