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The water gas shift (WGS) reaction is a key operation in the production of hydrogen from fossil fuels, biomass, and organic wastes through high temperature gasification processes. WGS reaction in hydrogen permselective membrane reactors has the potential to achieve both near-completion CO conversion (χco) and H2 separation in a single unit operation. In the literature, research and development efforts have been concentrated on membranes with high H2-selectivity, mainly the Pd-alloy and molecular sieve amorphous silica membranes. However, these membranes have serious issues of material instability in the conditions of syngas WGS reactions.This dissertation investigates high temperature WGS reaction in MFI-type zeolite membrane reactors (MR). The research includes extensive experimental and simulation studies on WGS membrane reaction in a temperature range of 400 – 550°C and a reaction pressure range of 2 – 6 atm. The goal of this research is to understand the effects of membrane properties, i.e. selectivity and permeance, and WGS MR operation conditions on the χco in the MR and evaluate the feasibility of the zeolite MR for achieving nearly complete χco at high temperatures. Macroporous α-alumina disc supported MFI zeolite membranes have been synthesized and modified by a novel on-stream catalytic cracking deposition (CCD) method previously developed in our laboratory to reduce the effective zeolitic pore size for improved H2 selectivity without large decrease in permeance. Modified MFI-type zeolite disc membranes were prepared and tested for high temperature WGS reaction using a cerium-doped ferrite (Fe/Ce) catalyst. It was found that increasing reaction pressure in the MR benefits the enhancement of the χco and dramatically reduce the undesirable side reactions of methanation. Both the enhancement of the χco and the inhibition of methanation in the MR are attributed to the more effective removal of H2 from the catalyst bed at higher pressures. The modified MFI-type zeolite membrane showed long term (tested for >4000 h) stability in high temperature WGS conditions, which is so far the best compared to other membranes in literature reports. It is clear that the improvement of the WGS MR depends not only on the development of better membrane and catalyst but also on the optimization of the operation conditions. A one-dimension PFR model has been established for simulation of the WGS reaction performance in the zeolite disc MR. The model gives good results in quantitatively describing the WGS MR performance and predicting the relationships between the χco and membrane properties as well as the performance dependences on operation conditions. The simulation also demonstrates the feasibility of using the modified MFI-type zeolite MRs to achieve χco above 99.5% under realistic operation conditions.Results of this dissertation research prove that the modified siliceous MFI zeolite MR can achieve nearly complete CO conversion and dramatically reduced methanation by single unit operation. The zeolite MR must be operated under conditions of high temperature and pressure where fast react rate is achieved in the reactor to prevent excessive permeation of unreacted CO. The optimal operation conditions for WGS in a zeolite MR, and porous membranes in general, depend on the membrane quality. |
