The Effect of Pressure in Membrane Reactors: Trade-Off in Permeability and Equilibrium Conversion in the Catalytic Reforming of CH4 with CO2 at High Pressure

The antagonistic effects of pressure on reaction equilibrium and permeability were studied for the first time in a membrane reactor (MR). The reaction employed was the catalytic dry-reforming of methane with carbon dioxide (CH₄ + CO₂ ? 2CO + 2H₂) which produces a net increase in moles and is disfavored by high pressure. The studies were conducted at non-equilibrium conditions in a MR containing a hydrogen-selective ceramic membrane and a packed-bed reactor (PBR) at various pressures (1–20 atm) and temperatures (873 and 923 K) using a Rh/Al₂O₃ catalyst. Because of the concurrent and selective removal of hydrogen from the reaction in the MR significant enhancements over the PBR in the yields for H₂ (>170%) and CO (>130%) in the reaction products were obtained. However, as pressure was increased the enhancement in H₂ and CO yields in the MR went through a maximum and then declined. This occurred because, although the rate of hydrogen separation increased with increasing pressure, the conversions of the reactants decreased with increasing pressure. Thus, the maximum was due to a tradeoff between a transport property (hydrogen separation) and a thermodynamic quantity (hydrogen production) which had opposing pressure dependencies. It was also found that the reverse water–gas shift (RWGS) reaction (H₂ + CO₂ ? CO + H₂O) occurred simultaneously with the reforming reaction, and at high pressures significantly reduced the amount of hydrogen production in favor of water. The results are general and make the dry-reforming reaction impractical for commercial hydrogen generation regardless of the type of catalyst or reactor used.