Mechanisms of solids drawdown in stirred tanks

Abstract

Agitated tanks are used in several industrial processes to achieve complete drawdown of floating solids in liquids. The design requirements for this process are not completely defined, and are currently limited to heuristics regarding the use of a surface vortex and the effect of wettability on the difficulty of mixing, along with several initial studies in the literature. In this study, the effect of the type of impeller, particle size and shape, solids concentration, impeller submergence, and baffle configuration on the minimum drawdown speed (N jd ) are investigated. It was found that the formation of a large surface vortex acts to hold particles close to the surface. Suppression of the surface vortex is recommended. In baffled tanks where the formation of a large surface vortex is suppressed, the intensity of turbulence and mean circulation velocity of the liquid are responsible for solids drawdown and distribution in the tank. The submergence of the impeller relative to the liquid surface and the pumping mode of the pitched blade turbine (PBT) were found to be the controlling parameters. CFD simulations were carried out to obtain a better understanding and interpretation of the flow patterns and drawdown mechanisms for the different baffle configurations.Les réservoirs agités servent dans plusieurs procédés industriels pour réaliser l'immersion complète des solides flottant sur les liquides. Les besoins de conception pour ce procédé ne sont pas complètement définis et sont actuellement limitésà une approche heuristique utilisant un vortex de surface età l'effet de la mouillabilité sur la difficulté de mélange, en complément de plusieursétudes initiales dans la littérature scientifique. Dans cetteétude, l'effet du type de turbine, de la taille et de la forme des particules, de la concentration de solides, de la submersion de la turbine et de la configuration des chicanes sur la vitesse d'immersion (N jd ) estétudié. On a trouvé que la formation d'un vortex de surface important a pour effet de garder les particules proches de la surface. La suppression du vortex de surface est recommandée. Dans les réservoirs munis de chicanes où la formation du vortex de surface important est supprimée, l'intensité de la turbulence et la vitesse de circulation moyenne du liquide sont responsables de l'immersion des solides et de leur distribution dans le réservoir. On a trouvé que la hauteur d'immersion de la turbine par rapport a la surface du liquide et le mode de pompage de la turbineà pales inclinées (PBT)étaient les paramètres gouvernants. Des simulations en CFD ontété menées pour une meilleure compréhension et interprétation des profils d'écoulement et des mécanismes d'immersion pour différentes configurations de chicanes.

A novel geometry for solids drawdown in stirred tanks

“…Two impellers and two submergences were examined, all using full height baffles and surface baffles with the impeller rotating at N jd . The fully baffled results were previ- ously reported in Khazam and Kresta (2008), but are included here to allow direct comparison with the surface baffle configuration.…”

“…In Khazam and Kresta (2008) it was also shown that the fully baffled configuration performs better than either an unbaffled tank or a tank with a single full height baffle. The unbaffled tank draws a strong stable central vortex, while the flow with a single full height baffle is not as stable as it is in a fully baffled tank.…”

“…The mechanisms of solids drawdown, the interactions between the design variables, and the effect of configuration on performance are only beginning to be discussed in the literature. The reader might expect at least a partial analogy between the drawdown of floating solids and the off bottom suspension Greek characters ε local rate of dissipation of turbulent kinetic energy per unit mass at any location (m 2 /s 3 ) kinematic viscosity (m 2 /s) B Bayol density (kg/m 3 ) L liquid density (kg/m 3 ) S solid density (kg/m 3 ) W tap water density (kg/m 3 ) density difference, L − S (kg/m 3 ) Khazam and Kresta (2008) identified three mechanisms for solids drawdown: a stable central vortex, turbulent engulfment, and mean drag. In turbulent engulfment, small energetic eddies break up lumps of particles on the surface and pull the particles down into the circulating flow; in mean drag, strong bulk circulation at the surface moves particles to the walls, or to the center of the tank, and drags them down into the bulk of the tank in a continuous stream.…”

“…

a b s t r a c tTwo mechanisms for the drawdown of floating solids in baffled stirred tanks have been identified [Khazam, O. and Kresta, S., 2008, Mechanisms of solids drawdown in stirred tanks. Can J Chem Eng, 86(4): 622-634]: turbulent engulfment and mean drag.

…”

CFD prediction of mean flow field and impeller capacity for pitched blade turbine

“…The distortion of free liquid surface(gas-liquid interface)was beyond the scope of this research since its effect was negligible in the fully baffled tanks. For single baffle cases, a more accurate treatment of free surface was required to predict the turbulence and transient eddies at the surface, but for full baffle cases, this approximation was reasonable [18] . Hence, the 'symmetry' boundary condition with zero normal velocity gradients was imposed on the free liquid surface.…”