Process Puzzler: Sidestep a Scale-up Scare

Readers grapple with a reactor production change.

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Even though bench-scale tests show that adding all the monomers at once may be OK, it will be unsafe to directly implement it in the actual plant reactor. This will be mainly due to inadequate mixing and hence very high localized reaction rates in certain parts of the reactor. This situation may pose problems with reaction temperature control and hence may lead to a possible runaway reaction (due to exothermic nature of the reaction).

The best way to tackle this problem is to increase the monomer addition rate to maximum possible value (with present reactor and cooling system design) in small steps. Each time increase the monomer addition rate by a small step based on actual reaction controllability and product quality. If successful, increase the monomer addition rate further with a careful eye on the reaction controllability and product quality.
C. C. S. Reddy, lead process design engineer
Singapore Refining Company Pte. Ltd., Singapore

The process manager has a right to be concerned. As many others will mention, the surface area to volume ratio will change as reactor size increases. For a process that is cooled by a jacketed shell (surface cooling), the relative cooling capacity will decrease as a reactor gets larger. For this reason the results of a bench-scale test may not be an accurate predictor in a larger reactor. The other complication in this scenario is the fact that it’s an exothermic polymerization. As the reaction proceeds the increase in viscosity will hinder cooling at the same time the evolved heat will increase the reaction rate. This can lead to a runaway reaction. It would be best to proceed to the final goal of all monomer in at once in small steps with careful monitoring. One potential way to monitor this is to measure the heat removal from the jacket and the top-mounted condenser separately. The top-mounted condenser would be a faster indicator of heat evolution because it cools the boiled solvent, which is an indicator of the bulk temperature. One potential approach would be to see if it would also be possible to temporarily decrease the polymer concentration to help mitigate the effects of viscosity on cooling.
Rob Vermeulen, engineer
Metron Technology, Boulder, Colo.

Changing from semi-batch, by slowly adding monomer, to batch, by charging all the monomer at once, can be dangerous. In semi-batch mode, a limited amount of energy in the form of unreacted monomer is available at any point in time. In batch mode, the entire energy release of the reaction is available at once. Should the reaction proceed adiabatically, as it would in the case of loss of cooling or reflux, the final temperature and therefore pressure due to the vapor pressure of the final mixture could be quite high. The elevated temperatures will also greatly increase reaction rate. Worse, at this elevated temperature, exothermic or gas-producing decomposition reactions may initiate, exacerbating an already bad situation. Before instituting batch-mode production, you must: 1) quantify the heat of reaction for the desired reaction; 2) determine the final temperature and pressure (due to vapor pressure) if the reaction should occur adiabatically; and 3) test the final product for thermal stability from room temperature up to the maximum achievable adiabatic temperature rise. If the final temperature or pressure of the adiabatic reaction exceeds the vessel’s design capacity or if thermal stability testing shows a decomposition reaction, adequate emergency relief must be designed for the system (as the existing emergency relief system my not be adequate). This will likely require testing in an accelerating rate calorimeter or vent sizing package testing to obtain maximum rates of temperature and pressure rise, as well as consideration of two-phase relief flow.
John C. Wincek, process safety manager

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