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8 - Heat & Power Integration1
Heat Exchanger Network Synthesis, Part III
Ref: Seider, Seader and Lewin (2004), Chapter 10
8 - Heat & Power Integration2
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
8 - Heat & Power Integration3
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).
8 - Heat & Power Integration4
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.
8 - Heat & Power Integration5
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.
8 - Heat & Power Integration6
Example – Dealing with Real Systems
Equivalent, piece-wise flowing heat
capacity:k 1 k
k
k 1 k
h hC
T T
Evaporation of n-heptane
Heating of vapor
Heating of liquid
8 - Heat & Power Integration7
Example – Dealing with Real Systems
Equivalent, piece-wise flowing heat
capacity:k 1 k
k
k 1 k
h hC
T T
Cooling of
vapor
Condensation
8 - Heat & Power Integration8
Example – Dealing with Real Systems
k 1 k
k
k 1 k
h hC
T T
Equivalent, piece-wise flowing heat capacity:
8 - Heat & Power Integration9
Example – Dealing with Real Systems
(b) MER Targeting:
8 - Heat & Power Integration10
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
network to meet MER.
W
C
H
HC
H = 100
H = 100
CP = 0.6
CP = 0.4
CP = 1.0
130o 100o
40o
50o
125o
140o
150o 30o
8 - Heat & Power Integration11
Class Exercise 7 - Solution W
C
H
HC
H = 100
H = 100
CP = 0.6
CP = 0.4
CP = 1.0
130o 100o
40o
50o
125o
140o
150o 30o
Stream TS
(oC) TT
(oC) H
(kW) CP
(kW/oC)
Feed Bottoms
Cond Recyc Reb
Stream TS
(oC) TT
(oC) H
(kW) CP
(kW/oC)
Feed 130 100 30 1.0 Bottoms 150 30 72 0.6
Cond 40 40 100 Recyc 50 140 36 0.4 Reb 150 150 100
Tmin = 10 oC
8 - Heat & Power Integration12
Class Exercise 7 - Solution (Cont’d)
Stream TS
(oC) TT
(oC) H
(kW) CP
(kW/oC)
Feed 130 100 30 1.0 Bottoms 150 30 72 0.6
Cond 40 40 100 Recyc 50 140 36 0.4 Reb 150 150 100
Tmin = 10 oCT1 = 150oC QHQH
H = 0
Q1
H = 4
Q2
H = 36
Q3
H = 8
H = 12
Q4
Q5
AssumeQH = 0
-100
-96
-60
-52
60
Eliminate infeasible(negative) heat transfer
QH = 100
0
4
40
48
160
T2 = 140oC
T3 = 120oC
T4 = 90oC
T5 = 50oC
T6 = 30oC
H = 6
QC
T7 = 20oC66 166
H = -100
H = +100
This defines:Cold pinch temperature = 140oCQHmin = 100 kW
QCmin = 166 kW
8 - Heat & Power Integration13
Class Exercise 7 - Solution (Cont’d)
Feed 130o 100o
150o
140o
150o
30o
150o
CP
1.0
0.6
0.4
40o
QHmin = 100 QCmin = 166
Botts
Cond
Recy
Reb
40o
HEN Representation of existing flowsheet
8 - Heat & Power Integration14
Class Exercise 7 - Solution (Cont’d)
Feed130o 100o
150o
140o
150o
30o
50o
150o
CP
1.0
0.6
0.4
40o
QHmin = 100 QCmin = 166
Botts
Cond
Recy
Reb
40o
125o
H
H
C
C
100
6 30
100
72
Tmin violation
HEN Representation of existing flowsheet
Feed130o 100o
150o
140o
150o
30o
50o
150o
CP
1.0
0.6
0.4
40o
QHmin = 100 QCmin = 166
Botts
Cond
Recy
Reb
40o
H
C
C
C
100
30
36
100
36
Retrofi tted flowsheet – one additional match f or MER
90o
8 - Heat & Power Integration15
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
8 - Heat & Power Integration16
The Grand Composite Curve (Cont’d) The Grand Composite Curve presents the same
enthalpy residuals, as follows:
Internal heat exchange
Internal heat exchange
TC,pinch
Minimum external heating, at 310 oC
8 - Heat & Power Integration17
The Grand Composite Curve (Cont’d) Alternative heating and cooling utilities can be used, to
reduce operating costs:
8 - Heat & Power Integration18
The Grand Composite Curve (Cont’d) Example:
GCC:
8 - Heat & Power Integration19
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?
8 - Heat & Power Integration20
GCC Example (Cont’d) Returning to the GCC:
8 - Heat & Power Integration21
GCC Example (Cont’d) Possible designs using CW, BFW, LPS and
HPS:
8 - Heat & Power Integration22
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.
Note: Qreb Qcond for columns with saturated liquid products.
Need to position column
carefully on composite
curve
8 - Heat & Power Integration23
Heat-integrated Distillation (Cont’d) Option A: Position distillation
column between hot and cold composite curves:
(a) Exchange between hot and cold streams
(b) Exchange with cold streams
8 - Heat & Power Integration24
Heat-integrated Distillation (Cont’d) Option B: 2-effect distillation:
(a) Tower and heat exchanger configuration; (b) T-Q diagram.
8 - Heat & Power Integration25
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).
8 - Heat & Power Integration26
Option C: Distillation configurations involving compression:
(a) heat pumping (b) vapor
recompression (c) reboiler flashing
Heat-integrated Distillation (Cont’d)
(b) vapor recompression
(a) heat pumping
(c) reboiler flashing
8 - Heat & Power Integration27
Option C: Distillation configurations involving compression:
Heat-integrated Distillation (Cont’d)
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.
(a) heat pumping (b) vapor recompression (c) reboiler flashing
8 - Heat & Power Integration28
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