Process Design and Optimization of LNG Subcooling Systems

The boil-off gas (BOG) generated during large-scale liquefied natural gas (LNG) transport increases storage-tank pressure and compromises operational safety. Conventional BOG reliquefaction and LNG subcooling systems are confronted by structural issues, including flash-gas formation and multiphase flow during compression, thus resulting in reduced efficiency and equipment reliability. This study aims to introduce a high-efficiency BOG treatment that addresses these limitations by adopting a subcooling configuration with vapor–liquid separation compression and dual-expansion to enhance energy performance and safety. Process modeling and simulation were performed using Aspen HYSYS, and operating variables were optimized using the particle swarm optimization algorithm to minimize the coefficient of performance (COP). Additionally, the total annual cost (TAC) for each case was analyzed to provide a comprehensive evaluation of system performance. The results show that the proposed process improves the COP by 68% and 6.8% compared with the conventional BOG reliquefaction system and an existing LNG subcooling process, respectively. These improvements are due to reduced energy consumption enabled by phase-separation compression and enhanced heat-exchange efficiency through dual expansion. Additionally, TAC analysis indicates potential annual cost savings of approximately $2 million. In conclusion, the proposed system offers an efficient and economically competitive solution for LNG BOG management.