(421b) A Study of Solids Suspension Homogeneity in Agitated Tanks Fitted with Disk Type Impellers Using Electrical Resistance Tomography

Electrical resistance tomography (ERT), a non-invasive flow visualization and measurement technique based on the difference in electrical resistivity of mixed phases, is employed to study solid particle suspension characteristics in agitated tanks. Transient 2D horizontal tomograms of solids concentration along six horizontal planes are obtained directly from measured voltages using Maxwell’s equation. A 3D tomogram of the temporal distribution of solids in the tank can also be obtained using the linear back projection algorithm. From the ERT data, solid suspension homogeneity is determined by calculating the average of the standard deviation of measured solids concentration in both axial and radial directions of the tank, as well as in the bulk.

A lab-scale agitated tank of 12 inch in diameter (D), which is fitted with a top-entry disk-type impeller, is used in all experiments. The homogeneity index is determined for two different disk-type impellers, namely Rushton impeller and standard froth flotation impeller, three impeller diameters, namely D/5, D/4, and D/3, and two impeller off-bottom clearances (T/12 and T/6). Glass ballotini impact beads with mean diameters of 160, 520, and 1000 µm are considered, with the volume loading (v/v) ranging from 5 to 10%. Impeller rotation speed is increased from 500 to 1500 rpm to assess the effect of agitation rate on homogeneity.

This study is a first of its kind to report the detailed temporal evolution of solids distribution and suspension homogeneity under a wide range of conditions in agitated tanks fitted with flotation-type impellers. The results show that the bulk homogeneity is improved with the increase in impeller rotation speed, and the higher homogeneity index is reached for finer particle sizes and larger impeller off-bottom clearance. For a single particle size, once the maximum homogeneity is achieved, a further increase in impeller agitation rate has no positive effect on solid suspension dynamics.