Vacuum swing adsorption
Encyclopedia
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., zeolite
s) 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.
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 utilized. 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.
, 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. 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.
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.
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., zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s) form a molecular sieve
Molecular sieve
A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...
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
Gas separation
Gas mixtures can be effectively separated by synthetic membranes. For other methods see adsorption, absorption, cryogenic distillation.Membranes are employed in:* separation of nitrogen or oxygen out of air...
as well as pressure swing adsorption
Pressure swing adsorption
Pressure swing adsorption is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation...
(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
Pressure swing adsorption
Pressure swing adsorption is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation...
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 utilized. 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 hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, and to manufacture oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, 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
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...
and high-altitude work environments, including in the mining industry or the Goldmud-Lhasa railroad
Qingzang railway
The Qinghai–Xizang railway, Qingzang railway, or Qinghai–Tibet railway , is a high-elevation railway that connects Xining, Qinghai Province, to Lhasa, Tibet Autonomous Region, in People's Republic of China....
in Tibet. VPSA technology has allowed the development of a portable oxygen concentrator
Oxygen concentrator
An oxygen concentrator is a device providing oxygen therapy to a patient at minimally to substantially higher concentrations than available in ambient air. They are used as a safer, less expensive, and more convenient alternative to tanks of compressed oxygen. Common models retail at around US$800...
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.
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.