Friday, January 4, 2013

Hand Cream And Low Shear Mixers

English: Mixing - Axial flow and radial flow. ...
English: Mixing - Axial flow and radial flow. Italiano: Miscelazione - Flusso assiale e flusso radiale. (Photo credit: Wikipedia)
About 15 years ago I was tasked with the need to improve the mixing of a top selling, over the counter hand cream. This was a text book oil-in-water emulsion in that it used triethanolamine and stearic acid to form a triethanolammonium stearate. The mixture was approximately a molar ratio of 2:1. It even used salt as a viscosity modifier (see my previous post here The Salt Curve. These formulas are stable, cheap and fairly high pH. They are definitely "out of fashion" for hand creams but are still widely used as a cream base for pharmaceutical preps.

The cream was made in an eight thousand gallon, jacketed tank with one high shear mixer (a Lightnin A200 if my memory is correct) fitted to the end of a very long shaft. The mixing tanks were built in the 1940's. The mixers were newer, probably manufactured in the seventies. The process was slow to heat and cool. The motor maxed out at 5 horsepower. Typically, 4.2 horsepower was needed to move the material to form the emulsion.

The issue for me was to improve the mixing to increase contact time with the tank walls so that heating and cooling would go faster. Also, increasing pumping action to move the dry adds from the surface to the high shear mixer. There was no budget to change the motor. Changing or adding a mixer would be difficult or impossible without affecting motor load.

That requirement led me to a low shear impeller designs. My goal was to place the impeller midshaft so that material draw-down was improved. I also hoped that a 90 degree pumping action would better supply the hi-shear mixer more efficiently. are intended to impart motion to the product while using the least amount of energy possible.  Since I could not exceed 5 horsepower on the motor, I needed to use a low shear, high efficiency hydrofoil which minimized power consumption while keeping the product in motion. I found that in the Lightnin A-310. Here is a site for approximate horsepower conversions: Motor Conversions.

I was able to shave off some the manufacturing time (about 45 minutes) but it was not enough enough to justify the cost of the retrofit and re-qualification needed to make the changes. Still, I liked the "out of the box" approach and it was worth investigating.
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