The Irradiation Field in Photocatalytic Reactors

Abstract

The evaluation of absorption photon rates in slurry reactors is a rather challenging task since light can experience a combination of reflection, scattering and absorption in the TiOi particle suspension. 4.1.1. Determination of Absorption of Radiation Several approaches with different degrees of complexity are reported in the technical literature to estimate the radiation distribution and consequently the light absorption in slurry reactors. a) Measurements at the Inlet Boundary. This approach relies on radiative flux radio-metric or actinometric measurements performed in the near reactor wall region. This method assumes that all the radiation entering the reactor is fully absorbed inside the heterogeneous medium. This approach does not consider heteroge-neous medium light scattering and therefore it only provides an estimation of the upper limit of the absorption of radiation (Matthews and McEvoy, 1992; Pruden and Ollis, 1983; Turchi and Ollis, 1989). b) Homogeneous Actinometry. This method makes use of homogeneous photo-chemistry to evaluate the radiation absorbed in the entire reactor volume. Us-ing this approach, the photocatalyst is removed and irradiation is evaluated via homogeneous actinometry using a liquid phase free of TiOi particles. This is equivalent to neglecting the particle radiation scattering in the evaluation of light absorption (Hahnemann et al., 1991; Mills and Hoffmann, 1993). c) Partial Application of the Radiation Transfer Equation (RTE). This is a more complex method that employs the Beer-Lambert equation with true absorp-tion and scattering coefficients and a radiation model for the source to estimate the amount of light absorbed in the heterogeneous medium. True extinction 63 64 CHAPTER 4 coefficients are determined from special spectrophotometric measurements. This approach tends to underestimate the Hght absorption since the radiation scatter-ing by the semiconductor particles is included as radiation losses (Brucato and Rizzuti, 1997a and 1997b; Martin et al., 1996a and 1996b). d) Radiation Balance and Measurements at the Outlet Reactor Boundaries. Using this approach, the light absorption is determined via the Beer-Lambert equa-tion for the media with apparent or effective extinction coefficients obtained via spectroscopic measurements. A combination of homogeneous actinome-try and photodetectors, placed at the reactor walls, is used to measure exiting radiation at the system boundaries (Brandi et al., 2000). On this bas…