an exergy calculator tool for process simulation76_ftp

7/29/2019 An Exergy Calculator Tool for Process Simulation76_ftp

1/7

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2007; 2: 431437Published 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, T2N1N4, Canada2Department of Chemical & Materials Engineering, The University of Auckland, Auckland, New Zealand

Received 1 December 20 06; 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 designteams 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 importantstep in the creation of a new chemical process or plant.The constant tightening of environmental regulationsand the ongoing need to reduce operating costs haveposed a challenge for the design of any chemicalprocess; this is also the case for existing processesthat have to be retrofitted to comply with the changingenvironmental regulations.

In process design, the capabilities provided by com-puters (e.g. fast calculation, large data storage, logicaldecisions) allow engineers to solve larger problems andto do it much more rapidly; furthermore, with the aidof computer software the engineers role can be shiftedfrom problem solving to planning, concept develop-ment, interpretation and implementation (Peters andTimmerhaus, 1991).

The intention of this article is to show the potentialhelp obtainable in process design by using the compu-tational tools available to chemical engineers today and

*Correspondence to: Juan M. Montelongo-Luna, Department ofChemical & Petroleum Engineering, Schulich School of Engineer-ing, University of Calgary, 2500 University Dr. NW, Calgary AB,T2N 1N4, Canada.E-mail: [email protected]

applying the concept of exergy as a means of findingthe most inefficient parts of a given process or plant.

A comparison between a simple exergy analysis andan energy balance on an ideal process will show thebenefits of the proposed tool.

Exergy: the concept

The most common analysis for energy efficiency ofa plant or process is based on the first law of ther-modynamics (i.e. energy conservation). However, thisanalysis does not provide enough information regardingthe potential work that a form of energy can produce

or the potential work lost in energy transformation pro-cesses (Kotas, 1985).Exergy, however, is based on the first and second

laws of thermodynamics, which allows accounting forirreversibilities in a process providing a more detailedtracking mechanism for the energy usage.

Kotas (1985) defined the exergy of a stream of matteras follows.

. . . the maximum amount of work obtainable whenthe stream is brought from its initial state to the deadstate by processes during which the stream may interactonly with the environment. (Kotas, 1985, p. 37)

2007 Curtin University of Technology and John Wiley & Sons, Ltd.


2/7

32 J. M. MONTELONGO-LUNA, W. Y. SVRCEK AND B. R. YOUNG Asia-Pacific Journal of Chemical Engineering

The dead state is that of unrestricted equilib-rium conditions of mechanical, thermal and chemicalequilibrium between the system and the environment. Itis worth noting that the processes this definition refersto are reversible processes.

There are two main ways to calculate exergy. Onedivides exergy into physical and chemical compo-nents (Kotas, 1980) and the other considers exergy as

being composed of three components, namely, physicalexergy, chemical exergy and exergy change of mixing(Hinderinket al ., 1996). For the present work, the latterapproach was used because it presents more advantagesfor composition-changing processes.

The main advantage of considering exergy as beingcomposed of three components is that the exergycomponents are calculated independently of each otherand the calculation appears to be clear with no hiddencomponents in each step. So the exergy, B , is calculatedvia Eqn (1).

B = Bchem

+ Bphys

+ mix

B (1)

Exergy will be calculated then as the sum of threecomponents; chemical and physical exergy and theexergy change of mixing. Each of these componentsis described in the following section.

Exergy components

The chemical exergy is calculated based on the so-called standard chemical exergy for the chemical ele-ments, which can be calculated from standard formationenthalpy and Gibbs energy or obtained from standardtables (Van Gool, 1998). Calculation of chemical exergyalso requires a flash calculation at reference conditions.The chemical exergy is then given by Eqn (2).

Bchem = L0

ni=1

x0,iB0lchem,i + V0

ni=1

y0,iB0vchem,i (2)

The physical exergy term requires a flash calculationat both the reference (T0, P0) and the actual conditions(T, P ). A mixing term is avoided by consideringonly the contribution of the pure components to theenthalpy and entropy of the mixture at reference (T0, P0)and actual (T, P) conditions. The physical exergycomponent is then given by Eqn (3).

Bphys =

L

n

i=1

xiHl

i T0n

i=1

xi Sl

i

+

V

n

i=1

yiHv

i T0n

i=1

yi Sv

i

T,P

T0,P0

(3)

For the determination of the exergy change of mixing,the concept of property change of mixing is used; this

is shown for an arbitrary thermodynamic property, M,in Eqn (4).

mixM = L

Ml

ni=1

xiMl

i

+

V

Mv

ni=1

yiMv

i

(4)

Thus, enthalpy and entropy changes can be calculatedto obtain the exergy change of mixing, which is thecontribution due to the pure components being in amixture, at actual conditions. This is calculated byapplying Eqn (5).

mixB = mixH T0mixS (5)

The necessary calculations can be easily done in achemical process simulator, which inherently performs

thermodynamic calculations in a very efficient manner.The design of the exergy calculator and the imple-

mentation of these equations are presented in the nextsection.

EXERGY CALCULATOR TOOL

As previously mentioned, exergy can be easily cal-culated with the help of a process simulator. For thepresent work Sim42 (Cota Elizondo, 2003) was used asthe chemical process simulator. Since Sim42 is an open-

source program, this permitted the seamless inclusion ofthe exergy calculations into the source code of the sim-ulator without having the inconvenience of linking anyexternal computer routines to the simulator or writing amacro-like routine inside the simulators own program-ming or scripting language. It is also freely available toany interested user or developer.

As mentioned before, the approach by Hinderinket al . (1996) where the calculation of the exergy isdivided into three components was implemented in theopen-source chemical process simulator Sim42 to createthe exergy calculator.

The exergy calculator performs the following stepsin order to get the exergy of a material stream(Montelongo-Luna et al ., 2005):

1. Identify the thermodynamic property package andthe chemical species used in the simulation.

2. Identify which elements within the simulation rep-resent material streams.

3. Calculate thermodynamic properties for each of thechemical species at standard conditions.

4. Get thermodynamic properties for each of thechemical species at actual conditions.

2007 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2007; 2: 431 437DOI: 10.1002/apj


3/7

Asia-Pacific Journal of Chemical Engineering ENERGY CALCULATOR TOOL FOR PROCESS SIMULATION 43

5. Calculate thermodynamic properties for the mate-rial streams at reference conditions.

6. Get thermodynamic properties for the materialstreams 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 eachstream.

In Sim42 it is not necessary to have MaterialStreams; instead, the information is propagated through

Ports. The Material Port represents streams of matterand it carries all the information regarding physi-cal, chemical and thermodynamic properties (Cota Eli-zondo, 2003).

The implementation for the exergy calculation was setup in the call to the thermodynamic property package.The exergy property was added to the Sim42 listof variables and then calculated in the thermodynamic

provider interface. This allowed inserting the exergyinto the material ports and propagating the valuesthroughout the simulation.

Figure 1 depicts a simplification of the algorithm asimplemented for the exergy calculator.

Start

Define T0, P0

Process Simulator Engine

CalculateThermodynamic

properties for chemicalspecies at Standard

Conditions

Get Thermodynamicproperties for chemical

species at ActualConditions

YES

Is all requiredinformation available?

Calculate ChemicalExergy

Calculate PhysicalExergy

Calculate ExergyChange of Mixing

Calculate Exergy forthe Material Stream

Displayresults

End

NO

Calculate

Thermodynamicproperties for MaterialStreams at Reference

Conditions

Get Thermodynamicproperties for Material

Streams at ActualConditions

Figure 1. Exergy calculator algorithm.



4/7


It is worth noting that at actual conditions theexergy calculator just needs to take values for thethermodynamic properties already calculated by thenatural flow of the process simulator. However, thevalues for the standard conditions need to be calculatedbecause they are not included in the regular processsimulator calculation steps. Note also that the exergycalculator does not take into account energy streams

modeled as pure or direct energy. In order to take theexergy of the utilities into account, it is necessary tomodel them as the actual material streams they represent(e.g. high pressure steam, low pressure steam, hot oil,etc.).

CASE STUDY: NATURAL GAS CONDENSATESTABILISATION

Natural gas containing considerable amounts of liq-uefiable 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 anda saleable gas for small production of condensate thatdoes not justify a full NGL recovery train. Figure 2shows a schematic of this process.

A rich gas is heated and sent to a separator wherethe liquid stream is sent to a heater and then to asecond separator where the pressure is reduced. Theliquid stream from this separator is heated again andsent to a third separator where the pressure is furtherreduced. The liquid stream from this separator is the

stabilised condensate. The gas streams from the secondand third separators are compressed to the pressure ofthe first separator and all three gas streams are thenblended to get a gas product stream which can be sold.

For the purposes of this work a rather idealisedfeed stream and process conditions were assumed. Thishowever, as will become apparent, does not limit thecapabilities of the exergy analysis. Table 1 shows thecomposition of the inlet gas. The gas is fed at 10 Cand 4125 kPa.

The first, second and third heaters increase the streamtemperature to 68, 124 and 134 C, respectively.

Other specifications for the simulation are as follows:

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 inthe 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.

Table 1. Inlet gas composition.

Compound Mole fraction

Methane 0.316Ethane 0.158Propane 0.105i -Butane 0.105n-Butane 0.105i -Pentane 0.053n-Pentane 0.053n-Hexane 0.027

n-Heptane 0.026n-Octane 0.026n-Nonane 0.026

Feed

Heater 1

Steam 1

Hot Feed 1

Stage 1

Liq

Stage 2

Liq

Steam 3

Hot Feed 3

Liquid

Product

Steam 2

Hot Feed 2

Stage 2

Vap

Stage 1Vap

Stage 3

Vap

Stage 2

Stage 3

Comp 1

Comp 2

Comp 2Out

Comp 1Out

Gas Product

Gas

Mixer

Heater 2

Heater 3

Stage 1

Figure 2. Stabilisation train.



5/7


Results

For this work the reference state as given by Van Gool(1998) was used. The reference pressure is 100 kPa andthe reference temperature is 25 C.

On the basis of the parameters described in theprevious section the exergy tool was run on the casestudy simulation to obtain the exergy numerical values.

Table 2 summarises the exergy flows of the materialstreams in the process.

Table 3 shows the energy supplied for each of theheaters and compressors.

For analysis purposes these energy feeds are treatedas exergy delivered to each of the equipment (i.e. it isassumed to be electricity).

Table 4 summarises the results by presenting theexergy flows in and out the process equipment.

Equation (6) defines the simple exergetic efficiencyused in Table 4.

=Bout

Bin

(6)

The overall exergetic efficiency for the process con-sidering 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 theenergy flows in and out of each unit operation. This isthe information needed to carry out an energy balance.

Table 2. Material streams exergy.

StreamExergy

(kJ/kmole)Exergy flow

(kW)

Feed 2 164 629 29 880Hot feed 1 2 165 016 29 886Stage 1 Liq 2 788 008 21 539Stage 1 Vap 1 373 173 8346Hot feed 2 2 790 204 21 556Stage 2 Liq 3 461 023 13 456Stage 2 Vap 2 109 329 8096Hot feed 3 3 462 995 13 463Liquid product 4 490 758 6280Stage 3 Vap 2 882 634 7176Comp 1 out 2 111 298 8103Comp 2 out 2 889 874 7194Gas product 1 905 276 23 636

Table 3. Energy input.

EquipmentEnergy feed

(kW)

Heater 1 118Heater 2 88Heater 3 31Comp 1 9Comp 2 22

Table 4. Equipment exergy flows.

EquipmentExergy in

(kW)Exergy out

(kW)Exergeticefficiency

Heater 1 29 998 29 886 0.996Stage 1 29 886 29 885 0.999Heater 2 21 627 21 556 0.997Stage 2 21 556 21 552 0.999Comp 1 8105 8103 0.999Heater 3 13 487 13 463 0.998Stage 3 13 463 13 456 0.999Comp 2 7198 7194 0.999Gas mixer 23 643 23 636 0.999

Table 5. Material streams energy.

StreamEnthalpy

(kJ/kmole)Energy flow

(kW)

Feed 128443 1773Hot feed 1 119891 1655

Stage 1 Liq 145093 1121Stage 1 Vap 87 858 534Hot feed 2 133768 1033Stage 2 Liq 162832 633Stage 2 Vap 104326 400Hot feed 3 154758 602Liquid product 204168 286Stage 3 Vap 126 998 316Comp 1 out 101 929 391Comp 2 out 118 355 295Gas product 98 330 1220

Table 6. Equipment energy flows.

EquipmentEnergy in

(kW)Energy out

(kW)Energy

efficiency

Heater 1 1655 1655 1Stage 1 1655 1655 1Heater 2 1033 1033 1Stage 2 1033 1033 1Comp 1 391 391 1Heater 3 602 602 1Stage 3 602 602 1Comp 2 295 295 1Gas mixer 1220 1220 1

Equation (7) defines the simple energetic efficiencyused in Table 6.

=Hout

Hin(7)

The overall energetic efficiency for the process con-sidering the energy flows for inlet and outlets is 1.0(which was expected from an idealised simulation withno heat losses).



6/7


DISCUSSION

The results from Tables 2 and 3 can be used to carryout a simple overall exergy analysis, which has shownthat the overall exergetic efficiency of the process is justabove 99%. One could argue that this number might bedue to round-off errors or numerical instability in theproperties calculations; however, if this were true for

a given thermodynamic property package, the energybalance would also be affected in the same manner.From Table 6 it is clear that the energy balance resultsin 100% efficiency. That means that even an idealisedmodel 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 plantit can be seen how much exergy is lost in every step ofthe process.

It is interesting to note the exergy losses in theheat exchangers; even though they are increasing thetemperature of the stream, and therefore increasing itsexergy, most of the energy supplied cannot be recoveredin the form of work (i.e. entropy is created). Thisindicates a good point to focus a more thorough designin terms of temperature differences and heating media.

Another interesting result is the loss of exergy inthe mixer; this loss is mainly due to the change ofcomposition 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 processin order to blend more composition-similar streams ornot blending at all.

These results confirm that the plant is governed byirreversible processes and that the capacity of producing

work is decreased.

CONCLUSIONS

It has been shown that the exergy can be easilycalculated with the aid of a chemical process simulator(Sim42). The results provided from this simple exergyanalysis show the areas where the exergy consumptionis the greatest, thereby allowing for improvement.

There is potential for applying exergy calculationsinto the early stages of process design to take into

account inefficiencies so that design engineers can takeactions to correct them. It is also evident that thisapproach can be used in retrofitting industrial processesas it can give a better perspective on where the energy isbeing wasted. Embedding the exergy calculation into aprocess simulator created a tool that can be extensivelyused in the early stages of process design to rapidlyevaluate different scenarios to find the most energyefficient ones.

An idealised process simulation showed that exergylosses are always present and should be taken intoaccount.

Acknowledgments

This work was in part supported by the COURSEprogram from the Alberta Energy Research Instituteunder the agreement No. 1512.

SYMBOLS USED

B ExergyH Molar enthalpyH Energy flow

L Liquid fractionM Arbitrary thermodynamic propertyn Total number of compoundsP PressureS Molar entropyT TemperatureV Vapour fraction

x Liquid molar fraction

y Vapour molar fraction

GREEK

Difference or change Efficiency

SUBSCRIPTS

0 Standard conditions

i Compoundschem Chemicalphys Physicalmix Mixturein Inletout Outlet

SUPERSCRIPTS

0 Standard conditionsl Liquid phase

v Vapour phase

REFERENCES

Cota Elizondo RC. Development of an Open Source ChemicalProcess Simulator. M.Sc. Thesis. 2003; University of Calgary,Calgary, AB.

Hinderink AP, Kerkhof FJPM, Lie ABK, De Swaan Arons J, VanDer Kooi HJ. Exergy analysis with a flowsheeting simulatorI.Theory; calculating exergies of material streams. Chem. Eng. Sci.1996; 51: 46934700.

Kotas TJ. Exergy concepts for thermal plant. Int. J. Heat Fluid Flow1980; 2: 105114.



7/7


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 SourceExergy 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 forChemical Engineers, 4th edn. McGraw-Hill: New York, 1991.

Van Gool W. Thermodynamics of chemical references for exergyanalysis. Energy Convers. Manag. 1998; 39: 17191728.


an exergy calculator tool for process simulation76_ftp

Documents