Fixed-bed reactors with small ratio of tube to pellet diameter (D/d(p)=N) are applied especially for highly exothermic or endothermic reactions. In this contribution, aspects of an algorithm-based approach are investigated, which models spatially resolved fixed-bed reactors with small N. This approach is compared with experimental data in terms of bed morphology, resulting velocities, heat transfer, and heterogeneous catalysis. Finally, the question is discussed whether CFD can be applied as a design tool for small N fixed-bed reactors.