Thermoeconomic analysis on a cascade energy utilization system for compression heat in air separation units

The performance of air compression is crucial to the overall efficiency of cryogenic air separation units. A cascade utilization system of compression heat (CUSCH) is proposed for air separation units, in which the compression heat is used in situ to improve the operating condition of the compressor through inlet air cooling and dehumidification. The compression heat of relatively higher-grade (> 75 degrees C) is used to drive an organic Rankine vapor compression (ORVC) refrigeration cycle for air cooling, while the remaining lower-grade heat (40-75 degrees C) is used to regenerate the desiccant solution for the dehumidification system. A thermodynamic model is built for the CUSCH and a case study is conducted based on a practical compression process for a 60,000-Nm(3)/ h scale air separation unit. The influences of air dehumidification and cooling on the compression performance are analyzed. The results indicate that the CUSCH not only reduces the compression power but also prevents water condensation in the compressors and heat exchangers, which is beneficial for operational safety. It is estimated that the proposed CUSCH can reduce about 4.9% of the total compression power, and the isothermal efficiency of air compressor is increased by about 5.0%. In addition, the CUSCH is economically viable and environmentally friendly compared with traditional compression systems, with a payback time of 5.0 years, a levelized cost of energy of 0.0339 $/kWh, and a CO2 emission reduction potential of 6340 tCO(2)/year, which shows its prospects for practical applications in particular for large scale air separation units.