## 1 Introduction  
Industrial gas plays an increasingly important role in the development of the economy. The market size of industrial gas in China grew rapidly from CNY 106.5 billion in 2016 to CNY 155.3 billion at the end of 2020, with an average annual compound growth rate of 10.5% [1]. The rapid growth of the industrial gas market has promoted the development of the cryogenic air separation unit (CASU), which remains the status quo for the industrial production of high-purity tonnage quantities of oxygen and nitrogen. Gas production via CASU is a capital and energy-intensive process, mainly including an air compression section (ACS), purification, heat exchange, pre-cooling, rectification, and a storage process. Energy consumption of a CASU accounts for 15-20% of the total energy consumption in iron and steel enterprises [2]. Thus, a great effort has been made to reduce the energy demand and cost of the CASU. At earlier stages, efforts mainly focused on the reduction in pressure drops through the optimization of the purification system and distillation columns, and successfully reduced the specific energy consumption of liquid oxygen from 0.7 kWh/kg to around 0.4 kWh/kg [3, 4]. Further effort has been made to improve the efficiency of compressors, as 70-80% of the overall power consumption of the CASU comes from the power requirement of feed air compressors [5]. Researchers found that a 19% reduction in specific power consumption could be achieved if the compressor isentropic efficiency increased from 0.74 to 0.9 [6]. Efforts have also been made to optimize the system process through cold energy utilization from liquefied natural gas (LNG). [PERSON] et al. [7] introduced a novel three-column CASU process integrated with LNG regasification, and reduced the specific power consumption of liquid oxygen and liquid nitrogen to 0.252 kWh/kg and 0.258 kWh/kg, respectively. [PERSON] et al. [8] studied the integration of a CASU with LNG as a heat sink, and found an 8.04% reduction in the amount of power requirement, and a 17.05% reduction in the initial capital cost of the CASU plant. Through continuous development, molecular sieves for purification, booster expanders, and the structured packing of distillation columns for reducing pressure losses, have become relatively established, and the methods of improving compression efficiency through the optimization of design and control technology of the compressor impeller have approached their technical bottlenecks [9]. Consequently, researchers have turned their attention to the waste heat utilization from the ACS. During the ACS, feed air is compressed by double- or triple-stage compressors to 0.6-0.7 MPa, and the outlet air temperature of each stage reaches 100-120 \({}^{\circ}\)C. As a result, 90% of the compression consumption is lost as thermal energy of the compressed air [10]. Taking a CASU producing 120,000 Nm\({}^{3}\)/h oxygen as an example, the air compressor intake flow is about 600,000 Nm\({}^{3}\)/h, and the heat removed by cooling water is as high as 30 MW. At the same time, intercoolers have to be installed to cool down the compressed air to approximately 40 \({}^{\circ}\)C, with the aim of preventing a high suction temperature for the next stage of compression [11]. Therefore, the ACS is a high energy consumption process accompanied by considerable waste compression heat. On the other hand, a 40 \({}^{\circ}\)C suction temperature is too high for the compressors, thus leading to a large potential for a reduction in suction temperature, and compression power saving [12]. However, the waste compression heat is low-grade, with low energy density and strong volatility, which makes its recovery technically and economically challenging. Methods of waste compression heat utilization that have been studied mainly include direct heat utilization and energy conversion methods.