The fluid dynamic operating behaviour of a multistage fluidized bed column was experimentally studied with the aid of a cold flow model. The model consists of a modular column that comprises four consecutive stationary fluidized bed stages with exchangeable gas distributor plates and weir plates for the internal downcomers. A solids recirculation system has been used to extract bed material from the bottom stage at a controlled rate and to redirect the material to the top stage. The focus of this work was to study the operation boundaries for different downcomer geometries with the aim to maximise the achievable solids flux through the downcomers. The maximum solids circulation rates were measured for several superficial gas velocities and the operating regimes resulting within the downcomers were monitored. Different mechanisms were identified to limit the stable operating range of the studied downcomer configurations that will be presented and discussed in this work. Finally, conclusions on the operability of the different downcomer configurations will be drawn and suggestions will be made for the design of internal downcomers of multi-stage fluidized bed columns. (C) 2016 Elsevier B.V. All rights reserved.