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. It is different from a common filter in that it operates on a molecular level. For instance, a water molecule may be small enough to pass through while larger molecules are not. Because of this, they often function 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, such as 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 starting materials. The mercury content of natural gas is extremely harmful to the aluminum piping and other parts of the liquefaction apparatus – silica gel is used in this case.
Type: Molecular Sieve 3A
Description: The 3A form is made by substituting potassium cations for the inherent sodium ions of the 4A structure, reducing the effective pore size to ~3Å, excluding diameter >3Å, e.g., ethane
- 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 agent in polar and nonpolar media.
Type: Molecular Sieve 4A
Description: This sodium form represents the type A family of molecular sieves. Effective pore opening is 4Å, thus excluding molecules of effective diameter >4Å, e.g., propane.
- 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.
Type: Molecular Sieve 5A
Description: Divalent calcium ions in place of sodium cations give apertures of ~5Å which exclude molecules of effective diameter >5Å, e.g., all 4-carbon rings, and iso-compounds.
Separation of normal paraffins from branched-chain and cyclic hydrocarbons; removal of H2S, CO2 and mercaptans from natural gas. Molecules adsorbed include nC4H10, nC4H9OH, C3H8 to C22H46, and dichlorodifluoro-methane
Type: Molecular Sieve 13X
Description: The sodium form represents the basic structure of the type X family, with an effective pore opening in the 910¼ range. Will not adsorb(C4F9)3N, for example.
Commercial gas drying, air plant feed purification (simultaneous H2O and CO2 removal) and liquid hydrocarbon/natural gas sweetening (H2S and mercaptan removal).
|Model||Pore diameter (Ångström)||Bulk density (g/ml)||Adsorbed water (% w/w)||Attrition or abrasion, W (% w/w)|
|5Å small oxygen-enriched||5||0.4–0.8||≥23|
Forms of Molecular Sieves
Molecular sieves comes in two forms ; one is cylindrical form and other one is beads form.
Regeneration (activation) of Molecular Sieve
Regeneration in typical cyclic systems constitutes removal of the adsorbate from the molecular sieve bed by heating and purging with a carrier gas. Sufficient heat must be applied to raise the temperature of the adsorbate, the adsorbent and the vessel to vaporize the liquid and offset the heat of wetting the molecular-sieve surface.
The bed temperature is critical in regeneration. Bed temperatures in the 175-260° range are usually employed for type 3A. This lower range minimizes polymerization of olefins on the molecular sieve surfaces when such materials are present in the gas. Slow heatup is recommended since most olefinic materials will be removed at minimum temperatures; 4A, 5A and 13X sieves require temperatures in the 200-315 °C range.
After regeneration, a cooling period is necessary to reduce the molecular sieve temperature to within 15° of the temperature of the stream to be processed. This is most conveniently done by using the same gas stream as for heating, but with no heat input.
For optimum regeneration, gas flow should be countercurrent to adsorption during the heatup cycle, and concurrent (relative to the process stream) during cooling. Alternatively, small quantities of molecular sieves may be dried in the absence of a purge gas by oven heating followed by slow cooling in a closed system, such as a desiccator.
Industries in which this product is very widely used:
For Adsorption: 1) New adsorbents for sieving. 2) Hydrophobic adsorbents. 3) Gas storage systems
Consumer applications: 1) Beverage carbonation. 2) Electrical conductors. 3) Ceramics 4) New catalysts. 5) Laundry Detergents. 6) Fire Extinguishers
Agriculture: 1) Fertilizers and soils 2) Animal culture
Molecular sieves has critical applications in across the industries.