Dispersion coefficients and settling velocities of solids in slurry vessels stirred with different types of multiple impellers

Equipment dealing with stirred suspensions are broadly used in the process industry. Out of the various aspects affecting their performance, solids distribution is addressed in this paper. The behaviour of a tank of high aspect ratio stirred with multiple impellers of three types is analysed. The purpose is to acquire fundamental information on parameters useful for their description, namely the axial dispersion coefficient of the solids phase and the particle settling velocity. Both transient and steady-state experiments were performed with a variety of dilute suspensions. The data were analysed with the axial sedimentation-dispersion model. The axial dispersion coefficient of the solid phase was found to differ from that of the liquid by less than 15% for all the impellers and conditions tested. The effective particle settling velocity in the stirred medium was then determined. As happens in equipment stirred with multiple radial turbines, the value of this parameter was found to be different, in general, from the terminal settling velocity. It is shown that a correlation among these parameters and Kolmogoroff length microscale and particle size obtained previously for a specific geometric configuration is actually independent of the impeller type.