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The bottom level must be kept within a range.

The usual manipulated variable is the outlet flow rate at the bottom, but it cannot be used in the present case because it is manipulated by another control loop downstream.

The available action is then the set point of the column feed rate.

The bottom flow rate is a disturbance variable for the level controller. It is taken into account by a feedforward PCR block.




Difficulties :

• Difficult plant tests because the level is much disturbed by the bottom outlet flow rate and by the bottom temperature fluctuation (whose effect is the vaporization of the bottom product)
• Large time delay between the feed flow rate set point and the bottom level.
• The system is integrator (it is a difficulty for a PID, but not for a PCR controller)