(288k) A Mechanistic View of Pressure Swing Adsorption Processes

Separation of gas mixtures by pressure swing adsorption (PSA) is gaining commercial acceptance as an alternative to other methods. Some of the PSA processes bear similarities to absorption and the others to distillation. Recognition of these similarities would help in designing processes to achieve high purities. In this paper, we show how to enhance the performance of the Skarstrom cycle and its variants on one hand and the Duplex cycle and its variants on the other, to obtain sharp separation of binary mixtures. Where only one of the components is adsorbed, the phase diagram is similar to the phase diagram of the gas-liquid equilibria (absorption) and the selectivity is infinite. It is possible to carry out the simplified analysis of the process using the McCabe-Thiele diagram. On the other hand, when both components are adsorbed, the phase diagram is similar to phase diagrams of the vapor-liquid equilibria and the selectivity is finite. It is possible to analyze the process using the diagram similar the Ponchon-Savarit diagram and on a McCabe-Thiele diagram with appropriate assumptions. The former does not require reflux, whereas the latter requires both raffinate and extract refluxes to achieve clean separation. The PSA cycles may be broadly classified as the Skarstrom cycle and its variants, and Duplex PSA cycle and its variants. The Duplex PSA proposed by Leavitt (1992), Dual-reflux PSA by Hirose and his coworkers (1995), Molecular-gate PSA by Keller and Kuo (1982), or the Piston PSA (2002) falls under the latter category. An absorption-like PSA process with Skarstrom cycle requires a minimum of three beds; if the adsorption, rinse and regeneration, and pressurization steps are carried out simultaneously. Here, we can get sharp separation if we ?rinse' the bed after adsorption with the adsorbable component. In contrast, the distillation-like process requires four beds for stripping, enriching, regeneration and presaturation. The presaturation of the bed before the stripping provides the ?raffinate' reflux and the reflux of the strongly absorbable component is the ?extract' reflux to the enriching bed. The distillation-like process can be operated between total and minimum reflux as in distillation. However, in the Duplex cycle both absorption-like and distillation-like processes can be carried out in two beds to get clean separation. The mechanism of separation appears to have no parallel in separation processes, but it is similar to the exchange of heat in regenerators employed for incineration of volatile organic compounds. We present a mechanistic view of the separation with the aid of diagrams and examples to show how clean separation can be achieved without a waste stream unlike in conventional PSA.

References: 1. F.W. Leavitt, US Patent 5,085,674 (1992) 2. D.Diagne, M. Goto and T. Hirose, Ind. Eng. Chem. Res., 34 (1995), 3083. 3. G.E. Keller II and C.H.A. Kuo, US Patent 4,354,859 (1982) 4. R. Arvind, S. Farooq and D.M. Ruthven, Chem. Engg. Sci., 57 (2002), 419.