The influence of the design of dual-zone ZoneFlow type structured catalytic reactors (Fig.1) on their performance in natural gas steam reforming under commercial operating conditions is studied by means of detailed Computational Fluid Dynamics (CFD) simulations, including detailed reaction kinetics.The paper describes the development of a hybrid CFD model that combines a detailed description of the reactor internals in the structured near-wall casing zone with a porous medium type description of the reactor core zone.Specific pressure drop measurements with the reactor core and the complete structure in a sufficiently wide flow rate range are carried out for determining two porous medium model parameters that are introduced and for model validation (Fig.2).