Cryogenic NGL recovery (GERG-2008 EoS)

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 flowsim/solver/thermo/gerg.py.