Cryogenic helium recovery / nitrogen rejection

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. Honesty note: helium'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.