Dense cermet membranes for hydrogen separation

Abstract

Dense cermet (i.e., ceramic–metal composite) membranes have been developed for separating hydrogen from mixed gases, particularly product streams generated during coal gasification and/or steam methane reforming. Hydrogen separation with these membranes yields high-purity hydrogen, thereby eliminating the need for post-separation purification steps. Extensive tests have been conducted with cermet membranes made by mixing ≈50–60vol.% Pd with Y2O3-stabilized ZrO2. Using several feed gas mixtures, the hydrogen permeation rate, or flux, for the membranes was measured in the temperature range 400–900°C. With pure hydrogen at ambient pressure as feed gas, an ≈18-μm-thick membrane on a porous substrate gave a measured flux of ≈26cm3[STP]/min-cm2 at 400°C and ≈52cm3[STP]/min-cm2 at 900°C. We also measured the hydrogen flux through a thicker (≈150μm) membrane at 400°C using a mixture of H2, CO, CO2, H2O, and He at ≈200 psig as feed gas. Hydrogen flux measurements in H2S-containing atmospheres showed that the cermet membranes are stable at 900°C in gases that contain ≈80% H2/400ppm H2S. Because formation of palladium sulfide (Pd4S) can seriously degrade hydrogen permeation through Pd-containing cermet membranes, the Pd/Pd4S stability phase boundary of the cermet membrane was determined in the temperature range 450–650°C using various feed gases that contained 10–73% H2 and 8–400ppm H2S. Given these promising results, longer studies using real-world coal gasification conditions should be pursued.