**3. Methodology**

In order to achieve retrofit design proposals that can operate cost efficiently at multiple operating points, a framework was developed. The framework is shown in Figure 2 and will be outlined in the following subsections. The proposed framework utilizes single-period and well-proven retrofit design methodologies but also "experience-based" retrofit design proposals can be considered. Design proposals that are structurally infeasible are discarded after a structural feasibility assessment. To avoid suboptimal solutions due to unnecessary overdesign of equipment, the proposed framework is based on the identification of critical points. Cost efficiency for the entire operating period is ensured, by considering representative operating points besides the identified critical points when solving the eventual design problem. A final feasibility check is performed to ensure that all critical points have been identified (i.e., the set of critical points is complete); otherwise, the new critical point, which is identified during this check, is added to the set of critical point(s) and the design problem is solved again.
