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, NTNU Department of Energy and Process Engineering, NO-7491 Trondheim, Norway. CHISA/PRES 2006 in Prague.

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An extended pinch analysis and design procedure utilizing pressure exergy for subambient cooling

CHISA/PRES 2006 in Prague


Outline of the presentation
Outline of the Presentation Pressure Exergy for Subambient Cooling

  • 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


Motivation and background
Motivation and Background Pressure Exergy for Subambient Cooling

  • 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)


The expand methodology ex tended p inch an alysis and d esign
The ExPAnD Methodology Pressure Exergy for Subambient Cooling(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”


Objectives and scope
Objectives and Scope Pressure Exergy for Subambient Cooling

  • 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


Classification of exergy
Classification of Exergy Pressure Exergy for Subambient Cooling

e(tm) = (h – h0) – T0 (s – s0)

Thermomechanical Exergy can be decomposed into

Temperature based and Pressure based Exergy


What can we do with pressure

T Pressure Exergy for Subambient Cooling

T

T

Q

Q

Q

T

Q

What can we do with Pressure?

Consider a Cold Stream: Ts Tt and Ps  Pt


So we can shape the composite curves to best suit our purpose

T Pressure Exergy for Subambient Cooling

T

Q

Q

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

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


General process synthesis revisited

Glasser, Hildebrand, Crowe (1987) Pressure Exergy for Subambient Cooling

Attainable Region

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

Hauan & Lien (1998)

Phenomena Vectors

Applied to design reactive

distillation systems by using

composition vectors for

the participating phenomena

reaction, separation & mixing

General Process Synthesis revisited

We would like to “ride” on a “Pressure Vector”

in an Attainable Composite Curve Region

for Design of Subambient Processes


Possible tq routes from supply to target state
Possible TQ Routes from Supply to Target State Pressure Exergy for Subambient Cooling

Target

State

Supply

State

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 versa

b) A (Cold) Process Stream temporarily acts as a Utility Stream

c) 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


The onion diagram revisited

The “forgotten” Onion Pressure Exergy for Subambient Cooling

The “traditional” Onion

S

H

U

R

C

&

E

S

H

R

Smith and Linnhoff, 1988

The User Guide, 1982

The “subambient” Onion

C

&

E

S

H

U

R

Aspelund et al., 2006

The Onion Diagram revisited


A brief overview of the methodology
A brief Overview of the Methodology Pressure Exergy for Subambient Cooling

  • 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


The paper has 2 examples
The Paper has 2 Examples Pressure Exergy for Subambient Cooling

  • 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


Liquefied energy chain based on lng

O Pressure Exergy for Subambient Cooling2

Air Separation

ASU

Oxyfuel

Power Plant

W

Air

NG

LNG

LIN

H2O

NG

LNG

Natural Gas

Liquefaction

CO2

Liquefaction

CO2

LCO2

This Presentation

Liquefied Energy Chain based on LNG


The base case
The Base Case Pressure Exergy for Subambient Cooling

Heat Recovery first, Pressure Adjustments subsequently


Pa for the base case
PA for the base case Pressure Exergy for Subambient Cooling

Heuristic 7: A fluid with Ps < Pt should be compressed in liquid phase if possible to save compressor work.


Pumping the lco 2 to 65 bar prior to hx
Pumping the LCO Pressure Exergy for Subambient Cooling2 to 65 bar prior to HX


Epa after pumping the lco 2 to 65 bar
EPA after pumping the LCO Pressure Exergy for Subambient Cooling2 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


Pumping the lin to 100 bar prior to hx
Pumping the LIN to 100 bar prior to HX Pressure Exergy for Subambient Cooling


Epa after pumping the lin to 100 bar
EPA after pumping the LIN to 100 bar Pressure Exergy for Subambient Cooling

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


Two stage expansion of the lin
Two stage expansion of the LIN Pressure Exergy for Subambient Cooling


Epa after two stage expansion of the lin
EPA after two stage expansion of the LIN Pressure Exergy for Subambient Cooling

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


Compression of the natural gas to 100 bar
Compression of the natural gas to 100 bar Pressure Exergy for Subambient Cooling


Epa after compression of natural gas to 100 bar
EPA after compression of natural gas to 100 bar Pressure Exergy for Subambient Cooling

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


Re compression of the nitrogen
Re-compression of the nitrogen Pressure Exergy for Subambient Cooling


Epa after re compression of the nitrogen
EPA after re-compression of the nitrogen Pressure Exergy for Subambient Cooling

 We are done !


The offshore lng process
The offshore LNG process Pressure Exergy for Subambient Cooling


The natural gas path
The natural gas path Pressure Exergy for Subambient Cooling


The co 2 path
The Pressure Exergy for Subambient CoolingCO2path


The nitrogen path
The nitrogen path Pressure Exergy for Subambient Cooling


The composite curves
The Composite Curves Pressure Exergy for Subambient Cooling


Conclusions lng process
Conclusions LNG Process Pressure Exergy for Subambient Cooling

  • By using LIN and LCO2as 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.


Conclusions expand
Conclusions Pressure Exergy for Subambient Cooling 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


Thank you for your attention audun aspelund@ntnu no truls gundersen@ntnu no
Thank You for Your attention Pressure Exergy for Subambient CoolingAudun.Aspelund@ntnu.noTruls.Gundersen@ntnu.no


Design basis lng process
Design basis LNG process Pressure Exergy for Subambient Cooling


Results
Results Pressure Exergy for Subambient Cooling


The offshore lng process1
The offshore LNG process Pressure Exergy for Subambient Cooling

  • In the novel LNG process, CO2and 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.