Application of Exergy Efficiencies in Complicated Cryogenic Processes
Donghoi Kim1, Truls Gundersen1
1Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Kolbjoern Hejes vei 1B, NO-7491, Trondheim, Norway
Our world today relies heavily on low temperature processes for energy, food, medical, and even space science. To improve such systems, an energy balance has been used as a traditional tool for performance measurement. However, the method based on the first law of thermodynamics does not consider the quality of energy. Unlike the traditional method, exergy analysis is a relatively new way of evaluating processes, which applies both the first and second law of thermodynamics. Exergy is the maximum available work acquired by bringing a system to equilibrium with its environment. Exergy analysis allows identifying where exergy is destroyed in a process, in other words, the location of entropy generation. This gives guidelines to improve efficiency by highlighting units having the largest exergy destruction. In cryogenic processes, refrigeration is produced by consuming compression power, and power is pure exergy. Thus, it is evident to use exergy analysis for low temperature processes.
Such exergy has a distinctive behaviour between above and below ambient temperature when it comes to heat transfer. Above ambient, the maximum work (exergy) acquired is always less than the heat transferred, but asymptotically approaching the value of heat with increasing temperature. In contrast, below ambient temperature, exergy of heat is increasing exponentially with decreasing temperature, so the exergy value can be equal and even larger than the amount of heat. Thus, attention has to be given when using exergy as a performance parameter due to this discontinuity in exergy of heat. In addition, the temperature and pressure based physical exergy affect each other when a process stream experiences changes in its temperature or pressure, which requires caution when applying exergy analysis. As physical exergy relates to ambient conditions, it is also important to know whether a process is operated above, across, or below ambient.
In exergy analysis, there have been various definitions suggested for exergy efficiency. However, the existing definitions of exergy efficiency do not fully handle the above-mentioned issues. Most of these efficiencies are defined and formulated for above ambient processes. Thus, a new exergy efficiency referred to as the Exergetic Transfer Effectiveness (ETE) has been developed in our research group by carefully defining exergy sources and sinks . Depending on the operating temperature level of a system, this efficiency thoroughly identifies the components of the exergy sources and sinks by decomposing the physical exergy into temperature and pressure based exergy. As a result, the ETE can be used for any processes operating above, across, and below ambient.
The main objective of this paper is to compare the ETE with other exergy efficiency definitions for some selected natural gas liquefaction processes such as dual mixed refrigerant (DMR) processes. The calculation of both system and equipment efficiencies are performed in order to comprehend the main sources of exergy destruction. In addition, the extension of the ETE for process units having a change in chemical exergy is presented. A discussion is also made on the practicality of the new exergy efficiency for design and optimization of cryogenic processes.
 Marmolejo-Correa, D. and T. Gundersen, “Low Temperature Process Design: Challenges and Approaches for using Exergy Efficiencies”, Computer Aided Chemical Engineering, vol.29, pp. 1909-1913, 2011.