Guidelines for the assessment of the rate law of slurry photocatalytic reactions

The assessment of the rate law of slurry photocatalytic reactions appears to be a hard task, mainly because in this type of reactions the average rate of reaction, which is experimentally observed in a real reactor, could be very different from the “true” (intrinsic) rate of reaction, which cannot be measured directly. In the present work, it is shown how a proper mathematical model allows the utilization of the differential and/or the integral methods of kinetic analysis. The mathematical model must take into account not only the momentum and the mass balances, but also the radiative transfer equation. However, the discrimination among different proposed kinetic laws remains difficult since these laws usually predict very similar behaviors. In the case of “photodifferential” reactors, kinetic analysis can be carried out considering just the measured average rates of reaction without utilizing the mathematical model. For instance, it is shown that, with a photodifferential reactor, the fitting of the average rate of reaction vs. the substrate concentration gives a satisfactory appraisal not only of the kinetic parameters but also of the average rate of photon absorption. A sufficiently low value of the optical thickness (less than 2.5) is the main requirement to approach the photodifferential condition. Anyway, a strategic choice of the experimentally investigated values of the independent variables (e.g. substrate concentration and irradiance to the reactor) is important for the comprehensiveness of the kinetic analysis.