Comparative study of waste heat recovery systems based on thermally driven refrigeration cycles in cryogenic air separation units

Recovering compression heat generated in the air compression process is expected for energy saving of cryogenic air separation units. However, the effective utilization of compression heat remains a challenge due to the low-grade feature of the available heat source. Therefore, the selection of the waste heat recovery system configuration is critical to the optimum utilization of the compression heat. In this study, two waste heat recovery systems based on different thermally driven refrigeration cycles for compression heat recovery are proposed, including an organic Rankine vapor compression-based air compression system (ORVC-ACS) and an absorption refrigeration-based air compression system (ARS-ACS). A 60,000-Nm3/h scale cryogenic air separation unit is considered as a case study for assessing and comparing the energetic, exergy and economic potential of the two proposed systems. The results reveal that the ORVC-ACS is preferable to the ARS-ACS in terms of energy-saving and economic performance. Operation optimization is conducted to identify the maximum energy saving of the preferred system. The optimization results show that the ORVC-ACS is capable of saving a total electricity of 8.52 GWh/year, corresponding to 4.53% of the annual electricity consumption for air compressors, while the values are 6.43 GWh/year and 3.37% for the ARS-ACS. This work may provide guidelines for configuration selection and optimum design of waste heat recovery systems in cryogenic air separation units.