Pressure swing adsorption (PSA), also known as carbon molecular sieves, is the second most commonly used biogas upgrading technology in Europe. A typical system is composed of four vessels in series that are filled with adsorbent media which is capable of removing water vapor, CO2, N2 and O2 from the biogas stream.
During operation, each adsorber operates in an alternating cycle of adsorption, regeneration and pressure buildup. Dry biogas enters the system through the bottom of one of the adsorbers during the first phase of the process. When passing through the vessel, CO2, N2 and O2 are adsorbed onto the surface of the media. The gas leaving the top of the adsorber vessel contains more than 97% CH4
The upgrading takes place over 4 phases: pressure build-up, adsorption, depressurization and regeneration. The pressure buildup is achieved by equilibrating pressure with a vessel that is at depressurization stage. Final pressure build up occurs by injecting raw biogas. During adsorption, CO2 and/or N2 and/or O2 are adsorbed by the media and the gas exits as CH4.
Depressurization is performed by equalizing with a second pressurizing vessel, and regeneration is achieved at atmospheric pressure, leaving a gas that contains high concentrations of CH4 to be re-circulated. During the regeneration phase, the bed must be regenerated by desorbing (or purging) the adsorbed gases. Purging is accomplished by reducing the pressure in the bed and back-flushing it with some of the concentrated gas product. The gas pressure released from one vessel is used by the other, thus reducing energy consumption and compressor capital costs.
Special adsorption materials are used as a molecular sieve, preferentially adsorbing the target gas species at high pressure. The adsorbent media is usually zeolites (crystalline polymers), carbon molecular sieves or activated carbon. 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.