The crystallization of potassium aluminum sulfate dodecahydrate (potash alum) in a fluidized bed crystallizer is studied both with experiments and simulations. A population balance system with three spatial coordinates and one internal coordinate (mass) is utilized as our model. The simulations are performed with a stochastic-deterministic method with novel extensions, where the fluid dynamics of the crystallizer (flow field, temperature, concentration) are solved deterministically and the particles are simulated with a stochastic method. In experiments of 30 min duration, the average crystal diameter increases by growth and agglomeration from about 130 mu m to 210 mu m. This observation agrees qualitatively well with our simulation results. A quantitative difference between simulation and experiment leaves room for future improvements in modeling. (C) 2019 Elsevier Ltd. All rights reserved.