How Can We Better Control an Exotherm?

We have to hold tight temperature control on a highly exothermic suspension polymerization process in order to maximize reaction rates. We tried using PID on the catalyst flow, but temperature fluctuation was too high. We added PID control to the cooling water and can now control the temperature within +/-5 Degrees C, but now the batch time is too long. What can we do to achieve tighter control on this kind of process?

,"From August Chemical Processing

Focus on heat transfer
The problem does not lie in the method of control (PID in this case) or the tuning of the controllers. It is a heat transfer issue. Consider the enthalpy balance:

where subscript R refers to the reactor conditions and subscript c refers to the coolant conditions. It is critical to note that in polymerization reactions the rate of reaction, R, is typically a highly nonlinear function of both TR and the catalyst feed rate, C, while the cooling capacity is determined by the flow of coolant. In addition, the ability to cool the reactor is limited by the reactor surface area and the overall heat transfer coefficient.

There are a number of options to improve reactor cooling.

First, one may marginally improve the overall heat transfer coefficient by increasing agitation speed. Since the main purpose of the agitation is to maintain the suspension, a higher agitator speed should not hurt the process, although it might improve the mass transfer from the bulk liquid into the reaction sites and result in a higher reaction rate. Higher turbulence at the wall will enhance the heat transfer slightly, but I believe that this would not be sufficient to improve control and reduce batch time.

Second, one can change to a cooling media with a higher heat capacity. This option will be capital intensive in that a new system is required.

Third, by adding a pump-around loop to the reactor vessel one can add additional heat exchange surface by either jacketing the pump around or putting a heat exchanger in-line. This option might not be practical due to increased maintenance resulting from fouling the heat exchange area.

Fourth, find a suspending fluid that will boil at the appropriate reaction temperature to take advantage of the latent heat effects. The vapors should be condensed and returned to the vessel. This will require an intensive development effort.

Fifth, scale down the process. A smaller reaction vessel will have a larger surface area to volume ratio. This will have the effect of increasing the heat transfer capability of the reactor, and ultimately allow shorter batch times with better temperature control. In addition, two smaller reactors will probably provide more throughput than a single reactor of equivalent size. This option might be difficult to sell to management due to capital investment and the potential for higher operational cost brought on by increasing the number of operators.
Keith Dackson, Ph.D., Engineering Scientist
Henkel Technologies/Loctite Electronics, Olean, N.Y.

Learn From a scale up
We successfully scaled up a highly exothermic and "unusual" batch isomerization reaction from 1 liter in a calorimeter to a 30-gal. pilot-scale batch and from there to a 500-gal. full-scale reaction. We were able to abandon a $20-MM plant modification based on a continuous reactor and substituted a $5-MM (1980s $$) modification based on a couple of batch reactors.

I would proceed as follows:

Run the reaction in a batch-heat-flow/heat-compensation calorimeter. This let's you run the reaction under conditions where the heat-transfer surface to reaction-volume ratio is very large. You can run the reaction in batch or semibatch mode; you can add the catalyst all at once, or add it slowly, i.e., you can explore heat evolution (and yields) as a function of time under various conditions. Since heat evolution is always some function of reaction rate, you're actually getting a real-time view of the reaction kinetics.