Development of a model for the wet air oxidation of phenol based on a free radical mechanism

A model for the wet air oxidation of phenol is proposed, based on elementary radical reactions that are known to occur in the oxidation process. By applying this model to associated experimental data, unknown rate constants have been calculated using numerical techniques to solve the system of differential equations that describes the proposed mechanism. The activation energy and pre-exponential factor for the elementary reaction between phenol and the phenoxyperoxyl radical (PhOO .) were estimated to be 47 +/- 3 kJ mol(-1) and 1.7 +/- 0.1 x 10(10) l mol(-1) s(-1) respectively, while values of 126 +/- 4 kJ mol(-1) and 4.7 +/- 0.2 x 10(10) s(-1) were found for the decomposition of phenoxy hydroperoxide (PhOOH). The operating parameters of temperature, oxygen concentration, pH and addition of free radical promoters (H2O2) have been studied, and in general, a good agreement has been obtained between experimental data and calculated concentration profiles. According to the proposed mechanism, it is shown that hydroxyl radicals (OH .) and phenoxyperoxyl radicals (PhOO .) are primarily responsible for the destruction of phenol, with perhydroxyl radicals (HO2.) having a less significant role to play in the process. The principles described in this paper may be applied to understand the wet air oxidation mechanism of other organic compounds. (C) 1998 Elsevier Science Ltd. All rights reserved.