158 research outputs found
Analysis of the fluid dynamic behavior of the liquid and gas phases in reactors stirred with multiple hydrofoil impellers
No full textLiquid- and gas-phase macromixing behavior was studied in gas - liquid high-aspect-ratio reactors stirred with multiple hydrofoil impellers pumping downward. Water, a sodium sulfate solution, and poly(vinylpyrrolidone) solutions of viscosity up to 110 mPa·s were used as the liquid. For characterizing the liquid phase, mixing time experiments were conducted at various operating conditions, while detecting the response curves at several positions inside the tank. Comparison of the experimental curves with the theoretical ones provided by simple fluid dynamic models showed that the axial dispersion model is quite acceptable. The influence of impeller speed, gas flow rate, and viscosity on the model parameter was studied, and dimensionless relationships are given. The gas behavior was studied by means of the RTD and modeled with the axial dispersion model, which proved good for water and acceptable with coalescence-inhibiting electrolyte solutions. The model parameter dependence on the operating conditions was studied. Comparison between hydrofoil impellers and radial Rushton turbines is also attempted
Solids distribution in stirred slurry reactors: Influence of some mixer configurations and limits to the applicability of a simple model for predictions
No full textThe features of solids concentration distribution were investigated in baffled and unbaffled tanks of high aspect ratio, stirred with multiple radial, mixed-flow or axial impellers. In the baffled tanks the average profiles can be predicted with the sedimentation-dispersion model regardless of impeller type - in spite of slight systematic departures from the average trend at the impeller plane and/or midway between the impellers. This is hardly possible for the unbaffled tanks due to lack of physical foundation of this simple model. When the unbaffled tank is stirred with hydrofoil impellers, an inverted profile is even obtained. The mentioned departures and this last anomalous behaviour are qualitatively discussed with reference to particle-turbulence interaction
Scale-up criteria for the solids distribution in slurry reactors stirred with multiple impellers
No full textScale-up criteria for obtaining the same vertical concentration profiles in agitated suspensions are discussed. The experiments were carried out in reactors of two scales (V=39.6 and 261 l) characterised by a high aspect ratio and stirred with multiple, evenly spaced impellers of two different types. The profiles were determined under different conditions: at constant tip speed, at constant specific power consumption, and at an intermediate condition (i.e., N∝D-0.93). The experimental profiles were compared with different approaches, namely on a qualitative basis, in terms of standard deviation and the parameter of the axial dispersion model with sedimentation. In all cases, the same criterion based on the aforementioned intermediate condition (closer to constant tip speed) was confirmed. The experimental data were also examined in terms of effective particle settling velocity, which is a basic parameter for modelling, and fair agreement of the data obtained at the two scales with the different impellers was obtained. © 2003 Elsevier Ltd. All rights reserved
Diagnosis of solid distribution in vessels stirred with multiple PBTs and comparison of two modelling approaches
No full textThe features of solids concentration distribution were investigated in two baffled vessels of different scale. The vessels were of high aspect ratio and were stirred with multiple PBTs. Both steady- and unsteady-state experiments were performed. The experimental data were compared with the previsions of the one-dimensional sedimentation-dispersion model and of CFD tools. The former approach provides good estimates only of the average, steady-state vertical profiles, while the latter describes the local variations much more accurately. Both approaches give fairly good estimates of the transient solids concentration distribution. The dynamic CFD simulations allowed us also to tune the value of the turbulent Schmidt number as a relevant parameter. Finally, both simulation approaches have confirmed that the particle settling velocity in a stirred liquid is a correct parameter to be used instead of the terminal settling velocity
Axial Force at the Vessel Bottom Induced by Axial Impellers
No full textThis paper deals with the axial force affecting the flat bottom of a cylindrical stirred vessel. The vessel is equipped with four radial baffles and is stirred with a four 45° pitched blade impeller pumping downwards. The set of pressure transducers is located along the whole radius of the flat bottom between two radial baffles. The radial distribution of the dynamic pressures indicated by the transducers is measured in dependence on the impeller off-bottom clearance and impeller speed.It follows from the results of the experiments that under a turbulent regime of flow of an agitated liquid the mean time values of the dynamic pressures affecting the bottom depend not on the impeller speed but on the impeller off-bottom clearance. According to the model of the flow pattern of an agitated liquid along the flat bottom of a mixing vessel with a pitched blade impeller, three subregions can be considered in this region: the liquid jet streaming downwards from the impeller deviates from its vertical (axial) direction to the horizontal direction, the subregion of the liquid flowing horizontally along the bottom and, finally, the subregion of the liquid changing direction from the bottom upwards (vertically) along the wall of the cylindrical vessel, when the volumetric flow rates of the liquid taking place in the downward and upward flows are the same
Experiments and CFD predictions of solid particle distribution in a vessel agitated with four pitched blade turbines
No full textThe distribution of solid particles in a high aspect-ratio baffled tank agitated with four 45° pitched blade turbines (PBT) was investigated using both experimental measurements and CFD simulations. Dilute suspensions of glass beads in water and moderately viscous liquids were considered. The measurement of axial particle concentration profiles was conducted by means of a light attenuation technique. Fully predictive simulations of solid-liquid suspensions were performed using a Sliding-Grid approach coupled with the Eulerian-Eulerian Two Fluid Model and the 'homogeneous' two-phase k-ε turbulence model. The simulated particle axial concentration profiles were compared with the experimental data and good agreement was found
Modelling of solids distribution in stirred tanks: analysis of simulation strategies and comparison with experimental data
No full textThe predictive capabilities of CFD techniques as applied to solid–liquid stirred vessels are investigated.
The distribution of solid particles was simulated in three baffled stirred tanks agitated with single and multiple impellers. Suspensions of glass beads of different diameters and average concentration up to 6 vol. % in water were studied. The simulations of solid–liquid suspensions in the stirred vessels were performed by using fully predictive approaches. Eulerian multiphase models were adopted for modelling the solid–liquid flow, coupled with three different extensions of the standard k-1 model to the case of multiphase flows. The simulated particle axial concentration profiles are compared with experimental data and critically discussed. The most successful simulation strategy and one possible implementation are described
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