Reactive Bubbly Flows – Understanding the Influence of Fluid Dynamics on Gas-Liquid Reactions

16.04.2021

-  Felix Kexel – Institute of Multiphase Flows, Hamburg University of Technology -

Due to the steadily increasing demand for high-quality chemicals all around the globe, gas-liquid reactions are of great importance, producing those highly demanded bulk chemicals such as alcohols, carboxylic acids and many more. Although such reactions are widely used, a detailed understanding of the complex interplay of gas-liquid mass transfer, mixing, and chemical reaction is still missing. This hinders improvement of yield and selectivity in a targeted reaction system. Closing this knowledge gap has been addressed in recent years by the DFG priority program SPP 1740 “Reactive Bubbly Flows”. In gas-liquid reactions, the gaseous species needs to be transferred across the bubble interfaces and through the liquid boundary layer into the liquid bulk phase. These transport processes are dominated by the bubble size and shape as well as the bubble-induced turbulence within a bubble swarm. Due to the interplay of local residence time distributions and the kinetics of competitive reactions, yield and selectivity can be affected by fluid dynamic conditions and different initial concentrations of reacting compounds [1,2]. Constant and reproducible conditions are desirable to gain the necessary information for tailoring the fluid dynamics to the desired chemical reaction or vice versa. Furthermore, the interplay of gas-liquid mass transfer, fluid dynamic conditions and chemical reactions should be acquired in industrially relevant model systems, containing, for example, organic solvents. In order to connect the fluid dynamics with the ongoing chemical reaction, two things are essential, first a model reaction that is easy and safe to handle and secondly an experimental setup to visualize the fluid dynamic conditions and the ongoing chemical reaction. A ferrous reaction system in methanol is used as a fast gas-liquid model reaction that follows a competitive consecutive scheme as shown in equations (1) and (2).

A so called Taylor bubble setup is used to visualize the necessary information. Taylor bubbles are large elongated gas bubbles surrounded by a liquid film in small capillaries. The bubble diameter is only slightly smaller than the diameter D of the capillary and the bubble is self-centering while staying elongated. 

The used setup consisted of narrow glass capillaries that allow a high degree of optical accessibility. This enables measurements such as high-speed imaging UV/VIS spectroscopy [3,4], Particle Image Velocimetry (PIV) in the bubble wake [1,5] and thus the visualization of the concentration fields and the selectivity (Figure 1) of the intermediate product P1 (green) and the final product P2 (red). The results provide a new, precise basis for a more detailed modeling of the interplay between fluid dynamics and chemical reactions. This will lead to more reliable correlations between process parameters and improve future process design and operation.The used setup consisted of narrow glass capillaries that allow a high degree of optical accessibility. This enables measurements such as high-speed imaging UV/VIS spectroscopy [3,4], Particle Image Velocimetry (PIV) in the bubble wake [1,5] and thus the visualization of the concentration fields and the selectivity (Figure 1) of the intermediate product P1 (green) and the final product P2 (red). The results provide a new, precise basis for a more detailed modeling of the interplay between fluid dynamics and chemical reactions. This will lead to more reliable correlations between process parameters and improve future process design and operation.

References

[1]     A.v. Kameke, S. Kastens, S. Rüttinger, S. Herres-Pawlis, M. Schlüter, Chemical Engineering Science 2019, 207, 317–326.

[2]     C. G. Llamas, C. Spille, S. Kastens, D. G. Paz, M. Schlüter, A. Kameke, Chemie Ingenieur Technik 2020, 92 (5), 540–553.

[3]     F. Kexel, A. von Kameke, M. Oßberger, M. Hoffmann, P. Klüfers, M. Schlüter, Chemie Ingenieur Technik 2021, 93 (1-2), 297–305.

[4]     F. Kexel, A. von Kameke, J. Tenhaus, M. Hoffmann, M. Schlüter, Chemie Ingenieur Technik, in press.

[5]     S. Kastens, J. Timmermann, F. Strassl, R. F. Rampmaier, A. Hoffmann, S. Herres-Pawlis, M. Schlüter, Chem. Eng. Technol. 2017, 40 (8), 1494–1501.