APPLICATION OF THE CONDITIONAL QUADRATURE METHOD OF MOMENTS FOR THE SIMULATION OF COALESCENCE, BREAKUP AND MASS TRANSFER IN GAS-LIQUID STIRRED TANKS

Mass transfer has been investigated in a gas-liquid stirred reactor by considering the absorption of oxygen in the liquid phase (water). A multivariate population balance model (PBM), coupled with an Eulerian multi-fluid approach, has been employed to describe the spatial and temporal evolution of the bubbles size and composition distributions. The PBM has been solved by adopting the Conditional Quadrature Method of Moments (CQMOM), that has been implemented through user-defined functions (UDF) and scalars (UDS) in the commercial Computational Fluid Dynamics code Ansys Fluent 12. Model predictions were compared with experimental data of bubble size distributions and then with oxygen accumulation in the liquid phase. The simulation results show that, although coalescence and breakup tend to homogenize bubble composition, a multivariate PBM is necessary to properly describe mass transfer. In fact, due to the fact that smaller bubbles, generally located in regions characterized by high turbulent dissipation rates, exchange mass with a much faster rate than bigger bubbles, both bubble size and bubble composition should be included as internal coordinates.