Silica Membranes for Hydrogen Fuel Production by Membrane Water Gas Shift Reaction and Development of a Mathematical Model for a Membrane Reactor

One of the technologies for Pre-Combustion Decarbonisation is the production of hydrogen rich fuel gas from fossil fuel feed stock by means of a water gas shift membrane reactor system. A study to develop and test hydrogen selective membranes for use in a water gas shift membrane reactor operating with sour synthesis gas has been sponsored by the CO2 Capture Project. The aim of the project was to demonstrate a proof of concept water gas shift membrane reactor for this purpose. As one of the potential membrane options in such a membrane reactor tubular micro porous silica membranes have been made for testing with a simulated water gas shift mixture. With standard silica membranes the flux criteria can be met when no water is present in the feed. However, with water in the feed the flux drops to a value, which is a factor 3 below the target. At the start of the project it was clear that the perm selectivity criterion of 100 was too high for micro porous membranes, because a maximum H2/CO2 perm selectivity of 39 was thus far measured for standard silica membranes. Selectivity improvement was focused on higher sintering temperatures, but increase of the H2/CO2 selectivity has not been experimentally proven. It was shown that H2S has no detrimental effect on a standard silica membrane and the H2/H2S selectivity is very high. Under the process conditions, so including a relative high water concentration, the stability of the silica membrane is limited to days as expected. The hydrothermal stability has been improved by incorporating alkyl-groups in the silica structure (ECN patent pending). The modified silica membrane is stable for more than 1000 hours under simulated steam atmosphere testing. A software model of the water gas shift membrane reactor has been developed. The model simulates a counter current water gas shift membrane reactor with micro porous membranes (silica and zeolite) and dense (palladium and proton conducting) membranes and copes with the isothermal and non-isothermal operation of the membrane reactor. The model is implemented as an Aspen Plus User Model (Aspen Plus, version 11.1) and is written in FORTRAN.