Mass transfer limitations in photocatalytic reactors employing titanium dioxide suspensions I. Concentration profiles in the bulk

The problem of mass transfer limitations in slurry, photocatalytic reactors employing titanium dioxide alone and applied for pollution abatement is studied with simulation experiments resorting to an existing isothermal reactor and making use of a corrected and recalculated intrinsic reaction kinetics previously obtained from a complete reaction mechanism corresponding to the mineralization of dichloroacetic acid. The mathematical description of the reactor is made employing rigorous momentum, radiation and mass transfer models derived from fundamental principles. In the first part of the work the analysis is concentrated in the bulk of the fluid. External and internal catalytic particle (and possible agglomerations) mass transfer limitations are the subject of the subsequent study. The main explored variables were: (i) flow rate, (ii) catalyst loading, (iii) irradiation rates, (iv) virtual changes in one significant kinetic constant of the kinetic model, (v) total suspension volume, and (vi) virtual changes in the reactor illuminated length. Significant concentration gradients that could result in appreciable transport limitations derived from the intrinsic non-uniformity of the radiation field are observed. These concentration gradients are difficult to be avoided, and they can be eliminated only if the reactor is operated under fully developed turbulent flow or very strong mixing conditions. However, it can be concluded that when the photocatalytic reaction is not fast, employing catalyst loadings below 1 g L^−1, irradiation rates below 1.0×10^−7 Einstein cm^−2 s^−1 and very good mixing operation, it will be always safe to assume that mass transport limitations in the bulk of the fluid are inexistent.