Heat Management for Process Intensification of Fast Exothermic Reactions in Microstructured Reactors

Nowadays, the production in fine chemical and pharmaceutical industry is mostly carried out in large scale batch reactors having typically dimensions of a few meters to satisfy the demand of the market. Even though this technology has been widely used and developed for centuries, it is by far not optimal for every type of reaction. For example, when working with exothermic reactions, the produced heat can’t be always fully evacuated. To avoid run-aways, high amounts of solvents are used to increase the heat capacity of the mixture, or semi-batch mode with a slow addition of one of the reactants. In both cases, the space-time yield, i.e. mass of product produced per unit of time and per unit of volume, drastically diminishes. One of the main enabling technologies allowing process intensification are the microstructured devices, characterized by high heat and mass transport rates due to the small characteristic dimensions (< 1mm). Using this type of equipment, almost isothermal conditions can be achieved while carrying out fast exothermic reactions (with characteristic reaction times down to tr ≈ 10 s). Thereby, the target throughput is reached by numbering-up, i.e. parallel connection of several identical microreactors. For very fast exothermic reactions, especially for quasi-instantaneous reactions, dimensions smaller than 100 μm are needed to prevent the formation of unwanted hot spots. As such small dimensions are not suitable for industrial scale due to possible clogging and high pressure drops, other solutions are warranted. The aim of this thesis is to develop alternative microstructured reactors enabling quasi-instantaneous reactions to be carried out under intensified conditions while suppressing the large hot spots. The work is divided into two main parts: determination of suitable strategies for the microstructured reactor design via numerical simulations (Chapter 3) and the experimental validation of the best microstructured reactor concept (Chapter 4-6). Three strategies for enhanced temperature control within microstructured reactors for quasi-instantaneous reactions are taken for analysis using numerical simulation: 1) reduction of hot spot temperature by increased axial heat transfer in the reactor wall, 2) by injection of one reactant in multiple points along the reactor length and 3) by continuous injection of one reactant through a porous wall in a concentric reactor geometry. The multi-injection reactor (option 2) is the most effective design since with an optimized dosing with only 4 injection points the temperature rise is 5-fold smaller as compared to the adiabatic temperature rise. Furthermore, the key design requirements for an efficient multi-injection reactor are identified: 1) complete mixing after each injection and 2) evacuation of the produced heat before reaching the next injection point. To experimentally validate the simulation results, in the subsequent chapter, an experimental method to monitor temperature in mic