Process Puzzler: Sidestep a Scale-up Scare

Readers grapple with a reactor production change.

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Management wants to boost production from our batch polymer process by changing the operating procedure. Now, a portion of the acrylic monomer goes in with the bulk of the solvent and other ingredients. Then, monomer is slowly added until the reactor level rises to about 55%. Instead, management wants to increase the maximum level to about 65% and raise the final concentration of the polymer about 2%. The reaction is very exothermic; a chilled-water baffled jacket provides cooling. At the top of the reactor a horizontal shell-and-tube condenser that uses cooling tower water captures evaporated solvent. Agitation consists of a single axial impeller and the tank has reduced baffles to account for viscosity above 5,000 cP. To meet the production goal, it’s been proposed that we add all of the monomer at once; bench-scale tests have shown that this could work but our current approach seems safer. Management likes the plan and wants to put into practice immediately. Our production engineer is a little nervous — should he be concerned? How should we approach this problem? Are any process changes necessary? What do you think?

I would recommend using a static mixer with an integral heat exchanger to blend the materials before they go into the reactor. The static mixer/heat exchanger could aid in reducing the temperature of the exothermic reaction before fluid is introduced into the reactor. Controlling the temperature before it is introduced into the reactor is a safer solution [than adding the monomer all at once] by allowing the reactor and the mass inside to act as a heat sink/quench tank. Two mass flow meters, one for the monomer and the one for the solvents, would monitor flow. If the temperature in the mixer/exchanger increases above safe thresholds, you can continue to add the solvents while reducing the monomer level, thereby maximizing the monomer addition and eliminating risk. You may want to have multiple feed lines [for the solvent] with isolation into the reactor for faster addition, better control, and in case pluggage occurs or cleaning is required.

Fast temperature sensing at the exit of the exchanger and in the reactor, with either adaptive or artificial-intelligence temperature control, will be necessary to fine-tune and predict the reactor temperature.

Using this system eliminates the possibility of thermal runaway in the reactor. I agree with your production engineer, as once the reaction starts to run away it is too late. Piping is much easier to clean [and contain] than a reactor if an incident were to occur.
Joel Heidbreder, principal engineer
Monsanto Co., St. Louis

Your production engineer’s concern is perfectly right as far as safety is concerned. Adequate cooling has to be ensured as the rate of heat generation will be much greater [with the process change]. Try a continuous plug-flow-type reactor as it can handle higher concentrations and is safer.
Nandan Bhandari, managing director
Maple Biotech Pvt., Ltd., Pune, India

Yes, the engineer is right, they should be careful. If this is a free radical solution or emulsion reaction, as I assume, then it is a chain reaction that could lead to an explosion. This is especially true if all monomer is added at once; the temperature will rise out of control.

Instead, to increase the conversion, increase the initiator concentration, but not [enough] to affect the molecular weight, and lower the solvent concentration in the formula to increase polymer concentration. Use bubbling nitrogen inert gas and change tower water to chiller water in the condenser to avoid monomer evaporation further and lower inner pressure.
Emilio Malaguti, technical manager,
Chemtron, Hialeah, Fla.


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