Light intensity distribution in a photocatalytic reactor using finite volume

Abstract

A conservative variant of discrete ordinate model was used to solve the radiation transport equation. The model prediction was used to assess the effect of wall reflectivity, catalyst loading, and phase function parameter on the light intensity distribution in an annular heterogeneous photocatalytic reactor. For relatively low catalyst loadings, thewall reflectivity strongly influenced the light intensity distribution. However, for an optically thick medium, the wall reflectivity had very little or no effect. The volumeaverage light intensity distribution decreased rather sharply with the catalyst loading and an opposite trend was obtained for the local volumetric rate of energy absorption (LVREA). However, after the initial sharp increase, the LVREA appeared independent of catalyst loading. For nonreflecting reactor walls, phase function parameter did not show much influence. However, for the specularly reflecting reactor walls and optically thin mediums, a moderate dependency on the phase function parameter was observed. AIChE J, 50: 1273AIChE J, 50: -1288AIChE J, 50: , 2004 Keywords: photocatalytic reactor, radiant transport, light intensity distribution, wall reflectivity, light volumetric rate of energy absorption (LVREA) © 2004 American Institute of Chemical Engineers IntroductionThe involvement of radiation is the single most important factor that distinguishes the photochemical/photocatalytic reactor from the conventional thermally activated reactive processes. The rate of initiation (in case of photocatalysis, electron-hole formation) step in photochemical reaction is directly dependent on the light intensity (Cassano et al., 1995). Because the step of electron-hole formation is a fast one (time constant Ϸ 10 15 s Ϫ1 ), in a well illuminated reactor, it may be expected that light intensity will not be a rate determining step. However, it is highly difficult to maintain a uniform light intensity within a reactor space and the intensity distribution invariably determines the overall conversion and reactor performance. The radiation transport equation (RTE) that describes the light intensity distribution is an integrodifferential equation, and an exact analytical solution is possible only for highly ideal one-dimensional situations (Carvalho and Farias, 1998). For photocatalytic reactors, however, because of light scattering in the presence of titania particles, it is impossible to find an analytical solution for the RTE. Therefore, to reliably design a photocatalytic reactor, use of new and efficient numerical methods is desired to solve the RTE in the reaction domain of interest. Furthermore, an overall design of the reactor system will require a simultaneous solution of flow equations (e.g., Navier-Stokes equation). It is, therefore, important to investigate the photocatalytic systems using a computational fluid dynamics (CFD) approach so that the effects of flow and radiation attributes can be combined. Most of the current commercial CFD codes use finite-volume scheme to numerically solve the conserva...

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