Numerical modeling of hydrodynamics, mass transfer and chemical reaction in bubble columns

Physical and chemical absorption of pure and dilute CO2 bubbles in water and aqueous sodium hydroxide (NaOH) solution has been studied in a squared-sectioned bubble column using the commercial software package CFX-4.4. The sub-grid scale (SGS) turbulence model of Vreman (2004) was employed to evaluate the shear-induced turbulent viscosity in the liquid phase. An “Opening” boundary condition was applied at the outlet, whereas the previously studied interfacial coefficients (Zhang et al., 2006) were used in the simulations. The dependence of the overall mass transfer coefficient on the bubble diameter as well as the decrease of the bubble size under a given condition were theoretically analyzed. Subsequently, physical absorption of pure CO2 in water and chemisorption of pure and dilute CO2 bubbles in aqueous NaOH solution were numerically studied. It is found that the overall mass transfer coefficient does not change much with the bubble diameter in the range of 2 to 4 mm, and provided that the pH value of the alkaline solution is lower than 12, the bubble diameter decreases approximately linearly with the time. During their rise in the column, the bubble diameter reduces from 4 to 2 mm, which is still acceptable for assuming a constant mass transfer coefficient. When pure CO2 is absorbed into water, the hydrodynamics is similar to the case without mass transfer. High aqueous CO2 concentrations are found around the bubble plume. After the bubble plume arrives at the free surface, the aqueous CO2 is transported from the top part of the column to the bottom along the walls due to the down flow of the liquid phase. When pure CO2 is absorbed into aqueous NaOH solution with an initial pH value of 12, initially, the local pH value drops sharply in a short period and accordingly carbonate is produced rapidly. Subsequently the local hydroxyl concentration decreased slowly due to the chemical reaction. Finally, the local hydroxyl concentration decreases in an oscillatory manner, which depends on the chemical reaction rate and the convective mixing. In case dilute CO2 gas is used in the chemisorption process, the local pH value drops slower compared with pure CO2 gas, whereas the flow structure and hydrodynamics are similar to the chemisorption of pure CO2 into aqueous NaOH solution. All the numerical results are only qualitatively presented, a more detailed comparison of the E-E results with the available E-L simulated results or experimental data is still needed.