Catalytic wet air oxidation of phenol: Review of the reaction mechanism, kinetics, and CFD modeling

Advanced oxidation processes, specifically catalytic wet air oxidation (CWAO), are considered as useful and robust methods for the treatment of refractory organic compounds such as phenol in wastewater. This paper reviews reaction mechanisms, kinetics and computational fluid dynamics (CFD) modeling of CWAO of phenol. Different reaction mechanisms have been proposed by various researchers to account for possible intermediates during phenol oxidation. These mechanisms can be either direct or indirect; with the indirect path resulting in the formation of intermediates, while with the direct mechanism, no intermediates are formed, the reaction proceeds straight to carbon dioxide and water. Acetic acid is considered as the most stable intermediate of phenol oxidation. Power law and Langmuir–Hinshelwood models were used to account for the adsorption/desorption of the reactants on the surface of the catalyst. The reviewed studies show that phenol conversion increases with increased temperature, pressure, gas velocity and decreases with increasing liquid space velocity. The formation of hot spots next to the walls of the reactor leads to safety issues and may cause catalyst deactivation and reactor thermal run away. This prompted a review of liquid mal-distribution and the formation of hot spots inside the reactor using the CFD modeling technique. In most cases, liquid channeling was observed, resulting in the formation of hot spots and catalyst deactivation.