Multi Bed PSA Process

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From the last two decades or so Pressure Swing Adsorption (PSA) processes have gained commercial acceptance and recognition as an energy efficient separation technique. However the major challenge here has been to improve product gas yields. An immediate requirement for this has been to operate more complex PSA cycles. This has resulted in the need for multi adsorbent beds. However in determining the optimum process cycle for a said gas purification method, an economic trade off must be made between heightened efficiency and purity.

In multi-bed PSA systems also, the beds undergo the same steps as in a normal PSA cycle and are connected to the exchange material during the steps of blow down and pressure equalization. Most of the industrial PSA installations uses a complex network of valves for switching gases between the adsorbent beds. These valves are activated by solenoids. For example in a eight-bed hydrogen PSA process, 42 valves are typically used to switch gases. In a PSA cycle the number of valves required are in proportion the number of adsorbent beds. These days Multi bed rapid cycle PSA systems are efficiently packaged for getting an integrated, modular rotating bed design.

The combination of fast cycle speeds and multi bed designs have been able to produces successfully a uniform flow of product and exhaust gas. This has also resulted in removal of the large surge tanks required on the product and exhaust streams. Bed size can be increased depending upon the extent of purity requred. We describe here a process to recover and purify hydrogen by using multi bed PSA process.

The application of multi bed PSA process can result in production of hydrogen that is extremely pure almost 100%. Primarily the PSA process involves the adsorption of impurities from a feed gas that is hydrogen rich, onto fixed bed of adsorbents at high pressures. Gradually the impurities are desorbed at low pressure into an offgas stream. A number of hydrogen rich feed stocks can be treated using this process. That includes syn gas, refinery gases and coke oven gases etc. The PSA process is tailored in such a way to to suit the composition of the feed gas and purity requirements. This is done by manipulating the PSA cycle and adsorbents used.

Continuous product stream and offgas flows are achieved by using multiple adsorbers that is operated in a step by step process. The process of hydrogen recovery by this process is a typically batch operation. But as in any PSA process the basic cycles can be made to work in the following five steps:

• Adsorption: Feed gas is allowed to pass co-currently through the clean adsorbent bed. Here the impurities are selectively adsorbed. While pure hydrogen product at high pressure exits from the adsorbent bed.
• Co-Current Depressurization: After adsorption, the bed gets saturated with impurities and regenerating is needed. For recovering the hydrogen trapped in the void spaces, co-current depressurization makes hydrogen to pass into the repressurizing beds.
• Counter-Current Depressurization: Final depressurization is counter-current and it is able to blow down impurities into the offgas stream.
• Purge: The bed is cleaned at low pressure using hydrogen rich stream that was obtained from another adsorber during the step of co-current depressurization. Impurities are subsequently removed into the offgas stream.
• Counter-Current Repressurization: To prepare the bed again for adsorption it is repressurized with hydrogen rich gas obtained from a depressurizing adsorber and pure hydrogen product.