End Point Detection and pH End Point Control

End Point Detection and pH End Point Control  

End Point Detection

Determining reaction endpoint in polymer/resin reactors is critical to achieve desired product properties, such as molecular weight. Knowing exactly when the reaction is complete saves residence time and batch conversion time, and lowers the overall manufacturing cost.

One way to increase plant productivity is to reduce batch cycle time.

If the reaction continues beyond the optimal endpoint, the process yield can be reduced due to side reactions.

reactions, the endpoint is accompanied by the fall or rise of the reaction pressure. This change in pressure can be used to have a pressure switch or pressure controller to shut off the reactant flow

pH End Point Control  

pH is used in detection the end point. pH end point control is important to realize that during most of the reaction the measurement will be away from set point. Therefore, integral action must not be used, because it would saturate and overshoot would result. This is one of the few processes in which the proper selection of control modes is a proportional plus derivative with zero bias, modulating an equal percentage reagent valve.

If the derivative time is set correctly, the valve will shut quickly as the desired endpoint pH is reached. With excessively long derivative time setting, the valve will close prematurely, but this is not harmful because it will reopen as long as the pH is away from the set point.

For the measurement of pH, conductivity, and resistivity, on-line analyzers are available. The pH probe shown in Figure can be used for on-line measurement in a glass lined reactor

When maintenance or recalibration is needed or when the reactor is empty, the probe can be lifted by a pneumatic actuator and immersed into buffer a solution to protect it from getting dry. In some cases it might be preferred to keep the pH sensor outside the reactor and bring a sample of the reactor contents to it.