Principle

PSA is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation techniques of gas separation. Special adsorptive materials ( Carbon Molecular Sieves) are used preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbent material.

Process

Pressure swing adsorption processes rely on the fact that under pressure, gases tend to be attracted to solid surfaces, or "adsorbed". The higher the pressure, the more gas is adsorbed; when the pressure is reduced, the gas is released, or desorbed. PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly. If a gas mixture such as air, for example, is passed under pressure through a vessel containing an adsorbent bed that attracts Oxygen more strongly than it does Nitrogen, part or all of the Oxygen will stay in the bed, and the gas coming out of the vessel will be Nitrogen. When the bed reaches the end of its capacity to adsorb Oxygen, it can be regenerated by reducing the pressure, thereby releasing the adsorbed Oxygen. It is then ready for another cycle of producing Nitrogen.

Using two adsorbent vessels allows near-continuous production of the target gas. It also permits so-called pressure equalization, where the gas leaving the vessel being depressurized is used to partially pressurize the second vessel. This results in significant energy savings, and is common industrial practice.