Estimating limit conversion for methane steam reforming in a palladium membrane reactor using counter-current sweep gas

Generally speaking, conversion increases with increasing reactor length to approach a certain value: so-called limit conversion. Limit conversion for a membrane reactor with a cocurrent sweep gas has been studied extensively. However, characteristics of limit conversion for the countercurrent, which are examined in this study, remain unclear. First, conversions of MRs with different reactor lengths were calculated using a conventional mode of integrating differential equations. Results confirmed that limit conversion is not always 100%, even for the countercurrent. Long MRs were found to have a pseudo-constant state inside, characterized by an extremely low apparent reaction rate and permeation rate. Secondly, a novel approach to estimate limit conversion was developed: reaction equilibrium constants and hydrogen partial pressure balance between both sides were applied not for the end of the reactor but for the pseudo-constant state. This new approach suggests that limit conversion for the countercurrent depends on the reaction temperature, feed-side and permeation-side pressures, feed-gas composition, and the sweep rate. It is fundamentally independent of reaction kinetic equations and hydrogen permeation-rate properties.