(684g) An Experimental Investigation of Parallel Free-Radical and Ionic Reaction Kinetics of Propane Oxidation in Supercritical Water

Supercritical water is a promising medium for reaction processes which can be used in future energy and manufacturing technologies. For systems operating near the critical point, parallel free-radical and ionic chemical reaction pathways can be active and are affected by mixture pH. While observed qualitatively in several studies, the complex balance and interaction of these dual reaction regimes in supercritical water is poorly understood.

The objective of this work was to experimentally investigate the activation and effects of base-catalyzed ionic reactions during propane oxidation in supercritical water, and to utilize these observations to develop a chemical model which includes parallel free-radical and ionic reactions. Experiments were conducted in a constant volume reactor system at 375ºC, 220 and 400 bar, for residence times 8 to 30 minutes. Mixtures were composed of 0.1 to 0.4 % propane by vol., oxygen with equivalence ratio of 0.75, and the remainder water or 0.1M NaOH in water (pH ~13). The chemical kinetic model will be created by expanding an existing free-radical model for propane oxidation to include potential ionic reactions, with rates determined empirically via genetic optimization.

Experimental results indicate that while addition of NaOH had a minimal effect on the rate of consumption of propane, it had a profound effect on the reaction products - which varied with pressure. For both pressures, the addition of NaOH significantly shifted reaction products away from alkanes/alkenes toward oxygenates (ethanol, methanol, acetone). These effects increased with pressure for ethanol and methanol as expected, but surprisingly decreased with pressure for acetone. Broadly, the oxygenate shift observed here is in agreement with previous studies of both decomposition and oxidation processes in near critical water, suggesting a common underlying mechanism. Model development is underway, and will focus on identification of various hypothetical ionic pathways and rates which will motivate future studies.