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Mixing Methods Create Quite a Stir

The article [Three Steps to Better Mixing,"Feb. 03, p 42] by Pordal and Matice contains a number of erroneous statements and conclusions. I agree with the three-tiered path for mixing evaluation and design. However, in apply-ing these steps to the stated blending problem, the article is severely flawed.

In the Tier 1 analysis, the selected impeller diameter is too large for turbulent mixing applications. General guidelines for impellers in this service call for impeller diameters closer to 0.15 m, not 0.25 m. Further, spacing should be near 0.42 m, not 0.5 m as shown. Correct impeller selection is critical to assure optimum flow behavior in the tank.

The authors do not list the source of their results for the Tier 2 analysis, the cornerstone of their conclusions. While the impeller speeds are not listed, back calculation from flow and power-draw data suggests an operating speed for the Rushton Turbine near 114 rpm, the Lightnin A310 near 233 rpm. An experiment demonstrates a blend time: for the A310 under the selected conditions near 20 sec., and the Rushton Turbine near 25 sec., no vortices were observed. The authors' conclusion that a Rushton Turbine is the best blending device is not supported by experimental results or a multitude of production plant data.

Finally, tangential and axial velocity distributions are listed as important mixing parameters, without elaboration on how these data are useful in any practical sense. The Tier 3 analysis appears to be based on a 1.25-m liquid level, while the remainder of the problem is based on a 1.0-m liquid level. Figures 3 and 4 suggest that full convergence has not been reached, since the nondimensionalized velocity field is sharply delineated at what appears to be the MRF boundary. More importantly, if the velocity field is normalized by im-peller tip speed (as is customary), then the conclusion of valid scale-up based on the work presented is incorrect. As long as geometric similarity and flow regime are as shown, i.e., turbulent flow is held, then the normalized velocity fields are expected to be similar.
Bernie Gigas, Manager, Research & Development, Lightnin, Rochester, N.Y.

One of the authors responds:
The main purpose of the article is to illustrate a solution strategy to extract information at various scales. The article provides a discussion of analysis techniques that can be applied for scale-up or scale-down of stirred tank mixing processes. We used general guidelines for sizing, and currently available equipment to illustrate the solution techniques discussed. Optimization of equipment is not the purpose of this article, though these methods can be applied for optimization.

Several of Gigas' objections are based on his back calculation of rpm from the data presented in the paper. These calculations do not match those used in our study; an rpm of 250 was applied for the analysis reported in Table 2. Similarly, the mixing time he reported is for a smaller volume of liquid. Gigas's comment regarding software used can be addressed by referring to the bibliography included with the article.

Gigas makes a specific complaint that the CFD analyses are not converged, and cites as evidence a sharply delineated multiple reference frame (MFR) boundary. I believe he has mistaken lines corresponding to the edges of baffles for the MFR boundaries. In any case, all solutions were carefully converged and checked prior to publication, and represent sound application of CFD solution techniques. The analysis methods presented in the article have been compared with experimental data in other studies.
Harbi Pordal

Ready for Responsible Care
Although I'm not directly involved in Responsible Care, as you stated in the article [Can You See Me Now?,"June 03, p 24], changes to the code have the potential to affect everyone, even down to my level. That is why I like to keep current on changes in the chemical industry. Your article was very informative, and a good update on Responsible Care.

I especially appreciate the "at a glance" info on the metrics additions and the info on the new security code, which are handy for quick reference. Thanks for a wonderfully informative article.
Jeff Kruszka, Process Engineer, Crompton Corp., Geismar, La.

 

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