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P. BISWAS et al., Modeling and Simulation for Pressure Swing Adsorption System ..., Chem. Biochem. Eng. Q. 24 (4) 409–414 (2010) 409 Modeling and Simulation for Pressure Swing Adsorption System for Hydrogen Purification P. Biswas, S. Agrawal, and S. Sinha* Original scientific paper Department of Chemical Engineering, Received: November 20, 2009 Indian Institute of Technology Roorkee, India Accepted: October 28, 2010 Introduction The PSA process for hydrogen purification uti- lizes different loading capacities of adsorbent at dif- ferent pressures to separate a gas mixture into its components. The loading capacity of an adsorbent is the amount of gas that can be bound by mass unit of the adsorbent material. The extent to which an adsorbent can be loaded increases in general with the partial pressure of the gas component. When a gas mixture containing hydrogen enters the adsorbent bed at high pressure, most components are moved by the adsorption force to the adsorbent surface and bound to the adsorbent. The adsorbed gas components concentrate on the adsorbent surface while the lighter components, such as hydrogen, are not bound tightly to adsorbent materials, and pass the adsorbent bed without significant interaction. In a dynamic adsorption system, the better adsorbed components move slower through the adsorbent bed than the less adsorbed. By this, the concentration of impurities in processed gas is reduced while flowing through the adsorbent bed, which results in the de- sired separation of stream components. Commercially available adsorbents are granu- lates, beads or extrudates. Common to all adsorbent *Corresponding author. E-mail address: sshishir@gmail.com Tel.: +91-1332-285648; Fax: +91-1332-276535, 273560 materials is porous structure resulting in large sur- face. Through these pores, the molecules move to the internal surface, where they are adsorbed. Any adsorbent material has specific binding capabilities for different gas components. Therefore, adsorber beds are designed with up to four layers of different adsorbent materials to separate hydrogen from the mixture of various gases. Previous studies The original idea of using cyclic pressure vari- ation in adsorptive separation or purification of gases was proposed around 40 years ago. The growth in the research and development of PSA technology has been phenomenal. The first U.S. patent on the subject, authored by C. W. Skarstrom, was granted in 1960. During 1980–2000, more than 600 patents were issued in the U.S. alone, while during 1970–2000 the number of published papers relating to PSA was more than 800. The pressure swing adsorption system (PSA)1–4 in its basic form consisted of two beds, which were alternately pres- surized and depressurized according to a pro- grammed sequence. A number of modifications have subsequently been developed based on the Skarstrom cycle, vastly enlarging the area of practi- cal applications of the process. Pressure swing adsorption (PSA) is a very versatile technology for separation and purification of gas mixtures. Its key industrial application is the production of high purity (99.999 + %) hydrogen. For this, it utilizes different loading capacities of adsorbent at different pressures. Despite tremendous growth in practical applications of this technology, the design of PSA system is more of an experimental basis. The realistic model of a PSA cycle con- sists of flow rate which varies due to significant adsorption, while the heat effect needs also to be considered due to adiabatic thermal conditions. In this paper, an effort has been made to model and simulate a PSA cycle for hydrogen purification. For this pur- pose, a system consisting of 4 beds and 8 stages was considered using beds of activated carbon and zeolite. The feed stream was taken as a mixture of CO2, H2, CH4, CO, and N2, which is typ- ical for that used in hydrogen purification. The Newton-based approach was used in solving the model by discretization in space coordinate and time coordinate. The results were then compared for the columns having activated carbon and zeolite packing. Key words: Pressure swing adsorption, layered bed, mathematical modeling, adsorption, purificationPDF Image | Modeling and Simulation for Pressure Swing Adsorption System
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