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A discrete bubble model (DBM) has been used to study the effect of gas sparger properties on the hydrodynamics in a bubble column. As a first step the performance of the model was evaluated by comparison with experimental data. Subsequently, four different perforated plates with different sparged areas were used as a gas sparger. Distributions of liquid velocity, turbulent kinetic energy, and void fraction in the central plane were compared for the four different systems. Furthermore, the effect of the sparger location was also investigated. It was found that the liquid-phase circulation becomes more pronounced as the sparged area location is more distant from the center of the bottom plate. Finally, gas-phase residence time distributions (RTD) were obtained from the simulations. By employing standard axial dispersion model, the gas-phase mixing in the bubble column was characterized. Results show that the extent of mixing increased when the sparged area decreased. The axial dispersion coefficient increased as the sparged area was shifted to the edge of the bottom plate. [ABSTRACT FROM AUTHOR]

In the production and processing of granular matter, mixing of solids plays an important role. Granular materials such as sand, polymeric particles, and fertilizers are processed in different apparatuses such as fluidized beds, rotary kilns, and spouted beds. In the operation of these apparatuses, proper mixing is essential, as it helps to prevent the formation of hot spots, off-specification products, and undesired agglomerates. In this article, we discuss various methods that are available to give quantitative information on the solids mixing state in granular systems based on a discrete description of the solids phase. We apply the different methods to two-fluid model simulations. It is found that some of these methods are grid-dependent; not reproducible; sensitive to macroscopic flow patterns; and/or able to calculate only overall mixing indices, rather than indices for each direction. We compare some methods described in the literature, and in addition, we propose two new methods that do not suffer from the disadvantages mentioned above. Simulations are performed for seven different operating pressures. It is found that mixing improves with operating pressure as a result of increased porosity in the dense phase. [ABSTRACT FROM AUTHOR]

Physical and chemical absorption of pure CO2bubbles 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. Fluids2004, 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 CO2in water and chemisorption of pure CO2bubbles 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 CO2in 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 CO2is 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. [ABSTRACT FROM AUTHOR]