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

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.