Sam Firestone, CEO
Norstone Inc., Wyncote, Pa.

Add the oil last
The ribbon blender is putting a great deal of energy into the mixture. The combination of the coarse and fine materials is creating a dense product once the oil is added. The increased sheer is generating the heat. I would suspect that as the mass becomes wetter, the longer the blender needs to be operated. I would suggest first blending the coarse and fine materials and then adding the oil. Blend the mixture for five minutes or less.

Kief Hess, operations technical leader
Crompton Corp., Morgantown, W.Va.

Slowly add fine powder
I believe a simple change may solve the problem of higher torque and heat. Assuming there is no reaction taking place between the oil and solids, I would spray the oil on the coarse powder and run the blender until the oil thoroughly coats it. Once that is done I would slowly add the fine powder so that it starts to adhere and coats the oil-coated coarse particles.

Girish Malhotra, P.E.
EPCOT International, Pepper Pike, Ohio

Ask an expert
You could consult a rheologist who knows how your ingredients interact. Nevertheless, you have a non-Newtonian mixture with rising temperature due to increasing shear (or an exothermic reaction). Postulating the need for a very uniform mixture, one might try for uniformity by mixing the solids before adding the oil. But the better bet would be a double-cone mixer or a muller.
Take the ingredients to two or three mixing-equipment vendors&rsquo labs and let them decide how they would do it. If the cost of new equipment is high, you can look into buying used equipment.

Tom Murphy, P.E.

Add some more lubricant
It seems like your ingredients are reacting exothermically. It&rsquos also possible that the amount of lubricant is insufficient or is being atomized or used up.

David Goodson, president
Stealth Coatings Co., Hinsdale, Ill.

Check the driver&rsquos specs
The viscosity of your mixture may be changing as a result of a reaction between the oil and the solid material. Check the actual viscosity with a viscometer. Then check the specifications of the mixer driver to be sure that the mixture&rsquos viscosity is within the specs. You may have to increase the horsepower of the driver or use a different oil for the mix. You also may have a problem of over-mixing the material, thereby increasing the rate of reaction.

Have your drivers been failing prematurely? Evaluate the problem from all angles: viscosity as a function of reaction, mixing parameters and driver size.

Eric M. Roy, principal engineer
Westlake Group, Sulphur, La.

Look for a reaction
Is there a reaction causing the increase in temperature?
If the mixture becomes less viscous, you can anticipate an increase in torque. Consequently, the blending action will cause a temperature rise due to friction in a less viscous mixture.

Scott J. Evans, manager, process laboratories
F. L. Smidth Inc., Bethlehem, Pa.

The oil and powder are interacting
It appears that the interaction between the oil spray and the coal or the dry ingredient is causing the powder to swell or even soften. This often happens when dry-blending PVC powder and a plasticizer. Coal is a mass of chemicals, especially soft coal, and an interaction with petroleum-based oil is very likely.

Dan Ryan

DECEMBER'S PUZZLER We use a light stripping column for removing methane and lighter components from natural gas liquids (NGL). We are having flooding problems with the column that we think might be due to water (the feed is saturated). Simulation shows all the water should go overhead and no hydrates should form. The first tower has a condenser temperature of -20°C (-4°F, a bottoms temperature of 84°C (183°F) and an operating pressure of 20 bar (295 psig). Could our simulation be wrong? What else could be going on? Send us your comments, suggestions or solutions for this question by Oct. 29. We&rsquoll include as many of them as possible in the December 2004 issue. Send visuals, too &mdash a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., 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&rsquod like to pose to the readers, send it along and we&rsquoll be pleased to consider it for publication.