Thermodynamic evaluation of three mini-scale nitrogen single expansion processes for liquefaction of natural gas using advanced exergy analysis

Three mini-scale nitrogen single expansion processes for liquefaction of natural gas were investigated through conventional and advanced exergy analyses. Generally, energy intensive processes such as liquefaction cycles are considered to minimize energy consumption through improving efficiency. In the present study, total avoidable exergy destruction and total exergy efficiency as two of the most important indicators were calculated to obtain a deep understanding of the processes. The second law efficiency of the processes was calculated up to 44% which shows a great potential for improvement. Results of conventional analysis indicate that the air coolers produce a large irreversibility and have a small exergy efficiency. Results of advanced analysis show that for all the processes, most of the exergy destruction of the air coolers is unavoidable. Moreover, a large portion of total avoidable exergy destruction of the processes (up to 85%) belongs to the compressors and the expanders. Therefore, these components have the highest priority for improvement. The results also present a vast improvement potential for Statoil process. The sensitivity of the analysis indicators to some important operating variables including the maximum and the minimum pressures of the liquefaction cycles and the refrigerant mass flow rates were also studied.