Wet air oxidation: solubility of carbon dioxide and reactor design

Aqueous wastes containing organic pollutants and process wastewater from the chemical industry can be efficiently treated by wet air oxidation, ie oxidation by molecular oxygen in the liquid phase, under high temperature (200-325 degrees C) and pressure (up to 150 bar). In Europe, only a 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). The essentials of this design are discussed. Usually, the column is divided into a number of cells. Each cell is a perfectly mixed reactor connected with the contiguous cells. Reliable models are available for hydrodynamics (axial dispersion, gas hold up), mass transfer, and chemical reaction kinetics. The thermodynamics of the gas phase components are determined with the Peng-Robinson equation of state, and the Henry law for solubility of gases describes the equilibrium conditions. However, additional data are needed for water solutions containing salts. A method that allows such a determination to be made by establishing rigorous balances on a batch autoclave is described; 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.