Innovative propane-nitrogen two-phase expander refrigeration cycle for energy-efficient and low-global warming potential LNG production

Abstract

Nitrogen (N2) expander liquefaction process has the highest ecological and safety advantages over different types of available commercial natural gas liquefaction processes. However, its relatively low energy efficiency is a major issue. In this context, the optimum flow rate of propane as a high-boiling component with low-global warming potential was mixed with conventional refrigerant N2, resulting in a two-phase single mixed refrigerant appearing at the suction point of the conventional turbo expander. The potential application of a two-phase cryogenic expander was investigated to generate a cooling effect through the expansion of the high-pressure two-phase propane-nitrogen refrigerant. The proposed study was modeled using Aspen Hysys® and optimized by adopting a MATLAB coded particle swarm optimization approach that was linked to Aspen Hysys® using the ActiveX (also known as COM) functionality. The results revealed that the specific energy consumption and required refrigerant flow rate for liquefied natural gas (LNG) production can be reduced up to 46.4% and 27.7%, respectively, in comparison with the conventional N2 single expander LNG process. Furthermore, the overall energy can be reduced from 79.2% to 29.5% as compared to previously reported N2 single expander LNG processes, depending on feed conditions, composition, and design parameters. An exergy analysis of the proposed LNG process revealed that the compressors and LNG heat exchanger have the highest exergy loss, i.e., 34.0% and 29.7%, respectively.