(629b) 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.