Boil-off gas (BOG) recovery system, BOG reliquefaction plant
An onboard reliquefaction system that recovers boil-off gas in LNG carriers and returns it to the cargo tanks. The BOG, at about -143°C and atmospheric pressure, is taken from the cargo tanks and passed through a compressor, where its pressure is raised to 4.5 bar and temperature to -60°C. This is then passed through one side of a nitrogen heat exchanger where its temperature is reduced to -160°C at between 2 and 4 bar. This process converts the gas back into liquid but it must then be passed through a separator to remove any incondensibles. The cargo can then be returned to the tanks via loading or spray lines.