Turbulent blend times with off‐centre v‐mounted agitators

• Published in: Canadian journal of chemical engineering 2020-11, Vol.98 (11), p.2360-2367
• Main Authors: Chen, Peng, Myers, Kevin J., Janz, Eric E., Brown, Nicholas A.
• Format: Article
• Language: English
• Subjects: Agitators; blend time; blending; Diameters; eccentric agitator; Guidelines; Hydrofoils; Impellers; Turbines; unbaffled blending; vertical agitator
• ISSN: 0008-4034; 1939-019X
• DOI: 10.1002/cjce.23772

Experimental measurements in a flat‐bottom tank with narrow‐blade hydrofoil and pitched‐blade impellers are used to develop guidelines for off‐centre, or eccentric, placement of vertical agitators in unbaffled tanks. The guidelines are based on providing a turbulent blend time that is no more than 20% longer than that of the same impeller operating at the same rotational speed on the centreline of a baffled tank. In addition to investigating the effect of impeller type, impeller diameter and off‐bottom clearance are also considered. The results support the commonly noted rule of thumb that as off‐centre distance is increased, performance in an unbaffled vessel approaches that in a baffled tank. A notable exception to this axiom occurs when a large impeller is located close to the tank base (specifically, D/T = 0.40, C/T = 0.10, and O/T = 0.25). In this case, a stable impeller tip vortex forms with both impeller types, with slow exchange of material between the vortex and bulk liquid leading to long blend times. Besides blend time decreasing with increasing off‐centre distance, the uncertainty or run‐to‐run variation in blend time also decreases dramatically. In most cases, the pitched‐blade turbine requires a smaller off‐centre distance than the hydrofoil impeller to approximate the blending performance provided during baffled operation.