Thesis for Doctor of Philosophy

Sandra Cristina Petronilho Orvalho
“Hydrodynamics and Mass Transfer in Bubble Systems “
Ph. D. Chemical Engineerieng, IST, 2004
Supervisors: J.M.T.Vasconcelos; S.S. Alves

Abstract

The effect of surfactants on the hydrodynamics and mass transfer of bubbles was studied.

An innovative experimental set-up was used for single bubbles, which allows simultaneous monitorization of bubble diameter and terminal velocity for long periods of time. Bubble dissolution is initially fast, with kL corresponding to a mobile interface, with an apparently sudden transition to a slow dissolution regime, with kL corresponding to a rigid bubble. The drag coefficient evolution is similar, but with no discontinuity.

The results, different for soluble and insoluble sufactants, are interpreted in terms of surfactant accumulation at the bubble surface. Two alternative models are proposed, based on the stagnant cap concept. One of the models (SIM - Sudden Immobilization Model) assumes sudden immobilization of the bubble surface due to the Marangoni effect, while the other (GIM – Gradual Immobilization Model) does not involve regime change and has the advantage of relating hydrodynamics and mass transfer.

Finally, based on the previous results and on the SIM model, a model is proposed for the interpretation of experimental kL obtained in experimental conditions very diverse in bubble bed contactors (Airlift and Bubble Column).