Vacuum swing adsorption

Vacuum Swing Adsorption (VSA) is a non-cryogenic gas separation technology. Using special solids, or adsorbents, VSA segregates certain gases from a gaseous mixture under minimal pressure according to the species' molecular characteristics and affinity for the adsorbents. These adsorbents (e.g., zeolites) form a molecular sieve and preferentially adsorb the target gas species at near ambient pressure. The process then swings to a vacuum to regenerate the adsorbent material.

VSA differs from cryogenic distillation techniques of gas separation as well as pressure swing adsorption (PSA) techniques due to the fact that it operates at near-ambient temperatures and pressures.

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Advantages of VSA over PSA

The simplicity of the VSA process allows for greater efficiency and cost savings, and less maintainence vis-à-vis PSA systems. The VSA process extracts maximum sieve and power efficiencies. The integrated rotary lobe blower, which also serves as a vacuum regenerator, results in low feed pressure. The dramatically lower pressure swings in the VSA system eliminate the need for a feed air compressor, which translates into lower power consumption for VSA systems. As a result, power savings of as much as 50% can be achieved.
The low pressure air input into the adsorber vessel in combination with the high efficiency of the vacuum applied during the desorption stage means that a single adsorption vessel may be used. In contrast to traditional PSA systems, which require feed air compressors as well as process valves and associated dryers and feed air filtering systems, this single-vessel VSA system eliminates many of the design problems associated with PSA.
Maintenance issues typically associated with PSA systems are greatly reduced with VSA technology. VSA systems are less susceptible to sieve dusting because the pressure swings are of a lower order of magnitude. These lower operating pressures also eliminate any water condensate. Overall, VSAs are not as susceptible to humid environments as PSA systems, while PSA feed compressors require both oil and water removal hardware. The above-mentioned rotary lobe blower is a rotary device that does not require the high level of routine maintenance typical of air compression systems. The use of a vacuum step provides a superior regeneration of the molecular sieve, thus extending sieve life. Overall, the VSA adsorber vessel has much longer service life than PSA vessels, which commonly need re-packing of sieve material every 3-5 years.

Commercial uses

The design simplicity and efficiency that VSA technology offers has generated products that are more energy- and cost-efficient than traditional gas separation units. VSA processes are used at refineries, chemical and petrochemical plants, water treatment facilities, and landfills. VSA technology is used to purify air, soil, water, and hydrogen, and to manufacture oxygen, nitrogen, and hydrogen.

VSA technology plays an increasingly important role in the commercial production of oxygen. Oxygen concentrators that use VSA processes are a more lucrative and reliable option than oxygen cylinders for many industries. Its mobility and constant supply of oxygen makes it a perfect choice for governments and aid organizations in their emergency medicine and disaster relief operations, as well as for district hospitals in developing nations. Other commercial applications of oxygen concentrators include the fields of aquaculture and high-altitude work environments, including in the mining industry or the Goldmud-Lhasa railroad in Tibet.

For the oil and gas industry, the production of liquid nitrogen via PSA technology is key in cases where a high nitrogen flow rate and/or high discharge pressure is required. As an inert gas, nitrogen is preferred over air for cleaning out newly drilled wells as well as maintaining old wells. In addition, nitrogen is used for fracturing, pipeline purging and drying, cementing, and pressure maintenance.

See also

• Adsorption
• Oxygen concentrator

References

• Hutson, Nick D.; Rege, Salil U.; and Yang, Ralph T., “Air Separation by Pressure Swing Adsorption Using Superior Adsorbent,” National Energy Technology Laboratory, Department of Energy, March 2001
• Adsorption Research, Inc., “Adsorption is the Solid Solution,”[1]
• Ruthuen, Douglas M., Principles of Adsorption and Adsorption Process, Wiley-InterScience, Hoboken, NJ, 2004, p. 1
• Yang, Ralph T., “Gas Separation by Adsorption Processes,”Series on Chemical Engineering, Vol. I, World Scientific Publishing Co., Singapore, 1997
• Ruthuen, Douglas M.; Farooq, Shamsuzzaman; and Knaebel, Kurt S., Pressure Swing Adsorption, Wiley-VCH, Weinheim, Germany, 2001
• Santos, João C.; Magalhães, Fernão D.; and Mendes, Adélio, “Pressure Swing Adsorption and Zeolites for Oxygen Production,”in Processos de Separação, Universidado do Porto, Porto, Portugal