Process optimization, exergy and economic analysis of boil-off gas re-liquefaction processes for LNG carriers

• Published in: Energy (Oxford) 2022-03, Vol.242, p.122947, Article 122947
• Main Authors: Cao, Xuewen, Yang, Jian, Zhang, Yue, Gao, Song, Bian, Jiang
• Format: Article
• Language: English
• Subjects: BOG re-liquefaction; Brayton cycle; Composite curve analysis; Economic analysis; Efficiency; Energy consumption; Exergy; Exergy analysis; Genetic algorithms; Heat; Heat exchange; Heat exchangers; Heat transfer; Liquefaction; Liquefied natural gas; Operating costs; Optimization; Reverse Brayton cycle; Thermodynamics
• ISSN: 0360-5442; 1873-6785
• DOI: 10.1016/j.energy.2021.122947

A re-liquefier based on the reverse Brayton cycle is the first choice for the re-liquefaction process owing to its compactness, simplicity, and safety. However, due to the low efficiency of the heat exchangers, the energy consumption of the re-liquefaction process is relatively high. Therefore, a novel cascade nitrogen expansion re-liquefaction process with high heat exchange efficiency is proposed and compared with the parallel expansion process. In the designed process, the condensation and subcooling stages of the boil-off gas (BOG) are performed in different heat exchangers. The performance improvement of the designed process is analyzed using the Aspen HYSYS and the genetic algorithm is used to optimize the processes. The optimization results show that the specific energy consumption (SEC) and exergy loss rate of the designed process are 0.727 kWh/kgLNG and 148.14 kW, which are 11.7% and 16.7% lower than those of the reference process, respectively. Moreover, the exergy efficiency (EXE) of the designed process is 0.352, which is 13.2% higher than that of the reference process. Because of the higher EXE and lower refrigerant demand, the designed process saves 7.2% of the total annual costs consisting of annual capital and operating costs compared with those of the reference process. •A novel cascade nitrogen expansion BOG re-liquefaction process was designed.•The new process and the reference process were optimized by genetic algorithm.•The matching degree of composite curve of heat exchangers was evaluated.•The exergy loss and exergy efficiency of two re-liquefaction processes was analyzed.•The performance and economy of two re-liquefaction processes was compared.