How Can We Better Control an Exotherm? | Chemical Processing

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Steve Kortheuer
TLV Corp., Charlotte N.C.
Initially this sounds like the temperature setpoint (PID control) on the cooling water is too low. But I'm sure there is more to it than that. I suspect there is more than one process parameter that controls the reaction time.

Vary the setpoint

Perhaps the process requires maintaining one temperature when in steady state but could be allowed to raise a given amount when the catalyst is added. If this is the case, the controller could switch to a higher setpoint when triggered that the catalyst is being added. The setpoint could go back to the initial value after a set time period. Some PID controllers can store more that one setpoint and switch them based on time or logic.

For other situations, consider an over-ride controller. This consists of two PID controllers on one box that share an output (or set of outputs if both heating and cooling are controller). A simple rule set determines which PID controller had control of the output. Usually this is based on which controller output is calling for the lowest output, but this is configurable. Over-ride allows one output to be independently controlled by two parameters via two PID controllers.
Jim Overturf, VP of Marketing,
Eurotherm Inc., Leesburg, Va.
The problem is that a tight temperature control is difficult to achieve by indirect cooling through a jacket of the vessel ," because of the high energy produced by the reaction, the heat to be removed is also high. The limitation to removing that heat will be mainly on the product side (not in the jacket).

 

Improve the stirring

One simple way to improve the heat transfer is stirring. But in order to stir, the solution or suspension has to be diluted. Thus the concentration of active substance is lower and so too is conversion and reaction speed. If the suspension is particularly dense, the agitator will not move or, using a tougher, kneader type unit, the induced energy will be eventually even higher through mechanical dissipation.

One appropriate method to overcome this problem is evaporative cooling. This means the operating pressure is adjusted so that one of the components in the reaction mixture (likely the solvent or suspension media) simply boils. You will need a closed design of the equipment and pressure control. The evaporated solvent can be recycled internally through a simple heat exchanger. You have to check if there is danger that one of the active substances of the reaction is stripped out and chose your solvent accordingly.
Daniel Witte, Senior Process & Sales Engineer,
List USA Inc., Charlotte, N.C.
If a liquid is exposed to a pressure lower than its steam pressure, it starts evaporating and radiating heat through the developing vapors until the steam pressure is equal to the ambient pressure. In this process, the liquid cools down to the temperature corresponding to the prevailing pressure.

Vacuum recirculation cooling

If the homogeneous mix of the product is exposed to underpressure, water will evaporate from the mix until the pressure of the water is equal to the interior pressure in the mixer. With the developing vapor, a heat stream is dissipated from the paste in the form of evaporative heat. At this point, the paste cools down to the temperature corresponding to the prevalent pressure according to the water vapor chart.

With our vacuum cooling method, the cooling is generated in the product itself, independent from cooling surfaces (contact cooling) or from environmental conditions (evaporative cooling).

A rotating mixing pan and rotary mixing tool keeps the product constantly moving, thus supporting the evaporation process.

The temperature of the prepared product in the mixer is determined by the generated vacuum (underpressure), because pressure and temperature are related according to the water vapor chart. With a pre-selected defined pressure value, it is possible to automatically control the product temperature by means of the process controller.

Occurrence of partial temperature differences in the product is impossible. The product quantity is entirely exposed to the vacuum, so that the water evaporates evenly throughout.

The water vapor originated during the product cooling process is evacuated through a surface condenser, which consists of a bundle of thin-walled tubes with external circulation of cooling water. The developing condensate flows immediately back into the mixer, again being mixed homogeneously into the product. The processes of evaporation, condensation, and recirculation are occurring simultaneously during the cooling phase.
Nick Semitka
Eirich Machines, Gurnee, Ill.

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:

DT, while heat generated increases exponentially with reaction temperature). If your reactor has insufficient jacket area, incorporate a pump-around loop to get sufficient heat exchange area.DT between cooling media and reactants. By doing this, the cooling media temperature essentially determines the reaction temperature. By operating at low DT, when the batch temperature rises slightly, heat evolution increases, but the delta T increases more significantly, thus removing the extra heat and driving the reaction temperature back to the target temperature. 

 

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