Off-design performance analysis with various operation methods for ORC-based compression heat recovery system in cryogenic air separation units

Off-design performance of the ORC-based compression heat recovery system in cryogenic air separation units (ASU) with various operation methods is conducted in this paper, in order to explore the energy-saving potential during annual off-design operation. An operation method considering the temperature matching between the heat source and working fluid is proposed and compared with the other four operation methods (i.e., constant superheat operation method, constant pressure operation method, sliding pressure operation method and free operation method). With a 60,000-Nm3/h scale cryogenic ASU taken as the study case, the proposed operation method has the best energy-saving effect in all cases with a maximum annual energy saving reaching 13.83 GWh, corresponding to an energy-saving ratio of 9.6%, which produces a maximum energy-saving difference of 1.47 GWh with the other operation methods. At the same time, the system with the proposed operation method can realize normal operation under all annual working conditions. This study may provide guidelines for the design and operation of the compression heat recovery system in cryogenic ASU along with other waste heat recovery systems.