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RESEARCH PRODUCT

CFD Predictions of Sufficient Suspension Conditions in Solid-Liquid Agitated Tanks

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Abstract Most research efforts on mechanically agitated solid-liquid contactors have been devoted to the assessment of the minimum impeller speed for complete off-bottom suspension, N js . Actually, many industrial vessels are operated at impeller speeds slightly lower than N js (Oldshue, 1983; Rieger et al., 1988). This suggests that the sufficient suspension condition, which is quantitatively specified in this paper by introducing a suitably defined quantity N ss , may represent a valid alternative to that of complete suspension. In the present work time-dependent RANS simulations were carried out with the aim of predicting the achievement of sufficient suspension conditions. The Eulerian-Eulerian Multi Fluid Model along with the Sliding Grid algorithm were employed. The Unsuspended Solids Criterion (USC) (Tamburini et al., 2011a) was adopted to distinguish the suspended particles from the unsuspended ones. The experimental suspension curves (mass fraction of suspended particles versus impeller speed, i.e. x susp vs N) collected by Tamburini et al. (2011a) and the relevant N ss values were predicted by the present CFD simulations and a very good agreement was found, with maximum discrepancies of about 4%. Moreover, CFD results were used to test the ability of some N js assessment methods proposed in the literature to predict sufficient suspension conditions.