Operation optimization is an effective method to explore potential economic benefits for existing plants. The maximum potential benefit from operation optimization is determined by the distances between current operating point and process constraints

which is related to the margins of design variables. Because of various disturbances in chemical processes

some distances must be reserved for fluctuations of process variables and the optimum operating point is not on some process constraints. Thus the benefit of steady-state optimization can not be fully achieved while that of dynamic optimization can be really achieved. In this study

the steady-state optimization and dynamic optimization are used

and the potential benefit is divided into achievable benefit for profit and unachievable benefit for control. The fluid catalytic cracking unit (FCCU) is used for case study. With the analysis on how the margins of design variables influence the economic benefit and control performance

the bottlenecks of process design are found and appropriate control structure can be selected.