Design and Characterization of Photoreactors for Environmental Applications

Abstract

This work utilized a multiphysics approach combining computational fluid dynamics (CFD) and reduced-order modelling to perform intrinsic kinetic model fitting. Experimental results for the degradation of 4-chlorophenol (4-CP) on a flat plate photoreactor immobilized with TiO₂ were used. Using the fitted kinetic parameters, a simulation study examining both mass transfer effects and the absorbed radiation fields on a newly proposed surface-enhanced corrugated plate photoreactor (SECP) was conducted. CFD calculations using COMSOL Multiphysics were used to resolve the flow field as well as the local mass transfer fields by solving the incompressible continuity and Navier-Stokes equations along with the species transport equation with diffusion approximated by Fick's law. Monte Carlo ray tracing was used to solve the absorbed radiation fields on the reactive surface. A light-dependent Langmuir-Hinshelwood model was proposed as a suitable surface reaction rate. Nonlinear least squares regression was used to fit the kinetic parameters. Model predictions using the fitted parameters were found to agree well with the experimental data over a range of Reynolds numbers (431-5813) and light intensities (15-122 W m⁻²). A linear light dependence was found to fit the experimental degradation optimally. Oxygen was found to have a linear dependence on the degradation rate, leading to a simplification in the kinetic model. A baseline corrugated plate photoreactor was modelled for comparison. A fully coupled model was solved which included laminar flow, species transport, radiation modelling, and the intrinsic kinetics to compute the degradation of 4-chlorophenol over this geometry. Comparison of diffusive and normal radiation models were also conducted. Results revealed a slightly higher degradation overall due to a more uniform radiation field allowing for more efficient use of regions with sufficient mass transfer rates. The SECP reactor fitted with cavities and protrusions was modelled. The SECP reactor using a moderate depth-to-diameter ratio of 0.2 yielded higher degradation performance overall compared to the unmodified surface. This small improvement in the total consumption under the dimpled surface is mostly attributed to a higher catalytic surface area rather than any benefits to the mass transfer or radiation field. At the low end of intensity, the rate is kinetically limited by the available photon flux. As the intensity increased, the rate transitioned into a mass-transfer-limited regime. Results from this study show potential in using surface modifications to improve photoreactors, but significant geometric optimization is required to optimize the surface mass transfer augmentation and absorbed radiation field.