Optimization and thermodynamic analysis of a cascade PLNG (pressurized liquefied natural gas) process with CO2 cryogenic removal

Based on an increased liquefaction pressure, PLNG (Pressurized liquefied natural gas) technology has the advantages of energy-saving and space-saving. This paper proposes a novel cascade PLNG process with CO2 cryogenic removal, wherein a two-stage cascade refrigeration system is used to replace the conventional three-stage one and a CO2 cryogenic removal unit is integrated with the process to eliminate CO2 pre-treatment facilities. CO2 is removed to 0.5% level at a pressure of 1.5 MPa and temperature of 165.7 K. Three refrigerant options, namely CH4-C2H6, CH4-C2H4 and C2H4-C3H8, are selected for the novel process and three process configurations are designed based on the refrigerants. Taking the specific energy consumption as a target, the three designed processes are simulated and optimized. Results show that C2H4-C3H8 process has the lowest specific power consumption, the best matched composite curves and the smallest heat exchanger size. Under optimum condition, the specific power consumption of C2H4-C3H8 process is 0.2559 kWh/Nm3, 2.5% less than that of conventional CH4-C2H4-C3H8 process. In addition, thermodynamic analyses are conducted for the proposed processes. The COP and exergy efficiency of C2H4-C3H8 process are calculated to be 0.75 and 0.47, respectively, improving by 17% and 38% over that of conventional one.