A molecular sieve is a material containing tiny pores of a preciseand uniform size that is used as an adsorbent for gases andliquids. Molecules small enough to pass through the pores are absorbedwhile larger molecules are not. It is different from a commonfilter in that it operates on a molecular level. For instance, awater molecule may be small enough to pass through while largermolecules are not. Because of this, they often function as adesiccant. Molecular sieve can absorb water up to 22% of its ownweight.
Often they consist of aluminosilicate minerals or syntheticcompounds that have open structures through which small moleculescan diffuse, such as clays, porous glasses, microporous charcoals,active carbons etc...
Molecular sieves are usually utilized in the petroleum industry,especially with purification of gas streams. The mercury content ofnatural gas is extremely harmful to the aluminum piping and otherparts of the liquefaction apparatus - silica gel is used in thiscase.
Methods for Regeneration of molecular sieves include by pressurechange as in oxygen concentrators or by heating and purging with acarrier gas as when used in ethanol dehydration.
3A (pore size 3 Å): Adsorbs NH3, H2O, (notC2H6), good for drying polar liquids.
4A (pore size 4 Å): Adsorbs H2O, CO2,SO2, H2S, C2H4,C2H6, C3H6. Will notadsorb C3H8 and higher hydrocarbons. Good fordrying nonpolar liquids and gases.
5A (pore size 5 Å): Adsorbs normal (linear) hydrocarbons ton-C4H10, alcohols toC4H9OH, mercaptans toC4H9SH. Will not adsorb isocompounds or ringsgreater than C4.
13X (pore size 10 Å): Adsorbs di-n-butylamine (nottri-n-butylamine). Useful for drying HMPA.