Impellers for Agitating Lignocellulosic Slurries Revisited

by Gregory T. Benz, Benz Technology International, Inc. (affiliated with LEC Partners)
Special to The Digest

Several years ago, I published articles comparing pitched blade turbines to hydrofoils in lignocellulosic slurries (references 1 and 2). Since that time, I have continued to work with such slurries. Recently I came across a chapter in reference 3 that claimed that the agitated volume reaching the vessel wall depended on torque only, independent of impeller type. This contradicts my experience, so I decided to revisit this in a new test in my facility. Since most academic studies use a Rushton turbine, I decided to test 3 impellers: the Rushton turbine, a pitched blade turbine and a generic hydrofoil.

Cavern concept

When there is insufficient agitation, there will be movement in a zone around the impeller called the “cavern”, but this movement will not reach the vessel wall. With more agitation, the cavern can reach the vessel wall. This is illustrated in Figure 1, where the left side does not reach the wall, but the right side does.

Figure 1 Caverns

Fluid tested

The test fluid was a 22.1% lignocellulosic slurry consisting of several sources, including partially hydrolyzed bagasse, wood flour, sawdust, milled switchgrass and rice hulls. It was not intended to represent a particular customer slurry, but to be representative of the concept.

Impellers and vessel tested

The impellers tested, shown in Figure 2, consisted of a 6” Rushton, a 6” pitched blade and a 7” hydrofoil. The vessel was 11.375” ID with a liquid level of 10.5”.  Impellers were located 4” off bottom.

Figure 2 Impellers

Test procedure

Each impeller was operated  at various speeds until slow, steady movement was visible at the wall. The speed and torque were recorded, and the height of the cavern was also recorded. In Figure 3, the height of the cavern is shown for each impeller as the space between the green tape. From left to right, the results for the Rushton, pitched blade and hydrofoil are shown.

Figure 3 Caverns as tested

Results

Table 1 summarizes the results for torque, speed and cavern height. The differences between impellers were striking.

Key bullet points:

• The pitched and Rushton turbines had to operate at a higher shaft speed to drive the cavern to the wall
• The pitched blade turbine required 2.9 times the torque and more than 4 times the power as the hydrofoil
• The Rushton required 4.6 times as much torque and 5.6 times as much power as the hydrofoil to drive the cavern to the wall
• The hydrofoil also controlled a higher liquid height than the other impellers, and the position of the cavern was closer to the bottom.

Conclusions

The notion that torque is the sole determining factor for cavern size in a high yield stress fluid seems to be in error. It is clear in this study that there are massive differences in impeller performance. Though more study would be useful, it is clear for now that hydrofoils are the preferred impeller style for such slurries.

References

• “Hydrofoil Impellers vs. Pitched Blade Turbines in Lignocellulosic Slurries”, G. Benz, Biofuels Digest, May 8, 2017
• “Impeller Selection for Lignocellulosic Hydrolysis Reactors”, G.Benz, Biofuels Digest, 3/16/2017
• “Handbook of Industrial Mixing, Chapter 9, Eqn 9-39 and 9.40, pp 521-522, R. Grenville and A. Nienow, Published 2004

Author Bio

Gregory T. Benz is President of Benz Technology International, Inc. (www.benz-tech.com), located at 2305 S Clarksville Road, Clarksville, Ohio 45113. Phone 937-289-4504; fax 937-289-3914; e-mail g.benz@benz-tech.com. He received his BsChE from the University of Cincinnati in 1976, and has taken a course on Fermentation Biotechnology from The Center for Professional Advancement. A registered Professional Engineer in Ohio, he has over 45 years’ experience in the design of agitation systems, specializing in fermentation and bioreactors. Currently his company does general engineering, mixing and bioreactor design consultation, including equipment specification and bid evaluation. He is a member of AIChE, ISPE, SIMB, American Mensa and the American Chamber of Commerce