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. for nitrogen: from 100 Nm3/h at 99.9% purity, to 9000 Nm3/h at 97% purity ; 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. This is the process used in medical oxygen concentrators used by emphysema and COVID-19 patients and