Effect of impeller placement on solid–liquid mixing at high particle loadings

초록

This study investigates solid–liquid mixing at high particle concentrations, focusing on the determination of the just-suspended impeller speed ( N JS) and the practical usefulness of power consumption behavior as an operational indicator. While Zwietering’s classical correlation remains widely used for dilute suspensions, it becomes unreliable for dense slurries and opaque vessels where visual observation is difficult. To address these limitations, this work aims to (i) provide high-quality experimental data suitable for validating CFD-based N JS predictions and (ii) assess whether characteristic transitions in power consumption can reliably indicate the onset of complete particle suspension. Experiments were conducted in a flat-bottom cylindrical vessel using Rushton turbine, with solid mass fractions ranging from 20 to 70 wt%. The effects of impeller elevation (four vertical positions) and baffling were systematically examined. Power consumption was calculated from torque measurements, and N JS was determined visually following Zwietering’s definition. The results show that transitions in power consumption, although reproducible, do not consistently correspond to N JS in high-solids systems and therefore cannot serve as a universal criterion. A key finding is that the influence of impeller elevation on N JS reverses relative to classical expectations derived from dilute suspensions. Under baffled conditions, raising the impeller reduces N JS, with the minimum occurring when the impeller is located at the solid–liquid interface. This behavior is analogous to mixing phenomena observed in systems with extreme viscosity contrasts, such as Bingham fluids. In unbaffled vessels, bottom placement minimizes N JS at low solid concentrations, whereas at concentrations of 40 wt% or higher, positioning the impeller within the solid bed becomes more effective. Furthermore, the dependence of N JS on solid concentration was found to deviate markedly from Zwietering’s correlation. The exponent of the solid concentration term increased from 0.5 in the range 20–40 wt% to approximately 1.3 above 40 wt%, reflecting fundamental changes in flow and particle mobilization mechanisms at high loadings. These findings demonstrate that impeller elevation is a critical design and operational parameter for dense slurry mixing and highlight the need to reconsider classical N JS correlations when applied to industrial high-solids processes. © 2026 The Author(s).

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