Display omitted •The relationship of energy consumption with bubble diameter was constructed.•The phenol conversion in a sub-millimeter bubble-dominated WAO reactor can be well predicted.•The spatial distribution of variables (including temperature, etc.) were evaluated.•The initial bubble diameter lines between 0.2 ∼ 0.4 mm for the phenol WAO process. The huge investment in equipment and energy consumption required for wet air oxidation (WAO) severely limits its industrial application due to harsh operating conditions with high temperatures and pressures. In this work, the energy consumption of phenol WAO was calculated considering gas compression, pumping, preheating and bubble generation. The relationship between energy consumption, bubble diameter and phenol conversion was constructed to evaluate the main influencing factors. It was found that sub-millimeter bubbles can effectively improve the WAO process by enhancing the oxygen mass transfer, resulting in a decrease in operating temperature and pressure. Considering the maximum conversion and minimum cost, the initial air bubble diameter for phenolic WAO is preferably between 0.2 ∼ 0.4 mm. The model was validated using experiments with a WAO reactor dominated by sub-millimeter air bubble, and good agreement was obtained between simulated results and plant data.