(380b) A Comparison of the Performance of Mixing Systems for Viscous Solid-Liquid Mixing Using CFD-DEM

Solid-liquid mixing is a significant process that is critical to pharmaceutical, biomechanical, food processing and other industries. Despite the relevance of viscous solid-liquid mixing applications, the physics governing the suspension of particles in agitated vessels is still poorly understood and historically much less studied in the laminar regime than in the turbulent regime. This can lead to sub-optimal designs and important energy waste or even an incapacity to achieve the desired mixing state. Important criteria that characterize solid-liquid mixing, like the just-suspended speed (Njs) and the homogeneity of the suspension, are difficult to predict. Consequently, engineers often rely on correlations to design solid-liquid mixing vessels. There is a need for more studies on solid-liquid mixing and, more particularly, for new and robust simulation models to predict suspension dynamics.

Complex geometries can be used to meet specific process requirements. For example, the double helical ribbon has shown to achieve good mixing in the laminar regime. However, the simulation of solid-liquid flow in such rotating systems is challenging. To deal with this issue, an innovative Euler-Lagrange model in a non-inertial frame of reference that is suitable to a wide range of geometries regardless of their complexity was developed. It is based on an unresolved CFD-DEM approach that combines the Discrete Element Method (DEM) for the solid particle dynamics and CFD techniques for the fluid phase [1].

First, the full model is introduced with a description of the coupling strategy for the fluid and the solid phases and the validation of the CFD-DEM model against experimental data and numerical studies previously carried in our lab [1,2], Then, using this model, several agitator geometries commonly used for viscous liquid, such as the double helical ribbon, the anchor or the Paravisc, are compared. This comparison is based on typical solid-liquid mixing parameter such as the minimum agitation speed required to obtain a complete suspension (Njs), the cloud height (CH) or the power number (Np). Finally, we investigate various strategies that can be used to enhance solid-liquid mixing efficiency in the laminar regime.

[1] B. Blais, M. Lassaigne, C. Goniva, L. Fradette, and F. Bertrand, "Development of an unresolved CFD–DEM model for the flow of viscous suspensions and its application to solid–liquid mixing," Journal of Computational Physics, vol. 318, pp. 201-221, 2016/08/01/ 2016.

[2] M. Lassaigne, 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, vol. 108, pp. 55-68, 2016.