Water density effects on supercritical water oxidation.

Abstract

Phenol and methanol were oxidized in supercritical water using two isothermal plug-flow reactors to obtain data that would allow the most complete examination to date of the effect of water density on SCWO kinetics. Supercritical water oxidation (SCWO) data was obtained at 380--500°C, 103--310 bar, with water densities ranging from 1.6--28 mol/L. Reactor residence times ranged from 2--295 seconds. In the phenol experiments, the initial reactant concentrations were [&phis;OH] = 0.18 +/- 0.03 mmo1/L and [O₂] = 6.4 +/- 0.08 mmol/L. The initial reactant concentrations for methanol SCWO were [MeOH] = 1.04 +/- 0.05 mmol/L and [O₂] = 7.95 +/- 0.40 mmol/L. The deviation from the mean is at the 95% confidence level. Two different reaction media were used to decouple the total system pressure from the water density. The first reaction medium was pure water and the second was a 1/3 helium-2/3 water by moles mixture. Using these reaction media, it was determined that both phenol and methanol SCWO rates are affected by water density and not the total system pressure. For both methanol and phenol SCWO it was found that the water density can inhibit and accelerate the oxidation process, with the nature and magnitude of the effect of water being a function of the temperature and water density. A rate law was developed for the disappearance of phenol. This rate equation is the only one to date that captures the inhibitive and accelerative effect of water density on phenol SCWO. Detailed chemical kinetic models were used to explore the possibility that water participating in phenol and methanol SCWO as a reactant, product, or collision partner could explain the density effects observed. These roles did not appear to be potential explanations for phenol, but they did explain almost the entire density effect observed experimentally for methanol SCWO at 500°C. Several other chemical and physical phenomena were also examined in an attempt to determine why water density inhibits and accelerates phenol SCWO. It was determined that phenol SCWO most likely takes place via free radical chemistry and not ionic chemistry. Retained as possible explanations for water's ability to inhibit and accelerate phenol SCWO were dielectric constant effects on transition state stabilization, phenol dissociation into phenolate, and the possibility of partial diffusion control of a rate-determining reaction.