Efficient Configuration of a Natural Gas Liquefaction Process for Energy Recovery

One of the most important challenges in a natural gas liquefaction plants is to improve the plant energy efficiency. In particular, if part of the natural gas is used as a fuel gas or the liquefaction ratio is taken into account as a design factor in an liquified natural gas (LNG) plant, process design focusing on cold energy recovery is an attractive option. In this study, various energy recovery-oriented process configurations and the potential improvements of energy savings in LNG plants were analyzed. Our primary focus for energy recovery in the LNG liquefaction process was centered on utilizing the flash gas stream from the phase separator. The applicability of the proposed configurations was validated by modeling and simulation of the single mixed refrigerant (SMR), propane precooled mixed refrigerant (C3MR), and single nitrogen (N2) expander processes. The simulation results for all cases exhibited considerable reductions of refrigerant flow rates, seawater cooling duties, and the specific work. For example, when the liquefaction ratio was fixed at 0.90, the amount of refrigerant was reduced by 4-5% by employing configuration 1, which recovers cold energy from the flash gas in LNG heat exchangers. This also led to 4-5% reductions of the specific work and seawater duty. Any energy recovery configuration will result in a considerable energy consumption reduction as the natural gas liquefaction process consumes a large amount of energy. Therefore, the optimization of energy recovery configurations in the natural gas liquefaction process is highly recommended with the objective of maximized energy savings considering capital costs.