Euler-Euler Modeling of Flow, Mass Transfer, and Chemical Reaction in a Bubble Column

Physical and chemical absorption of pure CO2 bubbles in water and an aqueous sodium hydroxide (NaOH) solution has been studied in a square cross-sectioned bubble column using the commercial software package CFX-4.4. The subgrid-scale turbulence model of Vreman [PhYs. Fluids 2004, 16, 3670-3681] 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 were used in the simulations. Full coupling of fluid flow, mass transfer, and chemical reaction is achieved through the incorporation of a bubble number density equation. The capability of the bubble number density model to predict the bubble size is investigated first. Subsequently, physical absorption of pure CO2 in water and chemisorption of pure CO2 bubbles in an aqueous NaOH Solution are numerically studied. It was verified for a test case without absorption that the specified bubble size can be reproduced with the aid of a bubble number density equation. For the physical absorption of CO2 in water, it is found that generally the size of the bubbles in the core of the bubble plume is larger than that of the bubbles trapped in the downflow along the wall. In this test case, the bubble size ranges from 3 to 4 mm. As time proceeds, the differences in bubble size become smaller in both the horizontal and vertical directions. When pure CO2 is absorbed into an aqueous NaOH solution with an initial pH value of 12, the bubble size does not change very much with time. In this case, the bubble size ranges from 2.7 to 4 mm because the mass-transfer enhancement factor is on the order of unity, as a result of the relatively low pH. The pH history resulting from the numerical model is compared to that obtained from a simple macroscopic model. It is found that numerical results obtained from the case in which the bubble size is solved agree well with the simple model. The observed differences between the simple model and the simulated results obtained with constant bubble size are due to the lack of coupling of mass transfer and fluid flow.