Design of Photocatalytic Reactors

Photocatalysis is a photochemical reaction induced by photon–absorption of a solid material, a "photocatalyst", that remains unchanged during the reaction. Photocatalysis has a wide variety of applications, e.g., degrading contaminants in aqueous solutions and in air, oxidizing liquid hydrocarbons, and reducing carbon dioxide into valuable hydrocarbons. It has been successfully applied at lab scale and many advancements are achieved with respect to photocatalyst development and the effect of different parameters such as pH, temperature, catalyst loading, and light intensity on the photoreaction kinetics of various compounds. Yet, there are several issues to be resolved for the technology to be widely implemented at large scale. The current limited industrial application of the technology is attributed to, among other factors, the difficulty of quantifying and predicting the photonic efficiency at different steps of the chain of photocatalytic events. Moreover, the strategies for the scale–up of photocatalytic reactors are scarce. There is a lack of guidelines on how to carry out experimental studies, and how to design and operate photocatalytic reactors. This thesis focuses on addressing these issues in multiphase slurry photocatalytic reactors. In this work, we first focused on providing experimentalists with simple guidelines to properly measure kinetic data in well-mixed slurry photoreactors. Whereas in such reactors concentrations are independent of location, the light distribution may still be inhomogeneous. As the light travels through the photoreactor, it is scattered and absorbed by the photocatalyst particles and its intensity drops; since photons initiate the photoreaction, this results in a non–uniform reaction rate in the photoreactor. We calculated the local volumetric rate of photon absorption based on a so-called two-flux model. This model assumes that photocatalyst particles absorb and scatter photons, but scattering happens only in a direction opposite to the incident light. Based on the analysis of the rate of photon absorption, we developed analytical expressions that calculate the minimum optical thickness that is required to ensure no photon transmits through the reactor unused, both for low and for high photon fluxes. We concluded that for a reliable determination of the photoreaction rate, an optically differential photoreactor is needed. In such a photoreactor, the optical thickness is sufficiently small for the gradient in the rate of photon absorption to be less than 5%. In a photocatalytic reaction, the absorbed photons excite the electrons in photocatalyst particles, generating electron–hole pairs. The electron–hole pairs then initiate a set of redox reactions, or recombine to lose the absorbed energy to heat. Many photocatalytic processes require a supply of oxygen for the progression of the photocatalytic chain of events. The role of oxygen is to remove the excited electrons in order to suppress the recombination of electron–hole pairs. In practice, the aeration is done either externally in a recirculating–flow photoreactor, or internally via sparging air or oxygen into the slurry photoreactor. Considering that bubbles scatter photons significantly, the main research question was whether there is any advantage to separating the aeration and photoreaction units in a photoreactor setup. Thus, we aimed to determine at what gas fractions and bubble diameters, bubbles start having a significant effect on the photonic efficiency in a bubbly slurry photocatalytic reactor. Bubbles scatter the light mainly in the forward direction, and the two–flux model fails to consider that. To capture the scattering characteristics of bubbles, we developed a new optical model. We devised a bidirectional scattering model that accounts for scattering in both forward and backward directions. Based on the photon balances, we showed that for typical values of gas fraction and bubble diameter in bubbly slurry photoreactors, the effect of bubbles on the photon–absorption and photoreaction rate is negligible. Therefore, there is no advantage to separating the aeration and photoreaction units in a photoreactor setup. Moreover, the same guidelines for design and operation of two–phase slurries can be applied to bubbly slurry photocatalytic reactors. Following the analysis of photonic efficiencies in well–mixed photoreactors, we looked into the extent of diffusion limitations in unstirred photoreactors. As mentioned previously, the gradient in the rate of photon absorption results in a gradient in the photoreaction rate throughout the photoreactor. This consequently leads to a non-uniform concentration field in the photoreactor. Of course, vigorous stirring can eliminate such concentration gradients, but we find many examples of unstirred photocatalytic reactors for which neither forced nor natural convection is reported. Obviously, when the mass transport is not fast enough to keep up with photocatalysis, the overall reaction rate changes, and the measured kinetic data are obscured by diffusion limitations. Similar criteria were also developed for optically thin reactors. By applying a two–flux model, we showed that the effect of diffusion limitation in rectangular optically thick photoreactors is negligible when the Damkohler number based on reactor depth in the direction of incident light is smaller than the 10% of the product of optical thickness and the exponent that describes how the reaction rate varies with intensity. Finally, this thesis presents a case study for the scale–up of photocatalytic reactors for water remediation purposes. A successful implementation of photocatalysis at large–scale calls for an interdisciplinary view over the interplay between all the important parameters that affect the capture and utilisation of photons in a photoreactor Moreover, the engineering aspects of photocatalysis in terms of contacting patterns and mass transfer rates must be optimized to develop useful design procedures for the scale–up of photocatalytic reactors. In this case study, we chose the reactor configuration (i.e., rectangular slurry bubble column) as such to ensure a good mass transfer rate, a high photocatalytic surface area per reactor volume, and a high reactor surface–area–to–volume ratio for a good capture of sunlight. By implementing a bidirectional scattering model, we calculated the local rate of photon absorption and the photonic efficiencies for different steps of the photoreaction. The outcome of photonic efficiency studies revealed that the electron–hole trapping at the surface of the photocatalyst, and the surface reaction efficiency are the bottlenecks of the overall photonic efficiency. Future research efforts must be focused on improving these efficiencies. Despite the low photonic efficiencies, we showed that implementing the principles of process intensification, the large scale degradation of cyanide to below its allowable emission threshold set by European legislation is achievable. Throughout this thesis, we focused on implementing and developing simple models to capture the most relevant phenomena in (bubbly) slurry photocatalytic reactors. We developed analytical expressions and a set of easy-to-calculate criteria for the design and operation of photoreactors. Based on such criteria, this study helps answering some of the main questions related to analysis, design and scale–up of photocatalytic reactors. Finally, based on the analysis of photonic efficiencies, the limiting steps in the overall photonic efficiency were identified. Future research shall indeed focus on improving the efficiency of these limiting steps to make photocatalysis feasible for large–scale applications.Chemical EngineeringApplied Science