Solid liquid stirred tanl‹s are commonly used in the minerals industry for operations 1il‹e 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—Eule- rian 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 tanl‹. Simulations are con- ducted using commercial CFD solver ANSYS Fluent i 2.1. The CFD simulations are conducted for concen- tration TO and 7Z vJv and the impeller speeds above the “just suspension speed”. It is obseiwed 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 Itolmogorov length scale ratio, which, on a volume averaged basis, was found to be around 4 3 in the cases simulated. The modified drag law was found to be useful to simulate the low solids holdup in stirred tanl‹s. The predictions in terms of velocity profiles and the solids distribution are found to be in reasonable agreement with the literature experimental data. Turbu- lent 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 world provides an insight into the solid liquid flow in stirred tanks.70048

0 2012 The Society of Powder Technology ]apan. Published by Elsevier B.V. and The Society of Powder

1. Introduction

Solid—liquid mixing systems are amongst the common opera- tions used in the field of chemical and mineral industry. The main purpose of mixing is the contact between the solid and liquid phase for facilitating mass transfei-. In industi-ial processes effective mixing is necessary at both micro and macro level for adequate performance. At the micro level, micromixing governs the chemical and mass transfer reactions. Micromixing is facilitated by mixing at macro level. Numerous factors such as the just suspension speed, critical suspension speed, solids distribution, etc. dictate the mix- ing performance. CFD has proved to be a useful tool in analyzing the impact of these factors on the flow characteristics of such sys- tems [1—7]. Pt-oper evaluation of interphase drag is essential for accurate predictions using the CFD model. In this study four differ- ent drag models are analysed and their validity is checl‹ed by com- paring the results of CFD simulations at low concentrations of solid with  the  experimental  data  available  in  the  literature  [8]. The

* Corresponding author. Tel.: +64 892669837; fax: +64 892662684 .

£-mail  address:  r.util‹ar&curtin.edu.au   (R.P. Utikar). ' Present address.

behaviour of turbulence kinetic energy, suspension  quality  and cloud  height  are  extensively discussed.

2. Literature review

Micale et al. [5] used Settling Velocity Model (SVM) and Multi fluid Model (MFM) approaches to analyse the particle distribution in stirred tanks. In SVM, it is assumed that the particles are trans— ported as a passive scalar or molecular species but with a superim— posed sedimentation flow, whereas in MFM, momentum balances are solved for both phases. Computationally intensive MFM was found to be better than SVM, but for both the models it was neces- sary to take into account the inci ease in drag with the increasing turbulence. Micale et al. [4] simulated the solids suspension of 9.6% and 20 volume fractions using the MFM approach and slid— ing grid (SG) approach using the Schillar Nauman drag model. Schillar Nauman is applicable on spherical particles in an infinite stagnant fluid and accounts for the inertial effect on the drag force acting on it. It provided satisfactory results at low impeller speed. Derksen [9] conducted Eulerian—Lagrangian simulations to study the velocity field, turbulence, solid distribution and particle-impeller and particle—particle collision and frequencies