GRACE: Experimental Evaluation of Hydrogen Production by Membrane Reaction

Water gas shift reaction, widely used for upgrading H2 containing streams, was analyzed in a membrane reactor (MR) using tubular Pd/Ag, silica and zeolite-A supported Pd membranes supplied by SINTEF (Norway), the University of Twente (The Netherlands) and the University of Zaragoza (Spain), respectively. MR experiments were carried out investigating the effect of temperature (200-338°C), reaction pressure (up to 550 kPa), partial pressure difference, sweep factor (0-7.5) and space velocity (472-2,308 h-1) on CO conversion and identifying rate determining step (kinetics or thermodynamics). H2O/CO feed molar ratio was around the stoichiometric value. However, three different streams were fed to the MR: an equimolecular H2O/CO stream; an "ATR exit + Extra Steam" stream (20% CO, 20% H2O, 10% CO2, 50% H2); and the outlet stream (partially converted) of a traditional reactor (TR) placed before the MR. TR experiments were also performed at a high SV (15,050 h-1). A commercial, Haldor-Topsoe low temperature Cu-Zn oxides-based catalyst (LK821-2) was employed in both MR and TR. TR equilibrium conversion (TR-EC) was considered as reference because it is the upper limit for typical reactors. This constraint can be overcome by MR as a consequence of H2 removal by means of a selective membrane. CO conversion measured in MR experiments, using the SINTEF and Twente University membranes, significantly overcome the thermodynamic limit for TR, depending also on the operating conditions, mainly temperature, pressure and feed composition. In some cases a total conversion was obtained. Also the use of a TR before the MR allows the TR-EC to be overcome. The conversion showed by the Zaragoza University membranes slightly overcame the TR-EC. Other parameters such as reaction pressure or sweep factor have a positive effect on conversion. All the membranes were also characterized by means of permeation measurements with a pressure drop (for single gas) and concentration gradient (for gas mixture) methods. The experimental work provided valuable information about the different membrane types and gives useful experimental information on the membrane WGS reactor concept.