The use of simple compartmentalized population balance (PB) models was investigated for describing the crystal size distributions (CSD) of two different laboratory batch gibbsite crystallizers, one uniformly-mixed and the other inhomogeneously-mixed with respect to particulate phase. The compartments were selected on the basis of the shear rate and solids concentration distribution maps generated by the computational fluid dynamics (CFD). In the inhomogeneously-mixed case, the CSD predicted by the compartmental model was highly sensitive to the relationship between the agglomeration kernel and shear rate used to describe the local agglomeration rate. This observation is consistent with recent findings reported in the literature that to correctly account for the agglomeration process in crystallizers its kinetic formulation should be based on the local rather than the volume-averaged properties.