CFD simulation of solid-liquid stirred tanks

Solid liquid stirred tanks are commonly used in the minerals industry for operations like concentration, leaching, adsorption, effluent treatment, etc. Computational Fluid Dynamics (CFD) is increasingly being used to predict the hydrodynamics and performance of these systems. Accounting for the solid-liquid interaction is critical for accurate predictions of these systems. Therefore, a careful selection of models for turbulence and drag is required. In this study, the effect of drag model was studied. The Eulerian-Eulerian multiphase model is used to simulate the solid suspension in stirred tanks. Multiple reference frame (MRF) approach is used to simulate the impeller rotation in a fully baffled tank. Simulations are conducted using commercial CFD solver ANSYS Fluent 12.1. The CFD simulations are conducted for concentration 1% and 7% v/v and the impeller speeds above the "just suspension speed". It is observed that high turbulence can increase the drag coefficient as high as forty times when compared with a still fluid. The drag force was modified to account for the increase in drag at high turbulent intensities. The modified drag is a function of particle diameter to Kolmogorov length scale ratio, which, on a volume averaged basis, was found to be around 13 in the cases simulated. The modified drag law was found to be useful to simulate the low solids holdup in stirred tanks. The predictions in terms of velocity profiles and the solids distribution are found to be in reasonable agreement with the literature experimental data. Turbulent kinetic energy, homogeneity and cloud height in the stirred tanks are studied and discussed in the paper. The presence of solids resulted in dampening of turbulence and the maximum deviation was observed in the impeller plane. The cloud height and homogeneity were found to increase with an increase in impeller speed. The work provides an insight into the solid liquid flow in stirred tanks. (C) 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.