Sourcing » Engineering » Articles » What is PSA Process

What is PSA Process

Share |

Pressure-Swing Adsorption or PSA is a technical process widely used these days in gas production. It's a typically non-cryogenic way of purifying gas. PSA process differs from cryogenic techniques of gas separation as it can function under near-ambient temperatures. Under pressure, the PSA process separates some 'species' from a mixture, according to the molecular characteristics of those 'species'. This is because different gases has the propensity to be attracted to different solid surfaces more or less strongly.

More gas is adsorbed at higher pressure while the gas is desorbed as the pressure is reduced. Suppose air under pressure is passed through a vessel containing an adsorbent bed and this bed has the property to attract nitrogen more strongly than oxygen. What will happen is that a part or all of the nitrogen will remain in the bed, while the gas coming out of the vessel will be enriched in oxygen. As the bed reaches its optimum capacity to adsorb nitrogen, it can again be regenerated by reducing the pressure. Releasing the adsorbed nitrogen in the process. Thus readying itself for another cycle of oxygen enriched air. This cycle goes on.

Popular adsorptive materials like carbon, zeolites, silica etc. are used as the adsorbent beds. They selectively adsorbs the undesired gases at high pressure on their surface. Continuous production of the target gas is achieved by using more than one adsorbent chamber. However it is not a very easy process and difficult to model accurately. But once properly modeled, adsorption process can result in complex diffusion relationships.

Characteristics of PSA:
Purity: Can result in purity upto 99% with some plants offering control of purity to the user.
Speed: One Process cycle requires just few minutes.
Economy: One unit can, for example easily produce about 20,000 SCFH of Nitrogen. The number of units required is in direct proportion to the quantity of the production needed.
Initial Start up time: It requires around 2000 cycles requiring 3 to 4 days.

Important Factors to be considered before selecting a PSA process:
All the factors as enumerated below plays a decisive role, say when you opt for an on site source of gas generation:

Purity Considerations: It depends on the nature of industry. For example, an industry that can tolerate purity of 90-95% PSA is a preferred option.
Consumption and Volume pattern: The flow rate and usage pattern plays a vital part. For e.g. for nitrogen separation which can take a start up time upto several days, an on site Nitrogen generator can prove expensive in batch operations. Ideally non-cryogenic sources are ideal for lower flow rates (100-100,000 SCFH).
Expansion needs: We need to look at the future plans and expansion needs. Out of PSA, Cryogenic or Membrane separation the last one is easy to adapt but can prove expensive for high volume.
Cost of electricity: For areas where cost of electricity is high the process of on site gas generation can prove costly than having it delivered through cryogenic medium.
Other Costs: This can include back up system of gas generation and pressure flow as higher pressure from a large compressor means higher costs.

How a PSA device works:
Working of a typical PSA Device can be described by taking the example of oxygen separation from air. It works like this. Air separation starts at the PSA process when air from the atmosphere is compressed at a pressure of 100 psig. The compressed air is then directed to one of the two adsorbing tanks by valves. In the tank the Carbon Molecular sieve desorbs the oxygen selectively while letting go Nitrogen and Argon. Impurities like Carbon dioxide or water also gets adsorbed by the sieve at the beginning of the bed. As the adsorber starts producing the Nitrogen or Argon composition, a sensor opens the valve which lets the release of stream as a product.

Gradually the adsorbent becomes saturated with oxygen and is no longer in a position to adsorb further oxygen from air. Here at this point, the product stream from the adsorber is cut off. A Bleed stream from the adsorber is opened and the pressure in the tank is released. This reduced pressure makes the dedsorbed oxygen to be released through the vent. Two adsorbers work simultaneously in a PSA unit. While the one is busy de- adsorbing the oxygen, on the other air is fed for adsorption. This cycle of adsorption and de-adsorption results in a continuous stream of production.

Tags:- About Pressure Swing Adsorption Process, Psa Gas Separation Process

Add a comment