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Nitrogen Or Oxygen Generation Using Pressure Swing Adsorption (PSA)

PSA technology can be used to produce either nitrogen or oxygen by passing compressed air, at several atmospheres, through a vessel containing adsorbent materials. Adsorbants are chosen on the basis of their adsorption characteristics. A desirable adsorbant will have much greater affinity for non-product molecules than for the product gas (nitrogen or oxygen).  This characteristic results in most of the desired molecules passing through the bed and into the product stream, while undesired components (product impurities) are captured by the adsorbant. 

The PSA process is inherently a batch process, as the adsorbent bed requires periodic desorption. Consequently,PSA systems usually contain two adsorbant vessels to provide operational continuity.  At any time, one of the vessels will be making product by adsorbing undesired components of air; while the other vessel is undergoing regeneration by de-pressurization to atmospheric pressure. When the adsorbing vessel approaches saturation, a set of valves quickly switches the services.  A surge vessel (buffer vessel) downstream of the adsorbers ensures that delivery of product is continuous.  While the two-bed system is most common, mono-bed and three-bed configurations are sometimes used. The mono-bed system provides capital savings while the three-bed system provides for greater continuity of production.   

Nitrogen PSA vessels contain an activated carbon molecular sieve material which removes oxygen and other undesired components by adsorption. Nitrogen is typically delivered from the production unit at pressures of six to eight atmospheres and at a purity of 95 to 99.5%.  If higher purity is required, both the equipment size and the ratio of air feed to product make go up.  Alternatively, a "de-oxo" unit can be added that catalytically combines hydrogen with the oxygen in the nitrogen product leaving the PSA, producing water, which is removed by cooling and additional adsorption. 

The waste stream from a nitrogen membrane or PSA plant is enriched in oxygen - often to about 40% oxygen.  This stream is sometimes used for combustion enhancement or waste treatment equipment operation at the same site.

Oxygen PSA units typically use alumina to remove much of the water vapor from the air feed, together with zeolite molecular sieve to adsorb nitrogen, carbon dioxide, residual water vapor and other gases.  Typical oxygen PSA delivery pressures exiting the unit are one to three atmospheres.  Oxygen purity is typically between 90 to 95%;  limited primarily by the argon content, which will be 4.5 to 5%. 

   

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