Abstract

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The CO₂ and H₂ gas mixture is obtained from various sources such as refinery off gas, and during gasification of petroleum coke and coal. Efficient separation of CO₂ is required to use this gas mixture as fuel for energy production. Various methods are available to remove CO₂ from the mixture for example cryogenic distillation, membrane separation, absorption and adsorptive separation. Pressure Swing Adsorption(PSA), an adsorptive separation method, is one of the promising methods for CO₂ removal due to variety of adsorbent material available and various combinations of PSA cycles possible to fit the process requirement. Most of the PSA cycles proposed in the literature are the variants of the Skarstrom cycles involving four basic steps namely, pressurization, adsorption, blowdown and purge. Based on the nature of adsorption equilibria the adsorptive separation of binary gas mixture has been classified as distillation-like and absorption-like processes. In the distillation like process: both components show competitive adsorption in a binary gas mixture and the separation factor is moderate, whereas in the absorption like process one component preferentially adsorbs over the other and the separation factor is very high (∞). Separation of CO₂/H₂ using activated carbon is an example of absorption-like process. In this work simulation studies have been carried out to compare the performance of a 4-bed PSA cycle having steps: stripping, enriching, blowdown and pressurization with a 3-bed PSA cycle having steps: stripping, depressurization, blowdown and pressurization, for the separation of a binary mixture of CO₂/H₂, representing a typical refinery off gas (after treatment). The salient result shows that a moderately high productivity can be achieved with 3-bed PSA compared to the 4-bed PSA for a target purity of 97 mol% of H₂, but, the former requires a deep vacuum to facilitate this high productivity using activated carbon as a solid adsorbent.

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