Turbulence impacts synthetic methanol production in a square duct flow: a direct numerical simulation (DNS) study

Improving the synthetic methanol production efficiency is essential for adopting carbon-neutral methanol technologies. In the present computational fluid dynamics (CFD) study, we investigate synthetic methanol (CH$_3$OH) production in a milli-duct with a rectangular cross-section. The duct involves catalytic wall reactions with CO$_2$ and H$_2$ as reactants. Direct numerical simulations are carried out at bulk Reynolds numbers $Re_\mathrm{b} =$ 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 GHSV = 0.086, 0.44, 0.96 and 1.93 m$^3$/(kg$_\mathrm{cat}$ h). The results consist of the following features. 1) Turbulent mixing may significantly enhance methanol production under transport-limited chemistry, increasing the methanol mass flow rate at the outlet up to 56.4 % in the duct. 2) The reaction rate reaches local minima/maxima in the turbulent ejection/sweep structures observed in the proximity of the catalytic walls, respectively. 3) The single-pass conversion efficiency of CO$_2$ and the methanol yield across the duct decrease monotonically as the Reynolds number increases (lower residence time), while in contrast, the methanol mass flow rate at the outlet increases as $Re_\mathrm{b}$ increases (higher reactant inflow). These results suggest that turbulent transport could be leveraged to improve methanol yield in straight millichannels, albeit at the cost of increased reactant recycling.