## 3 Results and Discussion
### Thermodynamic Analysis of ORC System  
The operating condition at a high condensation temperature of 57 \({}^{\circ}\)C is considered as an example to perform a thermodynamic analysis of the ORC system. The heat exchanger networks are that two heat source streams heat the ethane tower reboiler and the low-pressure propane tower reboiler, and the exhaust steam at the expander outlet heats the reboilers of the two propylene towers. This paper investigates the performance of seven pure working fluid. After screening the pure working fluid,  
\begin{table}
\begin{tabular}{c c c} \hline
**Heat Sink** & **Temp (\({}^{\circ}\)C)** & **Heat Load (MW)** \\ \hline Propylene tower reboiler \#1 & 52 & 25 \\ Ethane tower reboiler & 62 & 9 \\ Propylene tower reboiler \#2 & 46.5 & 22 \\ Low pressure propane tower reboiler & 80 & 2 \\ Total & & 57 \\ \hline \end{tabular} Data in the table are obtained from the reference book [21].  
\end{table}
Table 1: Heat sink of methanol to olefin (MTO) process in low-grade temperature.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Heat Source** & **T\({}_{\text{in}}\) (\({}^{\circ}\)C)** & **T\({}_{\text{out}}\) (\({}^{\circ}\)C)** & **Heat Load (MW)** & **Flow Rate (t/h)** \\ \hline Wash water & 95 & 55 & 113 & 2300 \\ Quench water & 110 & 40 & 35 & 400 \\ \hline \end{tabular} Data in this table are simulated based on the reference book [21].  
\end{table}
Table 2: Heat source of the MTO process in low-grade temperature.  
the working fluid can be ranked according to the thermodynamic performance under each working condition. Then, the optimal working fluid is chosen to form a binary mixture with each inferior working fluid, respectively. The mass fraction of the optimal working fluid and the non-optimal working fluid are both 50%. The temperature-enthalpy diagrams (T-H diagram) of this working condition are shown in Figure 3. The two diagrams in Figure 3 show the cold and hot streams in the MTO process and ORC system when pure and mixed working fluid are used.  
From the figures we can clearly see the location of the pinch. By comparing the two figures, we can find the temperature slip caused by the mixed working fluid. This temperature slip is shown at the green line in Figure 3b. At this time, the enthalpy line of the ORC system is closer to the enthalpy line of the heat source, so it can bring higher exergy efficiency. The position of the temperature and enthalpy line of the stream in the reboilers reflects which stream they exchange heat with.  
It is shown from Figure 4 that the output power of the expander varies with the outlet pressure of the evaporator (expander outlet pressure) with pure working fluids and mixed working fluids under its operating condition. From the figure we could find that there is an optimal value (maximum value) of the output work. Because of the increase of the outlet pressure of the evaporator, the temperature is also increasing, and the working fluid flow will be reduced accordingly. Now there are two different effects on the output work: The inlet pressure has a positive effect on the output work, while the flow rate of the working fluid has a negative effect the output work. As a result of two opposite effects, the optimal value of the output work appears and so does its corresponding optimal pressure and optimal working fluid. The optimal output work in the following sections is calculated in this method.