Pressure Swing Adsorption

PSA, also referred as Pressure swing adsorption is one of the most rapidly growing industrial process for gas separation. PSA process is used to produce oxygen or nitrogen. This is done by passing compressed air at various atmospheres through a vessel consisting of adsorbent materials. Considering the adsorption characteristics, the adsorbents are chosen. An adsorbent which is desirable have much greater attraction for non-product molecules than for the product gases like nitrogen or oxygen. This leads to absorbing impure products or undesired components by the adsorbent, while most of the desired molecules pass through the bed and into the product stream. The PSA systems usually have two adsorbent vessels to provide continuous operation. At any time, one of the vessels will adsorb undesired components of air and produce the product while the other vessel undergoes regeneration by de-pressurization to atmospheric pressure. While the two-bed system is the most commonly used, there are single-bed and three-bed configurations as well which are used for greater capital savings or greater continuity of production.

Nitrogen PSA Unit
Nitrogen PSA vessels contain an activated carbon molecular sieve material. This material by adsorption is used to remove oxygen and other undesired components. Nitrogen is delivered at a purity of 95 to 99.5% by using PSA system. If higher purity is required, a "de-oxo" unit can be added. The "de-oxo" unit combines hydrogen with the oxygen in the nitrogen product leaving the PSA, which in turns produce water and this is removed by cooling and additional adsorption.

Oxygen PSA Unit
This unit make use of alumina and zeolite molecular sieve. While the alumina removes water vapor from the air feed, the zeolite sieve adsorbs nitrogen, carbon dioxide, residual water vapor and other gases. Oxygen purity is typically between 90 and 95% in PSA system.

A comparison between the PSA and Cryogenic Methods of Oxygen Production is given below:

Elements	PSA	Cryogenic production
Temperature	Ambient	Low
Pressure	Maximum is 150 kPa	Maximum is 13,000 kPa
Purity	95%	near 100%

Uses of the PSA Gas Separation

• PSA separation is typically used to produce 90 to 95% pure oxygen or nitrogen in small and medium sized gas plants.
  
  
• One of the primary use of PSA is in the removal of CO2 in the final process in the large-scale commercial synthesis of hydrogen which is used in oil refineries and in the production of ammonia (NH3).
  
  
• Another use of PSA is the separation of CO₂ from biogas which in turn increases the methane (CH₄) content.
  
  
• Through PSA technology, it is possible for biogas to be upgraded to a quality similar to natural gas.
  
  
• PSA system is widely used in the medical industry to produce oxygen. This becomes all the more advantageous in inaccessible areas where bulk cryogenic or compressed cylinder storage are not possible.
  
  
• PSA plants are used to produce nitrogen for purpose like blanketing fuel tanks at the airports.
  
  
• Presently PSA is used to remove CO2 from the flue streams of lime and steel industry. In future, there is every chance that such plants will be used to remove greenhouse carbondioxide from flue streams of coal fired power stations.
  
  
• Another reason for preference of this system is that it is a very clean process where there is no chance of polluting the environment.