When designing the control structure of a chemical plant, with optimal operation and control in mind, it is important to know how the active set of constraints changes with disturbances. The generation of optimal active constraints regions using optimization generally is very time-consuming, and this paper discusses how to use process knowledge to minimize the need for numerical calculations that is, minimize the number of optimization problems that must be solved. We consider the case of two disturbances, as this can be nicely represented graphically. In this paper, we study economically optimal operation of a reactor separator recycle process and show how the set of active constraints changes with feed flow rate and energy cost as disturbances. We also identify the economical and physical bottleneck of the process. For this case study process, we find five distinct regions with different active sets. This illustrates that, even for simple cases, the map of active constraint regions can be quite complex.