Selection of controlled variables and robust setpoints

Optimal operation of chemical plants is usually accomplished by rst nding the optimal steady-state using the present (nominal) set of disturbances. This is usually implemented by sending con-stant setpoints for selected variables to the control system. The setpoints are then the nominally optimal values. However, because of disturbances this may result in feasibility problems, which we here try to avoid by adjusting the setpoints ("backo"). First, we need to avoid infeasibility in the active constraints ("constraint backo") [1]. Second, we need to adjust the setpoints of the uncon-strained controlled variables. This may be done by oine computation of robust setpoints ("optimal backo") or by online feasibility correction ("exible backo"). As a case study we consider a reactor-separator-recycle process. For this process the control structures based on Luybens rule (x a ow in every recycle loop) are infeasible if we use the nominal setpoints, but are feasible with reasonable loss if we use robust setpoints.