Computational fluid dynamics modelling of micro- and macro-scale photoreactors for advanced oxidation processes

This thesis presents innovative multiphysics Computational Fluid Dynamics (CFD) modelling approaches for the removal of water contaminants of emerging concern (CECs) in micro, bench, pilot and industrial scale photoreactors by Advanced Oxidation Processes (AOPs). One major reason for the slow penetration of photochemical AOPs in the industry has been the complex numerical description of these systems, thus, in this thesis high-fidelity solutions are proposed by coupling the fluid dynamics, the species transport, the kinetics and the transport of radiation in the photoreactors. The AOPs systems studied included the degradation of common pharmaceuticals (Acyclovir, Stavudine, Zidovudine, Methylisothiazolinone, Benzisothiazolinone and Isoxazole) by the H2O2 process in helical microcapillary film photoreactors (MCF), the removal of CECs (O-desmethyltramadol, O-desmethylvenlafaxine and gabapentin) in pilot and large-scale solar photo-Fenton raceway pond reactors (RPR) and the photocatalytic oxidation of 2-hydroxybenzoic acid in bench-scale annular and flat channel photoreactors. The most relevant transport phenomena in each case study were identified and answers to gaps in the literature were provided. This study provided tailored model hypotheses of fundamentally different AOPs, photoreactors and reactor scales to reduce the computational cost of the simulations while maintaining a high-fidelity of the model results. These approaches and hypotheses were validated by comparisons with experimental results of hydrodynamics and removal rate of the water contaminants. The UV/H2O2 process in a helical shaped MCF was studied to unveil the impact of secondary flow (Dean Vortices) on radial mixing and process intensification in a safe, ultrafast and controlled microreactor environment. This was achieved using a 10-fold less CPU-demanding CFD model that neglected the torsion effects in the helical MCF. Although the UV/H2O2 process has reached full-scale applications, accurate estimations of intrinsic kinetics constants are needed, and this thesis showed how CFD modelling can be used to determine highly accurate kinetics constants necessary to design efficient UV/H2O2 photoreactors. The CFD modelling of pilot and large-scale solar photo-Fenton RPRs was performed to evaluate the impact of mixing and hydrodynamics on the removal of CECs from a real secondary Wastewater Treatment Plant (WWTP) effluent. A multiphysics CFD modelling of the solar photo-Fenton process was shown for the first time, and a 41-fold less CPU-demanding model was proposed using momentum source domain rather than a transient-multiphase solution. The interest in the requirements use of RPRs for the treatment of secondary WWTP effluents by the solar photo-Fenton process has increased in recent years, and this study revealed that the design of hydrodynamics in large-scale RPRs must be carefully examined to reduce power consumption while increasing mixing performance. Lastly, CFD was used to the determine intrinsic kinetics constants of the photocatalytic oxidation of 2-hydroxybenzoic acid in TiO2 suspension in an annular and in a flat channel photoreactors. The Local Volumetric Rate of Photon Absorption (LVRPA) was computed by solving the Radiative Transfer Equation (RTE) using the Six-Flux model (SFM) and the Discrete Ordinates Model (DOM). Results showed, for the first time, that refractive and reflective phenomena should not be neglected when using DOM, however, SFM provided fairly accurate evaluations of the LVRPA, the Electrical Energy Per Order Reduction (EEO) and estimations of the optimal optical thickness. This gives SFM a significant advantage for reactor design investigations, since it can calculate the LVRPA in few seconds in a personal computer, while the DOM requires parallel processing in several minutes/hours. In summary, this thesis established CFD as a fundamental tool for the computation of highly accurate kinetics parameters, for the comparison of different photochemical AOPs, for example by the estimation of the EEO, and for AOPs intensification. Deviations from the ideal flow reactors (e.g., plug flow or perfectly mixed) were disclosed at micro and macro-scale photoreactors, and the conditions to diminish mass transfer limitations were identified. The combined approach of experimental and numerical simulations is crucial to accomplish the design of reliable photochemical AOPs in industrial scale, and this thesis has strengthened the use CFD simulations of AOP photoreactors by the proposition of modelling simplifying hypotheses at different photoreactor scales.