Fixed bed catalytic reactors, such as those used for the example reaction of n-butane to maleic anhydride chosen in this paper, are generally operated under steady-state. Nevertheless, the activity and selectivity of the catalyst is changing dynamically during operation. However, since these changes occur on a much larger time scale compared to the reaction and transport phenomena, the system can be considered steady-state, if only one operating point is studied ). Therefore, steady-state governing equations are solved for the fluid and solid domain and coupled via the fluid-solid interface for energy and species mass transport.