(626h) Use Exergy Analysis to Increase Energy Efficiencies of LNG Processes

LNG processes are widely used all around the world.  Large LNG facilities are used to transport natural gas overseas.  Mid-scale and small-scale LNG facilities have the potential to compete with pipeline transportation for medium and small shale gas fields.  In a typical LNG value chain, the liquefaction section consumes 30%-45% of the total operating costs.  Reducing the operating cost of an LNG process could reduce the total cost of LNG value chain.

LNG processes are basically refrigeration systems.  Exergy is a very useful thermodynamic property to analysis refrigeration systems.  Exergy means the maximum useful work possible during a process that brings the system into equilibrium with environment.  From exergy point of view, LNG processes get exergy from mechanical work and transfer it to natural gas to make it far from equilibrium with environment.

In this paper, a new exergy analysis method was developed for LNG processes.  3-D exergy-temperature-pressure (B-T-P) diagram was developed for LNG processes.  The B-T-P diagram shows not only the overall exergy consumption of the whole process, but also exergy transfer between facilities in the process.  Exergy in the system is also classified as three categories: temperature related exergy, pressure related exergy and phase-change related exergy.  Changing of exergy types happens during exergy transfer.  Since every facility can only process certain types of exergy, the exergy loss during changing plays an important role for overall efficiency.  With B-T-P diagram and exergy changing, the loss of exergy for each facility can be evaluated.  Process improvement methods are used to reduce exergy loss and increase process efficiency.

A C3MR process was used as case study.  Two improvements were made for the original process.  The first improvement only changes operation conditions, and the second improvement changes process itself.  Results show that the overall operating cost could be reduced by 15% for the second improvement, and device investment for the second improvement is relatively small.