Thermodynamic evaluation of mixed refrigerant selection in dual mixed refrigerant NG liquefaction process with respect to 3E's (Energy, Exergy, Economics)

The dual mixed-refrigeration process makes it possible to achieve higher liquefaction of natural gas with the advancement in both the refrigeration cycles and refrigerant combinations of components. This is made possible by the fact that the process uses multiple mixed refrigerants. This has a significant impact on the overall improved performance of natural gas liquefaction by having a negative influence on the quantity of energy that is consumed. This study proposes a methodology for selecting the MR components based on their thermodynamic behavior in both warm and cold refrigerant streams. As a result, eighteen alternative scenarios are simulated for this study, each based on (i) fixing the warm loop components or (ii) fixing the cold loop components. The procedure was investigated from the point of view of process engineering, with the 3E model of energy, exergy, and economics serving as the decision-making factor. The findings indicate that increasing the number of components for pre-cooling and subcooling cycles from three to five results in specific energy consumption of 0.49 kW.kgLNG−1, which seems to be a reduction of 54% in terms of the amount of energy that is consumed in comparison to the process that is based on three components. The irreversibilities of the process were uncovered by doing an exergy analysis. It identified the cases based on five refrigerant components providing reduced exergy destruction of 3505.02 kW with 59% exergy efficiency. The viability of the proposed process is assessed even further through economic analysis. It was observed that five MR-based processes save 22.93% of the total capital cost, 43.56% of the overall operating cost, and 33.61 %of the total annualized cost.