Experimental and Numerical lnvestigation of Competitive Chemical Reactions on Different Scales in Milli Reactors
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The application of modular continuous-flow equipment enables more efficient and greener chemical processes through precisely defined process windows and new synthesis pathways. Despite the advantages, continuous-flow processes are sparsely favored over large-scale batch processes in the chemical industry due to today's lack of detailed understanding of micro mixing performance and selectivity of chemical reactions in laminar flow regimes. This thesis reviews and proposes numerical and experimental methods for quantifying micro mixing and its influence on selectivity in competitive chemical rea...
The application of modular continuous-flow equipment enables more efficient and greener chemical processes through precisely defined process windows and new synthesis pathways. Despite the advantages, continuous-flow processes are sparsely favored over large-scale batch processes in the chemical industry due to today's lack of detailed understanding of micro mixing performance and selectivity of chemical reactions in laminar flow regimes. This thesis reviews and proposes numerical and experimental methods for quantifying micro mixing and its influence on selectivity in competitive chemical reactions. The transport properties of a laminar-flow liquid-liquid mixing problem with chemical reactions is investigated by experimental and numerical methods. A modular split-and-recombine (SAR) mixing unit is used as a benchmark geometry. First, direct numerical simulations (DNS) are performed for a range of Reynolds and Schmidt numbers (1¿Re¿400, 1¿Sc¿3600) to determine micro mixing length scales required for full problem resolution. For validation, two different experimental setups are introduced. Confocal laser-scanning microscopy (CLSM) reveals three-dimensional concentration distributions and pH distributions of an acid-base reaction, confirming the simulation results and coinciding with the data from the literature. Furthermore, the two-dimensional concentration distributions of reagents and products of the said reaction are recorded simultaneously with imaging UV-Vis spectroscopy. Second, the Villermaux-Dushman protocol (VD protocol) is investigated in detail with CFD simulations, experimental methods, and micro mixing models to determine micro mixing time scales and reaction selectivity. The VD protocol is a well-established method for quantifying micro mixing performance in aqueous solution inprocess equipment. The one-dimensional micro mixing model (incorporation model) is used to link the global selectivity to the micro mixing time of the mixing unit. The local concentration distributions obtained from the imaging UV-Vis spectroscopy highlight the importance of local flow phenomena in the overall mixing process, which cannot be captured by the one-dimensional micro mixing model. Finally, the developed methods are transferred to a reaction system of the nitrosyl iron complex (NIC) in organic solution, which enables the direct measurement of local selectivity distributions by imaging UV-Vis spectroscopy. The incorporation model of the new reaction system is validated by the micro mixing times determined with the Villermaux-Dushman protocol. The investigated SAR mixing unit shows a wide range of local selectivity and micro mixing time distribution, and the unfavored product is formed almost exclusively in areas above the global mean mixing time. The approaches applied in this thesis paint a detailed picture of the micro mixing processes in the modular mixing unit. The locally resolved selectivity and micro mixing time distributions yield multiple anchor points in equipment optimization. Detailed knowledge of the local mixing phenomena is essential to predict selectivities and consequently accurately designing efficient continuous processes that are capable of outperforming existing batch processes.
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