Use of exergy efficiency for the optimization of LNG processes with NGL extraction

Abstract

In this paper, processes for liquefied natural gas (LNG) production with upstream or integrated natural gas liquids (NGL) removal have been optimized and compared. Since the NGL and LNG production systems use both work and heat to deliver products with different energy quality, it is challenging to measure accurately the thermodynamic efficiency by using conventional energy performance indicators. Thus, two different objective functions, specific energy consumption and exergy efficiency, have been applied in the optimization of these complex systems in order to evaluate the effectiveness of the two performance indicators. The results indicate that use of the exergy-based objective function results in a richer NGL and a larger amount of LNG production with a marginal increase in energy consumption, showing a higher thermodynamic efficiency than the result with the energy-based objective function. Besides, integrated NGL extraction shows a lower thermodynamic performance than upstream removal, indicating that further advanced schemes are required for effective integration of the NGL extraction part in the LNG process.