1 Single-cycle mixed-fluid LNG (PRICO) process Part I: Optimal design Sigurd Skogestad & Jørgen Bauck Jensen Qatar, January 2009

2 Single-cycle mixed fluid LNG (PRICO) process Natural gas: 45 kg/s (1.3 MTPA) Feed at 40 bar and 30 °C –89.7 mol% C1, 5.5% C2, 1.8% C3, 0.1% C4, 2.8% N2 Cooled to ~ -156 °C Expansion to ~ 1 bar –Flash gas may be used as fuel Liquefied natural gas (LNG) product at -162C -162 °C 1 bar 45 kg/s 30 °C 40 bar -156 °C 35 bar

3 Single-cycle mixed fluid LNG (PRICO) process Refrigerant: Mixed fluid: ~ 33mol% C1, 35% C2, 0% C3, 25% C4, 7% N 2 Partly condensed with sea water to ~ 30 °C Subcooled to ~ -156 °C Expansion to ~ 4 bar Evaporates in NG HX Super-heated ~ 10 °C Compressed to ~ 22 bar 4 bar Sup 10 °C -156°C 19 bar subcooled 22 bar 45 kg/s 30 °C 40 bar 475 kg/s 30 °C 22 bar -156 °C Sat. liquid

4 Compressor: Max. pressure: 22 bar / 30 bar Max. compressor suction volume*: m3/h Max. compressor head*: kJ/kg Or:Max. compressor ratio* Pr, e.g. 5.5 (Price) 4.Max. compressor work: 77.5 MW / 120 MW 5.Minimum superheating: 10C Design constraints -162 °C 30 °C 40 bar 1 bar 3.33 kg/s (7%) * Design constraint only 30 °C

5 Design constraints Max. Pressure –Manufacturing limit for compressor or heat exchangers, e.g bar seems to be the maximum limit for a conventional centrifugal compressor (GE) Max. Compressor suction volume –Compressor suction volume is limited by the physical size of the compressor, this seems to be m 3 /h for a single flow centrifugal compressor (GE) –This is a constraint that is not necessary to consider in operation since compressor head and suction volume will be linked through the compressor characteristic curve Max. Compressor head (specific enthalpy rise) –Simplified correlation for the maximum head over a single wheel Head = Δh = s·u 2 [kJ/kg] s is the work input factor, ~ 0.6 and u is the velocity of the wheel tip [m/s] –Not in operation Max. Compressor pressure ratio –An alternative simple way of limiting the compressor head –Less exact since head also depends on mass flowrate Max. Compressor shaft work –Available power supply (operation) –Physical limitation in the compressor drive system –Physical limitation in the compressor itself Additionally –LNG outlet conditions (saturation at 1.1 bar) –Fuel specification (maximum flowrate to avoid excess fuel) –Gproms, multiflash, srk

6 Optimal design: TAC minJ TAC = J operation + J capital subject toc ≤ 0 J operation [$/year] is the annual operating cost –J operation = J utility + J feeds + J products J capital [$/year] is the annualized cost of the equipment Total annualized cost (TAC) is minimized with respect to the design variables –Flowsheet structure –Areas, sizes –Operating parameters (pressures etc.) Requires mixed integer non-linear programming Our case  Fixed structure  Try a simpler approach Maximize total profit = Minimize Total Annualized Cost (TAC):

7 Idea: Specify ΔT min to balance between operating costs (favoured by a low value) capital costs (favoured by a high value) Simpler approach: Specify ΔT min -162 °C 30 °C 40 bar 1 bar 3.33 kg/s * Design constraint only ΔT min =2C* 30 °C

8 Simple ΔT min -method (Approach 1) ΔT min (=2C) is added as an extra design constraint + minimize compressor work (W s ) BUT: The resulting design parameters (pressure etc.) are not optimal for the resulting process! –Reoptimizing reduces ΔT min to about 1C and reduces work by about 5% (!) –Cannot be fixed by iterating on ΔT min Therefore: Approach 1 NOT USED

9 Simplified TAC (sTAC) Capital cost J capital = Σ i (C fixed,i + C variable,i ·S i ni ) / T T – capital depriciation time, e.g. 10 years 1.Structure of plant given  C fixed,i = 0 2.Main equipment: Heat exchangers and compressor 3.Scaling exponent n = 1 for compressor use largest compressor available can then combine operation and capital cost! n = 0.65 for heat exchangers 4.C variable,i = C 0 for all heat exchangers Approach 2: Adjust C 0 to get ΔT min = 2C

10 sTAC – Optimization problem Minimize cost Case I: Feedrate (NG) given Case II: Feedrate free Here: Consider Case II. Minimize cost= ”Max. single-train LNG feed” 3.33 kg/s 1 bar 30 °C 40 bar 30 °C -162 °C

11 “Max feed” sTAC: Minimization with respect –Heat exchanger areas (A HOT and A NG ), A NG : NG / cold refrigerant A HOT : hot refrigerant / cold refrigerant –refrigerant composition –operating parameters (P h, P l, m LNG ) Here: Adjust C 0 to obtain ΔT min = 2C Other constraints c: depend on specific case

12 Case 1 – Price and Mortko (1983) Data –LNG outlet temperature (before expansion) = -144 °C –77.5 MW compressor power –Maximum P h = 22 bar –Maximum Pr = P h /P l = 5.5 Differences / uncertainties –Feed composition –Neglected removal of heavy components –Pressure losses (especially important at low pressure, e.g. compressor suction) –Heating of fuel gas produces some LNG “for free” 3.7 % higher production compared with Price & Mortko –44.6 kg/s LNG production –Gives too much fuel gas (7.7 kg/s, ~230 MW) Want to limit fuel to 3.33 kg/s, ~100 MW

13 Case 2 – Limited fuel flow Limitation on fuel flow instead of outlet temperature –Maximum 3.33 kg/s of fuel (7.7. kg/s in Case 1) –Outlet temperature down from -144 °C to -156 °C to get sufficient cooling with less flash gas (fuel) –Production (with W s =77.5 MW and Pr=5.5) reduced by 6 % compared with case 1 From 44.6 kg/s to 41.7 kg/s 3.33 kg/s 77.5 MW -162C 41.7 kg/s -156C 22 bar 4 bar 45 kg/s 30C 475 kg/s 30C

14 Case 3,4 – Super-heating Wish to find the optimal degree of super-heating –10.0 °C super-heating used for all cases except 3 and 4 –Case 3; 11.6 °C super-heating increases production by 0.8 % compared with case 2 –Case 4; 25.7 °C super-heating decreases production by 1.3 % compared with case 3 Optimum is very flat in terms of super-heating Some super-heating is necessary to protect the compressor Some super-heating is optimal due to –Internal heat exchange in the main heat exchanger However, the heat transfer coefficient in the super-heating region is lower than in the evaporating region –This has not been considered here –Will tend to reduce the optimal amount of super-heating

15 Case 5 – No pressure constraint We have removed the following constraints –Maximum P h = 22 bar –Maximum Pr = P h /P l = 5.5 P h is increased to 50.4 bar and Pr is increased to 22 LNG production is increased by 11 % (from case 2) The high pressure ratio is not possible with a single compressor casing –The compressor head is too high –Two compressors in series will do the job Higher head [kJ/kg] gives lower refrigerant flow –Cooling duty per kg of refrigerant closely related to head –Less heat transfer area is needed since less warm refrigerant needs cooling The cost of an additional compressor casing is at least partly offset by the decreased heat transfer area and increased production

16 Case 6,7 – Real GE Compressor GE MCL1800 series compressor –Centrifugal compressor with 1800 mm casing diameter –Maximum suction volume is m 3 /h  active constraint –Maximum discharge pressure P h = 30 bar  active constraint Case 6 – 77.5 MW; Same production as case 5 –Compressor head is 216 kJ/kg may be feasible with a single compressor casing Case 7 – 120 MW; 71.1 kg/s of LNG product –Compressor head is 162 kJ/kg which is feasible with a single compressor casing –Corresponds to 2.0 million tons per annum (MTPA) with 330 operating days per year

17 Case 8 – Liquid turbines Expansion in liquid turbines –Takes the pressure down to 2 bar above the saturation pressure –Avoid vapour in the turbines –Possible with two phase turbines? Production increased by 6.6 % compared with case 7 –75.8 kg/s  ~ 2.2 MTPA per train

18 Production vs. feed pressure Results for case 8 Achievable feed pressure depends on –Location of heavy extraction Up-front or integrated Recompression after heavy extraction –Feed compressor? Complicates the optimization problem –Very important for production

19 Comment All the results presented here are with a minimum approach temperature ΔT min = 2.0 °C –This is achieved by adjusting C 0 in the optimization problem An alternative is to find a reasonable C 0 and the use the same value for all cases –These results are presented in the paper

20 Conclusion sTac method – better than specifying ΔT min Superheating is optimal Feed pressure very important for the achievable production A large PRICO train of 2.2 MTPA is feasible with a single compressor casing –2.0 MTPA without liquid turbines

21 Additional material 1.Table with results for all cases 2.Table with results for the alternative design method with constant C 0

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23 Fixed C 0 for all cases