Methane addition effect on a dual nitrogen expander refrigeration cycle for LNG production

•Genetic algorithm was applied to minimize energy consumption of a nitrogen double turbo-expander cycle.•The effect of methane addition (1–50 mol%) on the nitrogen cycle was studied.•The best concentration of methane in the refrigerant was obtained.•The effect of ambient temperature on the required work of the process was investigated.

Genetic algorithm was applied to optimize a typical nitrogen double turbo-expander cycle for LNG production plant. The net required work of the process was considered as an objective function. The effect of methane addition to the nitrogen cycle was studied from 1 to 50 mol percent. The results showed when the concentration of methane in the refrigerant is equal to 26 ± 1 mol percent, the work consumption of the process will be minimized. At this concentration, the net required work of the process will be 8% less than the optimized process case which uses pure nitrogen as a refrigerant. Moreover, the effect of ambient temperature on the energy consumption of the process was investigated from 30 °C to 50 °C. It was observed that when ambient temperature varies from 30 °C to 40 °C the nitrogen flow rate will be constant, approximately. Also the net required work increases significant from 40 °C to 50 °C ambient temperature.