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The use of laboratory-scale equipment to measure intrinsic oxidation kinetics in supercritical water environments was evaluated in this study. The objectives were two-fold:  (1) to compare the use of hydrogen peroxide with dissolved oxygen as an oxidant and (2) to characterize the dynamics and intensity of mixing organic reactant and oxidant streams. Methanol was used as the model organic as the oxidation rate exhibits a first-order dependence according to extensive earlier studies. No statistically significant difference was observed in the reaction rates or product distributions for the use of either dissolved oxygen gas or hydrogen peroxide that was preheated and fully decomposed before mixing with methanol at supercritical water conditions (500 °C, 246 bar). The intensity of mixing was shown to be an important factor in determining effective mixing times for the reactant and oxidant. Although hydrodynamic effects are certainly dependent on the design and geometry of the mixing tee in the reactor system, fully turbulent (Re > 10 000) cross-flow between entering oxidant and organic streams was found to reduce mixing times to 1 s or less.

Supercritical water (SCW) benzene oxidation data were modeled using a published, low-pressure (<1 bar) benzene combustion mechanism and submechanisms describing the oxidation of key intermediate species. To adapt the low-pressure, gas-phase benzene combustion mechanism to the lower temperature (<700 °C or 975 K) and higher pressure (>220 bar) conditions, new reaction pathways were added, and quantum Rice−Ramsperger−Kassel theory was used to calculate the rate coefficients and, hence, product selectivities for pressure dependent reactions. The most important difference between the benzene oxidation mechanism for SCW conditions and those for combustion conditions is reactions in SCW involving C6H5OO predicted to be formed by C6H5 reacting with O2. Through the adjustment of the rate coefficients of two thermal decomposition pathways of C6H5OO, whose values are unknown, the model accurately predicts the measured benzene and phenol concentration profiles at 813 K (540 °C), 246 bar, stoichiometric oxygen, and 3−7 s residence time. Comparison of the model predictions to benzene SCW oxidation data measured at several different conditions reveals that the model qualitatively explains the trends of the data and gives good quantitative agreement with no further adjustment of the rate coefficients. For example, the model predicts the benzene reaction to within ±10% conversion at temperatures between 790 and 860 K (515 and 590 °C) at 246 bar with stoichiometric oxygen and at pressures from 139 to 278 bar at 813 K (540 °C) with stoichiometric oxygen.

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