Determination of kinetic constants of a photocatalytic reaction in micro-channel reactors in the presence of mass-transfer limitation and axial dispersion

The main objective of this article is to evaluate quantitatively the kinetic constants of a photocatalytic reaction in micro-channel reactors despite the presence of a radial concentration profile (mass-transfer limitation) and axial dispersion. Photocatalytic micro-channel reactors with immobilized titanium dioxide as photocatalyst have been fabricated using stereolithography process. The photocatalytic degradation of salicylic acid is investigated as a function of rectangular micro-channel size, contaminant concentration, flow rate and incident UV light intensity. All the micro-channel reactors exhibit the same tendency. Higher degradation is observed for high incident light intensities, low pollutant concentrations and flow rates. A simple equation for the determination of the kinetic constants during the photocatalytic degradation is reported. The equation includes the hydrodynamic properties and a surface reaction model for the photocatalytic reaction (monomolecular Langmuir–Hinshelwood kinetics). In the experimental system, we demonstrate that some external mass-transfer limitation and axial dispersion occur. They are included in the modeling with calculated values of the mass-transfer coefficient of salicylic acid from the solution to the catalyst surface and the axial dispersion coefficient. A single couple of values of the reaction rate constant k and the adsorption equilibrium constant K represent properly the experimental degradation ratios for all reactor dimensions, flow rates and pollutant concentrations. The major parameter that controls the values of the reaction rate constant is the incident light intensity. The dependence of the reaction rate on the incident light intensity is first order.