(188k) Exergy Targeting for Complicated LNG Processes

Exergy is the maximum available work obtained by bringing a system to equilibrium with its environment. Such analysis, which is the combination of first and the second law of thermodynamics, is based not only on the condition of a system but also the environment. These features make exergy efficiency more reliable than energy efficiency, showing the real performance of processes. Thus, exergy analysis has been regarded as a valuable post design tool.

However, exergy can also be used in an early stage of design by calculating the minimum available work required in a process, especially in low-temperature processes. Refrigeration in cryogenic processes is produced by compression power and such power is pure exergy. Traditionally, pinch analysis has been widely used for designing systems related to heat integration. This analysis only contains temperature as a variable. Therefore, pinch analysis is inadequate for designing a refrigeration system where streams experience changes in both temperature and pressure [1].

Exergy targeting for designing a refrigeration process can be performed by combining pinch analysis and exergy analysis, having both temperature and pressure as variables. Previous work in our research group has suggested a new graphical diagram for exergy targeting, based on so-called exergetic temperatures [2]. The advantage of these new temperatures (actually new â??lumpedâ? temperature functions) is that there is a linear relation between exergetic temperature and exergy. Simple piecewise linear graphical diagrams similar to composite curves in pinch analysis can easily be drawn and used for exergy targeting.

Previous work studied only simple processes to test the new graphical diagram, such as N₂ gas expander processes assuming the ideal gas model. Thus, the main objective of this paper is to extend this exergy targeting method to multi-component refrigerant LNG processes, while using a real gas model. A discussion is also made on the practicality of the exergy targeting approach for design and optimization of complicated cryogenic processes.

References

[1] Aspelund, A., Berstad, D., Gundersen, T. â??An Extended Pinch Analysis and Design procedure utilizing pressure based exergy for subambient coolingâ?, Applied Thermal Engineering, vol. 27, pp. 2633-2649, 2007.

[2] Marmolejo-Correa, D. and Gundersen, T. â??New Graphical Representation of Exergy Applied to Low Temperature Process Designâ?, Industrial & Engineering Chemistry Research, vol. 52, pp. 7145-7156, 2013.