Modeling of Photochemical Processes in Continuous-Flow Reactors

Abstract

Despite the progress in the past decades in the area of light-driven reactions, only a limited number of photochemical reactions is implemented at large scale. [1,2] The major hurdle which needs to be overcome is linked to the scale-up strategy. [3] The limited light penetration (described by Lambert-Beer's law) rapidly decreases the transformation efficiency in large-scale batch reactors as only non-uniform light intensities are achieved in the vessel. However, using continuous flow reactors provides an avenue for efficient scale-up. Moreover, the characteristic reactor sizes on the micro-or milli-scale allow for a homogeneous light distribution in the reacting volume and advanced designs ensure sufficient productivity at the desired scale. [4] Another reason is the difficulty of modeling photochemical reactors. [3] Scaling-up can be realized by starting working with a laboratory-scale reactor and continue by gradually increasing the reactor size until reaching the desired productivity or by predicting the performance of the large scale reactor with mathematical modeling. Modeling is more precise and cost efficient, but is characterized by a higher complexity. [5,6] Consequently, most pilot-scale photoreactors are still designed using empirical or semiempirical methods. [7] Predicting the performance of a large-scale reactor of a different geometry than the one investigated in the laboratory requires knowing the intrinsic kinetic parameters of the reaction. [6,8] Alfano and Cassano described a methodology for scaling-up photoreactors which uses modeling at small and large scale: firstly for determination of the kinetic parameters and then for predicting the pilot-scale reactor performance. [8] This chapter aims to introduce the main concepts of a holistic scale-up strategy based on combined modeling and experiments. We will highlight the important steps of photoreactor modeling such as the kinetic model, radiance, mass and momentum balance etc., and which are the variables to be exchanged between them to ensure efficient and robust scaling. Furthermore, examples from various application fields of photochemistry, e.g. water treatment and synthetic chemistry, will be offered to illustrate the implementation of the described concepts. This chapter will appeal to both chemical engineers and chemists aiming to develop modeling as a tool for scaling-up photochemical reactors. Modeling continuous photochemical processes for photoreactor scaling-upAlfano and Cassano reported a scale-up methodology based on photoreactor modeling by coupling the radiation, mass and momentum balances with the kinetic model. [8] Figure 1.1 presents the main steps of this method and was adapted from the flow scheme reported by Ghafoori. [6]

Overcoming mass and photon transfer limitations in a scalable reactor: Oxidation in an aerosol photoreactor

“…Therefore, it gives an indication of how well the process is intensified. It has been used in recent studies to assess the photoreactor performance [2,[9][10][11].…”

Multiple wavelength (365–475 nm) complete actinometric characterization of Corning® Lab Photo Reactor using azobenzene as a highly soluble, cheap and robust chemical actinometer

“…Unfortunately, few actinometric protocols for the determination of the received photon flux density within flow microphotoreactors has been reported and most of them are based on costly actinometers with tedious procedures. [8][9][10][11][12][13] In a more fundamental context or for scale-up approaches, [14] actinometry is necessary for the determination of photochemical reactions quantum yield, [15][16][17][18] which is the physical parameter governing the reactivity. Indeed, the quantum yield represents the ratio between the number of molecules transformed and the number of absorbed photons at a certain wavelength in a period of time.…”