Comparative study of boil-off gas re-liquefaction processes for liquid ethylene vessels

A re-liquefaction plant is essential for a liquid ethylene gas (LEG) vessel to liquefy the boil-off gas (BOG) and return it to the cargo tank. In this study, three potential BOG re-liquefaction processes for LEG vessels have been compared in terms of coefficient of performance (COP) and exergy efficiency. The effects of the ethylene storage pressure, propane condensing temperature, propane evaporating pressure, intermediate pressures of refrigeration, and liquefaction cycles on the system performances were analysed. The parameters of the processes were optimised individually with the aim to minimise the system power consumption. The results showed that a cascade process denoted as Case-I is superior to other two processes. The optimal COP of Case-I is about 0.767 and the maximum exergy efficiency is 63.1%. This work could provide some reference for selection of the BOG re-liquefaction processes for LEG vessels.