(239a) Effective Single-Impeller Turbulent Blending In Tall Vessels

Liquid level is typically reported to have a limited effect on blend time for an agitator with a single impeller in turbulent operation (blend time proportional to the square root of liquid level).  However, this result seems to be based on limited data in shorter vessels with liquid levels less than or equal to the tank diameter.  In practice, taller vessels are often used to increase volume without increasing footprint, and little information is available concerning the effect of higher liquid levels on agitator blend time.

This work demonstrates that turbulent blend times with single radial-flow and down-pumping axial-flow impellers increase rapidly as the liquid level becomes greater than the vessel diameter.  One method to keep blend times from becoming excessive in tall vessels is to use multiple impellers.  However, multiple radial-flow impellers “stage”, with rapid blending occurring within the stage generated by each impeller but with significantly slower exchange between the stages.  Multiple down-pumping axial-flow impellers do not stage such that they provide rapid blending, but this comes at the increased cost of additional impeller(s), larger shafting and gear box, and higher operating costs due to increased power draw.

A different approach to effective blending in tall vessels is to use a single up-pumping axial-flow impeller.  By directing the impeller discharge flow into the majority of the liquid above the impeller, the velocities in the upper portion of the vessel are not as low as those produced by a down-pumping impeller such that blend time does not increase rapidly with increasing liquid level.  Similarly, turbulent blend times do not increase rapidly with increasing liquid level when a single down-pumping impeller is placed high in the vessel such that the impeller’s discharge flow is directed into the majority of the liquid below the impeller.  Placing the down-pumping impeller high in the vessel mimics the advantageous velocity field generated by an up-pumping impeller placed low in the vessel by eliminating extremely low velocities.

This work presents data that can be used to design for effective turbulent blending using a single-impeller agitator across the wide range of liquid levels encountered in practice.