Reliquefaction

Reliquefaction is the process of converting a vapor that has been evaporated back into a liquid, typically by removing latent heat through a refrigeration cycle. In cryogenic engineering, reliquefaction is applied to recover vaporized constituents that accumulate in a closed system, most commonly boil-off gas in liquefied natural gas (LNG) storage and transport. By recondensing the gas, reliquefaction minimizes product losses and methane emissions and helps maintain safe pressure and temperature conditions within containment.

In LNG facilities, boil-off gas is generated by heat input and is directed into a reliquefaction loop. The gas is compressed and cooled in a refrigeration system and condensed back to liquid, which is returned to the LNG product stream or stored. Reliquefaction is also used in other cryogenic applications and in gas processing plants to recover and reuse vented or evaporated components.

Technologies employed in reliquefaction include various refrigeration cycles, such as mixed-refrigerant cycles, cascade cycles, and Joule-Thomson expansion schemes. These systems rely on heat exchangers, compressors, and condensers to remove latent heat and promote phase change from gas to liquid. The choice of cycle depends on factors such as gas composition, required recovery rate, energy efficiency, and integration with existing plant infrastructure.

Benefits include reduced methane emissions, improved energy efficiency, and enhanced operational flexibility. Limitations involve energy consumption, capital and maintenance costs, and control complexity, particularly in systems handling variable feed temperatures and pressures.