CO₂ precooled dual phase expander refrigeration cycles for offshore and small-scale LNG production: Energy, exergy, and economic evaluation

Small-scale liquefied natural gas (LNG) processes have gained considerable attention owing to the development of offshore natural gas (NG) reserves. Low energy efficiency and high process costs are the major challenges associated with small-scale LNG processes. In this study, to reduce energy consumption and improve process economics, a dual-phase expander-based process assisted by carbon dioxide (CO₂) precooling has been proposed. Three proposed cases were developed using two precooling refrigerants (propane (C₃) and CO₂) and dual-phase expander-based liquefaction cycles utilizing a mixture of high-boiling (ethane (C₂) and propane (C₃)) and low-boiling refrigerant (nitrogen (N₂)). These processes were optimized with the multivariate Coggins algorithm to exploit the energy-saving opportunities. The results show that proposed case II (using CO₂ in precooling and C₂/N₂ in liquefaction) is the most energy-efficient, with a specific energy consumption of 0.3790 kWh/kg_NG. Proposed case II is 1.73% and 16.6% more energy-efficient than proposed cases I and III, respectively. The exergy analysis results show that proposed case II has the lowest exergy destruction, while the economic evaluation shows the lowest capital investment ($44.4 MM) compared to the other proposed cases. These results show the superior performance of case II among the proposed cases.