The Development of a Novel Wet Oxidation Process for the Treatment of Organic Industrial Waste

Abstract

The application of wet oxidation to the treatment of industrial waste historically has been hindered by the high pressures (2-22 MPa) and temperatures (160-374°C) necessary for substantial oxidation. Advances in engineering methods and materials, in addition to increasingly stringent laws governing the disposal of wastes has prompted further investigation of these processes. In the study presented here, the utilisation of wet oxidation to the treatment of organic industrial waste has been investigated. Model compounds were used to determine the reaction conditions necessary for substantial oxidation. Work was then extended to samples of waste from the food processing and woolscouring industries, soil contaminated with polycyclic aromatic hydrocarbons and deinking waste from the pulp and paper recycling industry. In all cases wet oxidation was found to be an effective means of lowering the biological load or toxicity of the waste. This was measured as the reduction in chemical oxygen demand (COD) and biological oxygen demand (BOD5) of the waste streams. The major oxidation product was acetic acid, which largely accounted for the remaining biological oxygen demand of the treated samples. In order to lower the reaction temperature and pressure, the use of catalysts in both batch and continuous processes was investigated. While effective in lowering the conditions necessary for substantial oxidation, catalysis was found to be unsuitable when used with the traditional methods for wet oxidation. due to the difficulties associated with recovery of the catalyst. A novel method has been developed whereby the limitations in the existing processes are largely overcome. The feedstock of a wet oxidation process is generally non-volatile, while the major products of reaction, acetic acid, formic acid, water and carbon dioxide, are volatile under the same reaction conditions. Analysis of the vapour-liquid phase composition of the acetic acid-water system showed that the concentration of each were nearly equal in the liquid phase as in the vapour phase. The reactants and oxidation products are therefore able to be separated on this basis. A novel process was thus developed, whereby the non-volatile feedstock entered the reactor in t-he liquid phase, and the major products of oxidation were removed from the reactor in the vapour phase. Operation in this manner was found to have significant advantages. A smaller reactor may be used than in conventional processes, due to the continuous removal of excess water from the feedstock. The instantaneous rate of reaction remains high due to the constant removal of reaction products and addition of new feedstock to the same volume. A catalyst may be used without contamination of the bulk of the downstream product, thus decreasing the temperature, pressure and average residence time necessary for substantial oxidation. This method has resulted in the filing of a provisional patent and the development of a pilot plant to accommodate the process on a larger scale.