The application of membrane assisted fluidized bed reactors for distributed energy production has generated considerable research interest during the past few years. It is widely accepted that, due to better heat and mass transfer characteristics inside fluidized bed reactors, the reactor efficiency can outperform other reactor configurations such as packed bed units. Although many experimental studies have been performed to demonstrate and monitor the long term performance of membrane assisted fluidized bed reactors, the hydrodynamics of membrane-assisted fluidized bed reactors has thus far only been studied in pseudo-2D geometries. In this work the solids concentration inside a real 3D fluidized bed reactor geometry was measured using a fast X-ray analysis technique. Experiments were conducted in absence and presence of two different membrane modules with different configurations and number of membranes (porous Al₂O₃ tubes) for two types of particles, viz. 400–600 μm polystyrene (Geldart B type) and 80–200 μm Al₂O₃ (Geldart A/B type). Results from the experiments with Geldart B type particles revealed that the membrane modules (both the membranes and the spacers) can significantly reduce bubble growth along the fluidized bed resulting in a smaller average bubble diameter, expected to improve the bubble-to-emulsion mass transfer, whereas for the experiments with fine Geldart A/B particles, and at a very high extraction values (40% of the inlet flow), a densified layer with high solids concentration was formed near the membrane, which may impose an additional mass transfer resistance for gas components to reach the surface of the membranes (concentration polarization). The results from this study help designing and optimizing the positioning of the membranes and membrane spacers for optimal performance of fluidized bed membrane reactors.