The aim of this work, consequently, is to correlate the methane formation rate with the catalyst pellet size for the hydrogenation of CO/CO2 mixtures and to validate the simulation results with experimental data. Therefore, the binary friction diffusion model is implemented based on our recent work, which accounts for multicomponent diffusion and viscous flux in the pellet . We have already shown that diffusion properties (e.g. pore diameter) affect reactant selectivity, significantly under dynamic operation conditions. However, a systematic study on the pellet size effect on effective reaction rate and reactant selectivity was not yet performed, which therefore is in the focus of the present work. Importantly, the reaction kinetics for the simultaneous methanation of CO and CO2 on Ni-based catalysts is used , which is parameterized with the same catalyst used for experimental validation in the present work. Hence, the current work provides the basis to use the catalyst pellet size as an additional degree of freedom for optimizing the reactant selectivity and CH4 formation rate for hydrogenation of CO/CO2 mixtures.