The hydrodynamic behaviour of a pseudo-2D Two-Section Two-Zone Fluidized Bed Reactor (TS-TZFBR) with an immersed tube bundle in its lower zone has been studied using non-invasive Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA). Coupled porosity distribution maps from DIA post-processing and solids velocity vector fields from PIV analysis allowed the reconstruction of the transient and time-averaged solid fluxes along the bed. Six different tube bank configurations at several different superficial gas velocities have been tested to evaluate the hydrodynamic behaviour within the lower zone of the TS-TZFBR. The solids axial mixing along the vertical bed position has been quantified for this novel reactor configuration. Besides, the effect of the internals on the gas-solid mass transfer has been estimated by means of bubble size. A five-staggered tube bundle configuration was able to diminish the average bubble size by 30% within the lower bed zone at usual TS-TZFBR gas flow rates, u(gas)/u(mf)=3.0, without increasing the solids circulation time dramatically. (C) 2015 Elsevier Ltd. All rights reserved.