Coupled influence of non-ideal diffusion and multilayer asymmetric porous supports on Sieverts law pressure exponent for hydrogen permeation in composite Pd-based membranes

The behaviour of the pressure exponent in the Sieverts-type empirical law is evaluated for hydrogen permeation through Pd-based membranes under the influence of non-ideal internal diffusion and transport in an asymmetric multilayer porous support. The results show that the non-ideal diffusion within the temperature range investigated (300-500°C) causes the pressure exponent to be non-monotone, with it showing a minimum point indicating the passage from slower internal diffusion to slower transport in support. Additionally, a detailed analysis shows that the maximum transmembrane pressure difference (in terms of maximum feed pressure with constant permeate pressure) is the only operating parameter affecting the degree of uncertainty by which the pressure exponent is evaluated. This analysis also shows that the Sieverts-type empirical law can be fully used in the presence of supports. This suggests that an overall transport mechanism resulting from a combination in series/parallel of different elementary mechanisms obeying this law can be also described by the same type of law with a different value of pressure exponent and permeance.