A New Approach to the Use of Exergy Analysis in Synthesis and Design - Focus On Low Temperature Processes

A new Approach to the Use of Exergy Analysis in Synthesis and Design – Focus on Low Temperature Processes

Truls Gundersen¹, Danahe Marmolejo Correa²

Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjoern Hejes v 1B; NO-7491, Trondheim, Norway.

¹corresponding author: truls.gundersen@ntnu.no;

²danahe.marmolejo@gmail.com.

Abstract

This paper presents a summary of some recent discoveries and developments related to the use of Exergy Analysis for the synthesis and design of industrial production plants with special emphasis on sub-ambient processes. The key elements of the paper are (1) the importance of decomposing certain exergy forms into their exergy components, (2) the importance of decomposing thermo-mechanical exergy at ambient temperature, (3) the systematic definition of exergy sources and exergy sinks for use in exergy efficiencies, (4) the establishment of linear Exergy diagrams using basic thermodynamic equations and simplifying assumptions, (5) the concept of targeting for Exergy similar to Energy targeting in Pinch Analysis, and (6) the use of Exergy as a design tool in conceptual process development. Since Exergy is defined as the maximum work that can be produced when a system reaches equilibrium with its environment through reversible processes, the concept of Exergy is particularly useful for sub-ambient processes. Here, external cooling is provided by refrigeration cycles, where the cooling duty is produced by a sequence of expansion and compression. This requires work or mechanical energy, which is pure exergy. The paper will use a very small example to illustrate the basic concepts, and the Reverse Brayton process for liquefaction of natural gas as an industrial case study.

Background

Despite a rich literature on Exergy (a term that was "coined" by Rant in 1956) in the last decades, and the fact that Exergy is closely related to the 2^nd Law of Thermodynamics (often formulated by the use of the logical counterpart Entropy), the use of Exergy in the process industries has been rather limited. One contributing factor to this situation can have been the lack of standardization in the field of Exergy. Different symbols and different names have been used across disciplines (such as chemical versus mechanical engineering) and across continents (such as Europe versus the US). This lack of standardization also includes issues of classification, reference conditions and exergy efficiency definitions. Another contributing factor can have been the fact that Exergy considerations in quite a few cases are in conflict with economic considerations. In the process industries, the bottom line is always economy, expressed either as maximum annual profit or minimum total annualized cost. Since, however, there are considerable uncertainties in both operating cost and investment cost, using measures such as Exergy at least has the advantage of developing a "sound" design from a thermodynamic point of view. For sub-ambient process, exergy losses translate into compression work, which again translates directly into economy.

When using the Exergy concept for sub-ambient processes such as liquefaction of natural gas (LNG) and separation of air by cryogenic separation (ASU), more limitations have been encountered (Marmolejo-Correa and Gundersen, 2011) in the available literature on Exergy Analysis that limit the ability to properly evaluate the quality of process designs. Some of the proposed exergy efficiencies are unable to account for the fact that sub-ambient processes utilize the option to switch between two components of thermo-mechanical exergy, i.e. the temperature based part and the pressure based part (Marmolejo-Correa and Gundersen, 2012a). By expansion in a turbine below ambient temperature, pressure based exergy can be transformed into temperature based exergy (refrigeration or cold exergy) and work. Similarly, compression can be used to "store" work (i.e. exergy) in the form of pressure based exergy. A very important part of this picture is the fact that temperature based exergy for a material stream as well as the exergy of heat exhibits a remarkable behavior below ambient temperature. The exergy of heat is always positive, and when passing ambient temperature from above to below, there is a discontinuity at ambient temperature. Temperature based exergy for a material stream is also always positive, and even though there is no discontinuity at ambient temperature, the exergy value of the stream increases more rapidly per degree Celsius or Kelvin when the temperature is reduced below ambient compared to increasing the temperature above ambient.

This is why decomposition of exergy into different components and to decompose exergy evaluations at ambient temperature are important for understanding the behavior of certain process equipment as well as for the development of exergy efficiencies that properly measures the quality of process designs.

Contributions and Conclusions

The paper describes some modest contributions to the field of Exergy Analysis with emphasis on sub-ambient processes. The issues of standardization, classification and decomposition were discussed in the Background section. Based on insight developed by studying the behavior of exergy components and certain pieces of equipment above and below ambient temperature, a new general exergy efficiency definition called Exergy Transfer Effectiveness (ETE) has been developed (Marmolejo-Correa and Gundersen, 2012c) that focus on exergy change and exergy transformation in processes. Changes in exergy, supply or removal of exergy and transformation of exergy are carefully divided into Exergy Sources and Exergy Sinks. A set of rules has been developed to make a proper classification into sources and sinks.

Existing Exergy diagrams (Linnhoff and Dhole, 1992) for overall processes have been developed from temperature vs. enthalpy diagrams in Pinch Analysis (i.e. from Composite and Grand Composite Curves) by replacing temperature with Carnot factor as the y-axis (i.e. Exergy Composite Curves and the Exergy Grand Composite Curve). These diagrams have a number of shortcomings. First, in their construction, a considerable number of points have to be calculated due to the nonlinear shape of the curves (the Carnot factor is a non-linear function of temperature). Then, in the interpretation of the curves, targets for maximum exergy recovery, minimum exergy losses, etc. are not readily available from the curves. Exergy losses can be measured in a cumbersome way geometrically by measuring the area between the Exergy Composite Curves.

Using basic thermodynamic equations and simplifying assumptions, new Exergetic Temperatures have been proposed (Marmolejo-Correa and Gundersen, 2012b) that can be used to develop (piece-wise) linear Exergy Diagrams that are (1) easy to construct from the process stream data, and (2) provide easily and explicitly various targets for Exergy such as maximum exergy recovery, minimum exergy losses, minimum exergy requirement, and minimum exergy rejection. These diagrams enable the use of Exergy Analysis in early stages of process design, while currently it is used as a post-design measure of quality. More specifically, the paper will discuss its use in combination with the ExPAnD methodology (Aspelund et al., 2007, and Wechsung et al., 2011).

There is considerable scope for increased use of Exergy in analysis, synthesis and design of processing plants; however, there is a lack of expertise in industry and a lack of suitable software for application in industrial environments. The fact that Exergy Analysis must be used with care, since reducing exergy losses caused by irreversibilities often is in conflict with investment cost minimization, may also have contributed to its rather limited industrial use in the past.

References

 ADDIN EN.REFLIST

Aspelund, A., Berstad, D.O., Gundersen, T. (2007). An extended pinch analysis and design procedure utilizing pressure based exergy for subambient cooling. Applied Thermal Engineering, 27(16), 2633-2649.

Linnhoff, B., Dhole, V.R. (1992). Shaftwork targets for low-temperature process design. Chem. Engng. Sci., 47(8), 2081-2091.

Marmolejo-Correa, D., Gundersen, T. (2011). Low temperature process design: Challenges and approaches for using exergy efficiencies, in Pistikopoulos, E.N. et al. (editors). Proceedings of the 21^st European Symposium on Computer Aided Process Engineering - ESCAPE 21, Thessaloniki, Greece, vol. 2, pp. 1909-13.

Marmolejo-Correa, D., Gundersen, T. (2012a). A comparison of exergy efficiency definitions with focus on low temperature processes. In review for Energy.

Marmolejo-Correa, D., Gundersen, T. (2012b). A new graphical representation of exergy applied to low temperature process design, in Karimi I.A., Srinivasan, R. (editors). Proceedings of the 11^th International Symposium on Process Systems Engineering - PSE 2012, Singapore.

Marmolejo-Correa, D., Gundersen, T. (2012c). Exergy transfer effectiveness for low temperature processes. To be submitted to International Journal of Thermodynamics.

Rant, Z. (1956). Exergy, a new word for technical work potential. Forsch. Ing. Wes., 22, 36-37 (in German).

Wechsung, A., Aspelund, A., Gundersen, T. (2011). Synthesis of Heat Exchanger Networks at Sub-ambient Conditions with Compression and Expansion of Process Streams. AIChE Journal, 57(8), 2090-2108.