Despite of its general use in industry, particle sedimentation is still a not well understood unit operation. Hydrodynamics is complex in essence, mainly because the possible volumes are depending on the operating conditions, which in turn has consequences on the stability of the equipment in unsteady state conditions. In this work, we attempt to model the hydrodynamics in the zone of separation of the flows in a pilot plant equipment by using different flow configurations. Use is made of fluid Residence Time Distribution (RTD) experiments performed in the absence of solids. The moments method is used to discriminate between four different models before any attempt to compare the RTD curves. A first model comprising two dispersion-convection zones, each in the upper and bottom part, is rejected for its poor physical meaning. The discussion analyzed the merits of the last three models representing the zone of separation of the inlet flow by various associations of perfectly mixed cells. The simplest configuration, a perfectly mixed zone plus a plug flow zone in each part of the apparatus, is also the more versatile. However, it is rivalled by models allowing ascending flows from the bottom part. The dependency of the internal volumes with flow rates and flow ratios is examined as well as the potentialities of application of such models in the presence of solids.