Molecular sieve

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 poresareadsorbed while larger molecules are not. It isdifferentfrom a common filter in that it operates on a molecularlevel andtraps the adsorbed substance. For instance, a watermolecule may besmall enough to pass through the pores while largermolecules arenot, so water is forced into the pores which act as atrap for thepenetrating water molecules, which are retained withinthe pores.Because of this, they often function as a desiccant. A molecular sieve can adsorb water up to22%of its own weight.[1]Theprinciple of absorption to molecular sieve particles issomewhatsimilar to that of size exclusion chromatography, except that withoutachanging solution composition, the adsorbed product remainstrappedbecause in the absence of other molecules able to penetratethepore and fill the space, a vacuum would be created by desorption.

Often they consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, or synthetic compounds that have openstructuresthrough which small molecules, such as nitrogen andwater candiffuse.

Molecular sieves are often utilized in the petroleum industry, especially for the purificationofgas streams and in the chemistry laboratory forseparatingcompounds and drying reaction starting materials. Themercury content of natural gas is extremely harmfultothe aluminum piping and other parts of the liquefaction apparatus-silica gel is used in this case.

Methodsforregeneration of molecular sieves include pressure change (asinoxygen concentrators), heating and purging with a carrier gas(aswhen used in ethanol dehydration), or heating under highvacuum.Temperatures typically used to regenerate water-adsorbedmolecularsieves range from 130°C to 250°C.

Adsorption capabilities

• 3A (pore size 3 Å) Molecular sieve : Adsorbs NH3, H2O, (not C2H6), good fordryingpolar liquids.
• 4A (pore size 4 Å) Molecular sieve : Adsorbs H2O, CO2, SO2, H2S, C2H4, C2H6,C3H6,ethanol.Willnot adsorb C3H8 and higher hydrocarbons. Good for dryingnonpolarliquids and gases.
• 5A (pore size 5 Å) Molecular sieve : Adsorbs normal (linear) hydrocarbons ton-C4H10,alcohols to C4H9OH, mercaptans to C4H9SH. Will notadsorbisocompounds or rings greater than C4.
• 10X (pore size 8 Å) Molecular sieve : Adsorbs branched hydrocarbons andaromatics.Useful for drying gases.
• 13X (pore size 10 Å) Molecular sieve : Adsorbs di-n-butylamine(nottri-n-butylamine). Useful for drying HMPA.