Sustainable distillation under feed variability: Embedding flexibility index in early-stage process design

Process flexibility has becoming increasingly important as the chemical industry shifts toward sustainable operations and variable feedstocks. Traditional process design often prioritizes economic objectives, such as total annual cost (TAC), while flexibility is considered only at later stages of design. To address this gap, this study integrates the Flexibility Index (FI) into the early-stage design (steady-state design), using both Aspen Plus and Aspen Plus Dynamics to assess whether steady-state FI can reliably reflect a system's ability to tolerate disturbances. A multi-objective optimization framework based on Nondominated Sorting Genetic Algorithm II (NSGA-II) was applied to both a binary distillation system (Case 1) and a reactive distillation system (Case 2). Results from both systems reveal a strong dependency of column diameter on FI due to its relationship with flooding, weeping, and reaction extent. Therefore, the design of the column diameter becomes more important when considering FI. Dynamic analysis using Aspen Plus Dynamics demonstrated that the steady-state FI is a reliable indicator of system flexibility, particularly in simple distillation systems. Although FI may not precisely capture the full dynamic behavior in the reactive distillation system, it still effectively distinguishes between configurations with higher or lower flexibility. These findings confirm that, despite its limitations, the steady-state FI remains a valuable tool for evaluating and comparing process robustness across different designs. Overall, this study not only confirms the viability of the steady-state FI but also demonstrates that incorporating FI into early design enables more resilient and adaptable process configurations, helping avoid cost-optimal designs that may fail under real-world variability.