Imaging method for the determination of the minimum agitation speed, N_js, for solids suspension in stirred vessels and reactors

N_js, the minimum agitation speed to just suspend dispersed solids in stirred liquids is commonly measured experimentally using the visual method, consisting of determining the agitation speed N at which the solids are observed to rest on the vessel bottom for 1–2 s (N_js–Visual). This method is observer dependent. Here a novel observer-independent method to experimentally obtain N_js was developed by capturing images of the vessel bottom at different agitation speeds and processing them with ImageJ software to quantify the area A_S covered by the solids. Plots of A_S vs. N, extrapolated for A_S going to zero, yielded the value of N_js (N_js–Imaging). The values of N_js–Imaging and N_js–Visual were found to be in excellent agreement with each other for a large number of different impeller types and locations, vessel geometries, and solids size and fractions. The imaging method proposed here could find applications in many solid–liquid mixing processes.