Optimization of a Single Expander LNG Process

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1 Optimization of a Single Expander LNG Process Bjørn Austbø and Truls Gundersen Department of Energy and Process Engineering Norwegian University of Science and Technology (NTNU) 3rd Trondheim Gas Technology Conference 4-5 June 2014

2 Single expander LNG process COOLER Refrigerant EXPANDER COMPRESSOR Natural gas I II III LNG HX-A HX-B 2

3 Outline Motivation Problem formulation Simplified model Rigorous model Comparison Conclusions 3

4 Outline Motivation Problem formulation Simplified model Rigorous model Comparison Conclusions 4

5 Motivation floating LNG Criteria for selection of liquefaction technology (Tangen and Mølnvik, 2009; Castillo and Dorao, 2010): Profitability Energy efficiency Environmental impact Safety Operability Compactness Equipment count Motion impact Expander processes an interesting alternative 5

6 Motivation expander processes Remeljej and Hoadley (2006) performed an exergy analysis of a dual expander process for natural gas liquefaction using the Peng-Robinson equation of state for modelling Shah and Hoadley (2007) proposed a shaftwork targeting method for expander processes with applications in natural gas liquefaction Marmolejo-Correa and Gundersen (2013) used an exergy diagram for targeting and design of a single expander process assuming ideal gas behaviour 6

7 Motivation Is a perfect gas model accurate for design and optimization of a single expander process for natural gas liquefaction? 7

8 Outline Motivation Problem formulation Simplified model Rigorous model Comparison Conclusions 8

9 Problem formulation Optimization problem: min W x = W x W x x s.t. T Constant isentropic efficiency: Compressor: η s,comp Expander: η s,exp ( ) ( ) ( ) NET COMP EXP HX ( x) T min Refrigerant: nitrogen 9

10 Problem formulation Process specifications natural gas: Variable Unit Value Flow rate kg/s 1 Feed pressure bar 55 Feed temperature K Product temperature K Molar composition: Methane Ethane Propane N-butane Nitrogen

11 Problem formulation COOLER Refrigerant ṁ R p H T 2 T 3 T 4 EXPANDER p L T 1 T 6 T 6 COMPRESSOR ṁ NG, p I, T I T II T III Natural gas LNG HX-A HX-B Uniform heat exchanger exit temperature (T 4 = T II ) Given cooler temperature (T 3 = T I ) Two heat exchanger energy balances Compressor equation Expander equation 11

12 Problem formulation COOLER Refrigerant ṁ R p H T 2 T 3 T 4 EXPANDER p L T 1 T 6 T 6 COMPRESSOR ṁ NG, p I, T I T II T III Natural gas LNG HX-A HX-B Four degrees of freedom for design optimization 12

13 Problem formulation Simplified model: Perfect gas model (ideal gas + constant c p,r ) Rigorous model: Soave-Redlich-Kwong equation of state Mean natural gas heat capacity (ṁ c p ) NG Solved analytically Process modelling: Aspen HYSYS (Aspen Technology, Inc.) Optimization: Sequential quadratic programming, NLPQLP (Schittkowski, 2006) 13

14 Outline Motivation Problem formulation Simplified model Rigorous model Comparison Conclusions 14

15 Simplified model T HX-A HX-B 15 Q

16 Simplified model Temperature T I ΔT warm T I ΔT warm T II ΔT stage T III T III ΔT cold ΔT cold Q A Q tot Heat Q B 16

17 Simplified model Decision variables: Stage temperature T II Cold end temperature difference ΔT cold = T III T 5 Warm end temperature difference ΔT warm = T I T 1 Pressure level p L or p H (does not influence power consumption) 17

18 Simplified model Calculations: Energy balance for the heat transfer process Equation for the compression process Equation for the expansion process Definition of isentropic efficiency Equation for entropy change for ideal gas Net power consumption as a function of the decision variables: (,, ) W = W W = W T T T NET COMP EXP NET II cold warm 18

19 Simplified model Case study: (ṁc p ) NG = 3.5 kj/k c p,r = 1 kj/kgk T I = 300 K T III = 115 K η s,comp = η s,comp = 0.8 ΔT cold = 4 K ΔT warm = 8K Studying the influence of the stage temperature T II 19

20 Simplified model Flow rate / pressure ratio Refrigerant flow rate (kg/s) Stage temperature (K) 12 Pressure ratio (-) 20

21 Simplified model Net power consumption: Total power (kw) Stage temperature (K) 21

22 Simplified model Optimal cold end temperature difference ΔT cold* : From thermodynamics: T * cold = T min Optimal warm end temperature difference ΔT warm* : Locating extrema (isentropic efficiency sufficiently high): d dw NET * ( Twarm ) * warm = 0 T = T min 22

23 Simplified model Optimal stage temperature T II* : Locating extrema: dw dt NET * II = 0 Optimal stage temperature T II * as a function of T I, T III, ΔT warm, ΔT cold, η s,comp, η s,exp 23

24 Simplified model LNG case study T I = T 3 = K T III = K η s,comp = η s,comp = η s ΔT cold = ΔT warm = ΔT min Optimal stage temperature plotted for different values of η s and ΔT min 24

25 Simplified model Optimal stage temperature T II* : Optimal stage temperature (K) Minimum temperature difference (K) ηs = 0.95 ηs = 0.90 ηs = 0.80 ηs = 0.70 ηs =

26 Simplified model Net power consumption at T II* : Net power consumption (kw) Minimum temperature difference (K) ηs = 0.60 ηs = 0.70 ηs = 0.80 ηs = 0.90 ηs =

27 Outline Motivation Problem formulation Simplified model Rigorous model Comparison Conclusions 27

28 Rigorous model Decision variables: Refrigerant flow rate ṁ R Stage temperature T II Pressure ratio p H /p L Low pressure level p L (Alternatively p H ) Pressure levels: 1 bar p 120 bar 28

29 Rigorous model Optimization results: Net power consumption (kw) Minimum temperature difference (K) pl=1bar ηs = 0.7 ηs = 0.8 ηs = 0.9 ηs = 1.0 ph=120bar ηs = 0.7 ηs = 0.8 ηs = 0.9 ηs =

30 Rigorous model ΔT min η s Ẇ T stage ṁ R p L p H p H /p L ΔT cold ΔT warm (K) (-) (kw) (K) (kg/s) (bar) (bar) (-) (K) (K)

31 Rigorous model Composite curves (ΔT min = 5 K, η s = 1.0): Temperature (K) Heat (kw) 31

32 Outline Motivation Problem formulation Simplified model Rigorous model Comparison Conclusions 32

33 Comparison Simplified Rigorous (p L = 1 bar) Rigorous (p H = 120 bar) ΔT min η s T stage Ẇ net T stage Ẇ net T stage Ẇ net (K) (-) (K) (kw) (K) (kw) (K) (kw)

34 Comparison Specific heat compression/expansion: 1.3 Specific heat (kj/kgk) Pressure range (-) ph=120bar Expansion Compression pl=1bar Compression Expansion 34

35 Outline Motivation Problem formulation Simplified model Rigorous model Comparison Conclusions 35

36 Conclusions Single expander process optimized for different values of ΔT min and η s for both simplified and rigorous thermodynamic model Two local optimal solutions observed for the rigorous model, of which one is close to the solution of the simplified model For most cases, the best solution found is significantly different for the two models 36

37 Future work Extensions to dual expander process COOL EXP-A EXP-B COMP Natural gas HX-A HX-B HX-C LNG 37

38 Acknowledgements This publication is based on results from the research project Enabling Low-Emission LNG Systems, performed under the PETROMAKS program. The authors acknowledge the project partners; Statoil and GDF SUEZ, and the Research Council of Norway (193062/S60) for financial support Per Eilif Wahl, SINTEF Energy Research, is acknowledged for providing the interface software required for the study 38

39 References Castillo L, Dorao CA. Influence of the plot area in an economical analysis for selecting small scale LNG technologies for remote gas production. Journal of Natural Gas Science and Engineering. 2010;2(6): Marmolejo-Correa D, Gundersen T. New Graphical Representation of Exergy Applied to Low Temperature Process Design. Industrial & Engineering Chemistry Research. 2013;52(22): Remeljej CW, Hoadley AFA. An exergy analysis of small-scale liquefied natural gas (LNG) liquefaction processes. Energy. 2006;31(12): Schittkowski K. NLPQLP (Version 2.2) [Computer program]; Available from: [accessed 2014 June 2]. Shah NM, Hoadley AFA. A Targeting Methodology for Multistage Gas- Phase Auto Refrigeration Processes. Industrial and Engineering Chemistry Research. 2007;46(13): Tangen G, Mølnvik MJ. Scenarios for remote gas production. Applied Energy. 2009;86(12):

Compression and Expansion at the Right Pinch Temperature

Compression and Expansion at the Right Pinch Temperature 1939 A publication of CHEMICAL ENGINEERING TRANSACTIONS VOL. 56, 2017 Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim Copyright 2017, AIDIC Servizi S.r.l., ISBN 978-88-95608-47-1;