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Mass Transfer from Growing and Oscillating Rising Bubbles

Mariano Martín, Francisco J. Montes, and Miguel A. Galán. Departamento de Ingeniería Química, Universidad de Salamanca, Plz. de los Caídos 1, 5, Salamanca, 37008, Spain

Mass transfer in gas liquid systems depends on the contact between the bubbles, and the surrounding liquid. Further knowledge of the hydrodynamics of the growing and rising bubbles will help understanding the mass transfer process in bubble columns.

An experimental study about bubble dynamics was carried out by means of high speed video techniques and computational fluid dynamics. Bubble growth and rise were studied separately in several desoxygenated liquids.

The growth stage depends on the physical properties of the liquid in which the bubble is generated. Liquid properties determine the formation time of the bubble and its initial size, critical for the rising stage. Not only the hydrodynamics is defined by the physical properties of the liquid but also the transport properties, the diffusivity, which is modified by the liquid viscosity. A model was proposed and verified so that the isolate effect of the physical properties of the liquid on the mass transfer coefficient could be explained. The Sherwood number increases with both viscosity and density. A dimensionless expression for the Sherwood number as function of the Reynolds and the Schmidt numbers was obtained. The exponents obtained in the theoretical equation agree with previous work.

Once the bubble has detached, it begins to oscillate due to the inertial forces resulting from the break of the bubble neck along with the turbulence of the system. The oscillation is the result of a complex dynamic process considering the surface tension and the kinetic energy. Bubble oscillations modify the concentration profile surrounding the bubble, which increases the mass transfer rates. However, the oscillation energy can be partially or totally absorbed if the liquid media is viscous. This fact and the decrease of diffusivity, reduces the mass transfer rate from rising bubbles in viscous fluids. An expression for the Sherwood number for oscillation bubbles in viscous fluid was developed and agrees well with the exposed facts.

In a bubble column, coalescence, which can occur either in the growing stage or in the rising stage, is an important disadvantage due to the decrease in the specific area. However, a bigger bubble is more deformable, and that leads to the onset of oscillations which consequently will speed up the mass transfer. The double effect of coalescence on the global mass transfer coefficient in bubble columns was studied. It was found that for certain initial bubble sizes the effect of the decrease of the specific area can be balanced by the increment in the oscillation amplitude of the bubble resulting from the coalescence.

Acknowledgment: The support of the Ministerio de Educación y Ciencia of Spain providing a F.P.U. fellowship to M. Martín is greatly welcomed. The funds from the project reference PPQ2000-0097-P4-02 are also appreciated.