The influence of dispersive effects(axial mixing and mass-transport resistance) on the performance of simulated moving bed (SMB) units is studied through detailed modeling. The results are represented in the space of the dimensionless operating parameters defined as the ratios between the fluid and the simulated stationary phase now rate in the different sections of the SMB unit. The complete separation region calculated by numerical simulations is compared to the one obtained through equilibrium theory (i.e., by assuming ideal behavior and neglecting dispersive effects). The differences are shown to be relatively small when realistic values of the process parameters are used; however, they must be taken into account for fine-tuned process optimization. This proves the usefulness of the equilibrium theory approach to provide a first solution that can then be refined through detailed model simulations. A thorough analysis of the optimal design of the operating conditions when only one compound is required pure is carried out and a rather interesting asymmetric behavior is evidenced and explained. Finally, the usefulness of the theoretical findings is assessed by using them to discuss and explain a set of experimental data reported in the literature.