Silica gel or zeolites molecular sieve – what to choose for drying processes?
Molecular sieves and silica gels are a group of adsorbents widely used in gas drying. Both materials have a highly developed porous structure of a hydrophilic nature. The key difference between silica gel and molecular sieves is the repeatability of the pore size, which in zeolite molecular sieves is of a strictly defined size, while in silica gel the pore size is characterized by a larger scatter of values. Which adsorbent to choose for the drying process? This question is often asked by our clients and the answer is – it depends…
Zeolite molecular sieves
Molecular sieves are highly porous synthetic aluminosilicates with a precisely defined porous structure. They have excellent adsorption properties, and thanks to the repetitive porous structure, this adsorption is highly selective.
The molecular sieve is available in four basic types: 3A, 4A, 5A and 13X. Individual types differ in pore size, and thus have different properties and are capable of selectively separating particles of different sizes.
Silica gels are porous forms of silicon dioxide with high water vapor adsorption efficiency. Their porous structure is characterized by a larger size distribution than in molecular sieves.
They are available with different pore sizes (Type A, B and C), special features (with or without a moisture indicator, waterproof WS and FNG).
Then what to choose?
The moisture adsorption of silica gel and molecular sieve differ in several key criteria. For applications with low relative humidity, the molecular sieve is second to none. The adsorption efficiency increases almost vertically already at a very low concentration of water vapor, the value of which does not change much with increasing concentration. Silica gel (silica gel) is a weaker material in these ranges. The situation changes for relative humidity above 40%. The molecular sieve quickly reaches its maximum and then only slightly increases. Silica gel, on the other hand, increases the adsorption value continuously with increasing relative humidity.
For applications where the speed of drying is important, a better material will again be a molecular sieve, whose kinetics of water vapor adsorption is excellent. Silica gel (silica gel) is average at best.
So is molecular sieve better in every case? Of course it is not. For typical industrial dehumidification applications, silica gel will be a good enough material, which also has a higher water adsorption capacity at relative humidity above 40%. It is also a cheaper and easier material to use.