Thermodynamic analysis and performance optimization of an ORC (Organic Rankine Cycle) system for multi-strand waste heat sources in petroleum refining industry

Low-grade waste heat source accounts for a large part of the total industrial waste heat, which cannot be efficiently recovered. The ORC (Organic Rankine Cycle) system has been proved to be a promising solution for the utilization of low-grade heat sources. It is evident that there might be several waste heat sources distributing in different temperature levels in one industry unit, and the entire recovery system will be extremely large and complex if the different heat sources are utilized one by one through several independent ORC subsystems. This paper aims to design and optimize a comprehensive ORC system to recover multi-strand waste heat sources in Shijiazhuang Refining & Chemical Company in China, involving defining suitable working fluids and operating parameters. Thermal performance is a first priority criterion for the system, and system simplicity, technological feasibility and economic factors are considered during optimization. Four schemes of the recovery system are presented in continuous optimization progress. By comparison, the scheme of dual integrated subsystems with R141B as a working fluid is optimal. Further analysis is implemented from the view of economic factors and off-design conditions. The analytical method and optimization progress presented can be widely applied in similar multi-strand waste heat sources recovery.