Desiccant

Source: Wikipedia, the free encyclopedia.
For the use of chemicals in agriculture to aid harvesting, see crop desiccation.
molecular sieves
used as desiccant in drug containers to keep contents dry.
Silica gel in a sachet or porous packet

A desiccant is a

hygroscopic substance that is used to induce or sustain 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.

Types of desiccants

Main article: List of desiccants

Although some desiccants are

calcium sulfate, calcium chloride, and molecular sieves (typically, zeolites). Desiccants may also be categorized by their type, either I,II,III,IV, or V. These types are a function of the shape of the desiccant's moisture sorption isotherm
.

Alcohols and acetones are also dehydrating agents. Development of desiccants made of treated rice husks is a promising use of waste agricultural products.[citation needed]

Performance efficiency

One measure of desiccant efficiency is the ratio (or percentage) of water storable in the desiccant relative to the mass of desiccant.

Another measure is the residual

relative humidity
of the air or other fluid being dried.

The performance of any desiccant varies with

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.

Colored saturation indicators

Indicating silica gel

Sometimes a

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.

Applications

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 refrigeration systems to absorb water carried by the refrigerant, whether residual water left over from the construction of the system, or water released by the degradation of other materials over time.

Bagged desiccants are also commonly used to protect goods in barrier-sealed

Pharmaceutical packaging
often includes small packets of desiccant to keep the atmosphere inside the package below critical levels of water vapor.

Desiccants induce dryness in any environment and reduce the amount of moisture present in air. Desiccants come in various forms and have found widespread use in the food, pharmaceuticals, packing, electronics and many manufacturing industries.

Air conditioning systems can be based on desiccants, as drier air feels more comfortable and absorbing water itself removes heat.[5]

Desiccants are used in livestock farming, where, for example, new-born piglets are highly susceptible to hypothermia owing to their wetness.[6]

Drying of solvents

Toluene is heated under reflux with sodium and benzophenone to produce dry, oxygen-free toluene. The toluene is dry and oxygen free when the intense blue coloration from the benzophenone ketyl radical is observed.
Main article: Air-free technique

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.[7][8]

See also

References

  1. doi:10.1002/14356007.a23_583.pub3
    .
  2. ^ Rollo, P (1996). A Protective packaging evaluation involving a high barrier film lamiation, desiccants and oxygen absorbers (MSc). Rochester Institute of Technology. Retrieved August 8, 2021.
  3. ^ MIL-D-3464E, MILITARY SPECIFICATION: DESICCANTS, ACTIVATED, BAGGED, PACKAGING USE AND STATIC DEHUMIDIFICATION, 1987, retrieved August 8, 2021
  4. S2CID 101082998
    . Retrieved 11 August 2021.
  5. doi:10.1016/j.rser.2004.09.010
    .
  6. PMID 33241187
    .
  7. ISBN 978-0-7506-7571-0
    .
  8. S2CID 17801540
    .

Further reading
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