The Mass Transfer Limitation Problem in a Slurry Photocatlytic Reactor for Water Treatment

BALLARI, M.; BRANDI, R.J.; ALFANO, O.M.; CASSANO, A.E.

Buenos Aires, Argentina

Congreso; XXII Congreso Interamericano de Ingeniería Química y V Congreso Argentino de Ingeniería Química; 2006

Photocatalytic reactions constitute one of the Advanced Oxidation Technologies applied to water and air purification. These processes involve a solid semiconductor catalyst, regularly titanium dioxide, which is activated with ultraviolet light of the appropriate wavelength. These reactions are very attractive for treating pollution problems because a wide range of contaminants may be fully destroyed. The model contaminant chosen in this work is the Dichloroacetic acid (DCA). The intrinsic reaction kinetic of the photocatalytic decomposition of DCA has been determined by Zalazar et al. (2005) from a complete reaction mechanism and under kinetic control conditions. The majority of suspended solid photoreactor studies have considered perfect mixing conditions and have not deeply studied the possibility of mass transport limitations. But sometimes mass transfer restrictions appear and the kinetic regime control is not guaranteed. These conditions may lead to loses in the reactor performance effectiveness. In Mehotra et al. (2003) and Serpone (1997) results, these phenomena came visible when the reaction rate decreased at high catalyst concentrations and very likely, poor mixing conditions. No quantitative explanations based on the fundamentals of chemical reaction engineering have been provided. In the present work, a flat plate, slurry photocatalytic reactor employing titanium dioxide is modeled considering the possibility of the existence of a diffusion control regime. The external and internal mass transfer limitations were considered. Only for very large particles, not usually employed with unsupported titanium dioxide, interfacial external mass transfer limitations in the boundary layer surrounding the catalytic particle and internal diffusion limitation into the catalyst agglomeration can be detected. A transient, two dimensional model was proposed to describe the photoreactor performance that is part of a batch recirculation system. This model considers axial convection, diffusion in the characteristic direction of radiation propagation, a transient term and a sink term that corresponds to the heterogeneous intrinsic reaction rate. In addition, the Radiative Transfer Equation (RTE) in a participating media with radiation absorption and scattering is solved because it is necessary for characterizing the initiation step of the reaction kinetic expression. The main explored variables were: (i) flow rate, (ii) catalyst loading, (iii) irradiation rates, (iv) virtual changes in the kinetic constants of the kinetic model, (v) porosity of the catalyst agglomeration, and (vi) diameter of agglomeration. The main mass transport limitations are the result of concentration gradients in the bulk that can be avoided only with very strong mixing conditions (or very high flow rates) and rather low reaction rates (resulting from low catalyst concentrations and low irradiation rates). As part of this work, the transient two dimensional model was compared with a pseudo-steady state model and a perfect mixing model. The latter is only accomplished under very strong stirring. The optimal reactor operation conditions were determined to keep the photoreactor under the most efficient performance.