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Add Other Options to the Mix

Readers suggest how to improve a blending operation.

THIS MONTH'S PUZZLER
We blend a partially dehydrated color paste with mineral turpentine (C₉–C₁₆ range) at a volume ratio of 15:85. The final flow is 25 gal/min. We then disperse solids, up to 20% by weight, in the solvent. The solids form clumps. The paste isn't shear sensitive. The mixture is chilled to reduce vaporization. As a result, solvent viscosity is somewhere between glue and milk after chilling. The final product is an oil-based paint that is heated before use in a sprayer. Turpentine is driven off and recycled. One engineer has proposed blending this mixture in one shot with a single hydrofoil impeller in a tank with a 1.5:1 length-to-diameter (L/D) ratio. We have baffled closed tanks available in the mix room with L/D ratios of 1:1,1.5:1, 2:1, 2:1 and 3:1 and volumes of 340 gal., 220 gal., 500 gal., 500 gal., 650 gal., respectively. Can you suggest a better way of mixing the products? How can we improve this process to make it safer, easier to operate and more environmentally friendly?

USE AN INLINE MIXER
Has an inline mixer set-up been considered? This doesn't appear to be a chemical reaction process, only a mixing process. Currently the process description indicates cooling to lower vaporization then heating. Use an inline mixer in a pipe, with the pump pressurization to reduce vaporization eliminating a heat exchange step. Feed components at various locations in the line. Design the process to allow recycling if more mixing time is needed.
Larry Stephans, retired
Rochester, N.Y.

GO ULTRASONIC
Why not try ultrasonic stimulation at various temperatures and pressures?
Tom Murphy, CEO
Puritrol, Inc., Centerville, Mass.

SPLIT THE SOLVENT
This process consists of two different mixing operations: mixing a soluble paste into a solvent and dispersing a solid into that solvent. Solids clumping and solid viscosity are problems we must handle.

Since the final mixture has a lot of solvent, why not disperse the solids in a portion of the solvent and the color paste in the remainder (see Figure 1). You may need to do a lot of lab work to find the minimum and maximum solvent needed to disperse the solid and mix the paste. Then, try blending the two solutions together to check for any problems.

Solid suspension is the most critical design issue. Chemineer suggests scaling up shaft speed, N, for solids suspension via N₂ = N₁ (V_batch/V_production)^3/4 where V is volume. Use mixing intensity, which relates power to operating volume, to compare equipment choices. Intensity is directly proportional to blending time.

Choose the tanks that provide the highest residence time for the suspension and adequate blending time for the paste. Initially operate the paste blending at a slow speed to avoid splatter. Go with as small an impeller diameter as possible because, for a given motor, speed affects blending time more than diameter does. Use the 3:1 tank for solid suspension. I estimate a 60-minute retention time for a 50:50 split. Consider one or both of the 2:1 tanks for paste blending. Blend at low speed and then pump to a second tank for high speed mixing.

Using a rotor/stator mixer is tempting but these mixers produce little flow. It's better to go with a conventional four-blade 45°-pitched-blade impeller with an adjustable speed drive.
Dirk Willard, consulting process engineer
Organic Technologies, Coshocton, Ohio

JUNE'S PUZZLER
We have two brick-lined vessels with an expansion joint installed between them (Figure 2). The joint consists of a protective outside layer of high-density nylon fiber, two layers of polytetrafluoroethylene (PTFE) and then an inside layer of carbon-impregnated PTFE. The process fluid is a combustion stream consisting of hydrochloric acid, flue gases and steam at a bulk temperature of about 180°C and a vacuum of about 2 psia. Our commissioning team is concerned about the installation because the joint accumulates water. Can you see any potential problems with this expansion joint?

Send us your comments, suggestions or solutions for this question by May 10, 2010. We'll include as many of them as possible in the June 2010 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.

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