The molar component balance of the gas bulk is consequently expressed by equation ( ) assuming steadystate conditions and the absence of homogeneous gas phase reactions. The first two terms refer to the convective molar inlet and outlet flow rate of component 𝑖, while the third term expresses the diffusive mass transfer across the external catalyst surface. Since external mass transfer limitations are excluded experimentally (see above), the interfacial mass transfer is governed by the effective surface flux density 𝐽 𝑖 e (𝑦 = 𝑟 p ), which is derived from the model at the pellet scale. Furthermore, 𝑛 p is the number of pellets within the reactor and 𝐴 p is the surface area of one single pellet.