(466a) Hydrodynamics Characteristics of a Stirred Tank Provided with Angle-Mounted Impellers Using Computational and Experimental Approaches

Most industrial stirred tanks and reactors are provided with a centrally mounted impeller to ensure adequate mixing and with wall baffles to prevent the swirling effect that would otherwise occur in an unbaffled vessel. However, an alternative approach to reduce swirling and promote mixing in an unbaffled tank is to place the impeller asymmetrically and mounted on an angled shaft. This type of configuration is common in a number of industrial operations and especially in the pharmaceutical industry, since the absence of baffles reduces the potential for contamination and ensures thorough cleaning of the vessel between batches while introducing adequate top-to-bottom liquid recirculation of the vessel content and effectively eliminating swirling effects. Although industrially relevant and relatively common in the industrial practice, only limited information is available on the operation of tanks provided with angle-mounted impellers and even less is known of their hydrodynamics. Therefore, the objective of this work was to study the hydrodynamics of a scaled-down version (diameter: 316 mm) of a tall industrial vessel with an elliptical bottom and provided with two angle-mounted (by 5° off the vertical) A310 Lightnin impellers under different operating conditions in order to determine the mixing characteristics of the industrial system. Both Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) were used to quantify the hydrodynamics and provide guidance on the optimal operation of these industrial system. The PIV apparatus was used to experimentally determine the velocity flow field in water at different agitation speeds so as to validate the CFD predictions. A multiple reference frame (MRF) computational approach and turbulence models based on the k-ε method were applied for all cases in the CFD simulations. The results revealed substantial agreement between the CFD predictions and the PIV experimental results. A complex flow in the vessel was observed. Due to the axial pumping action of the impellers, a downward flow impinging the vessel bottom was created resulting in the splitting of the flow. A portion of the zone near the bottom of the vessel, which is the preferential location for the sedimentation of settling solids in the liquid, was dominated a less well-mixed zone formation. Additionally, increasing agitation speeds had no significant effect on the presence of that zone.