How Can We Better Control an Exotherm? | Chemical Processing

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There was a real incentive to go batch,"it was much cheaper. I finally built my own heat-compensating calorimeter in one afternoon (a 1 liter resin kettle, submerged in a temperature-controlled bath, with an electric immersion heater in the reactor for temperature control). It was 1980, before commercial calorimeters were available. During the first run, we found that the reaction was not 1st order (as we had suspected). Here is what we observed: Initially, heat release was quite slow. Then, after about 30 min., the heat release started to pick up, but then diminished exponentially (as expected). It was that 30 min. phenomenon that caused the runaway reactions.

There was obviously an "initiation" period, where an intermediate compound was formed, which then further reacted to complete the isomerization. We never determined the mechanism (it wasn't really important) because we now knew why we had the runaway reactions. We scaled up directly to a 30 gal. pilot-plant reactor, with a pump-around loop. We actually started out at a higher temperature, to get the reaction going, and as the heat release increased, we lowered the reaction temperature to maintain a constant heat evolution.

On the second 30-gal. run, we only made a single temperature adjustment to the cooling loop,"and the reaction ran perfectly. When we then scaled up 500 gal. commercial batches (which required an external heat exchanger), we never had any problems with temperature control.
Gary Knapp, President GFK Consulting Ltd.,
San Clemente, Calif.
Subject to limitations due to reactor volume or agitation requirements, use a high-heat-capacity inert such as tabular alumina spheres to dampen temperature swings. Tabular alumina spheres of 1/2 to 3/4-in. diameter would provide a significant heat sink, a high void fraction and would not significantly impede flow from the reactor at the end of a cycle.
Bill Norman, Technology Manager
Porocel Corp.,
I think you meant solution polymerization. In the past, I have used an external cooler (heat exchanger) installed in the reactor circulation line. Depending on polymer properties and cleaning considerations, monomer injection can be on the inlet (preferred) or outlet of the cooler.

It's the media's fault

Add external cooling

Monomer injection should be interlocked with high/low temperature on the reactor to prevent run-away exotherm and accumulation of unreacted monomer. Circulation pump and cooling should also be interlocked.
Robert Bryant II, Manufacturing Engineer
Valent USA, Walnut Creek, Calif.
For a tight temperature control we often use a loop with a relative high-flow-capacity pump (about 5 to 10 times the vessel content per hour) and feed the loop with fresh cooling water over a PID-controlled three-way valve. This is a good and reliable solution in case the amount of heat to be removed gives a relatively small temperature rise in the cooling water. That's why this flow has to be as large as possible. The background of the story is that the heat buffer capacity of the cooling water is so large that the response time becomes also large. This gives the control loop the time to adjust the temperature before it runs away. I hope you can do something with this suggestion. However, the mixing action of the agitator is also a very important variable in this problem and in most cases the bottleneck.
Alan Ferraro, V.P. Engineering
Jongia Mixing/MPE Group USA, Pennsauken, N.J.
The reader is right about PID control. You can get rapid response, but have too much overshooting and swings or you can control the overshooting and swings but have long process times and very sluggish response. Most exothermic reactions only allow a very narrow temperature band but require a rapid response to "catch" and control the reaction when it occurs. Allowing the exothermic reaction to get too hot or too cold can ruin the product batch. It is a significant issue with polymers.

Lots of water flow buffers response time

Consider vacuum steam

We have a patented system that can control to +/- 1.8 Degrees F (1 Degrees C) and provide instant response to process fluctuations and temperature profiles. It is especially well-suited for exothermic reactions since it provides both heating and cooling capabilities. The heating system provides vacuum steam as low as 10 Degrees F. A pressure control valve uses proprietary control algorithms to allow rapid, measured response with minimal overshooting and swings. The cooling system uses an atomized water spray, which vaporizes under vacuum conditions to take advantage of the latent heat transfer at a much lower temperature (below 32 Degrees F if using brine solution). Product cooling times can be reduced by about 25%. Additionally, these systems provide energy and space savings when replacing hot water systems.

By the way, did you know that vacuum steam has more BTU/lb. to transfer in latent heat? Just an added bonus.

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