## 3 Results and Discussion
### Design of the Energy-Saving Processes  
#### 3.2.1 Design of the Energy-Saving Processes by Recovering the Sensible Heat from the Hot Streams  
Firstly, in the simulations of the conventional TCED process, two possibilities for the heat recovery of the sensible heat from the hot streams were identified, as shown in Figure 2. The optimal flowsheets for the energy-saving processes with the detailed operation parameters, material, and energy information are shown in Figure 9. In Figure 9a, the D3 hot water at 372.62 K was mixed with the B1 hot water at 380.7 K. Then, the hot flow at 372.85 K could preheat the PC feed from 313.15 K to 327.10 K. Thereafter, the hot water stream can be discharged with 323.15 K. In the same flowsheet, in Heater 3, some of the sensible heat of B3 hot extractive solvent was recovered to preheat the EDC feed. The D1 liquid (at bubble point, 355.86 K and 111.41 kPa) was preheated up to 386.25 K, while the temperature of B3 decreased from 464.93 K to 415.77 K. After the calculation, it was found that the energy saving for QR1 was only 2.9%, for QR2 29.8%, and for the QCooler 44.8%, compared with the conventional TCED process. Figure 9: _Cont._  
Another configuration for the heat recovery of the sensible heat from the hot streams is also shown in Figure 9b. Unlike the previous design, the B3 hot extractive solvent, after preheating the feed of EDC in Heater 3, still has excess heat. Thus, B3 can further preheat the PC feed after Heater 1, from 327.1 K to 359.25 K; this way, in Heater 2, more heat is recovered from the hot extractive solvent before recirculating to the top of EDC. Two benefits are identified in this configuration: it decreases the duty of the QR1 even more, but it also reduces the duty of the extractive solvent cooler (QCooler). Compared with the conventional TCED process, it was found that the energy saving for QR1 was 10.2% and for QCooler 50.34%. 2.2 Design of the Energy-Saving Processes by Using the Latent Heat of the Overhead Vapor and the Sensible Heat of the Hot Solvent  
Secondly, as mentioned in Section 2.2.2, the energy saving in the following simulations of the TCED process was evaluated by using both the latent heat of the overhead vapor and the sensible heat of the hot solvent. The optimal flowsheets for the energy-saving processes with the detailed operation parameters, material, and energy information are shown in Figure 10. In Figure 10a, the PC was operated under a vacuum pressure of 40 kPa, and the overhead pressure of the SRC was kept at 109.84 kPa. Under these operating conditions, the overhead vapor temperature of the SRC of 375.45 K was higher than the temperature of the PC bottom's product, which was 351.78 K. Therefore, the latent heat of the overhead vapor could provide 1499 kW of the 3270 kW--the total duty of the PC reboiler, while the PC column was working at 40 kPa. For the remaining reboiler duty (i.e., 1771 kW) of the PC, an auxiliary reboiler needs to be installed. It is not economical to use low-pressure steam in this auxiliary reboiler. As long as the sensible heat of the B3 stream is high enough and available, then it can be used as a heat source in this auxiliary reboiler of the PC. Thus, the B3 stream at 464.93 K gave out some of its heat in the auxiliary reboiler to ensure 1771 kW as the duty, and subsequently, the temperature of the B3 stream decreased at 421.09 K. In the B3 stream, the remaining excess heat was used to preheat the EDC feed (i.e., D1 stream) from 332.16 K to 358.39 K. After this heat exchange, the temperature of the B3 stream decreased at 365.23 K. Then, the B3 stream was cooled down at 348.15 K before being recirculated to the EDC top. Compared with the conventional TCED process, it was found that the energy saving for QR1 was complete, so no low-pressure steam was  
Figure 9: The TCED process with two configurations for energy saving by recovering the sensible heat from the hot streams: (**a**) SH1; and (**b**) SH2. consumed; for the QR2 reboiler the energy saving was 28.75%, and the energy saving at QCooler was 86.7%.