Methanol synthesis via direct CO2 hydrogenation in a square duct: A direct numerical simulation study

Efficient production of hydrogen (H2) derivatives, such as e-methanol, is essential to achieving carbon-neutrality targets. In the present computational fluid dynamics study, we investigate synthetic methanol (CH3OH) production via direct carbon dioxide (CO2) hydrogenation in a milli-duct with a rectangular cross section. The duct involves catalytic wall reactions with CO2 and H2 as reactants. Direct numerical simulations are carried out at bulk Reynolds numbers of Reb= 100, 500, 1100, and 2200 spanning laminar and turbulent flow regimes. We investigate a high catalyst loading, corresponding to gas hourly space velocity values of 0.086, 0.44, 0.96, and 1.93 m3/(kgcat h) at these Reynolds numbers, respectively, to study the impact of turbulence on methanol production in the limit of fast, transport-limited chemistry. The numerical results consist of the following features: (1) In the turbulent case (Reb=2200), the reaction rate reaches local minima/maxima in the turbulent ejection/sweep structures observed in the proximity of the catalytic walls, respectively. (2) Turbulent mixing may significantly enhance methanol production under transport-limited chemistry, increasing the methanol mass flow rate at the outlet up to 56.4% compared to laminar flow at a matched Reynolds number. (3) The single-pass conversion efficiency of CO2 and the methanol yield across the duct decrease monotonically as the Reynolds number increases (lower residence time). In contrast, the methanol mass flow rate at the outlet increases as Reb increases (higher reactant inflow).