Heat Exchanger Network
Synthesis, Part III
Ref: Seider, Seader and Lewin (2004), Chapter 10
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Instructional Objectives
• This Unit on HEN synthesis serves to expand on
what was covered in the last two weeks to more
advanced topics.
•
Instructional Objectives - You should be able to:
– Extract process data (from a flowsheet
simulator) for HEN synthesis
– Understand how to use the GCC for the optimal
selection of utilities
– Have an appreciation for how HEN impacts on
design
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Data Extraction
Process analysis begins with
the extraction of “hot” and
“cold” streams from a process
flowsheet
Required:
 The definition of the
“hot” and “cold” streams
and their corresponding
TS and TT
 CP for each stream is
either approximately
constant or H=f(T).
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What is considered to be a stream ?
 In general: Ignore existing heat exchangers
 Mixing: Consider as two separate
streams through to
target temperature.
 Splitting: Assume a split point
wherever convenient.
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Example – Dealing with Real Systems
o Toluene is manufactured by dehydrogenating n-heptane.
o Furnace E-100 heats S1 to S2, from 65 oF to 800 oF.
o Reactor effluent, S3, is cooled from 800 oF to 65 oF.
o Install a heat exchanger to heat S1 using S3, and thus
reduce the required duty of E-100.
a) Generate stream data using piece-wise linear
approximations for the heating and cooling curves for
the reactor feed and effluent streams.
b) Using the stream data, compute the MER targets for
Tmin = 10 oF.
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Example – Dealing with Real Systems
Heating of liquid
Equivalent, piece-wise
flowing heat capacity:
C 
k
h h
T T
k 1
k
k 1
k
Heating of vapor
Evaporation of n-heptane
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Example – Dealing with Real Systems
Equivalent, piece-wise
flowing heat capacity:
C 
k
h h
T T
k 1
k
k 1
k
Cooling of vapor
Condensation
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Example – Dealing with Real Systems
Equivalent, piece-wise flowing heat capacity: C 
k
8
h h
T T
k 1
k
k 1
k
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Example – Dealing with Real Systems
(b) MER Targeting:
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Class Exercise 7
a) Extract data for hot and cold streams from the
flowsheet below.
b) Assuming Tmin = 10o, compute the pinch temperatures,
QHmin and QCmin.
c) Retrofit the existing
50
o
W
CP = 0.4
network to meet MER.
40o
C
H = 100
100o
130o
H = 100
CP = 1.0
125
o
H
30o
150o
C
H
140o
10
CP = 0.6
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Class Exercise 7 - Solution
W
50o
CP = 0.4
40o
C
130o
100
H = 100
o
H = 100
CP = 1.0
125o
H
30o
150o
C
H
140o
11
CP = 0.6
Tmin = 10 oC
Stream
TSS
(ooC)
TTTT
((ooC)
C)
H
H
(kW)
(kW)
CP
(kW/oC)
Feed
Bottoms
Cond
Recyc
Reb
130
150
40
50
150
100
30
40
140
150
30
72
100
36
100
1.0
0.6

0.4

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Class Exercise 7 - Solution (Cont’d)
Tmin = 10
oC
Stream
T
o
( C)
T
(oC)
H
(kW)
CP
(kW/oC)
Feed
Bottoms
Cond
Recyc
Reb
130
150
40
50
150
100
30
40
140
150
30
72
100
36
100
1.0
0.6

0.4

S
T
T1 = 150oC
QH
Assume
QH = 0
Eliminate infeasible
(negative) heat transfer
QH = 100
H = -100
H = 0
Q1
-100
0
-96
4
-60
40
-52
48
60
160
66
166
T2 = 140oC
H = 4
Q2
o
T3 = 120 C
H = 36
Q3
o
T4 = 90 C
H = 8
This defines:
Cold pinch temperature = 140oC
QHmin = 100 kW
QCmin = 166 kW
Q4
T5 = 50oC
H = 12
Q5
T6 = 30oC
H = +100
H = 6
QC
T7 = 20oC
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Class Exercise 7 - Solution (Cont’d)
HEN Representation of existing flowsheet
CP
130o
100
150o
30o
40o
40o
o
1.0
Feed
Botts
0.6
Cond

140o
Recy
150o
150o

Reb
QHmin = 100
13
0.4
QCmin = 166
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Class Exercise 7 - Solution (Cont’d)
Retrofitted
HEN Representation
flowsheet – oneofadditional
existing flowsheet
match for MER
CP
100
130o
C
Feed
Botts
1.0
30
150o
90o
30o
140o
100
125o
H
H
100
QHmin = 100
14
6
Reb
40o
C
Cond
150o
0.6
C
72
36
40o
150o
o
50o
Recy

0.4
30
36

Tmin violation
QCmin = 166
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Heat Integration in Design
The Grand Composite Curve
 An enthalpy cascade for a process
is shown on the right.
 Note that QHmin = QCmin = 1,000
kW
 Also, TC,pinch = 190 oC
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The Grand Composite Curve (Cont’d)
The Grand Composite Curve presents the same enthalpy
residuals, as follows:
Minimum external
heating, at 310 oC
Internal heat
exchange
TC,pinch
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Internal heat
exchange
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The Grand Composite Curve (Cont’d)
Alternative heating and cooling utilities can be used, to
reduce operating costs:
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The Grand Composite Curve (Cont’d)
Example:
GCC:
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GCC Example (Cont’d)
Possible designs using CW and HPS:
Umin = 4 + 2 – 1 = 5
How many loops?
Does this design meet
Umin ? If not, what is
the simplest change
you can make to fix it?
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GCC Example (Cont’d)
Returning to the GCC:
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GCC Example (Cont’d)
Possible designs using CW, BFW, LPS and HPS:
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Heat Integration in Design
Heat-integrated Distillation
 Distillation is highly energy
intensive, having a low
thermodynamic efficiency (as
little as 10% for a difficult
separation), but is widely used
for the separation of organic
chemicals in large-scale
processes.
 Thermal integration of columns
can be done by manipulation of
operating pressure.
Need to
position column
carefully on
composite curve
 Note: Qreb  Qcond for columns
with saturated liquid products.
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Heat-integrated Distillation (Cont’d)
Option A: Position distillation
column between hot and cold
composite curves:
(a) Exchange between hot
and cold streams
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(b) Exchange with cold streams
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Heat-integrated Distillation (Cont’d)
Option B:
2-effect distillation:
(a) Tower and heat
exchanger
configuration;
(b) T-Q diagram.
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Heat-integrated Distillation (Cont’d)
Option B: Variations on two-effect distillation:
(a) Feed Splitting (FS)
(b) Light Split/forward heat integration (LSF)
(c) Light Split/Reverse heat integration (LSR).
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Heat-integrated Distillation (Cont’d)
Option C: Distillation
configurations involving
compression:
(a) heat
heat pumping
pumping
(a)
(b) vapor
vapor recompression
recompression
(b)
(c)
(c) reboiler
reboiler flashing
flashing
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Heat-integrated Distillation (Cont’d)
Option C: Distillation configurations involving compression:
(a) heat pumping
(b) vapor recompression
(c) reboiler flashing
 All 3 configurations involve the expensive compression of a vapor
stream.
 May not be cost-effective except where pressure changes required are
small. Example: separation of close-boiling mixtures
For further reading:
Smith, R., “Chemical Process Design and Integration”, Wiley, 2005, Chapter 11.
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Heat Integration - Summary
• Data Extraction
– Getting data for HEN synthesis from material
and energy balances (i.e., from simulator)
• Heat Integration in Design
– Use of Grand Composite Curves for selection
of utilities
– Options for heat-integrated distillation
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