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Two Bed PSA Process

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Using simple two bed PSA process high purity Oxygen, Nitrogen and Hydrogen could be obtained. From the economic point of view, for Oxygen production less than 700 Nm3/hr the 2-bed PSA process is conformed as a more feasible option. Generally Two adsorbent beds are arranged in parallel. One bed is made on-line while the other gets regenerated. Each bed then undergoes a cycle of phases.

The PSA process takes the following cyclic sequence of Pressurization with the feed gas (PR), High-pressure adsorption (AD), Depressurizing pressure equalization (DPE), Countercurrent depressurization (DP), Purge (PG), and lastly Pressurizing pressure equalization (PPE).

Using Double bed PSA for Helium Purification
Helium owing to its unique properties is gaining ground for its use in high profile technologies like Nuclear power plants, as a refrigerant for superconducting magnets, or transfer of energy to lasing gases.

Using Double bed PSA system for purification makes it possible to enhance the performance of Helium enrichment plant. The PSA system has been able to reduce impurities like nitrogen, which has led to obtaining pre-enriced helium concentration to the tune of 15 vol % from about 1.4 vol % earlier.

One of the primary benefits of using PSA system has been that unlike the enrichment and purification plants where nitrogen in the liquid form is used as an external refrigerant here liquid nitrogen is not needed to pre enrich helium

Each step of Two Bed PSA Process is described in the following table:

Elementary steps	Mode of operation	Principal features
Pressurization	Pressurization with feeding from the feed end Pressurization with raffinate product from the product end before feed pressurization	Enrichment completed, of the 'less selectively adsorbed species' in the gas phase at product end Sharpens the concentration front, that improves purity and recovery process of raffinate product
High-pressure adsorption	Product (raffinate) withdrawal is done at constant bed pressure The bed pressure is allowed to lessen as the raffinate product is drawn from the product end	Raffinate product gets delivered at high pressure Considerable recovery of the less selectively adsorbed species is achieved, but the end product is delivered at low pressure
Blowdown	Counter-current blowdown to very low pressure Co-current blowdown to an intermediate pressure before counter-current blowdown	Applicable only when raffinate product is required at high purity; can prevent contamination of the product end with more strongly adsorbed species Used when extract product is also needed with high purity; enhances extract product purity; can also increase raffiante recovery
Desorption at low pressure	Counter-current desorption with product purge Counter-current desorption without external purge Evacuation	Enhances raffinate product but at the expense of lower recovery; purge at sub atmospheric pressure reduces raffinate product loss but energy cost goes up. Recovery is done while maintaining high product purity is possible only during certain type of kinetic separation. High purity obtainable of both extract and raffinate products; advantage over product purge when the adsorbed phase is very held strongly
Pressure equalization	The low and high pressure beds are connected through their product ends or through feed end. Product ends of the high-pressure bed are joined to the respective ends of the low-pressure bed	Conserves energy and separative work.

Tags:- 2-bed PSA Process, Double Bed PSA System

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