Investigating CO2 hydrogenation to methanol in a water-selective membrane reactor considering multiscale dual-porosity transport effects

The CO2 hydrogenation to methanol (CHTM) represents a pivotal strategy for carbon utilization and energy storage, yet traditional fixed-bed reactors (TFBR) are constrained by thermodynamic equilibrium limitations and low yields. This article develops a particle-resolved numerical model incorporating a multiscale dual-porosity (MDP) framework to investigate coupled transport phenomena and reaction kinetics within a water-selective packed-bed membrane reactor (PBMR). Compared to conventional pseudo-homogeneous models, the MDP approach rigorously couples intra-particle diffusion with external transport, revealing significant concentration gradients between the catalyst pellet interior and the bulk bed, particularly at the reaction zone entrance. Results manifest that the water-selective PBMR significantly outperforms non-permeable TFBR thanks to in-situ water removal shifting equilibrium, achieving a CO2 conversion (XCO2) of 66.1% and a methanol yield (YMeOH) of 32.2%. Higher temperatures and pressures promote XCO2, however, YMeOH favors an optimum temperature around 220 °C. Increasing the H2/CO2 feed ratio and reducing space velocity further improve XCO2 and YMeOH; while larger pellet size and higher pellet porosity degrade performance, although the former reduces pressure drop and the latter elevates space-time yield. Dimensionless analyses demonstrate that both the Damköhler number (Da) and normalized water permeance (θH2O) positively influence XCO2 and YMeOH, with the most pronounced improvements occurring at low initial values. Replacing the water-selective membrane with a methanol-selective counterpart facilitates a substantial increase in methanol selectivity (SMeOH) to 91.3%, ultimately driving the YMeOH to 44.5%. This work provides theoretical guidance for the design and optimization of next-generation reactors for efficient e-methanol production.