CHEMICAL ENGINEERING TRANSACTIONS 
 

VOL. 61, 2017 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Petar S Varbanov, Rongxin Su, Hon Loong Lam, Xia Liu, Jiří J Klemeš
Copyright © 2017, AIDIC Servizi S.r.l. 
ISBN 978-88-95608-51-8; ISSN 2283-9216 

Calculation of Cogeneration Potential of Total Site Utility 
Systems with Commercial Simulator 

Xiao-Ying Rena, Xue-Xiu Jiab, Petar S. Varbanovb, Jiří J. Klemešb, Zhi-Yong Liuc,* 
a
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China 

b
Sustainable Process Integration Laboratory - SPIL, NETME Centre, Faculty of Mechanical Engineering, Brno University of 

 Technology - VUT Brno, Technická 2896/2, 616 69 Brno, Czech Republic 
c
School of Marine Science and Engineering, Hebei University of Technology, Tianjin 300130, China 
liuzhiyong@hebut.edu.cn 

Cogeneration potential of utility systems is one of the important indicators for the design of Total Site utility 
systems. In this paper, a new method is proposed to calculate cogeneration potential of Total Site utility 
systems. The temperatures of steam mains are calculated with Aspen Plus simulator first. Steam flow rates 
are calculated based on temperatures of steam mains and head loads of process, then. Based on the 
temperatures and steam flow rates obtained, shaft powers generated by the steam turbines in expansion 
zones can be obtained with the built-in block COMPR in Aspen Plus simulator. Compared to the existing 
methods, the method proposed needs not complex calculation procedure. For the illustrated case studies, the 
results obtained by this method are comparable to that obtained in the literature. It is shown that the 
calculation procedure proposed in this paper is simple, and the results are accurate.  

1. Introduction  
It is important to calculate cogeneration potential of utility systems before designing of Total Site utility 
systems (Klemeš et al., 1997).  
Sorin and Hammache (2005) introduced a thermodynamic model for targeting fuel consumption, cooling water 
requirement and shaft power production based on a modified Site Utility Grand Composite Curve (SUGCC). In 
the model, the targets are represented as special segments on the modified SUGCC. El-Halwagi et al. (2009) 
presented a systematic method for targeting cogeneration potential in steam systems before designing of the 
power generation network. With the method, combustible wastes are used effectively, and the utilisation of 
fuel sources, heating and power generation are coordinated. Bandyopadhyay et al. (2010) proposed a 
graphical method to estimate the cogeneration potential at the total site level. Kapil et al. (2012) developed a 
method by combining bottom-up and top-down procedures. In their method, steam levels are systematically 
optimised in the design of site utility configurations. An Iterative Bottom-to-Top Model (IBTM) was developed 
by Ghannadzadeh et al. (2011). The method considers the degree of superheat and also employs the same 
thermodynamic equations as Kundra (2005). The IBTM uses an iterative procedure to calculate cogeneration 
potential with less required data compared to the method of Kundra (2005). Oluleye et al. (2014) presented a 
mathematical model to evaluate the potential of a processing site for waste heat recovery. Ng et al. (2017) 
developed an algebraic method to determine cogeneration potential. 
The listed methods need complex calculation procedure or tedious steps for constructing of the curves. The 
calculation of the cogeneration potential with commercial simulators still desires more development. In this 
paper, a commercial simulator is used to calculate cogeneration potential of Total Site utility systems. In the 
calculation procedure, parameters are adopted from Ghannadzadeh et al. (2011). The results obtained by the 
proposed method are comparable to that obtained with the literature methods.  

2. New Method 
In the calculation procedure of targeting the cogeneration potential, many required data can be obtained from 
commercial simulator easily. In addition, many simulators have flexible and robust calculation framework in 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1761203

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Ren X.-Y., Jia X.-X., Varbanov P.S., Klemeš J.J., Liu Z.-Y., 2017, Calculation of cogeneration potential of total site 
utility systems with commercial simulator, Chemical Engineering Transactions, 61, 1231-1236  DOI:10.3303/CET1761203  

1231



which material and energy balances can be obtained easily. Simulators have many built-in model blocks which 
can be directly used in the simulation of utility systems. In this paper, Aspen Plus simulator is used to calculate 
the cogeneration potential. In the calculation, steam is the working fluid of turbine, the Property Method is 
STEAMNBS. 
In this work, the targeting of cogeneration potential for the site utility system is divided into three steps as 
shown in Figure 1, Step 1: calculation of temperatures of steam mains, Step 2: calculation of the mass flow 
rates of steam mains, and Step 3: calculation of shaft powers of steam turbines. The detailed procedure will 
be discussed as follows. 

T∆+T=T sat1 net
iQ

net
im

net
im

net
im

 

Figure 1: Overview of the new method for calculating cogeneration potential 

Step 1: Calculation of temperatures of steam mains  
The procedure for manipulating stream data to generate the Site Utility Grand Composite Curve (SUGCC) can 
be found in Klemeš et al. (1997). Figure 2 shows the diagram of steam turbines configuration with a set of 
steam mains in SUGCC. It can be seen that different level steams are required or generated by the process. 
To meet the process steam demands and utilise the energy effectively, higher-pressure steam can be used to 
generate shaft power via steam turbines and reduced to lower-pressure steam. In SUGCC, pressure and heat 

load of every steam level are shown. Net heat load netiQ is the difference between the process demand steam 

and the process generated steam in the same steam level. 

0>Qnet1

0>Qnet2

0<Q net3

 

Figure 2: An illustration of SUGCC with steam turbines 

Using Aspen Plus simulator, temperatures of all steam mains can be calculated based on the lowest-pressure 
steam main T1. The temperature of the lowest-pressure steam main T1 is specified by adding a degree of 
superheat (△T) to its saturation temperature because the lowest-pressure steam main would be used for 
steam heating. When steam is used for steam heating, it is desirable to have at least 10 °C superheat in 
steam mains to avoid excessive condensation in the transport. As shown in Figure 2, the very high pressure 
(VHP) steam expands through turbines to the low-pressure (LP) steam. When the isentropic efficiency of 
steam turbines is constant, the temperature of the VHP steam main determines temperatures of the lower-
pressure steam mains. When the temperature of the LP steam main is specified, the temperatures of VHP, HP 
and MP steam mains can be calculated based on the temperature of LP steam main. The simulation 
flowsheeting for calculating temperatures of steam mains is shown in Figure 3. The built-in block COMPR is 
used to model steam turbines with isentropic efficiency. In the simulation, the flow rate of VHP can be taken as 
1 t/h. In the “Design Spec” of Aspen Plus simulator, the temperature of LP steam main is set to the target 

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value and the temperature of the VHP steam main is the manipulated variable. By running simulator, 
temperatures of all steam mains can be obtained.  

TB3

TB2

TB1

VHP

HP

MP

LP

 

Figure 3: The simulation flowsheeting for calculating temperatures of steam mains 

Step 2: Calculation of the mass flow rates of steam mains 

For a given Total Site, the net heat loads ( netiQ ) at different pressure levels are often known except for the 

highest-pressure level that is aimed to be determined at the end of targeting stage. The process required 
steam is assumed to be turned into condensate water after being used. The steam generated by the process 

is assumed to be superheated steam that is obtained by heating condensate water. The net mass load ( netim ) 

at every pressure level will be calculated using the net heat load by Eq(1). 

fRe
i

net
inet

i
h-h

Q
m =  (1) 

where ih  is the enthalpy at the temperature and pressure of steam main i, and 
fReh is the enthalpy of the 

condensate water at the pressure of the steam main and temperature of boiler feed water.  
The net mass loads are mass flow rates of net process generation steam or net process demand steam at 
different pressure levels. The mass flow rates (msi) of steam i expanding through the (i-1) steam turbine to 
generate shaft powers can be calculated using net mass loads by Eq(2), as shown in Figure 2. It should be 
noted that flow rate of steam expanding through the top turbine is equal to the flow rate of VHP steam main. 

∑
1-i

1i

net
ii mms

=

=  (2) 

Step 3: Calculation of shaft powers of steam turbines 

FSP

TB1

TB2

TB3

MIX

LP

MP

HP

MP-USE

VHP

HP-IN

HP-GEN

 

Figure 4: The simulation flowsheeting for calculating shaft powers 

In Figure 4, the simulation flowsheeting for calculating shaft powers is established based on the SUGCC 
shown in Figure 2. The temperatures and flow rates of VHP, HP-GEN and MP-USE steam mains should be 
inputted. Temperatures have been obtained in step 1. The flow rates of HP-GEN and MP-USE steam mains 
can be calculated by Eq(1), and flow rate of VHP steam main is calculated by Eq(2) in step 2. Inputting these 
parameters, running the simulator, the shaft powers can be determined. 

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3. Case study 
In this section, the new method proposed will be demonstrated by three case studies presented by 
Ghannadzadeh et al. (2011). Figure 5 shows the required input parameters for cogeneration targeting of the 
site utility systems. In these cases, the temperature of condensate water is assumed to 105 °C. The 
condensate water is supplied to the boiler house or steam generated in the process. The isentropic efficiency 
of all turbines is taken as 0.7 and mechanical efficiency is assumed as 100 %. In Example 1, the degree of 
superheat (△T) in LP steam main is assumed to be 30 °C. In Examples 2 and 3, the degree of superheat (△T) 
in LP steam main is assumed to be 40 °C. 

 

Figure 5: Input data: (a) Example 1, (b) Example 2 and (c) Example 3. 

Step 1: Calculation of the temperatures of steam mains.  
In Example 1, the saturation temperature of LP steam main is 130 °C. The degree of superheat in LP steam 
main is 30 °C. Then, the temperature of LP steam main is 160 °C. In the “Design Spec”, specify the target 
temperature of LP steam main TLP as 160 °C, and TVHP is specified as the manipulated variable. After running 
simulator, the following results are obtained: TVHP, THP and TMP are 501, 417 and 302 °C, when TLP achieve 
the goal 160 °C, as shown in Figure 6. 
Step 2: Calculation of the mass flow rates of steam mains.  
Figure 5 shows the net heat loads at the HP, MP and LP through the SUGCC. In Example 1, the net mass 
loads for LP, MP and HP levels calculated are 24.96, 9.53 and 13.69 t/h, with Eq(1). The mass flow rates of 
steams expanding through the steam turbines of MP-LP, HP-MP and VHP-HP intervals calculated by Eq(2) 
are 24.96, 34.49 and 20.80 t/h. 

TB3

TB2

TB1

501

90.0

VHP 417

46.0

HP 302

15.5

MP
160

2.7

LP

Temperature (C)

Pressure (bar)

 

Figure 6: The simulation for calculating temperatures of steam mains of Example 1 

1234



 

TB3

W=-837

TB2

W=-1945

TB1

W=-1796

MIX

FSP

501

90.0

20794

VHP
417

46.0

20794

HP

417

46.0

34488

417

46.0

13694

HP-GEN

302

15.5

34488

MP

302

15.5

9530

MP-USE

302

15.5

24958

159

2.7

24958

LP

Temperature (C )

P ressure (bar)

M ass F low  Rate (kg/hr)

W P ower(kW)

 

Figure 7: The simulation flowsheeting for calculating shaft powers of Example 1  

Step3: Calculation of the shaft powers of the turbines.  
In Example 1, the temperature of VHP steam main is 501 °C, and the steam flow rate of VHP steam main is 
20.80 t/h. Net HP steam generated in the process is 13.69 t/h, and net steam demand in MP is 9.53 t/h. 
Inputting these constraint conditions into Aspen Plus simulator, shaft powers of steam turbines obtained are 
0.84, 1.95 and 1.80 MW, and are shown in Figure 7.  
Similarly, the temperatures of steam mains, steam flow rates and shaft powers generated by the steam 
turbines for Example 2 and Example 3 can be obtained. The results of three examples are listed in Tables 1, 2, 
3 and 4. 

Table 1: Temperatures of steam mains (°C) 

Example LP MP HP VHP 
Example 1 160 302 417 501 
Example 2 174 301 436 549 
Example 3 174 301 436 549 

Table 2: Net mass flow loads (t/h) of process generation or process demand steam 

Example LP MP HP 
Example 1 24.96 9.53 13.69 
Example 2 129.10 55.36 63.42 
Example 3 265.79 117.63 63.42 

Table 3: Mass flow rates (t/h) of steam expanding through the steam turbines 

Example MP-LP HP-MP VHP-HP 
Example 1 24.96 34.49 20.80 
Example 2 129.10 184.46 247.88 
Example 3 265.79 148.16 84.74 

Table 4: Shaft powers (MW) of steam turbines  

Example VHP-HP HP-MP MP-LP 
Example 1 0.84 1.95 1.80 
Example 2 13.47 12.28 8.32 
Example 3 4.61 9.86 17.13 

 

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Table 5: The shaft powers generated by turbines and the temperature of VHP steam main 

Example Method 
VHP-HP 

(MW) 
HP-MP 
(MW) 

MP-LP 
(MW) 

Temperature 
of VHP (°C)

Example 1 
New method 0.84 1.95 1.80 501 

IBTM 0.8 1.9 1.7 500 

Example 2 
New method 13.47 12.28 8.32 549 

IBTM 13.49 12.28 8.33 550 

Example 3 
New method 4.61 9.86 17.13 549 

IBTM 4.6 9.8 17.1 550 

 
The comparison of the results of the IBTM method and that of the new method are presented in Table 5. From 
Table 5, it can be seen that the results of the two methods are almost the same. However, the calculation 
procedure of the new method is simpler than that of IBTM method (Ghannadzadeh et al., 2011). The 
parameters required in the new method are less than the targeting approach of Kundra (2005). 

4. Conclusions 
In this paper, a new method is presented for calculating cogeneration potential of site utility systems by Aspen 
Plus simulator. In the new method, the temperatures of steam mains are calculated with the built-in block 
COMPR of the simulator. Then, steam flow rates are determined based on the temperatures of steam mains 
and heat loads. Shaft powers generated by the steam turbines are obtained with the temperatures of steam 
mains and steam flow rates obtained. By using the simulator, it is not necessary to develop mathematical 
programs or set up thermodynamics models, this makes the calculation procedure much easier. For the 
illustrated case studies, the results obtained by this method are comparable to that obtained in the literature. 
As a future work, the evaluation of the steam turbine capital cost and environmental footprints should be 
further added to the targeting procedure, to provide decision makers with more complete picture of the 
designed sites. This should allow them to efficiently screen and compare the major design options on the 
basis of economic and environmental performance. 

Acknowledgements 

The project "Sustainable Process Integration Laboratory – SPIL", project No. CZ.02.1.01/0.0/0.0/ 
15_003/0000456 funded by EU "CZ Operational Programme Research and Development, Education", Priority 
1: Strengthening capacity for quality research supported by the collaboration agreement with the Hebei 
University of Technology – Tianjin, China, has been acknowledged. 

References 

Bandyopadhyay S., Varghese J., Bansal V., 2010, Targeting for cogeneration potential through total site 
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El-Halwagi M., Harell D., Spriggs H.D., 2009, Targeting cogeneration and waste utilisation through process 
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Ghannadzadeh A., Perry S., Smith R., 2011, A New Shaftwork Targeting Model for Total Sites, Chemical 
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Kapil A., Bulatov I., Smith R., Kim J.K., 2012, Site-wide low-grade heat recovery with a new cogeneration 
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Kundra V., 2005, To develop a systematic methodology for the implementation of R-curve analysis and its use 
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Klemeš J., Dhole V.R., Raissi K., Perry S.J., Puigjaner L., 1997, Targeting and Design Methodology for 
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