(721b) Experimental Determination and Computational Prediction of Vortex Shape and Vortex Formation in an Unbaffled Stirred Vessels Provided with a Retreat-Bladed Impeller

Glass-lined, stirred reactors are frequently utilized in the pharmaceutical industry to perform various unit operations such as crystallization, fermentation, cell culture, chemical reactions, and others where product contamination and chemical compatibility are concerned. The vessel walls and corresponding impeller are lined with a glass coating to prevent corrosion and provide a smooth surface for superior cleanability. Typically, this reactor is fitted with a Retreat-Blade Impeller (RBI) with low impeller clearance and a single baffle. However, in some applications, the presence of baffles may create undesirable outcomes such as particle attrition in the crystallization process, cell damage in bioreactor applications, or promote product cross-contamination. Therefore, unbaffled stirred tank are also commonly used. Although unbaffled stirred reactors may offer some advantage in some cases, such as lower power consumptions per unit volume, the lack of baffles tends to generate high swirling motion leading to a vortex formation in the impeller shaft region, primarily when operating in the turbulent flow regime. The fluid flow in a stirred vessel can be complex. Vortex shape and depth are typically dependent on the agitation speed, the liquid medium viscosity and vessel geometry. Thus, the objective of this work was to experimentally characterize vortex formation in a scaled-down unbaffled RBI system at different agitation speeds in turbulent regimes. The depth and shape of the vortex we precisely determined using a leveled laser pointer system and all experiments were conducted in triplicates to verify the reproducibility and robustness of the experimental approach. Computational Fluid Dynamics (CFD) was additionally used to predict the vortex shape and depth using a Multiple Reference Frame (MRF) model coupled with a Volume of Fluid (VOF) model. The CFD modeling was validated by comparing the predictions from the simulations with the experimental data. In all case they were found to be in close agreement. As anticipated, increasing the agitation speed resulted in a significant variation in vortex shape and depth. This result of this work can help industrial practitioners optimize pharmaceutical processes and their reactor/bioreactor design.