(382a) Viscous Solid-Liquid Mixing in Agitated Vessels Via CFD-DEM Simulation: Suspension Dynamics and Influence of the Geometry

Solid-liquid mixing in mechanically agitated vessels is a unit operation found in many industries such as ore processing, pharmaceutical and cosmetics. The majority of the efforts concerning the study of solid-liquid mixing has been geared toward the assessment of the minimum agitation speed (or just-suspended speed) N_js required to suspend the particles in the turbulent regime. Moreover, Zwieteringâ??s correlation is known to perform poorly in the laminar and transitional regimes [1, 2]. In particular, experiments by Lassaigne et al. [1] revealed that the hydrodynamics of non-dilute suspensions depends on the flow regime, and the influence of the fluid viscosity and particle diameter is in opposition to what is predicted by Zwieteringâ??s correlation. The fact that it can be challenging to investigate experimentally the behavior of non-dilute suspensions, which usually are opaque systems, has provided the impetus for the development of simulation models.

A variety of models have been developed to simulate solid-liquid flows. These include the classical Eulerian-Eulerian (or two-fluid) model, and the combination of the Discrete Element Method (DEM) for the particles and CFD methods for the liquid phase. Recent work in our group has shown that such a Eulerian-Lagrangian CFD-DEM approach can be highly accurate in predicting the fraction of suspended solids and the flow patterns, when compared to experiments [3]. However, the potential of this model has yet to be exploited to improve our understanding of solid-liquid dynamics and help design and optimize such mixing operations.

In this work, an unresolved CFD-DEM model developed by our group, which is based on the CFDEM framework combining LIGGGHTS for the DEM and OpenFOAM for the CFD, is used to study the solid-liquid mixing of non-dilute suspensions stirred by a pitched blade turbine (D=T/3). The flow patterns and solids concentrations are investigated for three impeller clearances (T/3, T/4, T/5), with and without baffles. The fraction of suspended solids and the degree of mixing are assessed from the particle trajectories and the so-called Lagrangian suspended fraction analysis technique.

References

 

[1] Lassaigne M., B. Blais, L. Fradette, and F. Bertrand, Experimental investigation of the mixing of viscous liquids and non-dilute concentrations of particles in a stirred tank, Chemical Engineering Research and Design, in press.

[2] Ibrahiman S. and A. W. Nienow, Comparing impeller performance for solid-suspension in the transitional flow regime with Newtonian fluids, Chemical Engineering Research and Design, 77, 721-727 (1999).

[3] Blais B., M. Lassaigne, C. Goniva, L. Fradette and F. Bertrand, Development of an unresolved CFD-DEM model for the flows of viscous suspensions and its application to solid-liquid mixing, Journal of Computational Physics, in press.