(258h) Simulating Phenol Oxidation in An Annular Photoreactor Using Computational Fluid Dynamics

Advanced oxidation processes are a promising technology for degradation of organic compounds resistant to conventional treatments such as phenol. Computational fluid dynamics (CFD) has recently emerged as a powerful tool that allows a deeper understanding of photochemical processes in reactor engineering by solving the coupled momentum, mass and radiation balances. Little attention has been given to the homogeneous UV/hydrogen peroxide (H₂O₂) systems, which are simpler to operate and design than photocatalytic equipments. The current state-of-the-art of UV/H₂O₂process simulation in CFD commercial codes considers simplifications to its kinetic modeling, mainly the pseudo-steady-state hypothesis (PSSH) and equilibrium conditions for radical species.

This work aimed to investigate the UV/H₂O₂ process in an annular photoreactor using CFD and a more realistic kinetic model. The fluid dynamics was studied in a previous work. Next step was introducing the mechanism of phenol degradation proposed by Edalatmanesh, Dhib and Mehrvar (2008) into the CFD model. It considers a total of 15 reactions and 6 key components: phenol, hydrogen peroxide, hydroxyl radical (OH•), hydroperoxyl anion (HO₂^-), hydroperoxyl radical (HO2•) and superoxide anion radical (O2^•-); this kinetic model is based on dynamic equations for all species. The fluence rate field was calculated by the radial model. Simulations reproduced experimental data spanning a wide range of initial phenol concentrations (50-500 mg/L), H₂O₂/phenol molar ratios and three values for lamp power (36, 75 and 115 W, corresponding to 11.19, 14.89 and 17.66 W of 253.7 nm radiation emission). The volumetric inlet flow rate was fixed at 72 L/h. Convergence of the numerical solution was reached when the maximum of the scaled residuals of the equations reached less than 10^-4.

It was found that, for the 36 and 75 W lamps (diameter of 25 mm), 911,751 elements were necessary to reach grid independence, and 909,903 for the 115 W lamp (diameter of 37 mm). The velocity field depends on the volumetric flow rate: either it maintains a swirling motion through the whole reactor or might develop like a plug flow. The k-ε model did not represent the RTD data accurately, and the velocity field therefore, since it is not appropriate for swirling flows. The other turbulence models properly matched RTD data, especially the k-ω model.

The average conversion of phenol for a single pass is 14.3%, with an average deviation of 7.7% from experimental data. Fluence rate was symmetrical around the reactor axis. Due to the exponential relationship between distance and the fluid absorption coefficient, the radial model predicted a sharp decay of radiation intensity away from the lamp surface. It was possible to verify the concentration maps of the species through the reactor: due to the swirling inlet effects, reactants got concentrated close to the walls and migrated on the lamp direction along the reactor path. High radiation intensities close to the lamp surface created a layer around it where photolysis of H₂O₂ took place with higher rates. OH• radicals were generated in that layer and transported towards the outer wall by convection. This caused most of phenol to be consumed in the second half of the reactor and accumulation of the radical near the lamp and the reactor outlet, since the pollutant in this area was already oxidized. Satisfactory results indicated that CFD was an appropriate tool for analyzing this reactive flow.