(301d) Comparative Study of MFI and SAPO-34 Zeolite Membrane Reactors for Propane Dehydrogenation
Propane dehydrogenation (PDH) reaction is an attractive process to meet the increasing demand of propylene and polypropylene. PDH is an equilibrium-limited endothermic reaction, and the use of permselective membrane reactors (MRs) is thought to be advantageous for intensifying the process. Hydrogen-permselective zeolitic membranes are potentially useful for PDH MRs. Compared to Pd-alloy dense membranes, zeolite membranes have high thermal stability and chemical resistance. The main focus of this work is to compare and understand the effects of zeolite membranes with differing H2 permeation characteristics, on the MR performance in PDH reactions. In particular, we discuss in detail the effect of PDH reaction enhancement using MRs constructed with MFI (medium-pore) and SAPO-34 (small-pore) zeolitic membranes.
MFI and SAPO-34 zeolite membranes were synthesized on the inner surface of porous α-alumina tubes. Binary permeation data showed that SAPO-34 membranes exhibited much higher H2/C3H8 selectivity = 20 but slightly lower H2 permeance = 1.4×10−7 mol m-2 s-1 Pa-1 at 650°C, compared to the MFI zeolite membrane (H2/C3H8 selectivity = 7.2 and H2 permeance = 1.7×10−7 mol m-2 s-1 Pa-1). The MFI and SAPO-34 membrane tubes were packed with a Na2O doped Cr2O3/Al2O3 catalyst. The operation parameters studied in this work include reaction temperature, space velocity, sweep flow rate, sweep flow direction, H2/HC ratio and H2O/HC ratio. At 650oC, both zeolite MRs effectively exceeded the equilibrium conversion. In comparison to MFI MR, the propane conversion and propylene selectivity in SAPO-34 MR were significantly higher, due to its high H2 selectivity which allows for smaller permeation of HCs during the reaction. It is found that the enhancement of the propane conversion in MR depends strongly upon the H2 permeation characteristics of the membrane. We then discuss the optimal operation of the PDH MR system. Rapid catalyst deactivation and regeneration are important considerations for commercial PDH processes. To avoid the rapid activity decrease, hydrogen was added as diluent to the propane feed. This provided significantly more stable catalyst activity. PDH was also carried out in the presence of H2O, and its effects on reducing coke formation and catalyst deactivation are described. Furthermore, the MRs were operated in the counter-current and co-current sweep modes. It is observed that the counter-current mode leads to higher propane conversion but lower propylene selectivity. This finding is discussed in terms of competing reaction and transport effects.