Desiccant

Desiccant Capacity chart (hours); Sorbent Systems

Desiccant Capacity chart (RH); Sorbent Systems

Description

A hygroscopic substance that will remove water vapor from the air. Desiccants are used as drying agents. Some desiccants are chemically inert, which allows the to be dried in an oven and reused, such as silica gel. Other desiccants react with water; these require special handling techniques.

Examples are Activated carbon, Aerogel, Activated alumina, Calcium chloride, Calcium oxide, Calcium sulfate, Clay, Fumed silica, Molecular sieve, Potassium carbonate, Potassium hydroxide, Rice, Silica gel, Sodium, Sodium chlorate, Sodium chloride, Sodium hydroxide, Sodium sulfate, Sucrose and Zinc chloride.

Sorbent	Description	Risks	Adsorption Capacity	Regeneration conditions	Usage
Activated carbon	A form of carbon containing small, low-volume pores that produce a high surface area to volume of material	Non-toxic, non-corrosive; dust may cause irritation	15-25%		Purification of air (odors, pollutants, moisture) and liquids; filtration; spill clean-up; chromatography
Activated alumina	Dehydroxylated aluminum hydroxide producing a form of Al2O3 that is highly porous with high surface area; similar to silica gel but with lower capacity; preferentially adsorbs polar molecules	Non-toxic; non-corrosive; dust may cause irritation	7.50%	175-315 C	Purification of air (odors, pollutants, moisture) and liquids (heavy metals); filtration; selective adsorption in chemical production processes
Clay (montmorillonite or bentonite)	Crystalline aluminosilicate with uniform network of pores; large surface area; selective absorption; chemically stable; does not release water easily	Non toxic; recyclable when it does not contain toxins	15%-25%	May release moisture at temperatures below 50C	Readily absorbs most liquids and heavy metals; commonly used outdoors and in mixtures; cat litter
Calcium chloride	Hygroscopic crystalline salt producing an exothermic reaction with water; depresses the freezing point of water	Corrosive; use PPE for handling;		-5-90 C	Very high moisture absorption; often used in mixtures with clay; effective at high temperatures; de-icer
Calcium oxide (quicklime)	Hygroscopic white powder; adsorption occurs slowly with swelling and heat production	Caustic; use PPE for handling	28%		Very high moisture absorption
Calcium sulfate (anhydrite)	Dehydrated gypsum; primarily used in labs; chemically stable; does not release water easily as it alters to form gypsum	Non-corrosive; non-toxic and recyclable except when contains indicators	5-10%	Limited regeneration; 175+ C	Selectively absorbs water from air and liquids; gypsum is used in drywall because it forms anhydrite when burn, thus releasing water
Fumed silica	A powder consisting of microscopic droplets of amorphous silica fused into three-dimensional particles with high surface structure and low bulk density; chemically stable.	Non-corrosive; non-toxic	6%	Not readily regenerated	Widely used as thickener and filler in food storage and commercial products
Molecular sieve (synthetic Zeolite)	Crystalline aluminosilicate with uniform network of pores; large surface area; selective absorption; chemically stable; does not release water easily	Non-toxic; recyclable	10%-25%	230-330 C	Affinity for water but absorbs most VOCs (limited by molecule size vs. pore size); can be used at high temps
Silica gel	Partially dehydrated, amorphous form of silicic acid with interconnected micro pores producing very high surface structure; physical absorption; chemically stable; high mechanical strength; does not swell	Non-corrosive; non-toxic except when contains indicators	10%-27%; most efficient below 25c	120C	Widely used; food storage and commercial products; controlled humidity environments

Synonyms and Related Terms

drier; drying agent; dessicant (sp); sorbent; scavenger

Commercial Products:

• Art Preservation Services: Arten Gel (silica gel for use from 0-45 %RH), Rhapid Gel (silica gel for use above 40%RH)
• Clariant: Desi Pak (bentonite clay); Sorb-It (silica gel); Tri-Sorb (molecular sieve)
• Uline: Container-Dri (CaCl)
• McMaster Carr: Silica gel (indicating and non-indicting); Clay; CaSO4; Aluminosilicate (for low humidities); Starch/CaCl (for high humidities)
• Sorbent Systems: StayDry paks (indicating silica gel or non-indicting silica gel or clay or activated carbon or custom blends); DriBox (Polycarbonate boxes with blue silica gel or orange silica gel or molecular sieves)

Usage Calculators

For Silica Gel:

• Art Preservation Services: Silica Gel Recommendations
• Small Corp: [www.smallcorp.com/silica-gel-calculator Silica Gel Calculator]
• Gaylord: [www.gaylord.com/resources/silica-gel-calculators Silica Gel Calculator]

Resources and Citations

• AGM: Selecting the right desiccant
• Sorbent Systems: Desiccant Charts
• Theodore J. Reinhart, 'Glossary of Terms', Engineered Plastics, ASM International, 1988
• Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
• Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996
• The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998