## 1 Introduction
[PERSON] and co-workers [15] investigated the optimum design and configuration of the extractive distillation and recovery column. The sensitivity analysis, such as the effect of the stage number, binary feed stage, solvent feed stage, reflux ratio to the isopropyl alcohol purity, and reboiler-condenser duties, was realized using Aspen Plus. Extractive distillation is often used for the separation and purification of the azeotropic mixtures with a significant content of one component, such as the water content in the fermentation broth of C\({}_{2}\)-C\({}_{4}\) bio-alcohols. In most cases, before the extractive distillation process, another supplementary step must be realized to reduce the water content in the mixture in a preconcentration column. The literature reports two categories of techniques for reducing energy consumption: classic energy-saving technologies for ED, including thermally coupled ED [16], heat-integrated ED [3; 17], heat-pump-assisted ED [18], and extractive dividing wall column [19], and novel energy-saving methods such as combining columns [4; 6; 20; 21], adding a column [22], adding side streams [23], double annular column [24] and splitting the fresh feed [25; 26]. Although several methods were investigated to reduce the energy consumption of ED with three columns applied to the separation of binary azeotropic mixtures from dilute aqueous solutions (i.e., dehydration of C\({}_{2}\)-C\({}_{4}\) bio-alcohols from fermentation broth), throughout the research, it has been proven that the simplest and most efficient is the thermal integration of heat. Although not novel, this concept is supported by recent improvements in the study by [PERSON] [3], who investigated the IPA-water separation and purification with DMSO as a solvent in a three-column extractive distillation (TCED) conventional process with two forms of heat integration: valorization of the latent heat of the top vapors from solvent regeneration column (SRC) and/or from the extractive distillation column (EDC) to provide some heat in the preconcentration column (PC) reboiler, and the recovering of the sensible heat of the high-temperature solvent from the SRC bottom to preheat the PC feed, the EDC feed, the SRC feed, and then used them as a heat source for the low-temperature reboiler of the PC or the EDC. The study demonstrated that the fully improved heat integration can increase the energy savings from 26% to 52.5%. In this paper, propylene glycol (PG) is used as an extractive agent for IPA dehydration via a three-column extractive distillation (TCED) conventional process. Considering the commercial availability, low toxicity (PG replaces EG in deicer and antifreeze products due to its low toxicity) [27], and biodegradability of PG, it can be a proficient solvent in the alcohol-water separation process. PG can be easily obtained via propylene oxide hydrolysis (a common raw material); it has a lower boiling point compared with DMSO, EG and glycerol. Also, it has lower thermal conductivity, which leads to a smaller overall heat transfer coefficient and, in the last instance, to a lower heat exchanger area and smaller capital costs. First, the original idea is to investigate the suitability of PG as an extractive agent in the extractive distillation process for the removal of IPA from its azeotropic mixtures with water. To the best of our knowledge, no literature has reported the IPA-water system separation (or other alcohol-water systems) by the extractive distillation process using PG as solvent. PG has been reported in [PERSON]'s and [PERSON]'s papers [26; 28] as an extractive agent for the tetrahydrofuran-water separation system by ED. The optimal design of the IPA dehydration process is also studied. A flowsheet with a PC, an EDC, and an SRC is simulated in a steady state using the PRO/II process simulator. The optimal design of the conventional TCED process in our work is the one that maintains the lowest energy consumption while maintaining the process separation target.