Robust control of distillation columns: μ- vs H∞-synthesis

High-purity distillation columns are known to be difficult to control due to their ill-conditioned and nonlinear behaviour. Two approaches for the design of robust controllers yielding high performance are presented. For the first approach, first principles are used to develop an uncertainty model describing the nonlinear dynamics within the entire operating range of an industrial distillation column. This structured uncertainty model is used for/l-synthesis. In a second approach which is based on loop shaping ideas, an H -controller is designed. This controller performs as well as the y-controller. The H~-approach offers the advantage that the burden for uncertainty modelling and computation is greatly reduced. However, the GS/T augmentation scheme, which is developed in this paper, must be used for the design of the H -controller to avoid the inversion of the plant in the controller. The paper concludes with a comparison of the H=- and H-synthesis methods.