Étude expérimentale du mélange solide-liquide: Caractérisation des suspensions concentrées en milieu visqueux

摘要

To address a considerable lack of knowledge in solid-liquid mixing, the suspension of large concentrations of spherical particles in a viscous fluid is investigated through the experimental characterization of fundamental mixing parameters: the just-suspension speed (Njs), the homogenization speed (NH) and the homogenization time (t H). Known for its great efficiency for suspending particles in turbulent regime, and commonly used in the chemical industry, the pitched blade turbine is investigated. Njs is characterized by the pressure gauge technique (PGT), NH and tH are measured using a robust and simple technique of electrical resistance tomography (ERT). The description of the system is further characterized by means of power consumption and cloud height measurements, and by comparing the experimental data to numerical results.;First, the PGT method was successfully adapted to laminar and transitional regimes of operation, with good reproducibility and stability. To explore the effect of particle diameter (dp), solid mass concentration (Xw), viscosity (mu), impeller diameter (D) and bottom-off clearance (C), an optimal design of experiment was carried out, from which the effects of every factor was determined. In early transitional regime with non-dilute concentration of solid particles, an increase in D causes Njs and NH to considerably decrease, as also reported in turbulent regime. Unlike the prediction of the well-known Zwietering correlation (Zwietering, 1958), it is found that increasing dp or mu leads to a decrease of Njs. In addition, the effect of solid concentration is more complex than what is predicted by the Zwietering correlation, showing a dependency on dp. The large discrepancies observed between our experimental values and Zwietering correlation values of Njs are related to the differences in hydrodynamics and suspension mechanisms regarding the operating regime. The effects of dp and mu for solid suspensions in a viscous liquid appear to correspond to the erosion of a bed of particles. The impact of Xw, dp and C/T on NH are similar to those reported on turbulent regime. The study on the homogenization time t H shows that the erosion of the particle bed is the dominating phenomenon to consider in order to achieve a fully suspended state and an uniform distribution of the particles. Finally, the numerical model developed in our group is validated by comparison with experimental data, and is used to fully understand the mixing system investigated.;Keywords: solid-liquid mixing - viscous fluid - high solid loading -- stirred tank -- just suspended speed.