Plantwide control system design for the economically optimum operation of a recycle process with side reaction, consisting of a continuous stirred tank reactor (CSTR) followed by. two distillation columns, is studied. The steady-state operating profit is maximized over a large throughput range for an existing design. As the throughput is increased, constraints progressively become active until maximum throughput is reached where all the steady-state degrees of freedom are exhausted. A strategy of moving the throughput manipulator to the next constraint to become active as the throughput is increased is used to improve the control of the active constraint. This minimizes the variability and hence the back-off in the active constraint variables for economically optimal operation. The plantwide control system (CS1) so obtained is quantitatively compared with three other more conventional control systems (CS2-CS4) in terms of the required back-off to avoid hard constraint violation for disturbances. Results show that the most economical operation is achieved using CS1 and the traditional practice of fixing throughput at the feed (CS4) gives the highest (up to similar to 7%) economic loss. A simple switching scheme to transition from low to maximum throughput is also demonstrated.