by Gregory T. Benz
There are many sources of organic fibers that can be used as feedstock for biofuels and renewable chemicals. The author has tested many lignocellulosic materials for fluid flow and mixing characteristics. Examples include corn stover, ground wood, switchgrass, various types of cane, beet pulp, paper mill waste, municipal solid waste and corn fiber, among others. A number of different pretreatment methods have been used, such as acid, alkali, steam explosion and mechanical grinding. Though the exact fluid properties depend on feedstock, % solids, particle size and pretreatment method, they all have one thing in common: a yield stress. This means that such slurries are self-supporting to some degree, and will tend to be difficult to agitate at the vessel wall. Most have rheology that is well modelled using the Herschel-Bulkley equation:
µa = σ/(dv/dx)+M(dv/dx)(n-1) , where µa is the apparent viscosity, σ is the yield stress, M is the viscosity coefficient and n is the power law exponent. An example of such a slurry is shown in figure 1
Figure 1 Lignocellulosic slurry
Purpose of Equipment
There are two main purposes for the pilot agitation equipment: determining the minimum shaft speed required for creating full tank motion, and using the agitator as a viscometer. The latter is required because fibrous slurries are difficult to quantify using conventional viscometers. They tend to settle and dewater around the spindle. Techniques for both of these purposes are described in reference 1. A complicating factor is the wide speed range required to achieve both of these purposes, as the viscosity must be measured at the same shaft speed range as the production scale equipment. Production equipment might have speeds in the 10-45 rpm range, whereas a 5-10 gallon pilot unit may need speeds of hundreds of rpm to achieve minimal wall movement. Table 1 illustrates suggested agitator power, torque and speed range as a function of vessel diameter, for very difficult slurries at the edge of being solids rather than liquids.
Table 1 Suggested Pilot Agitator Drive Parameters
|Vessel diameter, in.
|Speed range, rpm
Since the minimum criterion for mixing is full tank motion, it is strongly preferred that the vessel be transparent so that motion, or lack thereof, can be readily observed. If sight glasses are used on a metal vessel, they should ideally be full length and made flush to the internal contour of the vessel. Even a small lip can stop local motion, so it is possible that intermittent motion in the center of the sight glass may indicate fairly vigorous agitation just a few cm away from the wall. Flush-mounted round sight glasses should be avoided on most pilot vessels, unless the vessel diameter is 20 times or more the sight glass diameter.
Ideally, the vessel bottom should be dished, so as to avoid dead spots or fillets at the junction of the bottom to the sidewall. This is true of the production vessel as well. If a transparent pilot vessel cannot be obtained with a dished head, then one should imagine a “virtual” dish made up of biomass, forming a non-moving fillet up to 1/6 of the tank diameter off bottom. If motion occurs above that, it will probably occur throughout the bottom of an actual dished bottom vessel.
For high % unhydrolyzed solids, baffles are detrimental. After hydrolysis, baffles might be needed. Ideally the process will have more than one stage of hydrolysis vessels so that later stages can use a smaller impeller and a baffled tank. If all mixing must be done in one vessel, the vessel and agitator should be set up for the most difficult material.
Size matters! If the vessel is too small, high-solids slurries will climb the walls and be self-supporting. As the tank radius increases, the solids will more easily fall back into the middle of the tank. Though the author has done some tests in 6” diameter vessels, he recommends 12” nominal diameter or larger.
As long as the slurry is barely liquid, turbine impellers can be used to agitate them. If there is not enough liquid to fill in the spaces between the solids, then it is no longer a slurry, but, rather, a damp solid. Such solids require solids mixing equipment to mix them, and such equipment cannot be scaled to large volumes. Helix or anchor impellers will not mix high solids slurries with a high yield stress. The batch merely rotates en masse with the impeller. Though several kinds of turbine impellers can work, the author has found, as described in reference 2, that narrow-blade hydrofoils can agitate the heaviest lignocellulosic slurries with ½ to 1/3 of the torque of pitched blade turbines, and can handle low viscosity slurries (as in hydrolyzed material) using about 40-50% of the torque of pitched blade turbines. For the high-solids slurries, the lower impeller should have a D/T (impeller to tank diameter) ratio of about 0.6 to 0.7, and all upper impellers should be a little smaller; about 85% of the lower impeller diameter. Use one impeller for each liquid height increment equal to ½ of the tank diameter. For low viscosity slurries, the D/T ratio should be 0.3 to 0.45, and one impeller can handle up to 1.2 times the tank diameter in height; additional impellers can handle an additional height equal to the tank diameter.
Two sets of measurements should be taken. The first set is used to determine rheology, as described in reference 1. After running at a high enough speed to ensure the slurry is uniform, the speed is slowed down to about 10 rpm, and torque and speed are recorded. This is repeated at several speeds up to the maximum speed anticipated in the production scale, which is typically below 60 rpm.
The second set is the mixing study. Since the upper region of the tank may agitate differently from the lower region (reference 3), the minimum speed that creates creeping movement at the wall in each zone should be recorded. In the case of a flat bottom tank, movement above a height of about 1/6 of the tank diameter is sufficient. If the two speeds are not the same, adjustments to relative impeller diameter will be made upon scale-up.
There are several ways to measure torque. The most accurate way is to use a strain-gage torque sensor. There are several ways to set this up. The most complex and difficult is on the output of the gear drive. In such a case, the shaft bending load must be eliminated from the torque readings, which involves a complex strain gage setup. A simpler way is to use a torque sensor on the input to the gearbox. Most gearboxes have losses of about 2% of the torque, and the loss can be calibrated in air. A third way, shown in figure 2, places the vessel on a torque sensor that can support the vessel. A similar option is to place the vessel on a turntable with low friction bearings (air bearings are the best) with a linear force gage attached to a torque arm. The least accurate way is to use electrical measurements on the motor, but in some cases it is the only way possible. Accuracy may be OK at higher speeds ald loads, but suffers when the motor is lightly loaded or running at very low speeds. Figure 3 show a typical small scale mixing test being run at Benz Technology International, Inc.
Because hydrolysis drastically affects fluid properties, tests may need to be done at several stages in the process to help determine mixing and vessel needs if the process is to be broken up in stages.
Apparatus for studying mixing of lignocellulosic slurries at high solids concentrations and low conversions is fairly simple but must be robust. The key requirements are to be able to visually observe the mixing and accurately measure shaft speed and torque. The mixer torque requirements are generally much higher than a typical lab mixer can provide.
About the Author: Gregory T. Benz is a member of Lee Enterprises Consulting, the world’s premier bioeconomy consulting group, with more than 100 consultants and experts worldwide who collaborate on interdisciplinary projects, including the types discussed in this article. The opinions expressed herein are those of the authors, and do not necessarily express the views of Lee Enterprises Consulting.
- “Agitation of Fibrous Materials”, G. Benz, Chemical Engineering Progress, June, 2010, pp 28-32
- “Hydrofoil Impellers vs. Pitched Blade Turbines in Lignocellulosic Slurries”, G. Benz, Biofuels Digest, May 8, 2017 (online publication).
- “Determining Torque Split for Multiple Impellers in Slurry Mixing”, G. Benz, Chemical Engineering Progress, February, 2012, pp 45-48