Material Properties and Operating Configurations of Membrane Reactors for Propane

Dehydrogenation

Seung-Won Choi, Christopher W. Jones, David S. Sholl, and Sankar Nair

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100

Jason S. Moore, Yujun Liu, Ravindra S. Dixit, and John G. Pendergast

Engineering & Process Sciences, The Dow Chemical Company, Freeport, TX 77541

Abstract

We consider the problem of defining physically realistic and technologically interesting ranges for catalytic and transport properties, and operating configurations, of packed-bed membrane reactors (PBMRs) for propane dehydrogenation (PDH). This work employs an isothermal PBMR simulation with simplified kinetics and transport terms to investigate the influence of a broad range of reaction and transport parameters on the PBMR performance, and thereby determine operating windows for such reactors when constructed with industrial PDH catalysts and nanoporous (e.g., zeolitic) membranes. First, dimensionless analysis of the simulation results is carried out using Damköhler (Da) and Péclet (Pe) numbers to represent the material catalytic properties, transport properties, and the reactant throughput. We also consider the effects of operating parameters such as different sweep flow rates and countercurrent versus cocurrent modes. An increased Da and a decreased Pe are useful for enhanced conversion in the PBMR. However, the conversion reaches a maximum and then decreases with further decrease in Pe if the membrane allows permeation of the propane reactant in addition to hydrogen, due to reactant depletion along the length of the PBMR. This is significant because of the consideration of nanoporous zeolite membranes as a leading candidate for PDH PBMRs. A highly hydrogen-selective, small-pore zeolite membrane (e.g., SAPO-34 or DDR) will show better performance over a wide range of weight-hourly space velocity (WHSV) conditions than a higher-flux, lower-selectivity medium-pore zeolite membrane (e.g., MFI) or a Knudsen-selective high-flux mesoporous membrane. H2-selective membranes show a plateau region of conversion at low Pe. This limit can be overcome by using a larger sweep flow rate (which enhances the driving force for membrane permeation) and countercurrent operation. However, due to a complex trade-off between the reaction kinetics and forward/back- permeation across the membrane, the countercurrent mode can be recommended only in a limited (but nevertheless realistic) window of material properties. The preference for small-pore H2-selective zeolitic membranes requires the fabrication of thin (~1 m or thinner) membranes with low defect densities in order to achieve an adequately high transport flux. Low-selectivity, high-flux membranes can only provide moderate enhancement of PBMR performance at low WHSV (< 1 hr-1). The above analysis is useful in quickly identifying desired material properties For example, in order to enhance the PDH conversion from 48 % (PBR) to 52 % (PBMR) at a WHSV of 1 hr-1 using Pt-Sn catalyst, the required dimensionless numbers in the operating window are Pe ~ 1.4 and Da ~ 28, which can be realized with a membrane surface area in the range of 4.5�10-3-4.5�10-4 m2/g catalyst and a small-pore zeolite
membrane with H2 permeance in the range of 10-8-10-7 mol.m-2.s-1. Pa-1.