Kinetics and Mechanism of Wet Air Oxidation of Butyric Acid

Low molecular weight acids are common intermediate reaction compounds formed during the wet air oxidation (WAO) of aqueous waste streams, and frequently their oxidation is the rate-controlling step in the overall reaction. The WAO kinetics of aqueous solutions of butyric acid was studied in a stainless steel 316 autoclave over a temperature range of 200-320 °C with a total pressure of 15 MPa of synthetic air, which provides an excess of oxygen. Kinetic models were developed with respect to various concentrations of butyric acid and chemical oxygen demand (COD). Oxidation reactions always obeyed a pseudo-first-order kinetic, but two different activation energies were needed to represent the temperature dependence in two ranges, namely 200-275 and 275-320 °C. This finding can be explained by the competition between two main reaction pathways in the oxidation of the acid, since both pathways take place simultaneously. A mechanism in accordance with the results obtained is proposed.