Effect of the impeller type on the break up of bubbles in a stirred tank
Advancing the chemical engineering fundamentals
Multifase Flows - I (T2-5a)
Keywords: Impellers, Bubbles, Population balances, Break up
So far, one of the most important problems in modelling gas – liquid dispersions generated inside stirred tanks using population balances has been the multiple parameters involved in the equations. These parameters refer to the coalescence and break up phenomena of the bubbles. Although in an industrial case the standard impellers, such as the Rushton turbine or the A series turbines, are widely used and so the particular parameters can be obtained, a more systematic study can provide some insight about the break up phenomenon, basic to improve the mass transfer efficiency in stirred tanks.
Using a CFD commercial code, CFX 5.7, discoloration and high speed video techniques, the hydrodynamics and the dispersions generated in a stirred tank were studied for five different impellers, standard ones, like the propeller or the Rushton turbine and non standard ones, several turbines and blades. Finally, a population balance model, based on the ones proposed in the literature, has been used to study the effect of each impeller on the bubble mean size inside the tank.
It was verified that each impeller shows a particular behaviour on the bubbles according to its geometry and the flow pattern developed inside the tank. The critical Weber number for each impeller depends on the break up mechanisms responsible for the bubble mean size of the dispersions generated by each one, lower values are found for direct break up of the bubbles, meanwhile the effective energy applied to the bubbles in the neighbourhood of each impeller, although within the range of the dissipated energy given by the literature, it depends on the physical effect of the impeller on the bubbles
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 CTQ 2005 – 01395/PPQ are also appreciated.
Presented Monday 17, 11:35 to 11:55, in session Multifase Flows - I (T2-5a).
