The effect of bubble-induced liquid flow on mass transfer in bubble plumes

The use of microbubbles to enhance mass transfer in a compact bubble column has become a valuable topic recently. When the liquid flow induced by the presence of microbubbles is taken into account, the behavior of the microbubbles may differ widely from simple estimations. One example is the change of the residence time, which is determined not only by slip velocity but also the velocity of the surrounding liquid. In the present study the effect of the bubble-induced liquid flow on mass transfer in microbubble plumes is analyzed numerically. A two-way coupling Eulerian–Lagrangian approach is used to simulate oxygen bubble plumes with initial bubble diameters from 100 μm to 1 mm and a maximum local void fraction of less than 2% in compact rectangular tanks. The simulations illustrate that the effect of bubble-induced liquid velocity on the residence time of microbubbles increases with the decrease of initial bubble diameters, and also increases with the reduction of initial water depth. The differences between the concentrated and uniform bubble injections are compared. The results show that the uniform injection of microbubbles provides much better mass transfer efficiency than the concentrated injection, because the bubble-induced liquid flow is suppressed when bubbles are injected uniformly over the entire bottom of the tank.