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The Use of Discrete Element Modeling (DEM) and Computational Fluid Dynamics (CFD) for Analysis of Solid-Fluid Mixing

Richard D. LaRoche and Wai Sam Wong. DEM Solutions, 24 Hanover Street, Suite 10, Lebanon, NH 03766

The use of computer aided engineering (CAE) tools for mixing analysis became mainstream with the advances in computational fluid dynamics (CFD) in the mid-1990's. CFD packages such as CFX, STAR-CD and FLUENT all introduced software capable of modelling a key mixing unit operation in the process industries – the stirred tank reactor. Progress over the last decade has led the validated use of CFD in modeling mixing in stirred tank reactors. CFD analysis is routinely used for determining 3D velocity fields, impeller power numbers and blendtime. The richness of this 3D analysis now gives engineers to generate probability statistics for turbulent energy dissipation rate and strain rate which are very useful complements to current scale-up methods in industrial fluid mixing practice.

CFD in stirred tank mixing analysis still faces some challenges in modeling multiphase mixing in stirred tanks using Eulerian-Eulerian CFD methods. For example, it has been problematic for researchers to predict the solids concentration profile in a solid-liquid stirred tank mixing application. Some researchers have had some success in adjusting multiphase parameters to match experimental data at a single scale -- the “belly curve” of solids concentration in a stirred tank as you traverse from bottom to top. But fixing these parameters has not led to a general ability to predict solids concentrations at different scales.

The author will outline an enhanced approach where the particle nature of the solids can be introduced through the use of Discrete Element Modelling (DEM) and coupled with CFD analysis. DEM is a numerical method which models the movement and contact between each particle. A DEM model particle can represent either a single particle or group of particles in the physical system. The most common and physically accurate implementation of DEM is the ‘soft contact' approach which calculates the forces acting on each particle using models which can account for the particle mechanical and surface properties.

DEM is very well established as a powerful tool for studying the mechanics of granular bulks. Advanced DEM software tools such as EDEM can model both particles and equipment which enables direct application to complex processes. DEM provides a framework for investigating the relative effect of particle material properties, environmental conditions and equipment design. Factors such as particle shape, surface properties (e.g. cohesion and electrostatic charge), temperature and moisture content can be accounted for using appropriate models. DEM coupled with CFD is explored as an avenue to incorporate more of the physics necessary to allow multiphase CFD to break through into solid-liquid mixing analysis.