Multi-objective optimization assessment of a new integrated scheme for co-production of natural gas liquids and liquefied natural gas

The most of introduced schemes for the co-production of natural gas liquids (NGL) and liquefied natural gas (LNG) are designed as integrated configurations from the base. Furthermore, due to the high complexity of these structures single-objective optimizations are usually applied in this field. The present paper proposes a new NGL/LNG configuration with appropriate specifications based on an already built industrial NGL unit. The optimization approach in this study has the potential to advance current knowledge about multi-objective optimization assessment of integrated schemes. In this regard, the accurate surrogate models that describe the plant performance towards the annualized profit, specific power consumption, and exergy efficiency are determined by the response surface methodology. A genetic algorithm is utilized for the single-objective optimizations of these models, while their dual and triple-objective optimizations are performed with a controlled NSGA-II. To gain a robust decision, the best point from Pareto solutions in each of the multi-objective optimizations are selected by two well-known types of decision-making methods, where the criteria importance through the inter-criteria correlation (CRITIC) is applied for weighting the objectives. According to the results, the proposed scheme has higher economic, thermodynamic, and exergetic efficiencies among similar integrated configurations. The economic analysis reveals that the profit is increased by 669.99 $ min(-1) with converting the existing NGL plant to the proposed integrated scheme. Meanwhile, the specific power consumption is set to its minimum value of 0.3473 kWh kg(-1) LNG and exergy efficiency is reached to its maximum value of 55.12%. Moreover, the compressors located in the mixed refrigerant (MR) cycle have the largest share of the costs increasing. The highest power consumption and exergy destruction rates also belong to these equipments. The findings of this paper help for better understanding of the roles of annualized profit, specific power consumption, and exergy efficiency when they are simultaneously considered in the final optimal design of NGL/LNG co-production scheme. Furthermore, the purpose of this study will become even more apparent when there is no choice but to reduce the production costs for the survival of the LNG industry.