A Lyapunov-based controller for the composition control of binary distillation columns has been developed. It takes into account physical constraints on the inputs and ensures the global asymptotic stability of the closed-loop system with robustness to modelling errors and with the capability of performing set-point tracking and (approximate) disturbance rejection. This controller requires the knowledge of the internal state of the model and this leads to the design of an exponentially converging ’high-grain’ observer. We arrive at the global asymptotic stability of the whole control structure (the controller + the observer) by proving a somewhat general nonlinear separation principle.