(679a) An Exploratory Study of the Use of Combined Lattice Boltzmann – Discrete Element Method Modeling of Agitated Solid-Liquid Suspensions

The use and value of computational methods to understand and design agitated systems has been growing steadily. At this point, the use of simulations is routine, but methods continue to evolve and improve. The lattice Boltzmann approach to flow simulation and the discrete element method (DEM) of particle simulation are relatively new developments. Coupling these two tools provides an exciting opportunity for application to agitated solid-liquid suspensions. It is hoped that these approaches can provide more rigorous solutions without the need to adjust modeling parameters and to be a modeling expert. To be valuable for everyday application, this approach must provide accurate results with a reasonable amount of computational power and effort.

One of the most fundamental parameters to be provided by simulation of an agitated system is impeller power draw because of its impact on both process and mechanical design. Simulations currently provide accurate results of impeller power draw in liquid-only systems, but results in multiphase systems are lacking. Extensive experimental data that illustrate the effects of impeller type and solid characteristics on impeller power draw in these systems* provide the opportunity to critically evaluate the capabilities of simulation tools and determine if they are developed to the point that they can be successfully and confidently applied on a routine basis for agitator design.

Examples of experimental data that will be used to test the simulation results include the following:

• Addition of settling solids that increase suspension density have been found to increase the power number of all impellers (when power number is based on liquid density); however, the increase in power number with suspension density is less with flat impeller blades that generate large trailing vortices (e.g. – pitched-blade and Rushton turbines) than with profiled impeller blades that generate small trailing vortices (e.g. – hydrofoils).
• Addition of large neutrally-buoyant particles that do not alter suspension density would not be expected to affect impeller power draw. This lack of effect has been observed with a Rushton turbine, but the power draw of pitched blade turbines and narrow-blade hydrofoils increased dramatically with increasing solid content.
• Large settling solid particles increase the power number of a pitched-blade turbine more than small particles.

*K. J. Myers, E. E. Janz, T. Bao, and M. M. Heigl, “Impeller Power Draw During Turbulent Operation in Solid-Liquid Systems”, Canadian Journal of Chemical Engineering, Volume 97, pages 2662 – 2670 (2019).