Oxidation of benzene in supercritical water : experimental measurements and development of an elementary reaction mechanism

Abstract

Supercritical water oxidation (SCWO) typically refers to a waste treatment or remediation process and derives its effectiveness from the unique solvent properties of water above its critical point. When organic compounds and oxygen are brought together in water well above its critical point of 221 bar and 374°C, the oxidation of the organic is rapid and complete to carbon dioxide and water with heteroatoms such as Cl, S and P converted to their corresponding mineral acids which can be neutralized using a suitable base. The research presented in this thesis addresses the general goal of characterizing the mechanisms and kinetics of reactions of model organic chemicals in supercritical water. This goal is achieved through a detailed experimental and theoretical investigation of the oxidation of benzene, a representative model aromatic compound, in supercritical water. Such basic research benefits the field by providing a better understanding of the SCWO process and can lead to better and more efficient designs of commercial waste treatment systems. In preparation for experiments on the SCWO of benzene, an investigation was undertaken to characterize the effects of mixing and oxidant choice on laboratory-scale kinetic measurements. The apparent induction time, previously reported in SCWO kinetics of various model compounds measured in the MIT laboratory-scale tubular reactor system, was found to be influenced by the geometry and flow conditions within the mixing region at the reactor entrance. Redesign of this mixing region led to a reduction in the apparent induction time measured during methanol SCWO from 3.2 to 0.7 seconds. In order to realize higher concentrations of oxygen in the reactor, the use of hydrogen peroxide as an oxidant was explored. The oxidation rate of methanol was found to be the same using hydrogen peroxide or dissolved oxygen, thus demonstrating the use of aqueous hydrogen peroxide solutions as a viable means of introducing molecular oxygen in situ into the laboratory-scale SCWO reactor system. Oxidation and hydrolysis reactions with benzene were thoroughly investigated in supercritical water using a laboratory-scale, plug-flow reactor system. Little to no conversion of benzene occurred in supercritical water at temperatures between 530 and 625°C by a hydrolysis pathway (in the absence of oxygen) for residence times up to 6 s. Oxidation reactions were studied at temperatures ranging from 479 to 587°C, pressures of 139 to 278 bar, reactor residence times from 3 to 7 s, and initial benzene concentrations of 0.4 to 1.2 mmol/L, and oxygen concentrations ranging from 40% of stoichiometric oxygen demand to 100% excess oxygen. The oxidation rate was found to be 0.40±0.06 order in benzene and 0.18±0.05 order in oxygen with an activation energy of 240± 10 kJ/mol. The primary oxidation product at all reaction conditions and levels of benzene conversion was carbon dioxide. Other important oxidation products were carbon monoxide, phenol and methane. Trace levels of additional light hydrocarbon gases and single- and multi-ringed aromatic species were detected as well. Prior to the theoretical investigation of benzene SCWO using an elementary reaction mechanism (ERM), the effects of uncertainty in the input parameters of these ERMs on their predictive capabilities was explored for hydrogen oxidation in supercritical water. Two methods, the Detenninistically Equivalent Modeling Method (DEMM) and Monte Carlo simulations, were applied for this purpose. Analysis revealed the presence of considerable uncertainty in the predicted species concentration profiles arising from the reported uncertainties in the forward rate constants and species enthalpies of fonnation. For example, at the point of maximum uncertainty, the predicted concentrations of hydrogen and oxygen deviated by ±70% from their median values at the upper 97.5% and lower 2.5% probability contours. Model predictions were found to be highly sensitive to two relatively uncertain parameters: the MIJ of H02 radical and the rate constant for H202 dissociation. An elementary reaction mechanism for the supercritical water oxidation of benzene was developed to provide mechanistic insights regarding key reaction pathways. An available, low-pressure combustion mechanism was adapted to the lower temperatures and higher pressures of SCWO through the addition of new reaction pathways and the calculation of the rate constants of pressure dependent reactions using quantum Rice-Ramsperger-Kassel (QRRK) theory. The resulting mechanism, after adjustment, accurately reproduces the exoerimentally measured benzene and phenol concentration profiles at 540°C and 246 bar with stoichiometric oxygen. Additionally, a comparison of the model predictions to benzene SCWO data measured at conditions other than those to which the model WuS fit revealed that the model qualitatively explains the trends of the data and gives good quantitative agreement at many conditions. For example, the model predicts the measured benzene conversion to better than ± 10% conversion at temperatures between 515 and 590°C at 246 bar with stoichiometric oxygen and at pressures from 139 to 278 bar at 540°C with stoichiometric oxygen. The most important difference between this benzene SCWO mechanism and those previously developed for combustion conditions is the inclusion of reactions involving the C6H500 radical. Without their inclusion, the predicted oxidation rate of benzene was too fast and the concentration of carbon monoxide was incorrectly predicted to exceed that of carbon dioxide.