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
Examples of the applications of the differing molecular sieve grades
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
(3) They perform more effectively as moisture adsorbers at higher
temperatures (greater than 25°C) than silica gel does.
We can product the following molecular sieve products: 3A, 4A,
5A, and 13X molecular sieve.