Ind. Eng. Chem. Res. 1997, 36, 2875
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CORRESPONDENCE Comments on “Catalytic Dehydrogenation of Propane in Hydrogen Permselective Membrane Reactors” J. Gunther Cohn Consultant, P.O. Box 240 WOB, West Orange, New Jersey 07052
Sir: The paper by Collins et al. (1996) describes catalytic dehydrogenation of propane to propylene employing several membrane reactors, one of which consisted of a macroporous alumina tube with its inner surface covered by a layer of R-alumina with a pore diameter of 200 nm onto which a palladium film of approximately 12 µm had been deposited. While the palladium reactor permeated more hydrogen at 500 °C than membrane reactors coated with dense silica (pores 5 µm or less), this was no longer the case when the gas mixture was 20% hydrogen, 80% propane instead of hydrogen. At more elevated temperatures the hydrogen permeation through palladium declined by coke laydown at the palladium surface, and at 575 °C the palladium film developed cracks, making the reactor unusable. The authors had no full understanding of the mechanism of the failure. A tentative explanation was that carbon diffusion into the palladium caused the cracking at more elevated temperatures. There seems to be no literature to support such a mechanism. The failure of the palladium film could have been caused by another mechanism. In the past Engelhard Corp. has developed structures similar to those of the authors: a ceramic body having coarse pores and a densely porous thin layer on the top of the ceramic body coated by a film of palladium or palladium alloys. The objective was to produce high throughput diffusers for hydrogen production from gas mixtures (Langley et al., 1971). It was critical for obtaining durable and crack-free systems that the palladium was well bonded to the underlying ceramic and that the thermal expansions of the film and the ceramic were closely matched. The former was achieved by firing the composite at 1000 °C and the latter by incorporating into the palladium film a glaze of appropriate properties and concentrations (Langley and Myers, 1968, 1969). The palladium or palladium alloy films were 2.5-10 µm thick and delivered 35-40 cm3/cm2‚min high-purity hydrogen at 600 °C and a pressure of 2.1 kg/cm2. The authors produced their palladium films by electroless plating following the procedure of Collins and Way (1993). These films adhere to the support but were not bonded. Thus, they became delaminated by gas fluctuations or mechanically by cutting through the
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composite. However, at temperatures of 500 °C and higher bonding might occur, which upon reaching 575 °C could rupture the film due to the mismatch of thermal expansion. The inhibiting coke laydown which was formed by the presence of propene proceeds via adsorption of propene on the palladium surface. According to Gryaznov (1992), there are palladium alloys which maintain hydrogen permeability in contact with dehydrogenation products. Gryaznov and co-workers (1983, 1986) have studied a number of palladium alloys in several dehydrogenation reactions. For instance, dehydrogenation of butane to butene and some butadiene over a commercial catalyst improved by hydrogen withdrawal by palladium-20% silver alloy (Gryaznov, 1992). Another way to suppress coking could be to operate at elevated hydrogen pressure and temperature. Hydrogen could be introduced into the feed by recycling part of the hydrogen produced through a palladium diffuser as described by Cohn and Pfefferle (1971) for hydrodesulfurization of petroleum hydrocarbons. Literature Cited Cohn, J. G. E.; Pfefferle, W. C. Steam Reforming with Preliminary Hydrodesulfurization. U.S. Patent 3,595,805, 1971. Collins, J. P.; Way, J. D. Preparation and Characterization of a Composite Palladium-Ceramic Membrane. Ind. Eng. Chem. Res. 1993, 32, 3006-3013. 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, 4398-4405. Gryaznov, V. M.; Ermilova, M. M.; Morozova, L. S.; Orekhova, N. V.; Polyakova, V. P.; Roshan, N. R.; Savitsky, E. M.; Parfenova, N. I. J. Less-Common Met. 1983, 89, 529. Gryaznov, V. M. Surface Catalytic Properties and Hydrogen Diffusion in Palladium Alloy Membranes. Z. Phys. Chem. 1986, 147, 123. Gryaznov, V. M.; Platinum Metals as Components of Catalytic Membrane Systems. Platinum Met. Rev. 1992, 36, 70. Langley, R. C.; Myers, H. Elements for the Separation of Hydrogen from Other Gases. U.S. Patent 3,413,777, 1968. Langley, R. C.; Myers, H. Forming Nonporous Hydrogen-Permeable Palladium-Alloy Films on Porous Ceramic Supports. U.S. Patent 3,428,476, 1969. Langley, R. C.; Lindenthal, J. W.; Myers, H.; Straschil, H. K. Apparatus for Separating Hydrogen from a Gaseous Mixture. Ger. Offenleg. 1,953,223, 1971.
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© 1997 American Chemical Society
