An innovative two-stage continuous bioreactor with cell recycle (TSCB) previously allowed us to reach very high ethanol productivity (41 kg m(-3) h(-1)) with the yeast Saccharomyces cerevisiae under high cell densities (similar to 100 gCDW/L) suitable for industrial transposition. Nevertheless, bioreactor performances and complex interactions between hydrodynamics (gas, liquid and solid phases) and biological activity (viability, conversion yield, ethanol stress) need to be investigated. In this work, an overall TSCB hydrodynamic identification (under abiotic conditions) was carried out by using residence time (RTD) and internal age (IAD) distribution studies, according to a suitable methodology. First, experiments led to identify and formulate the analytical RTD and IAD functions in continuous mode with and without recycling. Mixing time experiments were carried out in order to characterize batch mode for the liquid phase. Secondly, a systemic approach was used to identify the model parameters and characterize their evolution within the investigated operating conditions. and then to corroborate the reliability of the model by considering the experiments with recycling. For the gas phase, it was shown that each stage (R1 and R2) exhibited independent behaviour and could be modelled as a plug-flow reactor associated to j Continuous Stirred Tank Reactor, CSTR in series; the values of the model parameters (alpha,j) remained constant within the investigated operating range (VVM, Re and Re-A). For the liquid phase, the hydrodynamic behaviour of R1 and R2 could be represented by CSTR behaviour. A plug-flow reactor is thus proposed to describe the permeate compartment behaviour. (C) 2011 Elsevier B.V. All rights reserved.