Residence time distribution measurements in an external-loop airlift reactor: Study of the hydrodynamics of the liquid circulation induced by the hydrogen bubbles

Abstract A detailed study of the residence time distribution (RTD) analysis of liquid phase has been performed in an external-loop airlift reactor of 20 L nominal volume, regarded as a global unit and discriminating its different sections (riser, gas–liquid separator and downcomer) using the tracer response technique. The reactor was used as an electrochemical reactor in order to carry out the electrocoagulation/electroflotation (EC/EF). The gas phase created in the riser is the hydrogen produced by water electrolysis. In order to use this reactor for a continuous EC/EF, hydrodynamic studies were carried out to control the operating conditions and to help modelling the electrocoagulation. Current density, position of the electrodes in the riser and the volumetric liquid flow (inlet flow) are the key parameters for the hydrodynamics. The experimental results revealed that both in the downcomer and the riser–separator zones, the flow model is axial dispersion. Interesting results were obtained: – The superficial liquid velocity (ULd) at the downcomer, decreased when the volume inlet flow increased (0 – The Peclet number obtained in the downcomer was correlated to the current density and the electrodes position. – In the riser–separator zone the Peclet number decreased with the superficial liquid velocity in the riser indicating that the dispersion increased with an increase of turbulence created in the separator by an increase of liquid velocity. – The percentage of flow that quits the reactor without reacting increased when the main flow increased and the current intensity decreased. The global RTD can be reconstituted by the signal resulting from the junction and that from riser–separator and downcomer zone by using the convolution technique. The experimental results confirm this reconstitution. The experiments confirm also that the liquid crosses the reactor without achieving loops in the case of the continuous flow.

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