THE INTERFACE BETWEEN DESIGN AND CONTROL .2. PROCESS OPERABILITY

The optimum steady-state design fixes the equipment sizes based on the mean value of the disturbances, but as disturbances enter the plant the fixed equipment sizes may prevent the process constraints from being satisfied or may prevent the operating variables from being adjusted to significantly lower the operating costs. Operability problems can be overcome either by an appropriate amount of flexibility or by developing alternative operating policies, and we want to determine the alternative that has the smallest cost penalty. By evaluating operability alternatives at the preliminary stage of a process design, we can include some of the economic penalties associated with control as an additional criterion for screening flow-sheet alternatives.