Synthesis of N2-hydrocarbon refrigerant composition for maximum LNG production in PRICO processes

Several technologies are being researched to address the challenges of cryogenic heat transfer in liquefied natural gas (LNG) production. Single mixed refrigerant (SMR) or dual mixed refrigerant (DMR) systems have advantages vs. pure refrigerant-based systems (e.g., cascade or inverted Brayton cycle with nitrogen) due to compact design suitable for small-scale units. However, the selection of mixed refrigerant composition is a challenge. Several methods have been published, typically using heuristic and computational techniques; with power consumption, efficiency, and/or exergy as the figure of merit. This paper presents an alternative approach for the determination of optimum refrigerant mixture composition based on nonlinear thermodynamic equations relating heat exchanger composite curves and mass flow of LNG produced. Three case studies demonstrate the effectiveness of the approach, indicating that single mixed refrigerant systems can be used for cost-effective LNG production while improving LNG production per unit of refrigerant flow rate by as much as 6.5 % vs. conventional approaches. The research is applicable to current needs of the gas extraction industry because it helps increase LNG production in small-scale cryogenic systems, thereby enabling economic gas recovery and reducing flaring during fracking operation, as is typical in e.g., the Bakken formation of North Dakota.