(253c) Hydrogen Sulfide and Carbon Dioxide Removal with Facilitated Transport Membranes

Acid-gas removal is of great importance for various gas separation applications. H₂S is a common contaminant, in the hydrogen derived from synthesis gas, which poisons the fuel cell anode catalyst. Since this poisoning mechanism is irreversible, even trace concentrations of H₂S ( > 10 ppb) in the hydrogen can significantly degrade fuel cell performance. So it must be almost completely removed prior to feeding hydrogen to fuel cells. On the other hand, the increasing public concern over global warming has concentrated on the greenhouse gas emission. Therefore, it is highly desirable to capture CO₂ from the flue gas, a major source of the greenhouse gas. In this study, facilitated transport membranes containing amino groups were investigated to remove H₂S for hydrogen purification in fuel cell application and CO₂ for flue gas application.

We synthesized H₂S- and CO₂-selective polymer membranes by incorporating amino groups in polymer networks, which react with these acid gases reversibly and enhance their removal. A circular permeation cell with countercurrent gas flows was used to study the H₂S removal. The membranes showed high H₂S and CO₂ permeabilities and H₂S/H₂ and CO₂/H₂ selectivities in temperatures ranging from 110^oC to 140^oC. The transport properties for H₂S were significantly better than those for CO₂ because of the faster reaction mechanism with H₂S. Using this membrane cell with a membrane area of 45.6 cm², the H₂S concentration in the gases was reduced from 50 ppm to less than 80 ppb, or from 100 ppb to less than 10 ppb, at 120^oC. We have developed a one-dimensional isothermal model to evaluate the separation performance of a hollow fiber module composed of the H₂S- and CO₂-selective membrane. The modeling has shown that < 10 ppb H₂S is achievable from typical reforming synthesis gas with small membrane area requirement.

In the CO₂-capture experiments, the same circular permeation cell with countercurrent gas flows was used. The feed gas consisted 20% CO₂, 40% H₂, and 40% N₂. The permeate CO₂ dry concentration of greater than 98% was obtained by using steam as the sweep gas. We have also investigated the effects of feed flow rate and sweep-to-feed molar ratio on membrane separation performance. As the feed inlet flow rate increased, the permeate CO₂ dry concentration slightly increased, and the CO₂ recovery decreased owing to the reduced residence time. Increasing sweep-to-feed ratio enhanced the permeation driving force and resulted in a higher CO2 recovery, while the permeate CO₂ dry concentration did not change significantly. From the modeling results, a 2-ft hollow fiber module containing 1,050,000 fibers was sufficient to recover above 95% of CO₂ and obtain a permeate CO₂ concentration of larger than 98% (on the dry basis) from a 1000 SCFM flue gas stream containing 9% CO₂.