Mass transfer in bubble columns ? a single
bubble approach

Lutz Böhm, David Merker, Niklas Paul, Matthias Kraume, Technische Universität
Berlin, Chair of Chemical and Process Engineering, Berlin, Germany

Although bubble columns are common reactors in chemical
engineering, the design of such reactors is often based on rather roughly
estimated parameters. The priority program SPP1740 ?Influence of local
transport processes on chemical reactions in bubbly flows? was set up to get a
deeper insight in such systems. In the frame of the program, the chemistry
(with typical reactions where material is transferred from gas to liquid with a
consecutive reaction in the liquid phase), the process itself (investigation of
single bubble and bubbly flow behaviors) and the according mathematical
modeling (CFD modeling of the bubble behavior; correlations for mass transfer)
are of interest. A special focus in the program is not only on the generally
described global process but on the time and spatially resolved events, as e.g.
the concentration field surrounding a single bubble in a bubble swarm has a
crucial influence on the overall mass transfer in the reactor. Especially in
case of mixing sensitive reactions in a chemical reaction networks with fast
reactions the locally resolved concentration field is of special interest.

This sub-project has the goal to determine qualitatively and
quantitatively the local and integral mass transfer. Usually in literature,
integral, volume specific mass transfer coefficient were determined. The main
influencing factors were found to be the pressure, temperature, gas and liquid
velocity, liquid properties and column geometry. Here, single bubble
investigations were performed in a rising cell and a counter flow cell allowing
different contact times, overall pressures and ambient temperatures.
Additionally, the bubble size and the used gases and liquids can be varied and
turbulence promoters will also be tested.

As one part of the project, integral non-volume specific
mass transfer coefficients were determined based on fluid dynamic
investigations. A carbon dioxide bubble as dispersed phase rose in water used
as continuous phase. During its stay in the counter flow test cell (Fig.1a-c),
carbon dioxide was transferred from the gaseous to the liquid phase leading to
a decrease of the bubble size. The different according rise velocities can be
associated to a certain bubble size (Fig.1d). Therefore, the amount of transferred
carbon dioxide over time could be determined (Fig.2). As the surface area of
the bubble and the carbon dioxide concentration in the liquid phase were known,
a mass transfer coefficient can be calculated based on this data.

 

Fig.
1: Exemplary images of a single carbon dioxide bubble
dissolving in  water in a counter flow cell (a to c) and the according terminal
rise velocity referable to an equivalent bubble size (d)

 

Fig.
2: Development of the carbon dioxide concentration in a single
bubble over time

This bubble size determination based on the simple bubble rise
velocity measurement was validated by more complex direct optical bubble sizing.
Both, in the rising test cell and counter flow cell, serial measurements can be
done allowing the determination of statistically significant results.

Furthermore, the concentration profile in the vicinity and
wake of the bubble will be determined with the help of a colorimetric
measurement method. A reaction in the liquid phase will be investigated which consumes
a component transferred from the gas phase and leads to a discoloration of the
liquid phase.

  

ACKNOWLEDGEMENTS

Financial
support by DFG priority program SPP 1740 ?Einfluss lokaler Transportprozesse
auf chemische Reaktionen in Blasenströmungen? (?Influence of local transport
processes on chemical reactions in bubbly flows?) is gratefully acknowledged.