Multi-objective optimisation, exergy analysis and process intensification of side-stream extractive distillation processes

The separation of complex ternary azeotropic mixtures poses a significant challenge in the chemical industry, often requiring energy-intensive processes. This study proposes a systematic framework for the design, optimisation, and intensification of side-stream extractive distillation (SSED) to enhance the sustainability of such separations. A representative mixture of tetrahydrofuran/n-propanol/water was separated into three products with a purity of at least 99.9 mol% for each key component. Six novel SSED configurations, categorised into centralised (Series I) and decentralised (Series II) solvent recovery strategies, were conceptually designed. A multi-objective optimisation framework using the NSGA-II algorithm was employed to minimise total annual cost and CO2 emissions, with the TOPSIS method selecting the optimal solution. The optimal SSED designs were further intensified through the sequential integration of mechanical vapour recompression heat pumps and intermediate reboilers. Exergy analysis revealed that the strategic placement of side-streams mitigates remixing effects, reducing exergy destruction by up to 16% in the most thermodynamically efficient configuration. The results demonstrate that process intensification delivers substantial improvements; the heat pump-assisted SSED with intermediate reboiler process achieved a 17.4% reduction in TAC and a 31.0% reduction in CO2 emissions compared to the conventional extractive distillation benchmark. However, the decentralised strategy of Series II, while economically favourable in its base form, was found to limit heat pump applicability due to high column bottom temperatures. These results suggest that combining systematic process synthesis with thermal integration can support the development of more energy-efficient and environmentally friendly separation processes for complex multi-azeotropic systems.