Transcript Single-cycle mixed-fluid LNG (PRICO) process Part II: Optimal operation

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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
30 bar
4 bar
Sup 10 °C
Subcooled
-157 °C
26 bar
Sat. liquid
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
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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)
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Degrees of freedom
Assumptions:
1. Assume maximum
cooling in SW cooler
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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
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Minimize compressor work (Ws)
• Mode II: Free production rate
With reasonably high LNG prices:
Optimization problem simplifies to
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Maximize production rate (mfeed)
while satifying operational constraints (max. compressor load)
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Mode I: Nominal optimum
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Feed flowrate is given (69.8 kg/s)
– 8 - 1 = 7 steady-state degrees of freedom (incl. 4 compositions)
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Three operational constraints are active at optimum
1. Given temperature LNG (-157 °C)
2. Compressor surge margin at minimum (0.0 kg/s)
3. Compressor speed at maximum (100 %)
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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
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LNG production is maximized
– 8 steady-state degrees of freedom (incl. 4 compositions)
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Four operational constraints are active at optimum
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2.
3.
4.
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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
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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)
NG in
TNG-TC
LNG out
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Optimum with disturbances
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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)
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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
Max cooling
WC
m
Max speed
SC
Ws,max=120MW
Δmsurge=0
TC
Alternative: MPC
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Conclusion
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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
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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