Application of coalescence and breakup models in a discrete bubble model for bubble columns

In this work, a discrete bubble model (DBM) is used to investigate the hydrodynamics, coalescence, and breakup occurring in a bubble column. The DBM, originally developed by Delnoij et al. (Chem. Eng. Sci. 1997, 52, 1429-1458; Chem. Eng. Sci. 1999, 54, 2217-2226),1,2 was extended to incorporate models describing the breakup and coalescence along with a subgrid scale closure model for the turbulence. To validate the turbulence model, simulation results of the DBM are compared to experimental PIV data of Deen et al. (Chem. Eng. Scl. 2001, 56, 6341-6350).3 It is shown that incorporation of the subgrid scale model results in a better prediction of the mean and fluctuating velocity components in the bubble column, which can be subscribed to an increase of the effective viscosity. Furthermore, it was found that the predicted hydrodynamics are hardly altered when the subgrid scale velocity is taken into account in the evaluation of the interface forces. Finally, the bubble size distributions predicted by the DBM including the coalescence models of Chesters (Trans. IchemE 1991, 69, 259-270)4 and Lee et al. (Chem. Eng. Commun. 1987, 59, 65-84)5 are compared with experimental data that were obtained through digital image analysis in a pseudo 2D bubble column. It is found that the number of collisions between two bubbles that result in coalescence is 43% with the model of Chesters(4) and 85% with the model of Lee et al.(5) Coalescence occurs mostly in the lower part of the column. The mean diameter obtained from the DBM is higher than those measured experimentally, which is probably due to the lack of breakup.