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An exergy calculator tool for process simulation.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2007; 2: 431–437
Published online 13 September 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.076
Research Article
An exergy calculator tool for process simulation
Juan M. Montelongo-Luna,1 * William Y. Svrcek1 and Brent R. Young2
1
Department of Chemical & Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Dr. NW, Calgary AB, T2N
1N4, Canada
2
Department of Chemical & Materials Engineering, The University of Auckland, Auckland, New Zealand
Received 1 December 2006; Revised 23 February 2007; Accepted 23 February 2007
ABSTRACT: The constant tightening of environmental regulations and the ongoing need to reduce operating costs
have posed a challenge for the design of any chemical process. Process engineers use process simulators to help them
perform calculations that will, ultimately, result in design parameters or operating conditions for a plant or process.
Exergy is a potential indicator that can aid in the design of energy efficient chemical processes and plants. The exergy
concept has been increasingly used as a tool to locate the critical energy use in many industrial processes, both chemical
and non-chemical. However, currently most process simulators in the market do not offer the capability of calculating
the exergy of a process. An open-source exergy calculator has been created by embedding the calculation procedure in
an open-source chemical process simulator. This improves process simulation by including a potential tool for design
teams to quickly evaluate several process options in detail in order to understand their energy utilisation. A simple
exergy analysis for a gas processing facility is used to demonstrate the capabilities of the tool. The analysis shows
where the largest quantities of exergy are being consumed within the plant, thus pointing to areas where improvement
in energy usage can be made. The use of exergy as a potential design and retrofit tool is also discussed.  2007 Curtin
University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: exergy; exergy analysis; energy balance; process simulation; process design
INTRODUCTION
Process design has been always an extremely important
step in the creation of a new chemical process or plant.
The constant tightening of environmental regulations
and the ongoing need to reduce operating costs have
posed a challenge for the design of any chemical
process; this is also the case for existing processes
that have to be retrofitted to comply with the changing
environmental regulations.
In process design, the capabilities provided by computers (e.g. fast calculation, large data storage, logical
decisions) allow engineers to solve larger problems and
to do it much more rapidly; furthermore, with the aid
of computer software the engineers’ role can be shifted
from problem solving to planning, concept development, interpretation and implementation (Peters and
Timmerhaus, 1991).
The intention of this article is to show the potential
help obtainable in process design by using the computational tools available to chemical engineers today and
*Correspondence to: Juan M. Montelongo-Luna, Department of
Chemical & Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Dr. NW, Calgary AB,
T2N 1N4, Canada.
E-mail: jmontelo@ucalgary.ca
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
applying the concept of exergy as a means of finding
the most inefficient parts of a given process or plant.
A comparison between a simple exergy analysis and
an energy balance on an ideal process will show the
benefits of the proposed tool.
Exergy: the concept
The most common analysis for energy efficiency of
a plant or process is based on the first law of thermodynamics (i.e. energy conservation). However, this
analysis does not provide enough information regarding
the potential work that a form of energy can produce
or the potential work lost in energy transformation processes (Kotas, 1985).
Exergy, however, is based on the first and second
laws of thermodynamics, which allows accounting for
irreversibilities in a process providing a more detailed
tracking mechanism for the energy usage.
Kotas (1985) defined the exergy of a stream of matter
as follows.
‘. . . the maximum amount of work obtainable when
the stream is brought from its initial state to the dead
state by processes during which the stream may interact
only with the environment.’ (Kotas, 1985, p. 37)
432
J. M. MONTELONGO-LUNA, W. Y. SVRCEK AND B. R. YOUNG
The ‘dead state’ is that of unrestricted equilibrium conditions of mechanical, thermal and chemical
equilibrium between the system and the environment. It
is worth noting that the processes this definition refers
to are reversible processes.
There are two main ways to calculate exergy. One
divides exergy into physical and chemical components (Kotas, 1980) and the other considers exergy as
being composed of three components, namely, physical
exergy, chemical exergy and exergy change of mixing
(Hinderink et al ., 1996). For the present work, the latter
approach was used because it presents more advantages
for composition-changing processes.
The main advantage of considering exergy as being
composed of three components is that the exergy
components are calculated independently of each other
and the calculation appears to be clear with no ‘hidden’
components in each step. So the exergy, B , is calculated
via Eqn (1).
B = Bchem + Bphys + mix B
(1)
Exergy will be calculated then as the sum of three
components; chemical and physical exergy and the
exergy change of mixing. Each of these components
is described in the following section.
Asia-Pacific Journal of Chemical Engineering
is shown for an arbitrary thermodynamic property, M ,
in Eqn (4).
mix M = L M −
l
Mv −
n
+
xi Mil
i =1
V
n
yi Miv
(4)
i =1
Thus, enthalpy and entropy changes can be calculated
to obtain the exergy change of mixing, which is the
contribution due to the pure components being in a
mixture, at actual conditions. This is calculated by
applying Eqn (5).
mix B = mix H − T0 mix S
(5)
The necessary calculations can be easily done in a
chemical process simulator, which inherently performs
thermodynamic calculations in a very efficient manner.
The design of the exergy calculator and the implementation of these equations are presented in the next
section.
EXERGY CALCULATOR TOOL
Exergy components
The chemical exergy is calculated based on the socalled standard chemical exergy for the chemical elements, which can be calculated from standard formation
enthalpy and Gibbs energy or obtained from standard
tables (Van Gool, 1998). Calculation of chemical exergy
also requires a flash calculation at reference conditions.
The chemical exergy is then given by Eqn (2).
Bchem = L0
n
0l
x0,i Bchem,i
i =1
+ V0
n
0v
y0,i Bchem,i
(2)
i =1
The physical exergy term requires a flash calculation
at both the reference (T0 , P0 ) and the actual conditions
(T , P ). A mixing term is avoided by considering
only the contribution of the pure components to the
enthalpy and entropy of the mixture at reference (T0 , P0 )
and actual (T , P ) conditions. The physical exergy
component is then given by Eqn (3).
T ,P
 n
n
l
l
 L i =1 xi Hi − T0 i =1 xi Si + 

n
(3)
Bphys = 
n


v
v
yi Hi − T0
yi Si
V
i =1
i =1
T0 ,P0
For the determination of the exergy change of mixing,
the concept of ‘property change of mixing’ is used; this
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
As previously mentioned, exergy can be easily calculated with the help of a process simulator. For the
present work Sim42 (Cota Elizondo, 2003) was used as
the chemical process simulator. Since Sim42 is an opensource program, this permitted the seamless inclusion of
the exergy calculations into the source code of the simulator without having the inconvenience of linking any
external computer routines to the simulator or writing a
macro-like routine inside the simulator’s own programming or scripting language. It is also freely available to
any interested user or developer.
As mentioned before, the approach by Hinderink
et al . (1996) where the calculation of the exergy is
divided into three components was implemented in the
open-source chemical process simulator Sim42 to create
the exergy calculator.
The exergy calculator performs the following steps
in order to get the exergy of a material stream
(Montelongo-Luna et al ., 2005):
1. Identify the thermodynamic property package and
the chemical species used in the simulation.
2. Identify which elements within the simulation represent material streams.
3. Calculate thermodynamic properties for each of the
chemical species at standard conditions.
4. Get thermodynamic properties for each of the
chemical species at actual conditions.
Asia-Pac. J. Chem. Eng. 2007; 2: 431–437
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
ENERGY CALCULATOR TOOL FOR PROCESS SIMULATION
5. Calculate thermodynamic properties for the material streams at reference conditions.
6. Get thermodynamic properties for the material
streams at actual conditions.
7. Calculate the chemical exergy component.
8. Calculate the physical exergy and the exergy
change of mixing components.
9. Calculate the exergy for the material stream.
10. Display the results for the total exergy for each
stream.
‘Ports’. The ‘Material Port’ represents streams of matter
and it carries all the information regarding physical, chemical and thermodynamic properties (Cota Elizondo, 2003).
The implementation for the exergy calculation was set
up in the call to the thermodynamic property package.
The ‘exergy property’ was added to the Sim42 list
of variables and then calculated in the thermodynamic
provider interface. This allowed inserting the exergy
into the material ports and propagating the values
throughout the simulation.
Figure 1 depicts a simplification of the algorithm as
implemented for the exergy calculator.
In Sim42 it is not necessary to have ‘Material
Streams’; instead, the information is propagated through
Start
Process Simulator Engine
Define T0, P0
Calculate
Thermodynamic
properties for chemical
species at Standard
Conditions
YES
Is all required
information available?
Calculate Chemical
Exergy
Get Thermodynamic
properties for chemical
species at Actual
Conditions
NO
Calculate
Thermodynamic
properties for Material
Streams at Reference
Conditions
Calculate Physical
Exergy
Get Thermodynamic
properties for Material
Streams at Actual
Conditions
Calculate Exergy
Change of Mixing
Calculate Exergy for
the Material Stream
Display
results
End
Figure 1. Exergy calculator algorithm.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 431–437
DOI: 10.1002/apj
433
434
J. M. MONTELONGO-LUNA, W. Y. SVRCEK AND B. R. YOUNG
Asia-Pacific Journal of Chemical Engineering
It is worth noting that at actual conditions the
exergy calculator just needs to take values for the
thermodynamic properties already calculated by the
‘natural flow’ of the process simulator. However, the
values for the standard conditions need to be calculated
because they are not included in the regular process
simulator calculation steps. Note also that the exergy
calculator does not take into account energy streams
modeled as pure or direct energy. In order to take the
exergy of the utilities into account, it is necessary to
model them as the actual material streams they represent
(e.g. high pressure steam, low pressure steam, hot oil,
etc.).
For the purposes of this work a rather idealised
feed stream and process conditions were assumed. This
however, as will become apparent, does not limit the
capabilities of the exergy analysis. Table 1 shows the
composition of the inlet gas. The gas is fed at 10 ◦ C
and 4125 kPa.
The first, second and third heaters increase the stream
temperature to 68, 124 and 134 ◦ C, respectively.
Other specifications for the simulation are as follows:
•
•
•
•
•
•
•
CASE STUDY: NATURAL GAS CONDENSATE
STABILISATION
Feed flow: 49.7 kmol
Stage 1 pressure drop: 0 kPa
Stage 2 pressure drop: 2075 kPa
Stage 3 pressure drop, 1700 kPa
Gas Product pressure: 4125 kPa
Comp 1 adiabatic efficiency: 75%
Comp 2 adiabatic efficiency: 75%
The Peng–Robinson equation of state was used in
the simulation as the thermodynamic property package.
Note also that the simulation case study was set up with
no heat losses from any equipment to the environment.
Natural gas containing considerable amounts of liquefiable hydrocarbons (ethane, propane and heavier)
produces condensate upon cooling or compressing and
cooling (Manning and Thompson, 1991).
A simple stabilisation scheme is used to separate an
oil and gas mixture into a stabilised condensate and
a saleable gas for small production of condensate that
does not justify a full NGL recovery train. Figure 2
shows a schematic of this process.
A rich gas is heated and sent to a separator where
the liquid stream is sent to a heater and then to a
second separator where the pressure is reduced. The
liquid stream from this separator is heated again and
sent to a third separator where the pressure is further
reduced. The liquid stream from this separator is the
stabilised condensate. The gas streams from the second
and third separators are compressed to the pressure of
the first separator and all three gas streams are then
blended to get a gas product stream which can be sold.
Table 1. Inlet gas composition.
Compound
Mole fraction
Methane
Ethane
Propane
i -Butane
n-Butane
i -Pentane
n-Pentane
n-Hexane
n-Heptane
n-Octane
n-Nonane
0.316
0.158
0.105
0.105
0.105
0.053
0.053
0.027
0.026
0.026
0.026
Stage 1
Vap
Gas Product
Steam 1
Feed
Hot Feed 1
Comp 1
Out
Stage 1
Steam 2
Heater 1
Stage 1
Liq
Hot Feed 2
Gas
Mixer
Stage 2
Vap
Comp 2
Out
Comp 1
Stage 2
Heater 2
Steam 3
Stage 2
Liq
Hot Feed 3
Stage 3
Vap
Comp 2
Stage 3
Heater 3
Liquid
Product
Figure 2. Stabilisation train.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 431–437
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
ENERGY CALCULATOR TOOL FOR PROCESS SIMULATION
Results
Table 4. Equipment exergy flows.
For this work the reference state as given by Van Gool
(1998) was used. The reference pressure is 100 kPa and
the reference temperature is 25 ◦ C.
On the basis of the parameters described in the
previous section the exergy tool was run on the case
study simulation to obtain the exergy numerical values.
Table 2 summarises the exergy flows of the material
streams in the process.
Table 3 shows the energy supplied for each of the
heaters and compressors.
For analysis purposes these energy feeds are treated
as exergy delivered to each of the equipment (i.e. it is
assumed to be electricity).
Table 4 summarises the results by presenting the
exergy flows in and out the process equipment.
Equation (6) defines the simple exergetic efficiency
used in Table 4.
Bout
(6)
η=
Bin
The overall exergetic efficiency for the process considering the exergy flows for inlet and outlets is 0.992.
Analogously, Table 5 presents the energy (enthalpy)
flows of each process stream and Table 6 shows the
energy flows in and out of each unit operation. This is
the information needed to carry out an energy balance.
Table 2. Material streams exergy.
Stream
Feed
Hot feed 1
Stage 1 Liq
Stage 1 Vap
Hot feed 2
Stage 2 Liq
Stage 2 Vap
Hot feed 3
Liquid product
Stage 3 Vap
Comp 1 out
Comp 2 out
Gas product
Exergy
(kJ/kmole)
2 164 629
2 165 016
2 788 008
1 373 173
2 790 204
3 461 023
2 109 329
3 462 995
4 490 758
2 882 634
2 111 298
2 889 874
1 905 276
Exergy flow
(kW)
29 880
29 886
21 539
8346
21 556
13 456
8096
13 463
6280
7176
8103
7194
23 636
Table 3. Energy input.
Equipment
Heater 1
Heater 2
Heater 3
Comp 1
Comp 2
Energy feed
(kW)
118
88
31
9
22
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Equipment
Exergy in
(kW)
Exergy out
(kW)
Exergetic
efficiency
Heater 1
Stage 1
Heater 2
Stage 2
Comp 1
Heater 3
Stage 3
Comp 2
Gas mixer
29 998
29 886
21 627
21 556
8105
13 487
13 463
7198
23 643
29 886
29 885
21 556
21 552
8103
13 463
13 456
7194
23 636
0.996
0.999
0.997
0.999
0.999
0.998
0.999
0.999
0.999
Table 5. Material streams energy.
Stream
Enthalpy
(kJ/kmole)
Energy flow
(kW)
Feed
Hot feed 1
Stage 1 Liq
Stage 1 Vap
Hot feed 2
Stage 2 Liq
Stage 2 Vap
Hot feed 3
Liquid product
Stage 3 Vap
Comp 1 out
Comp 2 out
Gas product
−128 443
−119 891
−145 093
−87 858
−133 768
−162 832
−104 326
−154 758
−204 168
−126 998
−101 929
−118 355
−98 330
−1773
−1655
−1121
−534
−1033
−633
−400
−602
−286
−316
−391
−295
−1220
Table 6. Equipment energy flows.
Equipment
Energy in
(kW)
Energy out
(kW)
Energy
efficiency
Heater 1
Stage 1
Heater 2
Stage 2
Comp 1
Heater 3
Stage 3
Comp 2
Gas mixer
−1655
−1655
−1033
−1033
−391
−602
−602
−295
−1220
−1655
−1655
−1033
−1033
−391
−602
−602
−295
−1220
1
1
1
1
1
1
1
1
1
Equation (7) defines the simple energetic efficiency
used in Table 6.
Ḣout
(7)
η=
Ḣin
The overall energetic efficiency for the process considering the energy flows for inlet and outlets is 1.0
(which was expected from an idealised simulation with
no heat losses).
Asia-Pac. J. Chem. Eng. 2007; 2: 431–437
DOI: 10.1002/apj
435
436
J. M. MONTELONGO-LUNA, W. Y. SVRCEK AND B. R. YOUNG
Asia-Pacific Journal of Chemical Engineering
DISCUSSION
The results from Tables 2 and 3 can be used to carry
out a simple overall exergy analysis, which has shown
that the overall exergetic efficiency of the process is just
above 99%. One could argue that this number might be
due to round-off errors or numerical instability in the
properties calculations; however, if this were true for
a given thermodynamic property package, the energy
balance would also be affected in the same manner.
From Table 6 it is clear that the energy balance results
in 100% efficiency. That means that even an idealised
model of a process accounts for some (not all, however)
of the exergy destruction in the process.
By examining each of the unit operations in the plant
it can be seen how much exergy is lost in every step of
the process.
It is interesting to note the exergy losses in the
heat exchangers; even though they are increasing the
temperature of the stream, and therefore increasing its
exergy, most of the energy supplied cannot be recovered
in the form of work (i.e. entropy is created). This
indicates a good point to focus a more thorough design
in terms of temperature differences and heating media.
Another interesting result is the loss of exergy in
the mixer; this loss is mainly due to the change of
composition from the inlet streams to the outlet gas
(i.e. chemical exergy and exergy change of mixing).
This problem can be looked at by designing the process
in order to blend more composition-similar streams or
not blending at all.
These results confirm that the plant is governed by
irreversible processes and that the capacity of producing
work is decreased.
CONCLUSIONS
It has been shown that the exergy can be easily
calculated with the aid of a chemical process simulator
(Sim42). The results provided from this simple exergy
analysis show the areas where the exergy consumption
is the greatest, thereby allowing for improvement.
There is potential for applying exergy calculations
into the early stages of process design to take into
account inefficiencies so that design engineers can take
actions to correct them. It is also evident that this
approach can be used in retrofitting industrial processes
as it can give a better perspective on where the energy is
being wasted. Embedding the exergy calculation into a
process simulator created a tool that can be extensively
used in the early stages of process design to rapidly
evaluate different scenarios to find the most energy
efficient ones.
An idealised process simulation showed that exergy
losses are always present and should be taken into
account.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Acknowledgments
This work was in part supported by the COURSE
program from the Alberta Energy Research Institute
under the agreement No. 1512.
SYMBOLS USED
B
H
Ḣ
L
M
n
P
S
T
V
x
y
Exergy
Molar enthalpy
Energy flow
Liquid fraction
Arbitrary thermodynamic property
Total number of compounds
Pressure
Molar entropy
Temperature
Vapour fraction
Liquid molar fraction
Vapour molar fraction
GREEK
η
Difference or change
Efficiency
SUBSCRIPTS
0
i
chem
phys
mix
in
out
Standard conditions
Compounds
Chemical
Physical
Mixture
Inlet
Outlet
SUPERSCRIPTS
0
l
v
Standard conditions
Liquid phase
Vapour phase
REFERENCES
Cota Elizondo RC. Development of an Open Source Chemical
Process Simulator. M.Sc. Thesis. 2003; University of Calgary,
Calgary, AB.
Hinderink AP, Kerkhof FJPM, Lie ABK, De Swaan Arons J, Van
Der Kooi HJ. Exergy analysis with a flowsheeting simulator–I.
Theory; calculating exergies of material streams. Chem. Eng. Sci.
1996; 51: 4693–4700.
Kotas TJ. Exergy concepts for thermal plant. Int. J. Heat Fluid Flow
1980; 2: 105–114.
Asia-Pac. J. Chem. Eng. 2007; 2: 431–437
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
ENERGY CALCULATOR TOOL FOR PROCESS SIMULATION
Kotas TJ. The Exergy Method of Thermal Plant Analysis.
Butterworths: London, 1985.
Manning FS, Thompson RE. Oilfield Processing of Petroleum,
Volume One: Natural Gas. PennWell Books: Tulsa, Oklahoma,
1991.
Montelongo-Luna JM, Young BR, Svrcek WY. An Open Source
Exergy Calculator Tool. In 2nd CDEN International Conference
on Design Education, Innovation, and Practice. 2005; Kananaskis,
Alberta, Canada.
Peters MS, Timmerhaus KD. Plant Design and Economics for
Chemical Engineers, 4th edn. McGraw-Hill: New York, 1991.
Van Gool W. Thermodynamics of chemical references for exergy
analysis. Energy Convers. Manag. 1998; 39: 1719–1728.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 431–437
DOI: 10.1002/apj
437
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