Numerical Simulation and Comparative Analysis of Three Boil-Off Gas Reliquefaction Systems

In view of the problem that the boil-off gas (BOG) generated by small liquefied natural gas (LNG) ships affects the transportation safety, three BOG reliquefaction systems are improved, which are as follows: case 1: serial nitrogen expansion reliquefaction system (N₂-BOG); case 2: single mixed refrigerant reliquefaction system (SMR-BOG); and case 3: propane precooled mixed refrigerant reliquefaction system (C3MR-BOG). The basic process is simulated in aspen plus and combined with the mathematical model built up in matlab where the three cycles are optimized with the specific power consumption (SPC) as the objective function. The results show that the SPC of three cases are 0.8021 kWh/kg (LNG), 0.4150 kWh/kg (LNG), and 0.5063 kWh/kg (LNG); the coefficient of performance (COP) of cases 1, 2, and 3 are 0.20, 0.39, and 0.30; the exergy efficiency of 36.56%, 70.66%, and 57.93% along with the total product cost of 89.00 $/h, 55.73 $/h, and 77.54 $/h are achieved in Cases 1–3, respectively. In addition, the overall exergy destruction of the cycles and each equipment are discussed, and the matching of the cold and heat sources of the multistream heat exchanger in the three systems is analyzed. In conclusion, under the condition that the BOG mass flowrate is 350 kg/h, the improved SMR-BOG has advantages on thermodynamic performance and economic analysis, which makes it a better choice.