Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Mixing in stirred vessels of different density liquids is common operation in the process industry. The hydrodynamic behaviour of such a system could be crucial for the overall process performance. The aim of this work is to numerically predict the dynamics of the mixing process of initially stratified system of two different density liquids. Stratified conditions could occur in the stirred vessel, especially in the case of impeller malfunctioning, i.e. impeller breakdown. This effect might prove to be of significant importance, particularly in the case of reacting liquids for large-scale reactors operating in the industry.
The CFD analyses were performed for a non-baffled stirred vessel reactor, mechanically agitated by the Pfaudler impeller. The two main cases of passive and reactive tracer mixing behaviour were numerically predicted by the means of the CFD software CFX 5.7. In both cases the system of two miscible liquids with different densities was dynamically predicted in 3D from an initially stratified to a complete mixing condition. The full 3D simulation was applied in order to capture the flow instabilities associated with the impeller blade passage, especially pronounced in the first few seconds after the impeller start. In order to study the effect of the free surface deformation on the mixing process, the gas phase was also involved into the system via the free surface model. The different density liquids were comprised into the liquid phase by the means of the multicomponent model. In the case of the reactive tracer mixing, the alkali was diluted in the lighter liquid. The simulations were performed on different grids in order to obtain grid independent results.
The accuracy of the numerical simulations was evaluated experimentally using video visualisation technique. The lighter (alcoholic) coloured component and the heavier (water) transparent one which were initially stratified were brought into motion by the rotating impeller. In the case of the passive tracer mixing, the alcoholic phase was coloured by blue dye whereas in the case of the reactive mixing a phenolphthalein indicator was applied. The mixing process was captured by a digital camcorder and subsequently the images were digitally processed. The mixing of different initial lighter phase concentrations was visually investigated to obtain the colour calibration curve. The numerical predictions were evaluated against several locations, close to vessel central line and wall, for which the optical distortion was minimal.