Fuel-Cell Fuel Processing Membranes
We are developing new membranes and membrane processes, including a novel CO2-selective water-gas-shift (WGS) membrane reactor, for the purification of H2 for fuel cells. The new process is based on the facilitated transport mechanism, in which CO2 transport through the membrane is enhanced with reaction in the membrane, and H2 is rejected by the membrane.
The work involves the synthesis, characterization, and modeling of new CO2-selective membranes that would achieve the purification process. The membranes synthesized comprise primary, secondary, and tertiary amines in polymer networks. The membrane characterization includes CO2 transport enhancement due to reaction and reaction kinetics at gas-membrane interfaces. We have identified membranes with high CO2 permeability and CO2 / H2 selectivity at 50 – 100oC. As expected, the permeability has increased as temperature increases. However, the selectivity has also increased as temperature increases; this is impossible for a membrane based on the solution-diffusion mechanism. The selectivity increase is due to the facilitated transport with a higher CO2- amine reaction rate and lower H2 solubility in the membrane as temperature increases. We are extending the membranes to 150 – 200oC for the membrane reactor.
We have developed a non-isothermal model for the countercurrent WGS membrane reactor by taking material and energy balances and reaction into account. With this model, we have elucidated the effects of system parameters, including temperature, feed CO concentration, and feed-to-sweep gas flow rate ratio, on the membrane reactor for synthesis gases from steam reforming and autothermal reforming. The modeling results have shown that H2 enhancement via CO2 removal and CO reduction to 10 ppm or lower are achievable.
The investigation of CO2-selective membranes for other separations and CO2 sequestration is under way. Other separations include H2S from syngas, CO2 from flue gas, CO2 and H2S from biogas, and CO2 from confined space air.
Supported Liquid Membranes with Strip Dispersion
Recent advances have been on the development of supported liquid membranes (SLMs) with strip dispersion for the removal and recovery of heavy metals, including chromium, copper, zinc, and strontium, from wastewaters. The stability of the SLM has been ensured by an improved SLM with strip dispersion, where the aqueous strip solution is dispersed in the organic membrane solution in a mixer. The strip dispersion formed is circulated from the mixer to the membrane module to have a constant supply of the organic solution to the membrane pores.
The investigation of SLMs for bioprocessing and environmental areas is under way. Bioprocessing involves the recovery of products from fermentation broths via membranes. Environmental areas cover the removal and recovery of (1) heavy metals and organic compounds from wastewaters and (2) volatile organic compounds from air. The research involves the synthesis and characterization of new extractants with high specificity towards the target species. The characterization includes (1) transport enhancement due to the carrier-mediated mechanism and (2) reaction stoichiometry and kinetics at membrane interfaces.
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