Phase Envelope Generator PT Diagram
Generate pressure-temperature phase envelopes for multi-component hydrocarbon mixtures using the Peng-Robinson equation of state. Find critical point, cricondentherm, and cricondenbar.
Mixture Composition
Enter mole fractions that sum to 100%.
How this was calculated
Equation of State: Peng-Robinson EOS (1976). a = 0.45724 R²Tc²/Pc × α, where α = (1 + κ(1 − √(T/Tc)))² and κ = 0.37464 + 1.54226ω − 0.26992ω².
Mixing Rules: van der Waals one-fluid. amix = ∑∑ yiyj√(aiaj)(1−kij), bmix = ∑ yibi.
Binary Interaction Parameters: GPSA values for non-hydrocarbon pairs (N&sub2;, CO&sub2;, H&sub2;S). All hydrocarbon–hydrocarbon kij = 0.
Phase Envelope Tracing: Wilson K-value initialization, successive substitution convergence (≤100 iterations, tol 10−&sup6;), brentq root-finding for bubble/dew temperatures at each pressure step.
Critical Properties: GPSA Engineering Data Book (14th ed.) values for Tc, Pc, and ω.
Limitations: PR-EOS can over-predict liquid densities. Results near the critical point may be less accurate. Pseudocomponents (C7+) use lumped properties. Very heavy fractions (>C10) and water are not modeled.
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Groundwork Analytics specializes in production optimization, fluid characterization, and AI-driven reservoir analytics for E&P operators.
Book a free strategy call →Understanding Phase Envelopes in Petroleum Engineering
A phase envelope (also called a PT diagram or phase diagram) is one of the most fundamental tools in petroleum fluid characterization. It defines the pressure-temperature region where a hydrocarbon mixture exists as two phases — liquid and vapor — versus the regions where only a single phase is present. Every fluid sampling program, separator design, pipeline flow assurance study, and gas condensate reservoir management decision depends on understanding the phase behavior of the mixture.
The phase envelope is bounded by two curves: the bubble point curve (where the first bubble of gas appears as pressure drops below the bubble point at constant temperature) and the dew point curve (where the first drop of liquid condenses as pressure drops below the dew point). These two curves meet at the critical point, where the distinction between liquid and vapor phases disappears. Inside the envelope, the fluid is two-phase; outside it, single-phase gas or liquid exists.
Two special points define the extremes of the two-phase region. The cricondentherm is the maximum temperature at which two phases can coexist — above this temperature, no amount of pressure will condense liquid from the gas. For natural gas pipelines, if the flowing temperature exceeds the cricondentherm, liquid dropout is impossible regardless of pressure, which is a key design consideration for offshore subsea flowlines where ambient temperature is cold. The cricondenbar is the maximum pressure of the two-phase region — above this pressure, the fluid is always single-phase.
Gas condensates behave differently from conventional oils. They exist as single-phase gas at reservoir conditions but experience retrograde condensation as reservoir pressure declines below the dew point — liquid drops out in the reservoir, potentially causing permanent productivity loss because condensate saturation near the wellbore can be too low to flow. Accurate phase envelope calculation is critical for optimizing gas condensate development, whether through pressure maintenance (gas recycling), lean gas injection, or determining the optimal separator pressure for maximum liquid recovery.
This tool implements the Peng-Robinson equation of state (PR-EOS, 1976), the industry standard for hydrocarbon phase behavior calculations. PR-EOS provides excellent accuracy for vapor-liquid equilibrium at reservoir and surface conditions. The tool uses GPSA critical properties and binary interaction parameters, Wilson K-value initialization, and successive substitution equilibrium calculations with brentq root-finding for robust phase boundary tracing across the full pressure range from 72 psia to 7,250 psia.
Disclaimer: This tool is provided for educational and preliminary engineering purposes only. Results are based on the Peng-Robinson EOS with GPSA component properties and are not a substitute for full PVT laboratory analysis or professionally tuned EOS models. Critical point and cricondenbar/cricondentherm locations are estimates and may be less accurate near the critical region. Always validate phase behavior calculations against laboratory PVT data before use in reservoir management, facility design, or regulatory filings. Groundwork Analytics LLC assumes no liability for decisions made based on these calculations.