Multi-Objective Optimization and Exergy Analysis of Liquefaction of Natural Gas Process with Mixed Refrigerant

Mixed refrigerant is widely used in the natural gas liquefaction process, and this makes it important to optimize the parameters affecting this process. In this research, a PRICO process with mixed refrigerant was optimized by using Non-dominated Sorting Genetic Algorithm (NSGAII). In this optimization, energy consumption and the surface of heat exchanger were optimized simultaneously to eliminate the adverse effect of the surface of heat exchanger increase due to the single-objective optimization of the energy consumption observed in the previous papers. Molar flow rates of mixed refrigerant compounds including methane, ethylene, propane, isopentane and nitrogen, outlet pressures of the first and second compressors and outlet pressure of the cooling cycle valve were considered as optimization variables. Also, the considered objective functions were performed on sensitive analysis towards optimization variables Moreover, the optimization solutions were presented as a set of optimal solutions called Pareto optimal fronts whose results showed the contradiction of the two objective functions well. Afterwards, multi-objective optimization results were compared with single-objective minimization results. Ultimately, comparison results showed that the surface of heat exchanger in multi-objective optimization was reduced by 54% in comparison with  single-objective optimization (energy optimization).