# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 3 Kinetics Models and Their Development
### Dckm
#### 3.3.3 Acetic Acid  
[PERSON] and [PERSON] proposed a DCKM for the oxidation of acetic acid in SCW using oxygen as an oxidant [31; 122]. The DCKM consisted of 19 elementary reactions that were divided into eight reaction families. A reaction family is a set of reactions, which have similar transition states and changes in the reactivities of family members that are largely due to changes in reaction energetics. As a result,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**\#**} & \multirow{2}{*}{**Reaction 1**} & \multicolumn{4}{c}{**Sensitivity Coefficients**} \\ \cline{3-6}  & & **CH\({}_{3}\)OH** & **CH\({}_{2}\)O** & **CO** & **CO\({}_{2}\)** \\ \hline
1 & CH\({}_{3}\)OH+OH\(\rightarrow\)H\({}_{2}\)O+CH\({}_{2}\)OH & \(-\)0.101 & 0.313 & 0.106 & \(-\)0.131 \\ \hline
2 & CH\({}_{3}\)OH+OH\(\rightarrow\)H\({}_{2}\)O+CH\({}_{3}\)O & \(-\)0.074 & 0.210 & 0.091 & \(-\)0.052 \\ \hline
3 & CH\({}_{3}\)OH+HO\({}_{2}\)\(\rightarrow\)H\({}_{2}\)O\({}_{2}\)O+CH\({}_{2}\)OH & \(-\)0.224 & 0.017 & 0.773 & 1.47 \\ \hline
4 & CH\({}_{2}\)O+HO\(\rightarrow\)H\({}_{2}\)O+HCO & 0.106 & \(-\)0.535 & 0.111 & \(-\)0.151 \\ \hline
5 & CH\({}_{2}\)O+HO\({}_{2}\)\(\rightarrow\)H\({}_{2}\)O\({}_{2}\)+HCO & \(-\)0.205 & \(-\)0.106 & 0.819 & 1.50 \\ \hline
6 & HO\({}_{2}\)+HO\({}_{2}\)\(\rightarrow\)H\({}_{2}\)O\({}_{2}\)O\({}_{2}\) & 0.394 & 0.036 & \(-\)1.42 & \(-\)2.74 \\ \hline
7 & H\({}_{2}\)O\({}_{2}\)(+M)\(\rightarrow\)HO+HO(+M) & \(-\)0.797 & 0.042 & 2.77 & 5.31 \\ \hline
8 & OH+H\({}_{2}\)O\({}_{2}\)\(\rightarrow\)HO\({}_{2}\)+H\({}_{2}\)O & \(-\)0.463 & 0.013 & 1.62 & 3.10 \\ \hline \hline \end{tabular} 1  
\end{table}
Table 4: Reactions with Largest Normalized Sensitivity Coefficients [66].  
changes in entropies between reactants and transition states in a reaction family are proportional to those in enthalpies. Individual rate constants were estimated by adjusting the pre-exponential factor for steric effects and estimating the activation energies for each elementary reaction, which were constrained to follow simple structure/reactivity correlations. The structure/reactivity model for the oxidation of acetic acid predicted the experimental results well at 350 \({}^{\circ}\)C, while underpredicting and overpredicting the data at 350 and 415 \({}^{\circ}\)C, respectively. The temperature effect indicated that the refinement of values for the activation energies rather than pre-exponential factors would further increase correlation.  
[PERSON] et al. [80] coupled a quartz capillary micro-reactor with a mass spectrometer to study the reaction mechanism of acetic acid in the SCWO process, which could detect the intermediates, free radicals and products produced in the early stages of the reaction at conditions above 23 MPa and 400-500 \({}^{\circ}\)C. At lower temperatures below 500 \({}^{\circ}\)C, SCW was not involved in the reaction and only used to maintain a fully mixed single-phase reaction environment. Furthermore, hydrogen peroxide was completely decomposed to oxygen by 450 \({}^{\circ}\)C, whereas subsequent degradation of acetic acid began at 470 \({}^{\circ}\)C, which was consistent with the fact that oxidation of acetic acid began primarily by O\({}_{2}\) rather than the extraction of hydrogen from acetic acid by OH\(\bullet\). [PERSON] et al. [79] also inferred that the decarboxylation of acetic acid could not be carried out through reaction (20), but by the hydrogen extraction reaction by O\({}_{2}\) in reaction (21) to form HO\({}_{2}\bullet\). However, the deduction that acetic acid was more reactive with O\({}_{2}\) rather than HO\(\bullet\) may be attributed to oxygen concentration being much higher than HO\(\bullet\) concentration in their study [79, 80]. Developments in computational chemistry and molecular dynamics will play an important role in predicting the reactivity of O\({}_{2}\) and HO\(\bullet\).