Blending in unbaffled and baffled mechanically agitated vessels – Effect of Reynolds number and liquid height

Abstract

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 industry and the laboratory. The reasons given for using unbaffled vessels are primarily concerns around cleaning the vessel between batches and the possibility of stagnation zones in high viscosity fluids particularly when properties change over the processing time. In this paper we will examine the effect of baffles on the blend time using one vessel: with and without baffles. The blend time was measured using the iodine color change method using a range of viscosities and Reynolds Numbers. The depth of the vortex was also measured. The same axial flow-down pumping impeller is used. 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 the 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 baffled tank at the same rotational speed compared to the baffled tank. This difference decreases with increasing Reynolds Number. 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 and as the Reynolds Number decreases the difference increases. The vortex is also reduced with decreasing Reynolds number. We consider this work just a beginning step in a long study and hope this work will be picked up by other researchers as happened so many times with Professor Nienow’s work.