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Ind. Eng. Chem. Res. 1997, 36, 2876
Rebuttal to the Comments of J. Gunther Cohn on “Catalytic Dehydrogenation of Propane in Hydrogen Permselective Membrane Reactors” John P. Collins Advanced Materials Laboratory, Sandia National Laboratories, 1001 University Boulevard SE, Albuquerque, New Mexico 87106
Sir: Mr. Cohn has proposed that the thermal expansion difference between the palladium film and porous alumina support is a potential explanation for the membrane failure that was observed when our supported palladium thin film membranes were tested under propane dehydrogenation conditions. He has proposed that greater adhesion between the palladium and alumina support may have occurred during heating, which upon reaching 575 °C could have ruptured the film due to the mismatch of thermal expansion. He also argues for the use of palladium alloys instead of pure palladium as well as addition of hydrogen to the reactor feed gas to suppress coking and the resulting deactivation of the membrane. The results of our experimental studies and previously published literature do not support the arguments regarding the failure of the supported palladium membranes due to thermal expansion incompatibilities. The supported palladium membranes failed rapidly in only a few hours when exposed to propane dehydrogenation reaction conditions. However, no such failure was observed when similar membranes were successfully tested at temperatures from 450 to 600 °C for periods of 1-2 weeks in membrane characterization tests conducted with hydrogen, nitrogen, and helium and ammonia decomposition tests in a similar packed-bed membrane reactor (Collins and Way, 1993, 1994). Additional stability tests conducted in our laboratories indicate that simply heating the membranes from 500 to 575 °C under a hydrogen or inert atmosphere does not result in membrane failure or a significant reduction in the hydrogen permselectivity. Therefore, we conclude that the observed membrane failure is not due to a thermal effect as suggested but instead to problems associated with the chemical stability of pure palladium under propane dehydrogenation conditions. We do agree, however, that the thermal expansion difference between palladium and alumina may present a problem for the long-term durability of these membranes in applications where thermal cycling occurs frequently.
S0888-5885(97)00834-8 CCC: $14.00
Comments were also made regarding the use of palladium alloys which have better resistance to deactivation under dehydrogenation conditions than pure palladium. It should be noted, however, that the degree to which membrane deactivation occurs depends not only on the membrane composition but also on the reaction products and operating temperature. The cited results obtained for butane dehydrogenation in a palladium-silver membrane were at an operating temperature of 400 °C (Gryasnov, 1992) which is well below the desired operating temperature range of 500-600 °C for propane dehydrogenation. Finally, Mr. Cohn proposed the use of elevated hydrogen pressures to suppress coking in the palladium membrane by recycling part of the hydrogen recovered from the permeate stream. However, this operating configuration may not be competitive with the configuration where pure propane is fed to the reactor. In our study (Collins et al., 1996), we demonstrated that microporous silica-based membranes can be successfully operated at practical space velocities under conditions where pure propane is fed to the reactor (i.e., no hydrogen recycle). To successfully compete with other potential membrane candidates for this application, palladium-based membranes should be able to operate under similar conditions. Literature Cited Collins, J. P.; Way, J. D. Preparation and characterization of a composite palladium-ceramic membrane. Ind. Eng. Chem. Res. 1993, 32, 3006. Collins, J. P.; Way, J. D. Catalytic decomposition of ammonia in a membrane reactor. J. Membr. Sci. 1994, 96, 259. Collins, J. P.; Schwartz, R. W.; Sehgal, R.; Ward, T. L.; Brinker, C. J.; Hagen, G. P.; Udovich, C. A. Catalytic dehydrogenation of propane in hydrogen permselective membrane reactors. Ind. Eng. Chem. Res. 1996, 35, 4398. Gryaznov, V. M. Platinum metals as components of catalytic membrane systems. Platinum Met. Rev. 1992, 36, 70.
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© 1997 American Chemical Society
