Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, February 2004.Includes bibliographical references.(cont.) at well-defined operating conditions and to develop. both microscopic and macroscopic models, ranging from regressed global models to an elementary reaction mechanism, to quantify MPA oxidation kinetics in supercritical water. MPA hydrolysis and oxidation rates were experimentally measured in a laboratory-scale plug flow reactor. The effects of MPA concentration (0.5 to 1.0 mM), oxygen concentration (1.0 to 3.8 mM), temperature (478 to 572⁰C) and pressure (138 to 277 bar) on oxidation rates were determined for residence times ranging from 3.0 to 9.5 s. Conversion due to hydrolysis was less than 6% after [tau]=7 s at all temperatures studied. For [tau]=7 s at stoichiometric conditions and P=246 bar, low conversion (X<30% at [tau]=10 s) was observed at T<503⁰C, while almost complete conversion (X=99%) occurred at T=571⁰C. The only phosphorus-containing product was phosphoric acid, while the carbon-containing intermediates, carbon monoxide and methane, were present in varying concentrations in addition to final carbon-containing product, carbon dioxide. Methane was only a minor product, with a carbon yield less than 20% at all experimental conditions. MPA oxidation rates varied with oxygen concentration and pressure (or water density), but were relatively independent of initial MPA concentration. A global MPA oxidation rate law was regressed from the data with its dependence on temperature, MPA concentration, oxygen concentration, and water concentration quantified ...Above its critical point (Tc=374⁰C, Pc=221 bar), the physical properties of pure water change drastically from liquid-like to dense gas-like behavior. Supercritical water is a nonpolar solvent with moderate densities (approximately 0.1 g/mL) and gas-like diffusivities and viscosities. Above 450⁰C, radical pathways dominate due to the higher temperatures and decreased ionic reaction rates when the ion-dissociation constant of water is less than 10⁻¹⁹. Supercritical water is employed as an oxidation medium for the destruction of dilute organic aqueous waste streams because organic compounds and gases are both soluble in supercritical water. Oxidation proceeds quickly and completely without interphase mass transfer limitations, with characteristic reaction times of one minute needed for total conversion of C/H/N/O organic compounds to water, carbon dioxide, and molecular nitrogen. With this as a motivation, the supercritical water oxidation kinetics of the model organophosphorus compound, methylphosphonic acid (MPA or PO(OH)₂CH₃), was the primary focus of this thesis. Organophosphorus oxidation in supercritical water is being considered as a destruction method for stockpiled organophosphorus chemical warfare agents. MPA is a refractory intermediate and its reaction kinetics are important for the complete oxidation of larger organophosphorus compounds. Previous experimental MPA oxidation studies focused on determining the conditions necessary to achieve high destruction efficiencies at excess oxygen and long residence times. The primary goal of our research was to improve the mechanistic understanding of MPA oxidation kinetics in supercritical water. Our approach was to experimentally measure MPA oxidation rates and product yieldsby Patricia A. Sullivan.Ph.D
