The overall reactor model is illustrated in figure . Inside the reactor two phases are present, namely the bulk gas phase and the porous catalyst pellet phase, which are described by separate models at reactor and pellet scale, respectively. The inlet volumetric flow rate of 𝑉 ̇in with a molar composition of 𝑥 𝑖,in is fed into the reactor, while the respective quantities at the outlet are denoted as 𝑥 𝑖,R and 𝑉 ̇out . Since the total molar amount changes with reaction extent in the methanation reaction, 𝑛̇o ut < 𝑛̇i n holds for the overall molar flow rates at inlet and outlet. Since absence of gradients is assumed for the gas bulk, according to the experiment (see above), a simple ideal continuous stirred tank reactor (CSTR) model is appropriate on reactor scale.