(161b) Impeller Power Draw during Turbulent Operation in Liquid-Solid Suspensions

Impeller power draw in turbulent liquid-only operation has been studied extensively, while in multiphase operation, power draw during gas dispersion is relatively well understood. Far less work has been performed regarding impeller power draw when agitating liquid-solid suspensions with settling solids. Additionally, most work in liquid-solid systems has been limited in scope, focusing on a particular impeller type with a narrow range of particle characteristics and loadings.

The most common approach to predicting impeller power draw in liquid-solid systems is modification of the liquid-only power number to account for the effect of solids on mixture density. Typical of this approach is the viewpoint of Nagata (1975) that power draw can be estimated using the average density in the vicinity of the impeller, although this density is difficult to determine. Due to this complication, the average suspension density is often used in practice, but Barresi and Baldi (2000) reported that use of the average suspension density generally over estimates impeller power draw. Chudacek (1982) postulated that use of the suspension density approach requires proper determination of the suspended solids mass and the volume through which they’re distributed. Thus, unsuspended solids should not contribute to the solids mass and also reduce the suspension volume. Similarly, clear liquid above the suspended solids should not be included in the suspension volume. Rather than using slurry density, Micheletti et al. (2003) recommended using the liquid-only power number and liquid density with a correction factor based on the difference between the average slurry and liquid densities.

In an effort to develop a better understanding of impeller power draw during turbulent operation in liquid-solid systems, an experimental study was performed using six impeller types: a narrow-blade hydrofoil, a wide-blade hydrofoil, a pitched-bladed turbine, a straight-blade turbine, a disc turbine with straight blades, and a disc turbine with semicircular blades. To reduce solid distribution effects, data was taken only when solids were distributed throughout the liquid phase. A variety of solids were studied across a wide range of loadings, yielding significant density variations. Data was also taken with neutrally buoyant particles whose addition did not change suspension density.

This study indicates that the six impellers exhibit different behaviors with respect to the effect of suspension density on turbulent impeller power draw. For some impellers, the addition of solids leads to power draw increases that are less than the increases in suspension density. For other impellers, the increases in power draw are similar to the increases in suspension density. And in one case, the increases in power draw are larger than the increases in suspension density. The correlating approach of Micheletti et al. (2013) is found to be promising for all impellers, but again, each impeller behaves differently.

References

Barresi, A. A., and G. Baldi, “Power Consumption in Slurry Systems”, Proceedings of the Tenth European Conference on Mixing, pages 133-140 (2000).

Chudacek, M. W., “Formation of Unsuspended Solids Profile in a Slurry Mixing Vessel”, Proceedings of the Fourth European Conference on Mixing, pages 275-287 (1982).

Micheletti, M., L. Nikiforaki, K. C. Lee, and M. Yianneskis, “Particle Concentration and Mixing Characteristics of Moderate-to-Dense Solid-Liquid Suspensions”, Industrial and Engineering Chemistry Research, Volume 42, pages 6236-6249 (2003).

Nagata, S., Mixing: Principles and Practice, Kodansha Ltd, Tokyo / John Wiley and Sons, New York (1975).