In the next article I’m going to talk about a synthetically produced desiccant called molecular sieve.
Appearing as small opaque pinkish beads, molecular sieves are highly porous crystalline metal-alumino silicates. Compared with other desiccants, the distinctive feature is the uniformity pore size in the crystal-lattice structure which is part of the manufacturing process.
As the pore size on molecular sieve particles is controlled, it allows the end product to be more precise then other desiccants so different pore sizes can be found, each one with a few different properties. Absorption will occur only for molecules with smaller diameters than the molecular sieves’ pore size larger molecules being excluded from absorption. Preferentially absorbed are molecules of greater polarity which makes molecular sieve ideal for absorption of water from liquids and gases as water molecules are both polar and very small.
Molecular sieve is classified by its pore size in angstroms, some of the most used being 3A, 4A, 5A, 8A (10X) and 10A, also known as 13X. This feature allows the selection of a molecular sieve which can absorb water yet exclude most of other molecules or other desiccants which will absorb bigger molecules like aromatics or carbon dioxides.
Some applications of the different molecular sieves’ grades:
A3: Adsorbs NH3, H2O, good for drying gases and organic liquids. It is used to remove water from cleaning fluids in ultrasonic baths.
A4: It adsorbs H2O, CO2, SO2, H2S, C2H4, C2H6, C3H6 and ethanol. It is a general dryer of non-polar liquids and natural gases. It is an excellent absorber of carbon dioxide.
A5: Adsorbs from linear (normal) hydrocarbons to n-C4H10, alcohols to C4H9OH, mercaptans to C4H9SH.
A8 (10X): Adsorbs branched hydrocarbons and aromatics. It is useful for drying gases.
A10 (13X): Adsorbs di-n-butylamine and hexamethylphosphoramides better known as HMPA. It is used for water pre-purification due to its high affinity for water and carbon dioxide.
Molecular sieve absorbs up to 22% of its own weight and can hold moisture to temperatures well past 450°F (230°C), and because of its high affinity for moisture, molecular sieve is able to bring the relative humidity in packages down as low as 10% RH.
The United States FDA has not yet approved molecular sieve for direct contact with consumable items, although in Europe it is used with pharmaceuticals. Being man-made rather than naturally occurring, molecular sieve is slightly higher in cost per unit, but due to its extremely large range of absorptive capabilities, it might often be the best value, especially in applications of low relative humidity.
Lack of government approval for the use of molecular sieves in food and drug packaging has limited its more widespread use. Independent testing suggests that molecular sieves meet all government requirements but the industry has been unwilling to fund the expensive testing required for government approval.
Another classification of molecular sieves is by bead size which ranges from 1.6 – 2.5 mm to 2.5 – 5.0 mm. They can be found as silica gel in desiccant packs from 500 grams to 1 kg in polytubs or foil packs or in bulk packs from 3 kg to 10 kg in polytubs. Bigger quantities like 25 kg, 130 kg and 150 kg can be found in metal drums.
If you have some more information to add about molecular sieves, let me know in the comment section or via email.