Particle-Resolved CFD Simulation of Diluted Catalytic Fixed Bed Reactors for Formaldehyde Production

In catalytic fixed bed reactors for highly exothermic reactions, the bed is often diluted with inert particles to prevent thermal runaway and to distribute the reaction more homogeneously along the reactor length. The partial oxidation of methanol to formaldehyde is an example with high industrial relevance, in which diluted fixed beds are applied. In this work, particle-resolved computational fluid dynamics (PRCFD) simulations are conducted for the hotspot region (0–0.5 m) of an industrial scale fixed bed for formaldehyde production to systematically investigate the impact of dilution on integral reactor performance and locally distributed quantities, such as the temperature and catalyst effectiveness factor. PRCFD is the most detailed modeling approach for the simulation of diluted fixed beds since the spatial resolution of the fixed bed geometry allows the inert particles to be considered directly without the implementation of averaged activity factors. Different catalyst distributions have a significant effect on integral conversion, hotspot formation, and catalyst overheating while increasing the inert thermal conductivity has only a minor impact on heat transport and hence reaction. The difference between the maximum catalyst temperature of two different catalyst arrangements can reach 34 K. Finally, the present study demonstrates that even highly diluted fixed beds with industrial particle and tube dimensions are not suited to perform intrinsic kinetic measurements for the partial oxidation of methanol because of catalyst overheating (ΔT = 23.12 K) and pore diffusion limitation (η_i,FA < 0.5).