PSA Technology hits the Fast Lane

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Courtesy of Xebec Adsorption Inc.

Conventional pressure-swing adsorption (PSA) has been used ito separate and purify industrial gases for more than 35 years. The technology, first developed by ExxonMobil and Air Liquide in the late 1950s, is based upon the capacity of adsorbents to selectively adsorb and desorb particular gases as gas pressure is raised and lowered. PSA is currently used in applications ranging from the production of nitrogen and oxygen from air to dehydration and hydrocarbon recovery.

New fast-cycle Pressure Swing Adsorption technology is currently being developed, offering more-compact, less-expensive and more-energy-efficient gas separation equipment. This article looks at this new technology and its potential in areas such as hydrogen purification and recovery.

A cyclic process
The PSA process involves a cyclic repetition of four basic steps: production, depressurizing, purging and repressur-izing (Fig. 1). First, a gas mixture is fed under high pressure into a vessel containing a bed of 2-6-mm diameter adsorbent beads, typically alumina, silica gel, activated carbon or molecular sieves. Impurities in the feed gas adsorb onto the internal surfaces of the adsorbent, leaving purified product gas in the void spaces of the vessel. Product gas is then withdrawn from the top of the vessel under pressure.

The pressure in the adsorption vessel is then reduced, and product gas remaining in the void spaces of the vessel is removed. The adsorbed impurities are released back into the gas phase, regenerating the adsorbent bed. The vessel is then purged with a small amount of purified product gas, to complete regeneration of the adsorbent bed. Impurities exit the PSA process in a low-pressure exhaust stream.

Finally, the vessel is repressurized with a mixture of product gas from the depressurization step, feed gas and high-purity product gas. This cycle is repeated every 2-20 minutes in conventional PSA systems. Since each cycle is essentially a batch process, multiple pressure vessels are used together in sequence to provide a semicontinuous flow of product gas. In addition, large surge tanks are used to dampen variations in flows of feed, product and exhaust streams.

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