LNG front end and storage: guard beds, tank boil-off, driver limit — a PENG-ROBINSON process flowsheet
The parts of a 5.2 Mtpa LNG train that are not the cold box, and the four questions they answer that no bulk unit can — at real capacity, so the vessel sizes and duties are ones an engineer can check against their own plant. Guard beds: a sulfur-carbon bed takes mercury from 200 to 0.01 µg/Nm³ — the brazed-aluminium limit, because mercury attacks the plate-fin cold box by liquid-metal embrittlement — on a 4.8 m x 6.6 m bed with 4.2 years of life from a capacity balance — inside the 3-5 year window these non-regenerable beds are actually replaced on, which is what a turnaround plan needs. A 4A molecular sieve then dries the gas to 0.1 ppmv, the only route to a cryogenic water spec (a glycol contactor tops out near a −30 °C dew point), on a 4.9 m x 5.1 m bed running a 13.8 h cycle for 1.3 MW of regeneration duty. Storage: the LNG goes to a tank whose boil-off rate comes from a 0.05%/day guarantee but whose boil-off composition comes from a real equilibrium flash — and that is the interesting part, because the vapour leaves at ~11 mol% nitrogen against 1.1% in the feed. Nitrogen and methane are far more volatile at 113 K, so the boil-off is light and the stored liquid weathers heavier: nobody specified that, the flash found it. Recondensing: the boil-off goes back into the send-out LNG rather than to a flare, and the unit reports the LNG:BOG ratio (1999) against the minimum the energy balance demands (3.8), which is the constraint a terminal is actually operated against. The driver: a gas turbine burning plant fuel gas, rated 97.5 MW at ISO conditions, delivers only 84.5 MW at 35 °C — 5.5 MW short of its 90 MW refrigeration load, and flagged as such. That derate is computed from two physical effects (a fixed-geometry compressor swallows a fixed volume so mass flow follows air density, and hotter air costs more to compress), not from a vendor curve, and it comes out at 0.66%/K — mid-band for industrial machines. It is why a tropical LNG train makes less product in summer.
Modeling assumptions & limitations
- 1Liquefaction here is a plain cooler standing in for a refrigeration train — the real C3MR cold box is 'c3mr-lng-liquefaction', and this showcase is about what surrounds it
- 2Mercury is a unit-op PARAMETER in µg/Nm³, not a thermo component, because at 1e-9 mole fraction it is meaningless to an equation of state
- 3The guard bed is a capacity model, not a breakthrough model, and the sieve is an equilibrium design model, not a dynamic one
- 4The tank is well-mixed and steady-state — for what happens when that assumption fails, see the LNG-rollover analysis tool.
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- Rigorous PENG-ROBINSON thermodynamics, solved by the same engine every simulation runs on.
- 10 unit operations modeled: MRU, DEHY, 2× Liquefy, JT, DRUM, TANK, Sendout PUMP, BOG COMP, Recon, GT.
- Focus areas: LNG, Mercury removal, Dehydration, Boil-off gas, Storage, Gas turbine, Cryogenics.
- Thermodynamics
- PENG-ROBINSON
- Components
- n2, methane, ethane, propane, water
- Unit operations
- MRUDEHY2× LiquefyJTDRUMTANKSendout PUMPBOG COMPReconGT
Opens in a new tab, loaded straight into the app — no setup.
Read the step-by-step guideReproduce this exact result from Python — the real client.get_example() → run_and_wait() path, not a mockup.
from flowsim.sdk import FlowSimClient
client = FlowSimClient()
example = client.get_example("lng-front-end-and-storage")
sim = client.create_simulation(example["title"], example["flowsheet"])
result = client.run_and_wait(sim["id"])
print(result["status"]) # "converged"
streams = client.streams(sim["id"])Related models
Gas-condensate LNG train: condensate, LPG and LNG
The LNG plant that is also a liquids plant, at real capacity: this is one 4.4 Mtpa train (a two-train plant is two of these), not a scaled-down sketch. A condensate-rich field arrives by trunkline as a two-phase stream: the slug catcher drops ~20 mol% of it out as raw condensate before a single molecule reaches the cold end, and the plant then runs two trains side by side off one feed. Liquids: the raw condensate is let down to an MP flash that strips the dissolved methane (without it the stabilizer's shortcut total condenser tries to condense methane at 8 bar, which is not a real stabilizer overhead), then stabilized to a C₅/C₆ product. Gas: sweetened and dried, chilled to 245 K to knock out an NGL cut, and the NGL demethanized and split by a deethanizer / depropanizer / debutanizer sequence into commercial propane (~98 mol%) and butane (~98 mol%) LPG plus a natural-gasoline bottoms. LNG: the lean gas goes to the same APCI C3MR cold end as the 'c3mr-lng-liquefaction' showcase — propane precool, mixed-refrigerant MCHE to 120 K, JT letdown to a 1.5 bar end-flash drum. Five sold products come out of one flowsheet, each a stream the solver computed. Watch the end-flash boil-off: it leaves at ~13 mol% nitrogen against 1 mol% in the feed, because the flash drum is where an LNG train actually rejects its nitrogen — nobody specified that, the flash found it. Dehydration is the real molecular-sieve bed, not a stand-in: a 4A sieve sized by length-of-unused-bed, which is what actually takes the gas to the <=0.1 ppmv the cold box needs — 95 t of sieve on a 5.5 x 5.5 m bed, a 17.5 h cycle and 1.4 MW of regeneration duty, with 0.47 bar of Ergun pressure drop. The AGRU spec is checked against the physics rather than assumed: 99.9% CO₂ removal leaves 33 ppmv, and at the coldest point in the train (115.9 K) the solid-CO₂ solubility limit is 232 ppmv on the measured-data basis — a 7x margin, so the sweetening spec demonstrably clears freeze-out instead of merely looking tight.
High-CO2 LNG with amine capture and CO2 reinjection
One 5.3 Mtpa LNG train on a reservoir whose gas is 14 mol% CO₂, where the CO₂ is not vented but compressed to dense phase and injected — 2.8 Mtpa of it, at 200 bar, which is the scale that makes this a storage project rather than a gesture. Two things make this different from every other LNG example here. First, the acid-gas removal is real chemistry, in the same flowsheet as the cryogenics: per-node thermo_overrides run the absorber, the rich/lean loop and the stripper on the 'enrtl-mdea-pz' electrolyte package (MDEA protonation + piperazine carbamate/dicarbamate speciation) while the gas train and the cold end run Peng-Robinson — the amine unit and the MCHE are not two models bolted together, they are one solve. Second, the CO₂ goes somewhere: the stripper overhead is knocked back, two-stage compressed with interstage cooling and drying, and pumped to 200 bar as a ~96 mol% dense-phase stream ready for a reinjection well, rather than leaving as a vent. The sweet gas is water-washed, dried and liquefied in the C3MR cold end to LNG at ~116 K.
C3MR LNG liquefaction
A simplified APCI C3MR train: natural gas and the mixed refrigerant are precooled by a closed propane loop, liquefied to 120 K in a multi-stream main cryogenic exchanger against a closed N₂/C₁/C₂/C₃ refrigerant cycle, then let down to storage — LNG at ~115 K. Single-level precool and a single MCHE bundle (a real train uses three propane levels and two bundles); refrigerant charges are set via tear_specs.
TEALARC LNG liquefaction
A simplified Technip TEALARC train: two closed mixed-refrigerant loops — a heavy C₁/C₂/C₃ precool MR that also cools itself in a 4-stream exchanger, and a light N₂/C₁/C₂/C₃ liquefaction MR — bring natural gas to 120 K before letdown to storage (LNG at ~115 K). Both circulations are set via tear_specs; the precool MR runs rich for pinch feasibility (screening fidelity). From the ChemSep casebook (TEALARC).
Snohvit mixed-fluid-cascade LNG liquefaction
A simplified Statoil/Linde MFC train: three independent, cascaded closed refrigerant loops (propane precool, an N₂/methane/ethane liquefaction MR, and an N₂/methane subcooling MR) each condense/precool through the previous stage's cold box before their own JT expansion — the same cross-stage coupling TEALARC uses, one level deeper. Natural gas is cooled 300 K → 255 K → 175 K → 112 K across the three multi-stream exchangers before letdown to LNG storage (~114 K). All three circulations are set via tear_specs. From the ChemSep casebook (Snohvit MFC).
FLNG dual N2-expander liquefaction
A reverse-Brayton (turboexpander, not vapor-compression) liquefaction train — the compact, offshore-friendly FLNG alternative to mixed-refrigerant cycles like C3MR. Natural gas is acid-gas-scrubbed and mol-sieve-dried (fixed-recovery separators, the same simplification the existing carbon-capture example uses for amine treating), then cooled in two cascaded closed nitrogen loops: a warm loop precools the gas and the cold loop's own N₂ charge, and a cold loop finishes the liquefaction to ~113 K in a main cryogenic multi-stream exchanger. Both loops are pure compressor + turboexpander cycles — no phase change in the refrigerant, no JT valves — the real differentiator from every other LNG example here. Single precool level and a single MCHE bundle (a real train uses more); refrigerant charges are set via tear_specs.