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Single-cycle mixed-fluid LNG (PRICO) process

Part II: Optimal operation

Sigurd Skogestad & Jørgen Bauck Jensen

Qatar, January 2009

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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

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Single-cycle mixed fluid LNG process

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

30 bar

-157 °C

26 bar

4 bar

Sup 10 °C

Sat. liquid

Subcooled

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Degrees of freedom

Manipulated variables:

1. Compressor speed N

2. Choke valve opening z

3. Turbine power

4. Sea water flowrate

5. Natural gas feed flowrate

6-9. Composition of refrigerant (4)

6-9

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Degrees of freedom

Assumptions:

1. 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

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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

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Optimal operation

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

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Two modes of operation

• Mode I: Given production rate (mfeed)Optimization problem simplifies to

– Minimize compressor work (Ws)

• Mode II: Free production rateWith reasonably high LNG prices:

Optimization problem simplifies to

– Maximize production rate (mfeed)

while satifying operational constraints (max. compressor load)

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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 optimum1. Given temperature LNG (-157 °C)

2. Compressor surge margin at minimum (0.0 kg/s)

3. Compressor speed at maximum (100 %)

• Only the four degrees of freedom related to refrigerant compositions are unconstrained

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Nominal optimum

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Mode II: Nominal optimum

• LNG production is maximized– 8 steady-state degrees of freedom (incl. 4 compositions)

• Four operational constraints are active at optimum1. Given temperature LNG (-157 °C)

2. Compressor surge margin at minimum (0.0 kg/s)

3. Compressor speed at maximum (100 %)

4. 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

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Nominal optimum

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Nominal compressor operating point for mode II

N=100% (max speed)

N=50%

N=10%

* Surge limit

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Temperature profiles in heat exhanger (mode II)

TNG-TC

NG in

LNG out

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Optimum with disturbances

• 4 operational degrees of freedom– Refrigerant composition is constant during operation

Optimum with disturbances:

1. Given LNG temperature (all cases)

2. Given load (all cases)– Mode I: The production rate is given

– Mode II: The compressor work is at maximum (Ws = 120 MW)

3. Max. speed compressor (most cases)

4. Operate at surge limit (most cases)

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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

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Example of control structure

TC

Max cooling

Max speed

WCWs,max=120MW

SCΔmsurge=0

m

Alternative: MPC

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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:1. Maximum sea water cooling (valve fully open)

2. TLNG = -157 °C

3. LNG flowrate = 69.8 kg/s (mode I) or Ws = 120 MW (mode II)

4.

5.

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Additional material

1. Disturbances considered

2. Structure of model equations

3. Data used for the PRICO process

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Disturbances considered

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Structure of model equations

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Data used for the PRICO process