Blending in Baffled and Unbaffled Mechanically Agitated Vessels – Effect of Reynolds Number and Liquid Height

The standard vessel for mixing in the process industry is a baffled tank. This design is very common especially for low viscosity fluids. Nonetheless, unbaffled vessels are found in both industry and the laboratory. The reasons cited for using unbaffled vessels are primarily concerns around cleaning the vessel between batches and the potential for stagnation in high viscosity fluids, particularly when properties change over the processing time. We will examine the effect of baffles on overall blend time using one vessel with removeable baffles. The same axial flow-down pumping impeller is used. The blend time was measured using the iodine color change method over a range of viscosities and Reynolds Numbers. The depth of the vortex was also measured. In this work we observed that the flow patterns are dramatically different between unbaffled and baffled tanks. At high Reynolds Numbers with the baffled configuration, we see a familiar turbulent diffusion throughout the vessel. Without the baffles the fluid segregates into two zones: a swirling inner zone near the shaft and a more turbulent outer zone. The overall blend time is longer in the unbaffled tank at the same rotational speed compared to the baffled tank. We identified two blend times in using unbaffled tanks, a short blend time for the outer zone and a much longer time for the inner vortex centered zone which defines the overall blend time for the vessel. At low Reynolds Numbers the baffled blending is in the transitional flow regime between turbulent and laminar with a combination of eddies and striations. The last region of the tank to become completely mixed is located behind the baffles. Unbaffled blend times are shorter than baffled and as the Reynolds Number decreases the difference increases. The vortex is reduced with decreasing Reynolds number. We consider this work just a beginning step in a long study and hope this work will be continued by other researchers.