A comprehensive optimization and comparison of modified single mixed refrigerant and parallel nitrogen expansion liquefaction process for small-scale mobile LNG plant

Highlights: • A numerical comparison of two processes for small-scale mobile LNG plant. • Minimizing energy consumption cannot result in the minimum total cost. • MSMR has better economic performance than PENC in the life cycle period. • MSMR has better operational flexibility than PENC. • The maximum exergy efficiency of MSMR reaches 49.96%. -- Abstract: This paper aims to offer some criterions on selecting liquefaction process for small-scale mobile LNG plant. To this end, a comprehensive optimization and comparison was presented to show quantitive results on modified mixed refrigerant liquefaction process (MSMR) and parallel nitrogen expansion liquefaction process (PNEC) from the perspectives of specific energy consumption, exergy efficiency, techno-economy and operational flexibility. First, the MSMR and PNEC were optimized by using two different objective functions, namely minimizing specific energy consumption and total investment. The results showed that the minimum specific energy consumption of MSMR and PNEC were 0.411 kWh/kg and 0.618 kWh/kg, respectively. Then, the exergy efficiency comparison demonstrated that the exergy efficiency of MSMR reached 49.96%, while that of PNEC was only 33.19%. Furthermore, the techno-economic comparison was investigated on the small-scale mobile LNG plant by using total investment (TI) model in 20 years life cycle. The total investiment of MSMR was $6,126,133, while that of PNEC was $8,379,177. The total investiment of MSMR was 26.88% lower than that of PNEC, which indicated that MSMR had a better techno-economic performance than PNEC. Finally, the flexibility study was adopted to compare the capability of handling different feed gas conditions for liquefaction. The comparison results showed that MSMR had a lower specific energy consumption, higher exergy efficiency, lower total investment, and higher flexibility than PNEC. In conclusion, MSMR is a better choice for small-scale mobile LNG plant from the four perspectives.