Multicomponent pressure swing adsorption
Abstract
A new chromatographic pressure swing adsorption (PSA) process is developed to deliver multiple product streams. For ternary system, this process employs two pairs of columns with different amounts of absorbent. Six schemes are developed depending on the blowdown policy. This process can provide higher adsorbent productivity than the two modified schemes of the process developed by Nataraj and Wankat (1982), who used three columns. No one scheme is clearly superior; the advantages and disadvantages of each scheme are delineated. For the experimental study of chromatographic PSA, a fully automatic system for continuous production is designed. The feed is introduced to each column approximately 1/3 of the cycle. The operation of the system consisting of three columns is optimized by changing flow rates during each portion of the cycle for fractionation of methane and ethane in nitrogen or hydrogen. The trade-off between the purities for methane-rich and ethane-rich products is found. Increasing the feed pressure and repressurizing with high-pressure product instead of feed increase the product purities. Separation for the methane-ethane-hydrogen system is excellent since, unlike nitrogen, hydrogen does not adsorb. A new PSA process is developed to deliver a highly enriched product of strongly adsorbed component. This process employs an inert column between two adsorbent columns for compression and recycling of the product stream of the strongly adsorbed component by the product stream of the weakly adsorbed component. The process is analyzed for a dilute system with linear isotherms and no mass transfer resistances. The local equilibrium theory is extended to analyze theoretically a combined cocurrent-countercurrent blowdown cycle and binary PSA with coupled Langmuir isotherms. The combination of the two blowdown methods is most effective in cases where the mole fraction y$\sb{\rm f}$ of the strong adsorbate in the feed gas is lower than about 0.8 and the separation factor $\beta$ is larger than 0.01 (poor separation). The blowdown levels between the two blowdown steps are optimized at various feed conditions. The linear-driving-force model leads to approximately the same optimum pressure between the two blowdown steps as the local equilibrium model. An analytical expression for recovery in a binary gas separation with coupled Langmuir isotherms is obtained. There is a pressure ratio which maximizes the recovery for nonlinear systems. Since the nonlinearity as well as the adsorption capacity depends on temperature, an optimum operating temperature exists for concentrated feeds. The intensification of existing PSA processes is explored. The adsorbent productivity can be increased while other separation quantities are kept the same or better by appropriate scaling. The result is a rapid cycle with a short, fat column packed with small diameter particles.
Degree
Ph.D.
Advisors
Wankat, Purdue University.
Subject Area
Chemical engineering
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