Development of Low Cost Membranes (Ta, Nb & Cellulose Acetate) for H<sub>2</sub>/CO<sub>2</sub> Separation in WGS Reactors

Abstract

The main aim of this work is to synthesize low temperature bimetallic nanocatalysts for Water Gas Shift reaction (WGS) for hydrogen production from CO and steam mixture; and develop low-cost metal (Nb/Ta)/ceramic membranes for H{sub 2} separation and Cellulose Acetate membranes for CO{sub 2} separation. Cu-Ni-Ce/alumina, Fe-Ni-Ce/alumina granular WGS catalysts incorporating metal oxide nanoparticles into alumina support were prepared using sol-gel/oil-drop methods. The catalysts were characterized by Powder X-ray Diffractometer (PXRD), Scanning Electron Microscope (SEM), Differential Thermal Analyzer (DTA), Thermal Gravitational Analyzer (TGA), and Brunauer, Emmett and Teller (BET) techniques. TGA shows sharp weight loss at approximately 215°C and DTA shows dehydration of metal hydroxides between 200°C and 250°C. The PXRD spectra show an increase in crystallinity as a result of heating to 1000°C, and indicating a fine dispersion of the metal oxide nanoparticles in alumina supports during the sol-gel synthesis and calcination at 450°C. BET analysis indicated a mesoporous structure of the granules with high surface area. A gas-phase dynamic flow reactor is used to optimize the reaction temperatures. A gas-phase batch reactor was used to obtain kinetic data and the parameters for maximum CO conversion. In Cu-Ni-Ce/alumina category, Cu(0%)Ni(10%)Ce(11%) was found to be the best WGS catalyst among six Low Temperature Shift (LTS) catalysts with optimum temperatures between 200-300A‚°C, while Ni(5%)Cu(5%)Ce(11%) was found to be the best among four High Temperature Shift (HTS) catalysts with optimum temperature between 350-400°C. In the Fe-Ni-Ce/alumina category catalysts, Fe(8%)Ni(0%)Ce(8%)/alumina and Fe(6%)Ni(2%)Ce(8%)/alumina catalysts showed optimum WGS reaction temperature below 150°C. All Ni(8-x%)Fe(x%)Ce(8%) had lower WGS reaction efficiencies compared to Ni(8-x%)Cu(x%)Ce(8%). Metal (Nb or Ta)/ceramic membranes for hydrogen separation from the WGS reaction gas products have been prepared using a) sputtering and b) aluminothermic techniques. A polyvinyl-glass permeability tester was used with a gas chromatograph (GC) for H{sub 2}/CO permeability testing. Nb films showed a higher permeability than Ta at a given disk porosity. The aluminothermically deposited membranes have higher H{sub 2} permeability compared to the sputtered films, and Nb-film coated disks showed lower H{sub 2} permeability than Ta-film. A three-stage prototype stainless steel reactor with integrated housing for 1) WGS reaction catalysts, 2) H{sub 2}/CO{sub 2} separation metal/ceramic or metal/asbestos membranes, and 3) CO/CO{sub 2} separation cellulose acetate /filter-paper membranes has been designed and tested to have capabilities to perform WGS reactions at temperatures up to 400°C and withstand gas pressures up to 15 bars. The cracking of ceramic disks and gas leaks were successfully prevented by replacing ceramic disks with asbestos sheets that can easily withstand 400°C. Kinetic studies of H{sub 2} and CO permeabilities were performed through the single and double layer Nb and Ta membranes. Cellulose acetate (CA) films with 25% triethyl citrate (TEC) as plasticizer were prepared for H{sub 2}/CO/CO{sub 2} gas separation with varying thickness of the films by acetone solutions at different concentrations and by dip-coating onto filter papers. The AFM analysis of the CA membrane showed that the uniform coating had fewer and smaller pores as the film thickness increased, and corroborated by gas permeability studies. The CO{sub 2} permeability has decreased faster than CO permeability with the CA/TEC membrane thickness, and findings support that the CA membrane could be used to entrap CO{sub 2}. Several CA/TEC membranes were also staked to increase the separation efficiency. Positron Lifetime Spectroscopy (PLS) was used to estimate the micro-porosity (pore size and concentration) and fractional free volume changes of CA/TEC films, and used to understand the variations observed in the CO{sub 2}/CO permeabilities.