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Programmes in Advanced Process Design
MSc DesignProject
2011-2012Design of Heat Exchanger
Network -
14th March, 2012
Submitted To: Simmon Perry
Project Advisor
Submitted By: Farhan Hafeez
ID: 8197194
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Contents1. Introduction .............................................................................................................................................. 3
2. Site Description ......................................................................................................................................... 4
2.1 Plant A: ................................................................................................................................................ 4
2.2 Plant B: ................................................................................................................................................ 4
2.3 Plant C: ................................................................................................................................................ 4
3. Process Data Extraction ............................................................................................................................ 5
3.1 Stream Identification: ......................................................................................................................... 5
3.2 Temperature-enthalpy profile: ........................................................................................................... 5
3.3. Mixing: ............................................................................................................................................... 5
3.4 Effective Temperature: ....................................................................................................................... 6
.................................................................................................................................................................. 63.5 Soft Constraints: .................................................................................................................................. 6
4. Energy Target: ........................................................................................................................................... 7
4.1 Delta Tmin selection: ............................................................................................................................. 7
4.2 Composite Curves ......................................................................................................................... 8
4.3 Problem Table ............................................................................................................................... 9
4.4 GrandCompositeCurve: ............................................................................................................... 11
4.5 Utilities selection ............................................................................................................................... 11
4.5.1 Plant A ........................................................................................................................................ 11
4.5.2 Plant B ........................................................................................................................................ 12
4.5.3 Plant C ........................................................................................................................................ 12
4.6 Utilities Summary .............................................................................................................................. 13
5. Economics ............................................................................................................................................... 14
5.1 Capital Cost Estimation ..................................................................................................................... 14
5.2 Heat Exchanger Area Targeting ........................................................................................................ 15
5.3 HEN Area Cost Calculations............................................................................................................... 17
5.4 Energy Cost Calculations ................................................................................................................... 18
6. Capital – Energy cost Trade-off and ∆Tmin............................................................................................... 21
7. Heat Exchanger Network Design ............................................................................................................ 22
7.1 Plant Data .......................................................................................................................................... 22
7.2 HEN Design Methods ........................................................................................................................ 23
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7.3 HEN Design Plant A ........................................................................................................................... 24
7.3 HEN Design Plant B ........................................................................................................................... 27
7.4 HEN Design Plant C ........................................................................................................................... 29
7.5 Sensitivity Analysis ............................................................................................................................ 30
8. Conclusion ............................................................................................................................................... 30
9. References: ............................................................................................................................................. 30
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1. IntroductionThis report is written in reference to Research Techniques and Methods (skills) module for
MSc in Advanced Process Design. The work assigned in this course is to design a total site system and is
named as MSc Design Project.
Groups comprising of four students have been made to accomplish this task and the design
project is divided into two main sections based on individual member’s work and group work. The
individual member has to design heat exchanger network for each plant in the total site and group task
is to design site utility system.
This report covers the design of heat exchanger network for each plant on total site as an
individual task for the design project.
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Figure 3: Flow sheet and Stream Data Plant C
2. Site DescriptionThe total site includes three plants named as Plant A, B & C respectively.
The data provided for each plant is limited to feed and product streams, process streams temperatures
and heat flows associated with each stream. Thus the thermodynamic analysis of each plant is based on
given data and each plant is discussed in detailed in the following sections.
A brief introduction of each plant is as follows:
2.1 Plant A:
Plant is consisted of two overall feed streams and two product streams with number of
intermediate streams from one unit to the other. Each stream has its own heating or cooling duties with
specified inlet and out temperatures from heat exchangers. The flow sheet for plant A is shown below:
2.2 Plant B:
Plant B is the most complex plant in the given case which involves four product streams
from two distillation columns in sequence and a feed stream to first distillation column after passing
through two more process units. The flow sheet of Plant B is shown below:
Figure 2: Plant B Flow Sheet
2.3 Plant C:
Plant C has two cold streams entering into a reactor with a single outlet hot stream
which is being cooled down. Moreover there is downstream process involves in plant C which requires
10MW of MP steam and 48MW of LP steam which are being supplied at 19.07and 3.62bara respectively.
Figure 1: Flow sheet Plant A
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3. Process Data ExtractionFollowing basic rules have been employed in extracting data for each plant:
3.1 Stream Identification:
It is important to identify the streams which are to be included in
heat integration analysis from a process thus only streams with known temperatures and heat flows are
considered for heat integration analysis for each plant i.e. in fig 4 of plant A, the stream circled red is not
considered due to unknown material and energy flow.
Similarly in plant B the red circled streams are not considered in analysis due to unknown or not fully
defined stream properties.
3.2 Temperature-enthalpy profile:
As the physical properties data for the streams is
unknown so we have assumed that the heat capacity of the streams in each plant is constant over the
range of temperature change with gives straight line when plotted these streams on temperature-
enthalpy diagram for heat exchanger area targeting unless until specified in the given data as in case of
one stream in plant C which is shown as follows:
In this case we have assumed three streams based on heat capacity change with temperature
change from 100 to 264C, 264 to 264C with phase change and 264 to 650C of a single stream.
The purpose of selecting this data is to avoid significant errors in heat integration analysis and heat
exchanger area targeting.
3.3. Mixing:
Mixing of two streams causes hidden heat transfer and thus this fact is considered in data
extraction for our process. This problem can be seen in plant A feed streams heating as follows:
Figure 5: Plant A under defined stream Figure 4: Plant B Undefined and Unknown Streams
Figure 6: Plant C Stream 2 Temperature-Enthalpy Data
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Here we have assumed heating of Stream 1 from 58
to 150C and stream 2 from 25 to 150C instead of 58 to
165C and 25 to 80C respectively to avoid heat of mixing
and thus error in heat integration analysis.
However the CP value for each stream is calculated with initial temperature change from 58 to165C and 25 to 80C respectively which is required to calculate heat load from heating these streams
from initial 58 and 25C temperature to final 150C temperature.
Stream No Type TS – ⁰C TT – ⁰C ∆H – kW CP – kW/⁰C
1 Cold 58 165 3380 31.59
2 Cold 25 80 185 3.36Table 1: Stream Data Plant A
These values of CP are used to calculate heat load of heat exchangers for each stream to be heated to
150C as follows:
Stream-1: Q = CP x (TT – TS) Similarly for; Stream-2: Q = CP x (TT – TS)
Q = 31.59 x (150-58) Q = 3.36 x (150-25)Q = 2906.28 kW. Q = 420 kW.
3.4 Effective Temperature:
Data is extracted from each plant based on availability of heat at its effective
temperature which can be used for heat recovery opportunities, e.g. the feed entering into the process
unit is at 380C in a section of plant B as follows:
However the unit outlet stream is at 400C which is being cooled
to 225C. Thus the availability of heat for heat recovery
opportunity is at 400C temperature and thus by extracting databased on this rule for every stream in each plant has provided
an opportunity to get error free heat integration analysis.
3.5 Soft Constraints:
Considering the flexibility of product storage temperature as a soft constraint we have
extracted the data for plant A and B following streams at different temperatures than given to reducethe complexity of heat exchanger network design which is shown in the later part of the report.
Figure 7: Plant A Mixing Streams
Figure 8: A Stream Data from Plant B
Figure 9: A Stream from Plant A and B -Soft Constraint
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Following is the streams data from each plant for heat integration analysis based on above rules.
Plant A:
Stream No Type TS – C TT – C ∆H – kW CP – kW/C
1 Cold 58 150 2906.28 31.59
2 Cold 25 150 420 3.36
3 Cold 35 225 2150 11.324 Hot 258 257 4270 4270
5 Hot 280 110 5760 33.88
6 Hot 80 55 5200 208
7 Hot 45 35 2600 260
8 Hot 450 80 4193 11.33Table 2: Stream Data Plant A
Plant B:
Stream No Type TS – C TT – C ∆H – kW CP – kW/C
1 Cold 25 380 30350 85.5
2 Cold 210 280 6475 92.53 Cold 350 351 8400 8400
4 Cold 278 279 4960 4960
5 Hot 400 225 28880 165
6 Hot 40 39 4080 4080
7 Hot 184 183 7430 7430
8 Hot 350 200 9700 64.66
9 Hot 200 199 1760 1760
10 Hot 278 67.5 6084 28.9Table 3: Stream Data Plant B
Plant C:
Stream No Type TS – C TT – C ∆H – kW CP – kW/C
1 Cold 25 650 96000 153.6
2 Cold 100 264 15600
264 264.1 34800
264.1 650 21000
3 Hot 650 170 14800 30.833Table 4: Stream Data Plant C
4. Energy Target:Now we evaluate the energy target for each plant with the above data. We will use each
plant’s composite curve, grand composite curve and problem table analysis using SPRINT (CPI software)
to determine the energy targets for respective plant.
4.1 Delta Tmin selection:
To generate above mentioned tools for energy targeting using SPRINT
software we need an initial value of minimum approach temperature i.e. ∆Tmin. The value of ∆Tmin sets
the energy recovery target between hot and cold streams of the process which is quite evident from
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composite curves of the process and is discussed in following section. We select an initial value of 10C
for each plant based on experience and high energy cost these days.
4.2 Composite Curves:
The composite curves are produced by plotting all the hot and cold
streams in a process on temperature-enthalpy diagram and the enthalpy change is the sum of individual
stream enthalpy change in a certain temperature range. Two different curves are plotted one is termed
as hot composite curve which contains all the hot streams in a process and second is termed as cold
composite curve which contains all the cold streams of the process. Composite curve is a tool which is
used to determine the process-to-process heat recovery, hot and cold utility targeting at certain
minimum temperature difference (∆Tmin) between hot and cold streams.
Following are the composite curves of our system for individual plant at ∆Tmin 10C.
The red and blue curve in above figure is hot and cold composite curves respectively for plant A.
It is noticeable from the curve above that cold stream does not extend beyond the start of hot stream
and ends at same enthalpy value indeed. This type of problem is called as threshold problem where
there is no need of either hot or cold utility in the process. Here in this case the threshold value for both
the curves is at start of hot composite curve of the process thus there is no need of hot utility in this
system as all the heat required for the process will be provided by process-to-process heat recovery,
whereas relatively large quantity of cold utility is required. The overlap area of the hot and cold
composite curve shows the process-to-process heat recovery, In our case the heat recovery comes out
to be 5475kW.
Figure 10: Composite Curve Plant A
Figure 11: Composite Curve Plant B
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For plant B the composite curve shows that there is a huge process-to-process heat recovery
opportunity however in this case the hot composite curve extends beyond the start of cold composite
curve and vice versa. Thus we need both cold and hot utility for our process. The value for process-to-
process heat recovery in this case comes out to be 45821kW.
For plant C is also a threshold problem at ∆Tmin of 10⁰C with no cold utility requirement and
process heat recovery is 143800kW.
4.3 Problem Table:
The composite curve gives us the potential of process-to-process
heat recovery and hot and cold utility requirement but it does not clearly shows the temperatures at
which these utilities are required and exact values of utilities load as there could an error in reading
these values from graph. Moreover it’s difficult to construct composite curves without computer aided
programs. Thus to overcome this problem we haveused another technique which gives same results
with more accuracy and both heat loads with
required temperatures for utilities targeting.
In constructing problem table we follow the same
technique of dividing streams into temperature
intervals as in case of producing composite curves
but in this case we shift hot streams by -∆Tmin/2
and cold streams by ∆Tmin/2 to keep feasible
temperature difference for heat transfer. Theenthalpy change is then calculated in each interval
and finally the heat is cascaded down from high
temperature side to low temperature side of the
process. We get some negative heat flows in the
process of heat cascading down which is not
feasible for heat flow thus we add external heat
Plant A - ∆Tmin 10C
Interval Temperature* Enthalpy
[C] [kW]
445 0
275 1926.1
253 2920.72
252 7235.93
230 8230.55
155 10773.1
150 10767.8
105 10210.7
75 8822.97
63 10763.9
50 13277.2
40 13130.5
30 16546.9
Table 5: Problem Table Plant A
Figure 12: Composite Curve Plant C
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equal to highest negative value in the system to make it at least zero and the temperature at which the
heat flow value becomes zero is termed as pinch temperature. This external heat is actually the hot
utility target and the heat left at the end of the table is cold utility target. Adding ∆Tmin/2 in highest
temperature in the table gives us the required temperature of the hot utility and subtracting ∆Tmin/2
from lowest temperature gives the required temperature for cold utility.
SPRINT software is again used to construct the problem table for each plant.
The threshold problem for plant A is also evident from problem table as the
Value of external heat requirement for process heating is zero at the shifted temperature of 445C.
It is the pinch temperature of the process which divided the process into heat source and heat sink
which will be discussed further in following part of the report. Furthermore the maximum cold utility
temperature required for the process is at 25C which is achieved by subtracting ∆Tmin/2 from lowest
temperature in the problem table corresponding to minimum cold utility requirement i.e. 16546.7 kW.
In case of plant B the pinch
temperature is at 355C and 4363.65kWof hot utility is required at the
temperature of 400C. (Adding ∆Tmin/2 in
highest temperature in the table) The
area below the pinch is heat source
area and it require cold utility which in
this case is 12124.4kW at 25C.
(Subtracting ∆Tmin/2 in lowest
temperature in the table).
Similarly for plant C the minimum hotutility temperature comes out to be
660C from problem table and there is
no need for cold utility requirement as
all the process streams lie above the pinch region. The heat duty
of hot utility is found to be 23600kW.
Plant B -
Interval Temperature* Enthalpy
[C] [kW]
395 4363.65
385 6013.93
356 8320.46
355 0
345 795.356
285 9447.49
284 9499.2
283 4590.9
273 5107.92
220 9380.03
215 8957.91
195 9119.46
194 10822.9
179 9974.03
178 17347.4
62.1 10788.7
35 8471.86
34 12466.4
30 12124.4
Plant C
Interval
Temperature* -
C Enthalpy – kW
655 23600
645 21518.5
270 55807.8
269 21153.7
165 26443.3
105 11520
30 0
Table 6: Problem Table Plant B
Table 7: Problem Table Plant C
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4.4 GrandCompositeCurve:
After setting targets for process-to-process heat recovery
and hot and cold utility targets with their required temperature levels we need to select appropriate
utilities for the process. This objective is attained using grand composite curve which is obtained by
plotting problem table on temperature-enthalpy curve. The grand composite curve presents true
interface between the utilities and process conditions and clearly shows the process division into
heat sink and heat source regions in a graphical manner. The hot utility is supplied in heat sink area
and cold utility is supplied in heat source area. Moreover grand composite curve is used to target
multiple utilities in a process. Discussion on each plant grand composite curve is incorporated in the
following section of utilities selection.
4.5 Utilities selection:
SPRINT software is used again to produce grand composite curves for each plant as follows:
Figure 13: Balanced Grand Composite Curve Plant A
4.5.1 Plant A
Grand composite curve of plant A (only red curve in above figure) shows that sufficient
heat is available at higher temperatures so steam generation can be considered. Maximum saturated
steam temperature that can be achieved in plant A is around 250 which can be superheated to 400C at
∆Tmin of 10C, we need saturated MP steam at 19.07bara for downstream process heating of plant C so
we have considered to produce MP stream in plant A.MP steam produced in plant A is at 20 bar
Saturated conditions with 212.37C saturation temperature. MP steam pressure is considered to be a bar
above the required pressure in downstream process to accommodate pressure and temperature losses
and it is assumed that there is no more than one bar pressure drop in steam supply to downstream
process. The utilities targets obtained from balanced grand composite curve are as follows:
MP Steam = 9673.19kW and Cooling water = 6873kW
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4.5.2 Plant B
The balanced grand composite curve for plant B is shown in the following figure:
Similar to plant A the plant B has also potential to use steam as cold utility and here in this case
we have considered to produced MP steam at same level as at in plant A production and for plant C
downstream process heating. The utility targets for plant B from balanced grand composite curve are:
MP Steam = 8471kW and Cooling water = 3652kW.
Hot utility is also required in plant B at minimum temperature of 360C with a duty of 4363kW.
This temperature is high enough to make steam heating unsuitable because maximum steam saturation
temperature (supercritical temperature) at supercritical pressure is 372.15C which is practically
infeasible to use for process heating, moreover superheated steam at high temperature is also not
feasible for process heating due to low heat transfer coefficients and local condensation at lowertemperatures. For these reasons hot oil can be used as hot utility in plant B. Hot oils can achieve higher
temperature at relatively lower pressures and can be used both in vapor and liquid phase for process
heating. Thus hot oil at 400C will be used for this process heating. The return temperature of hot oil is
also vital in setting energy targets and it is considered to be at hot stream pinch temperature of 360C for
our case.
4.5.3 Plant C
The balanced grand composite curve for plant C is shown as follows:
Figure 14: Balanced Grand Composite Curve Plant B
Figure 15: Balanced Grand Composite Curve Plant C
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Two hot utilities have been used in this case one is flue gas at 750C and other is LP steam at
4 bar saturated conditions to produce balanced grand composite curve. It is clearly seen from the
balanced grand composite curve of plant C that there is a demand of hot utility at much higher
temperature but with relatively small heat load. The big heat pockets in the process have potential to
supply remaining high level heat. Moreover it is the most energy intensive process as total hot utility
requirement with maximum process-to-process heat exchange is 23600kW. Around 12905kW of heat
can be supplied by flue gas and remaining 10695kW can be provided by LP steam but we will not use LP
steam as a major portion of flue gas would be wasted at lower heat level so instead of using LP steam at
lower temperature level we can utilize lower temperature level flue gas heat left after exchanging at
higher temperature for heating. And if we look at the stream data of plant C then it is quite evident that
all the heat required for this plant is to be supplied to cold feed streams and the only heat source(hot
stream) is product stream which put a significant constraint on maximum process-to-process heat
recovery as more than 167400kW of heat would be required at least 660C for plant start-up which is to
be provided by some outside heat source. Thus for these practical constraints we have divided the plant
into two sections as follows:
Thus by dividing Plant C we can use furnace flue gas for cold stream heating and hot stream heat will
be used for high pressure steam generation which can be used for power production and plant C
downstream LP steam requirement, it can also supply heat to hot oil for plant B heating as heat is
available at high temperature and in sufficient quantity.
The MP steam requirement for plant C will be provided by MP steam generated in Plant A and B.
4.6 Utilities Summary
The following table contains the utilities selected for each plant:
Plant Hot Utility
Requirement -
kW
Cold Utility
Requirement - kW
Hot Utility
& Load - kW
Cold Utility
& Load – kW
A 0 16546 - MP Steam – 9673
CW – 6872
B 4363 12124 Hot Oil – 4363 MP Steam – 8472
CW – 3652
C 167400 143800 Flue Gas - 167400 HP Steam – 143800Table 8: Utilities Data - Plant A,B and C
Cold Streams
Hot Stream
Plant C +
Figure 16: Plant C Section
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5. EconomicsOne of the most important factors to be considered in designing a process is the
process economics. There are number of parameters to be evaluated in the economic analysis of a
process but we will consider the capital cost of heat exchanger network and energy cost for our
processes in this part of report. In second part of the design project includes the capital cost estimation
of other major equipment in total sight utility system.
5.1 Capital Cost Estimation
Capital cost involves the cost of constructing a new plant or a piece of
equipment. There are five generally acceptable methods to estimate capital cost in process industry as
follows:
a. Order of Magnitude Estimate.
b. Study Estimate.
c. Preliminary design Estimate.
d. Definitive Estimate.e. Detailed Estimate.
A brief introduction of each method is carried out in the following table:
Method Main Features
Order of Magnitude Estimate This estimate is usually based on the previous cost
data of same process and it involves the whole
process estimate which is then corrected by using
scaling factor.
Study Estimate The approach to this method is based on order of
magnitude estimate method but in this case costestimation of major equipment in a process based
on their approximate sizing is carried out.
Preliminary Design Estimate This estimate requires more accurate equipment
sizing with some approximation of piping,
electrical and instrumentation requirements
Definitive Estimate This estimate is based on preliminary specification
of major equipment, utilities, piping, electrical and
instrumentation.
Detailed Estimate This Estimate is as close to actual cost as possible
and is based on detailed engineering of the
equipment, utilities etc. with vendors quotes.Table 9: Description of Capital Cost Estimation Techniques
We have selected to do the cost estimation of heat exchanger network based on Study Estimate due to
limited time and data resource.
Cost data available for equipment would be out of date, based on some years ago and using this data
could result in significant error in cost estimation. To take this thing into account we use “Escalating
Factor” which is defined as below:
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A =
………………..eq-01
Where,
CEPCI is the chemical engineering plant cost index, it is published on monthly basis and includes major
equipment cost indices. CEPCIbase year is the year in which the equipment given cost is available.
There are other similar cost indices available in the market which can be used for updating the cost of equipment using same formula as given in equation 04,however we have data available for heat
exchangers based on CEPCI so our calculations are based on this index.
Thus by using escalating factor the cost of equipment in current year is calculated as;
Ccurrent year = Cbase year A
Where
Ccurrent year and Cbase year are the cost of equipment in current year and base year respectively and;
We might encounter a problem where the base year equipment cost would be based on certain capacity
which might be different from current equipment capacity. This problem can be resolve by using
following equation:
Ccurrent year = Cbase year ( ) ………………….eq-02
Where,
Ccurrent year and Cbase year are the cost of equipment in current year and base year respectively and;
Q current year and Q base year are the equipment size in current scenario and base year case
And “m” is the power factor which is mostly known for some major equipment in process industry and if
its value is unknown then it can be taken as 0.6 which turn the above equation into sixth-tenth rule as
most of the equipment cost due to change in size varies around sixth-tenth to the original equipment
size
Some other factors to be considered in cost estimation of an equipment are pressure factor(FP),
temperature factor(FT), design factor(FD) and material of construction factor(FCM). All these factors are tobe considered in cost estimation of equipment based on current case and base year case.
Thus the final equation for cost estimation of equipment can be written as:
Ccurrent year = Cbase year A FPFD FCM FT (
)
……………………eq - 03
5.2 Heat Exchanger Area Targeting
Heat exchanger area account for a significant cost of
manufacturing and it should be considered in designing a heat exchanger area network. The area is
inversely proportional to ∆Tmin so by increasing ∆Tmin the area can be reduced. Thus selection of ∆Tmin
for a given process heat exchanger network design is vital in terms of capital investment.
Composite curves not only give the energy targets for a process but these can also be used to
estimate the heat transfer area of a network. For area targeting we produce composite curve with
utilities included and these composite curves are termed as “balanced composite curves”. The balance
composite curves are divided into different enthalpy intervals based on kinks in the curve. Vertical heat
transfer in each interval is calculated and the cold and hot streams temperature considered are the
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terminal temperature of the interval. We also consider that the overall heat transfer coefficient remains
constant in each interval and thus with this assumption we calculate area of heat exchanger in each
interval by using following equation:
Ainterval =
Where,
Ainterval = the area of an interval
Q interval = heat transfer between hot and cold stream in one particular interval
U = overall heat transfer coefficient
LMTD =
= log mean temperature difference between cold and hot stream in one
particular interval, h1& h2 and c1&c2 are the hot and cold stream inlet and outlet terminal temperature
respectively.
The network area is calculated by the summation of each interval area as follows:
ANetwork = ∑
…………………………….eq-04
Where ANetwork = heat exchanger area of vertical heat transfer for whole network
K = total number of enthalpy intervals
Now we consider the area targeting for plant A heat transfer by using following balanced composite
curve:
The balanced composite curve for plant A is divided into 16 intervals and by assuming 4 kW/m 2- C value
of heat transfer coefficient (U), we calculated the area of heat transfer in each interval using eq-04 and
the results are tabulated in the following table:
Figure 17: Blocked Balanced Composite Curve Plant A
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3500
3550
3600
3650
3700
3750
0 10 20 30
A r e a - m 2
∆Tmin- ⁰C
HEN Area vs ∆Tmin
Similarly we have estimated the area of heat exchanger network for Plant A at different values of ∆Tmin.
The results are shown in the following table:
This data in above table and accompanying graph clearly shows that the area decrease with increase in
∆Tmin. This data is produced only for Plant A due to study the relationship of heat exchanger area vs
∆Tmin. Similarly same data can be produced from plant B and C but due to time constraint and complex
utilities involvement in plant B and C these calculations are restricted to Plant A only.
5.3 HEN Area Cost Calculations
Based on equipment cost estimation method as discussed
earlier we now calculate the cost of heat exchanger network (HEN) area (as calculated in above section )
for plant A.
The escalating factor is calculated for April, 2011 as the latest data available for CEPCI was in this month
and the base year is 2001.
Interval Q interval kW LMTD Ak – m2
1 2600 4.867923 133.5272
2 4290.34 9.259437 115.837
3 16.81 1.189795 3.532121
4 337.63 8.058595 10.47422
5 555.22 6.208295 22.35799
6 340 16.10384 5.278245
7 1279.39 27.97374 11.43385
8 2885.81 53.86313 13.39418
9 1819.9 50.50524 9.008472
10 661.6 2.00597 82.45387
11 4315.2 0.334114 3228.838
12 994.8 2.85725 87.04175
13 1784.9 17.55247 25.42234
14 141.7 12.52 2.829473
AreaNetwrok 3644.289Table 10: Enthalpy, LMTD and Area Data for Plant A - Decomposed Balanced Composite Curve at ∆Tmin 10C
∆Tmin - C Area – m2
2.5 3732.592
7.5 3663.14
10 3644.289
12.5 3626.003
15 3583.583
17.5 3565.436
20 3549.73
Table 11: HEN area vs ∆Tmin Data - Plant AFigure 18: HEN Area vs Tmin Graph
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A =
= = 1.69
FCM and FDthe material of construction and design factors are considered to be the same as per original
equipment.
The pressure factor (FP) is considered based on stream pressure i.e. 20 bar and temperature factor (FT) is
based on maximum temperature of the system i.e. 450C. The values for these factors and power factor“m” in eq-06 is taken from “Chemical Process Design and Integration” by Robin Smith†.
FP = 1.5, FT = 2.1 and m = 0.68
Base Equipment capacity = 1000m2 and Base equipment cost = $ 105,000
Thus cost of HEN with area 3644.289m2 (∆Tmin 10C) for Plant A is as follows,
C2011 = C2001 A FPFD FCM FT
Thus by putting all the values in above equation we get the cost of HEN
C2011 = 105000x1.69x1.5x1x1x2.1x
= 1347000 $ or 1.347 MM$similarly the cost for each network for different Tmin values are calculated and tabulated in the following
table,
The cost for heat exchanger network follows the same downward trend as of area for increase in ∆Tmin
value which is quite evident from the above table.
5.4 Energy Cost Calculations
High energy price has put a strict condition in giving priority to design
minimum energy requirement processes. For this reason heat exchanger network has become more
significant than ever in order to meet process heating and cooling demands with minimum external
energy requirements. For energy analysis of a system the cost of energy is vital and there are certain
ways in which we can calculate the utilities cost. For this report steam pricing and hot oil cost estimation
is considered. Price of some common fuels is given in the following table:
Fuel Price
Coal 4.72 $/GJ
∆Tmin - C Area – m2 Cost – MM$
2.5 3732.592 1.369
7.5 3663.14 1.352
10 3644.289 1.347
12.5 3626.003 1.343
15 3583.583 1.332
17.5 3565.436 1.327
20 3549.73 1.323
Table 12: HEN Cost Data for Different ∆Tmin Values
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Heavy Oil 909.01 $/t
Electricity 0.10608 $/kWh
Natural Gas 0.0319 $/kWhTable 13: Fuel Prices
Moreover the cost of cooling water is considered to be 1% of the price of electricity. The steam price
calculations are as follows:
Steam Pressure – bar a Temperature - C
HP 40 400 – super heated
MP 20 212.37 - Saturated
LP 4 143.7 - SaturatedTable 14: Steam Conditions for Total Site
The above table shows the different steam levels we have selected for our system. HP steam is at 40 bar
and 400C superheated temperature. Its cost calculation is as follows;
Amount of heat required to produce HP steam = Q HP = hsh-hBFW - kJ/kg
From steam table, hsh = 3214 kJ/kg
Boiler feed water is available at 105C, so hBFW = 4.2 (105-25)(relative to make-up water at 25C)
= 336 kJ/kg
Q HP = 3214 – 336
= 2878 kJ/kg
Thus the fuel required to produce HP steam = Q HP x Costfuel x
Assuming boiler efficiency of 0.90 and natural gas as fuel the cost of HP steam comes out to be
CostHP =
= 0.0283 $/kg or 28.3 $/t
Cost of MP steam = CostHP – Valuepower produced
HP steam is expanded isentropic ally to 20 bar MP steam conditions, thus;
W = hHP - hisp
Whereas hisp = isentropic enthalpy of steam after turbine
And hisp = hHP – ƞ(hHP – hg)
Whereas hg = enthalpy of steam at 20 bar and
Ƞ = turbine isentropic efficiency which is assumed to be 0.87 in our case
Again using steam table hHP = 3214 kJ/kg
Hg = 2799 kJ/kg
Thus, hisp = 3214 – 0.87(3214-2799)
= 2852.95 kJ/kg
From steam table this enthalpy value at 20 bar dictate the steam temperature of 231.5C which is higher
than the saturated temperature of 212.37C, thus the steam at turbine outlet is superheated and we
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calculate our steam price on these basis as some degree of superheat would be preferable to
accommodate thermal losses in steam supply.
So W = 3214 – 2852.95
= 361.05 kJ/kg
Values of power =
= 0.0106 $/kg
Cost of MP steam = 0.0283 – 0.0106
= 0.0177 $/kg or 17.7 $/ton
Similarly the cost of LP steam is calculated as,
CostLP = CostMP – Valuepower generated
MP steam is expanded isentropically to 4 bar LP steam conditions, thus;
W = hMP - hisp
Whereas hisp = isentropic enthalpy of steam after turbine
And hisp = hMP – ƞ(hMP – hg)
Whereas hg = enthalpy of steam at 4 bar and
Ƞ = turbine isentropic efficiency which is assumed to be 0.87 in our case
Again using steam table hMP = 2852.95 kJ/kg
Hg = 2739 kJ/kg
Thus, hisp = 2852.95 – 0.87(2852.95-2739)
= 2753.81 kJ/kg
From steam table this enthalpy value at 4 bar dictate the steam temperature of 150.46C which is higher
than the saturated temperature of 143.7C, thus the steam at turbine outlet is superheated and we
calculate our steam price on these basis as some degree of superheat would be preferable to
accommodate thermal losses in steam supply.
So W = 2852.95 – 2753.81
= 99 kJ/kg
Values of power =
= 0.00292 $/kg
Cost of LP steam = 0.0177 – 0.00292
= 0.0147 $/kg or 14.7 $/ton
Fuel Oil Calculations:
Cost of hot oil = Cost of pumping + cost of fuel
Cost of pumping is not considered in this system as it requires detailed engineering calculations, thus
only fuel cost is considered as follows,
CHO = Q HO x Cfuel x
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1.3
1.32
1.34
1.36
1.38
0 5 10 15 20 25
C o s t - M M $
∆Tmin - ⁰C
Capital-Energy Cost Trade-Off - ∆Tmin
Total Cost Capital Cost Energy Cost
Q HO = Cp (Ttarget – Tsupply)
Ttarget = 400C and Tsupply = 360C (temperatures selected using grand composite curve of plant B)
Cp = 2.527 kJ/kg K at 360C
Thus Q HO = 2.527 (400-360)
= 101.08 kJ/kg
CHO =
= 0.0001 $/kg or 0.1$/ton
Initial start-up cost of hot oil would be much higher than this running cost as more heat would be
required for hot oil heat up from storage temperature to required process heating temperature.
6. Capital – Energy cost Trade-off and ∆Tmin
We have seen so far that the capital cost
of heat exchanger network is associated with ∆Tmin, higher the value of Tmin less would be the overall
area but another significant parameter to be considered in selection of Tmin is energy cost. Energy cost
of a process increase with increase in Tmin due to missing opportunities of process to process heat
recovery in a heat exchanger network. Thus the optimum value of Tmin is determined by a trade-off
between capital and energy cost.
Now we consider the Tmin selection for Plant A. We have already calculated the heat exchanger
network area cost at different values of Tmin and we also know that Plant A has a threshold problem with
cold utility requirement only. The cold utility for plant A is MP steam generation and cooling water. Onlythe cost of cooling water will be used to evaluate the energy cost variation with change in Tmin as the
steam is being generated from process heating without external source.
Figure 19: Capital-Energy Trade Off Curve – Plant A
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0
10
20
30
0 5 10 15 20 25 30 C o o l i n g w a t e r
t e m p e r a t u r e - ⁰ C
Tmin - ⁰C
CW Temperature vs ∆Tmin
Cooling Water Temperature
From the figure above it is quite evident that there is no change in utility cost for plant A with T min
as it is a threshold problem for certain value of T min so the total cost follows the capital cost downward
trend with Tmin. Thus in this case it is no possible to select optimum value of T min for the heat exchanger
network. However another interesting fact related to change in Tmin is the change in utility temperature
requirement for process heating or cooling. Quality of heat is equally important as the quantity of heat
and it is observed that the maximum cold utility temperature decrease with increase in Tmin as shown in
the following figure:
Thus from the above figure we have selected the Tmin value of 10C for Plant A heat exchanger network
design as below this value the required cooling water temperature decreases our set target of 25C.
similarly for plant B and C we have selected the Tmin of 10C based on experience of determining this
value for plant A.
7. Heat Exchanger Network DesignNow as we have produced enough data for each plant,
we can design the heat exchanger network for plant A, B and C.
7.1 Plant Data
Following table contains all the data that we have generated so far from the given site processes.
Plant ∆Tmin - ⁰C Heat Recovery -
kW
Q Hmin & Utilities
(kW)
Q Cmin & Utilities
(kW)
A 10 5476.62 Total - 16546
MP Steam – 9673CW – 6873
B 10 45821 Hot Oil - 4363 Total – 12124
MP Steam – 8471
CW – 3652
C 10 - Flue gas -167400 HP steam –
143800Table 15: Data Summary Plant A,B and C
Figure 20: Cooling water vs ∆Tmin Curve
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As we have discussed already in utility selection section of this report that plant C has been divided
into two section containing cold and hot streams separately. Thus in above table all the hot utility
requirement will be fulfilled by furnace flue gas and HP steam will be generated from hot stream.
Another important information that is required during the design on heat exchanger network is
process and utility pinch temperatures(these are shifted temperature by ∆Tmin. Following table containsthis data for each plant:
Plant Process Pinch Temperature – ⁰C Utility Pinch Temperature - ⁰C
A 455 75
B 355 35
C 30 -Table 16: Pinch Data Plant A,B and C
7.2 HEN Design Methods
There are several methods available for design of heat exchanger network but we have discussed the
following methods briefly in this section:
a. Pinch Design Method.
b. Superstructure Optimization.
c. Stochastic Optimization (Simulated Annealing)
Pinch Design Method:
This method is based on pinch analysis in which a problem is divided across the pinch and design
starts from the pinch. The rules to design heat exchanger network based on pinch analysis are defined
near the pinch however there are no certain rules for design away from pinch. Energy targets are fully
met when a network is design on this method, moreover it gives a freedom to take decision by designerand incorporate all the process insight. However this method becomes complicated when a large
number of streams and multiple utilities are involved.
Superstructure Optimization:
In this method all the possible combinations for one stream’s heat exchange to other stream are
made using superstructure and the final result is optimized to acceptable number of heat exchangers
and streams. This method is based on automated design and it does not involve designer decisions thus
the process insight cannot be incorporated into this design method. Furthermore it become really
complex when large number of streams is involved due to difficult mathematical problem.
Stochastic Optimization:
In this method an initial feasible network is provided by the designer which is then exploited for
possible matches between each hot and cold streams. Perturbation moves like repiping, resequencing,
add/remove heat exchanger or stream split are set with their probabilities. Then the simulated
annealing parameters like initial temperature, final temperature, Markov chain length are set. This
method is analogous to physical annealing). Initial and final temperatures control the bad moves and
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markov chain length governs time for each match. Finally the design is optimized to get a feasible
solution. This method produce different design in every solution thus a designer can select the best
solution for a given process. This method takes relatively large time to generate the design and thus an
optimum solution is not possible in practical time, to overcome this problem non-linear programming is
used to obtain optimum results.
For this design project we have selected Pinch Design Method with maximum energy recovery in
plant A and B because the number of streams and utilities involved in each process are manageable and
it will give us a non-reducible energy efficient heat exchanger network design.
7.3 HEN Design Plant A
Plant A has five hot streams and three cold streams and it is a threshold
problem with no hot utility requirement. Based on pinch design method we first divide the process
across process and utility pinch. As a threshold problem the process pinch is at the beginning of heat
source area of the problem(shown in problem table section 4.3) thus the problem is divided across the
utility pinch only. Table 2 has stream data for plant A and Table 14 & 15 section 7.1 contains the datarequired to design maximum energy recovery heat exchanger network for plant A. Using this data we
generate the grid diagram using SPRINT which is the best representation of all the process streams to be
included for design of heat exchanger network with their temperature, enthalpy and CP data. Moreover
it also shows the process division across the pinch as follows:
However it was a threshold problem but multiple utilities have changed into a pinch problem. The grid
representation of the process streams shows some interesting facts about the location of process
streams. It is quite clear from the above figure that almost half of streams lie away the pinch and it
increase the complexity in the design process as no certain rules are defined for matching streams away
the pinch. Moreover we start the design of network from zero utility end of the process as there is no
utility to achieve the required temperature and then we move towards the rest of the process.
Following general rules are applied for making matches at pinch:
Above the Pinch: Number of hot stream, NH < Number of cold Stream NC and CPH< CPC
Figure 21: Grid Diagram Plant A
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1N: 1
258 257
25 7DH :4270 CP:4270
2N: 2
280 110
11 0DH :5760 CP:33.8 824
3N: 3
45 35
35DH :2600 CP:2 60
11
FF:0.152
13
FF:0.159
14
FF:0.629
12
FF:0.061
15
FF:1
4N: 4
80 55
55DH :5200 CP: 208
5N: 5
450 80.2 4
80D H:4193. 33 CP:1 1.3333
6N: 6
5815 0
15 0DH:379.065CP:31.5888DH:2527.1CP:31.5888
7N: 7
2515 0
15 0DH:151.364CP:3.36364DH:269.091CP:3.36364
8N: 8
3522 5
22 5DH:396.053CP:11.3158DH:1753.95CP:11.3158
9N: 9
2530
30DH:6873.52CP:1374.7
10N:10
10 5212.47
212.37DH:9673.19CP:90.0044
1
1
N:11
N:12
25 7
212.45
*Q:4270A:94.7223S: 0
2
2
N:13
N:14
184.58
212.39
*Q:3233.06A:67.3166S: 0
3
3
N:15
N:16
11 0
15 0
*Q:2527A:67.8843S: 0
8
8
N:23
N:24
35
26.89
*Q:2600A:190.4S: 0
7
7
N:25
N:26
68
70
*Q:379.065A:37.9018S: 0
9
9
N:27
N:28
68
70
*Q:396.053A:20.5583S: 0
10
10
N:29
N:30
67.99
70
*Q:151.364A:6.6912S: 0
11
11
N:31
N:32
68
30
*Q:1569.52A:35.3899S: 0
M1 N:33
68
12
12
N:34
N:35
55
28.86
*Q:2704A:81.1493S: 0
4
4
N:17
N:18
258.74
212.47
*Q:2167.57A:18.5349S: 0
5
5
N:19
N:20
23 5
15 0
*Q:269.091A:1.9944S: 0
6
6
N:21
N:22
80.24
22 5
*Q:1753.95A:173.318S: 0
80
70
H1
H2
H3
H4
H5
C1
C2
C3
Cooling Wate
MP Steam
Below the Pinch: Number of hot stream, NH > Number of cold Stream NC and CPH> CPC
CP rules are of significant importance as the temperature difference should increase on moving
away from pinch and this objective is achieved by following the CP rules. In matching streams away the
pinch it has been considered to supply heat from lowest minimum temperature e.g. in this case the
matches with hot streams and steam generation are made first at higher temperatures and then the low
temperature heat is supplied to cold process streams as they required heat at relatively lower
temperature than steam. Another problem associated with using steam generation as a cold utility is the
phase change of the water from liquid to gas. This change takes place at constant temperature and thus
a higher temperature than the saturation temperature of the steam is to be maintained in a relatively
bigger area of the exchanger to maintain minimum design approach temperature. This limit further
made the design more complex. Furthermore below the pinch only hot stream 4 (inlet temperature 80C)
can exchange heat with three cold streams(outlet temperature 70) due to temperature constraints and
one of cold stream i.e. stream 6 has inlet temperature of 58C which is higher than stream 6 outlet
temperature. This problem was addressed by setting stream 6 outlet temperature at 68C after
exchanging heat will all the streams through split stream rule. And to achieve exit temperature of 68C at
the end mixing point one of split stream is being cooled with cooling water. This constraint resulted in
an extra cooler for the process.
Following above mentioned rules and considering the process limitation we have design the
following maximum energy recovery heat exchanger network for plant A at Tmin of 10C:
This network has zero cross pinch heat transfer and following table contains the heat exchanger
network report generated by SPRINT software:
Total Hot
Utility =
0 [kW] MER Heat Exchanger Network Report Plant A
Total cold
Utility =
16544.1 [kW]
Figure 22: MER Heat Exchanger Network - Plant A
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Original Optimized
Each Hot Cold Minimum Hot Hot Cold Cold Area Area
No. Stm Stm Approach In Out In Out [m^2] [m^2]
[C] [C] [C] [C] [C]
F 1[H1 ]cu 1 10 44.61 258 257 212.39 212.45 94.72 94.72
2[H2 ]cu 2 10 28.79 280 184.58 105 212.39 67.32 52.53
3[<Unna] 2 6 34.58 184.58 110 70 150 67.88 59.53
4[<Unna]cu 5 10 46.3 450 258.74 212.45 212.47 18.53 10.44
5[<Unna] 5 7 108.74 258.74 235 70 150 1.994 2.126
6[<Unna] 5 8 10 235 80.24 70 225 173.3 49.69
7[<Unna] 4:11 6 10 80 68 58 70 37.9 0
F
8[<Unna]cu
3 9 10 45 35 25 26.89 190.4 190.4
9[<Unna] 4:13 8 < 10.00> 80 68 35 70 20.56 0
10[<Unna] 4:12 7 < 10.00> 80 67.99 25 70 6.691 0
11[<Unna]cu4:14 9 39.14 80 68 28.86 30 35.39 0
12[<Unna]cu
4:15 9 28.11 68 55 26.89 28.86 81.15 136.3
Original Optimized
Process Minimum Approach Temperature 10 [C] 44.94 [C]
Utility Minimum Approach Temperature 10 [C] 10 [C]
Overall Minimum Approach Temperature 10 [C] 10 [C]
Table 17: Network Report MER Heater Network - Plant A and Area Comparison with Optimized Network
As it is clear from the table above that there is no violation of Tmin in any heat exchanger of the networkbut the highlighted areas of exchanger are too small and practically infeasible thus we optimized the
network for minimum number of heat exchangers, the results are compared in same table above and
following is the optimized heat exchanger network:
Figure 23:Optimized MER Heat Exchanger Network- Plant A
1 N: 1
2 5 8 2 5 7
25 7D H : 4 2 7 0 C P : 4 2 7 0
2 N: 2
2 8 0 1 1 0
11 0D H : 5 7 6 0 C P : 3 3 . 8 8 2 4
3N: 3
4 5 3 5
35D H : 2 6 0 0 C P : 2 6 0
13
FF:0
11
FF:*****
12
FF:0
14
FF:1
15
FF:1
4 N: 4
8 0 5 5
55D H : 5 2 0 0 C P : 2 0 8
5N: 5
4 5 0 8 0 . 2 3
80D H : 4 1 9 3 . 3 3 C P : 1 1 . 3 3 3 3
6N: 6
5815 0
15 0D H : 3 7 9 . 0 6 5C P : 3 1 . 5 8 8 8D H : 2 5 2 7 . 1C P : 3 1 . 5 8 8 8
7N: 7
2515 0
15 0D H : 1 5 1 . 3 6 4C P : 3 . 3 6 3 6 4D H : 2 6 9 . 0 9 1C P : 3 . 3 6 3 6 4
8N: 8
352 2 5 . 0 1
22 5D H : 3 9 6 . 0 5 3C P : 1 1 . 3 1 5 8D H : 1 7 5 3 . 9 5C P : 1 1 . 3 1 5 8
9N: 9
2530
30D H : 6 8 7 3 . 5 2C P : 1 3 7 4 . 7
10N : 1 0
10 52 1 2 . 4 7
2 1 2 . 3 7D H : 9 6 7 3 . 1 9C P : 9 0 . 0 0 4 4
1
1
N : 1 1
N : 1 2
25 7
2 1 2 . 4 5
* Q : 4 2 7 0A : 9 4 . 7 2 6 3S: 0
2
2
N : 1 3
N : 1 4
1 9 5 . 7 7
2 1 2 . 3 9
* Q : 2 8 5 3 . 9 9A : 5 2 . 5 3 2 4S: 0
3
3
N : 1 5
N : 1 6
11 0
15 0
* Q : 2 9 0 6 . 0 7A : 5 9 . 5 2 8 1S: 0
8
8
N : 2 3
N : 2 4
35
2 6 . 6 7
* Q : 2 6 0 0A : 1 8 9 . 1 1 4S: 0
7
7
N : 2 5
N : 2 6
80
58
* Q : 0A: 0S: 0
9
9
N : 2 7
N : 2 8
80
35
* Q : 0A: 0S: 0
10
10
N : 2 9
N : 3 0
80
25
* Q : 0A: 0S: 0
11
11
N : 3 1
N : 3 2
80
30
* Q : 0A: 0S: 0
M1 N : 3 3
80
12
12
N : 3 4
N : 3 5
55
30
* Q : 5 2 0 0A : 1 3 6 . 3 1 6S: 0
4
4
N : 1 7
N : 1 8
3 0 7 . 0 5
2 1 2 . 4 7
* Q : 1 6 2 0 . 1 5A : 1 0 . 4 3 6 1S: 0
5
5
N : 1 9
N : 2 0
2 6 9 . 9 5
15 0
* Q : 4 2 0 . 4 5 5A : 2 . 1 2 6 1 9S: 0
6
6
N : 2 1
N : 2 2
8 0 . 2 3
2 2 5 . 0 1
* Q : 2 1 5 0 . 1A : 4 7 . 6 9 0 4S: 0
80
70
H1
H2
H3
H4
H5
C1
C2
C3
Cooling Wate
MP Steam
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1N:1
25380.1
380DH:427.465CP:85.493DH:27357.7CP:85.493DH:2564.79CP:85.493
2N:2
210280
280DH:6475CP:92.5
3N:3
350351
351DH:8400CP:8400
4N:4
278279
279DH:4960CP:4960
5 N:5
400 225
225DH:6601.14 CP:165.029DH:22278.9 CP:165.029
6 N:6
3 50 200
200D H: 97 00 C P: 64 .6 66 7
7 N:7
1 84 183
183DH:7430 CP:7430
8 N:8
40 39
39DH:4080 CP:4080
9 N:9
2 00 199
199DH:1760 CP:1760
10 N:10
2 78 67.1
67.1D H: 60 95 .7 5 C P: 28 .9 03 5
11N:11
2530
30DH:3652.54CP:730.507
12 N:12
400 360
360DH:4363.65 CP:109.091
13N:13
105212.47
212.37DH:8471.86CP:78.8266
360
350
40
30
C1
C2
C3
C4
H1
H2
H3
H4
H5
H6
CW
Hot Oil
MP
This network has eliminated all the process-to-process heat exchanger(dashed lines) below the
pinch and this load is shifted to process-to-process heat exchangers above the pinch but in doing so
there is 926kW of cross pinch exchanger heat flow which has resulted in load shift from MP steam utility
to cooling water thus total cooling requirement remained the same as shown in following cross pinch
heat transfer report of above network:
Minimum Hot Utility = 0 [kW]
Minimum Cold Utility = 16546.7 [kW]
Process heat recovery= 5476.62 [kW]
Total Hot utility = 0.00000 [kW]
Total Cold utility = 16544.1 [kW]
Total Cross Pinch heat transfer = 0.00000 [kW]
Exchanger cross pinch heat transfer
3 3 [<Unnamed> ] 379.065 [kW] Pinch Number 1
5 5 [<Unnamed> ] 151.364 [kW] Pinch Number 1
6 6 [<Unnamed> ] 396.053 [kW] Pinch Number 1Total exchanger cross pinch heat flow = 926.482 [kW]
Table 18: Cross Pinch Report - MER Heat Exchanger Network Plant A
As the cost of cooling water is not high and there is slight decrease in steam production load thus the
optimized network is feasible both in terms of exchanger cost and utility load. Additional benefit of
reduced area for some heat exchanger is obtained in optimized network due to high driving force as
increased process to process minimum approach temperature shown in table 16.
7.3 HEN Design Plant B
Following the same pinch design method as we used in plant A we designed
maximum energy recovery heat exchanger area for plant B. The grid diagram for plant B is as follows:
Figure 24: Grid Diagram Of Plant B
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Heat Exchanger Network Design Page 28
1N:1
25380.1
380DH:427.465CP:85.493DH:27357.7CP:85.493DH:2564.79CP:85.493
2N:2
210280
280DH:6475CP:92.5
3N:3
350351
351DH:8400CP:8400
17
FF:0.214
18
FF:0.786
19
FF:1
4N:4
278279
279DH:4960CP:4960
5 N:5
400 225
225DH:6601.14 CP:165.029DH:22278.9 CP:165.029
6 N:6
350 200
200DH:9700 CP:64.6667
7 N:7
184 183
183DH:7430 CP:7430
20
FF:0.105
21
FF:0.895
22
FF:1
8 N:8
40 39
39DH:4080 CP:4080
9 N:9
200 199
199DH:1760 CP:1760
10 N:10
278 67.1
67.1DH:6095.75 CP:28.9035
11N:11
2530
30DH:3652.54CP:730.507
15
FF:0.588
14
FF:0.412
16
FF:1
12 N:12
400 360
360DH:4363.65 CP:109.091
13N:13
105212.47
212.37DH:8471.86CP:78.8266
M2N:45
351
1
1
N:14
N:15
360
351
*Q:6601.14A:268.994S:0
6
6
N:24
N:25
318.05
350.08
*Q:6923.31A:283.138S:0
5
5
N:22
N:23
287.99
279
*Q:4960A:232.669S:0
7
7
N:26
N:27
247.02
269.1
*Q:6761.29A:195.901S:0
10
10
N:40
N:41
225
212.45
*Q:3633.97A:166.916S:0
4
4
N:20
N:21
249.87
280
*Q:6475A:120.961S:0
11
11
N:32
N:33
200
212.4
*Q:3225A:105.816S:0
8
8
N:28
N:29
183
169.43
*Q:7430A:166.983S:0
13
13
N:36
N:37
39
30
*Q:427.465A:35.9572S:0
15
15
N:38
N:39
39
30
*Q:3652.54A:307.245S:0
M3 N:46
39
9
9
N:30
N:31
199
190.01
*Q:1760A:97.543S:0
12
12
N:34
N:35
222.45
212.47
*Q:1605.72A:54.3733S:0
16
16
N:42
N:43
67.1
82.52
*Q:4489.9A:57.9618S:0
2
2
N:16
N:17
360
351
*Q:1798.46A:73.2969S:0
3
3
N:18
N:19
360
380.1
*Q:2566.3A:178.968S:0
M1 N:44
360
360
350
40
30
C1
C2
C3
C4
H1
H2
H3
H4
H5
H6
CW
Hot Oil
MP
The complexity of design in this case is increased due to two pinches a process pinch at 355C and
utility pinch at 35C. Moreover between these two pinch temperatures we can see from the grid diagram
that the process streams are quite far from pinch thus causing more design difficulty due to no certain
rules. In same manner as in plant A we will transfer heat at lowest possible temperature when matching
hot and cold streams. Steam generation here poses the same constant temperature phase change
problem in network. Another interesting feature of plant B is the availability of sufficient heat but not at
high temperatures which add more complexity to the process. Thus by considering the same phase rules
as in plant A with plant B process constraints the HEN design is as follows:
Since the hot oil returned temperature is selected by process pinch temperature of 360C so the hot oil
utility stream is spit to maintain Tmin constraint for heating cold streams whose inlet temperatures are at
360C. Hot streams match with MP steam generator heat exchanger is designed in a way that the outlet
temperature of hot stream remained above 222.27C i.e. 10C plus steam saturation temperature to
overcome constant temperature phase change problem. Streams at lower temperatures with high heat
content are used to heat cold streams to a certain temperature without violating Tmin constraint and
remaining heat to the cold streams is provided by hot streams at higher temperature. This approach
made the design more complex as one exchanger on each stream could not fulfill its heating or cooling
requirement.
The Network report for this heat exchanger network generated using sprint as follows:
Exch Hot Cold Minimum Hot Hot Cold Cold Area DutyNo. Stm Stm Approach In Out In Out [m^2] [kW]
[C] [C] [C] [C] [C]
1[H1 ] 5 3:18 10 400 360 350 351 269 6601
2[Hu1 ]hu 12:14 3:17 < 10.00> 400 360 350 351 73.3 1798
3[Hu2 ]hu 12:15 1 < 9.92> 400 360 350.08 380.1 179 2566
F 4[H2 ] 6 2 39.87 350 249.87 210 280 121 6475
Figure 25: MER Heat Exchanger Design Plant B
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F 5[H3 ] 5 4 < 9.99> 318.05 287.99 278 279 232.7 4960
6[H4 ] 5 1 < 9.92> 360 318.05 269.1 350.08 283.1 6923
7[H5 ] 5 1 18.89 287.99 247.02 190.01 269.1 195.9 6761
F 8[H6 ] 7 1 14.57 184 183 82.52 169.43 167 7430
F 9[H7 ] 9 1 < 9.99> 200 199 169.43 190.01 97.54 1760
10[<Unna]cu 5 13 12.6 247.02 225 212.4 212.45 166.9 3634
F 11[G1 ]cu 6 13 13.58 249.87 200 105 212.4 105.8 3225
12[G2 ]cu 10 13 < 9.99> 278 222.45 212.45 212.47 54.37 1606
13[H10 ] 8:20 1 < 10.00> 40 39 25 30 35.96 427.5
15[Cu1 ]cu 8:21 11 10 40 39 25 30 307.2 3653
16[<Unna] 10 1 37.1 222.45 67.1 30 82.52 57.96 4490
Process Minimum Approach Temperature 9.92 [C]
Utility Minimum Approach Temperature 9.92 [C]
Overall Minimum Approach Temperature 9.92 [C]Table 19: Network Report MER Heat Exchanger Network Plant B
The minimum approach temperature of this network is found to be 9.92C and this values is a result of
two heat exchangers with 9.92C Tmin i.e. exchanger 3 and exchanger 6 and this could be result of some
calculation error as the heat load and temperature are considered to 3 decimal places and the
complexity of design involved large number of fractional values for the heat load and temperatures.
Similarly the heat transfer across the pinch is negligible and might be the result of same error because
only 6.7kW of cross pinch heat transfer is carried out from same heat exchanger 6 as shown below:
*DTmin = 10 [C]
Minimum Hot Utility = 4363.65 [kW]
Minimum Cold Utility = 12124.4 [kW]
Process heat recovery= 5476.62 [kW]
Total Hot utility = 4364.76 [kW]
Total Cold utility = 12117.2 [kW]
Total Cross Pinch heat transfer = 1.11646 [kW]
Exchanger cross pinch heat transfer
6 6 [H4 ] Pinch Number 1 -6.74845 [kW]
Total exchanger cross pinch heat flow = -6.74845 [kW]
There is no cross pinch mixingTable 20: Cross Pinch Report - Plant B
7.4 HEN Design Plant C
As we have discussed earlier in utility selection part of this report that
plant C hot stream will used to produce HP steam and its cold streams will be heated by furnace flue
gas. Thus the design of heat exchanger network for plant C is straight forward and does not involve any
complexity as follows:
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All the cold stream heating is carried out by furnace flue gas and all the heat from hot stream is used to
produce steam.
7.5 Sensitivity Analysis
Impact of a change of process input on process out can be estimated by
defining equipment depended and equipment independent relationships. Equipment dependent
relations are usually called as design equation where equipment independent relationships are material
balance, energy balance etc. or those relationships which do not require equipment specification.
For heat exchangers equipment dependent relationships are area equations i.e. Q = U A ∆TLMTD.Thus change in heat load due to change in temperature or process demand in any plant can affect the
performance of heat exchanger area network.
8. ConclusionA number of tools and techniques used to process data in a way to achieve required
target of designing heat exchanger network for three plants in a total site. The design stages involved
complex data handling which seemed to be relatively simple and straight forward in the beginning.
Economic data gathering and analysis was the most challenging part of the design project followed by
maximum energy recovery heat exchanger design for each plant. However the results obtained fromheat exchanger network meet the design parameters and energy targets for the process.
9. References:1] Richard Turton/C.Bailie/Wallace B.Whiting/Joseph A Shaeiwitz “Analysis, synthesis, and Design of
Chemical Processes, Third Edition. P: 177-182, 230, 583-584, 923-941.
[2]Fuel Price Data –Departmen of Energy and Climate Change , Quarterly Energy Prices, December 2011-
Section3 –Industrial Prices Table 3.1.1
<http://www.decc.gov.uk/publications/basket.aspx?filepath=statistics%2fsource%2fprices%2fqep312.xl
s&filetype=4#basket
[3]Chemical Engineering (April 2011) “ Economic Indicators: Chemical engineering Plant Cost Index.”
Viewed 11/02/2012
< http://www.che.com/PCI>
[4]ROBIN SMITH. Chemical Process Design and Integration. P:17-20
Figure 26: Heat Exchanger Network Plant C
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[5]SIMON PERRY. Utility System Handouts 2011-2012. P:L08-6
[6]SIMON PERRY. Energy System Handouts 2011-2012. P:L03-L1
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