Eulerian Simulations of Mixing and Segregation of Binary Gas-Solid Flow of Particles with Different Densities

Several industrial processes employing gas-solid fluidized beds often involve a mixture of particles differing in size, density, and shape (e.g. coal and inerts, coal and ash, biomass and inert used in gasification process). The difference in the physical properties (size, density, and shape) of these particles give rise to non-uniform mixing and segregation which leads to the formation of hot-spots and agglomerates (e.g. clinker formation), thus deteriorating the performance of the gasifier. Therefore, the performance of these industrial processes is influenced by the local mixing of the gas and solid phases, which primarily is governed by the local fluidization/bubbling behaviour. Therefore, it is important to develop CFD models capable of predicting the bubbling characteristics, mixing and segregation phenomena of binary gas-solid flow accurately. Such models can then be extended to predict the local temperature and concentration distributions in gasifiers and eventually to optimize performance of such fluidized bed gasifiers/reactors.

The focus of the present work is on simulating the binary gas-solid flow differing in density and predicting the mixing and segregation phenomena. However, in order to predict the mixing and segregation phenomena, it is important to predict the bubbling characteristics (bubble size distribution, bubble rise velocity, and bubble frequency) as these are responsible for the mixing and segregation phenomena at a given superficial gas velocity (U_g) and bed composition. In our research group, we have been working on simulations of unary gas-solid flow in fluidized beds (Singh et al. (2018)). The Two-Fluid Eulerian simulations performed in our research group using different gas-solid drag models, for e.g. widely used model of Gidaspow et al. (1992), the multi-zonal drag on different values of local gas volume fraction (Gao et al. (2009), Singh et al. (2018)) and EMMS-based drag models (Hong et al. (2013), Li and Yang (2017)) that account for the heterogeneous flow structures. We found that the predicted time-averaged radial solid hold-ups for all drag models considered in the present work were in a reasonably good agreement with the experiments, whereas the predictions of bubbling characteristics (bubble size distribution, bubble frequency) were significantly different from the measurements. Therefore, such models are unlikely to predict the mixing and segregation phenomena as the latter are governed by the bubbling characteristics. As the existing gas-solid drag models mentioned above do not predict the bubbling characteristics well, it becomes important to investigate the effects of other closures and parameters such as particle frictional stress model, minimum and maximum frictional packing limits which are shown to have a significant impact on the bubble formation, growth and size (Passalacqua and Marmo (2009)).

The present work is carried in a pseudo 3-D (rectangular) fluidized bed because of its advantage in visualization and measurements of several parameters simultaneously. The objectives of the present work are (i) to measure the bubble size distribution, bubble rise velocity, time-evolution of average dispersed bed heights of individual solid phases and % segregation for binary systems differing only in density (same size), (ii) to analyze the contribution of particle frictional stress models (frictional pressure and frictional viscosity), maximum and minimum frictional packing limits on bubble size distribution and bubble rise velocities of two unary systems (both having same size but differing in density) and, (iii) to predict and to validate the time-evolution of individual solid-phase average dispersed bed heights, % segregation, bubble size distribution and bubble rise velocities for binary gas-solid flow differing in density (size ratio = 1, density ratio = 3.47) for a wide range of U_g and bed compositions.

Experimental and Computational methodology

Unary and binary gas-solid fluidization experiments were performed in a pseudo 3D (rectangular) column with height (H) of 1 m, width of 0.2 (m) and depth of 0.02 (m). Air was used as the fluidizing medium and glass beads (GB) (d_p = 2.26 mm; ρ_p = 2500 kg/m³), mustard seeds (MS) (d_p = 2.18 mm; ρ_p = 720 kg/m³) were used as the solid phases. Dynamics of fluidization of unary_GB, unary_MS and binary system (GB: MS) in the weight ratio of 70%:30%, 50%:50%, and 30%:70% solids was investigated for superficial gas velocities in the range 1.1-2 U_mf of the dense phase. The experimental images were recorded with a high–speed monochromatic camera at 640 × 800 pixel² resolution and at a rate of 400 fps. Further details of the experimental methodology will be presented in the full manuscript.

The Two-and Multi-Fluid Eulerian models were used to simulate the unary_GB, unary_MS and binary system_GB: MS under transient flow conditions. The Kinetic Theory of Granular Flow (KTGF) model was used to model the solid stresses. The model equations were solved using the commercial software “Ansys Fluent 19.1.0”. No-slip and Johnson-Jackson wall boundary conditions were used for gas and solid phases, respectively. The user defined functions were used to implement different closures models (gas-solid drag, frictional pressure, and frictional viscosity). Further details on the effects of the grid and time step-independence studies and simulation parameters will be presented in the full manuscript.

Results and discussion

For the binary gas-solid experiments performed for a system composed of GB: MS = 70:30 wt. % at a U_G of 1.73 m/s (1.1 U_mf of GB), it was observed that the time-evolution of average dispersed bed height of GB was found to reach 15 cm from the initial static bed height of 25 cm and the % segregation was found to reach 60 % after 25 sec. At U_G = 2.78 m/s (1.78 U_mf of GB), a complete mixing of the solid phases was observed as the operating U_G was much higher than the U_mf of GB and therefore segregation was not observed. From the information of the movement of bubble with respect to time and axial location, the bubble size distribution and bubble rise velocity are calculated for unary_GB, unary_MS and binary systems.

The Two-Fluid Eulerian simulations were performed using the gas-solid drag models of Gidaspow et al. (1992), Gao et al. (2009), Singh et al. (2018), and Li and Yang (2017) for unary_GB at U_g = 2.08 m/s (1.3 U_mf), 2.78 m/s (1.78 U_mf). Although the aforementioned gas-solid drag models predicted the dispersed bed heights in a satisfactory agreement with the measurements, the bubble size distribution predicted using Singh et al. (2018) was qualitatively in a better agreement with the experiments in comparison with the corresponding predictions of the other drag models. The drag model of Singh et al. (2018) predicted significantly higher solid shear stress over solid normal stress and this led to a higher compaction of solids around the bubble, thereby leading to less dispersion of gas into the solid phase. To further improve the predictions of bubble size, frictional pressure models of Syamlal et al. (1993) and Johnson and Jackson (1987) were used at U_g = 2.78 m/s for unary_GB system. At a minimum frictional packing limit of 0.61 and maximum packing limit of 0.63, the formation of large bubbles was observed in both the cases in comparison to the KTGF based frictional pressure model. Further investigations on the effect of frictional stress models (frictional pressure and frictional viscosity) at different frictional packing limits on bubbling characteristics and multi-fluid Eulerian simulations will be performed for a binary system (GB: MS) differing in density and the detailed analysis will be presented in full manuscript.

Conclusions

In this study the effect of different closures models for gas-solid drag and frictional stress models, minimum and maximum frictional packing limits on predictions of bubble size distribution, bubble rise velocity and time-evolution of % segregation at different U_G and bed compositions were investigated and compared quantitatively with corresponding experimental measurements. The frictional pressure model and frictional packing limit plays a significant role in the prediction of bubble size quantitatively in comparison to the gas-solid drag models. This study will provide a comprehensive analysis of the effect of gas-solid drag and particle frictional stress models on predictions of the bubbling characteristics, mixing and segregation phenomena.

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