Total organic carbon disappearance kinetics for the supercritical water oxidation of monosubstituted phenols

We oxidized phenols bearing single -CH₃, -C₂H₅, -COCH₃, -CHO, -OH, - OCH₃, and -NO₂ substituents in supercritical water at 460 °C and 25.3 MPa. The observed effects of the concentrations of total organic carbon (TOC) and oxygen on the global disappearance rates for TOC were correlated by using power-law rate expressions. This kinetics study revealed that the rate of TOC disappearance is more sensitive to the oxygen concentration than is the rate of reactant disappearance. Additionally, the rate of TOC disappearance is always slower than the rate of reactant disappearance, with the ratio of these rates ranging from 0.10 to 0.65 for the different phenols at the conditions studied. The rates of TOC disappearance during supercritical water oxidation (SCWO) of these substituted phenols varied by nearly 2 orders of magnitude, showing significant effects from both the identity and location of the substituent. These substituent effects are greater for TOC disappearance kinetics than for reactant disappearance kinetics. Additionally, all of the substituted phenols exhibit faster TOC disappearance rates than does phenol. Accordingly, phenol is a good 'worst case' model compound for SCWO studies. The pronounced substituent effects for TOC disappearance rates indicate that the oxidation of a common refractory intermediate is not an important feature of the SCWO networks for these phenols at the conditions studied.