Detailed modeling of oxalic acid degradation by UV-TiO2 nanoparticles: Importance of light scattering and photoreactor scale-up

•An integrated photoreactor model for the UV-TiO2 process has been advanced.•The model is validated for various TiO2 concentrations and scattering regimes.•An optimal TiO2 concentration for oxalic acid degradation is identified.•LVRPA and apparent quantum yield are both highly spatially anisotropic.•The UV-TiO2 process is heavily affected by both process and photoreactor scale-up.

A detailed computational fluid dynamics model is presented that integrates reactor hydrodynamics with advanced light models and UV-TiO2 advanced oxidation kinetics to yield the degradation of oxalic acid in a dispersed-phase photoreactor. Model predictions were first compared against experimental data obtained from the literature and subsequently used in a parametric study for investigating scale-up effects associated with both process and photoreactor variables. Investigated variables included: TiO2 concentration (5-400 mg L−1), initial oxalic acid concentration (0.9-32 mg L−1), lamp irradiance (100-10,000 W m−2), background fluid absorbance (0-30 m−1), reactor size (1/4-4 as relative scaling factor), lamp orientation (0-360°) and flowrate (2.5-10 m3 h−1). The analysis revealed that an optimum in oxalic acid degradation is observed when the TiO2 concentration was controlled in the 20-40 mg L−1 range (depending on lamp irradiance). While lamp orientation showed minimal impact, reactor size and flowrate emerged as key variables for photoreactor design. Moreover, an increase in initial oxalic acid concentration substantially reduced oxalic acid degradation performance observed at high loadings. Also, TiO2 activation and photoreactor degradation performance were impacted negatively by light competition with background fluid absorbance.

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