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Chapter 6. Design and Environmental Impact Analysis of a Hybrid PSA-Membrane Separation System reforming of natural gas, where PSA is usually employed in the final step to produce high-purity hydrogen. In the context of PSA processes, attaining high- est possible product recovery values without sacrificing product purity is highly sought after, as it directly improves the operating costs. During the blowdown and purge steps of the PSA cyclic operation, a fair amount of hydrogen can escape the system along with the impurity components as the off gas. For example, a typical PSA waste gas stream consists of CO2 ∼ 55%, H2 ∼ 35%, CH4 & CO ∼ 15%. This stream is not usually recycled since it would need to be re-compressed to the PSA feed pressure and is also considerably richer in impurities. Further- more, it also cannot be used for CO2 sequestration since it contains significant amounts of H2 and CH4. On the other hand, it is important to note that if CO2 emissions are captured as a relatively pure gas, they can further be sequestered or stored appropriately, thus reducing at the source,its amount emitted to the atmosphere. One common procedure to improve product recovery in PSA is to employ more depressurization steps, enabling the use of high pressure product to pressurize columns operating at lower pressures, thereby minimizing the need of external product stream. However, the maximum number of pressure equalization steps is strongly dependent on the total number of beds, limiting the scope of this method to large scale improvements. Another option is to integrate the PSA system with other separation devices such as membranes to reduce the hydrogen losses. A hybrid PSA-membrane process is expected to combine the high throughput and H2 product purity of a PSA process with the lower operating costs of a membrane process. In the past, only few studies have been reported in the literature towards this field. Sircar et al. [149] presented a PSA-membrane hybrid process strat- egy to achieve higher hydrogen recovery from the PSA purification section of 140PDF Image | Operation and Control of Pressure Swing Adsorption Systems
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