Abstract The design of simulated moving bed chromatography under reduced purity requirements is investigated. A parametric study indicates that all internal flow rates should be considered as free parameters in process design. This is confirmed by systematic numerical optimisations of two different design scenarios. A restrictive scenario requires complete regeneration of liquid and solid phases. In contrast, in a non-restrictive design all flow rates are optimised. The latter leads to significantly improved process performance. On the basis of the obtained observations, a simple design method for simulated moving bed processes with desired outlet purity values is derived from equilibrium theory.