1

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

2

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

3

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)

4

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”

5

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

6

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

7

What can we do with Pressure?Consider a Cold Stream: Ts Tt and Ps Pt

T

Q

T

Q

T

Q

T

Q

8

So, we can shape the Composite Curves to best suit our “Purpose”

T

Q

T

Q

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

9

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

10

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

11

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

12

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

13

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 idea

• A bit more involved problem taken from a real industrial situation (offshore LNG)– applies the ExPAnD Methodology– will be explained by Audun Aspelund

14

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

15

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

17

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

18

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

19

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

20

EPA after pumping the LIN to 100 bar

Heuristic 4: Expansion of a vapour 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

21

Two stage expansion of the LIN

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

22

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

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Thank You for Your attention

Audun.Aspelund@ntnu.no

Truls.Gundersen@ntnu.no

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Design basis LNG process

Compressors Polytropic eff. 82 % Pumps Adiabatic eff. 85 % Expanders Isentropic eff. 85 % Liquid expanders Isentropic eff. 85 % Heat exchangers TA > 0°C 5 °C 0>TA>-80°C 3 °C TA < - 80°C 2 °C Pressure drop 0.2 bar Flash drums Efficiency 100 % Mechanical to electrical

Efficiency 97 %

Stream Type Rate [kg/s]

Phase s Phase t Ts [°C]

Tt [°C]

Ps [bar]

Pt [bar]

Exergy s [kW]

Exergy t [kW]

Hot NG 8.1 Gas Liquid 15 -164 60 1 4457 7812 Cold 1 CO2 18.0 Liquid Dense -54.5 20 5.5 150 4009 3754 Cold 2 Nitrogen 8.0 Liquid Gas -177 15 6 1 5022 0 Total - 26.0 - - - - - - 9031 3754

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Results

Table 1 Power, utility and exergy balance

Unit A B C D E F Process Total duty HX MW 10.6 9.1 9.1 9.9 8.8 7.7 7.7 Cold Utility

MW 2.3 @ -100 C

2.5 @ -100 C

3.0 @ -160 C

1.0 @ - 75 C

0.2 @ -75 C

0 0

Hot Utility

MW 6.5 @ 15 C

2.5 @ 15 C

3.0 @ 15 C

2.2 @ 15 C

1.1 @ 15 C

0

0

Required power for compression

kW 6070 393 521 521 1170 1720 1720

Generated power in expanders

kW 115 115 115 1524 1548 1720 1720

Net process Power kW (-5955) (-278) (-406) 1003 378 0 Required power, cold utility

kW 3819 4151 11600 1230 176 0 0

Total Power balance including utilities

kW (-9774) (-4429) (-12006) (-227) 202 0 0

Exergy Efficiency % 49.7 64.6 51.8 84.3 87.1 85.7 85.7

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The offshore LNG process

• In the novel LNG process, CO2 and nitrogen are used as cold carriers.

• LCO2 and LIN are transported offshore in a combined carrier.

• Offshore, the LCO2 is pumped to injection pressure, heated by cooling of natural gas and injected in an oil field for EOR.

• The cooling duty in the cold end of the LNG process is provided by vaporization of LIN, which is emitted to the atmosphere.

• The LNG is transported to the receiving terminal, where LNG is used to liquefy nitrogen and CO2.

LIN CO2 CO2LNGLNGLNG