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Ivar J. Halvorsen and Sigurd Skogestad, ``Indirect on-line optimization through setpoint control'', AIChE Annual Meeting, 16-21 Nov. 1997, Los Angeles, Paper 194h.

We address the problem of optimizing operation of a process where there are extra degrees of freedom, and where the optimum is not at some constraint. In practice, there are always unknown disturbances and model uncertainties which make this task complicated. The key idea of this paper is to turn the non-linear optimization control problem into a setpoint control problem. If this can be done, the task of optimizing operation can be realized by simple standard control loops, without the need of solving complex nonlinear optimization problems online.
The integrated Petlyuk or dividing-wall distillation column is used as an example process. The Petlyuk arrangement is an interesting alternative to the conventional cascaded binary columns for separation of multi-component mixtures. As much as 30% energy savings have been reported. The industrial use has been very limited, and difficulties in control has been reported as one reason. The optimal operating point depend strongly on disturbances and design parameters. Thus it seems difficult, in practice, to achieve the potential energy savings without a robust optimizing control strategy. We discuss alternative candidate feedback variables for optimization by feedback control and test a particular variable with promising properties.