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ChE laboratory PRESSURE SWING ADSORPTION IN THE UNIT OPERATIONS LABORATORY PJason C. Ganley Colorado School of Mines • Golden, CO 80401 It is interesting to note that, despite serving an important role for many decades in the chemical process industry, PSA is not a central topic discussed in most chemical engineering educational resources, which instead focus on mass transfer unit operations. To the author’s knowledge, the present com- munication represents the first reported incorporation of PSA within an undergraduate unit operations laboratory. While PSA is commonly used for hydrogen and hydrocar- bon systems, it is also a popular process for the separation of air into product streams of enriched nitrogen, or of enriched oxygen. The process of air separation by PSA is an excel- lent illustration of the principles of gas-solid mass transfer within a unit operations laboratory course—there are no harmful chemicals involved, the operational pressures are not excessively high, the effective gas concentrations are easily measured and distinguished, and the equipment lifetime is practically unlimited (provided that the feed air is properly filtered and dried). The physical and chemical phenomena utilized in a PSA system are quite simple, and PSA units are usually designed to take advantage of one of two distinct aspects. 1) Kinetic Control: In porous solids, there may be a differ- ence in the rate at which various gases may diffuse to and from regions of the solid surface. 2) Equilibrium Control: Over a range of partial pressures of species within a mixture of gases, there may be dif- fering equilibrium surface concentrations of adsorbed gases on solid materials that are exposed to the gas mixture. Therefore, the nature of the adsorbent material will strongly influence the amount of adsorption and the speed at which adsorption occurs. The selection of a particular type of ad- sorbent may allow one component of a gas mixture to be preferentially adsorbed, the nature of surface and/or pore dif- fusion effects may vary for each gas in the mixture, and so on. PSA is a cyclic operation, and generally involves separa- tion of a gas mixture by taking advantage of differences in adsorption thermodynamics or in diffusion rates that exist for its various components. Cycling between higher and lower pressures allows components of the gas mixture to be removed from (and later released to) the gas phase over designated pe- riods of time. Students in the Unit Operations Laboratory may consider the PSA cycle primarily as a mass transfer (rather than heat transfer) experiment. The packed beds used in com- mercial PSA units are designed to be isothermal over an entire cycle. That is, although there are usually large exotherms for gas adsorption (and large endotherms for desorption), the adsorption exotherms are used to provide the necessary heat for desorption of the same gases during pressure reduction or purge steps later in the cycle. Additionally, large systems usually have sufficient heat capacity (or thermal mass) so that large temperature variations do not develop within the system. In this laboratory exercise, students use a custom-built, four-column PSA system; one pair of columns contains carbon molecular sieve (CMS) adsorbent, and the other pair is filled with 13X molecular sieve (sodium alumina-silicate). Students are able to vary a wide range of experimental parameters within the system. These include, but are not limited to: • Adsorption pressure • Purge gas type, flow rate, and pressure ressure swing adsorption (PSA) is an industrial process typically used for the bulk separation of gas mixtures. An outgrowth of temperature swing adsorption, PSA is one of only a few gas-surface adsorption processes that allows for the separation of mixtures of gaseous species or vapors that exist in relatively high (non-trace) concentra- tions in respect to one another. PSA has also been coupled to distillation processes for the separation of alcohol-water vapor azeotropes.[1] Jason Ganley is an associate teaching professor of chemical engineering at the Colorado School of Mines.He earned a B.S. degree from the University of Missouri-Rolla and M.S. and Ph.D. degrees from the Uni- versity of Illinois at Urbana-Champaign,all in chemical engineering. His current research focuses on the production of alternative fuels from renewable energy. 44 Chemical Engineering Education © Copyright ChE Division of ASEE 2018PDF Image | PRESSURE SWING ADSORPTION IN THE UNIT OPERATIONS
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