Performance evaluation of light emission models in light attenuating media

Abstract

The light distribution models developed in the literature have been mostly tested by conducting experiments in an annular reactor full of either air or deionized water, where the attenuating coefficients of the media were assumed to be zero. Although many of these models have predictive capability of the effect of absorbing media according to the Beer-Lambert law, their real performance in light attenuating media has seldom been investigated. In this study, the predicted local light intensities by three emission models in an attenuating medium were compared with experimental data. Potassium ferrioxalate actinometry was used to measure the light intensity profile within the reactor, and ozone at various concentrations was used as an attenuating medium, since it has a large molar absorption coefficient in the ultraviolet region of the electromagnetic spectrum. In addition, ozone is used as an external oxidant in many photochemical reactions. It was found that the predicted values using all three models were significantly lower than the experimental data, especially at high concentrations of ozone, while the performance of the emission models improved markedly at low concentrations of ozone. The discrepancy between the experimental data and the predicted values may be due to two reasons: (1) the Beer-Lambert law was developed for parallel light fields, and (2) violation of the prerequisite of the Beer-Lambert law (i.e., applicability of the law only at low concentrations). Although not discussed fully in the original Beer-Lambert law, it is frequently observed that the law is not obeyed when the light path length is relatively long and when light-absorbing species are present in the quartz cell of a spectrophotometer. The same phenomenon occurred in the test annular reactor, where the performance of the emission models worsened with the distance away from the light source. Thus, the application of light models in media with non-negligible light attenuating coefficients should be carefully reconsidered. Copyright © 2005 International Ozone Association.