Comparative study for air compression heat recovery based on organic Rankine cycle (ORC) in cryogenic air separation units

The annual energy consumption of the cryogenic air separation units (ASUs) reaches 205 TWh in China, over 80% of which is consumed in the compression processes while over 60% of the compression work is dissipated as waste heat. Efficient recovery and utilization of this amount of heat is expected to bring significant economic and environmental benefits. Organic Rankine cycle (ORC) based waste heat recovery systems for generating extra electricity or/and cooling the inlet air of the air compressors are proposed to achieve power saving and evaluated in terms of thermodynamic, economic and environmental metrics. These include an ORC-based electric generator (ORC-e) for extra electricity, an electrically coupled ORC and vapor compression refrigerator (ORC-e-VCR) and a mechanically coupled ORC and VCR (ORC-m-VCR) for extra electricity and compression power saving. A 60,000-Nm³/h scale cryogenic ASUs is selected for case studies and influence of the feed-air temperature and humidity is focused in the analyses. The results show that among these three systems, the ORC-m-VCR and ORC-e-VCR systems have similar performance when the expansion work-electricity conversion efficiency (η_e) is 90%, reaching the highest energy saving ratio of 11.7% and economic benefits with net present value achieving 154 million CNY. The ORC-m-VCR system outperforms the other two systems with η_e of 60% and 30%. This work presents comprehensive comparison of various heat recovery systems and provides practical guidance for configuration selection and design to achieve effective energy saving in air compression processes.