Blend Times with Dual-Impellers in a Stirred Vessel

In recent studies, researchers have made significant discoveries about the effects of baffles on the blend time in stirred tanks. In this study, the focus is on comparing, baffled and unbaffled vessels. Baffles change the direction of the flow of fluid from rotational motion to turbulent motion; to improve mixing. On the other hand, a vessel without baffles is usually considered to have undesirable fluid flows, leading to vortex formation, gas entrainment, segregation, and longer blend times. However, industries use unbaffled vessels for many process operations. The reasons cited for using unbaffled vessels are primarily concerned with cleaning the vessel between batches and the potential for stagnation in high-viscosity fluids, particularly when properties change over the processing time. Previous work has shown that the flow patterns are dramatically different between unbaffled and baffled vessels. There is a familiar turbulence throughout the vessel at high Reynolds numbers with the baffled configuration. Without the baffles the fluid segregates into two zones; a swirling inner zone near the shaft and a more turbulent outer zone. Two blend times were identified 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 a low Reynolds number, the baffled blending is in thew 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 vortex length is reduced with decreasing Reynolds number. Most of the blend time studies were reported on vessels with a single impeller. In an industrial setting, vessels may have a liquid height higher than the diameter of the vessel. In this vessel configuration, it is common in the industry to use multiple impellers placed on a single shaft. This research study examines the effect of dual impeller systems on blend time in both baffled and unbaffled tanks.