The operating regimes for a pilot-scale rotating-disc contactor (RDC) were investigated by a computational fluid dynamics/population balance model (CFD-PBM) simulation. The model successfully predicted the critical rotor speed, which divided the entire operating range into two regions. In the low rotor speed region, the input energy was insufficient to break droplets, resulting in an almost constant droplet diameter. Therefore, the increasing revolution slightly affected the interfacial area, while the axial mixing became severe. In contrast, the interfacial area increased significantly in the high rotor speed region because of the increased breakage rate. Moreover, the axial mixing extent increased slightly because the dispersed-phase accumulation enhanced the advection effect. The results indicate that the CFD-PBM approach can be applied to engineering practice for extractors.