Thermodynamic analysis of an organic Rankine–vapor compression cycle (ORVC) assisted air compression system for cryogenic air separation units

Abstract

Air compression is the main energy consumption process in cryogenic air separation units (ASUs), while compression waste heat occupies over 60% of the total compression power. However, the compression waste heat has not yet been recovered in existing ASUs, implying a great potential for energy efficiency improvement. In this paper, an organic Rankine-vapor compression cycle (ORVC) assisted three-stage air compression system (ORVC-ACS) is proposed. The compression waste heat is recovered by the ORVC to generate cooling capacity to precool the inlet air of the air compressors, which thus reduces its compression power consumption. Humidity constraint is specially considered in the thermodynamic model as the heat source is pressurized humid air. The feed air conditions (i.e., temperature and humidity) and operation parameters (i.e., the compressed air temperatures, evaporation temperatures and condensation temperature) are first investigated and optimized to study their effects on system performance. The operating ranges of the feed air and operation parameters are further analyzed and identified to ensure no condensed water during the air compressor operation. The optimization results show that the maximum energy saving ratio (ESR) of the ORVC-ACS reaches 4.2%, corresponding to a total annual energy saving of 7500 MWh for a 60,000-Nm3/h scale ASUs. It is found that the proposed ORVC-ACS is able to work under the feed air temperature range of 5.35–22.5 °C and relative humidity range of 0–0.8. In addition, economic and environmental assessment shows that a discounted pay-back period of 3.1 years and annual carbon-dioxide emission reduction of 5400 t are attainable with the proposed ORVC-ACS, indicating that it is economic viable with excellent decarbonization potential. This work would provide guidance for practical compression system operation in cryogenic ASUs.