A desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its local vicinity in a moderately-well sealed container.
Commonly encountered pre-packaged desiccants are solids, and work through absorption or adsorption of water, or a combination of the two. Desiccants for specialised purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules.
Pre-packaged desiccant is most commonly used to remove excessive humidity that would normally degrade or even destroy products sensitive to moisture. Silica gel, calcium sulfate, calcium chloride, montmorillonite clay, and molecular sieves are commonly used as desiccants.
Rice is a common "low-tech" alternative, frequently used for example in salt-shakers to maintain granularity of table-salt for effective pouring or shaking. Rice, however, is not a good general purpose desiccant since, unless immersed in an organism-hostile environment like pure salt, over time may be eaten by creatures that might in turn pollute the product that is being preserved. Salt itself is another effective desiccant, used for millennia in preparation of dried foodstuffs.
A desiccant may be chosen for a particular task based not only on its effectiveness at drying, but also perhaps for any natural antibiotic/fungicidal/pesticidal effect, or for a lack of harmful effect on humans -- for instance salt has a natural antibiotic and anti-fungal effect and is not harmful to humans when consumed in moderate quantities, hence its popular use as drying agent in preparation of dried foods.
Because of their common use in preservation, usually it is desirable for a desiccant to be substantially chemically stable or chemically inert, and a number of useful desiccants with this property have been identified and are in common use, such as silica gel, chalk- and clay-based desiccants.
One measure of desiccant efficiency is the ratio (or percentage) of water storeable in the desiccant, relative to the mass of desiccant (ie weight).
Another measure is the residual relative humidity of the air or other fluid being dried.
The performance of any desiccant varies with temperature and both relative humidity and absolute humidity. To some extent, desiccant performance can be precisely described, but most commonly, the final choice of which desiccant best suits a given situation, how much of it to use, and in what form, is made based on testing and practical experience.
Coloured saturation indicators
Often some sort of humidity indicator is included in the desiccant to show, by color changes, the degree of water-saturation of the desiccant. One commonly used indicator is cobalt chloride (CoCl2). Anhydrous cobalt chloride is blue. When it bonds with two water molecules, (CoCl2•2H2O), it turns purple. Further hydration results in the pink hexaaquacobalt(II) chloride complex [Co(H2O)6]Cl2.
Most useful desiccants can be recycled by thermally-induced drying, for example, in a conventional kitchen oven, or with solar energy. A cost-effective, low-energy, continuous-cycle desiccant dehumidifier or desiccant regeneration system can be easily designed from off-the-shelf component parts.
Drying of solvents
Desiccants are also used to dry solvents, typically used for moisture free reactions (e.g. the Grignard reaction). While various desiccants are used in a variety of ways, the method generally involves stirring the desired solvent with the drying agent.
Often, the drying agent will react with moisture to form an insoluble solid in the solvent being dehydrated, thus the precipitate can be removed by filtration. For more demanding applications, requiring very pure, anhydrous solvents a distillation rig is used to remove moisture and other impurities simultaneously.
Lavan, Z.; Jean-Baptiste Monnier, Worek, W. M.; 1982, "Second Law Analysis of Desiccant Cooling Systems", Journal of Solar Energy Engineering, Vol. 104, pp. 229-236.
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