A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids. Molecules small enough to pass through the pores are adsorbed while larger molecules are not. A molecular sieve can adsorb water up to 22% of its own weight.
Often they consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons or synthetic compounds that have open structures through which small molecules, such as nitrogen and water can diffuse.
Molecular sieves are often utilised in the petroleum industry, especially for the purification of gas streams and in the chemistry laboratory for separating compounds and drying reaction starting materials. Due to the mercury content of natural gas being extremely harmful to the aluminium piping and other parts of the liquefaction apparatus, silica gel is used in this case.
Methods for regeneration of molecular sieves include pressure change (as in oxygen concentrators), heating and purging with a carrier gas (as used in ethanol dehydration), or heating under high vacuum. EXHEAT electric heaters are commonly used to heat the carrier gas, for example nitrogen, that is used to regenerate the molecular sieve bed.