CO2-Selective Membranes for H2 Purification for Fuel Cells

This presentation covers an experimental and modeling study of carbon dioxide-selective membranes for hydrogen purification for fuel cells.  We have synthesized CO₂-selective membranes by incorporating amino groups into polymer networks.  The membranes have shown high CO₂ permeability and selectivity vs. hydrogen, carbon monoxide and nitrogen up to 180^oC.  We have elucidated the effect of amine steric hindrance in the solid membrane, showing significant enhancement for CO₂ transport.  Hydrogen sulfide permeates through the membrane much faster than CO₂, allowing H₂S removal in the treated synthesis gas before water-gas-shift (WGS) reaction.  Our initial experiments have shown a nearly complete removal of H₂S from 50 ppm in synthesis gas to about 10 ppb in the hydrogen product. 

In the modeling of the WGS membrane reactor in a spiral-wound membrane module based on both mass and enthalpy balances, the kinetics of the WGS reaction was incorporated using a published rate expression for the commercial CuO/ZnO/Al₂O₃ catalyst.  The resulting 1-D (countercurrent flow of feed and sweep streams) or 2-D (crossflow) system of differential equations were solved using COMSOL Multiphysics.  The model was validated by comparing the predicted CO results with previously published experimental data for a lab-scale flat rectangular membrane reactor, achieving <10 ppm carbon monoxide in the hydrogen product.  The effects of operating parameters like feed pressure, sweep to feed flow rate ratio and steam/CO ratio on membrane area and hydrogen recovery were carried out for reaching <10 ppm carbon monoxide in the hydrogen product.  Results show that although the countercurrent flow mode is the most efficient in terms of CO reduction; however, the crossflow mode might provide a better trade-off between CO reduction and heat management.