Analysis and efficiency enhancement for energy-saving re-liquefaction processes of boil-off gas without external refrigeration cycle on LNG carriers

Abstract

Re-liquefaction of boil-off gas (BOG) that is inevitably generated owing to heat leaks in liquefied natural gas (LNG) carriers can result in energy savings and protection of the environment. Because the direct re-liquefaction process without an external refrigeration cycle requires lower power and smaller size of the system than existing re-liquefiers, two novel direct re-liquefaction processes are proposed: (1) a dual-pressure Claude re-liquefaction process (DCRP) with BOG as the refrigerant, which adopts an improved BOG cycle structure and reasonable initial parameters; (2) a re-liquefaction process based on the LNG direct expansion refrigeration cycle (LERP), which, for the first time, completes the re-liquefaction of BOG using the cold energy of LNG directly. The two processes are simulated and analyzed using Aspen HYSYS. The results reveal that the specific energy consumption (SEC) and exergy efficiency (EXE) of the DCRP are 0.6338 kWh/kgLNG and 45.7%, respectively, while those of the LERP are 0.7012 kWh/kgLNG and 37.63%, respectively. The exergy losses of the two re-liquefaction processes are 1089.54 kW and 1325.60 kW, respectively. Compared with the indirect re-liquefaction processes, the direct re-liquefaction processes offer better performance and exhibit simpler structures and are thus more attractive re-liquefaction systems on LNG carriers.