The Importance of Nominal Operating Point Selection in Self Optimizing Control

A self-optimizing control (SOC) structure is when we can achieve an acceptable loss with constant setpoints for controlled variables. In the original paper (Skogestad, J. Process Control, 2000, 10 (5), 487-507), the operating point is selected first and then the controlled variables, using the optimal values for nominal disturbance as setpoints. In this work, the importance of nominal operating point selection is evaluated, showing that setpoints obtained by optimization along the disturbance region can provide a better self-optimizing structure. A simultaneous procedure is proposed where controlled variables are selected together with the nominal operating point. Despite the more expensive optimization procedure, this new policy leads to a better controlled variables set. This is corroborated by a reactor distillation case study and a cumene unit case study where the operating point selected decreases significantly the worst-case and mean losses for the process.