(398e) Exergy Analysis of a Single-Stage Mixed Refrigerant LNG Process

Liquefied Natural Gas (LNG) is the fastest growing energy carrier in the world, with an annual growth of 7% for the last two decades [1]. There are many reasons for this trend, such as flexibility in the market, low environmental impact compared with oil and coal, and the fact that natural gas reserves and extraction grow faster than oil. A wide variety of processes have been developed for natural gas liquefaction in order to handle different gas compositions, different climates (i.e. cooling water and air temperatures) and different working environments (offshore vs onshore).

LNG processes can be divided into three categories: (a) Cascade refrigerant cycles (with single component refrigerants); (b) Mixed refrigerant cycles and (c) Expander cycles. This work is focused on the study of the exergy flow in a Mixed Refrigerant Process (MRP) for LNG production. A simplified version of the PRICO process which is commercialized by the Pritchard Company [2] is used as the main case study. The PRICO process consists of a single refrigerant cycle with a multi-component mixture as refrigerant. The substance content of the refrigerant mixture should be similar to the LNG composition in order to reduce the thermodynamic irreversibility produced by heat transfer in the liquefaction process.

The simplified MRP used in this work is characterized by a two-stage compressor, one condenser, the main heat exchanger and one throttle valve. Due to the large amounts of energy needed to operate such a process, energy and exergy analysis should be performed to evaluate its efficiency. Exergy Analysis is used as an evaluator of the energy flow efficiency within the process. Chemical and thermo-mechanical exergies are calculated for all process streams [3]. The global process exergy efficiency is shown in addition to the exergy losses produced in each process unit.

The main intention of this work is to demonstrate a systematic procedure for the correct application of Exergy Analysis on energy intensive processes such as LNG plants using a simple liquefaction process. Based on the results from the Exergy Analysis, a discussion on how to improve global exergy efficiency by modification in key process units is given. Finally, the issue of standardized exergy definitions, including exergy efficiency, that are suitable for sub-ambient processes is also discussed.

References

[1] British Petroleum: Statistical review of world energy 2007; Available from: www.bp.com.

[2] Swenson, L.K., Single mixed refrigerant, closed loop process for liquefying natural gas, 1977, U.S. Patent 4,033,735.

[3] Kotas, T.J., The exergy method of thermal plant analysis. 1985, London.