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Single-cycle mixed-fluid LNG (PRICO) process

Single-cycle mixed-fluid LNG (PRICO) process. Part II: Optimal operation Sigurd Skogestad & Jørgen Bauck Jensen Qatar, January 2009. Single-cycle mixed fluid LNG process. Natural gas: Feed at 40 bar and 30 °C Cool to -157 °C (spec.) Δ P = 5 bar in main heat exchanger.

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Single-cycle mixed-fluid LNG (PRICO) process

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  1. Single-cycle mixed-fluid LNG (PRICO) process Part II: Optimal operation Sigurd Skogestad & Jørgen Bauck Jensen Qatar, January 2009

  2. Single-cycle mixed fluid LNG process Natural gas: • Feed at 40 bar and 30 °C • Cool to -157 °C (spec.) • ΔP = 5 bar in main heat exchanger

  3. Single-cycle mixed fluid LNG process 30 bar Refrigerant: • Partly condensed with sea water • Subcooled to ~ -157 °C • Expansion to ~ 4 bar • Evaporates in main HX • Super-heated 10°C • Compressed to ~ 30 bar 4 bar Sup 10 °C Subcooled -157 °C 26 bar Sat. liquid

  4. Degrees of freedom Manipulated variables: • Compressor speed N • Choke valve opening z • Turbine power • Sea water flowrate • Natural gas feed flowrate 6-9. Composition of refrigerant (4) 6-9

  5. Degrees of freedom Assumptions: • Assume maximum cooling in SW cooler • Realized by fixing T=30 °C • 8 degrees of freedom for optimization • 4 degrees of freedom in operation • Assume 4 constant compositions in operation

  6. Operational constraints • Some super-heating to avoid damage to compressor • But we find that super-heating is optimal anyway…. (constraint not active) • Maximum compressor power 120 MW • active • Maximum compressor rotational speed is 100 % • active • Minimum distance to surge is 0 kg/s (no back-off) • active

  7. Optimal operation • Minimize operation cost with respect to the • 8 degrees of freedom (u) • subject to the constraints c ≤ 0

  8. Two modes of operation • Mode I: Given production rate (mfeed) Optimization problem simplifies to • Minimize compressor work (Ws) • Mode II: Free production rate With reasonably high LNG prices: Optimization problem simplifies to • Maximize production rate (mfeed) while satifying operational constraints (max. compressor load)

  9. Mode I: Nominal optimum • Feed flowrate is given (69.8 kg/s) • 8 - 1 = 7 steady-state degrees of freedom (incl. 4 compositions) • Three operational constraints are active at optimum • Given temperature LNG (-157 °C) • Compressor surge margin at minimum (0.0 kg/s) • Compressor speed at maximum (100 %) • Only the four degrees of freedom related to refrigerant compositions are unconstrained

  10. Nominal optimum

  11. Mode II: Nominal optimum • LNG production is maximized • 8 steady-state degrees of freedom (incl. 4 compositions) • Four operational constraints are active at optimum • Given temperature LNG (-157 °C) • Compressor surge margin at minimum (0.0 kg/s) • Compressor speed at maximum (100 %) • Compressor work Ws at maximum (120 MW) • Note that two capacity constraints are active (3 and 4) • Only the four constraints related to refrigerant composition are unconstrained

  12. Nominal optimum

  13. Nominal compressor operating point for mode II N=100% (max speed) N=50% N=10% * Surge limit

  14. Temperature profiles in heat exhanger (mode II) NG in TNG-TC LNG out

  15. Optimum with disturbances • 4 operational degrees of freedom • Refrigerant composition is constant during operation Optimum with disturbances: • Given LNG temperature (all cases) • Given load (all cases) • Mode I: The production rate is given • Mode II: The compressor work is at maximum (Ws = 120 MW) • Max. speed compressor (most cases) • Operate at surge limit (most cases)

  16. Check Mode II(production vs. disturbance) • Dots are re-optimized • Lines are for different controlled variables constant • Constant distance to surge (0.0 kg/s) (ALL CASES) • N=Nmax gives highest production (CLOSE TO OPTIMAL) • N=Nmax only feasible structure in increasing load direction

  17. Example of control structure Max cooling Ws,max=120MW WC Δmsurge=0 SC m Max speed TC Alternative: MPC

  18. Conclusion • Maximum compressor speed and minimum distance to surge is nominally optimal for mode I and mode II • In practice one would have a back-off from surge, but this would still be an active constraint • This is also close to optimal or optimal for all disturbance regions •  Control the following variables: • Maximum sea water cooling (valve fully open) • TLNG = -157 °C • LNG flowrate = 69.8 kg/s (mode I) or Ws = 120 MW (mode II)

  19. Additional material • Disturbances considered • Structure of model equations • Data used for the PRICO process

  20. Disturbances considered

  21. Structure of model equations

  22. Data used for the PRICO process

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