Permeance inhibition due to reaction, coking and leakage of Pd membranes during methane steam reforming estimated from a micro-kinetic model

While permeance inhibition due to competitive co-adsorption of CO, hydrogen and other species have been documented and analyzed in Pd or Pd-alloy membrane separators and reactors, the effect of reaction on the Pd surface has not been analyzed. The contribution of the Pd membrane to the overall reaction rate of the catalytic packed bed in the reactor is negligible; yet, the adsorption and reaction on the Pd surface may inhibit permeation. The first aim of this study is to assess permeance inhibition due to this effect. This analysis requires a micro-kinetic model; and the estimation of the kinetic parameters of the Pd-membrane catalyzed reaction and using them to determine the surface composition. We will focus here on methane Steam Reforming (SR) CH4 + H2O -> CO + 3H(2) and the following Water Gas Shift (WGS) CO + H2O -> CO2 + H-2. We estimate that inhibition due to surface intermediates on Pd may be strong at relatively high temperatures and low hydrogen concentrations, and is negligible under other conditions due to the relatively faster water dissociation, but inhibition due to CO co-adsorption may be significant. We use the same micro-kinetic model to analyze two more long-term (slow) effects that were reported experimentally: we find that membrane coking due to methane dissociation (in the absence of water) is possible but it is unlikely to occur with steam in the feed. Leaking of CO, by its dissociation to its elements (C, O) followed by diffusion and association on the permeate side, is possible but is unlikely to account for the observed CO flux.

Automated summary

This study aims to evaluate the permeance inhibition caused by the effect on the Pd membrane surface, analyzing the kinetic parameters of Pd surface catalyzed reactions and surface composition using a micro-kinetic model. The research finds that inhibition due to surface intermediates on Pd may be strong at relatively high temperatures and low hydrogen concentrations, but negligible under other conditions.