Solid distribution in vessels stirred with multiple impellers

Abstract

Solid distribution for solid—liquid suspensions was investigated under batch and semibatch conditions in completely closed tanks. The tanks had an aspect ratio equal to four and were stirred with multiple Rushton turbines. Spherical particles of different materials and sizes were used as the solid phase in various liquids. Average solid concentrations along the vertical axis of the tank were measured by means of an optical technique. The solid profiles can be fairly well modelled with the one-dimensional sedimentation-dispersion model. A correlation of the solid Péclet number with dimensionless groups that include physical and operating parameters was obtained for batch conditions. A procedure to apply the model also in the case of semibatch conditions was checked. The results also give additional evidence of the reduction of the settling velocities of solid particles in a stirred medium with respect to that in a quiescent liquid—at least for some operating conditions. The interaction between the particles and the turbulence field is proposed as a means to explain this behaviour.