Abstract
Flow mixing of two miscible liquids with the addition of gas bubbles is a process often found in industrial chemical apparatus for the production of primary matter. The ongoing optimization of such processes also involves the transformation of batch to continuous mode operation. In that case, the use of helically coiled tubes is an interesting alternative, since those reactors have narrow residence time distributions, very good radial mixing properties and excellent mass transfer can be realized between gases and liquids. For these reasons, in this study the mixing of two miscible liquids with addition of air bubbles in gas–liquid flows has been characterized in a horizontal helically coiled reactor in the laminar flow regime at {text{Re}}_{{{text{total}}}} = 300 ldots 1088. Eight different superficial liquid velocities and five superficial gas velocities were investigated. In order to characterize mixing in the liquid plugs between two bubbles, laser-induced fluorescence of resorufin was used and particle image velocimetry has been employed to characterize the flow field. Pseudo-3D-visualizations of the resorufin concentration and the Q-criterion, representing the mixing efficiency and vorticity, respectively, were established for individual liquid plugs from the time-resolved measurement results. A time-resolved mixing coefficient, as well as a mean mixing coefficient obtained from multiple liquid plugs, is calculated from the fluorescence images for all examined flow conditions. The experimental results clearly show an increase in the mixing coefficient compared to single-phase conditions, caused by the bubbles. However, distinct mixing pattern, depending on the flow structure, can be recognized on different locations inside the liquid plug. Compared to a stationary case without air bubbles, mixing is worse behind the bubbles and increases inside the plug, reaching a maximum mixing coefficient in front of the next bubble. Overall the mixing coefficient is always increased by the presence of the bubbles. Pseudo-3D-visualizations of the Q-criterion and the vorticity show the presence of secondary vortices right in front of the bubbles, shifted to the outer tube walls, and in addition to the steady Dean vortices. In small plugs, these secondary vortices appear in the whole plug and increase the mixing coefficient drastically.Graphical
Highlights
Helical reactors with different shapes and sizes (Kockmann2020) have been a subject of interest since a rather long time, due to their practical use in industry for static mixing or heat and mass transfer
In order to characterize mixing in the liquid plugs between two bubbles, laser-induced fluorescence of resorufin was used and particle image velocimetry has been employed to characterize the flow field
The dark blue area represents a flow regime, in which the air content is so high and the overall flow rate so low that bubbles accumulate in the upper part of the helix reactor and an unsteady flow with very short liquid plugs occurs
Summary
2020) have been a subject of interest since a rather long time, due to their practical use in industry for static mixing or heat and mass transfer. Jokiel et al (2017) presented such a study for use in the hydroformulation of long-chain olefins. Kaiser et al (2017) proposed a semi-batch reaction network for this reaction including a continuously working helically coiled tubular reactor. This was the motivation of for further studies concerning this subject (e.g. Jokiel et al.2019; Mansour et al 2020b) and for this present experimental investigation
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