Fluid‐Borne entities in the impeller stream of a rushton turbine

Abstract

he breakage or disintegration of fluid-borne entities in stirred tanks is a key operation in the chemical process industries. But T because flow in stirred tanks is inhomogeneous and complex, no general method has been put forward that permits quantitative evaluation of size reduction events. The location of the interaction between a fluid-borne entity and the impeller stream largely determines the outcome of the encounter, and many other variables including size, density, and shape affect it. Calabrese and Stoots (1989) noted that globules, flocs, and fragile particles disintegrate in response to forces arising from the vortex system trailing the impeller blades, or from the region where the vortex system breaks down. This is generally confirmed by observation; for example, Chang et al.(1 981) demonstrated that the dispersion of oil in water can occur by stretching (and subsequent ejection) as oil drops flow over and around an oncoming turbine blade. Work carried out in our laboratory also shows that the deformation of large drops of a neutrally-buoyant oil dispersed in water can occur well out into the impeller stream as the entitiy is entrained in the impeller discharge.The performance of the Rushton turbine in mixing processes, emulsification, and chemical reactor operation has been the subject of intense examination for 40 years. Measurements of velocities and velocity vector orientation, as well as estimates of dissipation rate, have appeared in the literature; these efforts span the decades from Sachs and Rushton (1 954) to Lee and Yianneskis (1 998). The range and scope of the contributions can be gauged by consulting one of the excellent published reviews such as Chapter 4 in Tatterson 286The breakage or disintegration of suspended entities by energetk fluid motions in stirred tanks is an essential aspect of many operations in the chemical process industries. However, the hydrodynamic inhomogeneity of such tanks makes it extremely difficult to characterize the stresses experienced in any simple manner. This work provides a determination of both the location and the frequency of interaction of spherical fluid-borne entities with the discharge of a Rushton turbine. These data show how both particle size and impeller speed affect the severity of the exposure, setting the groundwork for improved descriptions of the dynamic behaviour of the particle size distribution in a wide variety of dispersed-phase processes.La rupture ou la desindgration de particules suspendues par des deplacements de fluides Cnergetiques dans des reservoirs agitCs est un aspect essentiel de nombreuses operations des industries de proc&des chimiques. Toutefois, la non-homogdnkite hydrodynamique de ces reservoirs rend exMmement difficile la caracterisation des forces en jeu par une methode simple. On determine dans ce travail la position et la frequence d'interaction de particules sphkriques transpottees par le fluide dans la zone de refoulement d'une turbine Rushton. Ces donnees montrent comment la taille des particules et la vitesse ...