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    June, 2005

Vol 1 Chapter 16: Hydrogen transport membrane technology for simultaneous carbon dioxide capture and hydrogen separation in a membrane shift reactor

Michael V. Mundschau et al, Eltron Research Inc, USA

Abstract: A wide variety of dense hydrogen transport membranes were tested for feasibility of resisting a minimum differential pressure of 3.10 MPa while extracting hydrogen from simulated high pressure water gas shift reactors operating at 693–713 K at an absolute pressure of 3.20 MPa and containing a hydrogen partial pressure of 1.31 MPa. Membranes were tested for compatibility with operating conditions of commercial water gas shift catalysts of 90 wt% Fe₃O₄/10 wt% Cr₂O₃. Best hydrogen flux results were achieved using select metal membranes of Group IVB and VB elements (i.e. Nb, Ta, V, Zr) and their alloys coated with submicron thick layers of palladium. Free standing, unsupported disks, 1.6 mm in diameter, of select metals and alloys were found to resist the target differential pressure of 3.10 MPa with the target partial pressure of hydrogen of 1.31 MPa while producing a hydrogen flux of 2.1 mol m^-2 s^-1 at 713 K at essentially 100% selectivity. At a 3.10 MPa differential pressure and a hydrogen partial pressure of 2.90 MPa, a record hydrogen flux of 2.5 mol m^-2 s^-1 was achieved at 713 K. It was concluded that the metal membranes appear superior to other classes of membrane tested for separation of H₂ from CO₂ at high pressure and are the most likely to be cost effective in scaled up reactors. Because commercial water gas shift catalysts are likely to be deactivated by sintering when used above about 713 K, proton conducting ceramic membranes, which typically require temperatures well above about 1000 K, were eliminated from consideration. Thin films of palladium supported on various porous materials were evaluated. In order to minimize interfacial stress between palladium and its potential substrates, which can lead to the formation of dislocations and cracks, a computer search of approximately 50,000 compounds was performed to select materials which would crystallographically match the cubic symmetry of the palladium crystal lattice and which would match the crystallographic lattice constants at the atomic level within about 2%. It was also desired to match coefficients of thermal expansion from room temperature to a maximum anticipated operating temperature of 713 K. From a dozen porous compounds tested, LaFe_0.90Cr_0.10O_3-x and LaFeO_3-x, performed best. However, it was concluded that, in general, hydrogen flux would likely be severely limited by gas phase diffusion of non-hydrogen gases through all conceivable thick porous supports needed to resist the extreme differential pressures, and that the predicted advantages of using micron-thin layers of palladium would be difficult to achieve. Also considered were dense cermets (ceramic metals) fabricated by sintering together powders of palladium or Group IVB–VB metals with ceramics which were lattice matched and matched for coefficients of thermal expansion. In the cermets tested, the hydrogen flux was predominantly through the metal phase (or along the metal ceramic phase boundaries) rather than through the ceramic phase. It was concluded that cost of scaled-up cermets of palladium might be prohibitive.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO₂ Capture Project Capture and Separation of Carbon Dioxide from Combustion Sources - Volume 1
Edited by: David C. Thomas, Senior Technical Advisor, Advanced Resources International Inc, USA

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