(301a) Effect of Impeller Type, Diameter, and Number on Turbulent Blend Time

There are two widely used correlations for predicting turbulent blend time in agitated systems.  The approach developed by FMP¹ concludes that all impellers provide the same blend time when compared at the same diameter and power input per mass, while the approach of Fasano and Penney², building upon the work of Khang and Levenspiel³, describes blending as a first-order process governed by a mixing rate constant that is characteristic of each impeller.  Data taken with ten impeller styles indicate that both approaches generally provide good turbulent blend time estimates, but do have limitations.  The impellers studied in this work include both axial-flow and radial-flow impellers, with turbulent power numbers that span a factor of twenty-five range.  In addition to considering the blending performance of single impellers in a square batch (liquid level equal to tank diameter), data is reported for the industrially significant case of multiple impellers in taller batches.  The aim of this portion of the study is to understand the effects of liquid level and number of impellers on turbulent blend time, how these parameters interact, and to determine when it is economically justified to add a second impeller.

References:

1.         Grenville, R. K., and A. W. Nienow, “Blending of Miscible Liquids”, Chapter 9 in Handbook of Industrial Mixing:  Science and Practice, edited by E. L. Paul, V. A. Atiemo-Obeng, and S. M. Kresta, Wiley-Interscience, Hoboken, New Jersey (2004).

2.         Fasano, J. B., and W. R. Penney, “Avoid Blending Mix-Ups”, Chemical Engineering Progress, pages 56-63 (October, 1991).

3.         Khang, S. J., and O. Levenspiel, “The Mixing-Rate Number for Agitator Stirred Tanks”, Chemical Engineering, pages 141-143 (October, 1976).