Hydrogen permeation experiments were performed in order to evaluate the influence of water vapor on hydrogen permeability in 80–20% by weight Pd–Ag membranes of 2.5μm in thickness. In particular, the hydrogen flux was measured in pure hydrogen permeation tests as well as in experiments involving binary mixtures also containing nitrogen or water vapor, that were performed at 473 to 723K and at transmembrane pressure differences of up to about 3bar. The membranes exhibited a very high hydrogen permeance and lifetime, as well as virtually infinite selectivity (exceeding 10000 for H2–N2 mixtures). The experiments in hydrogen–nitrogen mixtures were carried out at different temperatures, hydrogen concentrations and feed flow rates and confirmed the existence of a non-negligible concentration polarization phenomenon in the experimental module. The gas phase mass transport and the module fluid dynamics were thus analyzed and the dimensionless numbers characterizing these processes were evaluated at the different operative conditions; a linear correlation was found to hold between Sherwood and Péclet numbers. Interestingly, the hydrogen permeate fluxes measured with feeds containing H2–H2O mixtures resulted always lower than those obtained for the nitrogen–hydrogen mixtures performed at the same hydrogen mole fraction and operative conditions: in particular, the hydrogen flux depletion increased with decreasing temperature and/or increasing the concentration of water vapor. All the experimental evidences suggested a clear interaction between water vapor and metallic layer, causing a lower hydrogen adsorption capacity of the membrane surface. That phenomenon was reversible, since the original permeance of the membrane was restored once the water vapor was removed from the feed, and was apparently due to a competitive H2–H2O adsorption on the Pd–Ag surface. The hydrogen flux depletion was then modelled by considering the simultaneous effects of gas phase resistance and competitive adsorption on the surface, obtaining a rather good agreement between experimental data and calculated results.

Influence of Water Vapor on Hydrogen Permeation through 2.5μm Pd–Ag Membranes. J.Catalano, M.G.Baschetti, G.C.Sarti: International Journal of Hydrogen Energy, 2011, 36[14], 8658-73