Improving Batch Mixing

Dec. 18, 2003

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
I would recommend the addition of two pieces of equipment: a new suitably sized agitated tank for preparation of the cellulose solution and an external high-shear emulsifier. The use of a small tank is likely to make the hydration of the cellulose faster as the tank is better sized and the agitation will be more uniform. Use the available spare 500-gal. tank to make up two batches of cellulose at a time.

The emulsifier can be used for two purposes: blending the cellulose solution with the powder solution and for emulsifying the oil with the solution mixture. If the two applications have different mixing requirements, a variable speed drive could be used on the emulsifier. After solutions are blended, minor additions are made and hold time is satisfied, oil is added together with a recirculating stream of the blended solutions in the emulsifier to produce the final product.

I would suspect the batch time could be reduced substantially using this flowsheet.
Jim Lammers, principal metallurgical engineer,
Aker Kvaerner E & C, San Ramon, Calif.

Run-Focused Experiments
I just recently completed optimizing a multi-ingredient liquid formulation. The original recipe called for two tanks and myriad operations. Looking at available solubility data from the literature helped set the direction of some experiments designed to find the optimum mixing time without sacrificing solution stability, quality and operating times. If I had your problem, I would start out by combining the water into one charge and, prior to heating, adding in the "soluble solid." I would then follow this with the minor ingredients, the methyl cellulose thickener, the surfactant and finally the oil. I would play around with order of addition, need for temperature increase, agitation rate and agitation design. Note on agitation design: the hydrofoil blade entrains a lot of gas, which maybe the root cause for variability in emulsion stability; you may need to consider other blade designs. The ideal situation, of course, is to find the best order of addition that can produce a consistently stable emulsion using only one tank at ambient temperatures in the least amount of operating time as possible.
Marlon Carlos, process engineer
ZILA, Inc., Phoenix, Ariz.

Change the Order of Addition
I worked for a cosmetics and toiletries company 15 years ago and remember some things about emulsions because I was a development engineer/chemist. Here are some suggestions that may help improve batch times and reliability.

1. Add your soluble powder into your main batch tank before adding the thickener. This eliminates the time to fill a secondary tank and transfer the material to the primary tank.

2. If your formulation allows it, add the surfactant and other minor ingredients before the thickener. All these ingredients dissolve quickly in water before you increase viscosity. Add the thickener after all water-soluble ingredients are in, but before you add the oil phase.

3. I do not remember if higher temperatures affect methyl cellulose structure but, if not, I suggest that you heat your water to about 150 Degrees F to 160 Degrees F.

4. The oil phase of an emulsion usually is around 150 Degrees F. Adding oil to the same water temperature or higher water temperature will help improve dispersion of oil droplets.

5. Your mixing time will need to be established under these conditions, but once you decide on an optimum time, start cooling the batch and adjust the mixing speed. The batch should thicken up more as you cool; so, you may have to increase your agitation speed. If an rpm readout and speed control are available, you can always have consistent mixing at various stages from batch to batch. This should improve reliability and batch consistency.
Craig Kryzanowski, process/project engineer
MeadWestvaco Specialty Chemicals Group, Charleston, S.C.

Make Side Batches
You can simplify this operation by making large side batches ahead of time and using them when needed. This reduces the number of side batches, thus simplifying the operation. You can't really dispense with either the high-shear mixer or the large volume mixer.
Edward Bergen, contract engineer
PSGA Project Engineering, Raritan, N.J.

Go Continuous
Skip the batch process altogether, as it leads to too much wasted time and batch-to-batch variability. Go continuous. Utilize a staged rotor-stator design and reverse the process if chemistry allows, making the emulsion first, followed by addition of other ingredients and thickener. Direct the product into drums and avoid capital for storage systems. Overall process size should be minimal and the economics should be attractive.

It also sounds as though you have variables whose impact is not well understood when you say "mixing intensity and agitation time required to form the emulsion are highly variable." Do you have process variability or raw material variability? Run a well-designed series of experiments.
David S. Tascarella, process engineer
Dow Corning, Midland, Mich.

Opt for a Static Mixer
The emulsification process may be simplified and better controlled as follows:

1. All the water-soluble ingredients can be dissolved in one 500-gal. mixing tank. The methyl cellulose must be added last, because viscous solutions are harder to mix than dilute solutions. The existing agitator in the 500-gal. tank may have to be replaced with a high-shear mixer to speed up the dissolution process. The old agitator is no longer necessary, as explained below.

2. The emulsification process should be done in-line rather than inside a mixing tank. A few in-line emulsification technologies exist, but the simplest one is using a static mixer. The required emulsion droplet-size distribution was not specified in the problem, but a properly designed static mixer can achieve a fairly narrow droplet-size distribution. The water and oil phases must be fed into the static mixer at the proper flow rates, according to their relative volumes and the desired droplet size distribution.

3. The finished emulsion flows out of the static mixer directly into the next processing step. This process is now semi-continuous rather than batch.

4. Process control is very important, mostly due to the steep temperature-viscosity curve. Both water and oil temperatures must be controlled and monitored, starting at the tanks and all the way through the static mixer. This usually means jacketed tanks and pipes. Both flow rates must be carefully controlled. If pH is important, the water-phase pH should be adjusted prior to emulsification.
Ehud Zeigerson, process engineering consultant
Fort Collins, Colo.

Add an Eductor for the Powder
To eliminate the phase tank, improve the mixing times and the emulsion for the process you described, I would look at adding an inline high-shear mixer/disperser with an eductor for the powder introduction. The product would be pulled from the bottom valve of the main process tank and recirculated below product level. Depending on how much head the mixing has to overcome, you may need to add a boost pump to the recirculation loop after the discharge of the mixer.
Larry Overstreet, process engineer
Novozymes Biologicals, Inc., Salem, Va.

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