Types of Molecular Sieves in PSA, VPSA Oxygen Plant

Adsorption can be defined as selective concentration or retention of one or more components of a mixture on a solid surface. Molecular sieves, or synthetic zeolites, are crystalline aluminosilicates of metals like sodium, potassium, magnesium or calcium.

There're generally two types of molecular sieves in structure, commercially used for separation of oxygen and nitrogen in PSA, VPSA technology, (for removal of H2O, CO2 in cryogenic distillation, for removal of H2S from natural gas, etc.):

1. A-type: 5A (CaA);

2. X-type: (1) NaX, 13X, (2) CaX. (3) LiX.

The CaA, NaX and CaX molecular sieves are generally used for PSA oxygen generators. As of the end of the 1990s, oxygen production devices using CaA and NaX molecular sieves occupied a domiinant position in the market. But now the former two types have been obsolete owing to their disadvantages: The nitrogen adsorption capacity is small, the nitrogen-oxygen separation coefficient is small, leading to low oxygen recovery rate.

LiX - The high lithium low silicon aluminum X-type LiLSX molecular sieve with high oxygen recovery rate and greatly reduced energy consumption. The NaX type molecular sieve modified by Li+ ions, has larger air handling capacity and high oxygen yield. Li+ has the smallest radius among all metal ions, a higher charge density and a high polarizability, so the force between Li+ and N2 is greater.

LiX type zeolite is used in DOER's VPSA oxygen plant, having the key parameters as follows:

Diameter 1.6-2.6 mm;

Static nitrogen adsorption capacity: ≥22 ml/g;

Nitrogen-oxygen separation co-efficient: ≥6.2, at 760 mmHg;

Bulk density: 0.63±0.03 g/ml;

Crushing strength: 25±3 N/granule;

Water content: ≤0.5 %wt.

Life time: 10 years.