Tuning of two PID loops controlling a distillation column

Distillation column

 

Distillation columns are used to seperate substances from a liquid by evaporation and condensation. To do so, they require considerable amounts of heat (to create evaporation) and coolant (for condensation). The control challenge is to steer the heat and coolant inputs in the most efficient way.

Dishman Netherlands, located in Veenendaal in the Netherlands, manufactures (among others) Vitamin D, Cholesterol and lanolin for the pharmaceutical, cosmetic, feed, food, shrimp farming and industry applications. One of the steps in their production chain is to distil ethanol and water using a distillation column. The heat and coolant inputs to the distillation column used to be operated manually. Previous attempts to apply automatic control had failed due to the complexity of the PID tuning.
DotX prepared two PID loops to automatically control the quality variables (density and temperature) by adjusting two manipulatable variables: the steam flow valve and the coolant water valve. The PID loops (implemented in a Siemens S7 PLC) were prepared to allow tuning and monitoring by the PID Tuner.
The whole tuning procedure and verification took half a day. Figure 1 shows a screenshot of the PID Tuner with the response of the density to a step change in the heat valve. .

Figure 1: screenshot of the PID Tuner 'Fit Model' section showing the result of the step experiment (step on the coolant valve position)

Figure 2 shows the complete log. The first half hour was used for step/identification experiments. After half an hour we had gathered sufficient data to identify the process model. Immidiately after that, we tuned the PID controllers and switched them on. The temperature loop is relatively fast compared to the density loop, but both loops reduced the process variable variations with more than 80% (compared to manual control). Therefore, after approximately 1 hour, the operator agreed to increase the density setpoint, and to reduce the temperature setpoint. As a result, the heat valve position changed from 60% (at time 0) to 55% (at the end of the experiment) which boils down to approximately 8% reduction of steam flow. The automatic control system has been in operation ever since the tuning experiment.

Figure 2: complete log of the experiments. The first half hour was used for step experiments. Based on the results of those experiments, the PID controllers were tuned and switched on. After 2.2 hours the temperature setpoint was reduced resulting in lower steam flow consumption (lower valve position means less steam flow consumption).

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