## 3 Results and Discussion
### Economic Evaluation Results of TCED Processes  
In the previous section, the energy consumption advantages of the TCED energy-saving processes have been partially demonstrated compared to the conventional TCED separation process. In this section, the above results will be completed by the economic benefits of each heat-integration configuration performed via simulations of the extractive distillation process for IPA-water-PG separation. The results of the design calculations for all the variants can be found in the Supplementary File, in Tables S3-S7. The corresponding results for the economic evaluation are shown in Table 1.  
Table 1 shows that the total reboiler duty of the flowsheets of the TCED processes based on energy saving by recovering the sensible heat from the hot streams (designs SH1 and SH2) are about 15.66% and 18.18%, respectively, less than that of the conventional design. Moreover, a significant reduction is observed in the proposed flowsheets (LH-SH1 and LH-SH2) with energy saving by using both the latent heat of the overhead vapor and the sensible heat of the hot solvent. For these designs, a reduction of 41.12% and 45.60% saving in the total reboiler energy consumption was, respectively, observed compared to the conventional TCED flowsheet. In Table 1, it is also shown that there is a significant reduction in the utility cost, which is self-evident because the price of steam is much higher than that of cooling water for condensers and coolers. Therefore, in the fully thermally integrated flowsheet (i.e., LH-SH2), the total utility cost is 1841 (10\({}^{3}\) USD/year) compared to 3237 (10\({}^{3}\) USD/year) for the conventional process. This represents a reduction of about 43.13% of the steam and cooling water cost compared to the conventional TCED. The capital cost of the new energy-saving processes proposed increases gradually with the complexity of the flowsheets. New heat exchangers are added in the proposed flowsheets for heat recovery from the hot streams. The TAC of the fully thermally integrated flowsheet (i.e., LH  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Conventional** & **SH1** & **SH2** & **LH-SH1** & **LH-SH2** \\  & **TCED** & & & & \\ \hline Reboiler duty, QR, kW & 12 769 & 10 693 & 10 447 & 7519 & 6 946 \\ Energy saving & 0 & 15.66 & 18.18 & 41.12 & 45.60 \\ (\% difference) & 0 & 15.66 & 18.18 & 41.12 & 45.60 \\ Capital cost, USD & 8,218,000 & 8,193,000 & 8,236,000 & 9,485,000 & 10,518,000 \\ Utilities cost, USD/y & 3,237,000 & 2,726,000 & 2,669,000 & 1,983,000 & 1,841,000 \\ Utilities cost saving & 0 & 15.78 & 17.56 & 38.74 & 43.13 \\ (\% difference) & 5,976,000 & 5,457,000 & 5,414,000 & 5,046,000 & 5,248,000 \\ TAC, US/y & 5,976,000 & 5,457,000 & 5,414,000 & 5,046,000 & 5,248,000 \\ TAC saving & 0 & 8.69 & 9.41 & 15.57 & 12.18 \\ (\% difference) & 0 & 8.69 & 9.41 & 15.57 & 12.18 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Results of the design parameters and economic evaluation for IPA–water–PG separation by TCED processes.  
SH2) is 5248 (10\({}^{3}\) USD/year), signifying a total annual cost reduction of 12.18% compared to the conventional TCED. Meanwhile, for the LH-SH1 design, the TAC is 5046 (10\({}^{3}\) USD/year) and the total annual cost reduction is 15.57% compared to the conventional TCED.