393903 Minimum Speed for Off-Bottom Solids Suspension: Zwietering or P/V

Tuesday, November 18, 2014: 5:20 PM
313 (Hilton Atlanta)
Thomas Post, Post Mixing Optimization and Solutions, LLC, Pittsford, NY

Experiments were made to study the effects of solids suspension parameters on correlations such as
Zwietering and a constant P/mass approach. Tests were made in 3 geometrically similar tanks with tank
diameters of 0.3m, 0.5m, and 1.0m with various axial flow impellers. CFDs of each condition were also
made to determine what the conditions were near the locations of the last suspended particles.

There are many articles published about solids suspension in mixed tanks. Many are confusing, and
many contradict each other. We decided to launch an experimental and computational study to better
understand how disagreements between researched results can happen. The most commonly used
correlation for scale-up is based on the work of Zwietering. But many people have criticized it. More
recently, there have been claims that the scale-up should be based on constant P/V, and others that it
should be a hybrid between the two models.

Our study looks at 12”, 20” and 40” tanks and a variety of hydrofoil impeller designs and pitched
bladed turbines of various impeller to tank diameter ratios and various off bottom clearances. We
varied the solids concentration from 1 to 65%. We varied the particle sizes from 80 to 1000 microns.
Every experiment was duplicated with CFD to better understand the hydro-dynamics along the tank
base and the walls. Based on our results we plan on expanding our work to a 96” tank and investigate
the effect of solids as small as 20 microns on solids suspension.

The impeller design does have an impact on solids suspension, but we also found that for geometrically
similar setups, solid suspension was fundamentally different between the 12”, 20” and 40” tanks. We
also discovered a “sweet” spot for each impeller design, where the power of suspension was at a
minimum for that tank geometry and slurry characteristics. An amazingly simple little “trick” will be
demonstrated to improve solids suspension in many full-scale mining applications. Finally, we will also
show how constant P/V and Zwietering can both be correct scale-up methods.


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See more of this Session: The Use of CFD in Simulation of Mixing Processes II
See more of this Group/Topical: North American Mixing Forum