Pressure swing adsorption
Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperature and significantly differs from the cryogenic distillation commonly used to separate gases. Selective adsorbent materials (e.g., zeolites, (aka molecular sieves), activated carbon, etc.) are used as trapping material, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed gas.
Process
Pressure swing adsorption process (PSA) is based on the phenomenon that under high pressure, gases tend to be trapped onto solid surfaces, i.e., to be "adsorbed". The higher the pressure, the more gas is adsorbed. When the pressure is dropped, the gas is released, or desorbed. PSA can be used to separate gases in a mixture because different gases are adsorbed onto a given solid surface more or less strongly. If a gas mixture such as air is passed under pressure through a vessel containing an adsorbent bed of zeolite that attracts nitrogen more strongly than oxygen, a fraction of nitrogen will stay in the bed, and the gas exiting the vessel will be richer in oxygen than the mixture entering. When the bed reaches the limit of its capacity to adsorb nitrogen, it can be regenerated by decreasing the pressure, thus releasing the adsorbed nitrogen. It is then ready for another cycle of producing oxygen-enriched air.
This is the process used in medical oxygen concentrators used by emphysema and COVID-19 patients and others requiring oxygen-enriched air for breathing.
Using two adsorbent vessels allows near-continuous production of the target gas. It also allows a pressure equalisation, where the gas leaving the vessel being depressurised is used to partially pressurise the second vessel. This results in significant energy savings, and is a common industrial practice.
Adsorbents
Aside from their ability to discriminate between different gases, adsorbents for PSA systems are usually very porous materials chosen because of their large specific surface areas. Typical adsorbents are zeolite, activated carbon, silica gel, alumina, or synthetic resins. Though the gas adsorbed on these surfaces may consist of a layer only one or at most a few molecules thickness, surface areas of several hundred square meters per gram enable the adsorption of a large portion of the adsorbent's weight in gas. In addition to their affinity for different gases, zeolites and some types of activated carbon may utilize their molecular sieve characteristics to exclude some gas molecules from their structure based on the size and shape of the molecules, thereby restricting the ability of the larger molecules to be adsorbed.
Applications
Aside from its use to supply medical oxygen, or as a substitute for bulk cryogenic or compressed-cylinder storage, which is the primary oxygen source for any hospital, PSA has numerous other uses. 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 hydrotreating and hydrocracking units. Another application of PSA is the separation of carbon dioxide from biogas to increase the methane (CH4) ratio. Through PSA the biogas can be upgraded to a quality similar to natural gas. This includes a process in landfill gas utilization to upgrade landfill gas to utility-grade high purity methane gas to be sold as 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 medium for the preferred adsorption of methane and ethane over hydrogen.
· Industrial nitrogen generator units based on the PSA process can produce high-purity nitrogen gas (up to 99.9995%) from compressed air. However, such generators are more suited to supply intermediate ranges of purity and flows. Capacities of such units are given in Nm3/h, normal cubic meters per hour, one Nm3/h being equivalent to 1000 liters per hour under any of several standard conditions of temperature, pressure, and humidity.
o for nitrogen: from 100 Nm3/h at 99.9% purity, to 9000 Nm3/h at 97% purity ;
o for oxygen: up to 1500 Nm3/h with a purity between 88% and 93%.
In the frame of carbon capture and storage (CCS), research is also currently underway to capture CO2 in large quantities from coal-fired power plants prior to geosequestration, in order to reduce greenhouse gas production from these plants.
PSA has also been discussed as a future alternative to the non-regenerable sorbent technology used in space suit primary life support systems, in order to save weight and to extend the operating time of the suit.
(Content excerpted from Wikipedia)
