Process Puzzler: Sidestep a Scale-up Scare

Readers grapple with a reactor production change.

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Yikes! Sounds like “Synthron” all over again. Yes, there are significant process hazards if the reactor temperature is able to reach the “point of no return,” accelerating the exothermic reaction exponentially. Play the Chemical Safety Board video for management and see how they feel afterwards.
Chris Rentsch, process engineer
Dow AgroSciences, Midland, Mich.

With increased batch quantity, the agitation and cooling will have to increase. Normally, in a [batch] reactor, control of the rate of addition is very important to achieve the quality and yield. So to meet these requirements you have to make changes in the process procedures. You may want to look for an alternative catalyst. Finally if nothing works, adding another reactor would be better.
B.S. Gadodia, consultant
Indore, India

They should consider the following steps: 1) estimate or find by experiment the heat of reaction; 2) calculate the adiabatic temperature rise if all the reaction occurs instantly without cooling; 3) if the heat in step 2 is not enough to boil the reactor contents then all is okay to proceed with caution; 4) if the heat in step 2 can boil the reactor contents then proceed with great caution; 5) test the process using adiabatic reaction calorimetry to discover if the process’s behavior changes with the change in conditions (it is not uncommon for polymerizations to accelerate with temperature or to have competing reactions dominate at higher temperatures); 6) if maintaining a lower temperature becomes essential use the “efficiency method” rather than the usual log-mean, as it can handle transient conditions much more effectively; and 7) if in doubt, contact a chemical process engineer who understands the reasons for the steps above.
Ed Fish, associate consultant
Haztech Consultants, Winsford, U. K.

The engineer is right to be a little nervous. There are multiple changes happening here: addition rate, addition amounts, changing agitation pattern, and changing heat transfer surface. Small scale results often work well because the ratio of heat-transfer surface area to reactor volume is high but the ratio is much lower at larger scales. It shouldn’t hurt to make the changes incrementally, one at a time, over the course of weeks or a month. The results will show feasibility or demonstrate the system limits without making the six o’clock news.

Increasing the level and concentration means there is more reacting mass and likely a faster reaction rate. The level rise will increase the reactor heat-transfer area, but not in proportion to the additional duty. Further, the partial reactor baffling may mean that the increased level is in a part of the tank that is poorly mixed and has a low heat transfer coefficient.

On the other hand, it may be that the bulk of the heat transfer needs of the system are met by condensing evaporated solvent in the overhead condenser. In that case, someone needs to verify that the condenser and the vapor/condensate piping can support the additional load.

You can perform a number of calculations to check if the system can handle the extra load, but it’s easier, quicker and more accurate to slowly scale-up to the new conditions. I would start by increasing the rate of the initial addition; if all goes well, then either increase batch size or concentration or both in small increments. The traditional slow addition of the starting monomer charge may be needed to get the reaction initiated without “sandbagging” (no initial reaction followed by a sharp, maybe uncontrollable, spike in temperature). You want to demonstrate that this is not a problem before you have a lot of extra monomer in the vessel.
Alexander E. Smith Jr., engineer
Parsons Corp., Boston


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