Cryogenic NGL recovery (GERG-2008 EoS) — a GERG-2008 process flowsheet
A natural-gas dew-point-control / NGL knockout run on the GERG-2008 reference equation of state — the multiparameter Helmholtz model the LNG and custody-transfer industries use because cubic EoS (Peng-Robinson/SRK) are off by several percent on cryogenic natural-gas density. Rich pipeline gas is chilled, let down across a Joule-Thomson valve into the two-phase region, and flashed cold to drop out an NGL liquid (propane/butane-rich) from the methane sales gas. The value is accuracy: on this flowsheet GERG predicts an NGL liquid density ~10% different from Peng-Robinson at cryogenic conditions — enough to change vessel and exchanger sizing. Implemented via CoolProp's multiparameter Helmholtz mixture model (GERG-2008 binary reducing/departure functions), a validated implementation — not a hand-transcription of GERG's thousands of coefficients. Scope: natural-gas / light-hydrocarbon components only (every species must be a CoolProp fluid); see.
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- Rigorous GERG-2008 thermodynamics, solved by the same engine every simulation runs on.
- 3 unit operations modeled: Chill, JT, SEP.
- Focus areas: GERG-2008, NGL recovery, Cryogenic, Natural gas, Reference EoS.
- Thermodynamics
- GERG-2008
- Components
- methane, ethane, propane, n_butane, n2
- Unit operations
- ChillJTSEP
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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-ngl-recovery-gerg")
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
Deep ethane recovery with propane-refrigerated feed chilling
Associated gas is chilled by an external propane refrigeration package before a demethanizer recovers ethane and heavier as NGL bottoms from a methane-rich residue gas overhead — the cryogenic-chilling role Ortloff's CCS/GSP processes play ahead of the turboexpander in a real deep-ethane-recovery plant. The mechanical refrigeration loop itself (compressor/condenser/valve) isn't separately modeled here; the chiller's duty is represented directly as the feed's cooled outlet temperature (the same honest-simplification pattern used for LNG cold-box examples elsewhere in this library).
Associated gas conditioning
Field gas is compressed, chilled below its dew point, and flashed to knock out NGL/condensate — the sales-gas vs. liquids split every midstream gathering plant runs (Peng-Robinson).
Cold-methanol CO2 absorption at 30 bar (SRK + Huron-Vidal)
The Rectisol idea — CO₂ dissolves strongly in methanol at -35 C and 30 bar while methane does not — on the one Gᴱ-mixing package whose parameters were regressed for the rule: SRK with Huron-Vidal mixing driven by Equinor NeqSim's Apache-2.0 set (methanol/CO₂, methanol/methane fitted at infinite pressure), not by the low-pressure NRTL binaries the other Gᴱ packages borrow. 100 mol/s of 30% CO₂ gas against 150 mol/s of cold methanol: the treated gas comes out at 99.9% methane with 0.08% CO₂, the rich methanol holds 17 mol% CO₂, and a first-stage flash to 2 bar releases 6.3 mol/s of 97% CO₂ — a real Rectisol regenerates in several flash stages plus stripping, which is why this solvent still carries 14% CO₂. K-values on the package: CO₂ 0.60 at 238 K against plain SRK's 1.64 (the mixing rule is the whole difference), methane 3100. Two things worth knowing: this card found and fixed a defect in every cubic package — the K-value refinement collapsed onto the trivial solution (K = 1 for everything) when a gas was seeded at the 1e-4 placeholder the Kremser absorber uses, and that K sent 27 mol/s of methane into the solvent; the refinement now retries from the lightest component's own vapour, the way a stability analysis does. The lean methanol still carries the 0.1% dissolved methane a flash-regenerated solvent really has. And the absorber is a Kremser stage model at the mean of the two inlet temperatures.
Cold-separator NGL recovery / dew-point control
A rich natural gas is chilled in the gas/gas exchanger + propane-chiller train (lumped here as one cold box) to -33 degC and let into a cold separator. The heavy hydrocarbons (C3+) drop out as a raw NGL liquid, leaving a leaner sales gas that meets its hydrocarbon dew-point spec — so no more liquid forms as the gas cools in the export pipeline. The cold separator is modelled as an adiabatic flash fed by the chiller, so the single knob (the chill temperature) drives both the NGL recovered and the refrigeration duty: colder recovers more NGL but costs more refrigeration (the built-in sensitivity). Peng-Robinson handles the hydrocarbon VLE; the companion gas-conditioning utilities (hydrate risk, water content, Joule-Thomson choke cooling, and compressor sizing for the sales-gas recompression) quantify the rest of the plant around it.
Multistream exchanger: one core, three streams, one free outlet
A cryogenic cold box is not a network of two-stream exchangers — it is one brazed-aluminium core with several streams exchanging heat simultaneously, and the multistream exchanger models it as such. Three streams share this core: 300 mol/s of 300 K feed gas being chilled, 250 mol/s of 190 K residue gas being rewarmed, and 60 mol/s of 200 K cold liquid. You specify outlet temperatures for all but one — 225 K for the feed gas, 280 K for the residue — and the remaining stream is the free one: its outlet, 230.1 K, is not specified but SOLVED, because it is what closes the adiabatic energy balance across the core. That is the correct number of degrees of freedom for an adiabatic exchanger, and specifying all three would over-specify it. The 1.08 MW duty and a composite-curve pinch check against a 3 K minimum approach come with it, so a specification that would need heat to flow the wrong way is rejected rather than reported. Bounded: this is a thermal feasibility and duty model, not a mechanical design. There is no core geometry, no layer stacking, no fin type, no per-stream pressure drop from passage dimensions — a real cold box datasheet needs all of those, and a vendor computes them from proprietary correlations.
LNG front end and storage: guard beds, tank boil-off, driver limit
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.