Supercritical water oxidation (SCWO) performances are limited by salt precipitation and corrosion when it comes to treating real wastes. A porous reactor has been developed to overcome these problems. It consists of a concentric double-wall reactor in which the corrosive reactants are maintained inside an alumina porous tube, whereas pressure resistance is ensured by a stainless steel external vessel. Hydrodynamics in the reactor is thought to be rather complex. Thus, a residence time distribution study was performed to investigate the flow pattern. The experiments were carried out in supercritical carbon dioxide for experimental consideration. According to the experimental results, the axially dispersed-plug-flow model as well as the tanks in the series model can be used to describe the tubular reacting zone when there is no radial flow through the porous barrier. The whole reactor can then be considered as a cascade of equal ideally mixed tanks with a stage radial feed. This proposed hydrodynamic model was validated by the experiments of ethanol oxidation in supercritical water. Our model enables us to estimate and optimize the conversion of a waste if its oxidation kinetics is known.