Wet air oxidation: kinetics of reaction, carbon dioxide equilibrium and reactor design - an overview

Abstract

Aqueous wastes containing organic pollutants can be efficiently treated by wet air oxidation (WAO), i.e. oxidation by molecular oxygen in the liquid phase, under high temperature (200 to 325°C) and pressure (up to 150 bar). In western Europe, only an handful of industrial plants are in operation. Most of them were designed by extrapolating results from pilot plants. Only a very few studies have been devoted to the scientific design of such reactors (bubble columns). This paper discusses what needs to be known for this scientific design. The usual design methods assume that the column is divided into a number of cells. Each cell is a perfectly mixed reactor connected with the contiguous cells. Reliable data and models are available for hydrodynamics (axial dispersion, gas hold up) and mass transfer. Data are also needed for the kinetics of chemical reactions and are available from numerous sources, but have not yet been generalised. The thermodynamics (fugacities and enthalpy) of the gas phase can be determined with the Peng Robinson equation of state, and the Henry law for the solubility of gases in water describes the equilibrium conditions. But additional data are needed for water solutions containing salts. This paper describes a method that allows such a determination to be made by establishing rigorous balances on a batch autoclave. At present, these data are being used to implement a computer program aimed at determining the performance of a wet air oxidation reactor depending on the various operating parameters. Some typical profiles within the reactor are presented, establishing that the pH of the solution is a very important parameter.