Take out torque

Question from August's Chemical Processing We are experiencing high torquing problems and an increase in material temperature in our 6-ft.-diameter ribbon blender. We mix a dry, coarse ingredient with oil and then add a fine powder and mix for about 30 minutes. We feed the coarser material into one end of the blender and add the powder to another area. The oil is sprayed into the mixture using fan sprays. About 20 minutes into the mixing process, the torque increases substantially and the bulk mixture&rsquos temperature rises to about 90°F. What could be causing the torquing problem? Will changing the mixture reduce the overload?

Add ingredients separately
When fine powder is introduced separately on one end of the blender, it will concentrate and form a layer between the ribbon and the blender wall. You can introduce the coarser material, which already contains some oil, at the other end of the blender. Then spray the entire mixture with oil, which will migrate toward the fine powder and create a sludge. This sludge will increase the blender&rsquos torque and, consequently, the mixture&rsquos temperature.

A simple solution might involve introducing the coarse ingredient first, then spraying that material with oil. Powder should then be introduced evenly along the entire length of the ribbon blender while blending.

Lee Dudley, president
Diamondback Technology Inc., San Luis Obispo, Calif.

Evenly distribute the liquid
We perform a similar operation at our plant. First, we add all the liquid ingredients to the coarsest fraction. Once this is thoroughly mixed, we add the finer powders. This procedure eliminates about 90% of the heat buildup we used to experience.

The tendency to thicken and bulk is likely due to the oil absorption capacity of the finer powder. By predistributing the liquid, the bulking takes place throughout the mixer rather than in the middle zone. As a result, the torque required to mix is substantially reduced for any small, finite volume. If you need to add additional oil, do so after the pre-wet, coarse and fine powders have been thoroughly mixed.

Gary Hall, manager, organic technology

Add materials in a different order
It sounds as if your process is creating a dilatant solution, not unlike a mixture of cornstarch and water where the more you mix it, the thicker and more viscous it becomes. The increased heat most likely is caused by the high viscosity and friction between the ingredients, or a pure chemical reaction between your materials.

You may have to limit the mixing or change the sequence in which ingredients are added. If you can get the powders mixed before adding the oil, you should get the best final mixture with the least amount of mixing time and lessen your torque issues.

Kurt Lemman, maintenance engineering manager
Schreiber Foods Inc., Tempe, Ariz.

Vary the concentration
I suspect that you may be reaching the static packing point of the particles. The coarse material is packing so close that the individual particles have to move up and over each other instead of just flowing past in near proximity. It could help to vary the concentration to make the mixture a little more fluid.
Mont Dutson, staff engineer

DuPont Chemical Co., Fort Madison, Iowa

Consider a few things first
Many blending and drying processes go through a nasty &ldquoballing&rdquo stage that would explain the high torque and temperature. If the balls that form are very sticky, it makes the situation much worse. As the wet material contacts the dry, it will roll and fold causing balls to form. As more wet material is added, the balls break up and the blending becomes more uniform and reduces the torque.

The order and rate of material addition are the main factors. The type of blender and the blender impeller also have an effect. The torque and temperature can be related using equations developed by the English scientist James Prescott Joule. His work with water was so precise despite his primitive equipment that even modern equipment and methods have hardly changed the values that he determined.

There are some questions you should ask: Are all the meters for material additions calibrated? Or were they recently calibrated for the first time? If material rates have been modified even slightly, the effects can be dramatic. Have you tried adjusting the addition rates or order? Can you see inside during operation? Can you stop processing at various points and take a sample? You should consider non-linear addition rates. Has the process changed in any way or has it always behaved this way? Were the issues manifested during scale-up? Scaling up blending operations is notoriously non-linear. You can measure the temperature or torque over time and use it to determine if the changes you are making show any progress.

Eric Janson, project engineer