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

2 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

3 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

4 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) 6-9

5 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

6 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)

7 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

8 Two modes of operation

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 1.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

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 1.Compressor work W s 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

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

13 Nominal heat exchanger profiles for mode II

14 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 (W s = 120 MW) = 2 unconstrained degrees of freedom for both modes

15 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

16 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

17 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=N max gives highest production N=Nmax is the only feasible control structure in the increasing load direction

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: 1.Maximum sea water cooling (valve fully open) 2.T LNG = -157 °C 3.LNG flowrate = 69.8 kg/s (mode I) or W s = 120 MW (mode II) 4. 5.

19 Additional material 1.Disturbances considered 2.Structure of model equations 3.Data used for the PRICO process

20 Disturbances considered

21 Structure of model equations

22 Data used for the PRICO process