Study Guide
Pinch Technology Heat Exchange Networks
-
University:
Kentucky State University -
Course:
CHE 102 | General Chemistry I Academic year:
2023
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Views:
234
Pages:
42
Author:
Madelyn Bradshaw
Chapter 15 – Pinch Technology
Heat Exchange Networks
Chemical Process Design
1 •
•
Outline
Heat Integration
Design Procedure for MUMNE
–
–
–
–
–
Temperature interval diagram
Cascade diagram
Temperature-Enthalpy diagram
Minimum number of exchangers
Design above and below pinch
2 Heat Integration
• Heat exchange networks
• It saves money to match streams rather
than pay to heat one and pay to cool
another
• You have already done this on ad hoc
basis in design projects
3 Heat Integration
• There is a rigorous methodology
• We will learn MUMNE (Minimum
Utility, Minimum Number of
Exchangers) method
• Not necessarily (and unlikely to be)
economic optimum
4 Design Procedure
1. Complete energy balance on all streams to
& p values, and heat
determine all temperatures, mC
flows.
2. Choose minimum approach temperature. Typically,
this is between 5°C and 20°C, but any positive
number is valid.
3. Complete temperature interval diagram, Each
stream is drawn and labeled. The heat flow in each
interval is calculated.
5 Design Procedure
4. Complete the cascade diagram. The energy excess
or deficit is calculated for each interval on the
temperature interval diagram.
5. Find the minimum hot and cold utility
requirements and identify the pinch temperature.
6. Complete the composite temperature enthalpy
diagram. This is a T-Q diagram for the entire
process.
6 Design Procedure
7. Determine the minimum number of heat
exchangers required above and below the pinch.
8. Design the heat exchanger network.
7 Example Problem
Stream Properties for Example Problems
Stream
Tin
Tout
C p
m
kW/°C
Q
kW
1
200
120
3
240
2
140
100
5
200
3
100
170
3
-210
4
110
190
2
-160
Net heat
flow
70
8 Example Problem
1. The value of Q might not be given in above table,
& p is missing. These are
or Q is given and mC
calculated from the energy balance. The sign
convention is positive for heat available from a
stream and negative for heat needed by a stream.
2. Choose the minimum approach temperature. For
this problem, it is 10°C.
9 Example Problem
3. Draw and label the temperature interval diagram.
Label the intervals beginning with “A” for the
highest temperature interval. The heat flow for
& p T , where
each interval is calculated from, Q mC
the sum is over all streams existing in that interval.
10 11 12 Example Problem
4. Draw the cascade diagram. This represents the
cascade of heat flowing down from high to low
temperatures. Add utilities where needed. Label
the heat flows. The net utility flow should agree
with the net heat flow on the earlier table.
5. On the cascade diagram, there will be a location
where the heat-flow cascade is not continuous.
This represents the pinch temperature
13 Cascade Diagram
A
20
Hot
Utility
60
20
B
-80
Pinch Temp
130-140C
C
60
60
D
20
80
E
50
130 Cold
Utility
14 Example Problem
6. Construct the composite temperature enthalpy
diagram. This provides useful information, but it is
not required to solve the problem.
15 Example Problem
Hot
interval
E
D
C
B
A
T
(°C)
100
110
120
140
180
200
Cold
Q
(kW)
50
100
260
380
440
T
(°C)
90
100
110
130
170
190
Q
(kW)
30
130
330
370
16 Example Problem
In the table, the temperature shown is at the lower end
of the interval. The Q values are obtained by summing
& p T existing on the interval and adding it to the
all mC
previous interval. The temperature difference is for
& p value is the sum of all existing
that interval. The mC
streams on that interval.
17 Example Problem
The hot and cold stream lines are plotted, as shown on the
following figure. Clearly, there is a temperature cross, so the
cold stream line is shifted to the right until the minimum
approach temperature of 10°C exists at one point. (It could
exist at more than one point by coincidence.) For this
problem, all Q values for the cold stream must be increased
by 130 kW, as shown in the figure. Note how the hot and
cold utility requirements are apparent from the diagram.
18 QH=60
200
180
T(°C)
160
140
10°C - minimum approach temp.
120
100
Qc=130
0
100
200
300
400
500
600
Q(kW)
hot stream
cold stream
cold stream adjusted
Copyright - R. Turton and J. Shaeiwitz,
Composite
T-Q
Diagram
2012
19 Example Problem
7. By representing the heat available in each stream
and from the utilities both above and below the
pinch, the minimum number of heat exchangers
can be determined. This identifies the minimum
number, but not necessarily the correct stream
matches. The correct number of heat exchangers
is the number of process streams + the number of
utility streams – 1.
20 above pinch - 3 exchangers
1
HU
180
60
below pinch - 4 exchangers
60
60
60
3
120
120
4
120
2
200
1
60
3
90
30
40
4
40
130
CU
130
each arrow identifies one heat exchanger,
total number of arrows is total number of hxs.,
but not necessarily the correct stream matches
numbers in boxes are energy in streams
numbers with arrows are energy transferred in
a heat exchanger
21 Example Problem
7. Note that if a “direct match” is found, i.e.,
where sets of two streams match heat flows
exactly, one fewer exchanger may appear to
be possible. However, be careful, the
minimum approach temperature may be
violated.
22 Example Problem
8. Design the heat exchange network. There
may not be unique streams here. The
design is started at the pinch and you work
away from the pinch. Above the pinch, for
any streams that exist at the pinch, streams
can only be matched such that
C p m
C pC
m
H
23 Example Problem
8. When dealing with streams away from the
pinch, this criterion is no longer needed.
Any streams can be matched as long as the
temperatures are valid. If the criterion at
the pinch appears impossible to satisfy,
streams can be split to satisfy the criterion.
24 Copyright - R. Turton and J. Shaeiwitz,
2012
25 Copyright - R. Turton and J. Shaeiwitz,
2012
26 27 28 Example Problem
8. The same procedure is done below the pinch,
except that the criterion is
& pH mC
& pC
mC
9. Streams are matched and heat exchangers are
added until all required heat transfer is
accomplished. The entire network, both above
and below the pinch, can then be represented on
one diagram.
29 30 31 32 In-class Example Problem
Stream Properties for In-class Example Problem
Stream
Tin
Tout
m
Cp
kg/s
kJ/kg°C
1
250
100
1
1
2
280
120
2
2
3
100
200
1
2
4
120
230
1
5
33 In-class Example Problem
Determine (minimum approach T = 20°C)
a. minimum hot and cold utility consumption
b. pinch temperatures
c. minimum number of heat exchangers
required above and below the pinch
d. design of heat exchange network above and
below the pinch
34 35 Cascade Diagram
A
120
120
B
0
120
Hot
Utility
40
C
-160
Pinch Temp
120-140C
D
60
60
E
20
80
Cold
Utility
36 above pinch - 4 exchangers
2
560
1
110
10
110
3
160
550
HU
40
40
4
550
below pinch - 2 exchangers, if possible
1
40
40
3
40
2
80
80
CU
80
37 1
4
split
38 39 40 41 Summary
• Heat Exchange Networks
• Well-established procedure
• Not necessarily (and unlikely to be) economic
optimum but a very good starting point
• Straight forward, but must be careful when
matching streams at pinch
• Different correct answers possible
42
Pinch Technology Heat Exchange Networks
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