A reaction network model for phenol oxidation in supercritical water

Abstract

Dilute aqueous solutions of phenol were oxidized in a flow reactor at 420, 440, 460 and 480°C at 250 atm. Phenol disappearance kinetics followed the trends exhibited by previously published data obtained at T < 420°C. By merging the two sets of data, a global rate low for phenol disappearance kinetics valid between 380 and 480°C was determined to be rate = 10 2.34 exp( −12.4/RT) [φOH] 0.85 [O 2 ] 0.50 [H 2 O] 0.42 . Undesired multiring products, whose formation was reported previously at the lower temperatures, continued to form in high selectivities at these higher temperatures. Reaction products were classified into three categories: dimers, gases, and a remainder that included products from ring-opening reactions. A global reaction network that describes the transformation of phenol into these product groups was developed. Steps in the network are: parallel oxidation paths for phenol that from dimers and ring-opening and other products, secondary decomposition of dimers of ring-opening and other products, and oxidation of the ring-opening and other products to carbon oxides. The experimental products yields were used to determine optimal values for the reaction orders and rate constants for each step in the network. This quantitative reaction model shows that dimerization is the dominant primary path for phenol consumption. High temperatures and long residence times reduce the concentration of dimers in the reactor effluent and maximize the gas yield. High oxygen concentrations also increase the gas yield. The quantitative reaction network model is consistent with previously published product yields for T = 380 – 420°C.