An Exergy Calculator Tool for Process Simulation76_ftp

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING Asia-Pac. J. Chem. Eng. 2007; 2: 431–437 Published Published online 13 September 2007 in Wiley InterScience 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. Svrcek 1 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 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. non-chemical. However, currently currently most process process simulators simulators in the market do not offer the capability capability of calculatin calculating g 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 largest quantities quantities of exergy exergy are being consumed consumed within the plant, plant, thus pointing to areas where improvement 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 constant tightenin tightening g of environme environmental ntal regulatio regulations ns and the ongoin ongoing g need need to reduce reduce operat operating ing costs have have pose posed d a chal challe leng ngee for for the the desi design gn of any any chem chemic ical al proces process; s; this this is also also the case for existi existing ng proces processes ses 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 from proble problem m solvin solving g to planni planning, ng, concep conceptt develo developpment, ment, interp interpret retati ation on and implem implement entati ation on (Peter (Peterss 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 Correspondence to : Juan Juan M. M. Montel Montelongo ongo-Lu -Luna, na, Departm Department ent of Chemical & Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Dr. NW, Calgary AB, T2N 1N4, Canada. E-mail: [email protected] 

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 compar compariso ison n betwee between n a simple simple exerg exergy y analys analysis is and an energy balance on an ideal process will show the benefits of the proposed tool.

Exergy: the concept The most most common common analys analysis is for energ energy y effici efficienc ency y of a plan plantt or proc proces esss is based based on the first first law law of ther ther-modynami modynamics cs (i.e. energy energy conservati conservation). on). However, 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). Exerg Exergy, y, howev however, er, is based based on the first and second 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)

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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). n

      l

x i M il

mix M = L M −

+

i =1 n

v

yi M iv

V M −

(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 − T 0 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). n

Bchem = L0

n



0l x 0,i Bchem ,i

+ V 0

i =1



0v y0,i Bchem ,i

(2)

i =1

The physical exergy term requires a flash calculation at both the reference ( T 0 , 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 ( T 0 , P0 ) and actual (T , P ) conditions. The physical exergy component is then given by Eqn (3).

    n

L

Bphys =

x i H il − T 0

i =1 n

V

yi H iv i =1

−

      n

x i S il

T ,P

+

i =1 n

T 0 yi S iv i =1

(3) T 0 ,P0

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


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


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. In Sim42 it is not necessary to have ‘Material Streams’; instead, the information is propagated through

‘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.

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. 


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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. 




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 η= (6) 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


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

−128443 −119891 −145093 −87 858 −133768 −162832 −104326 −154758 −204168 −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. ˙ out H η= (7) ˙ in H 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

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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. 


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 ˙ H 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 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




ENERGY CALCULATOR TOOL FOR PROCESS SIMULATION 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.


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An Exergy Calculator Tool for Process Simulation76_ftp

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