Critical review of different aspects of liquid-solid mixing operations

Mechanically stirred slurry tanks are utilized in several industries to perform various unit operations such as crystallization, adsorption, ion-exchange, suspensions polymerization, dispersion of solid particles, leaching and dissolution, and activated sludge processes. The major goal of this review paper is to critically and thoroughly analyse the different aspects of previous research works reported in the literature in the field of liquid-solid mixing. This paper sheds light on the advantages and limitations of various particle concentration measurement methods employed to assess the suspension quality and the extent of solid suspensions in slurry reactors. Attempts are being made to identify and compare various mathematical models and methods to quantify particle dispersion and distribution in slurry reactors. It has been shown that various factors such as geometric configurations, agitation conditions, and physical characteristics of liquid and solid have pronounced influence on local suspension quality and power consumption. Computational fluid dynamics (CFD) modeling can be extremely useful in assessing the suspension of solid particles in slurry tanks. A critical review of different scale-up procedures employed for solid suspension and distribution in liquid-solid systems is presented as well. The findings of this review paper can be useful for future research works in liquid-solid mixing.

Scale-Up of Shear Thinning Fluid Mixing in an Unbaffled Stirred Vessel with Eccentrically Located and Modified Impellers

Mixing of a shear thinning fluid was scaled-up by maintaining equal Reynolds number (Re), tip speed, and power per volume. A standard 45° four-blade pitch blade turbine (PBT) and a modified version of the same impeller (DF-PBT) which provided simultaneous upward and downward flow were used. The impellers were located eccentrically, and the effects of scale-up were determined. The distribution of a tracer solution to evaluate mixing progress, power consumption, and flow fields were analyzed to compare the three different scale-up rules. The same endpoint of mixing was achieved in all cases of PBT with noticeable differences in flow profiles. Higher power consumption and less time were necessary to complete mixing with equal power per volume compared to that with equal Re and tip speed rules. For DF-PBT, only equal Re rule resulted in values similar to that obtained at the small scale. At a scale-up ratio of 2, the selection of scale-up rule should be based on the time and power requirements of the process. As the effects of scale-up on eccentrically located impellers do not differ significantly from those of concentrically located impellers, impellers which have complex dynamics such as DF-PBT require further studies to understand scale-up effects.