Improving Batch Mixing

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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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