Page 187 - Pressure Swing Adsorption

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162 PRESSURE SWING ADSORPTION EQJJJLIBRJUM THEORY 163
on sern1emp1ncai parameters that can only be determined from data. Such References
methods can lead to excellent designs when kinetics are fast. Even when
kinetics are slow (though not controlling), such methods can predict overall I I. l>. H. Tumock and R. H. Kadlec, "Separnuon of Nitrogen and Methane via Pem)dic
performance (e.g., m terms of recovery and byproduct enrichment) very well. Adsorption," A!ChE J. 17,335 0971).
The main and oerhaos only drawback 1s that. when kinetic constraints are 2. L. H. Shendalman and J. E. Mitchell, ''A Studv of Heatless Adsorptton in the Model
imoortant, 1t becomes impossible to estimate product purity reliably. I Svstem CO He, I.," Chem, Eng. Sci. 21. 1449-58 (1972).
Nevertheless, a vanety of aspects of PSA operation can be taken mto
account by equilibrium-based theories. Some that are illustrated in this 3. Y. N. I. Chan, F. B. Hill, and Y. W. Wong, "Equilibrium Theorv of a Pressure Swing
Adsorpuon Process," Chem. Eng. Sci. 36. 243-51 (1981).
chapter are: a variety of cycle and step options, wide ranges of operating
conditions, isotherm nonlineanty, heat effects. and deadzones m PSA 4. J.C. Kavser and K. S. Knaebel, "Pressure Swing Adsorption: Development of an Equdih-
rium Theory for Binary Gas Mix1ures with Nonlinear Isotherms,"' Chem. Eng. Ser. 44, i -H
columns. In several instances, the simple theories have been verified experi-
(1989).
mentally, so there is little doubt as to their reliability when the assumotions
are reasonably valid. 5. D. J. Bait, M.S.Ch.E. Thesis, Ohio State Urnversny, Columbus. OH, ! 986.
Chilton once said, "The simpler things become m a 01ece of research or 6. C. W. Skarstrom, "Use of Phenomena m Automatic Plant Type Gas Analvzers," Ann. N.Y.
devekmment, the closer one has come to the truth." Pigford added to that, Acad. Sci. 72, 751-63 (1959).
"The simpler an exolanation IS, the more widely it will be understood,
7. M. J. Matz and K. S. Knaebel, "Pressure Swing Adsorptmn: Effects of Incomplete Purge."'
appreciated, and used. " 42 In pressure swing adsorption ;systems, it 1s impossi- A/Ch£ J. 34(9), 1486-92 (1988).
ble to achieve greater simplicity than 1ocai eouilibrium models provide and
8. P. C. Wankat, "Feed-Purge Cycles m Pressure Swmg Adsorpt1bn," Separ. Sci. and Tech.
still retam fundamental understanding of the process. Whether the model (submitted I 992).
ore<lict1ons are close to the truth or not depends on the extent to which the
9. I. Rousar and P. Ditl, "Pressure Swing Adsorption: AnaiytiCal Soluuon for Op11mum
assumptions arc valid. For all lhat, tt 1s seldom possible to improve perfor-
Purge," Chem. Enx. Sci. (suhm11ted 1992).
mance beyond the caoability predicted by an eauilibnum model because
dissipative effects nearly always diminish performance. Hence, striving to 10. J.C. Kayser and K. S .. Knaebei, "Integrated Steps m Pressure Swing Adsorpt10n Cvc!es,"
Chem, Eng. Sci. 44, 3015-22 (1988).
confonn to those assumptions can be worthwhile, not oniy because 1t will be
possible to predict performance accurately and simply, but, more important, 11. S.-S. Suh and P. C. Wankat, "Combined Cocurrent-Countercurrem Blowdown Cvcle in
Pressure Swmg Adsorption," A/Ch£ J. 35, 523-26 (1989).
because performance will be superior. Whether modeling via the local
eauilibrium approach can be "understood, appreciated, and used" depends 12. J. E. Collins and K. S. Knaebel, Paper presemed ar the AIChE Annual Meetmg. San
mamiy on whether the implied superior PSA performance can be achieved in Francisco, CA (1989).
real applications. 13. K. S. Knaebel and F. B. Hill, "Pressure Swrng Adsorption: Development ot an Equilibrium
Future efforts should be directed towards a unified treatment of eauilib- Theory for Gas Separations,'.' Chem. Eng. Sci. 40, 2351-60 (1985).
rium based separat10ns accounting for the several diverse factors that until 14. J. L. Wagner, "Sc:lect1ve Adsorptron Process," U.S. Patent No. 3,430,418 (1969).
now have been accounted for separateiy. An examole would be to account
15. P. C. Wankat, Large Scale Adsorptmn and Chromatography, I, CRC Press. Boca Raton, FL.,
for the effCds ·of·nonline'ar isotherms on seauential pressurization by feed
95 (/986).
and feed steps. Another facet to examme 1s the coupling of isotherms, since 1t
1s widely observed that the light comoonent adsorbs prooortmnately iess in a 16. R. T. Yang and S. J. Doong, "Gas Separation bv Pressure Swmg Adsorption: A Pore
Diffusion Model for Bulk Separation," A/Ch£ J. 31, 1829-42 (1985).
mixture than the amount due to uptake of the heavy component. This effect
would tend to improve PSA performance, and might partially compensate for 17. S. J. Doong and R. T. Yang, "Bulk Separation of Multicon·1ponent Gas Mixmres bv
the effects of being nearly adiabatic (which is ohysically realistic), as opposed Pressure Swmg Adsorption: Pore/Surface Diffusion and Equilibrium Models,'' A/Ch£ J.
32, 397-410 (1986).
to the assumed isothermal condition of the adsorbent bect. Furthermore,
experimental work could be done to validate the combined effects, to analyze 18. R. T. Yang, Gas Separatwn by Adsorption Processes, Butterwonhs, Boston, MA. 328 (I 987).
cycles of more complex steps, and to account for a wider variety of properties -f 19. N. F._ Kirkby and C. N. Kenney, "The Roie of Process Steps m Pressure Swing Adsorption,"
and conditions. One example would be to generalize the dependence of the Fimdam. of ,.4dsorptwn, Engng. Foundation; New York, 325 (1987).
observed "effective" separatmn capacity of an adsorbent, especially with
20, J. C. Kayser and K. S. Knaebel, "Pressure Swing /\dsorphon: Exper1ment<1l Study ot an
respect to fiow conditions, as alluded to in Figure 4.3. Equilibrium Theory," Chem. Hnf!,. Sci. 41. 2931-38 (1986).