Molecular sieves are synthetically produced, highly porous desiccants. These desiccants generally include aluminate silicates, porous glass, charcoals and zeolites.
The desiccants can adsorb moisture up to 22% of its weight. Regeneration is generally accomplished with pressure swing, called PSA in Oxygen/Nitrogen generation; and, with temperature swing at temperatures over 130 0C.
These desiccants have pores of precise diameter, measured in Angstroms – 3A, 4A, 5A, 8A and 10A or 13X.
Molecules smaller than pore sizes are withheld or adsorbed by the desiccant; whereas, larger molecules are free flow out. The process is used to separate unwanted molecules.
Molecular sieves can adsorb water vapor molecules from air and gases down to 1 PPM level.
Commonly used Molecular sieves:
Grade 3A: For drying liquids e.g. Ethanol, Ethylene, Propylene, Refrigerant, Air and Gases; also adsorbs NH3
Grade 4A: For drying Compressed Air & Gases to very low dew points, down to -120 0F; also adsorbs CO2, SO2, H2S and Heavy hydrocarbons
Grade 5A: For drying of Cracked gas, Ethylene, Propylene; also for a used for high purity oxygen generation in medical applications
Grade 10X (8A): For adsorption of hydrocarbons and aromatics and drying gases.
Grade 13X (10A): For air purification due to its high water and carbon dioxide adsorption capacity; also adsorbs Sulfur compounds to sweeten gas.