Mass transfer limitations in titanium dioxide slurry reactors

MARÍA DE LOS MILAGROS BALLARI; RODOLFO J. BRANDI; ORLANDO M. ALFANO; ALBERTO E. CASSANO

San Diego

Conferencia; The 16th International Conference on TiO2 Photocatalysis: Fundamentals and Applications (TiO2-16); 2011

Redox Technologies, Inc.

Mass transfer limitations in photocatalytic slurry reactors is a problem poorly studied. In most cases, a simple empirical verification is usually made by changing the stirring rate in batch reactors or the flow rate in continuous systems. A second evidence is also used. When analyzing the experimental data, sometimes it is found that they are well described by a zero order reaction with respect to the photonic absorption rate. This latter effect is usually explained as the absence of sufficient concentrationof reagents for maintaining the reactionafter a very high rate of generation of electrons and holes produced by a extremely intense photonic activation. In this work, a complete theoretical study is presented accompanied by experimental validations in a specially designed laboratory reactor. A continuous flow reactor, as part of a recycle with a precisely regulated flow rate was used. The reactor had a non-irradiated, very long entrance section to ensure fully developed flow operation. The first step is to have a well-known intrinsic reaction kinetics for the test reaction that will be used to validate the theory. For this purpose, the degradation of dichloroacetic acid was selected. Data were obtained under controlled operations of: (1) isothermal performance, (2) intermediate to low irradiation rates and (3) very high mixing conditions. The second step is to make sure that a precise evaluation of the local volumetric rate of the photon absorption can be obtained in the employed reactor. Thirdly, it is also necessary to take into account that under the most common experimental conditions, suspensions of titanium dioxide in water usually agglomerates forming porous grains having equivalent diameters much larger than the original elementary particles. Then, the problem was studied considering the following phenomena: (1) Concentration gradients along the radiation extinction direction. (2) External mass transport in the boundary layer surrounding each particle, for both laminar and turbulent flow operations. (3) Intra particle mass and photon transport limitations, defining with this purpose, a mass transfer effectiveness factor and a photon transfer effectiveness factor. Each of these phenomena was modeled mathematically using the corresponding transport theories. It was found that: (1) Concentration gradients in the reactor bulk can be avoided only when very intense stirring or very high flow rates are used. This result was experimentally corroborated. (2) External mass transfer limitations becomes important only for big particle or agglomerate sizes, normally not found in these systems. (3) For agglomerate sizes in the order of one micron or larger, intra particle mass transfer restrictions will be encountered. In these cases, it should be expected that the photonic effectiveness factor would usually be very much smaller than the mass transfer one. Experimental verification of this last effect is under way.