In industrial chemical processes, tall tank agitators are often required due to space limitations or process needs. Most tanks have baffles that alter the direction of fluid flow from rotational motion to a vertical motion, improving overall mixing. On the other hand, an unbaffled vessel usually exhibits undesirable fluid flows causing vortex formation, longer blend times, however, processes without baffles require less cleaning. This study investigates the optimal placement of a second impeller in a tall tank with baffles and without baffles for dimensionless blend time. The bottom impeller was set at a clearance equal to the impeller diameter, while the spacing of the upper impeller is moved closer to the liquid surface. Blend times were measured using an iodine neutralization color change over a range of spacings and turbulent Reynolds Numbers. Spacings were normalized to tank diameter to keep units dimensionless. Video analysis was used to quantify blend time, and a non-linear regression was performed to correlate blend times with dimensionless spacing. In the baffled tall tank, dimensionless blend time decreased with spacing. At dimensionless spacing equal to 0.875, air entrainment caused a significant increase in dimensionless blend time. Optimal placement for a baffle tall tank occurred at a dimensionless spacing of 0.765. In the unbaffled tall tank it was determined that a single impeller resulted in a smaller dimensionless blend time, than adding the second impeller to the system. These findings provide clearer insight into more complex mixing systems.
