Complex Gas-Liquid Reactions with Mass Transfer in Coiled Flow Inverters

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Taylor bubble flow in capillaries is often used in chemical reaction technology. Due to complex fluid dynamics, it is only partly understood regarding mass transfer and selectivity of complex chemical reactions. Particularly in helical capillaries, the additional Dean flow complicates the situation, but improves radial mass transfer. For this purpose, an experimental system consisting of perfluorinated ethylene-propylene capillaries (FEP with the same refractive index as water or acetonitrile) with an internal diameter of 0.5 to 6 mm is built and investigated with gas-liquid reactions. The oxidation of leuco indigocarmine with a two-step color change as a consecutive reaction and glucose oxidase with color change parallel to sulfite oxidation are investigated. For these systems the diffusion coefficients and kinetic parameters are determined in a separate test facility. Bubble formation and bubble flow can be optically observed in a straight, vertical capillary with an internal diameter up to 6 mm as a reference case. In the helical capillary, Taylor-Dean flow is investigated with mixing, mass transfer, and complex chemical reactions. The Taylor-Dean flow profile can be entirely characterized, while different reactions with their typical time scales can also be optically examined in the capillaries with different diameters. In parallel, complex transport processes are numerically simulated with FeatFlow CFD Software. Additional 90° deflections between individual coils (Coiled Flow Inverter CFI) improve mixing by shifting the regions of the Taylor-Dean vortices. Dimensionless numbers as well as segregation index or exposure level are used for the analytical description of the mixing process of complex reactions. The numerical simulation of the complex Taylor-Dean flow requires a complete 3D simulation of the flow and the concentration field without and with chemical reactions. The numerical grid generation and treatment of the interface imposes high demands on the numerical tools. The adjustment of the kinetics of parallel and consecutive reactions allows for determining the local selectivity to the target molecule, which in the experiment causes a color change of the solution. As associated partner in the priority program further chemical reactions are tested in the CFI setup. In the course of the project further measurement techniques will be available with X-ray tomography, Raman-spectroscopy, and high-speed camera with macro lens.