Mass transfer limitations in slurry photocatalytic reactors: experimental validation

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

CHEMICAL ENGINEERING SCIENCE

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2010 vol. 65 p. 4931 - 4942

0009-2509

In the present work the existence of mass transfer limitations in slurry, photocatalytic reactors is studied. The experimental validation is made in a flat plate reactor that is part of a recycling system. The reactor is described with a mathematical model previously developed [Ballari et al., Chem. Eng. J. 136, 50 (2008)] considering a transient, two-dimensional mass balance (TDM). The complete reactor model was developed to show the existence of these effects that result from the occurrence of concentration gradients in the reaction space. They develop when these reactors are operated under some operating conditions whose effects should be always analyzed before assuming the validity of the existence of perfect mixing in the reaction space. Dichloroacetic acid (DCA) was the adopted model compound. To solve the TDM, a kinetic expression for the DCA acid was determined before under well mixed conditions [Ballari et al., Ind. Eng. Chem. Res. 48(4), 1847 (2009)]. The studied variables are: flow rate, catalyst loading and irradiation rates. The experimental data agree quite well when they are interpreted in terms of the two-dimensional model (TDM) regardless the operating mode. The perfect mixing model (PMM), normally employed to describe this and other types of slurry photoreactors, does not have the same level of universal application; i.e., it is restricted to perfect mixing, but in many cases if far simpler to use. However, it can be concluded that when the photocatalytic reaction is not fast, employing catalyst loadings below 1 g L-1, irradiation rates at the reactor wall below 1 × 10-6 Einstein cm-2 s-1 and good mixing operation (Re > 1700) it will be always safe to assume that mass transport limitations in the bulk of the fluid are inexistent. In a typical batch reactor the above flow conditions are equivalent to very intense mixing. If the catalyst concentration is increased, the mixing conditions should be improved in the same proportion. Within limits, higher solid loadings can be compensated with lower irradiation rates [Ballari et al., Chem. Eng. J. 136, 50 (2008)]. In addition, with the validated model, additional simulations are shown, operating the reactor under different virtual reactor thicknesses to widen the amplitude of the reached conclusions. These findings will be useful in kinetic studies to prevent the incursion in certain ranges of experimental conditions that could lead to erroneous interpretation of the obtained kinetic data.