Inherent flexibility design strategy of extractive distillation for binary azeotropic separation: Novel integration of optimization approach with penalty values

In our previous work (Sep. Purif. Technol. 312 (2023) 123344), we incorporated flexibility index (FI) into steady-state extractive distillation (ED) design for binary azeotropic separation. We pointed out the significant trade-off between the cost and FI, whereby good operational flexibility (i.e., FI) is always accompanied by a higher cost. One shortcoming, however, was that we did not determine an optimum configuration that provides a compromise trade-off between the cost and flexibility. In this work, we address this by introducing a novel approach, i.e., integrated total annual cost and FI objective function (TAC(FI)), which combines cost and FI as a single objective function, to obtain the most economic configuration with the largest operational flexibility. Additional penalty terms based on values of 10, 100, and 1000, were also introduced to compensate for the unexpected offset when the system operates beyond its limit point. Using the same isopropyl alcohol (IPA) dehydration process with the same uncertainties of +/- 20% feed flowrate and +/- 20% IPA feed composition as before, the TAC(FI) with penalty terms enables a lower cost relative to the previous work by 39-41%. We also observe significant enhancement in operational flexibility, with a penalty value of 10, resulting in a 520-fold enhancement. However, there is no improvement for penalty values of 100 and 1000 due to the physical and thermodynamic constraints for producing electronic grade IPA. This approach allows us to locate a new optimum point with lower cost under the same FI value, and the IPA dehydration using ethylene glycol (EG) with a penalty value of 10 provides the most significant improvement in both FI and cost by about 520 times and 5%, respectively.