Charles Fivelson, engineer
Was the curing agent contaminated?
Contaminants can trigger the isocyanate to react with itself to form an isocyanurate, releasing a lot of heat. Typical triggers are potassium and sodium, but iron can coordinate the reaction as well. The metal must be in ionic form, which usually occurs at a particular temperature for a catalyst designed for the reaction (potassium octoate for instance). If the reaction starts in any part of the mixture, it would propagate due to the amount of heat released. This would occur whether there are fillers (the aluminum and oxidizer, i.e., the HMX and ammonium perchlorate) or not. If this is the cause of the explosion, you need to reduce the heat generated in the batch. A polyol can be pre-reacted to reduce this risk. For instance, if the isocyanate is at 24% NCO, react it with about 40% by weight of a high molecular weight polyol like Terathane 2000 (or PEG 2000 to lessen signature) to bring the NCO down to 12%. The exotherm potential will be cut in half and the entropy of the pre-polymer will help retard the isocyanurate reaction.
Another possible cause of the problem might be the metal in the mixer itself. Just before getting my doctorate at Clemson, I interviewed at Redstone Arsenal where a lot of propellant work is done. They mixed their batches in Tupperware to maintain smaller quantities, reduce friction and reduce spark potential. A plastic lined metal container would probably be better for heat transfer though. Another way to reduce this potential would be foaming the mixture with a gas like helium. This would reduce the temperature and greatly reduce shear since a much-lower-density material is being stirred. The problem would be getting the gas out of the polymer, especially as it starts to cure at the end of the batch. Perhaps a vacuum extruder could be used before casting the propellant.
Dr. Jim Yavorsky, consultant
Chemical Consultants Network, Mickleton, N.J.
Reduce the mixer speed or change the formulation
The viscosity rise caused more energy to be expended to mix the material and also reduced heat transfer to the cooled surfaces. A rise in temperature occurred. This rise caused the reaction rates (polymerization and others) to increase until the explosion happened. The mixer speed should be reduced or the propellant formulation should be changed. Lower the viscosity by reducing the solids (aluminum, HMX, or ammonium perchlorate) or increasing the plasticizer to reduce the viscosity. Changing the formulation may require significant development work. Reducing the solids loading will affect combustion efficiency and Isp or “specific impulse.” Isp is the ratio of thrust produced to the weight of the flowing gases. Raising the plasticizer/polymer ratio could be detrimental to long-term storage of the propellant. Both of these potential problems must be considered.
Richard H. Smith, P.E., team leader
Texas Commission on Environmental Quality, Austin, Texas
Change the batch procedure
Being unfamiliar with urethanes I can only speculate. If the viscosity increased, I can imagine that, similar to polymers such as epoxies and hardeners, once they mix, they react and generate heat, expand, etc. It appears that while mixing is occurring, like regular epoxy, heat built up very rapidly, material expanded, etc. as the viscosity increased and formed a chain reaction, due to the high reactivity of the mixture. As material was added and further mixing occurred, it is like adding fuel to a fire — feeding the reaction. If the viscosity rapidly increased, obviously the reaction was advancing at a fast pace as well. Although a slight vacuum was used, when the material was added, it may have acted like a pressure cooker, resulting in an explosion.
One suggestion is to slow the mixing rate, while the cooling is going on and add the later components at a slower rate as well to control the heat, expansion, ignition, etc. This would require some additional steps to the batch procedure but avoid a heat build-up.
Another suggestion, if the cause was an ignition, would be to blanket the mixing with an inert gas, e.g., nitrogen or argon. Of course changing from a vacuum system to a pressure system will require regulators, relief valves, etc. and a review of the vessel pressure stamp. Another advantage of using an inert is how it will solve the problem of trapped air. As the fine particles are added, any air that might be entrapped in these particles (interstitially, or from the process flow during its entry to the mix) won’t matter, since the inert gas is helping to prevent ignition. One problem with a blanket is preventing trapped air, and inert gas, from creating voids in the propellant. Moisture in the air also could cause bubbles to form. A vacuum process is needed. This will require a closed, thin-film extrusion with a vacuum prior to casting the propellant.
Rich Ashley, associate dir. QC/tech affairs
Barr Labs, Pomona, N.Y.
Review the process
Examine the overall system and equipment. Start by reviewing the instrumentation: the RTD, the current loads on the agitator. The mixer exploded because the heat transfer was poor: low conductivities from high viscosities. Go back to the drawing board.
Tom Murphy, CEO
Puritrol, Inc., Centerville, Mass.
The centrifuge separation at a bio-products plant is a bottleneck. Currently, two desludger centrifuges and two continuous (nozzle) centrifuges are available. The solution must be flexible since several products may be centrifuged. The product slurry concentration varies from 8% to as high as 15% by weight in an aqueous solution. Viscosity varies from 1-20 cP, for some products. Newtonian flow is not guaranteed. The desludgers offer better separation but at half the rate of the continuous units — at the moment one continuous unit can manage plant flow. There is some concern about nozzle selection for the continuous centrifuges; the feeling is that increasing the nozzle size will reduce the centrifuge efficiency but extend the run time between cleanings. 7-mm nozzles are being used but going to 11-mm may only increase the throughput by 10% for the current product. The maximum discharge pressure for the unit is about 50 psig. The maximum loss to byproducts is about 10% of the batch. Note that the typical level, produced from a batch, is about 50% in the product tank. Without buying more units, at about $330K for a continuous unit, how would you eliminate this bottleneck (Figure 3)?
Figure 3. Improve the capacity and quality of centrifuge separation.
Send us your comments, suggestions or solutions for this question by September 8, 2006. We’ll include as many of them as possible in the October 2006 issue and all on CP.com. Send visuals — 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.
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