Impeller Selection for Lignocellulosic Hydrolysis Reactors

By Gregory T. Benz, Lee Enterprises Consulting

Special to The Digest

Lignocellulosic slurries at high solids concentrations (10-30%) can be quite viscous. The author has seen some where the viscosity at a shear rate of 1/s is higher than 400,000 cP. Conventional agitator impeller selection guidelines would lead one to use helical ribbon impellers (figure 1; aka helix) for such viscosities. These are usually applied at a diameter of 94-98% of the tank diameter.

Figure 1 Helix impeller

However, the author has successfully used hydrofoil impellers (figure 2) for high solids lignocellulosic slurries. Feedstocks have included corn stover, woody biomass, switchgrass, corn fiber, sugar cane bagasse, energy cane bagasse, municipal solid waste and sugar beet pulp, among others. The key to success is using the right size and quantity of impellers, and at the right speed. “Wet” laboratory testing plus scale-up techniques are generally required. Considerable information on proper techniques is provided in references 1 and 2. In general, a D/T (impeller diameter/tank diameter) ratio of 0.6 to 0.7 is required for the high-solid slurries, and one impeller for each increment of height equal to ½ of the tank diameter. The bottom impeller will typically be about 5-10% larger than the upper impellers, as it must overcome drag on the bottom of the tank as well as the side walls.

Figure 2 Hydrofoil impeller

But why use a hydrofoil instead of a helix? There are a number of compelling reasons, enumerated below.

1) Hydrofoils work! Helixes do not. A helix works very well in Newtonian and mildly shear-thinning liquids. But lignocellulosic slurries generally have a yield stress as well as shear thinning behavior. (They are usually described as Herschel-Bulkley fluids). What this means in practice is that a helix will shear the fluid at the wall but not in the middle, resulting in mass rotation of the entire batch, with virtually no mixing.

2) Hydrofoils are inexpensive. Even if both impeller styles worked, a hydrofoil would have a large capital cost advantage.

3) Hydrofoils are scalable. Though some helix impellers have been applied in batches as large as 10,000 gallons, this would still be considered small compared to the batch sizes needed for the biofuels and renewable chemicals industry. It would be almost impossible to build really large vessels with helix impellers, not only due to impeller fabrication difficulty but also the precision required of the vessel in terms of roundness and diametral tolerance. Field fabricated vessels would be out of the question. There are no size limitations for hydrofoil impellers.

Some might ask, Why not use pitched blade turbines? Indeed, simple pitched blade turbines can also be used successfully. However, even in laminar flow, the hydrofoils can produce the same degree of liquid motion with about 25-30% less torque. (In turbulent flow, they can produce the same motion with 50-60% less torque). They also produce a more axial flow pattern (see figure 3) and are easier to apply at larger D/T ratios due to their lower power draw characteristic. So, for now, it appears that hydrofoil impellers are the best choice for lignocellulosic slurries that are actually liquid. Note that viscosity quickly drops as enzymes break down the cellulosic fibrous structure and liquefy the cellulose.

Figure 3 axial flow pattern created by hydrofoil in biomass slurry

A caveat: the above only applies if the slurry is nominally liquid. This usually means 1-2% more water than required to fill in the voids between the solids. Slurries with lower moisture content than that may require solids mixers, such as horizontal shaft paddle mixers.

Conclusion: for high-solids lignocellulosic slurries that are liquid, hydrofoil impellers are the current best choice for successful results, economy and scalability.

About the author: Gregory T. Benz, P.E., is President of Benz Technology International, Inc. and a member of the Lee Enterprises Consulting, the world’s premier bioeconomy consulting group with over 100 consultants worldwide. The group has expertise in many areas, including in the subject discussed in this report.  

Acknowledgement: The photos used to illustrate figures 1 and 2 were by Chemineer, a brand of NOV

References
1) “Agitation of Fibrous Materials”, G. Benz, Chemical Engineering Progress, June, 2010, pp 28-32
2) “Determining Torque Split for Multiple Impellers in Slurry Mixing”, G. Benz, Chemical Engineering Progress, February, 2012, pp 45-48