Optimal temperature range for the separation of a mixture of 96% protium and 4% deuterium with a palladium membrane

Abstract

Hydrogen isotope separation can be achieved using a palladium (Pd) membrane because protium (H) and deuterium (D) exhibit different solubility, diffusivity and, hence, mixed-gas permeability in Pd. The permeability of H (kH2) and D (kD2) were evaluated using a 4 vol% D2 – 96 vol% H2 feed mixture at temperatures of 293 K–473 K and pressures of 239 kPa–446 kPa in a continuous Pd membrane separation unit. The Pd foil membrane (0.1 mm thick) is shown to be selective toward H2 over the entire temperature range, with both kH2 and kD2 increasing with temperature. As temperature increases, the mixed-gas selectivity (kH2/kD2) rises from 4.0 at 300 K to a maximum value of 9.6 in the 363 K–373 K temperature range (which represents the operating temperatures that would render the highest concentration of H2 in the permeate and the greatest concentration of D2 in the retentate), then decreases to 6.2 at 480 K. Competitive transport during co-permeation is a likely cause for the maximum separation factor kH2/kD2 observed in this temperature range, being larger than the values predicted using pure gases (2 for diffusion dominant separation). The membrane selectivity presumably decreases from the observed maximum because the reverse solubility isotope effect decreases as the temperature increases. Additionally, isotope diffusion begins to increasingly affect the selectivity at higher temperatures.