Vacuum Swing Adsorption (VSA or VPSA)

The separation of air for the production of oxygen is an important operation in the chemical processing industry as well as for energy conversion processes. This separation has been done predominately by cryogenic distillation; though, as adsorption systems have become more efficient and new. Vacuum swing adsorption (VSA ) have become increasingly competitive and are already favorable for small-to-medium scale operations. Currently, approximately 20% of air separations are accomplished using this technology.

The system and method separate high-purity oxygen product from air by sequenced adsorption and desorption operations occurring solely under vacuum pressure connditions. This lets for significantly reduced kilowatt-hours of electric-oxygen power consumption per oxygen ton produced.

The most common adsorption materials:

• For oxygen service includes a variety of zeolite molecular sieve which selectively adsorbs nitrogen, moisture and carbon dioxide gas. This allows the oxygen molecules to pass through the unit and produce low purity oxygen, typically at 90 to 95% purity.
  
  
• For nitrogen service, the zeolite is replaced by carbon molecular sieve to preferentially adsorb oxygen, thereby delivering N2 at purities up to 99.99%.

Components of VSA
The main components are:

• Two carbon or zeolite sieve containers
  
  
• Nitrogen or oxygen receiver
  
  
• Refrigerated dryer
  
  
• Feed air compressor
  
  
• Air receivers
  
  
• Air filters
  
  
• Oxygen enriched air
  
  
• Vacuum blower

VSA Process
The process is similar to that of PSA. The only difference is that the differential pressure takes place at lower absolute pressures. The main steps are:

• The VSA process begins by charging the first vessel with low-pressure air, initiating the N2 adsorption process.
  
  
• Before the zeolite reaches equilibrium, or when O2 is adsorbed, the pressurized gas in the first vessel is vented to the second vessel at lower pressure (vacuum).
  
  
• Residual N2 in the first vessel is then "desorbed" from the zeolite and vented at atmospheric pressure.
  
  
• All required valving operations are done automatically by carefully calculated timing cycles controlled by a PLC.

Advantages of VSA

• VSA is cost effective when production rates exceeds 20 to 30 tons per day.
  
  
• VSA can produce product at near atmospheric pressure.
  
  
• Separation power is lower.
  
  
• Increase in overall process efficiency.
  
  
• Total power including product compression is low.

Applications of VSA

• Fuel cell applications
  
  
• Industrial gas adsorption process
  
  
• Separation strategies
  
  
• In chemical industry