Perturbed hydrogen permeation of a hydrogen mixture-New phenomena in hydrogen permeation by Pd membrane

The newly developed constant concentration method was adopted to study the hydrogen permeation of a hydrogen mixture of H₂ + Y through a palladium membrane tube. The hydrogen permeation of the hydrogen mixture differs from that of a single hydrogen feed. For the hydrogen mixture, the well-known Sieverts equation, Q = J_H / [(P_R^H)^{1 / 2} - (P_P^H)^{1 / 2}], fails to yield the correct hydrogen flux or permeance even after the pressure terms are adjusted to the partial pressure of hydrogen. The hydrogen concentration in the mixture affects both the flux and the permeance. Significant abnormal permeation of the non-hydrogen gas, Y, in the hydrogen permeates is detected during the hydrogen permeation of the mixture, H₂ + Y, even though Y-gas alone does not permeate through the defect-free palladium membrane. This Y-gas slippage in the presence of hydrogen in the mixture is tentatively attributed to the expansion of the palladium atomic lattice, enlarging inter-cluster openings. Y-gas permeates through the enlarged structure or grain boundary of the palladium atoms in the membrane. The permselectivity of a palladium membrane cannot be measured simply from the ratio of two separate permeation fluxes of H₂ and Y. The permselectivity of a hydrogen mixture, H₂ + Y, measured by the constant concentration method in the retentate, is compared with that measured by the flowing-through method and with that determining from two separate measurements of H₂ and of Y. The three methods give different permselectivities of the same palladium membrane tube when the concentration ratio of H₂/Y is adjusted to 1.