Isobutane dehydrogenation in a DD3R zeolite membrane reactor

Abstract

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 N 2 as sweep gas. Clear advantage of using the small-pore zeolite DD3R is that it offers an absolute separation of H 2 from isobutane by a molecular sieving mechanism. Experiments in a conventional packed bed reactor served as benchmark. Cr 2O 3 on Al 2O 3 was used as catalyst. The DD3R membrane showed an excellent H 2/isobutane permselectivity (>500 @ 773 K) and a reasonable H 2 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 kg feed kg cat −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 H 2 from the reaction zone by the membrane, up to 85% at the lowest space velocity. The removal of H 2 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 ( Pe δ ). For a significant improvement of the MR performance as compared to a PBR Da ≥ 10 and Pe δ ≤ 0.1. DaPe δ 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 H 2 lean conditions are the important limiting factors regarding application of MRs in dehydrogenation reactions.