Characteristics & Uses of Molecular Sieves

Molecular sieve is a porous solid, usually a synthetic or a natural zeolite derived from alumina silicates with a different attachments of compounds like sodium, Calcium, Potassium that separates particles of molecular dimension which are defined to its pore diameter openings like 3A,4A,5A,10A. A true Molecular sieves are hydrated metal alumino-silicate compounds with well-defined crystalline structures. 

The silicate and aluminates groupings form three-dimensional crystal lattices surrounding cavities in which the metal ions and the water molecules are loosely held. Channels run through the entire crystal, interconnecting the cavities and terminating at the crystal surface. Upon heating above 200 deg C, the zeolites lose their inbound water content with little or no change in their crystal structure. The dehydrated Molecular sieves can reversibly absorb water or other molecules that are small enough to pass through the channels or pores. The metal ions are also readily replaceable by oterionic units of similar charge and size..

Molecular Sieves

 The sizes of a 4 to 8-mesh sieve is normally used in gas phase applications, while the 8 to 12-mesh type is common in liquid phase applications. The powder forms of the 3A, 4A, 5A and 13X Molecular sieves are suitable for specialized applications. Long known for their drying capacity (even to 90°C), molecular sieves have recently demonstrated utility in synthetic organic procedures, frequently allowing isolation of desired products from condensation reactions that are governed by generally unfavorable equilibria. These synthetic zeolites have been shown to remove water, alcohols (including methanol and ethanol), and HCl from such systems as ketamine and enamine syntheses, ester condensations, and the conversion of unsaturated aldehydes to polyenals.

Molecular sieves are used for drying gases and liquids and for separating molecules on the basis of their sizes and shapes. When two molecules are equally small and can enter the pores, separation is based on the polarity (charge separation) of the molecule, the more polar molecule being preferentially adsorbed.

3A type with potassium base is widely used in commercial dehydration of unsaturated hydrocarbon streams, including cracked gas, propylene, butadiene, acetylene; drying polar liquids such as methanol and ethanol. Adsorption of molecules such as NH3 and H2O from a N2/H2 flow. Considered a general-purpose drying agents in polar and nonpolar media.

Type 4A Molecular Sieve is most preferred for static dehydration in closed liquid or gas systems, e.g., in packaging of drugs, electric components and perishable chemicals; water scavenging in printing and plastics systems and drying saturated hydrocarbon streams. Adsorbed species include SO2, CO2, H2S, C2H4, C2H6, and C3H6. Generally considered a universal drying agent in polar and nonpolar media.

Commercial gas drying, air plant-feed purification (simultaneous H2O and CO2 removal) and liquid hydrocarbon/natural gas sweetening (H2S and mercaptan removal- sweet smell ) which is well taken care by 13X type.

If you plan to remove or separation of normal paraffin’s from branched-chain and cyclic hydrocarbons; removal of H2S, CO2 and mercaptans from natural gas. Molecules adsorbed include nC4H10, nC4H9OH, C3H8 to C22H46, and di-chlorodifluoro-methane (Freon 12®) then the most recommended product is a 5A type. Which is widely accepted.