CFD Simulation of Gas-Liquid-Solid Flow in Slurry Bubble Columns with EMMS Drag Model

Slurry bubble columns are widely used as contacting devices in chemical and biotechnological applications, e.g., Fischer-Tropsch (FT) synthesis. The Eulerian-Eulerian two-fluid model is usually used in Computational fluid dynamics (CFD) simulation by assuming the liquid-solid phases as a pseudo-homogeneous phase (Grevskott et al., 1996; Michele et al., 2002; Feng et al., 2005; Troshko et al., 2009). To extend the two-fluid model to three-fluid framework to simulate the three phases, one of the issue is to accurately describe the inter-phase momentum exchange terms, i.e., the drag models for each two-phase couple. For the gas-liquid flow in bubble columns, Yang et al. (2007) proposed the EMMS model for gas-liquid systems (also termed as DBS model), and the model has been coupled into the two-fluid CFD model to improve the simulation of the distribution of gas holdup (Yang et al., 2011; Xiao et al., 2013) and the model proved to be superior to the traditional empirical drag correlations. This work further explores the model applicability in gas-liquid-solid three phase flows. low-solid-concentration systems and then high-solid-concentration systems.

The cylinder column (Rados, 2003) and square column (Ojima et al., 2014) are selected for CFD simulation. The former operated at the superficial gas velocities (0.08 m/s, 0.30 m/s) with the solid loading of 9.1%, and the latter at the superficial gas velocities (0.020 m/s, 0.034 m/s) with the solid loading vary from 0% to 40%. For the low-solid-concentration system, the DBS model performs well and is better than the empirical drag correlations in the prediction of radial distribution of gas holdup with the relative error ranging form 0 to 8.3%. The Schiller-Naumann drag model underestimates the local gas holdup in low superficial gas velocities (0.020 m/s, 0.034 m/s, 0.08 m/s) and overestimates the local gas holdup in a higher gas velocity (0.3 m/s). This conclusion is also consistent with our previous work (Xiao et al. 2013) for gas-liquid bubble columns. For high-solid-concentration system, the DBS model overestimates the local gas holdup while Schiller-Naumann model underestimates the local gas holdup, implying that the model need to be modified to consider the effect of solid on the the gas-liquid momentum exchange. This paper then tried to modify the DBS model with a correction factor as a function of solid concentration. The new drag model agrees well with the experimental data with the relative error ranging from 0 to 14%, and we also tested other experimental systems for the applicability of the new model. In conclusion, this paper validates the applicability of DBS model for CFD simulation of low-solid-concentration systems (9.1 vol %, 10 vol %) over a wide range of superficial gas velocities (0.020 m/s, 0.034 m/s, 0.08 m/s, 0.3 m/s) and explores a modified DBS drag model for high-solid-concentration system(40 vol %). More experiments need to be carried out for high-solid-concentration three-phase system for the validation of modified DBS model in the next stage.

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