CO₂(H₂S) membrane separations and WGS membrane reactor modeling for fuel cells

Acid-gas removal is of great importance in many environmental or energy-related processes. Compared to current commercial technologies, membrane-based CO₂and H₂S capture has the advantages of low energy consumption, low weight and space requirement, simplicity of installation / operation, and high process flexibility. However, the large-scale application of the membrane separation technology is limited by the relatively low transport properties.

In this study, CO₂(H₂S)-selective polymeric membranes with high permeability and high selectivity have been studied based on the facilitated transport mechanism. The membrane showed facilitated effect for both CO₂and H₂S. A CO₂ permeability of above 2000 Barrers, a CO ₂/H ₂ selectivity of greater than 40, and a CO₂/N₂ selectivity of greater than 200 at 100 – 150 °C were observed. As a result of higher reaction rate and smaller diffusing compound, the H₂S permeability and H₂S/H₂ selectivity were about three times higher than those properties for CO₂. The novel CO₂-selective membrane has been applied to capture CO₂ from flue gas and natural gas. In the CO₂ capture experiments from a gas mixture with N₂ and H₂, a permeate CO₂ dry concentration of greater than 98% was obtained by using steam as the sweep gas. In CO₂/CH₂ separation, decent CO₂ transport properties were obtained with a feed pressure up to 500 psia. With the thin-film composite membrane structure, significant increase on the CO ₂ flux was achieved with the decrease of the selective layer thickness.

With the continuous removal of CO₂, CO₂-selective water-gas-shift (WGS) membrane reactor is a promising approach to enhance CO conversion and increase the purity of H₂ at process pressure under relatively low temperature. The simultaneous reaction and transport process in the countercurrent WGS membrane reactor was simulated by using a one-dimensional non-isothermal model. The modeling results show that a CO concentration of less than 10 ppm and a H₂ recovery of greater than 97% are achievable from reforming syngases. In an experimental study, the reversible WGS was shifted forward by removing CO₂ so that the CO concentration was significantly decreased to less than 10 ppm. The modeling results agreed well with the experimental data.