THERMAL DESIGN AND PERFORMANCE OPTIMIZATION OF THE ORC SYSTEM FOR THE WASTE HEAT RECOVERY IN REFINING PETROLEUM INDUSTRY

As the increase of the energy consumption and the deterioration of environment, a carbon tax will be imposed in China to reduce carbon emissions strictly and the industrial waste heat recovery has been getting more attention. The Organic Rankine Cycle (ORC) system has been proven to be a promising solution for the utilization of the low-grade heat sources. There are five waste heat sources from a 1.2 million ton reforming and extraction unit in Shijiazhuang Refining & Chemical Company of China. The temperatures of the waste heat sources are 98 similar to 80 degrees C, 104 similar to 80 degrees C, 147 similar to 80 degrees C, 205 similar to 80 degrees C and 205 similar to 80 degrees C, and the heat loads are 6.5MW, 11.5MW, 8.6MW, 3.8MW and 2.2MW, respectively. This paper studies the thermal design and performance optimization of a comprehensive utilization system for these waste heat sources, using ORC technology. The selection of suitable organic fluid is studied and the working parameters are designed and optimized with the application of the first law and the second law of thermodynamics. When the ORC systems are designed separately for the recovery of five waste heat sources, and the total power output is 3338.89kW with different organic working fluids. However this kind of designs leads to a very complex recovery system which needs large investment and space occupation. To reduce the overall system complexity, a single ORC system is proposed to recover all five heat sources, and the total amount of output power will only be 2813.02kW, due to the large exergy loss. With the above results shown, and for the purpose of simple system with large power output, this paper further studies the dual ORC systems heat recovery plan, with R245fa as the top cycle working fluid and R141b as the bottom cycle working fluid. The total amount of power output to 3353.27kW. The dual systems with single working fluid heat recovery plan is also studied, and with R141b as the working fluid for both the top cycle and the bottom cycle, the total amount of power output is 3325.03kW, and the heat recovery system is simple and compact, with good economical benefit.