Analysis and Optimization of Carbon Dioxide in a Propane Precooled Mixed Refrigerant Liquefaction Plant

Abstract This study explores the optimization of carbon dioxide (CO₂) as a refrigerant to enhance heat transfer and improve efficiency in the precooling and liquefaction stages of a Propane Precooled Mixed Refrigerant (C3MR) Liquefied Natural Gas (LNG) plant. Given the high energy demands of LNG production, a comprehensive sensitivity analysis of CO₂ behaviour within the system was conducted. The research focuses on the CO₂ precooling cycle, liquefaction cycle, and determining the optimal CO₂ concentration required to achieve energy savings without compromising LNG output. A C3MR LNG plant was designed, modelled, and optimized by introducing CO₂ as a natural refrigerant. The injected mole fractions of CO₂ was varied between 0 to 0.4, followed by sensitivity analysis and process optimization. The results indicated that increasing the CO₂ mole fraction up to 0.18 enhances the coefficient of performance (COP) by 1.30% and reduces specific energy consumption by 0.75%. It was also observed that increasing CO₂ mole fraction in the mixed refrigerant loop resulted in increased specific energy requirement and reduced coefficient of performance. Additionally, increasing the natural gas mass flow rates up to 413,951 kjmole/hr further improved the COP by 33.33% and lowered the energy consumption by 23.55%. Process Optimization was carried out using Aspen HYSYS to reduce the specific energy consumption from 1572 kJ/kg in the base case to 1363kJ/kg and further to 1322 kJ/kg in the optimized case, representing a 15.84% improvement from the base case. These findings demonstrate the potential of CO₂ as an effective refrigerant in C3MR LNG plants, contributing to improved energy efficiency and sustainability. Further process optimization can be carried out using complex and robust algorithms to increase the efficiency of the optimization.