Dept. of Chemical and Biomolecular Engineering National University
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 NTNU by Truls Gundersen Department of Energy and Process Engineering Norwegian University of Science and Technology Trondheim, Norway 21.02.20 T. Gundersen Slide no. 1 Trondheim in Summer Time People: NTNU SINTEF Students 4.300 2.000 20.000 Budgets: NTNU - NTNU - SINTEF 3,5 bill NOK 1,6 bill NOK NTNU/SINTEF is the Norwegian Center of Gravity for Science & Technology and Research & Development 21.02.20
T. Gundersen Slide no. 2 Trondheim in Winter Time NTNU We sometimes get a lot of Snow . . . . 21.02.20 T. Gundersen Slide no. 3 Trondheim in Winter Time NTNU Then we need proper Equipment . . . . 21.02.20 T. Gundersen Slide no. 4 Trondheim in Winter Time NTNU But Snow is not all that bad . . . . 21.02.20 T. Gundersen Slide no. 5 Norway - an Energy Nation . NTNU 3 Generations of Energy Development: Hydro Power, Petroleum, Renewables 21.02.20 T. Gundersen
Slide no. 6 Brief Outline NTNU 21.02.20 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
Slide no. 7 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 NTNU 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 21.02.20 T. Gundersen Slide no. 8 The Onion Diagram revisited The forgotten Onion The traditional Onion R S
H U R Smith and Linnhoff, 1988 NTNU S C & E H The User Guide, 1982 The subambient Onion R S C & E H U Aspelund et al., 2006 21.02.20 T. Gundersen Slide no. 9 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 NTNU 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.) 21.02.20 T. Gundersen Slide no. 10 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 NTNU 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. 21.02.20 T. Gundersen Slide no. 11 Classification of Exergy Exergy Mechanical Kinetic Thermal Potential Thermo-mechanical Temperaturebased Chemical Pressurebased NTNU e(tm) = (h ho) To (s s0) = e(T) + e(p) Thermomechanical Exergy can be decomposed into Temperature based and Pressure based Exergy 21.02.20 T. Gundersen Slide no. 12 Exergy Balance in (ideal) Expansion Exp
W m (e( p ) e(T ) ) ambient NTNU Exp 21.02.20 W m (e( p ) e(T ) ) T. Gundersen Slide no. 13 Temperature/Enthalpy (TQ) Route from Supply to Target State is not fixed Target State Supply State NTNU The Route/Path from Supply to Target State is formed by Expansion & Heating as well as Compression & Cooling a) b) c) d) Hot Streams may temporarily act as Cold Streams and vice versa A (Cold) Process Stream may temporarily act as a Utility Stream The Target State is often a Soft Specification (both T and P) The Phase of a Stream can be changed by manipulating Pressure The Problem is vastly more complex than traditional HENS 21.02.20 T. Gundersen Slide no. 14 General Process Synthesis revisited
NTNU Glasser, Hildebrand, Crowe (1987) Hauan & Lien (1998) Attainable Region Phenomena Vectors 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 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 21.02.20 T. Gundersen Slide no. 15 How can we Play with Pressure? Given a Cold Stream with Ts = - 120C, Tt = 0C, ps = 5 bar, pt = 1 bar Basic PA and the 2 extreme Cases are given below: 160 159.47C 120 Temperature, [C] 80 NTNU
40 Heating before Expansion Expansion before Heating 0 -40 -80 Heating only -120 -160 -176.45C -200 0 100 200 300 400 500 600 700 800 Duty, [kW] PA 21.02.20
-120 T. Gundersen 159 Slide no. 16 How can we Play with Pressure? Given a Cold Stream with Ts = - 120C, Tt = 0C, ps = 5 bar, ps = 1 bar Preheating before Expansion increases (mCp): 20 Temperature, [C] -20 NTNU -60 -100 -140 -180 0 100 200 300 400 500 600 Duty, [kW] PA 21.02.20 -120
-90 T. Gundersen -60 -30 0 Slide no. 17 How can we Play with Pressure? Given a Cold Stream with Ts = - 120C, Tt = 0C, ps = 5 bar, ps = 1 bar Heating beyond Target Temperature before Expansion: 140 Temperature, [C] 100 NTNU 60 20 -20 -60 -100 -140 -180 0 100 200 300 400 500 600 700
Duty, [kW] PA 21.02.20 0 60 T. Gundersen 120 159 Slide no. 18 How can we Play with Pressure? Given a Cold Stream with Ts = - 120C, Tt = 0C, ps = 5 bar, ps = 1 bar Attainable Region with One Expander: 140 Temperature, [C] 100 NTNU 60 20 -20 -60 -100 -140 -180 0 100 200 300 400
500 600 700 Duty, [kW] 21.02.20 PA -120 -90 -60 -30 0 60 120 159 AR- 1EXP T. Gundersen Slide no. 19 How can we Play with Pressure? Given a Cold Stream with Ts = - 120C, Tt = 0C, ps = 5 bar, ps = 1 bar Attainable Region with Two Expanders: 140 100 60 20 -20 NTNU
-60 -100 -140 -180 0 21.02.20 100 200 300 400 500 600 700 PA Min T Max T (-120, -120) (-100, -100) (-80, -80) (-50,-50) (0, 0) (70, 70) AR -2EXP T. Gundersen Slide no. 20
Attainable Region for infinite # Expanders 140 100 Temperature, [C] 60 NTNU 20 -20 -60 -100 -140 -180 0 100 200 300 400 500 600 700 Duty, [kW] PA AR- 1EXP 21.02.20 Min T AR- 2EXP T. Gundersen Max T Simple AR Slide no. 21
The simplest possible Example H1: Ts = -10C Tt = -85C mCp = 3 kW/K QH1 = 225 kW Ps = 1 bar Pt = 1 bar C1: Ts = -55C Tt = 10C mCp = 2 kW/K QC1 = 130 kW Ps = 4 bar Pt = 1 bar 20 40 CC 0 0 T (C) T (C) -20 NTNU GrCC 20 -40 -60 -20 -40 -60 -80 -80 -100 -100 0 50 100
150 Q (kW) 200 250 300 0 50 100 Q (kW) 150 200 Insufficient Cooling Duty at insufficient (too high) Temperature, but we have cold Exergy stored as Pressure Exergy !! 21.02.20 T. Gundersen Slide no. 22 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 = 10C): C1: EXT = -20 kW Exergy Surplus is then: NTNU EXP = -228 kW EXLoss = 14 kW
EXtm = -248 kW EXSurplus = 248 (65 + 14) = 169 kW Required Exergy Efficiency for this Process: X = 79/248 = 31.9 % 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 21.02.20 T. Gundersen Slide no. 23 After pre-expansion of the Cold Stream Modified Composite and Grand Composite Curves 20 CC 0 -40 T (C) T (C) -20 -60 -80 -100 NTNU -120 -140 0 50 100 150
Q (kW) 200 250 40 20 0 -20 -40 -60 -80 -100 -120 -140 300 GrCC 0 20 40 Q (kW) 60 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: 21.02.20 The Cold Stream is now much colder than required (-126C vs. -85C - Tmin) T. Gundersen Slide no. 24 80 Pre-heating before Expansion of C1 Modified Composite and Grand Composite Curves
GrCC 0 20 40 Q (kW) 60 Evaluation: New Targets are: Power produced: Notice: 21.02.20 QH,min = 60 kW (unchanged) , QC,min = 0 kW (eliminated) W = 155 kW (ideal expansion) The Cold Stream was preheated from -55C to -37.5C Temperature after Expansion is increased from -126C to -115C T. Gundersen Slide no. 25 80 Expanding the Cold Stream in 2 Stages to make Composite Curves more parallel 40 20 0 -40 T (C) T (C) -20 NTNU
Q (kW) 60 Evaluation: New Targets are: Power produced: QH,min = 64 kW (increased) , QC,min = 0 kW (unchanged) W = 159 kW (ideal expansion) Reduced Driving Forces improve Exergy Performance at the Cost of Area This was an economic Overkill 21.02.20 T. Gundersen Slide no. 26 An Industrial Application - the Liquefied Energy Chain Air Air Separation ASU LNG NTNU NG NG Oxyfuel Power Plant LIN W H2O LNG Natural Gas Liquefaction This Presentation
21.02.20 O2 LCO2 CO2 Liquefaction CO2 Power Production from stranded Natural Gas with CO2 Capture and Offshore Storage (for EOR) T. Gundersen Slide no. 27 The Base Case - using basic Pinch Analysis N2-2 N2-3 CO2-2 CO2-1 NG-1 NTNU N2-1 NG-3 NG-2 CO2-3 K-101 HX-101 HX-102 LNG
LIQ-EXP-102 Heat Recovery first, Pressure Adjustments subsequently 21.02.20 T. Gundersen Slide no. 28 Base Case Composite Curves Seawater NTNU Temperature [C] 50 NG 0 CO2 -50 -100 LNG -150 Hot CC Cold CC N2 -200 0 2 4 6
8 10 12 Duty [MW] External Cooling required for Feasibility External Heating is free (Seawater) 21.02.20 T. Gundersen Slide no. 29 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. 100 Temperature [C] 50 NTNU 0 -50 -100 Hot CC Cold CC -150 -200 0 2 4 6 8
Duty [MW] The Composite Curves have been massaged by the use of Expansion and Compression 21.02.20 T. Gundersen Slide no. 30 A novel Offshore LNG Process N2-7 EXP-101 N2-8 K-101 N2-9 N2-4 EXP-102 N2-5 N2-10 N2-12 N2-11 N2-6 NTNU N2-3 CO2-4 CO2-3 NG-2 N2-2 N2-1 CO2-2 NG-3 NG-5
NG-4 P-102 NG-1 K-100 NG-6 LIQ-EXP-101 Self-supported w.r.t. Power & no flammable Refrigerants 21.02.20 NG-PURGE P-101 T. Gundersen CO2-1 LIQ-EXP-102 V-101 P-100 LNG Slide no. 31 The Natural Gas Path 100 Pressure [bar] NG-2 (-67 C) (45 C) Cooling in HX - 101 Compression in
K-100 80 NTNU NG-3 Expansion in LIQ-EXP-101 NG-1 CP 60 NG-5 (-164 C) Cooling in HX - 102 (20 C) NG-4 (-77 C) 40 Expansion in LIQ-EXP-102 20 NG-6 (-164 C) 0 0 2000 4000 6000 8000
10000 12000 14000 Enthalpy [kJ/kmol] 21.02.20 T. Gundersen Slide no. 32 The CO2 Path CO2-4 140 (32 C) 120 Pumping in P-103 NTNU Pressure [bar] 100 CP 80 CO2-2 Heating in HX-101 (-52.5 C) CO2-3 (18 C) 60 40
Pumping in P-102 20 CO2-1 0 0 (-54.5 C) 2000 4000 6000 8000 10000 12000 14000 16000 18000 Enthalpy [kJ/kmole] 21.02.20 T. Gundersen Slide no. 33 The Nitrogen Path 100 N2-2 N2-4 Heating in HX - 102 and HX - 101
(-40 C) (-171 C) NTNU Pressure [bar] (Logarithmic) Pumping in P-101 CP Expansion in EX-101 Cooling in HX - 101 N2-8 N2-10 (56 C) (-40 C) 10 Compression in K-100 N2-1 N2-5 (-177 C) N2-7 (-160 C) (-40 C) Expansion in EX-102 N2-11 (-160 C)
1 0 2000 4000 6000 Heating in HX - 102 & 101 N2-13 8000 10000 (20 C) 12000 14000 Enthalpy [kJ/kmole] 21.02.20 T. Gundersen Slide no. 34 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 NTNU
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 21.02.20 T. Gundersen Slide no. 35
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