Reliquefaction On LNG carriers

Reliquefaction on LNG carriers cools and condenses boil-off gas (BOG) back into liquid methane using a closed-loop nitrogen refrigeration cycle (reverse Brayton cycle). It maintains cargo tank pressure, prevents vapor loss, and maximizes cargo delivery, usually by compressing the BOG and cooling it to below -161°C in a cryogenic heat exchanger.

Key Working Principles

• BOG Removal & Compression: BOG is removed from cargo tanks by a multi-stage centrifugal compressor and compressed, raising its pressure and temperature.
• Cryogenic Cooling (Cold Box): The compressed BOG flows into a cryogenic heat exchanger (cold box), where it is cooled down to temperatures approaching -160°C to -170°C.
• Nitrogen Expansion Cycle: A separate nitrogen loop provides the cooling capacity. Nitrogen gas is compressed, cooled by seawater, and then expanded in a turbine to generate extremely low temperatures for the heat exchanger.
• Condensation & Return: The cooled BOG liquefies, is separated from non-condensable gases (like nitrogen) in a separator vessel, and is returned to the cargo tanks.

• Reduced Loss: Re-liquefaction increases the amount of LNG delivered, making it more profitable than burning BOG for propulsion, common on modern diesel-electric (DFDE/MEGI) vessels.
• System Control: The system is often fully automatic, adjusting the nitrogen mass flow and compressor capacity (0–100%) based on the BOG production rate.
• Alternative Methods: Some systems use a mixed refrigerant cycle (similar to onshore plants) for improved efficiency.