Research Project within the DFG project SCHL-617_LI-899:
"Characterization of Fine Bubbles for biocatalytic processes"

Simon Matthes, M.Sc.

 Many chemical and biocatalytic reactions are consuming gaseous species like oxygen, provided by the mass transfer across interfaces of multiphase contact apparatuses. In large-scale processes the gas is often supplied to the liquid bulk phase by bubble aeration. Especially for biocatalytic reactions the macroscopic aeration can lead to reduced enzyme activity by foaming and induced shear forces. For fast chemical reactions in multiphase flows, the mass transfer limitation is often the bottleneck for a process optimization. Compared to large-scale bubble aeration, the potential of using bubbles smaller than 100 μm is less explored so far.

Fig.1 Endoscopic image from micro bubbles and their detected edges by post processing.  

At that point this project starts investigating the aeration with fine bubbles, due to the rising demand in process engineering for aeration with high mass transfer performance, low pressure drop, low shear stress and the avoidance of foaming. Bubbles with diameters less than 100 μm offer large volume-specific interfacial areas a and therefore high mass transfer rates βLa of the gaseous reactant on its way to the bulk phase. Furthermore, the rise velocity of a bubble decreases with decreasing bubble diameter leading to higher residence time and low induced shear stress. To determine the potential of fine bubbles for biocatalytic processes, the promising properties of fine bubble aeration compared to conventional aeration will be investigated. The project focuses on the examination of the physical properties of microbubbles within a diameter range from 1 μm to 100 μm and their characterization in water. The challenging task is the visualization and measurement of single bubbles and bubble swarms at this diameter range using optical measurement systems. The important parameter to determine are bubble number concentration, bubble size distribution, bubble shape, bubble rise velocity and agglomeration and coalescence behavior of multiple bubbles.

Fig.2 Typical bubble size distribution for fine bubble generators.

The project is in close collaboration with the Institute of Technical Biocatalysis (ITB) focusing on the application of the achieved findings for biocatalytic processes. Furthermore the leading expert in investigating fine bubbles Prof Koichi Terasaka from Keio University, Japan is supporting the project enabling the exchange between both countries.