## 1 Introduction
Many researchers have done a lot of research on the working fluid of ORC systems. [PERSON] et al. [16] examined ten different working fluid in terms of the thermal efficiency for a modified ORC. They found ammonia (R717), which is characterized by both the highest specific volume of saturated vapor flowing into the tanks and the largest difference between working pressures, obtained the maximum efficiency. Also, a mixture of ammonia and water has always been used in the Kalina Cycle, which has been investigated as a competitor of ORC, to recover the waste heat [17]. [PERSON] et al. [18] developed a thermo-economic CAMD-ORC framework and applied to subcritical and non-recuperated ORC systems in waste-heat recovery applications represented by a range of heat-source temperatures from 150 \({}^{\circ}\)C to 350 \({}^{\circ}\)C. [PERSON] et al. [19] established a new working fluid classification method based on the existence and relative location of some characteristic points of the vapor-liquid coexistence curve in T-s diagram. [PERSON] et al. [20] performed thermodynamic modeling and multi-objective optimization for three different configurations of the ORC running by ten zeotropic mixtures and one pure refrigerant as the working fluid. They found using zeotropic mixtures as the working fluid instead of a pure fluid such as R245 fa could generate more power. In China Shenhua's 1.8 million tons MTO process, a large amount of low-temperature energy has not been fully utilized. This part of the energy comes from the water washing tower and the quench tower. The water temperature of the quench tower is 110 \({}^{\circ}\)C, and the water temperature of the water washing tower is 95 \({}^{\circ}\)C [21]. At present, the low temperature energy of this part is mainly used for energy supply of the reboiler of the subsequent olefin separation unit. From the first law of thermodynamics, about 60% of low-grade energy cannot be recovered. From the perspective of the second law of thermodynamics, the low-grade energy is degraded while the heat source (about 100 \({}^{\circ}\)C) is used to heat the reboilers (about 50 \({}^{\circ}\)C). Therefore, it can be seen that this part of the energy has not been used reasonably from the perspective of the first law of thermodynamics and the second law of thermodynamics. In the recent research on the recovery of low-temperature waste heat by the ORC system, the heat sources are often considered as single stream, and most of them are directly condensed by cooling water or cooling air. In the MTO process, there is a heat source of multiple streams, and part of the energy is used by subsequent unit operations (distillation units). Therefore, a new energy utilization process needs to be designed, so that when the energy demand of the process meet itself, more low-temperature energy could be recovered. This paper is based on the current energy utilization status of the MTO process, and introduces the ORC energy recovery technology into the energy recovery in MTO process. According to the above literature, there is currently no ORC energy recovery for the low temperature waste heat of the MTO process. And less research on the overall optimization of ORC energy recovery schemes for multi-stream heat sources. In order to make up for the shortcomings in the literature, this paper first proposes five ORC energy recovery design schemes for multi-stream low-temperature heat sources while ensuring the energy supply of the energy-demanding unit. Then, parameters optimization and working fluid screening are performed for each scheme. Finally, the mixed working fluid was introduced to explore the effect of mixed working fluid on the thermal efficiency of ORC system.