The control structure selection for an air separation plant is discussed using the robust performance number (RPN) as a controllability index. The RPN indicates how potentially difficult it is for a given system to achieve the desired performance robustly. It reflects 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 verified by closed loop simulations. The analysis was performed using a linear nominal model, but can be extended to include nonlinearities and uncertainties. (C) 2000 IFAC.