Conventional PSA Process

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Conventional pressure-swing adsorption (PSA) process has been successfully used to separate and purify industrial gases. The technology, first developed in the late 1950s, is based on the premise that adsorbents can selectively adsorb and desorb particular gases under high or low pressure as the case may be. However most of the conventional PSA processes yield just one of the components as a pure product and at low recoveries. As the PSA cycles are the main factors in a PSA process, constant work has gone to make the PSA cycles quicker and more efficient.

PSA Cycle:
The control of the cycling is critical for the success of a PSA cycle. The basic principle underlying the PSA process is that the adsorbent is regenerated by quickly reducing the partial pressure of the adsorbed component. This is done by lowering the total pressure or by using a purge gas. The first PSA cycles of the 1960s, applied two steps of adsorption and depressurization/purge and carried out separation in two adsorbent beds. In the modern PSA processes generally three or more beds are used in addition to those used in the first PSA cycle. Generally the PSA cycle that are used in the in the industries involve the cyclic repetition of four basic steps of Production, Depressurizing, Purging and Repressurizing.

New concepts emerging in the PSA cycle process:
Heavy Reflux PSA cycle or Enriching Reflux PSA cycle for producing pure heavy sections.
Dual Reflux PSA cycle for the production of two pure products.

Working of a Conventional PSA process:
A gas mixture, under high pressure is fed into a vessel that contains a bed of adsorbent beds, whose diameter is 2-6-mm. These adsorbent beds, are typically made up of alumina, silica gel, activated carbon or molecular sieves. The impurities of the feed gas gets adsorbed onto the internal surfaces of the adsorbent. This leaves the purified product gas in the empty spaces of the vessel. The required product gas is withdrawn from the top of the vessel by applying pressure. The pressure that was generated in the adsorption vessel is then lowered, and the product gas remaining in the empty spaces of the vessel is subsequently removed.

For regenerating the adsorbent bed, the adsorbed impurities are released back into the gas phase. The vessel is then purged with little quantities of purified product gas, for getting regeneration of the adsorbent bed completed. A low-pressure exhaust stream is used for exiting the impurities in the PSA process. Lastly, the vessel is repressurized again with a mixture of product gas obtained from the depressurization step, feed gas and high-purity product gas. This completes the whole cycle and is repeated every 2-20 minutes in conventional PSA systems. As each cycle is essentially a batch process, multiple pressure vessels are used in tandem sequentially to provide a semi continuous flow of product gas. Additionally, big surge tanks are used to check variations in flows of feed, product and exhaust streams.

However, to improve the yield of the product gas, more-complex PSA cycles can be used, involving intermediary steps like cocurrent and countercurrent depressurization. A multiple number of adsorbent beds are needed for these complex PSA cycles.

Limitations of Conventional PSA process:

• Limited PSA cycle speed: Gases flowing through the adsorbent beds, are limited by the fluidization velocity of the adsorbent beads. Quicker cycle speeds and fast gas flows result in fluidization, that in turn leads to abrasion, accumulation of dust and lastly the disintegration of the adsorbent beads. This leads to slow cycle speeds (2-20 minutes per cycle) and correspondingly large pressure vessels. The big container sizes mean that conventional PSA systems have to be field erected, with additional costs and on-site construction problems.
• Conventional Valves: The series of individual switching valves, with instrumentation, control systems and process piping, make conventional PSA systems costlier and complex to operate. This reduces the product yield and overall process efficiency. Solenoid-actuated valves also involve high costs due to high replacement rate of internal valves.
• Kinetic Limitations: The fluidization factor as discussed, limits the minimum adsorbent bead size that can be used in commercial PSA processes to around 2 mm in diameter. This bead size makes the adsorbed gas to take a longer diffusion path. From the bulk gas stream to adsorption sites present on the internal surfaces of adsorbent macro-structure. As a result there is relatively slow mass transfer between the bulk gas and adsorption sites.