(353b) Hydrogen Purification for Fuel Cells by Carbon Dioxide Removal Membrane Followed by Water Gas Shift Reaction

Hydrogen is receiving increasing attention worldwide not only because of its role as an important raw material in chemical and petroleum industries, but also because of its prospect as a primary energy carrier in the 21st century. For many applications, especially for proton-exchange membrane fuel cells (PEMFC), the CO concentration of hydrogen-rich gas mixture must be reduced to ppm levels to avoid poisoning anode catalysts. In the commercial scale, most feasible strategies to generate hydrogen from hydrocarbon fuels consist of a reforming step followed by the water gas shift (WGS) and a CO clean-up step. The current known approaches for CO clean-up include methanation and preferential oxidation, both of which consume a significant amount of hydrogen. In this study, a process combining CO₂ removal using a polymeric membrane with subsequent CO conversion using water gas shift reaction was developed to purify hydrogen. This process converts CO to H₂ instead of consuming H₂ and removes more than 93.5% of the total CO₂ (for the synthesis gas consisting of 17% CO₂ and 1% CO), which makes subsequent CO₂ concentration and sequestration possible.

We have synthesized CO₂-selective polymer composite membranes with high CO₂ permeability and CO₂/H₂ selectivity in temperatures ranging from 50^oC to 150^oC. A rectangular flat-sheet membrane cell with well-defined countercurrent gas flows was used to study the CO₂ removal. A feed gas consisting of 17% CO₂, 1.0% CO, 45% H₂, and 37% N₂ was used to simulate the synthesis gas from autothemal reforming of natural gas with air. With this membrane cell running at 120^oC, the CO₂ concentration in the gas mixture was reduced from 17% to as low as 30 ppm. The CO₂ data have been in good agreement with model predications for the feed flow rates ranging from 10 to 130 cc/min. Then, another feed gas of 53.9% H₂, 0.1% CO₂, 1.2% CO, and 44.8% N₂ was used to simulate the synthesis gas from the CO₂-removal step. With this feed gas, a conventional low temperature water-gas-shift reactor packed with commercial Cu/ZnO catalyst was operated at 130 to 160^oC to shift CO to H₂. With more than 99% CO₂ removed in the synthesis gas, the reversible WGS was shifted forward so that the CO concentration was decreased from 1.2% to less than 10 ppm (dry), which is the requirement for PEMFC. The WGS reactor had a GHSV of 7650 h^-1 at 150^oC and the H₂ concentration in the outlet was more than 54.7% (dry).