Chemical processes are operated within a set of practical constraints and the most profitable operation point lies an the interception of these constraints. Enabling operation to be as close as possible to the optimum point is a challenging advanced control problem. This work is designed to achieve a suitable control strategy for handling constraints in a systematic way and illustrate the effectiveness of the approach by reference to a fluid Catalytic Cracking (FCC) system. Essential features of the proposed approach are : classifying constraints into input and output constraints; handling output constraints using the optimisation layer of the hierarchical control system structure; handling input constraints using the experience of operators. Computational complexity is reduced by handling the different types of constraints differently. In order to enhance the robustness and flexibility of a control system, the approach also tries to keep the openings of the key control valves away from their limits without losing control.