1 an extended pinch analysis and design procedure utilizing pressure exergy for subambient cooling...
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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, NTNUDepartment of Energy and Process Engineering, NO-7491 Trondheim, Norway
CHISA/PRES 2006 in Prague
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Outline of the Presentation
• 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 Thermal
Potential Kinetic Thermo-mechanical Chemical
Temperature-based
Pressure-based
e(tm) = (h – h0) – T0 (s – s0)
Thermomechanical Exergy can be decomposed intoTemperature 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
Q
T
Q
T
Q
T
Q
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So, we can shape the Composite Curves to best suit our “Purpose”
Given a Stream with Supply and Target State, thereis a Geometric Region of the Composite Curves that shows all possible TQ-paths in the Diagram
T
Q
T
Q
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General Process Synthesis revisited
Glasser, Hildebrand, Crowe (1987)
Attainable Region
Applied to identify all possiblechemical compositions one can get
from a given feed compositionin a network of CSTR and PFR
reactors as well as mixers
Hauan & Lien (1998)
Phenomena Vectors
Applied to design reactivedistillation systems by using
composition vectors forthe participating phenomenareaction, 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
SupplyState
TargetState
The Route/Path from Supply to Target State is formed by Expansion & Heating as well as Compression & Cooling
a) A Hot Stream temporarily acts as a Cold Stream and vice versab) A (Cold) Process Stream temporarily acts as a Utility Streamc) The Target State is often a Soft Specification (both T and P)d) Phase can be changed by manipulating Pressure
The Problem is vastly more complex than traditional HENS
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The Onion Diagram revisited
R S H U
The “traditional” Onion
Smith and Linnhoff, 1988
R SC&E
H
The “forgotten” Onion
The User Guide, 1982
R S H
The “subambient” Onion
Aspelund et al., 2006
UC&E
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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 ExampleH1: Ts = -10C Tt = -85C mCp = 3 kW/K QH1 = 225 kW Ps = 1 bar Pt = 1 bar
C1: Ts = -55C Tt = 10C mCp = 2 kW/K QC1 = 130 kW Ps = 4 bar Pt = 1 bar
-100
-80
-60
-40
-20
0
20
0 50 100 150 200 250 300Q (kW)
T (
°C)
CC
-100
-80
-60
-40
-20
0
20
40
0 50 100 150 200Q (kW)
T (
°C)
GrCC
QH,min = 60 kW QC,min = 155 kW for Tmin = 10C
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 EX
P = 0 kW EXtm = 65 kW
Inevitable Losses due to Heat Transfer at Tmin = 10C: EXLoss = 14 kW
C1: EXT = -20 kW EX
P = -228 kW EXtm = -248 kW
Exergy Surplus is then: 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 Processthat 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
-140
-120
-100
-80
-60
-40
-20
0
20
0 50 100 150 200 250 300Q (kW)
T (
°C)
CC
-140
-120
-100
-80
-60
-40
-20
0
20
40
0 20 40 60 80Q (kW)
T (
°C) GrCC
Modified Composite and Grand Composite Curves
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: The Cold Stream is now much colder than required (-126C vs. -85C - Tmin)
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Pre-heating before expansion of C1
-140
-120
-100
-80
-60
-40
-20
0
20
0 50 100 150 200 250 300Q (kW)
T (
°C)
CC
-140
-120
-100
-80
-60
-40
-20
0
20
40
0 20 40 60 80Q (kW)
T (
°C) GrCC
Modified Composite and Grand Composite Curves
Evaluation:
New Targets are: QH,min = 60 kW (unchanged) and QC,min = 0 kW (eliminated)Power produced: W = 155 kW (ideal expansion)
Notice: The Cold Stream was preheated from -55C to -37.5C Temperature after Expansion is increased from -126C to -115C
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Expanding C1 in two Stagesto make CCs more parallel
Modified Composite and Grand Composite Curves
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
-120
-100
-80
-60
-40
-20
0
20
0 50 100 150 200 250 300Q (kW)
T (
°C)
CC
-120
-100
-80
-60
-40
-20
0
20
40
0 20 40 60Q (kW)
T (
°C)
GrCC
This was an economic Overkill
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Example 2:
Sustainable utilization of Natural Gas
2. Fuel toelectricity withCO2 capture
1. Productionand transportof natural gas
3. Handlingand transport
of CO2
Possible interactions
Chain flow direction
Well
Oil reservoar/geological
structure etc.
Electricity grid
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Liquefied Energy Chain based on LNG
OxyfuelPower Plant
CO2
LiquefactionNatural GasLiquefaction
Air SeparationASU
NG
Air
LINLNG
W
CO2
NG
LCO2
O2
LNG
H2O
This Presentation
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The Base Case
NG-1 NG-2
LIQ-EXP-102
NG-3
LNG
N2-2 N2-1
CO2-1CO2-2
HX-101 HX-102K-101
CO2-3
N2-3
Heat Recovery first, Pressure Adjustments subsequently
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PA for the base case
Heuristic 7: A fluid with Ps < Pt should be compressed in liquid phase if possible to save compressor work.
-200
-150
-100
-50
0
50
0 2 4 6 8 10 12
Duty [MW]
Tem
pera
ture
[C
]
Hot CCCold CC
49.7transient
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Pumping the LCO2 to 65 bar prior to HX
NG-1 NG-2
LIQ-EXP-102
NG-3
LNG
N2-2 N2-1
CO2-2
CO2-3
HX-101 HX-102
N2-3
P-100
CO2-1
P-101
CO2-4
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EPA after pumping the LCO2 to 65 bar
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
-200
-150
-100
-50
0
50
0 2 4 6 8
Duty [MW]
Tem
pera
ture
[C
]
Hot CCCold CC
64.6transient
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Pumping the LIN to 100 bar prior to HX
NG-2
LIQ-EXP-102
NG-3
LNG
N2-3 N2-2
CO2-2HX-101 HX-102
P-100
CO2-1
P-101
N2-1
NG-1
CO2-3
N2-3
P-101
CO2-4
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EPA after pumping the LIN to 100 bar
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
-200
-150
-100
-50
0
50
0 2 4 6 8
Duty [MW]
Tem
pera
ture
[C
]
Hot CCCold CC
51.8transient
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Two stage expansion of the nitrogen
P-102NG-1 NG-2
LIQ-EXP-102
NG-3
LNG
P-101
N2-2
N2-5
N2-10
N2-11
N2-6
N2-3
EXP-101
EXP-102
N2-1
P-100
CO2-2
CO2-1
CO2-3
N2-12
N2-4
N2-7
CO2-4
HX-101 HX-102
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EPA after two stage expansion of the LIN
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
-200
-150
-100
-50
0
50
0 2 4 6 8
Duty [MW]
Tem
pera
ture
[C
]
Hot CCCold CC
84.3transient
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Compression of the natural gas to 100 bar
K-100
P-102NG-2
NG-1
NG-3
LIQ-EXP-102
NG-4
LNG
P-101
N2-2
N2-5
N2-10
N2-11
N2-6
N2-3
EXP-101
EXP-102
N2-1
P-100
CO2-2
CO2-1
CO2-3
N2-12
N2-4
N2-7
CO2-4
HX-101 HX-102
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EPA after compression of natural gas to 100 bar
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
-200
-150
-100
-50
0
50
100
0 2 4 6 8
Duty [MW]
Tem
pera
ture
[C
]
Hot CCCold CC
87.1transient
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Re-compression of the nitrogen
K-100
P-102NG-2
NG-1
NG-3
LIQ-EXP-102
NG-4
LNG
P-101
N2-2
N2-5
N2-10
N2-11
N2-6
N2-3
EXP-101
EXP-102
N2-1
P-100
CO2-2
CO2-1
N2-9K-101N2-8
CO2-3
N2-12
N2-4
N2-7
CO2-4
HX-101 HX-102
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EPA after re-compression of the nitrogen
-200
-150
-100
-50
0
50
100
0 2 4 6 8
Duty [MW]
Tem
pera
ture
[C
]
Hot CCCold CC
85.7transient
We are done !
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The offshore LNG process
K-100
P-102
V-101
NG-2
LIQ-EXP-101
NG-1
NG-3 NG-4
LIQ-EXP-102
NG-5
NG-6
NG-PURGE
LNG
P-101
N2-2
N2-5
N2-10
N2-11
N2-6
N2-3
EXP-101
EXP-102
N2-1
P-100
CO2-2
CO2-1
N2-9K-101N2-8
CO2-3
N2-12
N2-4
N2-7
CO2-4
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The natural gas path
0
20
40
60
80
100
0 2000 4000 6000 8000 10000 12000 14000
Enthalpy [kJ/kmol]
Pre
ssur
e [b
ar]
Compression in K-100
Cooling in HX - 101
Expansion in LIQ-EXP-101
Expansion in LIQ-EXP-102
Cooling in HX - 102
CPNG-1
NG-2NG-3
NG-4NG-5
NG-6(-164 °C)
(-164 °C) (-77 °C)
(-67 °C) (45 °C)
(20 °C)
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The CO2 path
0
20
40
60
80
100
120
140
0 2000 4000 6000 8000 10000 12000 14000 16000 18000
Enthalpy [kJ/kmole]
Pre
ssur
e [b
ar]
CP
Pumping in P-103
Heating in HX-101
Pumping in P-102
CO2-1
CO2-2
CO2-4
(-52.5 °C) (18 °C)CO2-3
(32 °C)
(-54.5 °C)
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The nitrogen path
1
10
100
0 2000 4000 6000 8000 10000 12000 14000
Enthalpy [kJ/kmole]
Pre
ssur
e [b
ar]
(Lo
gar
ithm
ic)
Heating in HX - 102 and HX - 101
Heating in HX - 102 & 101
Compression in K-100
Expansion in EX-101
Expansion in EX-102
Pumping in P-101
CP
N2-1
N2-2 N2-4
N2-5 N2-7
N2-8
N2-11
N2-10
N2-13
Cooling in HX - 101
(-177 °C)
(-171 °C) (-40 °C)
(-160 °C)
(56 °C)
(-40 °C)
(-160 °C)
(-40 °C)
(20 °C)
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The Composite Curves
-180
-130
-80
-30
20
0 1 2 3 4 5 6 7 8
Heat flow [MW]
Tem
pera
ture
[C]
Hot CC Cold CC
HX-101
HX-102
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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
