A case study for control structure selection: air separation plant

The control structure selection for an air separation plant is discussed using the robust performance number (RPN) as a con- trollability index. The RPN indicates how potentially dicult it is for a given system to achieve the desired performance robustly. It re¯ects both the attainable performance of a system and its degree of directionality. The RPN criterion is compared with other well- known and new methodologies. These other criteria do not take the dynamic characteristics of the plant into account, hence they can be misleading. The predictions made by RPN are veri®ed by closed loop simulations. The analysis was performed using a linear nominal model, but can be extended to include nonlinearities and uncertainties.