A general, rigorous dynamic model is described for studying the interactions of design and control in a double-effect distillation system. Two approaches are adopted. In the first, the steady-state process design and the control system are optimized sequentially; potential operability bottlenecks are identified and the economic advantage of double-effect systems over conventional single column systems is demonstrated. In the second approach, the process design and the control system are optimized simultaneously leading to a more economically beneficial system than that obtained using the sequential approach. (C) 2000 IFAC.