Energy and Exergy Optimization of a mini-scale Nitrogen Dual Expander Process for Liquefaction of Natural Gas

Nitrogen expansion processes are suitable for mini or small-scale liquefied natural gas plants, due to their simplicity and less equipment. However, they consume a high amount of energy and any attempt to reduce the energy consumption and improve the quality of energy (work potential of energy), leads to enhance the process efficiency and profitability. A mini-scale nitrogen dual expander natural gas liquefaction process is simulated and analyzed by Aspen HYSYS simulator. Then, in order to optimize energy performance of the process, some influencing variables are adjusted using the genetic algorithm approach provided by MATLAB software in two separate optimization cases with different objective functions. Specific energy consumption and total exergy destruction are considered as the objective functions of the optimization cases (namely energy and exergy cases), which represent quantity and quality of energy, respectively. The most important operating variables of the process, refrigerant molar flow, refrigerant temperatures and refrigerant pressures, are selected via a sensitivity analysis. The results indicate that in both of the optimization cases, the specific power consumption of the process is reduced 7.1%. However, the total exergy destruction for exergy case decreases 9.55% which is slightly a more desirable result than the energy case. Also, total exergy efficiency of the process in exergy case is 4.4% higher than the other case which reveals that considering the quality aspect of energy as the objective can improve the performance of the process more appropriately.