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 because it operates at near-ambient temperatures and pressures. VSA may actually be best described as a subset of the larger category of PSA. It differs primarily from PSA in that PSA typically vents to atmospheric pressures, and uses a pressurized gas feed into the separation process. VSA typically draws the gas through the separation process with a vacuum. For oxygen and nitrogen VSA systems, the vacuum is typically generated by a blower. Hybrid VPSA systems also exist. VPSA systems apply pressurized gas to the separation process and also apply a vacuum to the purge gas. VPSA systems, like one of the portable oxygen concentrators, are among the most efficient systems, measured on customary industry indices, such as recovery (product gas out/product gas in), productivity (product gas out/mass of sieve material). Generally, higher recovery leads to a smaller compressor, blower, or other compressed gas or vacuum source and lower power consumptions. Higher productivity leads to smaller sieve beds. The consumer will most likely consider indices which have a more directly measurable difference in the overall system, like the amount of product gas divided by the system weight and size, the system initial and maintenance costs, the system power consumption or other operational costs, and reliability.

Comparison of VSA to PSA

The simplicity of the VSA process may allow for greater efficiency and cost savings, and less maintenance than PSA systems. The VSA process operates on the steepest part of the isotherm curves and thus has the potential to extract 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, when compared to the most simple PSA systems. However, VPSA systems typically have comparable or better power efficiencies.

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 absorption 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 two-bed PSA.

Maintenance issues typically associated with two-bed 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 water removal hardware, and oil-removal hardware if an oil-lubricated compressor is used. Oil-less compressors are available, but are typically higher priced than oil-lubricated compressors. 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 two-bed 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. Due to their mobility and constant supply of oxygen, they are often used by governments and aid organizations in emergency medicine and disaster relief operations, as well as by 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. VPSA technology has allowed the development of a portable oxygen concentrator weighing less than 15 pounds (7 kg), but with continuous flows of oxygen up to 3 LPM and pulse flows up to an equivalent of 7.2 LPM.

VSA is also used in hypoxic air fire prevention systems to produce air with a low oxygen content.

See also

• Oxygen concentrator

References

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