# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 3 Kinetics Models and Their Development
### Semi-Empirical Model
#### 3.2.2 Benzene and Its Substituents  
Although [PERSON] and [PERSON] [100; 101] deduced the reaction pathway of benzene decomposition in SCWG that benzene formed phenol and gas, directly or through naphthalene as an intermediate medium, into char and other liquid products, and mainly transformed to phenol under the oxygen-rich condition in SCWO. [PERSON] et al. [102] found benzene reacted almost exclusively by C\({}_{6}\)H\({}_{6}\)+OH\(\rightarrow\)C\({}_{6}\)H\({}_{5}\)OH+H and C\({}_{6}\)H\({}_{6}\)+OH\(\rightarrow\)C\({}_{6}\)H\({}_{5}\)+H\({}_{2}\)O, with the latter accounting for over 97% of the oxidation rate of benzene at 813 K and 246 bar. The C\({}_{6}\)H\({}_{5}\) benzyl radical captured O\({}_{2}\) to form C\({}_{6}\)H\({}_{5}\)O\({}_{2}\) and underwent unimolecular dissociation to form C\({}_{6}\)H\({}_{5}\)O phenoxy radical and OH\(\bullet\) with C\({}_{6}\)H\({}_{5}\)O\({}_{2}\)H as an intermediate in subsequent steps. Due to the stability of phenoxy radical, its quenching reaction led to the formation of phenol at low temperatures [32]. [PERSON] and [PERSON] [103] investigated the degradation pathway for phenol oxidation in SCW based on the analysis of the free-radical chemistry of phenol oxidation in both gas and solution phases. The degradation of phenol included two main competing reactions, dimer formation and ring-opening reaction, in which the formation of dimers including 2- and 4-phenoxyphenol, dibenzofuran and 2,2'-biphenol was caused by the reaction between phenoxy radicals. The gas-phase combustion reaction for phenol at above 1000 \({}^{\circ}\)C would decay at 710 \({}^{\circ}\)C [104], which was inconsistent with the reaction temperature of SCWO. The reaction products of SCWO were also inconsistent with the gas-phase combustion mechanism because the formation of peroxy radicals was reversed under the SCWO condition, indicating a large number of peroxy radicals in the SCWO reaction could not be generated by this mechanism. However, the mechanism of phenol for ring-opening in the liquid phase involved aldehydes and acids as intermediates [105], which may be applied under SCWO conditions since formic, glyoxalic, oxalic, propionic, hydracrylic, succinic, and maleic acids have all been detected in phenol SCWO. [PERSON] et al. [106] analyzed the effect of phenol concentration on reaction intermediates at constant oxygen content. The reaction product distribution varied with concentration and tarry material production was observed in high phenol concentration phenol, which was probably because the concentration of phenol did not affect the initial phenol decomposition reaction but the subsequent radical reaction. The high phenol concentration accelerated the additive reaction between aromatic compounds. Additionally, the reaction pathways for the SCWPO of phenols were also investigated [107]. [PERSON] et al. [107] developed the pathways and a kinetic model of partial oxidation phenol at 450 \({}^{\circ}\)C and 24 MPa, which included nine reaction paths and eight kinetic equations. The model precisely depicts the concentration changes of CO, H\({}_{2}\) and CO\({}_{2}\), in which CO concentration initially increased rapidly, peaked, and then dropped slowly, while H\({}_{2}\) and CO\({}_{2}\) concentration increased slowly after a rapid increase in the beginning. Figure 4 depicts the four types of main reactions for partial oxidation phenol, including (1) phenol oxidation (both dimerization and ring-opening), (2) acid oxidation, (3) acid gasification, and (4) gaseous products interconversion. The kinetics model again presented the experimental results well and had good predictions for the composition of the gas effluent. It is worth noting that the rate constant for dimers formation (k\({}_{1}\)) was extremely small, indicating phenol barely converted to dimers in the presence of oxygen.