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In any oxy-generation process, air is passed through a bed of zeolite sieve beads, an adsorbent material derived from naturally occurring crystalline inorganic materials, similar to ordinary clays. Each bead is approximately 2 mm in diameter. This sieve is specifically designed to separate oxygen from nitrogen and other gases in air, by adsorbing the nitrogen molecules, while allowing the oxygen molecules to pass through to a collection vessel. When the bed of zeolite sieve is full of nitrogen molecules, it must be regenerated to allow the process to continue. The collected oxygen at 90% purity is the product, while the impure nitrogen combined with any water and carbon dioxide is vented. Most generators are of the Pressure Swing Adsorption (PSA) type, oxygen is produced from the sieve bed under pressures as high as 8 barg (116 psig), and the zeolite sieve is regenerated against ambient pressure. Such high pressures require an air compressor and air dryer to be used; major running costs that are eliminated with VSA technology. The VSA process produces oxygen at lower pressure (1.5 barg) and totally regenerates the sieve in a mild vacuum (0.5 barg). In a VSA generator the compressor can be replaced by a blower, meaning that capital and operating costs are reduced. The overall costs of buying and operating a VSA generator are lower than PSA systems.

While more affordable, VSA generation also reduces the probability of contamination of the zeolite bed, since it is a dry process; no water droplets are formed and any moisture present in the incoming air is kept in the vapour phase. On the other hand, a PSA process creates water molecules from the incoming compressed air and this can contaminate the zeolite sieve. If the compressed air supply to the PSA is from an oil injected screw compressor, which usually are the most economic method of compressed air supply then there is the added risk of oil contamination of the zeolite sieve. Another disadvantage of oil-injected compressors is that the condensate removed from the filtration system is contaminated with compressor oil and can present a hazardous waste disposal problem in some locations.

While PSA generators have many pressure activated pneumatic valves and switches, Air Products' newly patented VSA oxygen generators use a single electrically actuated four-way valve which is more reliable than pneumatic valves. So there is less to go wrong.

Download the PDF file to see some of the other differences between VSA and PSA generators.

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The following schematic illustrates how the process works:

Air enters the generator via an inlet vent, passes through the four way valve V-1, through a single stage, dry rotary lobe blower, an air cooler and then into the adsorber bed, again via valve V-1. On entering the adsorber vessel water vapour, carbon dioxide and hydrocarbons are removed in a desiccant layer. Thus only oxygen and nitrogen molecules flow over the sieve bed, where the nitrogen is adsorbed and the oxygen passes out of the vessel and through the open valve CV-2 to fill the O₂ buffer tank.

The bed quickly becomes saturated with nitrogen molecules. These now have to be removed in a vacuum step to regenerate the bed. Valves CV-1 and CV-2 are closed, and the four way valve V-1 rotates such that the blower now acts in reverse and "sucks" nitrogen molecules off the sieve bed, in addition to removing water vapour, carbon dioxide and hydrocarbons off the desiccant layer. All these are exhaust gases that are vented out of the system through the same blower (now acting as a vacuum blower) that the air originally entered.

In the purge step valve CV-2 opens to allow oxygen molecules to flow back into the adsorber vessel to help remove any last traces of nitrogen in the system.

Finally, in the re-pressurisation step, the whole system is brought back up to atmospheric pressure by valve V-1 rotating into a position such that air can rush back into the inlet. At the same time oxygen is allowed into the top of the bed, thus ensuring that there is no movement of the sieve bed (sometimes a problem in PSA systems). The product valve PIC-1 remains open throughout the whole of the above cycle so that a continuous flow of oxygen product is maintained.

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