## 2 Methodology
### Energy-Saving Methods  
#### 2.2.1 Energy Saving by Recovering the Sensible Heat from the Hot Streams  
As presented in Figure 1, the high-temperature solvent from the SRC bottom must be cooled down before being recycled to the EDC. It can be used as a heat source to preheat the EDC and PC feed streams. The preheating of the feed streams is expected to decrease the reboiler duties of the EDC and PC columns and, consequently, to reduce the saturated steam consumption. At the same time, regarding the solvent, giving up as much sensible  
Figure 1: Steady-state PRO/II flowsheet for the IPA–water separation process.  
heat as possible, additional cooling before recirculation to EDC should be performed with a much lower consumption of cooling water.  
On the other hand, in the conventional TCED process flowsheet, two hot residual water streams can be identified, namely, the B1 and D3 streams, with some sensible heat that could also be used for preheating the PC feed stream. Moreover, these water streams should be cooled prior to being discharged.  
In the conventional TCED process, we have identified two process flowsheet diagrams (PFD) of the heat recovery of the sensible heat from the hot streams, as seen in Figure 2. The processes are named as follows:  
* SH1 for the D3 + B1 hot waters to preheat the PC feed and B3 hot solvent to preheat the EDC feed (depicted in Figure 2a).
* SH2: H1 for the D3 + B1 hot waters to preheat the PC feed and H3 + H2 for B3 hot solvent to preheat the EDC feed and next to preheat the PC feed (depicted in Figure 2b).  
Figure 2: Two process flowsheet diagrams proposed for the recovery of the sensible heat from the hot streams: (**a**) SH1; and (**b**) SH2.  
2.2.2. Energy Saving by Using the Latent Heat of the Overhead Vapor and the Sensible Heat of the Hot Solvent  
If the PC is operated under vacuum pressure and the SRC is under higher pressure, the overhead vapor temperature of the SRC is higher than that of the PC bottom. Therefore, the latent heat of the overhead vapor can be recovered to supply part of the heat necessary at the PC reboiler. However, since the SRC's condenser duty (QC-3) is much smaller than the PC's reboiler duty (QR-1), an auxiliary reboiler needs to be installed for the PC. As long as the sensible heat of the B3 bottom solvent of the SRC (noted Q\({}_{\text{sen}}\)) is high enough and available, it can be used as a heat source in the auxiliary reboiler of the PC and/or the EDC's auxiliary reboiler. It is expected that after the hot solvent has given up some of its sensible heat in the auxiliary reboilers, the remaining excess heat can be reused to preheat the EDC feed and save even more energy.  
In this paper, the complex heat integration between the PC and SRC coupled with hot solvent sensible heat recovery, hereafter referred to as LH-SH, can take two PFDs, as shown in Figure 3. In both flowsheets, the entire latent heat of the overhead vapor from the SRC is used to supply part of the heat needed for the PC reboiler, noted as QR1-1. The differences between the two flowsheets from Figure 3 are as follows:  
* In the first flowsheet, labeled as LH-SH1 (Figure 3a), the flow of hot solvent B3 has given up some of its sensible heat in the auxiliary reboiler of PC, and then B3 is used to preheat the feed of the EDC.
* In the second flowsheet, labeled as LH-SH2 (Figure 3b), the flow of hot solvent B3 has given up some of its sensible heat in the auxiliary reboiler of the EDC (tagged as QR2-1), then B3 was used to complete with its sensible heat the rest of the duty of QR1 (the reboiler of the PC). Finally, the rest of the sensible heat of B3 was used to preheat the feed of the EDC. A supplementary reboiler (QR2-2) with saturated steam is required to ensure the entire reboiler duty of the EDC.  
Figure 3: \(Cont.\)