*4.1. Structural Feasibility Assessment*

The proposed retrofit designs can be considered as different pathways in a superstructure, and in a next step, the structural feasibility of the different proposals was evaluated. For the evaluation, variations in the four inlet temperatures and heat capacity flow rates were assumed. The variations are shown in Table 3.


**Table 3.** Variations of the average stream data.

The structural feasibility was evaluated by calculating the flexibility index using the active set approach developed by Floudas and Grossman [39]. Since all constraints i ∈ I and j ∈ J (see Section 2.1) are linear, the active set approach guarantees the globality of the solution [39]. In Table 4, the structural flexibility index for each retrofit proposal is listed.

**Table 4.** Results of the structural feasibility assessment of the different retrofit proposals.


The flexibility analysis revealed that retrofit proposal 5 is structurally infeasible, i.e., the structural flexibility index is smaller than 1 (see Table 4), meaning that for some operating points within the uncertainty span, the target temperature of at least one stream cannot be reached. This retrofit proposal was therefore removed from the initial superstructure of retrofit proposals, yielding a reduced superstructure.
