Modelling approaches to predict light absorption in gas-liquid flow photosensitized oxidations

Abstract

The photosensitized production of singlet oxygen is a benchmark reaction for characterizing flow photoreactors. Light absorption is the driving force of the photooxidation reaction and modelling represents a fast and cost-efficient method for its prediction and optimization. In this study, we report two models of photon absorption in single- and two-phase flow. Firstly, a three-dimensional ray tracing model was developed and validated by experimental 9,10-dimethylanthracene (DMA) photooxidation in presence of Rose Bengal. By comparing the simulated and experimental DMA conversion, the flow and illumination conditions affected by mass transfer limitations in liquid and gas–liquid Taylor flows were identified. Furthermore, the ray tracing model was used to compare the light absorption in Taylor, annular, and two bubbly flow configurations. At a low attenuation coefficient of 2.3 cm−1 a minor difference between flow patterns was observed irrespective of liquid hold-up. At the same liquid hold-up and between 26 cm−1 and 200 cm−1, the relative volumetric absorbed photon flux strongly depends on the flow pattern. Secondly, the ray tracing model was used as reference to develop a simplified absorption model which only requires the geometry and optical properties of the multiphase flow. The novel predictive method is based on the geometrical approximation of the optical pathlength in the liquid slugs, the liquid film and the liquid volume next to the bubble caps. As it requires minimum computational power and provides good accuracy, the geometry-based model has the potential to assist future automated optimization of gas–liquid photooxidations.