Turbulent Blending with a Vertical Off-Center Agitator in an Unbaffled Tank


Peng Chen

Date of Award


Degree Name

M.S. in Chemical Engineering


Department of Chemical and Materials Engineering


Advisor: Kevin Myers


Turbulent blend time in 0.445-m diameter flat-bottom cylindrical baffled and unbaffled tanks was investigated. Hydrochloric acid - sodium hydroxide neutralization reaction indicated by phenolphthalein was used to determine the blend time. Two types of impellers (HE-3 and P-4) with three diameters (0.114 m, 0.152 m, and 0.178 m corresponding to dimensionless impeller diameter D/T = 0.26, 0.34, 0.40) were tested at three off-bottom clearances (C/T = 0.10, 0.25, 0.40). For baffled tank, dimensionless blend time (Ntb) was found to decrease with increasing off-bottom clearance, which is not mentioned in most previous studies. For HE-3 impeller, dimensionless blend time Ntb, decreases by more than 20% as C/T increases from 0.10 to 0.40. For P-4 impeller, Ntb decreases by less than 10% as C/T increases from 0.10 to 0.40. Correlations for dimensionless blend time (Ntb) were developed, and the maximum error and average error of predicted blend time are 13% and 4% for HE-3 impeller, 6% and 2% for P-4 impeller.For unbaffled tank, experiments were performed with the same impellers at the same off-bottom clearances and various off-center distances (C/T = 0.00, 0.13, 0.17, 0.20, 0.25, 0.30). It was found that in most cases blend time in unbaffled tank decreases with increasing off-center distance and reaches a blend time close to that in a baffled tank when the off-center distance is high enough. The run-to-run variability of blend time, as indicated by the coefficient of variation, also decreases with increasing off-center distance. Blend time in unbaffled tank less than or equal to 1.15 times that in baffled tank at the same off-bottom clearance was considered acceptable performance. Recommended off-center locations for HE-3 and P-4 impellers in unbaffled turbulent blending operations are provided. Smaller impeller was found to have a wider range of off-center distance that works well compared to larger impeller. For HE-3 impeller, higher off-bottom clearance was found to have a wider range of off-center distance where the blend time is acceptable than lower off-bottom clearance. In general, HE-3 impeller requires higher off-center distance in unbaffled tank than P-4 impeller to reach a blend time close to that in the baffled tank. For large HE-3 and P-4 impellers (e.g. D/T=0.40), when the off-bottom clearance is low (i.e. C/T=0.10) and the off-center distance is high (i.e. O/T=0.25), a stable impeller tip vortex forms. Exchange of liquid between inside and outside the vortex is very slow and extends the value and variability of the blend time. Thus, this geometry is not recommended.


Chemical Engineering, turbulent blending, vertical blending, off-center blending, unbaffled tank, blend time

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Copyright 2019, author