Influence of the gas phase resistance on hydrogen flux through thin palladium-silver membranes

Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at different experimental conditions. In particular the permeance of pure hydrogen as well as of binary and ternary mixtures containing hydrogen, nitrogen and methane was measured, at temperatures ranging from 673 to 773 K and at pressure differences up to 6 bar. The membranes, supplied by NGK Insulators Ltd., Japan, were formed by a selective Pd-Ag layer (20 wt% Ag) deposited on a tubular ceramic support, and showed very high hydrogen permeance and a practically infinite selectivity toward hydrogen. Interestingly, the permeance values measured in pure gas experiments resulted always higher than those obtained in permeation tests with gas mixtures; in the latter case, moreover, the permeate flux significantly deviates from Sieverts' law based on the hydrogen partial pressure in the bulk gas phase. Both facts suggest the existence of non-negligible resistances to hydrogen transport in the gas phase itself, in addition to that offered by the metallic membrane. Experiments performed with increasing feed flow rates, showed also an increase in hydrogen permeance thus revealing the importance of the concentration polarization effects inside the module. Gas phase mass transport coefficients were calculated and used to determine the role of such a resistance in the overall mass transport process. The Sherwood number was also evaluated and was found to follow a boundary layer type of correlation. A general sensitivity analysis was performed in order to compare the effects on the transmembrane hydrogen flux of the two resistances, with different physical dimensions, offered by the gas phase and the metallic membrane. The concentration polarization number thus introduced allows for an a priori identification of the leading resistance at any operating conditions and gives clear indications on the actions required to improve the module performance.