Mixed refrigerant components selection criteria in LNG processes; thermodynamic analysis and prioritization guidelines

The most important operating variables for the optimization of natural gas liquefaction processes are the flow rate of the mixed refrigerant (MR) components and operating pressures. In this paper, for the first time, the thermodynamic principles of selecting MR components were investigated. For this purpose, the single mixed refrigerant process was simulated in thirteen cases with different MR components and optimized with the genetic algorithm to minimize process power consumption. The optimization results indicated that the boiling point temperature (T-b), specific heat capacity (c(p)), and latent heat of vaporization (lambda) were the most essential thermodynamic properties. The lower T-b and c(p) in the selection of lightweight components among nitrogen, methane, ethane, and ethylene, and higher T-b, c(p), and lambda in the selection of heavy components among propane, i-butane, and i-pentane caused better performance. The results of the process optimization with MR, including nitrogen, methane, ethane, propane, and i-butane, showed that the process power consumption would be reduced up to 11.28% and 8.91% by replacing ethane with ethylene and i-butane with i-pentane, respectively. In addition, MR components must be selected in such a way that the difference between their T-b is not significant. Therefore, propane and ethylene are a priority for heavy and lightweight components, respectively.