Effect of impeller type and density difference on the draw down of low density microspheres

Experiments on dispersion of floating solids into continuous liquid medium were carried out with low density microspheres with particle density of 680 kg/m3 and size 100, 230 and 325 μm in different liquids having density varying from 778 to 1830 kg/m3 and kinematic viscosity varying from 0.3 to 19 cP. A four-bladed radial impeller and axial paddle impellers with upward and downward flow directions were employed. The critical impeller speed (Ncrit) required for uniform dispersion of particles throughout the continuous medium was experimentally determined for various conditions. Experimental results indicate that the radial impeller with impeller location near the surface required minimum impeller speed for uniform dispersion. Strong effect of the density difference between the particles and the liquid and submergence of the impeller was noted. Based on the experimental results, a correlation in terms of relevant dimensionless groups was proposed for calculating Ncrit for the three impellers. The overall correlation indicates that the critical impeller speed is not significantly affected by liquid viscosity and particle diameter but that it is strongly influenced by the impeller diameter and the density difference. These results are in agreement with trends reported in the literature.