Mixing time in high aspect ratio vessels stirred with multiple impellers

Mixing time is a key parameter for evaluating the blending effectiveness of stirred tanks but, in contrast with the case of single impeller configurations, a general purpose correlation for multiple impeller geometries is not available to date. In this work, a lumped parameter equation for mixing time in the turbulent regime in high aspect ratio tanks stirred with multiple identical impellers is presented. The experimental data are collected by conductivity probes on a variety of stirred tank configurations, dimensions and impeller types with the purpose of identifying the influence of two main geometrical parameters: (i) the impeller number at fixed vessel aspect ratio, and (ii) the vessel aspect ratio at constant impeller distance. The results show that a single equation adding the impeller efficiency, the impeller number and the liquid height to the well-known correlation due to Grenville and Nienow (Grenville, R.K., Nienow, A.W., 2004. Handbook of Industrial Mixing: Science and Practice. In: Paul, E.L., Atiemo-Obeng, V.A., Kresta, S.M. (Eds.), Wiley-Interscience, Hoboken, NJ, USA) properly represents the whole experimental data relevant to axial and mixed-flow impellers collected in this work and in some of previous literature studies. For multiple radial turbines giving rise to compartmentalization of the vessel volume, a modified model based on a simplified fluid dynamic schematization of the vessel as a cascade of ideal stages with backmixing is proposed.