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DSpace Institution
DSpace Repository http://dspace.org
Chemical engineering Thesis and Dissertations
2020-03-11
Design of Heat Exchanger Network
Using Pinch Analysis Method: case
study on Awash Melkassa sulfuric acid
production factory
Tesfay, Hailay
http://hdl.handle.net/123456789/10190
Downloaded from DSpace Repository, DSpace Institution's institutional repository
BAHIR DAR UNIVERSITY
BAHIR DAR INSTITUTE OF TECHNOLOGY
SCHOOL OF RESEARCH AND POSTGRADUATE STUDIES
FACULTY OF CHEMICAL AND FOOD ENGINEERING
MSc Program in Process Engineering
Design of Heat Exchanger Network Using Pinch Analysis Method: case study
on Awash Melkassa sulfuric acid production factory
By
Hailay Tesfay Sheka
Bahir Dar, Ethiopia
March, 2019
i
Design of Heat Exchanger Network Using Pinch Analysis Method: case study
on Awash Melkassa sulfuric acid production factory
Hailay Tesfay Sheka
A Thesis in partial fulfillment of the requirements for the Degree of Master of Science in
Chemical Engineering (Process Engineering specialization)
Presented to the faculty of chemical and food engineering, Bahir Dar Institute of
1.1 Back Ground ..................................................................................................................................... 1
1.2. Problem statement .............................................................................................................................. 3
1.3.2 Specific objectives ....................................................................................................................... 4
1.4 Scope of the study ............................................................................................................................ 4
1.5 Significance of the study ................................................................................................................. 4
CHAPETR TWO .......................................................................................................................................... 6
LITERATURE REVIEW ............................................................................................................................. 6
2.1. Process description of sulfuric acid production plant ................................................................ 6
CHAPTER FOUR ....................................................................................................................................... 28
RESULTS AND DISCUSSION ................................................................................................................. 28
CHAPTER FIVE ........................................................................................................................................ 51
CONCLUSION AND RECOMMENDATION .......................................................................................... 51
temperature (because it is threshold problem), Total minimum number of units = 7 and other
target values can observe from table 3.4 targets view tab above.
3.4. Stream splitting
Grid diagram is the most common representation scheme of HEN, in which each heat exchange
unit is represented as a vertical line connecting two streams. It represents the countercurrent
nature of the heat exchange and it is a useful visual tool to apply the rules of pinch analysis.
In a grid diagram as it shown in figure 3.2 below, horizontal lines at the top of the diagram
represent hot streams. These streams flow from the left to the right of the grid diagram.
Horizontal lines at the bottom of the diagram represent cold streams. These streams flow from
the right to the left of the diagram. Vertical lines represent heat exchange unit and each line
connect a hot and a cold stream, a hot stream and a cooling utility, or a cold stream and heating
utility in this work hot stream will connect with cold utility.
Figure3.2 Grid diagram representation for process streams The pinch analysis provides a strategy for developing the network in a sequential manner
deciding on one heat exchanger at a time, with rules for matching hot and cold streams for these
heat exchangers. In the application of the pinch analysis, situations are commonly encountered
where stream splitting is an absolute requirement in order to design HEN that achieves minimum
external utilities.
26
This threshold problem is treated as one half of a pinched problem (follow rules of below the
pinch). The rules of pinch analysis below the pinch are: CP and stream numbers of hot are
greater or equal to that of cold. Stream splitting rule is represented in the figure 3.3 below for
both number and heat capacity criteria.
Number of streams criterion: NH≥ NC
CP criterion: CPH≥ CPC
Where, NH is number of hot streams, NC is number of hot streams, CPH is heat capacity of hot
streams and CPC is heat capacity of hot streams
Figure3.3 Flow diagrams for splitting of streams As it shown from figure 3.2 the numbers of hot streams are greater than cold streams and it
illustrates that no stream splitting is required to develop a HEN design. In this case, the number
of streams criteria rule is satisfied. But considering the CP values, it is impossible to split any of
the cold streams into two branches that both have CP values large enough to bring a hot stream
to target temperature. So stream five must split into two streams (including branches) but, still
the CP rule is not satisfied. Then based on the CP rule stream five has to be split to three streams
(including branches). The result is then return to the problem where the number of cold streams
is larger than the number of hot streams, thus this is violating the pinch rule. So further stream
splitting is required and one of the hot streams will have to be split. Finally stream four is split to
27
two streams (including branches). Therefore, after making stream splitting using the above figure
3.3, the result is shown below in grid diagram figure 3.4 of process streams.
Figure3.4 Grid diagram representation of process streams after stream splitting
28
CHAPTER FOUR
RESULTS AND DISCUSSION
4.1 Composite curves
In heat integration, plot gives a visual analysis of important variables in a given stream data.
Pinch analysis gives composite curves (CC) for cold and hot streams separately, as shown below
in figure 4.1. The CC graph shows below in figure 4.1 the temperature profile with respect to
enthalpy indicating how much heat is recovered in the process and how much utility is needed.
Figure 4.1 Composite Curves From the above figure 4.1, it shows that the overlap between hot and cold composite curves
represents the maximum amount of heat that can be recovered within the process. The overshoot
of the hot composite curve represents the minimum amount of external cooling required in the
process and at the cold end the composite curves are in alignment, indicating that there is no
demand for hot utility. Therefore, from the above CC curve, the sulphuric acid process is a
threshold problem that requires only cold utility. This implies that there should be no net
requirement for heating of process streams with hot utility. There is no pinch in this problem
because it is a threshold problem with non-utility end.
Composite curves provide overall energy targets, but CC does not indicate the amount of energy
that should be supplied at different temperature levels through utilities. Grand composite curve
(GCC) is plotted with net enthalpy against shifted temperature from the data of shifted
temperature level composite curves as it shown below in figure 4.2. From the GCC graph it can
29
also be easily identified the point where enthalpy is zero; the GCC graph touches the temperature
axis. Also the GCC graph show that the problem needs only cold utility; this indicates the nature
of problem is a threshold problem.
Figure 4.2 Grand composite curves Utility composite curve and GCC are similar, but utility composite curve contains hot and cold
utility streams. From the utility composite curve graph (figure 4.3), it determines the minimum
hot and cold utility requirements for the network and check how much of each utility contributes
to the total utility target.
Figure 4.3 Utility composite curves 4.2 Effect of ∆Tmin
4.2.1 Effect of ∆Tmin on utilities
In heat recovery problems having a pinch point, the selection of ∆Tmin values has special
significance in the design. But, in this study the problem is threshold problem, so for threshold
30
problem the utility heat load remains constant (as a ∆Tmin value varies utility requirement
remains constant) as it is shown in figure 4.4 below.
Figure4.4 Effect of ∆Tmin on utilities 4.3 Heat Exchanger Network
With performance targets for energy and units, the next step is the actual design of the HEN. One
of the most important features of pinch analysis is that the insight obtained in establishing
performance targets ahead of design actually forms the core of the design methodology. From
the targeting step it was found that the problem is threshold problem which needs only cooling
utility. So the idea in pinch design is to start the design where it is most constrained. If the design
is pinched problem, the problem is most constrained at the pinch. If there is no pinch, the most
constrained of this type of problem is the non utility end. This is where temperature difference is
smallest. So the threshold problem is treated as one half of a pinched problem (follow rules of
below the pinch).
To design a heat exchange network is performing matching between streams. And five different
matches between process streams were developed.
a) Matching of stream 3 (H-3) and stream 5(1)(C-1)
Number of streams criterion: 5≥ 5
CP criterion: 0.79≥ 0.7479036
So both number of streams and CP criterion are satisfied. Stream three has 158KW total heat
amount. A vertical line is drawn from stream three to stream five (1) and 39.639KW amount of
heat duty of stream three is transferred to stream five (1) in exchanger (E-111) to reach the
31
interval target temperature. The remaining 118.36KW heat duty of stream three not removed in
exchanger (E-111) is removed in the next match.
b) Matching of stream 3(H-3) and stream 5(2)(C-2)
Number of streams criterion: 5≥ 5
CP criterion: 0.79≥ 0.7479036
Both number of streams and CP criterion are satisfied. Now Stream three has left with118.36KW
heat amount. A vertical line is drawn from stream three to stream five (2) and 39.639KW
amount of heat duty of stream three is transferred to stream five (2) in exchanger (E-114) to
reach the interval target temperature. The remaining 78.72KW heat duty of stream three not
removed in exchanger (E-114) is removed in the next utility match.
c) Matching of stream 2 (H-4) and stream 5(3)(C-3)
Number of streams criterion: 5≥ 5
CP criterion: 0.78≥ 0.75419287
Both pinch analysis criterion are satisfied and stream two has 42.9 KW heat amount. A vertical
line is drawn from stream two to stream five (3) and 39.97 KW amount of heat duty of stream
two is transferred to stream five (3) in exchanger (E-112) to reach the interval target temperature.
The remaining 2.92 KW heat duty of stream two not removed in exchanger (E-112) is removed
in the next match with stream seven.
d) Matching of stream 2 (H-5) and stream 7(C-5)
Number of streams criterion: 5≥ 5
CP criterion: 0.78≥ 0.054
Both the golden rules of pinch analysis criterion are satisfied. And now Stream two has left
with2.92 KW heat amount. A vertical line is drawn from stream two to stream seven and
2.92KW amount of heat duty of stream two is transferred to stream seven in exchanger (E-115)
to reach the target temperature of both streams.
e) Matching of stream 1(H-3) and stream 6(C-4)
Number of streams criterion: 5 ≥ 5
CP criterion: 0.63≥ 0.05
32
Both pinch analysis criterion are satisfied. Stream one has 75.61 KW heat amount. A vertical
line is drawn from stream one to stream six and 3.45 KW amount of heat duty of stream two is
transferred to stream six in exchanger (E-116)to reach the interval target temperature. The
remaining 72.16 KW heat duty of stream one not removed in exchanger (E-116) is removed in
the next match with utility. The maximum energy recovery is designed by transferring heat
between the process streams as shown below in figure 4.5.
Figure4.5 Grid diagram of HEN for process to process heat transfer After the maximum energy recovery is designed by transferring heat between the process
streams but, until now some streams are unsatisfied and these are displayed below in table4.1.
Table 4.1 Unsatisfied streams
When the heat recovery is maximized, the remaining thermal needs are supplied by external heat
utility and three different matches between process streams and utility are developed.
f) Matching of stream 1 (H-1) and cold utility
Stream one was left with 72.16KW heat and this amount of heat is removed in exchanger (E-
117) by external utility air which is available at 80 oC.
33
g) Matching of stream 3 (H-3) and cold utility
The amount of heat that has not matched with the process streams was satisfied with external
utility. 78.73KW amount of heat was left in stream three and this amount of heat is removed in
exchanger (E-118) by external utility water available at 81 oC.
h) Matching of stream 4(1) (H-1) and cold utility
Since stream four was split in to two streams, each with 8.4KW of heat amount. Stream four (1)
has 8.4KW of heat and it is removed in exchanger (E-119) by matching with external utility
water available at 30 oC. Stream four (2) has 8.4KW of heat and exchange with exchanger(E-
120) by matching with external utility water available at 30 oC. The design for both heat transfers
between process to process and process to utility is shown below in the grid diagram figure 4.6.
Figure4.6 Heat exchanger networks for MER design AEA performs a heat integration using pinch technology. This heat integration is displayed in a
HEN diagram, showing which process streams or utilities enter and leave a given heat
exchanger. The heat exchangers on the grid diagram appear as colored disc lay on top of the
stream flowing through it. Each color indicates a type of heat exchanger: Grey color defines that
heat exchanger as a process to process exchanger. The heat exchanger is attached to two process
streams.
Blue color defines that heat exchanger as a cooler. In other words, the heat exchanger is attached
to a hot process stream and a cold utility stream. Therefore, from figure 4.6 the minimum energy
requirement is 167.69KW, the designed network in figure 4.6 above is meet the energy target.
34
4.3.1Network interval temperature calculations
The temperatures between each exchanger can be calculated using the energy balance equation.
Interval temperatures for process to process and for process to utility matches are calculated
below in table 4.2.
Match a (Heat exchanger E-111)
The supply and target temperature of stream 3 are 390 oC and 190
oC respectivly and with heat
capacityrate 0.79KW/ o
C. But match a was perform to cool stream 3 from 390 oC to unknown
temperature X with 39.639KW amount of energy from stream five(1). So to calculate this X
value the energy balance equationis used.
Q = CP*∆T
(390 - X)*0.79 = 39.635
X=339.82oC
Th interval values of other heat exchanger ara summerized in below in table 4.2
Table 4.2 Exchangers interval temperatures in the network before optimization
4.3.2 Optimization Of ∆Tmin Value
Range targeting contains information relevant to the optimization of the minimum approach
temperature. An optimum minimum approach temperature is calculated by minimizing the total
annual cost and it is finding the best balance between utility requirements, heat exchanger area
35
and unit and shell number. As the minimum approach temperature is varied the total annual cost
of the network is calculated and there is a ΔTmin which yield a minimum total annual cost.
For threshold problem, the optimum value occurs at the threshold temperature or it can be higher
than the threshold value and cannot be below the threshold temperature. Energy cost is constant
below this temperature but, only capital cost varies with ∆Tmin values. But if we increase the
optimum value to greater than threshold, the system changes from non utility end to near/pseudo
pinch problem or to pinched problem and needs both hot and cold utility as the result of
increasing the value of ∆Tmin value and this increases the operating cost. Therefore, for
threshold problem the optimum value occurs at the point where the summation of capital and
energy cost or total cost becomes minimum which is 13 oC as it shows in table 4.3 below. So the
optimum value of ΔTmin with minmum value of cost is at 13 oC.
Table 4.3 Optimization of ΔTmin value
Therefore, from the optimization of ΔTmin value we can understand that the minimum energy
requirement is not change with the variation of minimum temperature value as it shown in figure
4.4 above. Therefore, the designed HEN doesn’t need farther design at 13 oC as new value of
ΔTmin, because no change is observed at this value.
4.3.3 Optimization of Heat Exchanger Network
Network evolution is performed by optimizing the preliminary HEN by identifying loops and
paths within preliminary designs and shifting heat loads away from small, inefficient heat
exchange units to create less and more cost effective units. When optimization is carried out,
36
HEN with the maximum energy recovery from the initial design is simplified in terms of cost.
Decomposition at the pinch normally results in networks with at least one more unit than the
minimum number in the target. Manipulating with heat load loops and paths, stream splitting and
restoring ΔTmin, the final solution is improved in order to achieve an optimal HEN design.
Optimization of HEN begin by relaxing the restrictions imposed on preliminary heat exchanger
networks and allowing individual exchangers to operate below minimum approach temperatures
or transfer heat across the pinch, because during loop breaking pinch rules are not applied.
Energy relaxation is a procedure of allowing the energy usage to increase in exchange for at least
one of the following reasons: reduction in area and number of heat exchangers, and reduction in
complexity (typically less splitting).
Therefore, as it is shown from figure 4.6, the minimum energy requirement is 167.69KW and
maximum energy recovery (MER) value is 251.24KW, the designed network meets the energy
target. However, the minimum numbers of heat exchanger in the network in figure 4.6 are nine
which are greater than the targeted one that is seven. This may be due to the additional split
streams and existing of loops. Therefore, two heat exchangers should be removed. So the design
needs farther optimization step that is to design the Non-MER design. In order to fulfill the
prediction in the targeting stage, the number of exchangers has to be reduced.
Reducing the number of exchangers will definitely lower the capital cost of exchangers.
However, it will increase the cost for utilities (operating cost) for pinched problems but in this
problem which is threshold, the operating cost is constant.
4.3.3.1Loop breaking
Loop is a circuit in the network which starts at one exchanger and ends in the same exchanger.
Path is a circuit in the network that starts at a heater and ends at a cooler. The important feature
of loop is that, heat loads can be shifted around the loop from one unit to another to cause loop
breaking. The presence of loops in a HEN design may involve two statements. The designed
HEN has more units than the minimum number required, and it has more constraints in its
controllability. Based on this to design HEN, it is better to avoid loops whenever possible.
During loop breaking, the load is subtracted from the next and so on around the loop and this
load shift always keep the correct stream heat loads but the exchanger duties are changed
(Mohanty, 2010; K. Singh and R. Crosbie, 2011).
37
A loop exists between exchanger E-111 and E-114 and this loop must be broken but no path
exists in the HEN design as shown in figure 3.7 below.
Figure4.7 Loop in designed HEN Two heat exchangers are reduced during the optimization stage. The first exchanger is removed
by combining E-111 and E-114 into one exchanger. And the second exchanger is reduced by
adding up stream four into one branch.
Figure4.8 Optimized HEN design Therefore, the final optimized design is shown above in figure 4.8 with seven numbers of units
that meets the number of exchanger units in the targeting stage.
Detail information about each heat exchanger like connectivity and parameters are displayed as
below in figure 4.9.
38
Figure4.9 Detail information of heat exchanger 4.3.4 Network performance and controllability analysis
4.3.4.1 Network performance analysis
The performance tab on AEA brings up the table shown below in table 4.4 which gives the detail
about the effectiveness of the base case (target) heat integration calculation.
Table 4.4 Network performance before optimization
Table 4.4 provides the total amount of heating and cooling requirements, as well as the number
of heat exchangers and their shells. Also includes the summation of heat exchanger area in the
network. The % of target column in the table is significant, because it shows whether an
optimization to the HEN is achieved. From the design before optimization, the percent target of
heating and cooling, number of units and shells, and total area is displayed on the performance
tab view. Therefore, the heat exchanger network needs farther optimization, because 9 numbers
of units represent 128.6% of the target units which is 28.6% above target. The number of units
39
can be reduced by as much as 28.6% through optimization of the heat exchanger network.
Similarly, reduction on total shells and area is an outcome once optimization is performed; the
total area of the heat exchangers in the network will decrease by up to 15.3%.
Table 4.5 Network performance after optimization
The performance of the network after optimization is shown in table 4.5 above. From the design
after optimization, the percent target of cooling, number of units and shells, and total area is
displayed. The result shows that the energy requirement and number of units in the design
matches with the target value. But, number of shells in the design is 29.17% below the target and
total area is 15% above the target. This design can further be optimized to reduce the total area
but again it turns to increasing in ∆Tmin which results on increasing energy requirements and
also on number of shells and units. Therefore, HEN design is a matter of trade off between
∆Tmin, energy cost and capital cost to find the optimum design.
4.3.4.2 Network controllability analysis
Controllability status of the HEN design can be affected by different factors. The main factors
are: manipulated variables, sub networks, controlled variable, control constraints and number of
degree of freedom.
In a HEN design, the variables to be control are the process streams' outlet temperatures
(controlled variables). If the output temperatures are in control, then no possibility of
temperature fluctuation from the process streams that can affect the rest of the process. To
control the output temperature of the streams in the HEN design, it needs well manipulated
variables and degrees of freedom (DOF) to implement controls on to the design. The number of
manipulated variables in the HEN design equals the total number of heat exchangers in the
40
design and the number of control constraints equals to the number of loops that exists within the
design. However, each loop reduces the number of manipulated variables by one.
Sub networks are another factor that affects the controllability status of the design. A sub
network in the grid diagram is a set of streams that are heated or cooled within the set and does
not affect other streams in the entire HEN and three sub networks exist in this work as shown
below. The value of the degrees of freedom indicates whether the HEN design can be controlled
or not. The number of DOF is the difference between the manipulated variables (units) and the
sum of controlled variables (controlled streams) and number of loops for each sub networks.
Table 4.6 Network controllability status before optimization
From the above table 4.6, before optimization, the number of degree of freedom is greater than
zero in sub network three, indicates that there are enough manipulated variables in the HEN
design and can implement more sophisticate control structures.
41
Table 4.7 Network controllability status after optimization
From the above table 4.7 after optimization the number of degree of freedom is zero in all sub
networks, indicates that there is enough manipulated variables in the HEN design to control the
target streams which are process streams whose output temperature is controlled.
4.3.5 Potential heating and cooling savings of the network
The process has a minimum cooling demand of 167.69 KW and a heating demand of 0 KW as it
is calculated before. By comparing the minimum utility demands with the utility demands of the
existing system, it is possible to establish the potential for savings, as shown below in Table 4.8.
Table 4.8 Potential heating and cooling savings
Utility Present demand
(KW)
Minimum demand
(KW) by AEA
Potential for saving
(KW)
Potential for
saving (%)
Heating 125.62 0 125.62 100
Cooling 293.31 167.69 125.62 42.83
Total 418.93 167.69 251.24 59.97
4.4 Network Economic Analysis
4.4.1Network Cost Estimation
The economic parameters in AEA are required to calculate the capital cost and the annualization
factor of the heat exchangers in the HEN. Depending on the type of heat exchanger used in the
HEN the economic parameters changes. AEA has two types of heat exchangers. Each type has
its own formula for calculating the capital cost. Heat exchanger: This option considers the shell
and tube type exchangers, which uses convection to transfer energy. The capital cost is based on
42
the heat transfer area. Fired heater: This option considers the fired heater type exchangers; which
uses radiation to transfer energy. But, this study is based on shell and tube type of exchanger,
because the heat transfer mechanism between the fluids is convection transfer mechanism. A
typical HEN can have multiple heat exchangers types, and may be different material used to
construct the heat exchangers. Aspen energy analyzer provides a default cost set based on a
shell& tube exchanger type with carbon steel as the construction material. The basic economic
parameters used to calculate the cost of the heat exchanger network are capital cost, operating
cost and total annualized fixed cost (Burlington, 2011; S B Thakore and B I Bhatt , 2007).
a) Capital cost of heat exchangers
Capital cost is the fixed cost for purchasing and installing the heat exchangers. For each
exchanger in the network the capital cost is calculated below based on the following heat
exchanger capital cost formula by equation 4 below (S B Thakore and B I Bhatt , 2007; Silla,
2003; Max S. Peters, and Klaus D. Timmerhaus , 1991).
CC = a + b ∗ Area
Nshell 𝑐
∗ Nshell eq(4)
Where,
CC= installed capital cost of a heat exchanger ($)
a = installation cost of heat exchanger ($)
b, c = duty/area related cost set coefficient of the heat exchanger
Area (A) = heat transfer area of heat exchanger in meter square
NShell = number of heat exchanger shells in the heat exchanger
The heat exchanger capital cost index parameters from the aspen energy analyzer economics tab
view are displayed in table 4.9 below.
Table 4.9 Economics tab view for heat exchanger capital cost index parameters
43
The economics tab displays the cost set and economic parameter values used to calculate the
capital cost of the exchangers. A default set of economic parameters is supplied by AEA. And
the heat exchanger cost index parameters are:
a = 10000, b = 800, c = 0.8
And the plant life and operation days are taken 10 years and 300day/year respectively. Capital
cost for each exchanger is calculated below and all costs are in dollar. The annualization factor
accounts for the depreciation of capital cost in the plant. It must be considered since the capital
cost and operating cost of a heat exchanger network do not have the same units. Annual capital
cost is capital cost of the exchanger times the annualization factor(S B Thakore and B I Bhatt ,
2007). Both capital and annual capital cost are calculated below for each heat exchanger.
Heat exchanger (E-129)
Table 4.10 Parameters for heat exchanger E-129
From the above table 4.10 the values of area and cost parameters for E-129 are given below:
Area = 2.89m2
Capital cost ($) = = 1.187*10^4
44
Annual capital cost ($/s) = 9.764*10^-5
The total fixed capital cost and total annualized fixed capital cost of the heat exchangers are
summarized in the table 4.11below.
Table 4.11 Total annualized fixed capital cost of heat exchangers
Exchanger Duty (KW) Area (m2) Fixed capital
cost($)x104
Annualized
fixed capital
Cost ($/s) x10-5
E-129 79.28 2.89 1.178 9.764
E-128 39.97 0.945 1.081 8.854
E-127 2.92 0.08 1.010 8.31
E-116 3.45 0.075 1.010 8.307
E-118 78.722 6.92 1.376 11.32
E-117 72.166 2.57 1.170 9.625
E-120 16.8 3.80 1.233 10.14
Total 8.06 66.282
So the total annualized fixed capital cost is the summation of all heat exchangers cost which is
$66.282x10-5
/s or $17,180.2944/year.
b) Operating cost of utilities
The operating cost is a time dependent cost that represents the energy cost to run the exchangers
(S B Thakore and B I Bhatt , 2007 ;Max S. Peters, and Klaus D. Timmerhaus , 1991). For AEA,
the operating cost is dependent on the calculated energy targets in the HEN. On the utility
streams tab, utilities have costs associated with them. This cost information is required to
calculate the operating cost for the design. Operating cost of minimum heating & cooling utilities
are as follows:
Total operating cost is the summation of operating cost of heating and cooling utilities.
OC = ∑ (QHmin*Chu) + ∑ (QCmin*Ccu)
Where,
45
OC =operating cost ($/s)
QHmin=minimum energy required of hot utility (KW)
Chu = utility cost for hot utility ($/KJ)
QCmin=minimum energy required of cold utility (KW)
Ccu = utility cost for cold utility ($/KJ)
Since the problem is a threshold problem with only cold utility, there is no hot utility
requirement, so minimum energy required of hot utility is zero this means no operating cost for
heating utility.
OC = ∑ (QCmin*Ccu)
The cost index for the cold utilities is given below in table 4.12 in the utility stream tab on AEA.
Table 4.12 Utility streams tab for cost index of cold utility
From the final HEN design some process streams consumes utility to get their final target
temperature. Exchanger E-118, E-117 and E-120 are the exchangers that are connected with the
utility streams.
46
Table 4.13 Energy consumed and cost index of cold utility streams
SN Exchanger Energy
consumed
(KW)
Cost index
($/KJ)
x10-6
1 E-118 78.72 3.171
2 E-117 72.17 0.001
3 E-120 16.8 0.02125
Total 167.69
The operating cost is the energy consumed times the cost index for each utility streams given in
table 4.13 above.
Which is expressed as OC= energy (KJ/s) * cost ($/KJ).
Operating cost for exchanger E-118 is calculated as
OC= energy (KJ/s) * cost ($/KJ) = 78.72KJ/s*3.171x10-6
$/KJ =0.0002496211($/s)=
$6470.179/year
Operating cost for exchanger E-117
OC= energy (KJ/s) * cost ($/KJ) = 72.17KJ/s*0.001x10-6
$/KJ =7.217x10-8
($/s)= $1.87/year
And also the operating cost for exchanger E-120 is calculated below.
OC= energy (KJ/s) * cost ($/KJ) = 16.8KJ/s*2.125x10-7
$/KJ =3.57x10-6
($/s) = $92.5344/year
The total operating cost is the summation of all the above operating cost and is $ 6564.58/year.
4.4.2 Network Profitability Analysis
The maximum energy recovered during pinch analysis in the heat exchanger network design is
the amount of energy saved. The amount of energy saved by transferring heat from process to
process streams in HEN design is 251.24KW. The amount of saved energy is the amount of
income multiplying by its cost index of each stream.
47
Income from the saved energy is calculated as:
Income = energy saved (KW)*cost index ($/KJ)
Cost index is the summation of the two utility streams.
Income from exchanger E-129 is:
Income = 79.28KJ/s *6.342x10-6 =$4.964391x10-4/s =$12867.7015/yr and the income for other
exchangers is calculated in the table 4.14 below.
Table 4.14 Energy saved and cost index of cold utility streams
Exchanger Energy saved
(KW)
Process
Stream
Utility Stream Income
($/yr) Cost
index($/KJ)
x10-6
Cost
index($/KJ)
x10-6
E-129 79.28 3&5 3.171 3.171 12867.70
15
E-128 39.97 2&5 3.171 3.171 6570.453
02
E-127 2.92 2&7 3.171 2.2 406.5116
54
E-116 3.45 1&6 0.001 3.5 313.8782
4
Total 125.62 20157.73
96
Total income is the summation of the above incomes, which is $20157.7396/yr.
Then gross profit is calculated from total income (I) minus total production cost (Pc). But, in this
study the total production cost represents only operating and depreciation cost. Operating cost is
calculated before which is $6564.58 /yr and the depreciation cost is calculated below:
The uniform annual payment which made at the end of each year is the annual depreciation cost
(D). Analysis of costs and profits for any business operation requires recognition of the fact that
physical assets reduce in value with age. This decrease in value may be due to physical
deterioration, technological advances, economic changes, or other factors which ultimately affect
life of the property (Max S. Peters, and Klaus D. Timmerhaus , 1991). Depreciation cost is
calculated using the formula below.
D = (V-Vs)*i/ (1+i) N
-1
48
Where,
V =original value, assume it is equal with the fixed capital cost=$17180.2944
Vs =salvage value at the end of service life, assume zero value
i = annual interest rate, 7%
N = number of years
D = (17180.2944)*0.07/ (1+0.07)10
-1 = $1243.4668/yr
Gross profit (GP) is the profit before tax and is calculate below.
GP =I-Pc =20157.7396 – (6564.58 +1243.4668) =$12349.693/yr
Net profit is the profit after tax, with tax rate (t) of 30%.