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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” by Truls Gundersen Department of Energy and Process Engineering

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Dept of chemical and biomolecular engineering national university of singapore

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”

by

Truls Gundersen

Department of Energy and Process Engineering

Norwegian University of Science and Technology

Trondheim, Norway

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

People:

NTNU4.300

SINTEF2.000

Students20.000

Budgets:

- NTNU3,5 bill NOK

- SINTEF1,6 bill NOK

Trondheim in Summer Time

NTNU/SINTEF is the Norwegian Center of Gravity

for Science & Technology and Research & Development

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

Trondheim in Winter Time

We sometimes get a lot of Snow . . . .

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

Trondheim in Winter Time

Then we need proper Equipment . . . .

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

Trondheim in Winter Time

But Snow is not all that bad . . . .

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Dept of chemical and biomolecular engineering national university of singapore

Norway - an Energy Nation …….

3 Generations of Energy Development: Hydro Power, Petroleum, Renewables

T. Gundersen


Brief outline

Brief Outline

  • 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

T. Gundersen


Motivation and background

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

      • 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

T. Gundersen


The onion diagram revisited

The “forgotten” Onion

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

T. Gundersen


Limitations of existing methodologies

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

    • 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.)

T. Gundersen


The expand methodology

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”

  • 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.

T. Gundersen


Classification of exergy

Classification of Exergy

e(tm) = (h – ho) – To (s – s0) = e(T) + e(p)

Thermomechanical Exergy can be decomposed into

Temperature based and Pressure based Exergy

T. Gundersen


Exergy balance in ideal expansion

Exp

Exp

Exergy Balance in (ideal) Expansion

ambient

T. Gundersen


Temperature enthalpy tq route from supply to target state is not fixed

Target

State

Supply

State

Temperature/Enthalpy (TQ) ”Route”from Supply to Target State is not fixed

The Route/Path from Supply to Target State is formed by Expansion & Heating as well as Compression & Cooling

a)Hot Streams may temporarily act as Cold Streams and vice versa

b)A (Cold) Process Stream may temporarily act as a Utility Stream

c)The Target State is often a Soft Specification (both T and P)

d)The Phase of a Stream can be changed by manipulating Pressure

The Problem is vastly more complex than traditional HENS

T. Gundersen


General process synthesis revisited

Glasser, Hildebrand, Crowe (1987)

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

T. Gundersen


How can we play with pressure

Heating

before

Expansion

Expansion

before

Heating

Heating

only

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:

159.47ºC

-176.45ºC

T. Gundersen


How can we play with pressure1

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):

T. Gundersen


How can we play with pressure2

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:

T. Gundersen


How can we play with pressure3

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:

T. Gundersen


How can we play with pressure4

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:

T. Gundersen


Attainable region for infinite expanders

Attainable Region for infinite # Expanders

T. Gundersen


The simplest possible example

GrCC

CC

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

Insufficient Cooling Duty at insufficient (too high) Temperature,

but we have cold Exergy stored as Pressure Exergy !!

T. Gundersen


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): EXLoss = 14 kW

C1: EXT = -20 kW EXP = -228 kW EXtm = -248 kW

Exergy Surplus is then: EXSurplus = 248 – (65 + 14) = 169 kW

Required Exergy Efficiency for this Process: X = 79/248 = 31.9 %

Targeting by Exergy Analysis (EA)

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

T. Gundersen


After pre expansion of the cold stream

CC

GrCC

After pre-expansion of the Cold Stream

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 (-126C vs. -85C - Tmin)

T. Gundersen


Pre heating before expansion of c1

CC

GrCC

Pre-heating before Expansion of C1

Modified Composite and Grand Composite Curves

Evaluation:

New Targets are: QH,min = 60 kW (unchanged) , QC,min = 0 kW (eliminated)

Power produced: W = 155 kW (ideal expansion)

Notice: The Cold Stream was preheated from -55C to -37.5C

Temperature after Expansion is increased from -126C to -115C

T. Gundersen


Expanding the cold stream in 2 stages to make composite curves more parallel

CC

GrCC

Expanding the Cold Stream in 2 Stagesto make Composite Curves more parallel

Evaluation:

New Targets are: QH,min = 64 kW (increased) , QC,min = 0 kW (unchanged)

Power produced: W = 159 kW (ideal expansion)

Reduced Driving Forces improve Exergy Performance at the Cost of Area

This was an economic Overkill

T. Gundersen


An industrial application the liquefied energy chain

O2

Air Separation

ASU

Oxyfuel

Power Plant

W

Air

NG

LNG

LIN

H2O

NG

LNG

Natural Gas

Liquefaction

CO2

Liquefaction

CO2

LCO2

This Presentation

An Industrial Application- the Liquefied Energy Chain

Power Production from ”stranded” Natural Gas with CO2 Capture and Offshore Storage (for EOR)

T. Gundersen


The base case using basic pinch analysis

The Base Case- using basic Pinch Analysis

Heat Recovery first,

Pressure Adjustments subsequently

T. Gundersen


Base case composite curves

Seawater

NG

CO2

LNG

N2

Base Case Composite Curves

External Cooling required for Feasibility

External Heating is ”free” (Seawater)

T. Gundersen


After a number of manipulations

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.

The Composite Curves have been ”massaged”

by the use of Expansion and Compression

T. Gundersen


A novel offshore lng process

A novel Offshore LNG Process

Self-supported w.r.t. Power

& no flammable Refrigerants

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

The Natural Gas ”Path”

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

The CO2 ”Path”

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

The Nitrogen ”Path”

T. Gundersen


Dept of chemical and biomolecular engineering national university of singapore

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

  • 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

T. Gundersen


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