Post on 03-Jan-2016
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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