A double-wall reactor for hydrothermal oxidation with supercritical water flow across the inner porous tube

Supercritical water oxidation (P > 22.1 MPa, T > 647 K) is an efficient process to treat hazardous organic compounds: high destruction rates ( > 99.9%) with no NOx production, rapidity and a good confining of the reaction. Its performances are limited by salt precipitation and corrosion. A new reactor has then been developed to solve these problems. It consists in 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. A water flow through the porous pipe prevents sticky solid particles from depositing on the wall. The performances of this reactor were investigated. At 723 K and 25 MPa, the destruction of methanol, used as a model compound, reached 99.9%. Due to high thermal gradients generated by the exothermic reaction, the pipe which plays an important role in the decrease of salt precipitation and corrosion can be broken. Thus, its behaviour must be controlled in-situ. The pressure drop measurement across the porous wall was used to check whether the inner pipe was still intact. In fact, the experimental results show that supercritical water flow through the tube follows the Darcy's law. The experiments also confirm that the porous medium permeability is a characteristic constant of this medium. The permeability value remains equal whichever the fluid used, liquid water or supercritical water. In addition, the pressure drop measurement across the porous wall allows the control of the tube integrity. (C) 2003 Elsevier B.V. All rights reserved.