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Single-cycle mixed-fluid LNG (PRICO) process

Part II: Optimal operation

Sigurd Skogestad & Jørgen Bauck Jensen

Quatar, January 2009

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Single-cycle mixed fluid LNG process

Natural gas:• Feed at 40 bar and 30 °C• Cooled to -157 °C• ΔP = 5 bar in main heat

exchanger

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Single-cycle mixed fluid LNG process

Refrigerant:• Partly condensed with sea

water• Cooled to ~ -157 °C• Expansion to ~ 4 bar• Evaporates in main HX• Super-heated 10 °C• Compressed to ~ 30 bar

30 bar

-157 °C

19 bar

4 bar

Sup 10 °C

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Degrees of freedom

Manipulated variables:

1. Compressor speed N

2. Choke valve opening z

3. Turbine speed

4. Sea water flowrate

5. Natural gas feed flowrate

6-9. Composition of refrigerant (4)

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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• Maximum LNG temperature before expansion -157 °C

– Gives the amount of flash gas

• Maximum compressor power 120 MW• Maximum compressor rotational speed is 100 %• Minimum distance to surge is 0 kg/s (no back-off)

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Optimization problem

• Assume same prize for feed and fuel– Reasonable since feed may be used as fuel

• Neglect income of turbine work– The main effect of the liquid turbines is the extra cooling effect, not

the power production

• Neglect cost of cooling with sea water– Sea water requires pumping which is cheap in operation compared

with compressors

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Two modes of operation

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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. Temperature of natural gas after cooling at maximum (-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. Compressor work Ws at maximum (120 MW)

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

3. Temperature of natural gas after cooling at maximum (-157 °C)

4. Compressor speed at maximum (100 %)

• Note that two capacity constraints are active (1 and 4)• Only the four constraints related to refrigerant

composition are unconstrained

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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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Nominal heat exchanger profiles for mode II

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Optimum with disturbances

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

• -1; Always optimal to have minimum cooling– Natural gas is cooled to -157 °C

• -1; One degree of freedom is used to set the load– Mode I: The production rate is given

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

• = 2 unconstrained degrees of freedom for both modes

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Optimum with disturbances

• Two additional degrees of freedom were at constraints at the nominal optimum– Compressor rotational speed at maximum (100 %)

– Compressor surge margin at minimum (0.0 kg/s)

• We also find that controlling these constraints gives close to optimal operation with disturbance

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Optimum with disturbances

• Strictly speaking we would need to consider the following four regions:

• This is complicated and we prefer to have the same controlled variables in all four regions

• Control the nominal active constraints

and

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Mode II; production vs. disturbance• Dots are re-optimized• Lines are for different controlled variables constant• Constant distance to surge (0.0 kg/s)• N=Nmax gives highest production• N=Nmax is the only feasible control structure in the increasing

load direction

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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