(488c) 70 Years of Research and Application in Solid-Liquid Mixing

Suspension of solid particles in liquids is an important mixing operation in the process industries. These include crystallization and precipitation where particles are formed, dissolving of solids to create solutions and reactions where solid particles are catalysts. The optimum agitator speed for mass transfer between the particles and liquid is called the just suspension speed and was defined by Zwietering (Chem. Eng. Sci., 8, 244-253, 1958) as the speed required to ensure that no particle is stationary on the base of the vessel for longer than 1-2 seconds. Zwietering produced an empirical correlation, based on dimensional analysis, which correlated his data and has been used for next 50 years to design agitators for these applications. The correlation was recommended for use in the Handbook of Industrial Mixing (ch.10, John Wiley & Sons Inc., Hoboken NJ, 2004).

In the late 1970’s and early 1980’s physical models were proposed by Baldi et al. (Chem. Eng. Sci., 33, 21-25, 1978) and Davies (Chem. Engng. Proc. 20, 175-181, 1986) which related the properties of the turbulent eddies generated in the flow from the impeller to the negative buoyancy of the particles. Grenville et al. (ChERD, 100, 282-291, 2015) applied the Baldi / Davies physical modeling approach to a large data set measured by the Fluid Mixing Processes consortium in the UK. Measurements were made at three scales with liquid viscosities ranging from 1 to 2000 cP. The model captures the variability in the data very well taking into account scale, impeller type and geometry and physical properties of the particles and liquid.

Zwietering’s correlation predicts that, on scale-up, the power input by the impeller per mass of slurry will decrease while Grenville et al. show that it is constant, and this is an important conclusion for designers of industrial scale equipment. Giacomelli et al. (Flow. Turb. Comb., 105, 31-62, 2020; AIChEJ., 67, 2, 2021; ChERD., 180, 318-332, 2022;ChERD., 190, 793-813, 2023) have developed a CFD model to confirm the underlying assumptions of the Baldi / Davies model and extended the applicability of the Grenville et al. correlation to vessels with other, industrially relevant geometries.

In this presentation we will review the history of this important area and discuss recent developments in research.