Solid distribution and mixing time in stirred tanks. The case of floating particles.

The dispersion of solid particles and the liquid homogenization dynamics in a slurry stirred tank equipped with different impellers is investigated by Electrical Resistance Tomography (ERT). The selected experimental technique allows to overcome the typical limitations of optical methods and to gain insight into the complex behaviour of two-phase systems, without restriction on the upper value of the average volumetric fraction of the dispersed phase, this conditions being of interest for several chemical and biochemical processes. A laboratory scale baffled stirred tank equipped with different impellers, namely a Rushton Turbine, a Pitched Blade Turbine and a Lightnin A310 impellers, is investigated, in order to evaluate the dispersion features under radial, axial and mixed prevailing flow fields. The analysis concerns dense solid-liquid systems (mass of solid to liquid ratio up to 0.43) under fully or partially suspended solids. The dispersed phase distribution data are based on the measurement of the conductivity of the mixture as compared with the conductivity of the pure liquid, while the mixing time is estimated from the time evolution of the averaged conductivity on each measurement plane after the injection of a small amount of salt aqueous solution. The analysis of the experimental results allows to suggest simple correlations for the interpretation of both the concentration distribution and the dimensionless mixing time. Beside, the experimental data can be adopted as a benchmark for advanced modelling techniques based on Computational Fluid Dynamics, whose scant validation out of dilute conditions is often due to limited information on the local dispersion features.