Vacuum-Pressure Swing Adsorption

A vacuum pressure swing adsorption (VPSA) system is one of the popular technique, a reliable and economic on-site supply method, used in producing oxygen 5,000 and 200,000 cubic feet per hour or more. This technology is used in non cryogenic air separation. This process is used to produce oxygen and nitrogen with purity levels ranging from 90 to 95%. The system use proprietary molecular sieve adsorbents which is considered to give the most advanced air separation process of its kind. This type of non-cryogenic oxygen production process is also referred to as VSA (Vacuum Swing Adsorption) or PVSA (Pressure-Vacuum Swing Adsorption) systems.

How the Process Works?
A typical VPSA system usually comprises the following:

• Vacuum blower
• A rotary-lobe feed air blower
• An oxygen surge tank
• One or two adsorbent vessels
• Switching valves
• Computer controls

As already mentioned, VPSA produces gaseous products at purities between 90 and 94%. In case of oxygen, the impurities are usually nitrogen and argon with the argon content between 4.5 and 5%. The process cycle is almost similar to PSA air separation. The only difference is that here vacuum blowers are used to reduce the desorption pressure. Compared to a PSA, the lower desorption pressure reduces the required inlet pressure, as a result of which a typical oxygen VPSA produces oxygen at just 0.2 atmospheres, gauge. In other words, the system regenerate the sieve material under vacuum conditions and a higher percentage of available oxygen is recovered, and therefore less air has to be processed. Higher oxygen delivery pressures are produced when you add an oxygen booster compressor or blower to the VPSA system.


Bed systems
There are two bed systems for air separation. These are:

Single-bed system: In the single-bed system, the blower is used to draw in air, which is compressed and send to the adsorbent vessel to remove impurities. This separates the oxygen from air. The blower removes the gas by reducing the pressure inside the vessel and the waste products like carbon dioxide, nitrogen, water is then discharged into the air. Since oxygen is not produced during regeneration, the single bed system consists of a low-pressure surge tank to allow for continuous oxygen supply.

Two-bed system The two-bed system uses a similar adsorption process cycle like a single bed system that relies on swings in pressure. The swings in pressure is from above one atmosphere to below atmospheric pressure or vacuum. The cycle continues for each bed sequentially from adsorption to desorption.

Comparison of VPSA with PSA and cryogenic separation

• Though VPSA systems are more costly to build, they are comparatively more energy efficient than PSA systems for the same product flow, pressure and purity conditions.
  
  
• Oxygen VPSA units are economical than oxygen PSA units only when the desired production rate is greater than about 20 tons per day, up to 60 tons per day or more.
  
  
• They are more preferred in those cases when providing high purity oxygen is not required. · Cryogenic plants are preferred production rate is 60 tons or above per day.
  
  
• However, two oxygen VPSA plants can also perform in the same manner when they allow for better matching of large step-changes in demand.
  
  
• PSA specific power is approximately 1/3rd less than that for oxygen PSA plants, but almost same to the specific power of cryogenic air separation units.