Melt suspension crystallization is used for ultrapurification of caprolactam from caprolactam-water mixtures. The influence of hydrodynamics, cooling profile, impurity concentration, and residence time on the crystal bed permeability were modelled, and measured in a cooling disk crystallizer. Permeability gives a good indication of the efficiency of the solid-liquid separation. Reducing backmixing improves permeability. Permeability increases with increasing residence times and decreasing water content.