(31b) Carbon Dioxide-Selective Membranes for Hydrogen Purification with Water Gas Shift Reaction

This talk is in honor of Dr. Yi Hua Ed Ma for his many contributions in Chemical Engineering including membranes with chemical reaction, and it covers CO2-selective membranes for hydrogen purification with water gas shift reaction (WGS). The membranes can be used for hydrogen purification through two approaches: (1) WGS membrane reactor and (2) CO2 removal followed by WGS reaction. This presentation will describe these two approaches.

CO2-selective membranes have various applications, including synthesis gas purification for high purity H2, natural gas sweetening, and CO2 capture from flue gas. For fuel cell applications, it is a key issue to generate high purity H2 from available hydrocarbon fuels, especially for proton-exchange membrane fuel cells (PEMFCs) with the high purity requirement of less than 10 ppm CO. In commercial scale, most feasible strategies consist of a reforming step followed by water gas shift (WGS) reaction. The resulting synthesis gas still consists of about 0.5 to 1% of CO, which needs be reduced to < 10 ppm to meet the requirement of PEMFCs. The current approaches for CO clean-up include methanation and preferential oxidation, both of which consume a significant amount of H2. In this study, we synthesized new CO2-selective membranes and developed new approaches for H2 purification using these membranes.

We have synthesized CO2-selective polymeric composite membranes with high CO2 permeability and CO2/H2, CO2/CO and CO2/N2 selectivities in temperatures ranging from 25oC to 170oC. The synthesized membranes showed a permeability of 8200 Barrers (1 Barrer = 10-10 cm3(STP) · cm/(cm2 · s · cm Hg)) and a CO2/H2 selectivity of 450 at 120oC. A rectangular flat-sheet membrane cell with the membrane was used to remove CO2 from a gas stream consisting of 17% CO2, 1.0% CO, 45% H2, and 37% N2 (on dry basis), which simulated the syngas from the autothermal reforming of gasoline with air. In the experiments, the CO2 concentration in retentate was decreased from 17% to < 30 ppm. With such membranes, two approaches were used to convert the CO and decrease the CO concentration in syngas to obtain high purity H2.

In the first approach, i.e., WGS membrane reactor, the membrane removed CO2, and the commercial Cu/ZnO/Al2O3 catalyst catalyzed the WGS reaction. By removing CO2 simultaneously, the reversible WGS was shifted forward so that CO was converted to hydrogen and the CO concentration was then reduced significantly. A CO concentration of less than 10 ppm and a H2 concentration of greater than 50% (on the dry basis) were achieved at various feed gas flow rates. A one-dimensional model was developed to simulate the reaction and the transport process, and its results agreed well with the experimental data.

The second approach was a process consisting of a CO2-selective membrane module followed by a conventional WGS reactor. The CO2-selective membrane module removed more than 99% of the CO2 in the syngas. A feed gas consisting of about 53.9% H2, 0.1% CO2, 1.2% CO, and 44.8% N2 was used to simulate the syngas after this CO2-removal step. With this feed gas, a conventional WGS reactor packed with the commercial Cu/ZnO/Al2O3 catalyst was operated at 130 to 160oC to shift CO to H2. With this process, CO was converted to hydrogen and the CO concentration was then reduced to < 10 ppm (dry) at various feed gas flow rates. The WGS reactor had a maximum GHSV of 7650 h-1 at 150oC, and the H2 concentration in the product was about 55% (dry).