Hydrogen permeation through Pd-based composite membranes: Effects of porous substrate, diffusion barrier and sweep gas

• Bulk membrane resistance increases due to physical obstruction of substrate.
• A well-designed diffusion barrier does not always cause a decrease in H2 flux.
• Sweep gas increases H2 flux by dilution and penetration through the substrate.
• A superior sweep gas is one with small Schmidt number.
• High porosity and large pores are preferred for the membrane substrate.

Gas transport in the diffusion barrier and porous substrate layers is modeled in order to evaluate its impacts on the performance of palladium-based composite membranes for H2 separation. A pseudo-2-dimensional model is created for a cylindrical membrane module with counter-current flow. It is hypothesized that displacement of proton transport paths may occur in the bulk metal membrane due to a non-permeable solid phase in direct contact with it. A flux-weighted effective membrane thickness is proposed to account for this effect, which has not been considered in previous literature. Both the dusty gas model (DGM) and binary fraction model (BFM) are used to simulate gas transport in the porous media. Predictions show that under certain circumstances, a well-designed diffusion barrier may cause a slight increase in H2 flux, contrary to common belief. Two mechanisms are identified which enhance membrane performance by sweeping the permeate side. Though gases with small Schmidt number were found to be more likely to penetrate through the membrane substrate and build significant concentration at the membrane-substrate interface, the choice of sweep gas may depend on other practical considerations, especially gas separation downstream. Combinations of porosity and pore size are recommended for the membrane substrate and diffusion barrier based on modeling results.