​Pressure Swing Adsorption (PSA) is a technology used to separate some gas species from a mixture of gases under different pressure according to the species molecular characteristics and affinity for an Adsorbent material. It usually operates at near-ambient temperatures and differs significantly from Cryogenic Distillation techniques of gas separation. Special absorptive materials (e.g., Zeolites) are used as a Molecular sieve, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure at different stages to desorb the adsorbed material.

Process Description

Pressure Swing Adsorption processes rely on the fact that under high 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 of Zeolite that attracts Nitrogen more strongly than it does Oxygen, part or all of the Nitrogen will stay in the bed, and the gas coming out of the vessel will be enriched in oxygen. When the bed reaches the end of its capacity to adsorb nitrogen, it can be regenerated by reducing the pressure, thereby releasing the adsorbed nitrogen. It is then ready for another cycle of producing oxygen enriched air.

The pressure of the adsorber is decreased in one or more expansion sequences. The released gas is used to pressurize already purged adsorbers.

The further adsorber depressurization provides the necessary purge gas for Purging Sequences. The released gas is fed to adsorber at low pressure from top and used to purge out the residual impurities from the adsorbent material.

The last depressurization down to the tail gas. During this sequence the adsorbent material starts to release the adsorbed impurities.

Final desorption is performed at lowest cycle pressure by purging the adsorbent with almost pure hydrogen provided by the Depressurizing adsorber.

The pressurization is performed by the gas released during corresponding Depressurization sequences. Therefore, the final pressurization to the adsorption pressure to be accomplished by a hydrogen product split stream.

Aside from their ability to discriminate between different gases, adsorbents for PSA systems are usually very porous materials chosen because of their large surface areas. Typical adsorbents are Activated carbon, Silica gel, Alumina and Zeolite. Though the gas adsorbed on these surfaces may consist of a layer only one or at most a few molecules thick, surface areas of several hundred square meters per gram enable the adsorption of a significant portion of the adsorbent's weight in gas. In addition to their selectivity for different gases, zeolites and some types of activated carbon called carbon Molecular sieves may utilize their molecular sieve characteristics to exclude some gas molecules from their structure based on the size of the molecules, thereby restricting the ability of the larger molecules to be adsorbed.

Applications

One of the primary applications of PSA is in the removal of Carbon dioxide (CO2) as the final step in the large-scale commercial synthesis of Hydrogen (H2) for use in Oil Refineries and in the Production of Ammonia (NH3). Refineries often use PSA technology in the removal of Hydrogen Sulfide (H2S) from hydrogen feed and recycle streams of Hydro-Treating and Hydro-Cracking units. Another application of PSA is the separation of carbon dioxide from Biogas to increase the Methane (CH4) content. Through PSA the biogas can be upgraded to a quality similar to Natural Gas.

PSA is also used in Hypoxic Air Fire Prevention Systems to produce air with a low oxygen content.

On purpose propylene plants via propane dehydrogenation. They consist of a selective media for the preferred adsorption of methane and ethane over hydrogen.

Small-scale production of reasonable purity Oxygen or Nitrogen from Air. PSA technology has a major use in the medical industry to produce oxygen, particularly in remote or inaccessible parts of the world where bulk cryogenic or compressed cylinder storage is not possible.

Nitrogen generator units which employ the PSA technique to produce high purity nitrogen gas (up to 99.995%) from a supply of compressed air.