SOLID-LIQUID SUSPENSIONS IN UNBAFFLED TANKS

Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled tanks. However, they might provide significant advantages in a wide range of applications like crystallization processes as well as for food and pharmaceutical industries, where the presence of baffles is often undesirable. In the present work solid-liquid suspension in an unbaffled stirred tank was investigated. A novel experimental method (steady cone radius method) was devised to ease the evaluation of the minimum impeller speed for complete particle suspension (Njs). Experiments encompassed a quite wide range of particle sizes, densities and solids concentration. The Njs values obtained were compared with the relevant values pertaining to baffled vessels, estimated by means of the well known Zwietering’s correlation. Dependence of Njs on particle density and concentration was found to be similar for both types of tank. Conversely, a negligible influence of particle diameter on Njs was observed in the unbaffled tank, thus marking a difference from baffled tanks with important application implications. Finally the Njs values obtained in the unbaffled vessel are generally smaller than the corresponding Njs in baffled tanks. This finding, in conjunction with the much smaller Power Numbers exhibited by the former, suggests that the power consumption required to achieve complete suspension in unbaffled tanks may be much smaller than in baffled systems.