Sorbents are solid or liquid materials which have the property of extracting and holding other substances (usually water vapour) brought into contact with them.
Sorbents can be classified into two general categories.
Absorbent : A sorbent which changes either physically, chemically or both during the sorption process. Lithium Chloride is an example of solid absorbent. When water is absorbed on this material it changes to hydrated state. In liquid sorption dehumidification system, the air is passed through sprays of a liquid sorbent such as lithium chloride or glycol solution.
The sorbent in an active state has a vapour pressure below that of the air to be dehumidified and absorbs moisture from the air stream. The sorbent solution during the process of absorption becomes diluted with moisture which during regeneration is given up to an air stream in which the solution is heated.
Typically absorbent used is lithium chloride in either liquid form or as solid crystals in a honey-comb shell.
Adsorbent : A sorbent which does not change physically or chemically during the sorption process. Adsorbents are normally granular beads or solids with porous structures making it capable of holding large amounts of water on their surface.
The principle behind desiccant dehumidification is that the desiccant is exposed to moisture laden air, from where it extract the moisture. The saturated desiccant is heated, which drives off the collected moisture into the exhaust air stream. The regenerated desiccant is ready for use again. .
Thus, a continuous cycle of sorption and regeneration can be set up giving very low dew points. Typical absorbents used are Silica gel, Molecular Sieve and Activated Alumina.