Experimental investigation of the mixing of viscous liquids and non-dilute concentrations of particles in a stirred tank

Despite the importance for the process industry of solid-liquid mixing operations involving viscous liquids and high solids concentrations, most of the reported results have been obtained in the turbulent regime with low solids loadings. In this work, the suspension of non-dilute concentrations of spherical particles in viscous liquids is investigated through the determination of the just-suspended speed Njs, the homogenization speed NH and the homogenization time tH. The pitched blade turbine, which is a common and suitable agitator for the suspension of solids in the turbulent regime, is chosen. Njs is obtained using the pressure gauge technique, and NH and tH via electrical resistance tomography. The impact of the particle diameter dp, the solids mass concentration Xw, the liquid viscosity μ, and the impeller diameter D and off-bottom clearance C are assessed. In particular, the effect of dp and μ on Njs are observed to be in contradiction with the Zwietering correlation, which was derived in the turbulent regime. This is attributed to the hydrodynamics and mechanisms prevailing in the laminar and early transitional regimes, which are similar to those for the erosion of a particle bed. This also explains the discrepancies between our experimental values and the values of Njs predicted by the Zwietering correlation. Also, increasing Xw affects Njs in a more complex manner than what is predicted by this correlation. Finally, our results indicate that particle bed erosion is the dominating phenomenon to consider both to suspend the particles and achieve a uniform suspension in the tank.