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I.J. Halvorsen and S. Skogestad, ``Use of feedback for indirect optimizing control: Application to petlyuk distillation'', IFAC symposium DYCOPS-5, Corfu, Greece, 08-10 June 1998, 399-404.

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 so that we achieve self-optimizing control. 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 on-line. The integrated Petlyuk or dividing-wall distillation column is used as an example process. The column has two extra degrees of freedom at steady state, and the challenge is to find a set of feedback variables, which makes the operation insensitive to disturbances so that the reported 30% energy savings can be achieved in practice.