Kinetics of wet air oxidation of phenol and substituted phenols

Abstract

Wet air oxidation (WAO) of aqueous solutions of phenol and substituted phenols namely, o-, m- and p-chlorophenols, o-, m-cresols, o- and p-methoxyphenols, o-ethylphenol and 2,6-dimethylphenol, were carried out. The process was studied in a 1 litre stainless steel autoclave at temperatures in the range of 150-180°C. The oxygen partial pressure was varied from 0.3 to 1.5 MPa and the initial phenol concentration was 200 mg/l. The oxidation of phenols in water involves a free-radical mechanism and proceeds in two steps. The oxidation reaction was found to be first order in oxygen and also first order with respect to phenolic substrates in both steps. The values of activation energy were found to be in the range of 12.4 × 10³-201 × 10³ kJ/kmol. The conditions have been found under which the overall oxidation reaction becomes reaction controlled or mass transfer controlled. The values of mass transfer coefficient have been obtained. The data based on bench scale shows wet air oxidation of phenols can achieve destruction efficiencies exceeding 99.9%. The reduction of COD during oxidation of all phenols was also measured. Greater than 90% COD reduction was achieved. Some aspects of the process design of the oxidation reactor have been discussed.