Elementary Reaction Mechanism for Benzene Oxidation in Supercritical Water

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 C 6 H 5 OO predicted to be formed by C 6 H 5 reacting with O 2 . Through the adjustment of the rate coefficients of two thermal decomposition pathways of C 6 H 5 OO, 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.