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retained due to its bigger size. In this context, the membrane exhibited outstanding H2/i-butane ideal selectivity (based on the single gas permeation fluxes) of over 500 at 773 K. However, slightly lower mixture selectivity was recorded since the driving force for permeation of H2 in the mixture was reduced by the increased partial pressure of H2 in the permeate and its decreased partial pressure in the feed. About 50% increase in yield compared to the equilibrium value was recorded mainly due to the effective removal of H2 from the reaction zone. On the other hand, the studies provided evidence of a minor decrease in the catalyst activity compared to the conventional reactor. Moreover, a slightly increased coke formation, however with selectivity towards coke being still low than 5% was observed attributed by the authors to the lower H2 partial pressure at the reaction side. Despite the fact that the catalyst activity and the H2 removal hold a good balance, the authors concluded that both parameters limit to some extent the overall performance. Such being the case, further improvements in the catalyst activity and stability as well as in the permeation fluxes of the membrane are necessitated for successful application in dehydrogenation reactions at industrial scale. The performance of large-pore FAU type zeolite membranes prepared on porous α-Al2O3 support tubes in the catalytic dehydrogenation of cyclohexane conducted in membrane reactors was evaluated experimentally [148] and theoretically [149] by the group of Kusakabe. The membranes were able to simultaneously remove hydrogen and benzene from the reaction zone filled with a Pt/Al2O3 catalyst. Moreover, higher sweep or lower feed flow rates affected positively the cyclohexane conversion. The authors evaluated mathematically the trade-off effect of membrane permeance and separation factor on the conversion and concluded that in terms of industrial practice high membrane permeability accompanied with reasonable selectivity might be the more favorable option than a high selectivity at the expense of low permeability. Furthermore, it was highlighted that the H2 addition to the cyclohexane feed compensates the reduced H2 partial pressure on the reaction side, caused by its continuous extraction, hence preventing the catalyst from coking [149]. Defect-free silicalite-1 zeolite membranes were used for the catalytic dehydrogenation of ethylbenzene to styrene in membrane reactors packed with Fe2O3 exposing their advantages (74.8% at 610 ◦C) over a conventional reactor (67.5% at the same temperature) in regard to conversion due to the instant extraction of the produced H2 across the membrane [150]. However, the benefit of the membrane reactor diminished with increasing space velocities ranging from 0.5 to 1.5 and approached the performance of the conventional reactor configuration. On the contrary, rising the ratios of sweep gas to reactant feed from 0.5 to 2 contributed to increase in the ethylbenzene conversion (from 70.5% to 74.8%) since higher ratios induce generally larger driving force for permeation resulting in higher H2 permeation rates. Further rise in the sweep/feed ratio from 2 to 6 resulted in a nearly 25PDF Image | Zeolite Catalysis
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