High pressure performance of thin Pd–23%Ag/stainless steel composite membranes in water gas shift gas mixtures; influence of dilution, mass transfer and surface effects on the hydrogen flux

Abstract

The hydrogen permeation and stability of tubular palladium alloy (Pd–23%Ag) composite membranes have been investigated at elevated temperatures and pressures. In our analysis we differentiate between dilution of hydrogen by other gas components, hydrogen depletion along the membrane length, concentration polarization adjacent to the membrane surface, and effects due to surface adsorption, on the hydrogen flux. A maximum H 2 flux of 1223 mL cm −2 min −1 or 8.4 mol m −2 s −1 was obtained at 400 °C and 26 bar hydrogen feed pressure, corresponding to a permeance of 6.4 × 10 −3 mol m −2 s −1 Pa −0.5. A good linear relationship was found between hydrogen flux and pressure as predicted for rate controlling bulk diffusion. In a mixture of 50% H 2 + 50% N 2 a maximum H 2 flux of 230 mL cm −2 min −1 and separation factor of 1400 were achieved at 26 bar. The large reduction in hydrogen flux is mainly caused by the build-up of a hydrogen-depleted concentration polarization layer adjacent to the membrane due to insufficient mass transport in the gas phase. Substituting N 2 with CO 2 results in further reduction of flux, but not as large as for CO where adsorption prevail as the dominating flow controlling factor. In WGS conditions (57.5% H 2, 18.7% CO 2, 3.8% CO, 1.2% CH 4 and 18.7% steam), a H 2 permeance of 1.1 × 10 −3 mol m −2 s −1 Pa −0.5 was found at 400 °C and 26 bar feed pressure. Operating the membrane for 500 h under various conditions (WGS and H 2 + N 2 mixtures) at 26 bars indicated no membrane failure, but a small decrease in flux. A peculiar flux inhibiting effect of long term exposure to high concentration of N 2 was observed. The membrane surface was deformed and expanded after operation, mainly following the topography of the macroporous support.