It is desirable to know the variations around the normal feed conditions that a real plant can handle ensuring feasible operation and to detect the plant bottlenecks for these perturbations. This information is provided by a flexibility analysis of the real plant. Perturbations on total flow rate and ethane composition are expected in the feed of an ethylene plant. Due to the fact that only two uncertain parameters are contemplated, the maximum feasible displacement from the nominal point in different directions of the uncertain space can be evaluated. The maximum displacement is determined solving a nonlinear programming problem where the main operating conditions of the plant are the optimization variables. An active capacity constraint indicates a plant bottleneck. Therefore the flexibility analysis of a real plant allows the quantification of the perturbations that the plant can accommodate for different directions of interest, associating bottlenecks with scenarios in which they will take place.