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Reaction Engineering

Controlling photochemical reactions with the objective of high yields and high process efficiency is more challenging as the same task for thermally initiated reactions.[1;2] This stems from the requirement to consider the radiation field in addition to all parameters that are relevant for conventional reactions. This leads to an increase of the number of variables relevant for reaction control. Hence, reaction engineering of photochemical reactions is a challenging task. The concerns raised on photochemical technology can be attributed to a large extent to this.

A typical sequence of points to be considered during the development of photochemical processes is compiled in Figure 1. The prerequisite for a thorough design of such processes is the knowledge of the reaction mechanism (or at least a reasonable hypothesis), the absorption spectra of the relevant species and the solvent as well as the quantum yield of the reaction. The emission spectrum of the light source has to be matched with this information. Subsequently, the first design of the reactor geometry can be chosen, preferably under consideration of the geometry of the light source. For this first process design the (global) mass, radiation and heat balances can be used to set up a mathematical model of the photochemical process. Under optimal conditions, the BEER-LAMBERT-law can be used to describe the radiation field sufficiently detailed. Depending on the complexity of the reactor geometry and therewith the positioning of the light sources, modeling of the radiation field might be required to support modeling of the complete process. By implementing the technological and economic constraints, a numerical solution can be obtained that gives insights on the required reactor design, reactor operation, possibilities of process optimization and scale-up. As for all numerical methods, the gained results have to be validated against experimental data. As a consequence of the complex interactions involved as well as uncertainties in experimental or physicochemical data, an experimental verification of the data predicted by the models might be required for an intermediate plant size.

For simple reactions, where irradiation of the reactant leads to excitation followed by reaction of only this reactant, solutions for the above described balances can be obtained easily. This is not the case for reactions that are only initiated by a photochemical step or where sensitization drives the reaction. For processes using such photochemical reactions, mass transport effects become relevant. For optimal process control it is required to minimize the impact of secondary reactions that can become dominant in dark zones of the reactor. Hence, either mass transport has to be intensified to such an extent that conditions equal to an ideally mixed tank reactor can be assumed, or the reactor geometry has to be changed to avoid dark zones. For reactions where radicals are involved, consideration of unwanted side reactions becomes even more important, because the resulting byproducts can deposit at the reactor wall. This leads to a filter effect, that increases with time. Consequently, reaction conditions, i.e. the photon flux received in the reaction solution, change and process control is lost. It becomes clear, that sophisticated reaction engineering is crucial for conducting photochemical processes efficiently.



Figure 1: Reaction engineering aspects for photochemical processes.[1]


  1. BÖTTCHER, H.: Technical Applications of Photochemistry, 1991, Deutscher Verlag f?r Grundstoffindustrie, ISBN 978–3–3420–0627–5.
  2. BRAUN, A. M.; MAURETTE, M.-T.; OLIVEROS, E.: Photochemical technology, 1991, Wiley, ISBN 0471926523, DOI: 1002/ange.19921041147.

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