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Oxygen By Vacuum-Pressure Swing Adsorption (VPSA, VSA Or PVSA)

Vacuum-pressure swing adsorption units are used to produce oxygen at purities between 90 and 94%.  Impurities are argon and nitrogen, with the argon content typically between 4.5 and 5%.The process cycle is similar to that described above for PSA plants, except that vacuum blowers are used to reduce the desorption pressure. The lower desorption pressure reduces the required inlet pressure, compared to a PSA.   As a consequence, a typical oxygen VPSA produces oxygen at just a few psig (about 0.2 atmospheres, gauge).  When higher oxygen delivery pressures are required, an oxygen booster compressor or blower is added to the system.

Overall, VPSA systems are more costly to build, but more energy efficient than PSA systems for the same product flow, pressure and purity conditions.   

VPSA units regenerate the sieve material under vacuum conditions because it results in more fully regenerated molecular sieve material, which is more selective than material subjected to the classic PSA regeneration process. As a result, a higher percentage of available oxygen is recovered, and less air has to be processed.  Air compressor power is greatly reduced compared to an oxygen PSA unit due to lower air flow and lower compressor discharge pressure (typically less than half an atmosphere, gauge). There is an off-set to the air compression power savings however, due to power needed to operate the vacuum-generation machinery. 

Oxygen VPSA units are usually more cost effective than oxygen PSA units when the desired production rate is greater than about 20 tons per day. They are often the most cost-effective oxygen production choice up to 60 tons per day or more, providing high purity oxygen is not required.  Above about 60 tons per day, cryogenic plants are usually the oxygen production technology of choice, although in some cases, two oxygen VPSA plants allow for better matching of large step-changes in demand.  VPSA specific power is about one-third less than that for oxygen PSA plants.  It is usually similar to the specific power of a comparable capacity cryogenic oxygen plant.

Units using this type of non-cryogenic oxygen production process may be referred to as VPSA (Vacuum Pressure Swing Adsorption), VSA (Vacuum Swing Adsorption) or PVSA (Pressure-Vacuum Swing Adsorption) systems.

   

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