Particle Suspension in Top-Covered Unbaffled Tanks

Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled vessels.However, they may be expected to provide significant advantages in a wide range of applications (e.g.crystallization, food and pharmaceutical processes, etc.),where the presence of baffles is often undesirable. In the present work solid–liquid suspension in an unbaffled stirred tank is investigated. The tank was equipped with a top-cover in order to avoid vortex formation. A novel experimental method (the "steady cone radius method", SCRM) is proposed to determine experimentally the minimum impeller speed at which solids are completely suspended. Experimental Njs and power consumption data are provided over fairly wide ranges for particle size, density and concentration. Dependence of Njs on particle density and concentration is similar to that observed in baffled tank. Conversely, a negligible dependence of particle diameter on Njs is observed in the unbaffled tank, a difference from baffled vessels with important practical implications. Finally, the mechanical power required to achieve complete suspension in unbaffled tanks is shown to be much smaller than in baffled vessels. This, in conjunction with the previously ascertained excellent particle-fluidmass-transfer promotion, could make unbaffled tanks a best choice for many solid–liquid operations,where mass transfer is the main limiting factor.