Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Bubble columns and bubble column reactors are well established in the field of chemical and biochemical engineering. As the flow in such apparatus influence the performance of mixing or heat and mass transfer processes, numerous studies focus on the hydrodynamic description of the flow as well as the spatial distribution of bubbles inside the vessel. For the latter, the shear-induced lift force plays a key role as it acts lateral to the bubble rise direction and correspondingly affects the lateral distribution of bubbles. However, studies regarding the lift force usually focus on rather clean or uncontaminated systems, although even small amounts of surfactants are known to significantly change bubble characteristics like drag, coalescence and breakup behavior as well as the bubble shape in the case of ellipsoidal bubbles.
In previous studies we investigated the influence of surfactants on the lift force of ellipsoidal single bubbles and could reveal strong changes of the lift coefficient 𝐶 𝐿 in comparison to clean bubbles 1 . To elaborate the effect of the lift force in buoyancy driven bubbly flows, we present measurements conducted in a 2.0 m tall column with a polydisperse bubble size. This allows to study the development of bubble size-dependent gas fraction profiles along a distance that is sufficient for the effect of the lift force to unfold sufficiently. In these experiments, we added different dozes of 1-Pentanol to the flow in order to investigate a surfactant-related modification of the lift force in dependence on the contamination degree. We use shadowgraphic high-speed imaging to determine liquid velocity and gas phase properties. For the former we use a PIV-like technique called Particle Shadow Velocimetry, while for the latter we use a novel deep learning based procedure that uses Convolutional Neural Networks to
determine bubbles and reconstruct occluded bubble parts in the images. Although the addition of surfactants affects bubbles in multiple aspects, we link some of the findings to our previous studies on the shear-induced lift force and demonstrate the influence of surfactants on several flow characteristics in bubble columns.