Co-oxidation effects of methanol on acetic acid and phenol in supercritical water

Methanol acting as a co-oxidation component was introduced in supercritical water oxidation experiments of acetic acid and phenol which were identified as the most common and refractory intermediates during supercritical water oxidation of many complex organics. Experiments were performed in a tubular flow reactor at 25 ± 0.5 MPa. The concentrations of acetic acid and phenol were constant at 0.17 and 0.067 mol/L, respectively, with varying methanol concentrations ranging from 0.0313 to 0.6563 mol/L. Computational simulations based on the elementary reaction model for the supercritical water oxidation of two binary mixtures were also carried out to understand the co-oxidation mechanism of methanol for these two compounds. For the binary mixture of methanol/acetic acid, the experimental results showed that the acetic acid conversions in the mixture were higher than those in the absence of methanol, and as the methanol concentration increased, the accelerating effect was more notable. For the methanol/phenol mixture experiment, a low concentration of methanol (0.0313 mol/L) was found to have no measurable effect on phenol conversion, while higher concentrations (0.1563 and 0.3125 mol/L) caused increases in phenol conversions. Calculation results based on the elementary reaction model could reasonably reproduce the experiment trends. Model analyses showed that the methanol-derived HO₂·and OH· radicals seemed to play an important role in accelerating acetic acid or phenol decomposition.