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, NTNU
Department 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 Long 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
e(tm) = (h – h0) – T0 (s – s0)
Thermomechanical Exergy can be decomposed into
Temperature 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”Q T Q
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

9. 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
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
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

11. The Onion Diagram revisitedU
The “traditional” Onion
Smith and Linnhoff, 1988 R S C
& E H
The “forgotten” Onion
The User Guide, 1982 R S H
The “subambient” Onion
Aspelund et al., 2006 U C
& 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 ideas
A bit more involved problem taken from a real industrial situation (offshore LNG)
applies the ExPAnD Methodology
will be explained by Audun Aspelund

14. 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 CC
GrCC
QH,min = 60 kW QC,min = 155 kW for Tmin = 10C
Insufficient Cooling Duty at insufficient (too high) Temperature,
but we have cold Exergy stored as Pressure Exergy !!

15. Targeting by Exergy Analysis
Exergy Analysis using simplified Formulas and assuming Ideal Gas with k = 1.4 gives:
H1: EXT = 65 kW EXP = 0 kW EXtm = 65 kW
Inevitable Losses due to Heat Transfer at Tmin = 10C: EXLoss = 14 kW
C1: EXT = -20 kW EXP = -228 kW EXtm = -248 kW
Exergy Surplus is then: EXSurplus = 248 – ( ) = 169 kW
Required Exergy Efficiency for the Heat Exchange Process: X = 65/248 = 26.2 %
It should be possible to design a Process
that does not require external Cooling
First attempt:
Expand the Cold Stream from 4 bars to 1 bar prior to Heat Exchange

16. After pre-expansion of C1
Modified Composite and Grand Composite Curves CC
GrCC
Evaluation:
New Targets are: QH,min = 60 kW (unchanged) and QC,min = 12.5 kW (down from 155 kW)
Power produced: W = kW (ideal expansion)
Notice: The Cold Stream is now much colder than required (-126C vs. -85C - Tmin)

17. Pre-heating before expansion of C1
Modified Composite and Grand Composite Curves CC
GrCC
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 -55C to -37.5C
Temperature after Expansion is increased from -126C to -115C

18. Expanding C1 in two Stages to make CCs more parallel
Modified Composite and Grand Composite Curves CC
GrCC
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
This was an economic Overkill

19. Example 2: Sustainable utilization of Natural Gas

20. Liquefied Energy Chain based on LNG
Oxyfuel
Power Plant
CO2
Liquefaction
Natural Gas
Air Separation
ASU NG
Air
LIN
LNG W
LCO2 O2
H2O
This Presentation

21. The Base Case
Heat Recovery first, Pressure Adjustments subsequently

22. PA for the base case
Heuristic 7: A fluid with Ps < Pt should be compressed in liquid phase if possible to save compressor work.

23. Pumping the LCO2 to 65 bar prior to HX

24. 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

25. Pumping the LIN to 100 bar prior to HX

26. 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

27. Two stage expansion of the nitrogen

28. 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

29. Compression of the natural gas to 100 bar

30. 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

31. Re-compression of the nitrogen

32. EPA after re-compression of the nitrogen
 We are done !

33. The offshore LNG process

34. The natural gas path

35. The CO2 path

36. The nitrogen path

37. The Composite Curves

38. 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.

39. 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

40. Thank you for your Attention  Audun. Aspelund@ntnu. no Truls
Thank you for your Attention 