Cryogenic helium recovery / nitrogen rejection — a PENG-ROBINSON process flowsheet
A helium-rich natural gas is let down through a real isentropic turboexpander, then cascaded through two cryogenic flash stages (125 K, then 105 K) that progressively concentrate helium in the vapor while methane and nitrogen condense out.
Modeling assumptions & limitations
- 1Helium's near-infinite K-value at these conditions is numerically pathological for this solver's rigorous multi-stage column MESH (verified: even a trace of helium in a column feed reliably crashes or hangs the inside-out/Newton solvers) — so the methane/nitrogen split is modeled as a shortcut recovery-fraction separator, not a tray-by-tray column, and helium enrichment comes from cascaded flash equilibrium alone, not a true nitrogen-rejection distillation column.
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- Rigorous PENG-ROBINSON thermodynamics, solved by the same engine every simulation runs on.
- 4 unit operations modeled: Expander, 2× Cool2, 2× Flash2, NRU.
- Focus areas: Cryogenics, Helium, Nitrogen rejection, Turboexpander, Natural gas processing.
- Thermodynamics
- PENG-ROBINSON
- Components
- methane, n2, helium
- Unit operations
- Expander2× Cool22× Flash2NRU
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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("cryogenic-helium-recovery")
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
LNG front end and storage: guard beds, tank boil-off, driver limit
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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.
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Helium liquefaction (Linde-Hampson cycle)
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C3MR LNG liquefaction
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Two-stage cascade (−100 °C)
Propylene + ethylene cascade: the ethylene loop condenses inside a heat exchanger against the evaporating propylene loop and serves a −100 °C load. Two coupled closed cycles. From the ChemSep casebook (Refrigeration_2-Stage-100C).