Computerized thermodynamic and economic modeling of a multistage cascade refrigeration cycle used for natural gas liquefaction

As is known, liquefied natural gas (LNG) is a rapidly growing energy carrier that is practical and economically suitable for the transportation of natural gas in situations where the distance from the source to the market is long. When natural gas is liquefied, it must be cooled to a lower temperature of around -150°C or lower. by using the EES program, the thermal properties and equations of exergy destruction were calculated for all components of the cycle: compressors, coolers, expansion valves, and condensers for a multi-stage refrigeration cycle. In the analysis of the three models, a hypothetical temperature is 25°C and a natural gas inlet mass flow rate of 1 kg/s were taken at a pressure of 60 bar, in the first model based on thermodynamic analysis and thermo-economics results using (EES) modeling software, the minimum work is calculated as 459.5 kJ/kg, and the ideal coefficient of performance (COP) is calculated as 1.76. When the actual analysis of the designed system is performed, it is seen that the work required for the liquefaction of natural gas is 1095 kJ/kg, and the COP value of the cycle is calculated as 0.73. The exergy efficiency of the cycle is calculated as 42%. In addition, as a result of the Thermo economic analysis, the unit cost of liquefied natural gas is calculated as 0.40 $/kg LNG. For the second model, the minimum work is calculated as 472.4 kJ/kg, and the ideal coefficient of performance (COP) is calculated as 1.714. and the work required for the liquefaction of natural gas is 936.3 kJ/kg, and the COP value of the cycle is calculated as 0.86. The exergy efficiency of the cycle is calculated as 50%. the unit cost of liquefied natural gas is calculated as 0.36 $/kg LNG.for the third model the minimum work is calculated as 420.4 kJ/kg, and the ideal coefficient of performance (COP) is calculated as 1.92. the work required for the liquefaction of natural gas is 991.1 kJ/kg, and the COP value of the cycle is calculated as 0.81. The exergy efficiency of the cycle is calculated as 42%. In addition, as a result of the Thermo economic analysis, the unit cost of liquefied natural gas is calculated as 0.34 $/kg LNG.