PSA technology utilizes towers which are filled with carbon molecular sieve (CMS). Compressed air enters the bottom of the "online" tower and flows up through the CMS. Oxygen and other trace gases are preferentially adsorbed by the CMS, allowing nitrogen to pass through. After a pre-set time the on-line tower automatically switches to the regenerative mode, venting contaminants from the CMS. Carbon molecular sieve differs from ordinary activated carbons as it has a much narrower range of pore openings. This allows small molecules such as oxygen to penetrate the pores and separate from nitrogen molecules which are too large to enter the CMS. The larger molecules by-pass the CMS and emerge as nitrogen gas.
PSA nitrogen generators are typically used in applications where the purity requirement is higher than 99.5%, up to 99.9995%.