Decision Making on Liquefaction Ratio for Minimizing Specific Energy in a LNG Pilot Plant

Natural gas liquefaction is a highly energy-intensive process due to its cryogenic operation. Therefore, a major concern in designing and optimizing natural gas liquefaction processes is the enhancement of energy efficiency. A liquefaction ratio in the natural gas liquefaction process affects the specific energy consumption, which is the energy consumption per unit mass of liquefied natural gas (LNG). However, most of the previous research has not considered the liquefaction ratio of the natural gas. This study focuses on minimizing specific energy consumption of a LNG plant considering the liquefaction ratio. To analyze the effects of the liquefaction ratio, four different cases are set and the optimizations are performed. The objective function to be minimized includes the total energy consumption and specific energy consumption. The result of minimizing total energy consumption converges on the lower bound of the liquefaction ratio, but the result of minimizing specific energy consumption converges between the lower and upper bound. This shows that the total energy minimization does not always have the same meaning as the specific energy minimization when the liquefaction ratio is not fixed. Through this analysis, the relationships between the energy consumption and the liquefaction ratio are found. As the result of the optimization, the optimal compression ratio, temperature, intermediate pressure, and refrigerant flow rate are found, and the specific power was reduced by 16.40% over that of the LNG pilot plant design while yielding the natural gas liquefaction ratio of 86.9%.