Desiccant

A desiccant is a hygroscopic substance that induces or sustains 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.

Molecular Sieve5
Canisters are commonly filled with silica gel and other molecular sieves used as desiccant in drug containers to keep contents dry.
Silica gel bag open with beads
Silica gel in a sachet or porous packet

Types of desiccants

Although some desiccants are chemically inert, others are extremely reactive and require specialized handling techniques. The most common desiccant is silica, an otherwise inert, nontoxic, water-insoluble white solid. Tens of thousands of tons are produced annually for this purpose.[1] Other common desiccants include activated charcoal, calcium sulfate, calcium chloride, and molecular sieves (typically, zeolites).

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 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.

Colored saturation indicators

Indicating-silica-gel
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 air conditioning systems to help maintain the efficacy of the refrigerant. Desiccants are also commonly used to protect goods in shipping containers against moisture damage. Hygroscopic cargo, such as cocoa, coffee, and various nuts and grains, are particularly susceptible to mold and rot when exposed to condensation and humidity. Because of this, shippers often take precautionary measures to protect against cargo loss.

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 made based on desiccants.[2]

Desiccants are used in different kinds of livestock farming to dry newborn animals, such as piglets. The use of a good desiccant can help them dry quicker and save energy, which can be crucial for the animal's development. Another use is to reduce bacteria and pathogens that thrive in wet surfaces, reducing bacteria pressure. However, some desiccants have a very high pH-level, which can be harmful for an animal's skin.

Drying of solvents

Toluene with sodium-benzophenone
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.

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.[3][4]

See also

References

  1. ^ Otto W. Flörke, et al. "Silica" in Ullmann's Encyclopedia of Industrial Chemistry, 2008, Weinheim: Wiley-VCH, . doi:10.1002/14356007.a23_583.pub3.
  2. ^ Daou, K; Wang, Xia (2005). "Desiccant cooling air conditioning: a review". Renewable and Sustainable Energy Reviews. 10 (2): 55–77. doi:10.1016/j.rser.2004.09.010.
  3. ^ Chai, Christina Li Lin; Armarego, W. L. F. (2003). Purification of laboratory chemicals. Oxford: Butterworth-Heinemann. ISBN 0-7506-7571-3.
  4. ^ Williams, D. B. G., Lawton, M., "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants", The Journal of Organic Chemistry 2010, vol. 75, 8351. doi: 10.1021/jo101589h

Further reading

  • Lavan, Z.; Jean-Baptiste Monnier & Worek, W. M. (1982). "Second Law Analysis of Desiccant Cooling Systems". Journal of Solar Energy Engineering. 104 (3): 229–236. doi:10.1115/1.3266307.
  • S. Sadik; J. W. White (1982). "True potato seed drying over rice". Potato Research. 25 (3): 269. doi:10.1007/BF02357312.
Activated alumina

Activated alumina is manufactured from aluminium hydroxide by dehydroxylating it in a way that produces a highly porous material; this material can have a surface area significantly over 200 m²/g. The compound is used as a desiccant (to keep things dry by absorbing water from the air) and as a filter of fluoride, arsenic and selenium in drinking water. It is made of aluminium oxide (alumina; Al2O3). It has a very high surface-area-to-weight ratio, due to the many "tunnel like" pores that it has. Activated alumina in its phase composition can be represented only by metastable forms (gamma-Al2O3 etc.). Corundum (alpha-Al2O3), the only stable form of aluminum oxide, does not have such a chemically active surface and is not used as a sorbent.

Air dryer

See Also: Compressed air dryerA compressed air dryer is used for removing water vapor from compressed air. Compressed air dryers are commonly found in a wide range of industrial and commercial facilities.

The process of air compression concentrates atmospheric contaminants, including water vapor. This raises the dew point of the compressed air relative to free atmospheric air and leads to condensation within pipes and the compressed air cools downstream of the compressor.

Excessive water in compressed air, in either the liquid or vapor phase, can cause a variety of operational problems for users of compressed air. These include freezing of outdoor air lines, corrosion in piping and equipment, malfunctioning of pneumatic process control instruments, fouling of processes and products, and more.

There are various types of compressed air dryers. Their performance characteristics are typically defined by the dew point.

Regenerative desiccant dryers, often called "regens" or "twin tower" dryers

Refrigerated dryers

Deliquescent dryers

Membrane dryersWater vapor is removed from compressed air to prevent condensation from occurring and to prevent moisture from interfering in sensitive industrial processes.

Arsenic acid

Arsenic acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO(OH)3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but is only found in solution, where it is largely ionized. Its hemihydrate form (H3AsO4·​1⁄2H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.

Calcium hydride

Calcium hydride is the chemical compound with the formula CaH2, and is therefore an alkaline earth hydride. This grey powder (white if pure, which is rare) reacts vigorously with water liberating hydrogen gas. CaH2 is thus used as a drying agent, i.e. a desiccant.CaH2 is a saline hydride, meaning that its structure is salt-like. The alkali metals and the alkaline earth metals heavier than beryllium all form saline hydrides. A well-known example is sodium hydride, which crystallizes in the NaCl motif. These species are insoluble in all solvents with which they do not react. CaH2 crystallizes in the PbCl2 (cotunnite) structure.

Calcium sulfate

Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.

Dehumidifier

A dehumidifier is an electrical appliance which reduces and maintains the level of humidity in the air, usually for health or comfort reasons, or to eliminate musty odor and to prevent the growth of mildew by extracting water from the air. It can be used for household, commercial, or industrial applications. Large dehumidifiers are used in commercial buildings such as indoor ice rinks and swimming pools, as well as manufacturing plants or storage warehouses.

Desiccation

Desiccation (from Latin de- "thorougly" + siccare "to dry") 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.

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 with water from humidity.

The contents of desiccators are exposed to atmospheric moisture whenever the desiccators are opened. It also requires some time to achieve a low humidity. Hence they are not appropriate for storing chemicals which react quickly or violently with atmospheric moisture such as the alkali metals; a glovebox or Schlenk-type apparatus may be more suitable for these purposes.

Desiccators are sometimes used to remove traces of water from an almost-dry sample. Where a desiccator alone is unsatisfactory, the sample may be dried at elevated temperature using Abderhalden's drying pistol.

Dry box

A dry box is a storage container in which the interior is kept at a low level of humidity. It may be as simple as an airtight and watertight enclosure, or it may use active means to remove water vapor from the air trapped inside.

Dry boxes are used to safely store items that would otherwise be damaged or adversely affected by excessive humidity, such as cameras and lenses (to prevent fungal growth), and musical instruments (to prevent humidity induced swelling or shrinkage of wooden instrument parts). They are also used in the storage of surface mount electronic components prior to circuit board assembly, to prevent water absorption that could flash into steam during soldering, destroying the part.

Drying tube

A drying tube or guard tube is a tube-like piece of apparatus used to house a disposable solid desiccant, wherein at one end the tube-like structure terminates in a ground glass joint for use in connecting the drying tube to a reaction vessel, for the purpose of keeping the vessel free of moisture.The tube-like structure is often bent and can also widen to form a bulb/desiccant reservoir. Typically a drying tube is 1–2 cm wide and 5–10 cm long and bent at an angle of about 90 degrees. In use, the drying tube is filled with a rechargeable desiccant such as calcium chloride, and the open end of drying tube is partially blocked (e.g. with wool, glasswool or a bung/cork which has a small bore made in it) and the drying tube is connected to the apparatus to be kept dry via the ground glass joint. If the drying tube is bent the bend is oriented so that solid desiccant does not fall into the reaction vessel. Some drying tubes have a glass sinter to prevent desiccant falling into the reaction vessel. Drying tubes are often prepared in advance and the desiccant can be replaced when exhausted.

Reactions which are being heated, or which evolve gases, must never be sealed because an overpressure may shatter the vessel. Drying tubes are usually fitted on top of the reflux condenser, allowing the pressure to be relieved while excluding atmospheric moisture.Drying tubes are often used in less-demanding applications, typically in organic syntheses. While the reaction is often carried out at room temperature, the solvent, usually volatile diethyl ether or tetrahydrofuran is already able to displace air directly, making additional measures to exclude atmospheric moisture less important.

An oil bubbler may be a useful substitute. In this case, gases are allowed to escape, but air is not able to enter because the bubbler acts as a one-way valve. Oil bubblers can tolerate an underpressure in the reaction vessel. Oil is sucked into a sump in lieu of air. However, if the pressure in the reaction vessel falls too low, the oil may be sucked into the reaction vessel, contaminating it.For more demanding applications, a Schlenk line or glovebox may be used to provide an atmosphere of dry, inert gas such as argon or nitrogen.

Effervescent tablet

Effervescent or carbon tablets are tablets which are designed to dissolve in water, and release carbon dioxide. They are products of compression of component ingredients in the form of powders into a dense mass, which is packaged in blister pack, or with a hermetically sealed package with incorporated desiccant in the cap. To use them, they are dropped into water to make a solution. The powdered ingredients are also packaged and sold as effervescent powders or may be granulated and sold as effervescent granules. Generally powdered ingredients are first granularized before being made into tablets.Effervescent medicinal beverages date back to the late 1800s and originally arose to mask the taste of bitter waters taken as curatives, during the water cure craze of that era.

List of desiccants

A desiccant is a substance that absorbs water. It is most commonly used to remove humidity that would normally degrade or even destroy products sensitive to moisture.

List of desiccants:

Activated alumina

Aerogel

Benzophenone

Bentonite clay

Calcium chloride

Calcium oxide

Calcium sulfate (Drierite)

Cobalt(II) chloride

Copper(II) sulfate

Lithium chloride

Lithium bromide

Magnesium sulfate

Magnesium perchlorate

Molecular sieve

Phosphorus pentoxide

Potassium carbonate

Potassium hydroxide

Silica gel

Sodium

Sodium chlorate

Sodium chloride

Sodium hydroxide

Sodium sulfate

Sucrose

Sulfuric acid

Lithium bromide

Lithium bromide (LiBr) is a chemical compound of lithium and bromine. Its extreme hygroscopic character makes LiBr useful as a desiccant in certain air conditioning systems.

Magnesium perchlorate

Magnesium perchlorate is a powerful oxidizing agent, with the formula Mg(ClO4)2. It is also a superior drying agent for gas analysis.

Magnesium perchlorate decomposes at 250 °C. The heat of formation is -568.90 kJ mol−1.The enthalpy of solution is quite high, so reactions are done in large amounts of water to dilute it.

It is sold under the trade name anhydrone. Manufacture of this product on a semi-industrial scale was first performed by G. Frederick Smith in his garage in Urbana Illinois, but later at a permanent facility in Columbus, OH called G. Frederick Smith Chemical Co. He sold the magnesium perchlorate to A. H. Thomas Co., now Thomas Scientific, under the trade name Dehydrite.

It is used as desiccant to dry gas or air samples, but is no longer advised, for use as a general desiccant, due to hazards inherent in perchlorates. It is dried by heating at 220 °C under vacuum.

Magnesium perchlorate is created by the reaction of magnesium hydroxide and perchloric acid.

In 2011, a study conducted at the Georgia Institute of Technology revealed the presence of magnesium perchlorate on the planet Mars. Being a drying agent, magnesium perchlorate retains water from the atmosphere and may release it when conditions are favorable and temperature is above 250K. Because briny solutions that contain magnesium perchlorate have a lower melting point than that of pure water, their abundance on Mars could serve as evidence that liquid water may exist on its surface, where temperature and pressure conditions would ordinarily cause water to freeze.

Phosphorus pentoxide

Phosphorus pentoxide is a chemical compound with molecular formula P4O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

Silica gel

Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other liquids, or may be filled by gas or vacuum. In the latter case, the material is properly called silica xerogel.

Silica xerogel with an average pore size of 2.4 nanometers has a strong affinity for water molecules and is widely used as a desiccant. It is hard and translucent, but considerably softer than massive silica glass or quartz; and remains hard when saturated with water.

Silica xerogel is usually commercialized as coarse granules or beads, a few millimeters in diameter. Some grains may contain small amounts of a substance that changes color when they have absorbed some water. Small paper envelopes containing silica xerogel pellets, usually with a "do not eat" warning, are often included in dry food packages to absorb any humidity that might cause spoilage of the food.

'Wet' silica gel, as may be freshly prepared from alkali silicate solutions, may vary in consistency from a soft transparent gel, similar to gelatin or agar, to a hard solid, namely a water-logged xerogel. It is sometimes used in laboratory processes, for example to suppress convection in liquids or prevent settling of suspended particles.

Sodium-potassium alloy

Sodium-potassium alloy, colloquially called NaK (commonly pronounced ), is an alloy of two alkali metals, sodium (Na, atomic number 11) and potassium (K, atomic number 19), and which is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water (like its constituent elements) and may catch fire when exposed to air, so must be handled with special precautions.

Solar air conditioning

Solar air conditioning refers to any air conditioning (cooling) system that uses solar power.

This can be done through passive solar, solar thermal energy conversion and photovoltaic conversion (sunlight to electricity). The U.S. Energy Independence and Security Act of 2007 created 2008 through 2012 funding for a new solar air conditioning research and development program, which should develop and demonstrate multiple new technology innovations and mass production economies of scale.

Thermal wheel

A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of an air-handling system or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.

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