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Dept. of Chemical and Biomolecular Engineering
National University of Singapore
Seminar 10 March 2008
“A new Process Synthesis Methodology
utilizing Pressure based Exergy
in Subambient Processes”
NTNU
by
Truls Gundersen
Department of Energy and Process Engineering
Norwegian University of Science and Technology
Trondheim, Norway
20.07.2015
T. Gundersen
Slide no. 1
Trondheim in Summer Time
People:
NTNU
SINTEF
Students
4.300
2.000
20.000
Budgets:
NTNU
- NTNU
- SINTEF
3,5 bill NOK
1,6 bill NOK
NTNU/SINTEF is the Norwegian Center of Gravity
for Science & Technology and Research & Development
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Slide no. 2
Trondheim in Winter Time
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We sometimes get a lot of Snow . . . .
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Slide no. 3
Trondheim in Winter Time
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Then we need proper Equipment . . . .
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Slide no. 4
Trondheim in Winter Time
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But Snow is not all that bad . . . .
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Slide no. 5
Norway - an Energy Nation …….
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3 Generations of Energy Development: Hydro Power, Petroleum, Renewables
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Slide no. 6
Brief Outline
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
Motivation and Background

Limitations of existing Methodologies

Subambient Process Design

The ”ExPAnD” Methodology

How to play with Pressure?

Attainable Region for Composite Curve
Contributions from individual Streams

Small Example to illustrate the Procedure

Industrial Example to demonstrate the Power

Concluding Remarks
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Slide no. 7
Motivation and Background

Stream Pressure is an important Design Variable in
above Ambient Heat Recovery Systems
 Pressure Levels in Distillation & Evaporation affect the
Temperature of important (large Duties) Heat Sinks & Sources

Pressure is even more important below Ambient
 Phase changes link Temperature to Pressure
 Boiling & Condensation
 Pressure changes link Temperature to Power
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 Expansion & Compression

Why do we ”go” Subambient?
 To liquefy volatile Components (LNG, LH2, LCO2)
 To separate Mixtures of volatile Components (Air)

Subambient Cooling is provided by Compression
 Yet another important Link to Pressure
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Slide no. 8
The Onion Diagram revisited
The “forgotten” Onion
The “traditional” Onion
R
S
H
U
R
Smith and Linnhoff, 1988
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S
C
&
E
H
The User Guide, 1982
The “subambient” Onion
R
S
C
&
E
H U
Aspelund et al., 2006
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Slide no. 9
Limitations of Existing Methodologies

Pinch Analysis is heavily used in Industry
 Only Temperature is used as a Quality Parameter
 Exergy Considerations are made through the Carnot Factor
 Pressure and Composition are not Considered

Exergy Analysis and 2nd Law of Thermodynamics




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
Considers Pressure, Composition and Temperature
Focus on Equipment Units not Flowsheet (Systems) Level
No strong Link between Exergy Losses and Cost
Often a Conflict between Exergy and Economy
ExPAnD Methodology is under Development
 ”Extended Pinch Analysis and Design”
 Combines Pinch Analysis, Exergy Analysis and (soon)
Optimization (Math Programming and/or Stochastic Opt.)
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Slide no. 10
The ExPAnD Methodology


Currently focusing on Subambient Processes
A new Problem Definition has been introduced:
 ”Given a Set of Process Streams with a Supply and Target
State (Temperature, Pressure and the resulting Phase), as
well as Utilities for Heating and Cooling  Design a System
of Heat Exchangers, Expanders and Compressors in such a
way that the Irreversibilities (or later: TAC) are minimized”

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Limitations of the Methodology (at present)
 Relies Heavily on a Set of (10) Heuristics, 6 different Criteria
(Guidelines) and suffers from a rather qualitative approach
 Strong need for Graphical and/or Numerical Tools to
replace/assist Heuristic Rules and Design Procedures
 Using the Concept of Attainable Region is a small Contribution
towards a more quantitative ExPAnD Methodology
A. Aspelund, D.O. Berstad and T. Gundersen, ”An Extended Pinch Analysis and Design Procedure utilizing
Pressure based Exergy for Subambient Cooling”, accepted for Applied Thermal Engineering, April 2007.
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Slide no. 11
Classification of Exergy
Exergy
Mechanical
Kinetic
Thermal
Potential
Thermo-mechanical
Temperaturebased
Chemical
Pressurebased
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e(tm) = (h – ho) – To (s – s0) = e(T) + e(p)
Thermomechanical Exergy can be decomposed into
Temperature based and Pressure based Exergy
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Slide no. 12
Exergy Balance in (ideal) Expansion
Exp
W  m  (e( p)  e(T ) )
ambient
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Exp
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W  m  (e( p)  e(T ) )
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Slide no. 13
Temperature/Enthalpy (TQ) ”Route”
from Supply to Target State is not fixed
Target
State
Supply
State
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The Route/Path from Supply to Target State is formed by
Expansion & Heating as well as Compression & Cooling
a)
b)
c)
d)
Hot Streams may temporarily act as Cold Streams and vice versa
A (Cold) Process Stream may temporarily act as a Utility Stream
The Target State is often a Soft Specification (both T and P)
The Phase of a Stream can be changed by manipulating Pressure
The Problem is vastly more complex than traditional HENS
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Slide no. 14
General Process Synthesis revisited
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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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Slide no. 15
How can we Play with Pressure?
Given a ”Cold” Stream with Ts = - 120ºC, Tt = 0ºC, ps = 5 bar, pt = 1 bar
Basic PA and the 2 ”extreme” Cases are given below:
160
159.47ºC
120
Temperature, [C]
80
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40
Heating
before
Expansion
Expansion
before
Heating
0
-40
-80
Heating
only
-120
-160
-176.45ºC
-200
0
100
200
300
400
500
600
700
800
Duty, [kW]
PA
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-120
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159
Slide no. 16
How can we Play with Pressure?
Given a ”Cold” Stream with Ts = - 120ºC, Tt = 0ºC, ps = 5 bar, ps = 1 bar
Preheating before Expansion increases (mCp):
20
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Temperature, [C]
-20
-60
-100
-140
-180
0
100
200
300
400
500
600
Duty, [kW]
PA
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-120
-90
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-60
-30
0
Slide no. 17
How can we Play with Pressure?
Given a ”Cold” Stream with Ts = - 120ºC, Tt = 0ºC, ps = 5 bar, ps = 1 bar
Heating beyond Target Temperature before Expansion:
140
Temperature, [C]
100
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60
20
-20
-60
-100
-140
-180
0
100
200
300
400
500
600
700
Duty, [kW]
PA
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0
60
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120
159
Slide no. 18
How can we Play with Pressure?
Given a ”Cold” Stream with Ts = - 120ºC, Tt = 0ºC, ps = 5 bar, ps = 1 bar
Attainable Region with One Expander:
140
Temperature, [C]
100
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60
20
-20
-60
-100
-140
-180
0
100
200
300
400
500
600
700
Duty, [kW]
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PA
-120
-90
-60
-30
0
60
120
159
AR- 1EXP
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Slide no. 19
How can we Play with Pressure?
Given a ”Cold” Stream with Ts = - 120ºC, Tt = 0ºC, ps = 5 bar, ps = 1 bar
Attainable Region with Two Expanders:
140
100
60
20
-20
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-60
-100
-140
-180
0
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100
200
300
400
500
600
700
PA
Min T
Max T
(-120, -120)
(-100, -100)
(-80, -80)
(-50,-50)
(0, 0)
(70, 70)
AR -2EXP
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Slide no. 20
Attainable Region for infinite # Expanders
140
100
Temperature, [C]
60
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20
-20
-60
-100
-140
-180
0
100
200
300
400
500
600
700
Duty, [kW]
PA
AR- 1EXP
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Min T
AR- 2EXP
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Max T
Simple AR
Slide no. 21
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)
T (°C)
-20
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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
Insufficient Cooling Duty at insufficient (too high) Temperature,
but we have cold Exergy stored as Pressure Exergy !!
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Slide no. 22
200
Targeting by Exergy Analysis (EA)
EA with simplified Formulas and assuming Ideal Gas (k = 1.4) gives:
H1: EXT = 65 kW
EXP = 0 kW
EXtm = 65 kW
Inevitable Losses due to Heat Transfer (Tmin = 10C):
C1: EXT = -20 kW
Exergy Surplus is then:
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EXP = -228 kW
EXLoss = 14 kW
EXtm = -248 kW
EXSurplus = 248 – (65 + 14) = 169 kW
Required Exergy Efficiency for this Process: X = 79/248 = 31.9 %
It should be possible to design a Process
that does not require external Cooling
First attempt:
Expand the Cold Stream from 4 bar to 1 bar prior to Heat Exchange
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Slide no. 23
After pre-expansion of the Cold Stream
Modified Composite and Grand Composite Curves
20
0
CC
-40
T (°C)
T (°C)
-20
-60
-80
-100
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-120
-140
0
50
100
150
Q (kW)
200
250
40
20
0
-20
-40
-60
-80
-100
-120
-140
300
GrCC
0
20
40
Q (kW)
60
Evaluation:
New Targets are: QH,min = 60 kW (unchanged) and QC,min = 12.5 kW (down from 155 kW)
Power produced: W = 142.5 kW (ideal expansion)
Notice:
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The Cold Stream is now much colder than required (-126C vs. -85C - Tmin)
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Slide no. 24
80
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
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-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)
60
Evaluation:
New Targets are:
Power produced:
Notice:
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QH,min = 60 kW (unchanged) , 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
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Slide no. 25
80
Expanding the Cold Stream in 2 Stages
to make Composite Curves more parallel
40
20
0
0
T (°C)
T (°C)
-20
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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)
60
Evaluation:
New Targets are:
Power produced:
QH,min = 64 kW (increased) , QC,min = 0 kW (unchanged)
W = 159 kW (ideal expansion)
Reduced Driving Forces improve Exergy Performance at the Cost of Area
This was an economic Overkill
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Slide no. 26
An Industrial Application
- the Liquefied Energy Chain
Air
O2
Air Separation
ASU
LNG
NTNU NG
NG
Oxyfuel
Power Plant
LIN
W
H 2O
LNG
Natural Gas
Liquefaction
This Presentation
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LCO2
CO2
Liquefaction
CO2
Power Production from ”stranded”
Natural Gas with CO2 Capture and
Offshore Storage (for EOR)
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Slide no. 27
The Base Case
- using basic Pinch Analysis
N2-2
N2-3
CO2-2
CO2-1
NG-1
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N2-1
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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Slide no. 28
Base Case Composite Curves
Seawater
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Temperature [C]
50
NG
0
CO2
-50
-100
LNG
-150
Hot CC
Cold CC
N2
-200
0
2
4
6
8
10
12
Duty [MW]
External Cooling required for Feasibility
External Heating is ”free” (Seawater)
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Slide no. 29
After a number of Manipulations
A. Aspelund, D.O. Berstad and T. Gundersen, ”An Extended Pinch Analysis and Design Procedure utilizing
Pressure based Exergy for Subambient Cooling”, accepted for Applied Thermal Engineering, April 2007.
100
Temperature [C]
50
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0
-50
-100
Hot CC
Cold CC
-150
-200
0
2
4
6
8
Duty [MW]
The Composite Curves have been ”massaged”
by the use of Expansion and Compression
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Slide no. 30
A novel 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
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N2-3
CO2-4
CO2-3
NG-2
N2-2
N2-1
CO2-2
NG-3
NG-5
NG-4
P-102
NG-1
K-100
NG-6
LIQ-EXP-101
Self-supported w.r.t. Power
& no flammable Refrigerants
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NG-PURGE
P-101
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CO2-1
LIQ-EXP-102
V-101
P-100
LNG
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The Natural Gas ”Path”
100
Pressure [bar]
NG-2
(-67 °C)
(45 °C)
Cooling in HX - 101
Compression in
K-100
80
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NG-3
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
10000
12000
14000
Enthalpy [kJ/kmol]
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Slide no. 32
The CO2 ”Path”
CO2-4
140
(32 °C)
120
Pumping in
P-103
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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
12000
14000
16000
18000
Enthalpy [kJ/kmole]
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Slide no. 33
The Nitrogen ”Path”
100
N2-2
N2-4
Heating in HX - 102 and HX - 101
(-40 °C)
(-171 °C)
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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
10000
12000
14000
Enthalpy [kJ/kmole]
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Slide no. 34
Concluding Remarks


Current Methodologies fall short to properly
consider important options related to Pressure
in the Design of Subambient Processes
The Problem studied here is considerably
more complex than traditional HENS
 TQ behavior of Process Streams are not fixed
 Vague distinction between Streams and Utilities
 HEN is expanded with Compressors & Expanders
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
The Attainable Composite Curve Region is an
important new Graphical Representation
 Provides Insight into (subambient) Design Options
 Quantitative Tool in the ExPAnD Methodology
 Small Contribution to the area of Process Synthesis
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Slide no. 35