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Desiccant Molecular Sieve

Molecular sieves are desiccants with differing properties to those of silica gel. With the appearance of small opaque pinkish beads, molecular sieves are synthetically produced, highly porous crystalline metal-alumino silicates. They have many internal cavities that are linked by window openings of precise diameters. It is these diameters (measured in Ångstroms) that classify molecular sieves - 3A, 4A, 5A, and 10A (also known as 13X). Adsorption occurs only of molecules with smaller diameters than these cavity openings. Larger molecules will be excluded from adsorption. Preferentially adsorbed are molecules of greater polarity. This makes molecular sieves ideal for adsorption of water from air and liquids, as water molecules are both polar and very small.
Molecular sieves will adsorb water molecules and other contaminants from liquids and gases down to very low levels - often just 1 part per million.

Examples of the applications of the differing molecular sieve grades are :
Grade 3A. Dries unsaturated gases and organic liquids (e.g methanol). Used to remove water from cleaning fluids in ultrasonic baths.
Grade 4A. Is a general dryer of liquids, natural gases. Also an excellent adsorber of carbon dioxide.
Grade 13X. Is used in air prepurification (due to its high water and carbon dioxide adsorption capacity) and also adsorbs sulphur compounds ('sweetens'). It will remove decomposition products following the quenching of arcing in electrical products.

Molecular sieves properties as desiccants differ from silica gel in a number of ways :
(1) They adsorb water vapour more rapidly than silica gel.

(2) They will reduce water vapour to much lower levels than silica gel, making their use essential when a very dry product or atmosphere is required. 

(3) They perform more effectively as moisture adsorbers at higher temperatures (greater than 25°C) than silica gel does.