Molecular sieve is a type of synthetic zeolite or natural zeolite with function of molecular sieve selection.Its chemical formula is (M′2M)O·Al2O3·xSiO2·yH2O. M′、M is monovalent and divalent cation separately such as K+、Na+ and Ca2+、Ba2+. It has many holes with uniform aperture and arranged caves. Different aperture molecule divide different size and shape of molecule. Based on the different rate of SiO2 and Al2O3, there are different aperture molecular sieve. The common models are molecular sieve 3A, molecular sieve 4A, molecular sieve 5A, molecular sieve 13x.

Zeolite molecular sieve has high absorption capacity, high selection capacity and high temperature resistance so it is widely used in organic and petrochemicals. It is also a excellent absorbent for gas dehydration and it is also used for waste-gas cleaning.

Absorption capacity

The absorption ability is the fundamental characteristics of the molecular sieve. It is a physical change process. The reason of absorption is mainly because of a kind of surface force which is caused by molecule attraction on solid surface. When the fluid pass through, because of the irregular movement, some molecule beat on the absorbent surface and cause molecular concentrations on surface. The certain type of molecule will decease to get purpose of separation and cleaning up . Because the absorption doesn’t has chemical change, we only need to drive away the molecule concentrated on the solid surface to recover the absorption capacity of the molecular sieve. It is called the resolution or regeneration of the molecular sieve.

Because the pore size of the zeolite molecular sieve is uniform, it is easy to run into the crystal and be absorbed only when the molecule diameter is smaller than the zeolite molecular sieve aperture. So molecular sieve is like sieve for gas and liquid molecule.The size of the molecule determines whether it can be adsorbed.

Application in the field of adsorption separation

  • Separation of mixed xylene

Mixed xylene is generally used as solvent and asoline admixture for cheap sale. The waste of resources is very serious. However ,the fore isomers of mixed xylene including ethylbenzene, p-xylene, m-xylene and o-xylene are the most chemical raw materials, so it is necessary to separate them one by one.
Adsorption separation is an efficient separation method.The key is the preparation of adsorbent. Because of the special structure and various types,zeolite molecular sieve is widely used as an adsorbent to separate mixed xylene.

  • Separation of N2 / O2

In pressure swing adsorption (PSA) process, zeolite molecular sieve selectively adsorb N2 taking advantage of the difference of equilibrium adsorption of the two type of gases N2 and O2 on surface. Because N2 has higher polar ability, the effect of N2 on the cation and its polar surface in zeolite is stronger than O2. Type A and X molecular sieve has higher selection adsorption ratio for N2/O2, and it also have higher N2 adsorption capacity and higher thermal stability. It is widely used for PSA of N2 / O2.

General types of zeolite molecular sieve

Zeolite molecular sieve generally include molecular sieve 3A, molecular sieve 4A, molecular sieve 5A, molecular sieve 13x, XH molecular sieve, insulating glass molecular sieve, Oxygen concentration molecular sieve and so on.