Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Thermoeconomic analysis on a cascade energy utilization system for compression heat in air separation units Yangyiming Rong a , Xiaoqin Zhi a , Kai Wang a , Xia Zhou a , Xingwang Cheng a , Limin Qiu a, ⁎ , Xuelin Chi b a Institute of Refrigeration and Cryogenics, Zhejiang University, Hangzhou 310027, China b Hangzhou Oxygen Plant Group Co., Ltd., Hangzhou 310014, China ARTICLE INFO Keywords: Compression heat Liquid desiccant dehumidification Organic Rankine vapor compression Cryogenic air separation unit ABSTRACT 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 ℃) is used to drive an organic Rankine vapor compression (ORVC) refrigeration cycle for air cooling, while the remaining lower-grade heat (40–75 ℃) 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 CO 2 emission reduction potential of 6340 tCO 2 /year, which shows its prospects for practical applications in particular for large scale air separation units. 1. Introduction Cryogenic air separation units are widely developed and applied worldwide due to the growing industrial gas demand in metallurgy, coal chemical, medical treatment, machining, and other industries. The cryogenic air separation unit is an energy intensive facility, and the energy consumption accounts for 14% of the total electricity usage in the Chinese iron and steel industry [1]. Although there are many var- iations in cryogenic air separation processes, all of these consist of a series of similar steps, including air compression, precooling, purifica- tion, heat exchange, distillation, and storage processes [2,3]. In the past two decades, many experts have explored various energy conservation methods for cryogenic air separation units. These mainly include pro- duction scheduling optimization [1,4–8], main equipment optimization [9–15], control strategy for variable working conditions [16–20], cold energy utilization from liquefied natural gas (LNG) [21–26], and pro- duct energy reuse [27–30]. Production scheduling optimization refers to the process of setting different phase parameters of the system in order to minimize the en- ergy consumption and maximize the profitability according to changes in product demand and local electricity price. Tong et al. [1] proposed a combined variable oxygen supply method by adding another liquefac- tion subsystem to the original air separation unit. By adjusting the gas- liquid ratio of the oxygen product, the demand for the oxygen product can be fully met, and the exergy efficiency can be increased by 11%- 31%. Caspari et al. [4] proposed a mechanistic dynamic process model with an additional liquefaction assisted cycle. When the electricity price is high, the liquefaction cycle is turned off and the auxiliary cycle is turn on; when the electricity price is low, the liquefaction cycle is turned on, and the power demand can be reduced by up to 88% from the minimum to the maximum power demand. Main equipment optimization refers to the performance optimiza- tion of specific key equipment of air separation units to reduce the system power consumption, including distillation columns [12–15], compressors, main heat exchangers [9,11] and purifiers [10], etc. Among these, the distillation column is the most widely studied https://doi.org/10.1016/j.enconman.2020.112820 Received 26 November 2019; Received in revised form 5 April 2020; Accepted 6 April 2020 ⁎ Corresponding author. E-mail address: limin.qiu@zju.edu.cn (L. Qiu). Energy Conversion and Management 213 (2020) 112820 Available online 15 April 2020 0196-8904/ © 2020 Elsevier Ltd. All rights reserved. T