Solid−Liquid Suspensions in Top-Covered Unbaffled Vessels: Influence of Particle Size, Liquid Viscosity, Impeller Size, and Clearance

Particle suspension in liquids is a unit operation commonly encountered in the process industry. Although it is usually carried out in baffled stirred tanks, there are some specific applications where the presence of baffles may be undesirable. In the present work solid−liquid suspensions are investigated in a radially stirred unbaffled tank provided with a top cover. The minimum impeller speed at which all solid particles get suspended (Njs) and the relevant power requirements (Pjs) are assessed. The dependence of these two parameters on physical properties (liquid viscosity, particle concentration, and size) and system geometrical configurations (impeller diameter and clearance) is investigated, and a correlation for the Njs estimation is proposed. Results mark some notable differences with respect to baffled tanks: Njs appears to be almost independent of particle diameter, a feature that may suggest the adoption of unbaffled tanks when large heavy particles are to be dealt with. Also, Njs was found to decrease as liquid viscosity increases, once again marking a favorable discrepancy with respect to baffled vessels. As concerns the system geometrical configuration, a tank stirred by a D = T/3 six-bladed Rushton turbine offset by T/3 from the tank bottom is found to be the most convenient configuration among those investigated in this work. Present results are finally compared with literature information on solids suspension in baffled or unsteadily agitated unbaffled systems. In both cases, top-covered unbaffled stirred vessels are found to be a convenient choice, provided that mixing times are not a controlling factor.