*3.5. Feasibility Check*

In a final step, the feasibility is checked by means of the methodologies presented in Section 3.2. This step is necessary to ensure that all critical points have been identified (i.e., the set of critical points is complete). As outlined in Section 2.1, the previously derived HEX design characteristics are included, i.e., design constraints and solution obtained by solving the multi-period design problem. Since including design characteristics is likely to increase the complexity of the problem, solutions obtained by the methodologies presented in Section 3.2 should be analyzed and it is advisable to compare the achieved results of different methodologies to reduce the probability of failures. If feasibility for the respective design cannot be guaranteed, the result of the feasibility check can provide useful insights. Besides the flexibility index, the feasibility check provides a point in the space of the uncertain parameters, which limits the flexibility. The vector between this point and the average operating point (see Figure 1) can be used to generate candidates of critical points, which can be added to the multi-period design problem. If the newly generated design is feasible, the critical point has been successfully identified; otherwise, the new flexibility limit is analyzed, and the step is repeated until a feasible design is obtained.
