April 11, 2003

RE: "Three Steps to Better Mixing," February 2003

Ms. Canning:

I found the article on liquid,"liquid mixing to be of interest. I have frequently used Visimix for design and troubleshooting purposes such as crystallization and precipitation operations.

However, I believe that a critical factor that was not mentioned in the article when comparing the PBT (45 degrees), A310 and the Rushton turbine is the pitch angle for the Rushton turbine. When I reproduced the example, it appears that the Rushton-turbine impeller was specified with a 20-degree pitch angle as opposed to the standard pitch angle of 90 degrees.

The latter results in a power number for the Rushton of only approximately 0.86, while the standard Rushton has a power number of approximately 5.2 (confirmed by running Visimix at 90 degrees pitch). This results in more than six times the power draw and very significant differences in mixing parameters for the standard Rushton turbine.

I bring this to the attention of readers since it is my experience that most chemical engineers would initially believe the Rushton to be at the standard pitch angle of 90 degrees.

Wayne J. Genck, Ph.D.

Genck International

Richton Park, IL

Chemical Processing

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