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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

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Dept. of Chemical and Biomolecular Engineering National University of Singapore

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  1. 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

  2. People: NTNU 4.300 SINTEF 2.000 Students 20.000 Budgets: - NTNU 3,5 bill NOK - SINTEF 1,6 bill NOK Trondheim in Summer Time NTNU/SINTEF is the Norwegian Center of Gravity for Science & Technology and Research & Development T. Gundersen

  3. Trondheim in Winter Time We sometimes get a lot of Snow . . . . T. Gundersen

  4. Trondheim in Winter Time Then we need proper Equipment . . . . T. Gundersen

  5. Trondheim in Winter Time But Snow is not all that bad . . . . T. Gundersen

  6. Norway - an Energy Nation ……. 3 Generations of Energy Development: Hydro Power, Petroleum, Renewables T. Gundersen

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Exp Exp Exergy Balance in (ideal) Expansion ambient T. Gundersen

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. Attainable Region for infinite # Expanders T. Gundersen

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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

  27. 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

  28. The Base Case- using basic Pinch Analysis Heat Recovery first, Pressure Adjustments subsequently T. Gundersen

  29. Seawater NG CO2 LNG N2 Base Case Composite Curves External Cooling required for Feasibility External Heating is ”free” (Seawater) T. Gundersen

  30. 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

  31. A novel Offshore LNG Process Self-supported w.r.t. Power & no flammable Refrigerants T. Gundersen

  32. The Natural Gas ”Path” T. Gundersen

  33. The CO2 ”Path” T. Gundersen

  34. The Nitrogen ”Path” T. Gundersen

  35. 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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