Thursday, October 12, 2017



The Molecular Sieves are synthetic zeolites or metal alumino silicates, with a porous tridimensional crystallized network. Molecular sieve have regular pore sizes. Based on the pore size, they are called 3, 4, 5 or 10 A although the real pore opening is somewhat inferior to these values.

By “ activation “ a highly active adsorbent is obtained. The activation aims to desorb the zeolitic water contained in the adsorbent.

The combination of a uniform porous structure with a high adsorption capacity, enables the selective adsorption of the molecules.

“ Molecular Sieves “, are so called because of their property of screening molecules at the molecular scale. Molecular sieves differ from other adsorbents such as silica gel or activated alumina, which have a very wide pore distribution.

A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as anadsorbent  for gases  and liquids.

Molecules  small enough to pass through the pores are adsorbed  while larger molecules are not. It isdifferent from a common filter in that it operates on a molecular level. For instance, a water moleculemay be small enough to pass through while larger molecules are not. Because of this, they oftenfunction as a desiccant. 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, suchas nitrogen and water can diffuse.

Molecular sieves are often utilized in the petroleum  industry, especially for the purification of gas streams and in the chemistry laboratory for separating compounds and drying reaction startingmaterials. The mercury  content of natural gas is extremely harmful to the aluminum piping and otherparts 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 when used in ethanol  dehydration), or heating under high vacuum.

The different types of Molecular Sieves

1.  4 A is the basic molecular sieve obtained by synthesis of type A zeolite.

2.  3 A is obtained by replacing part of the sodium cations of the 4 A molecular sieve by potassium cations.

3.   5 A is obtained when potassium is replaced by calcium as the substitution cation.

4.   10 A ( type 13 x or Siliporite G 5 ) is the basic molecular sieve obatained by synthesis of type X Zeolite.

Adsorption capabilities :
  • ·    3 A ( pore size 3 ångströms ) : adsorbs NH3, H20, , ( not C2H6 ), good for drying polar liquids.
  • ·    4 A ( pore size 4 ångströms ) : adsorbs H2O, CO2, H2S, C2H4, C2H6, C3H6, , EtOH. Will not adsorb C3H8,  and  higher hydrocarbons. Good for drying non polar liquids and gases.
  • ·    5 A ( pore size 5 ångströms ) : adsorbs normal ( linear ) hydrocarbons to n-C4H10, alcohol to C4H9OH, mercaptans to C4H9SH. Will not adsorb isocompounds or rings greater than C4.
  • ·    10 X ( pore size 8 ångströms ) : adsorbs branched hydrocarbons and aromatics. Useful for drying gases.
  • ·    13 X ( pore size 10 ångströms ) : adsorbs di-n-butylamine  ( not tri-n-butylamine ). Useful for drying HMPA.

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