To better understand the complicated phenomena in a precipitation process, computational fluid dynamics (CFD) is utilized to account for reactions, crystallization, mixing, and their interactions. The moment transformations of the population balances taking into account the chemical and crystallization kinetics are integrated into a CFD solver to describe the generation and transportation of the crystal phase. The current work applies this CFD tool to simulate the reactive precipitation process in a semibatch crystallizer, which is widely used in chemical and pharmaceutical industries. BaSO4 precipitation is employed as an example system. The influence of hydrodynamics in the stirred tank, as characterized by the impeller speed and feed location, on the distribution of supersaturation and the subsequent crystal size distribution is investigated. The numerical predictions are validated with measurements and can be used as an aid in the optimization of semibatch precipitator design.