Use of dual pressure Claude liquefaction cycles for complete and energy-efficient reliquefaction of boil-off gas in LNG carrier ships

Abstract

Onboard reliquefaction of boil-off gas (BOG) generated due to heat in-leak in LNG carrier ships saves its calorific value and protects environment. Direct cryogenic liquefaction cycles, which use BOG as the working fluid, have the potential to reduce power and size of the system much below the level of existing reverse Brayton refrigerator-based reliquefiers. Though cryogenic liquefiers are not used for liquefaction of natural gas because of large changes in composition of mixture gases after Joule-Thomson expansion in direct liquefaction, it can be safely employed for BOG reliquefaction because large LNG volume acts as the buffer maintaining the composition of BOG throughout the voyage that consists primarily of methane and nitrogen only. Systems are simulated on Aspen HYSYS V8.6®. Dual pressure Claude cycle at BOG compressor exit pressure of 60 bara show 53.7%, 29.5% and 16.3% higher exergy efficiency when compared with reverse Brayton based, Cascade based and mixed refrigerant based indirect reliquefiers respectively. Heat exchanger sizes reduce with increased BOG compressor pressure. Economic analysis of the cryogenic BOG reliquefier with 60 bara compressor pressure shows about 2 years period of return on investment.