Alternative Compartment Models of Mixing in Tall Tanks Agitated by Multi-Rushton Turbines

Abstract

Liquid mixing in tall tanks agitated by multiple radial turbines can be modelled by a cascade of well-mixed compartments with backflows. Models have been proposed with different numbers of compartments per stage of agitation. In this paper, the five simplest models are compared in their ability to predict mixing times and concentration transients, and in their requirement for adjusted parameters. Examined models have from 1 to 4 compartments per agitation stage. The model which consists of 3 compartments per agitation stage is shown to be the best, since it predicts reasonably accurate turbulent mixing times and concentration transients, regardless of injection point position, without the need for any parameter adjustment. Compartments within an agitation stage are connected by the liquid circulation rate, known from the literature, while interchange of liquid between agitation stages was measured. The other models either require adjustment of one parameter for mixing time prediction or do not have a structure adequate to account for the effect of tracer injection point.