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An Extended Pinch Analysis and Design Procedure
utilizing Pressure Exergy for Subambient Cooling
A. Aspelund, D. O. Berstad, T. Gundersen
The Norwegian University of Science and Technology, NTNU
Department of Energy and Process Engineering, NO-7491 Trondheim, Norway
CHISA/PRES 2006 in Prague
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Outline of the Presentation
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Motivation and Background
Introducing the ExPAnD Methodology
Objectives and Scope
Exergy and what we can do with Pressure
General Process Synthesis revisited
The Onion Diagram revisited
Briefly about the Methodology
A liquefied Energy Chain based on LNG
Application of ExPAnD to the LNG Process
Concluding Remarks
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Motivation and Background
• Stream Pressure is an important Parameter in above
Ambient Heat Recovery Systems
– Pressure Levels of Distillation Columns and Evaporators affect
important Heat Sources and Heat Sinks (i.e. large Heat Duties)
• Below Ambient, Pressure is even more important
– Temperature is closely related to Pressure through Boiling and
Condensation
– Temperature is closely related to Power through Expansion and
Compression (i.e. changing Pressure)
• Basic Pinch Analysis only considers Temperature
• Exergy Analysis can handle both Temperature and
Pressure, as well as Composition (Process Synthesis)
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The ExPAnD Methodology
(Extended Pinch Analysis and Design)
• Will combine Pinch Analysis (PA), Exergy Analysis
(EA) and Optimization/Math Programming (OP)
– PA for minimizing external Heating and Cooling
– EA for minimizing Irreversibilities (thermodynamic Losses)
– OP for minimizing Total Annual Cost
• Preliminary and Extended Problem Definition
– “Given a Set of Process Streams with Supply State
(Temperature, Pressure and the resulting Phase) and a Target
State, as well as Utilities for Heating and Cooling  Design
a System of Heat Exchangers, Expanders and Compressors
in such a way that the Irreversibilities are minimized”
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Objectives and Scope
• Short Term Objective
– Utilize Pressure Exergy for Subambient Cooling
• Long Term Objective
– Develop a more general Methodology with Graphical and
Numerical Tools for Analysis, Design and Optimization of
complex Energy Chains and Processes, where Pressure
is included as an important Design Variable
• Current Scope
– Do not consider Systems with Chemical Reactions, thus
Composition Effects and Chemical Exergy is omitted
– Assume that changes in Kinetic and Potential Energy are
neglectable, thus Mechanical Exergy is omitted
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Classification of Exergy
Exergy
Mechanical
Kinetic
Thermal
Potential
Thermo-mechanical
Temperaturebased
Chemical
Pressurebased
e(tm) = (h – h0) – T0 (s – s0)
Thermomechanical Exergy can be decomposed into
Temperature based and Pressure based Exergy
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What can we do with Pressure?
Consider a Cold Stream: Ts  Tt and Ps  Pt
T
T
Q
T
Q
T
Q
Q
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So, we can shape the Composite
Curves to best suit our “Purpose”
T
T
Q
Q
Given a Stream with Supply and Target State, there
is a Geometric Region of the Composite Curves
that shows all possible TQ-paths in the Diagram
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General Process Synthesis revisited
Glasser, Hildebrand, Crowe (1987)
Hauan & Lien (1998)
Attainable Region
Phenomena Vectors
Applied to identify all possible
chemical compositions one can get
from a given feed composition
in a network of CSTR and PFR
reactors as well as mixers
Applied to design reactive
distillation systems by using
composition vectors for
the participating phenomena
reaction, separation & mixing
We would like to “ride” on a “Pressure Vector”
in an Attainable Composite Curve Region
for Design of Subambient Processes
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Possible TQ Routes from Supply to Target State
Target
State
Supply
State
The Route/Path from Supply to Target State is formed by
Expansion & Heating as well as Compression & Cooling
a)
b)
c)
d)
A Hot Stream temporarily acts as a Cold Stream and vice versa
A (Cold) Process Stream temporarily acts as a Utility Stream
The Target State is often a Soft Specification (both T and P)
Phase can be changed by manipulating Pressure

The Problem is vastly more complex than traditional HENS
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The Onion Diagram revisited
The “forgotten” Onion
The “traditional” Onion
R
S
H
U
R
Smith and Linnhoff, 1988
H
The User Guide, 1982
The “subambient” Onion
R
S
C
&
E
S
C
&
E
H U
Aspelund et al., 2006
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A brief Overview of the Methodology
• Exergy Analysis is used for Targeting
– Can the Cooling be done without External Utilities with maximum
utilization of Pressure (including Heat Transfer Irreversibilities)?
– If yes, what is the required Exergy Efficiency of the System?
• Pinch Analysis is used after each change (Expansion
or Compression) to evaluate the Progress of Design
• Would like to develop Limiting TQ Profiles
• 10 Heuristic Rules have been developed
• A Design Procedure (as a flow diagram) for utilizing
Pressure Exergy in a Cold Stream to cool a fixed Hot
Stream (starting in the Cold End) has been developed
• 6 different Design Criteria can be used
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The Paper has 2 Examples
• A simple 1 hot and 1 cold stream problem
– illustrates the use of Pressure Exergy for
Subambient Cooling
– suggested reading to catch our ideas
• A bit more involved problem taken from a
real industrial situation (offshore LNG)
– applies the ExPAnD Methodology
– will be explained by Audun Aspelund
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The Simplest possible Example
H1: Ts = -10C Tt = -85C mCp = 3 kW/K QH1 = 225 kW Ps = 1 bar Pt = 1 bar
C1: Ts = -55C Tt = 10C mCp = 2 kW/K QC1 = 130 kW Ps = 4 bar Pt = 1 bar
20
40
CC
0
0
T (°C)
-20
T (°C)
GrCC
20
-40
-60
-20
-40
-60
-80
-80
-100
-100
0
50
100
150
Q (kW)
200
250
300
0
50
100
Q (kW)
150
QH,min = 60 kW QC,min = 155 kW for Tmin = 10C
Insufficient Cooling Duty at insufficient (too high) Temperature,
but we have cold Exergy stored as Pressure Exergy !!
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Targeting by Exergy Analysis
Exergy Analysis using simplified Formulas and assuming Ideal Gas with k = 1.4 gives:
H1: EXT = 65 kW
EXP = 0 kW
EXtm = 65 kW
Inevitable Losses due to Heat Transfer at Tmin = 10C:
C1: EXT = -20 kW
Exergy Surplus is then:
EXP = -228 kW
EXLoss = 14 kW
EXtm = -248 kW
EXSurplus = 248 – (65 + 14) = 169 kW
Required Exergy Efficiency for the Heat Exchange Process: X = 65/248 = 26.2 %
It should be possible to design a Process
that does not require external Cooling
First attempt:
Expand the Cold Stream from 4 bars to 1 bar prior to Heat Exchange
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After pre-expansion of C1
Modified Composite and Grand Composite Curves
20
0
CC
-40
T (°C)
T (°C)
-20
-60
-80
-100
-120
-140
0
50
100
150
Q (kW)
200
250
300
40
20
0
-20
-40
-60
-80
-100
-120
-140
GrCC
0
20
40
Q (kW)
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Evaluation:
New Targets are:
Power produced:
Notice:
QH,min = 60 kW (unchanged) and QC,min = 12.5 kW (down from 155 kW)
W = 142.5 kW (ideal expansion)
The Cold Stream is now much colder than required (-126C vs. -85C - Tmin)
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Pre-heating before expansion of C1
Modified Composite and Grand Composite Curves
20
0
CC
-40
T (°C)
T (°C)
-20
-60
-80
-100
-120
-140
0
50
100
150
Q (kW)
200
250
300
40
20
0
-20
-40
-60
-80
-100
-120
-140
GrCC
0
20
40
Q (kW)
Evaluation:
New Targets are:
Power produced:
Notice:
QH,min = 60 kW (unchanged) and QC,min = 0 kW (eliminated)
W = 155 kW (ideal expansion)
The Cold Stream was preheated from -55C to -37.5C
Temperature after Expansion is increased from -126C to -115C
60
80
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Expanding C1 in two Stages
to make CCs more parallel
Modified Composite and Grand Composite Curves
40
20
0
0
T (°C)
-20
T (°C)
20
CC
-40
-60
-20
-60
-80
-80
-100
-100
-120
-120
0
50
100
150
Q (kW)
200
250
300
GrCC
-40
0
20
40
Q (kW)
Evaluation:
New Targets are: QH,min = 64 kW (increased) and QC,min = 0 kW (unchanged)
Power produced: W = 159 kW (ideal expansion)
Reduced Driving Forces improve the Exergy Performance at the Cost of Area
This was an economic Overkill
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Example 2:
Sustainable utilization of Natural Gas
Well
1. Production
and transport
of natural gas
2. Fuel to
electricity with
CO2 capture
Oil reservoar/
geological
structure etc.
3. Handling
and transport
of CO2
Possible interactions
Chain flow direction
Electricity grid
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Liquefied Energy Chain based on LNG
Air
Air Separation
ASU
LNG
NG
O2
NG
Oxyfuel
Power Plant
LIN
H2O
LNG
Natural Gas
Liquefaction
LCO2
W
CO2
Liquefaction
This Presentation
CO2
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The Base Case
N2-2
N2-3
CO2-2
N2-1
CO2-1
NG-1
NG-3
NG-2
CO2-3
K-101
HX-101
HX-102
LNG
LIQ-EXP-102
Heat Recovery first, Pressure Adjustments subsequently
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PA for the base case
Temperature [C]
50
0
-50
 transient  49.7
-100
-150
Hot CC
Cold CC
-200
0
2
4
6
8
10
12
Duty [MW]
Heuristic 7: A fluid with Ps < Pt should be compressed in
liquid phase if possible to save compressor work.
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Pumping the LCO2 to 65 bar prior to HX
N2-2
N2-3
CO2-4
N2-1
CO2-3
NG-1
NG-3
NG-2
P-101
HX-101
HX-102
CO2-2
CO2-1
P-100
LNG
LIQ-EXP-102
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EPA after pumping the LCO2 to 65 bar
Temperature [C]
50
0
-50
 transient  64.6
-100
Hot CC
Cold CC
-150
-200
0
2
4
6
8
Duty [MW]
Heuristic 9: If a cold liquid stream to be vaporized does not create a
Pinch point, it should be pumped to avoid vaporization at constant
temperature, reduce the total cooling duty and increase the pressure
exergy. Work and cooling duty should be recovered by expansion of
the fluid in the vapor phase at a later stage
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Pumping the LIN to 100 bar prior to HX
N2-3
N2-3
CO2-4
N2-2
N2-1
CO2-3
NG-1
NG-3
NG-2
P-101
P-101
HX-101
HX-102
CO2-2
CO2-1
P-100
LNG
LIQ-EXP-102
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EPA after pumping the LIN to 100 bar
Temperature [C]
50
0
-50
 transient  51.8
-100
-150
Hot CC
Cold CC
-200
0
2
4
6
8
Duty [MW]
Heuristic 4: Expansion of a vapor or dense phase stream in an
expander will provide cooling to the system, and at the same time
generate power. Hence, expansion should preferably be done below
Pinch. In subambient processes, a stream with a start pressure higher
than the target pressure should always be expanded in an expander
(not a valve) if the stream is located below the Pinch point
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Two stage expansion of the nitrogen
N2-7
EXP-101
N2-4
EXP-102
N2-5
N2-10
N2-12
N2-11
N2-6
N2-3
CO2-4
N2-2
N2-1
CO2-3
NG-3
NG-2
NG-1
P-101
P-102
HX-101
CO2-2
HX-102
CO2-1
P-100
LNG
LIQ-EXP-102
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EPA after two stage expansion of the LIN
Temperature [C]
50
0
-50
 transient  84.3
-100
-150
Hot CC
Cold CC
-200
0
2
4
6
8
Duty [MW]
Heuristic 10: Compression of a hot gas stream to be condensed will
increase the condensation temperature. The latent heat of vaporization
will also be reduced. Hence, work is used to increase the driving forces
and reduce the heating requirements
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Compression of the natural gas to 100 bar
N2-7
EXP-101
N2-4
EXP-102
N2-5
N2-10
N2-12
N2-11
N2-6
N2-3
CO2-4
N2-2
N2-1
CO2-3
NG-4
NG-3
NG-2
P-101
P-102
HX-101
NG-1
CO2-2
HX-102
CO2-1
K-100
P-100
LNG
LIQ-EXP-102
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EPA after compression of natural gas to 100 bar
100
Temperature [C]
50
0
-50
 transient  87.1
-100
Hot CC
Cold CC
-150
-200
0
2
4
6
8
Duty [MW]
Heuristic 6: A gas or dense phase fluid that is compressed above the
Pinch point, cooled to near Pinch point temperature and then expanded
will decrease the need for both cold and hot utilities. Additional work is,
however, required
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Re-compression of the nitrogen
N2-7
EXP-101
N2-8
K-101
N2-9
N2-4
EXP-102
N2-5
N2-10
N2-12
N2-11
N2-6
N2-3
CO2-4
N2-2
N2-1
CO2-3
NG-4
NG-3
NG-2
P-101
P-102
HX-101
NG-1
CO2-2
HX-102
CO2-1
K-100
P-100
LNG
LIQ-EXP-102
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EPA after re-compression of the nitrogen
100
Temperature [C]
50
0
-50
 transient  85.7
-100
Hot CC
Cold CC
-150
-200
0
2
4
6
Duty [MW]
 We are done !
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The offshore LNG process
N2-7
EXP-101
N2-8
K-101
N2-9
N2-4
EXP-102
N2-5
N2-10
N2-12
N2-11
N2-6
N2-3
CO2-4
CO2-3
NG-2
N2-2
N2-1
CO2-2
NG-3
NG-5
NG-4
P-101
P-102
NG-1
K-100
NG-PURGE
NG-6
LIQ-EXP-101
CO2-1
LIQ-EXP-102
V-101
P-100
LNG
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The natural gas path
NG-3
100
(45 °C)
Cooling in HX - 101
Compression in
K-100
80
Pressure [bar]
NG-2
(-67 °C)
Expansion in
LIQ-EXP-101
NG-1
(20 °C)
CP
60
NG-5
(-164 °C)
Cooling in HX - 102
NG-4
(-77 °C)
40
Expansion in
LIQ-EXP-102
20
NG-6
(-164 °C)
0
0
2000
4000
6000
8000
Enthalpy [kJ/kmol]
10000
12000
14000
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The CO2 path
CO2-4
140
(32 °C)
120
Pumping in
P-103
Pressure [bar]
100
CP
80
CO2-2
Heating in HX-101
(-52.5 °C)
CO2-3
(18 °C)
60
40
Pumping in
P-102
20
CO2-1
0
0
(-54.5 °C)
2000
4000
6000
8000
10000
Enthalpy [kJ/kmole]
12000
14000
16000
18000
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The nitrogen path
100
N2-2
N2-4
Heating in HX - 102 and HX - 101
(-40 °C)
(-171 °C)
Pressure [bar] (Logarithmic)
Pumping in
P-101
CP
Expansion in
EX-101
Cooling in HX - 101 N2-8
N2-10
(56 °C)
(-40 °C)
Compression in
K-100
10
N2-1
N2-5
(-177 °C)
N2-7
(-160 °C)
(-40 °C)
Expansion in
EX-102
N2-11
(-160 °C)
Heating in HX - 102 & 101
N2-13
(20 °C)
1
0
2000
4000
6000
8000
Enthalpy [kJ/kmole]
10000
12000
14000
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The Composite Curves
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Temperature [C]
HX-101
-30
HX-102
-80
-130
-180
0
1
2
3
4
5
Heat flow [MW]
Hot CC
Cold CC
6
7
8
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Conclusions LNG Process
• By using LIN and LCO2 as cold carriers, LNG can be
produced offshore with an exergy efficiency of 85.7 %
• The offshore process:
–
–
–
–
–
Is self-contained with power
Can operate with little rotating equipment
Can operate without hazardous refrigerants
Can operate without offshore cryogenic loading
Allows a higher fraction of CO2 and HHC in the LNG, reducing the
need for offshore gas conditioning and treatment.
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Conclusions ExPAnD
• The ExPAnD methodology integrates Pinch Analysis and
Exergy Analysis (in the future, also Optimization)
• The ExPAnD methodology has proven to be an efficient
tool for developing energy processes
• The methodology shows great potential for minimizing
total shaft work in subambient processes
• The savings are obtained by optimizing the process
streams compression and expansion work together with
the work needed to create necessary cooling utilities
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Thank you for your Attention

[email protected]
[email protected]