The simplified diagram above outlines the major components in a Vacuum Swing Adsorption Process (VSA). The key to selective gas separation is the choice of sieve material packed in the dual containers or vessels. The process is quite similar to Pressure Swing Adsorption systems, except that differential pressures take place at lower absolute pressures.

For oxygen service, specially treated zeolites are used as the adsorption sieve. Even at low pressure, nitrogen is preferentially adsorbed to the surface of the zeolite, allowing enriched oxygen to be produced. The rate of loading is faster for the nitrogen (kinetic separation effect), so the surface of the sieve adsorbs most of the N₂. The produced oxygen also contains unadsorbed argon and residual N₂, so that attainable O₂ purities range between 90 to 95%.

The VSA process begins by charging the first vessel with low-pressure air, initiating the N₂ adsorption process, similar to a sponge soaking up water. Before the zeolite reaches equilibrium, when O₂ will also be adsorbed, the pressurized gas in the first vessel is vented to the second vessel that is at lower pressure (vacuum). Residual N₂ in the first vessel is then “desorbed” from the zeolite and vented at atmospheric pressure. All required valving operations are done automatically by carefully calculated timing cycles controlled by a PLC.

For nitrogen service, the zeolite is replaced by carbon molecular sieve to preferentially adsorb oxygen, thereby delivering N₂ at purities up to 99.99% with the same process diagram shown above.