(701d) Relevance of Concentration Polarization on Hydrogen Flux through a Thin, Highly Selective Pd80-Ag20 Ceramic Supported Membrane

Permeation experiments using pure hydrogen and several mixtures of hydrogen, nitrogen, and methane were performed on a Pd-based membrane deposited on a ceramic support. In particular, the permeance of hydrogen was measured using binary and ternary mixtures containing also nitrogen and methane, at temperatures ranging from 673 K to 773 K at different feed flow-rates and H2 concentrations, in a custom-built, tube-in-shell apparatus. The membranes, supplied by NGK Insulator Ltd Japan, consisted of a 2.5 µm Pd-Ag layer deposited on a tubular ceramic support. They showed extremely high hydrogen permeability and practically infinite selectivity toward hydrogen. Permeability and permeance were calculated from the linear regression of pure hydrogen data, according to Sieverts' law. Permeability values ranged from 3.08 x10-6 to 4.66 x10-6 mol s-1 m-1 bar-0.5 for 673 K and 773 K respectively.

Surprisingly, experiments showed that hydrogen permeability is much lower for gas mixtures feeds than for the pure hydrogen case, and also that the hydrogen flux for mixture experiments deviates significantly from Sieverts' law. The above features reveal the presence of a significant mass transfer resistance in the external gas phase and that is clearly confirmed by the increase in hydrogen permeability obtained with increasing the feed flow-rate of the gas mixtures.

The data collected were then used to calculate the overall mass transfer coefficient in different experimental conditions. The gas phase mass transfer coefficient follows closely a correlation of Lévéque type proportional to the cubic root of the Peclét Number.