Economic impact of disturbances and uncertain parameters in chemical processes - A dynamic back-off analysis

Optimal operating conditions in chemical plants are characterized by operation on a number of active constraints. The presence of disturbances and model uncertainties can easily cause constraint violations. Thus, it is necessary to move the operating point away from the active constraints into the feasible region (back-off). Recently a strategy for detemining the necessary open-loop steady-state back-off from the nominal optimum has been developed by the authors, which consists of solving a joint steady-state optimization-flexibility problem. This paper extends these ideas to consider dynamic situations, thus leading to a joint dynamic optimization-flexibility problem. Having determined the economic penalty associated with open-loop back-off in dynamic systems, the next step is to estimate the potential recovery of this penalty that various control schemes might provide. The proposed approach also includes the optimization of the controller parameters. Results are given for a simple flowsheet example.