Desiccants
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A desiccant is a
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...
substance that is used to induce or sustain a state of dryness (
desiccation Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...
) in its vicinity; it is the opposite of a
humectant A humectant is a hygroscopic (water-absorbing) substance used to keep things moist. They are used in many products, including food, cosmetics, medicines and pesticides. When used as a food additive, a humectant has the effect of keeping moistu ...
. Commonly encountered pre-packaged desiccants are solids that absorb
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
. 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

Although some desiccants are
chemically inert In chemistry, the term chemically inert is used to describe a substance that is not chemically reactive. From a thermodynamic perspective, a substance is inert, or nonlabile, if it is thermodynamically unstable (negative standard Gibbs free en ...
, others are extremely reactive and require specialized handling techniques. The most common desiccant is
silica gel Silica gel is an amorphous and porosity, porous form of silicon dioxide (silica), consisting of an irregular three-dimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain wate ...
, an otherwise inert, nontoxic, water-insoluble white solid. Tens of thousands of tons are produced annually for this purpose. Other common desiccants include
activated charcoal "Activated" is a song by English singer Cher Lloyd. It was released on 22 July 2016 through Vixen Records. The song was made available to stream exclusively on ''Rolling Stone'' a day before to release (on 21 July 2016). Background In an inter ...
,
calcium sulfate Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate ...
,
calcium chloride Calcium chloride is an inorganic compound, a Salt (chemistry), salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with cal ...
, and
molecular sieve A molecular sieve is a material with pores of uniform size comparable to that of individual molecules, linking the interior of the solid to its exterior. These materials embody the molecular sieve effect, in which molecules larger than the pore ...
s (typically,
zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a meta ...
s). 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 The relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm. For each humidity value, a sorption isotherm indicates the corresponding wate ...
. Alcohols and acetones are also dehydrating agents.
Diethylene glycol Diethylene glycol (DEG) is an organic compound with the formula (HOCH2CH2)2O. It is a colorless, practically odorless, and hygroscopic liquid with a sweetish taste. It is a four carbon Dimer (chemistry), dimer of ethylene glycol. It is miscible in ...
is an important industrial desiccant. It absorbs water from
natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...
, minimizing the formation of
methane hydrate Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large am ...
s, which can block pipes.


Performance efficiency

One measure of desiccant efficiency is the ratio (or
percentage In mathematics, a percentage () is a number or ratio expressed as a fraction (mathematics), fraction of 100. It is often Denotation, denoted using the ''percent sign'' (%), although the abbreviations ''pct.'', ''pct'', and sometimes ''pc'' are ...
) of water storable in the desiccant relative to the
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
of desiccant. Another measure is the residual
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
of the air or other fluid being dried. For drying gases, a desiccant's performance can be precisely described by the
dew point The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air. When the air at a temperature above the ...
of the dried product.


Colored saturation indicators

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 (), which is blue when
anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
, but turns purple upon bonding with two water molecules (·2). Further hydration results in the pink hexaaquacobalt(II) chloride complex . However, the use of cobalt chloride raises health concerns, being potentially
carcinogenic A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and Biological agent, biologic agent ...
.


Applications

Applications of desiccants are dominated by the petrochemical industry. Hydrocarbons, including natural gas, often must be anhydrous or nearly so for processing or for transport. Catalysts that are used to convert some petroleum fractions are generally deactivated by even traces of water. Natural gas tends to form solid
methane hydrate Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large am ...
s which can block pipes.


Domestic uses

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
shipping container A shipping container is a container with strength suitable to withstand shipment, storage, and handling. Shipping containers range from large reusable steel boxes used for intermodal shipments to the ubiquitous corrugated box design, corrugated b ...
s against moisture damage: rust, corrosion, etc. Hygroscopic cargo, such as cocoa, coffee, various nuts and grains, and other foods can be particularly susceptible to mold and rot when exposed to condensation and humidity. Because of this, shippers often take measures by deploying desiccants to protect against loss. Pharmaceutical packaging often includes small packets of desiccant to keep the atmosphere inside the package below critical levels of water vapor. Air conditioning systems can be based on desiccants, as drier air feels more comfortable and absorbing water itself removes heat. Desiccants are used in livestock farming, where, for example, new-born piglets are highly susceptible to hypothermia owing to their wetness.


Laboratory uses

Desiccants are also used to remove water from
solvents A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
. Drying generally involves mixing the solvent with the solid desiccant.
Molecular sieve A molecular sieve is a material with pores of uniform size comparable to that of individual molecules, linking the interior of the solid to its exterior. These materials embody the molecular sieve effect, in which molecules larger than the pore ...
s are superior as desiccants relative to chemical drying reagents such as sodium-benzophenone. Sieves offer the advantages of being safe in air and recyclable.


See also

*
Desiccator Desiccators are sealable enclosures containing desiccants used for preserving moisture-sensitive items such as cobalt chloride paper for another use. A common use for desiccators is to protect chemicals which are hygroscopic or which react wit ...
* Humidity buffering * Humidity indicator card *
Moisture sorption isotherm The relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm. For each humidity value, a sorption isotherm indicates the corresponding wate ...
* Solar air conditioning *
Oxygen scavenger Oxygen scavengers or oxygen absorbers are added to enclosed packaging to help remove or decrease the level of oxygen in the package. They are used to help maintain product safety and extend shelf life. There are many types of oxygen absorbers ...
(oxygen absorber) *
Sorbent A sorbent is an insoluble material that either absorbs or adsorbs liquids or gases. They are frequently used to remove pollutants and in the cleanup of chemical accidents and oil spills. Besides their uses in industry, sorbents are used in comm ...
* Volatile corrosion inhibitor * Cromer cycle


References


Further reading

* * {{Authority control Packaging