Dehydrogenation of isobutane has been studied in a DD3R zeolite membrane reactor (MR) at 712 and 762 K, using pure isobutane at 101 kPa as feed gas and N2 as sweep gas. Clear advantage of using the small-pore zeolite DD3R is that it offers an absolute separation of H2 from isobutane by a molecular sieving mechanism. Experiments in a conventional packed bed reactor served as benchmark. Cr2O3 on Al2O3 was used as catalyst.
The DD3R membrane showed an excellent H2/isobutane permselectivity (>500 @ 773 K) and a reasonable H2 permeance (∼4.5 × 10−8 mol m−2 s−1 Pa). At low residence times isobutene yields 50% above the equilibrium could be obtained. At 762 K and 0.13 kgfeed kgcat−1 h−1, the isobutene yield in the membrane reactor (MR) is 0.41, where the equilibrium yield is ∼0.28. The increased performance is attributed to removal of H2 from the reaction zone by the membrane, up to 85% at the lowest space velocity. The removal of H2 mildly promotes coke formation, suppresses hydrogenolysis reactions and appears to slightly reduce the catalyst activity. The membrane permeation parameters and reaction rate constants have been estimated independently from membrane permeation and packed bed reactor (PBR) experiments, respectively. From these parameters the behaviour of the MR can be simulated well. Two important dimensionless parameters determine the MR performance primarily, the Damköhler (Da) and membrane Péclet number (Peδ). For a significant improvement of the MR performance as compared to a PBR Da ≥ 10 and Peδ ≤ 0.1. DaPeδ should be ≈1 to optimally utilize both catalyst and membrane. In the current MR unit both the hydrogen removal capacity and catalyst activity stand in the way of successful application. Using a more active catalyst and a more favourable area to volume ratio could greatly improve the MR performance. Operation at a higher feed pressure could be a possible solution. Since membranes with higher fluxes are already available, the limited catalyst activity and stability under relative low temperature and H2 lean conditions are the important limiting factors regarding application of MRs in dehydrogenation reactions.