Improving Batch Mixing

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We put 225 gal. of water in a 500-gal. tank, which has an angled, offset-mounted mixer with two hydrofoil-style impellers, and heat the water to about 120F. Then, a methyl cellulose thickener (<3%) is added and allowed to hydrate for about 1 hr, yielding a moderately viscous (approx. 2,500 cP) solution. Meanwhile, in an about 150-gal. tank, a high-shear rotor-stator mixer is used to dissolve a soluble powder in about 75 gal. of water. The powder's bulk volume initially is about three times that of water but, when dissolved, only increases the side-batch volume to about 100 gal. The side-batch then is fed into the main batch, which is mixed for about 45 min to ensure uniformity. Next, a small quantity of a liquid surfactant and a couple of other minor ingredients are added, followed by about 175 gal. of an oil, to form a stable emulsion. The mixing intensity and agitation time required to form the emulsion are highly variable and may be influenced by other factors such as pH and temperature. Finally, the mixer maintains gentle motion as the tank empties. We would like to simplify or eliminate the separate dissolving operation, improve the reliability of the emulsion formation and reduce the batch time if possible. We have a 500-gal. tank and are willing to buy additional equipment.
-- From October's Chemical Processing

Change the Mixer
Replacing the angle-mounted mixer in the 500-gal tank with an adequately sized rotor-stator mixer may be the key to improving this batch process. The high-shear characteristics of a rotor-stator mixer can be effective for both powder addition and emulsion formation. While changing the style of mixer shouldn't affect heating requirements, addition of the methyl cellulose thickener may be improved by the increased shear. The final viscosity of 2,500 cP is near the limit for rotor-stator mixers, but those with high pumping capacities may be suitable.

Elimination of the separate dissolving operation is an obvious simplification, but depends on the solubility of the added powder. Adding the powder directly to the main batch has both good and bad features. The 75 gal. of water used to dissolve the soluble powder may make the thickener step easier, but the thickener could make the dissolving step difficult or impossible. Assuming the powder is readily soluble, increasing the initial volume of water from 225 gal. to 300 gal. should lower the viscosity in the thickener step and the rotor-stator mixer should still dissolve the powder. The rate of powder addition must be controlled and possibly reduced because of the increased viscosity of the batch, but the time required to dissolve the powder may not change.

Addition of the surfactant and minor ingredients should work well with the new mixer. Finally, the addition of the oil and the emulsion formation should be easier and more reliable with the rotor-stator mixer because of consistent high shear. However, consistency of the final emulsion may be a problem because of limited pumping characteristics of typical rotor-stator mixers.

Improvements to a multi-step batch process always come with a series of "can't do's," especially when powder addition is involved. This recommendation is offered with the understanding that some or all these suggestions may be impractical due to powder handling, wetting characteristics, solubility and possible changes to product quality. Alternatives include recirculating liquid through an in-line powder addition device, use of an inline disperser for the oil addition and emulsion formation, a different formulation with the thickener addition at the end of the process, increasing temperature to reduce dissolving time and other combinations of mixing and addition. Complicated batch processes often require combinations of improvements.
David S. Dickey, senior consultant
MixTech, Inc., Dayton, Ohio

Air Might Help
Consider introducing air or inert gas into the tank via a tap just above the outlet valve used for draining the tank. This might give you the extra agitation you need, especially close to the tank walls.
Jim Henry, production scheduler
Reichhold Inc.

 

Start with the Solvent
In work I have done to simplify mixing problems of this type, I have had some success in changing the usually accepted order of addition of ingredients. The quick and dirty solution is to begin with your solvent portion in your mixing tank and add ingredients. Disperse the methyl cellulose in the solvent along with detergent (if it is oil soluble) and the water-soluble powder. Then add water to this mixture. I am currently making a waterless hand cleaner by this method. Of course, all this being said, it is important to know if the final product is an oil-in-water or water-in-oil emulsion. The type of solvent affects the emulsion stability as well as the emulsifier system. Depending on the end use of this emulsion, formulation changes could mean that some of the ingredients may be eliminated. I also don't know the viscosity of the final product. Usually there are many ways to make an emulsion, and you can formulate an equivalent product and greatly simplify mixing issues in the process.
Keith Rhodes, technical director
Cheatham Chemical Co., Stone Mountain, Ga.

 

Add a Tank and High-Shear Mixer

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