Znd. Eng.Chem. Res. 1994,33,2881
2881
Response to Comments on “Concentrationand Recovery of COa from Flue Gas by Pressure Swing Adsorption” E. S. Kikkinides and R.T.Yang’ Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
S. H. Cho Korea Znstitute of Energy Research, Taedok Science Town, Taejon, Korea
Sir: We are indeed gratified t o see that this reader has conducted experiments and that his results are in good agreement with our original theoretical predictions. We also appreciate his interest in investigating the problem further and his conclusions are, in fact, not a t all in disagreement with our original, simple and intuitive conclusions. For those who are interested in the problem, we offer further clarifications as follows. 1. The Cod02 kinetic competition is indeed an important factor in determining the C 0 2 product purity (during desorption). However, for a pressure swing adsorption (PSA) cycle with a long cycle time, the kinetic competition is no longer important, and this was the meaning of our statement that activated carbon (AC) gives better performance due to the lack of diffusional resistance. In other words, when using the carbon molecular sieve (CMS) sorbent, short cycle time will favor kinetic separation whereas long cycle time will favor equilibrium separation. The system under investigation contained three gases with favorable equilibrium selectivities for separation but with unfavorable kinetic selectivities. Thus a trade-off needs to be made
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in selecting the sorbent and the operating conditions. 2. The second point is concerned with whether C o d N2 separation is improved by using a large-pore sorbent (AC) over a narrow-pore sorbent (CMS). “his point was not addressed in our investigation but is discussed by the preceding communication. In the preceding communication, the author takes selected results from the work of Richter et al. (1984, cited in the preceding communication)for a dicferent cycle and speculates that better CO2 product concentrations and recoveries can be obtained by using a narrow-pore CMS over a widepore CMS. This speculation is based on a different PSA cycle with the additional cocurrent CO2 purge step, which is run a t nearly equilibrium conditions (i.e., by allowing a long time). The conclusion one should reach here is that for the C O D 2 separation by PSA when a C02 cocurrent purge step is used, this step should be done in a way such that the diffusional resistance is minimized. In our original work only equal time steps were used; it was obvious that we obtained superior performance when all steps were operated under equilibrium conditions.
0 1994 American Chemical Society
