(732c) Modeling and Analysis of Zeolite Membrane Reactor for Water Gas Shift Reaction for Hydrogen Production

Water gas shift reaction (WGS) is an important step in hydrogen production from fossil fuels. Due to its exothermic nature, it is known that the reaction tends to shift to the left side at high temperatures, which limits the conversion of the reaction. Membrane reactors, which integrate reaction and separation into the same unit, have the ability to improve the conversion and overcome the thermodynamic limit due to the ongoing selective removal of the product during reaction. For the commercial application of the high temperature (>400 oC) WGS process, the membrane reactor needs to have sulfur tolerance and hydrothermal stability. Crystalline microporous zeolite membranes, especially the siliceous MFI-type zeolite membranes, can meet these requirements [1]. The primary mass transport channels in the MFI-type zeolites have an effective diameter of 0.56 nm, which offer high selectivity by size discrimination for critically sized molecules. The latest results reported by Tang et al. [2] showed that the H2 selectivity was dramatically enhanced at the cost of a moderate decrease in the H2 permeance of the ?Ñ-alumina supported siliceous MFI-type zeolite membranes by selective depositing molecular silica in the zeolitic channels via in-situ catalytic cracking deposition (CCD) of silane precursor. Additionally, the modified membrane exhibited good stability in the presence of water vapor at temperatures >400 oC because of the chemical bonds between the silicious molecular deposits and the zeolite surface [2]. These promising experimental results show that the modified MFI-type membranes offer a great potential to construct a membrane reactor for WGS. In this work, the potential and the features of the WGS MFI-type membrane reactor will be analyzed by a modeling method.

An isothermal steady-state model has been developed for simulating WGS in an isothermal tube-shell MFI-type zeolite membrane reactor. The model considers the permeation properties of the MFI membrane measured in the literature [2] as a reference, and the kinetic equation developed for the Fe-Cr-O based catalyst [3] for WGS reaction. The effect of the membrane reactor design and operational parameters, such as pressure, temperature, inlet space rate, steam to CO ratio (S/C), and the relative values of membrane permselectivity and permeation flux, on the performance of a WGS membrane reactor has been studied. The performance was evaluated based on the CO conversion and H2 recovery.

The results show that the improvements in CO conversion and H2 recovery can be achieved with the reference permeation properties. For a space velocity of 10,000 h-1, with a steam to CO ratio of 1 and pressure difference between the reaction side and the sweep side of 30/1 atm, a CO conversion of 92% and H2 recovery of 97% can be obtained at 500oC. A pressure increase allows for the attainment of a higher conversion at the same space time. The increase of S/C has a negative effect on both the reaction rate and the gas fluxes due to its dilution effect when operating under kinetic limited conversion conditions with a constant CO inlet flow rate. When operating under equilibrium limited conversion conditions, high S/C values allow a better performance to be obtained in terms of reaction but worse preformance in terms of separation when performed at a low pressure (1.5/1 atm). When operating at a high pressure (30/1 atm), there is no obvious advantages of a high S/C ratio even under equilibrium limited conditions.

Keywords: Modeling and simulation; WGS; Zeolite membrane reactor; Hydrogen production

Reference [1] J. Dong, Y.S. Lin, M. Kanezashi, Z. Tang, J. Appl. Phys. 104 (2008) 121301. [2] Z. Tang, T.M. Nenoff, J. Dong, Langmuir, 25 (2009) 4848. [3] W.F. Podolski, Y.G. Kins, Ind. Eng. Chem. Process Des. Dev. 13 (1974) 414.