Get out of the Soup

Readers clarify a consistency complication.

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This Month’s Puzzler
We add a polymer agent as a coagulant to a cosmetic tank. During development, the ingredient was added by hand. In a large bulk tank, this is too inconvenient. So, we enlisted an open-impeller centrifugal pump to batch the agent to the bulk mixing tank, i.e., reactor. Now, however, the product, which should have a consistency of tapioca pudding, resembles runny soup. How should we investigate this problem and what could be the cause? How do you suggest we can improve the consistency?

It seems that process development work has been done well but things are not working as planned on the commercial scale. The problem as stated does not tell how and where the coagulant is being added and if it is a liquid or a solid. If it is a liquid, it could be added using an eductor on the inlet side of the pump. This should disperse the liquid coagulant as it goes through the pump. If the coagulant is a solid, it can be added using similar devices that are commercially available. They generate sufficient vacuum to suck a liquid or solid. In both cases, the addition should be close to the cavity of the pumping device.
Girish Malhotra, president
EPCOT International, Pepper Pike, Ohio

When you use a centrifugal pump to move the agent, you are essentially adding a step to the batch process. The pump impeller whips the polymer, much like eggs in a blender. You still get eggs but they are different. The centrifugal pump is very likely adding heat and, quite possibly, air to the agent. If the polymer agent is tolerant and it suits the process flow, you could use a diaphragm pump. A peristaltic pump is even gentler on the pumped fluid. Both pumps I mentioned are very accommodating to adjustment of flow rate and pressure.
Tyrone Riley, maintenance supervisor
The Shepherd Chemical Co., Norwood, Ohio

The pump may be shearing the thickening agent and reducing its effectiveness. This could be tested in the lab using a blender to shear the material before adding it. Mixing intensity can often be a problem with thickeners.
Bennett Willis, assistant professor
Brazosport College, Lake Jackson, Texas

You are either imparting too much shear, tearing apart the coagulant, or not enough shear, allowing two phases to separate (although you do not mention any lumps). In a situation like this, I always go to the lab, testing small additions/unit time, with an adjustable-rate mixing device — getting an idea of “wetting” capabilities. Most times, I insist on master batches, in so far as possible, to be fed into the production batch tank/piping (venturi flow), creating a smoother mixing and more uniform product. Miscibility is the key.
Tom Murphy, CEO
Puritrol, Inc., Centerville, Mass.

There is a good chance that the polymer agent is being broken down by shear from the centrifugal pump. This could be investigated in the lab by making two batches of the product. One would be blended using virgin polymer agent, the other would use a polymer additive that has been stressed using a high shear agitator. If this is indeed the case, replacing the centrifugal pump with a positive displacement pump, such as a progressive cavity or hose pump, should solve your problem.
Tom Patten, process engineer
O’Neal Inc., North Charleston, S.C.

It is probable the polymer you are using is a long chain hydrocarbon. These molecules fragment and lose their effectiveness when exposed to even the slightest of shear forces. Centrifugal pumps are exactly what you do not want to use when moving polymers like this — you need a low shear pump, e.g., a positive displacement pump. Find another way to do your mixing with low shear conditions. I bet if you contact your polymer vendor it can give you recommendations on how to do this; if not, there are other more qualified vendors out there. Talk to one of them.

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