Impact of mixed refrigerant selection on energy and exergy performance of natural gas liquefaction processes

摘要

Selection of mixed refrigerant has a significant impact on the performance of the single mixed refrigerant process for natural gas liquefaction. However, current studies mainly focus on optimizing the fraction of the mixed refrigerant with pre-fixed components which cannot guarantee the optimal solution. To address this issue, a simulation-based optimization methodology was proposed for simultaneously determining the components and their respective fractions in a mixed refrigerant. The methodology selected the optimal mixed refrigerant components from a database of the potential refrigerants with different objective functions. Then, four case studies of mixed refrigerants with three, four, five, and six components were optimized to determine their optimal combination of components and the fraction of each component by minimizing the specific energy consumption. Subsequently, the energetic and exergetic analyses were conducted to reveal the relationships between the mixed refrigerant components and the process performance. The results showed that the minimum specific energy consumption for the mixed refrigerant comprising three, four, five and six components was 0.721 (A3), 0.403 (B10), 0.392 (C10) and 0.343 (D14) kWh/kg respectively. The specific energy consumption declined drastically by 44.11% in moving from three (A3) to four (B10) components, with a less pronounced decrease of 14.9% in moving from four (B10) to six (D14) components. Moreover, the exergy efficiency of A3, B10, C10, and D14 was 36.74, 55.39, 56.44, and 61.66%, respectively. The results indicated that the energetic and exergetic efficiencies were severely influenced by the selection of components for the mixed refrigerant. Therefore, the proposed methodology provides a viable method of simultaneously determining the refrigerant components and fractions for mixed refrigerant-based processes to achieve the minimum specific energy consumption and maximum exergy efficiency.