CFD Simulation of Solid Liquid Suspensions in Baffled Stirred Vessels Below Complete Suspension Speed

Suspension of solid particles into liquids within stirred vessels is a complex phenomenon encountered in several industrial applications. Most researchers have focused their attention on the assessment of the minimum impeller speed (Njs) able to guarantee the suspension of all particles. Actually, in a number of industrial cases an impeller speed N lower than Njs is chosen as typical operating condition (Oldshue, 1983) as the resulting energy savings well counterbalance the loss of active interfacial area. Therefore, the assessment of the amount of unsuspended particles at any given impeller speed represents the key to quantify the economical advantage/disadvantage of adopting an impeller speed N < Njs. The present work focuses on the prediction of the fraction of suspended particles at rotational speeds N < Njs by means of Computational Fluid Dynamics (CFD) for the case of a dense solid-liquid suspension in a baffled tank stirred by a standard Rushton turbine. An Eulerian-Eulerian Multi Fluid Model coupled with a standard k-epsilon turbulence model was adopted. The Multiple Reference Frame (MRF) approach was employed to simulate the rotating impeller. Experimental data collected by using the Pressure Gauge Technique (Brucato et al., 1997) were employed for the validation of the CFD model. A very good agreement between experimental data and CFD results was found.