Gas Phase Behavior — Dew Point, Phase Envelope & CO₂ Freeze-out

1. The Phase Envelope

A multicomponent gas does not have a single boiling point — it has a two-phase region bounded by a bubble-point line (where the first vapour bubble forms in a liquid) and a dew-point line (where the first liquid droplet forms in a gas), meeting at the critical point. The right-most extreme of the envelope — the highest temperature at which liquid can exist — is the cricondentherm (the maximum dew-point temperature); the highest pressure is the cricondenbar. Pipeline and plant specs are written to keep the operating path outside this envelope (or, for an NGL plant, deliberately inside it).

2. Retrograde Condensation

Near the critical point, gases behave counter-intuitively: between the critical point and the cricondentherm, dropping the pressure at constant temperature can cause liquid to form (and then re-vaporize) — retrograde condensation. This is why a gas can be single-phase at the wellhead yet drop liquids across a pressure-reducing station, and why dew-point control matters for transmission.

3. The Equation of State

Phase behavior is computed from a cubic equation of state. The Peng-Robinson (PR, 1976) EOS is the workhorse for hydrocarbon dew-point work:

with a, b from each component's critical temperature, critical pressure and acentric factor, combined through mixing rules and binary interaction parameters; α(T) is the temperature function. PR is solved for the compressibility roots and equal fugacities give the dew point. (PR — properly tuned with accurate component criticals, e.g. n-hexane P_c ≈ 30.25 bar — is a different and lighter, screening-level method than the reference-quality multi-parameter GERG-2008 equation; a PR-based tool should be labelled as PR, not GERG.) Soave-Redlich-Kwong (SRK) is the common alternative.

4. Hydrocarbon Dew Point

The hydrocarbon dew point (HCDP) is the temperature, at a given pressure, where the first hydrocarbon liquid condenses. It is dominated by the small heavy-end fraction (C₆+), so an accurate C₆+ characterization (and its split into pseudo-components) matters far more than the methane number. Transmission tariffs cap HCDP (and the cricondentherm) to prevent liquid dropout; gas plants set the dew point by JT/refrigeration/turboexpander processing. Note HCDP is distinct from the water dew point, which is controlled separately by dehydration (glycol/molecular sieve).

5. CO₂ Freeze-out

In cryogenic NGL recovery and LNG liquefaction, the process reaches temperatures (−80 to −160 °C) where CO₂ can solidify and plug exchangers, the expander, or the demethanizer. The solid-formation limit is a solid-vapour/solid-liquid equilibrium: for each temperature there is a maximum tolerable CO₂ concentration. This sets the upstream CO₂ removal target — typically CO₂ must be reduced to roughly ≤ 50–100 ppm (often ~50 ppmv for deep LNG service, higher for moderate NGL service) to keep a safe margin from the freeze-out curve at the coldest point. The GPSA Engineering Data Book gives the CO₂ solubility/freeze-out correlations used for this screening.

6. References

• Peng, D.-Y. & Robinson, D.B. (1976) — "A New Two-Constant Equation of State," Ind. Eng. Chem. Fundam.
• GPSA Engineering Data Book (14th Ed) — Sections 23 (Physical Properties) & 25 (Equilibrium); CO₂ freeze-out correlations.
• ISO 23874 — natural gas, gas-chromatographic requirements for hydrocarbon dew-point calculation.
• Soave, G. (1972) — SRK equation of state; Kunz & Wagner — GERG-2008 reference EOS (for contrast).