Design and analysis of a process for directly Re-liquefying BOG using subcooled LNG for LNG carrier

Abstract

A process for directly re-liquefying boil off gas (BOG) using subcooled liquefied natural gas (LNG) for LNG carrier is designed. A part of LNG, pumped out from the bottom of the LNG storage tank is subcooled by the reverse Brayton nitrogen cycle and sent back into the tank by spraying to re-liquefy the BOG. The process is simulated by Aspen HYSYS and optimized by the genetic algorithm in order to find the minimum specific energy consumption (SEC). The optimization results show that the SEC and the exergy efficiency (FOM) of the BOG re-liquefaction process are 1.101 kWh/kgLNG and 25.40%, respectively. Furthermore, the flexibility study of the process is used to compare the system performance for different mass flows of the BOG. The results show that it can also satisfy the system requirements only by adjusting the flow rate of the refrigerant and LNG. In addition, the SEC and the FOM of the six different conditions are all about 1.10 kWh/kgLNG and 26%. Finally, the comparison of the SEC, FOM and the exergy losses of the process in this paper with other systems, demonstrates that the process after compression by the cryogenic BOG compressors has lower energy consumption and exergy losses and higher efficiency.