Determination of Minimum Agitation Speed for Complete Solid Suspension Using Four Electrode Conductivity Method

In many solid liquid operations, it is required to keep all the solids in suspension to avoid localised reactions, prevent the solid build-up at a particular location on the base of the vessel or just avoid clogging the outlet port. In operations like dissolution of a solid in a liquid, the employment of agitation speed over the just suspension speed (Njs) does not necessarily increase the effective surface area of the solids and hence increase mixing efficiency. Many workers have proposed the correlations for the Njs but it is widely accepted that the phenomenon is best represented by Zweitering correlation. The discrepancies in the correlations are probably due to lack of accurate Njs measurement instruments. In this paper, a sensor based on the four electrode conductivity measurement has been proposed to directly measure the Njs. This sensor relies on the change in the electrical resistivity as the solids settle over the electrodes. A relationship to determine the Zweitering constant has been investigated for the Rushton impeller using this technique. A 5-10% discrepancy in Njs values is normally found between the sensor prediction and the visual determination.