Phenol oxidation in supercritical water: Reaction kinetics, products, and pathways.

Phenol was oxidized in subcritical and supercritical water, using isothermal plug flow reactors and batch reactors. The temperatures examined were from 300 to 420$\sp\circ$C, and the pressures were from 188 to 278 atm. With plug flow reactors, residence times from 1.2 to 111 seconds were tested. Batch reactor work tested holding times from 5 minutes to approximately 8 hours. The initial phenol concentrations were between 1.7 $\times$ 10$\sp{-4}$ and 5.3 $\times$ 10$\sp{-3}$ M, and the oxygen concentrations were between 6.5 $\times$ 10$\sp{-3}$ and 6.4 $\times$ 10$\sp{-2}$ M at reaction conditions. The oxidation experiments covered essentially the entire range of phenol destruction conversions. Analysis of the kinetics data for the destruction of phenol revealed that the reaction was first order in phenol and one-half order in oxygen. The effect of pressure was evaluated using two methods. The first method considered the global reaction order for water to be nonzero with a pressure-independent rate constant. This resulted in a global rate law that was 0.7 order in water with an activation energy of 12.4 kcal/mol. In the second method, the rate constant was allowed to be a function of pressure. This resulted in a rate constant that had an activation volume of $-$1100 cm$\sp3$/mol and an activation energy of 14.7 kcal/mol. Carbon dioxide formation was also evaluated for this process, and the kinetics of carbon dioxide formation was correlated to that for phenol destruction. The carbon dioxide formation rate was much slower than that for the destruction of phenol. Phenol oxidation products included carbon monoxide, mono- and di-carboxylic acids, catechol, hydroquinone, p-benzoquinone, 2- and 4-phenoxyphenol, dibenzofuran, and dibenzo-p-dioxin. An analysis of the reaction product selectivities revealed that the phenoxyphenols and dibenzofuran were the major primary products. A reaction network consistent with the experimental data was established for the formation of higher molecular weight products. Dibenzofuran was determined to be one of the most persistent products, with a holding time of over 1 hour at 420$\sp\circ$C and 278 atm required to reduce it to less than 100 parts per billion.Ph.D.Applied SciencesChemical engineeringEnvironmental scienceHealth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128780/2/9135705.pd