Chris Moreton, VP, Pharmaceutical Sciences
Idenix Pharmaceuticals, Cambridge, Mass.
Install a VFD, replace the impeller
At the bare minimum the 20% increase in specific gravity will call for 20% more horsepower, and most likely the 10-fold increase in fluid viscosity will also require additional horsepower. The assumption being that an increase will take us into transitional flow regime in the Reynolds number vs. impeller drag coefficient curve, where increasing viscosity results in increased drag (power draw) on the impeller. The existing mixer will probably not work and if it does, the mixing will take longer. Here are a few options, some of which may be possible with the tight deadline:
- If mixer is equipped with a variable frequency drive, or a VFD is available, perhaps the mixing requirements can still be met by slowing the mixer shaft draw and shaft speed. The mixer power draw is proportional to the cube of the shaft speed. Therefore a small decrease in shaft speed would result in a noticeable drop in horsepower drawn at the shaft. I estimate that a 7% drop in RPM would be required to account for the increase in density.
- Perhaps the "shear" mixer utilizes a "saw tooth" type (a.k.a Cowles) impeller, which is readily available and relatively inexpensive. A number of vendors can supply impellers quickly. If so, we can take advantage of the relationship between mixer motor draw and impeller diameter — mixer shaft horsepower is proportional to the impeller diameter to the 5th power (D5). A very small decrease in impeller diameter causes a noticeable drop in mixer horsepower; roughly a 4% drop in impeller diameter would be required to account for the increase in density.
- Check with the manufacturer of the high shear mixer. The power frame of the mixer may in fact allow for a larger-hp motor to be put on the machine and still stay within the mechanical limits of the shaft system.
Shear mixers normally are most effective when their impeller peripheral speeds are in the 3,000 to 5,000 ft/min. range. Dropping the shaft speed or impeller diameter would effect the “tip speed” of the blade so if both new impeller and VFD are available one perhaps could re-tune the existing mixer to provide the proper process result or at worst case make up smaller batch sizes.
Tim Geiger, engineer
Mattoon & Lee Equipment, Farmington Hills, Mich.
Meter in the concentrate
The first thing I would try is to replace the motor on the high shear mixer. It should be possible to get a 5-hp motor to replace the 2-hp motor. Motors are relatively inexpensive and readily available on both the new and used market. The only problem might be the mounting plate; making a new one might be easy or hard, depending on your shop. Another solution is to meter the additive in. Add it slowly instead of all at once. By working with R&D, the people who created this situation, it might be possible to change the order of formulation. That way, the additive is more readily dispersed and diluted, hence not as viscous. It might also be possible to dilute the additive in one of the other components prior to addition to the main batch. The final suggestion is to go on vacation just before roll out, and claim it was working fine before you left.
Kevin Cappo, CHMM Corp. HSE director
Spraylat Corporation, Chicago, Ill.
The high shear mixer might be okay
Power curve calculations are often not useful in these situations especially in non-Newtonian fluids; the 2-hp mixer may do the job. If not, you could split the additive into two smaller batches. This may require two small tanks with portable mixers since the high sheer mixer will not be submerged. One other problem will be additional labor costs because of the time spent to split the batches. An alternative might be to change the size of the mixing blade in the high shear mixer. You can run a couple of experiments with the different rotors and stators (impellers) to measure the current draw using water in the production batch tank.
Nate Blackburn, technical director
Increte Systems, Inc., Odessa, Fla.
Replace the motor
Most motors come in different horsepower on the same frame and aren’t hard to find on the market. I would change the motor horse power to what is needed. This solution will work well as long as the motor shaft and agitator shaft don’t appreciably change. Another problem might be the draw for a larger motor if the wiring or heaters are sized too close to limit. Starters should not be a problem as long as the motor isn’t too large. Alternatively, a smaller impeller could be used, though this would affect the mixing time and reduce shear from agitation. This would reduce the energy input into the mixer so that the existing motor would work with the new material.
Michael J. Kirkman, chemical engineer
AREVA NP Inc., Richland, Wash.
Put some heat on it
Increase liquid temperature before the blender. Check viscosity-temperature data to see if increasing temperature will lower the viscosity enough to stay within the power draw. A bayonet heater or some simple retrofit might suffice. As a precaution, check if there is any negative effect on the product by being heated during processing.
Charles W. Lamb, senior process engineer
Teris L.L.C., El Dorado, Ark.
Raise the temperature
A solution might be to raise the temperature of the mix tank, assuming a higher temperature reduces the viscosity and the power draw. Some tests would be required before considering heat-tracing the tank and insulating. I thought about splitting the stream between two tanks but that would have its own problems. Another thought was to use a different mixer without such a high power draw but in practice this approach tends to slow down the process and you do not get good dispersions — so it creates more issues that it solves.