Molecular sieves are used commercially to purify gas streams and as desiccants to remove water or reduce humidity. This article focuses chiefly on what you need to know when buying a desiccant dryer, although the basic principles may apply to other applications as well.
How a Molecular Sieve Works
The material used in an industrial molecular sieve has small uniform pores. When other substances come in contact with the molecular sieve, the molecules that are the right size to fit in the pores will be adsorbed. The molecules that are too large to fit will not. Molecular sieves function at the microscopic level, therefore their sizes are measured in angstroms. Pore sizes 3Å and 4Å will adsorb water while larges sizes remove larger hydrocarbons.
Molecular Sieve Materials
In a strictly scientific sense, many natural desiccant dehumidifiers like lime, clay and silica gel also work by sieving molecules of water vapor, but commercial molecular sieves are made of synthetic crystalline aluminosilicates. Unlike desiccants found in nature, the control of pore size during manufacture produces selective adsorption characteristics.
The Advantages of Molecular Sieves
Molecular sieves typically adsorb water much faster than other desiccant air dryers and they can reduce the humidity to much lower levels that the standard silica gel. They are also more effective than natural desiccants for applications that exceed normal room temperature. When properly used, they can be effective in reducing water molecules as low as 1ppm in specialized containers or to 10% relative humidity in packaging.
Regeneration and Reuse of Molecular Sieves
While some molecular sieves that remove alcohols and aromatic hydrocarbons use pressure to regenerate the sieve, the molecular sieves that are used for water-adsorption are usually regenerated by heating. For most industrial purposes, these temperatures range from about 250° to 450°F, similar to baking temperature settings for a standard kitchen oven.
Disadvantages of Molecular Sieves
Prices are higher than other forms of desiccant dehumidification; however, molecular sieves are also more efficient. The actual costs per unit and final value will depend upon other factors such as the volume to be dehumidified and level of dryness needed. Molecular sieves, while approved for use with pharmaceuticals in Europe, have not been approved by the FDA for either foodstuffs or pharmaceuticals in the US.
Molecular sieves have excellent capacity for and rates of adsorption, even at elevated temperatures. They are the only desiccant that is selective for molecular size.