Effect of disturbances in optimizing control: Steady‐state open‐loop backoff problem

Abstract One of the key components in the operation of chemical plants is the ability to operate over a range of conditions while satisfying performance specifications. A method for determining the necessary open‐loop backoff from a steady‐state nominal optimum is introduced to ensure that disturbances cause no constraint violation. This approach consists of defining a joint optimization‐flexibility problem that can be solved within an optimization framework based on an iterative procedure. By formulating the optimization problem in economic terms, the backoff in the objective function is a measurement of the open‐loop economic penalty that is necessary to be paid to achieve feasible operation over the disturbance range of interest (considering the worst combination). An upper bound on the economic potential available for closed‐loop control can then be established, providing a valid reference for ranking different control schemes. Three examples presented illustrate the application of this approach: (1) a simple linear example, (2) a system of two CSTRs; and (3) an industrial distillation column.