(358g) Fluid Dynamics with Superimposed Mass Transfer of Single Bubbles in Reacting Liquids

The design of reactors with a gaseous phase dispersed in a liquid phase (e.g. bubble columns) is often based on rather roughly estimated parameters. The DFG priority program SPP 1740 “Influence of local transport processes on chemical reactions in bubbly flows” was set up to get a deeper insight in such systems. The interdisciplinary program allows a close cooperation between chemists, process engineers and mathematicians for both experimental work and simulation e.g. using CFD. Aim of the SPP 1740 is to develop a fundamental understanding of the interactions in gas/liquid-reactions, especially concerning fluid dynamics, mass transfer and reaction rate and their influence on yield and selectivity.

In this sub-project, the local and integral mass transfer is determined qualitatively and quantitatively. A considerable amount of literature has been published on the analysis of volume specific mass transfer coefficient or integral mass transfer in gas/liquid systems. The main influencing factors were found to be the pressure, temperature, gas and liquid velocity, liquid properties and column geometry.

In this work, single bubble investigations were performed in two different setups: a rising cell with a length of 2 meters and a counter flow cell. These setups allow different contact times to be applied, varying between a couple of seconds depending on the rise velocity of the bubble and up to several minutes in case of the counter flow setup. Additionally the overall pressure and temperature can be changed. The bubble size and the applied gases and liquids have been varied during the investigations.

To get a profound understanding of the local effects influencing the fluid dynamics and mass transfer during the rise of a bubble, the rising test cell uses two high-speed cameras on a carriage, to track and observe the bubble during its ascent. The velocity of the carriage is regulated in real-time by the position of the bubble in the captured picture. The view of the second camera has an angle of 90°, thus allowing the measurement of the trajectory, velocity, size and shape of a free rising bubble. The contact time is limited by the height of the column (H = 2 m). Two reactive systems were investigated. Carbon dioxide / water and nitric oxide bubbles in a solution of iron(ii) sulfate heptahydrate (FeSO₄) and EDTA (ethylenediaminetetraacetic acid). This chemical reaction has been chosen due to the color change of the reaction product in order to visualize local effects and the wake of the bubble. The reaction rate can be adjusted by the choice of ligand (e.g. EDTA).Depending on the concentration of the educts, the mass transfer can be enhanced by the chemical reaction in the liquid phase. This enhancement can be determined qualitatively.

In the counter-current measuring cell the preparation of the liquid phase can also play a significant role. Due to the dissolved gases in the liquid, the mass transfer from the continuous phase into the gas bubble must also be taken into account.

Acknowledgement

The authors gratefully acknowledge the financial support provided by the German Research Foundation within the Priority Program ‘‘Reactive Bubbly Flows’’, SPP 1740.