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 - 3Å, 4Å, 5Å, and 10Å (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 one part per million.
They are most often used in dynamic applications, where gases or liquids are passed over/through the molecular sieve.
Examples of the applications of the differing molecular sieve grades are :
Grade 3Å. Dries unsaturated hydrocarbons and organic liquids (e.g methanol).
Used to remove water from cleaning fluids in ultrasonic baths.
3Å molecular sieves can adsorb up to 22% of their own weight in water.
Grade 4Å. Is a general dryer of liquids, vapours and natural gases.
It can also remove other contaminants which have molecules with diameters of less than four Ångstroms.
Grade 5Å. In addition to an increased capacity to hold water molecules,
this molecular sieve will also adsorb carbon dioxide and hydrogen sulphide.
Grade 10Å (13X). With high water adsorption capacity (up to 29% weight for weight)
and carbon dioxide adsorption capacity (19% weight for weight) ) this grade is used for air purification.
It also adsorbs sulphur compounds ('sweetens'), up to 11% of it's own weight -
it will remove the decomposition products following the quenching of arcing in electrical products.
Molecular sieves properties as desiccants differ from silica gel in a number of ways :
They adsorb water molecules from liquids, as well as from air.
They adsorb water molecules more rapidly than silica gel.
They will reduce water molecules to much lower levels than silica gel will,
making their use essential when a very dry product or atmosphere is required.
They perform more effectively as moisture adsorbers at higher temperatures (greater than 25°C) than silica gel does.
Molecular sieves of grades 3Å, 4Å and 10Å (13X) are stock items.
They are sub-classified by their bead sizes, typically 1.6 - 2.5 mm bead and 2.5 - 5.0 mm bead.
They are available in the same bulk pack sizes as silica gel:
500 gram and 1 kg plastic containers or sealed foil bags
3 kg and 10 kg poly-tins
25 kg steel drums
140 kg and 130 kg steel drums
Molecular sieve sachets are made to customers requirements.
They can be made with any of the grades mentioned,
in most sachets from 3 gram to 500 gram and above.