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Peng-Robinson Hydrocarbon PT Flash Calculator

Flash a C1-C6 hydrocarbon mixture with Peng-Robinson: equilibrium phase compositions, vapor mole/mass fractions, densities and closure checks.

Inputs

Results

Calculate to view linked results. Default values are examples, not recommended operating inputs.

Learn, compare and practice

Worked explanations, controlled studies and printable exercises connected to this calculation.

Phase fraction versus phase composition in a flash resultWhy phase stability is different from solver convergenceSame feed two equations of state a reproducible comparisonCompositional flash teaching cases with component balances

Calculation sequence

  1. Enter the mixture's equilibrium absolute pressure and temperature, then mole fractions for methane, ethane, propane, normal butane, normal pentane and normal hexane. Fractions must sum to one within 1e-10; percentages and mass fractions are not accepted as mole fractions. Set an omitted component to zero. At least two components must be present, each at 0.001 or greater. No heavy fraction is silently assigned the properties of hexane.
  2. Peng-Robinson uses the attractive denominator v squared plus 2bv minus b squared and its standard acentric-factor alpha function. The native CoolProp 8.0.0 library supplies its cubic component parameters and performs PT stability analysis followed by an equilibrium phase split when required. PR is not an ideal-gas law with a liquid fraction attached; the phase compositions feed back into the mixing rules and fugacity coefficients. All binary interaction parameters are explicitly zero, not tuned to a reservoir sample.
  3. A two-phase result contains liquid x and vapor y compositions, vapor mole fraction beta, vapor mass fraction, both phase densities and equilibrium bulk density. The component table separately shows the moles in each phase per mole of feed; adding those two allocation columns must reproduce the feed. The composition plot uses fixed C1-C6 labels, with normal isomers identified by nC4, nC5 and nC6. Zero components remain visible with zero allocations.
  4. Before returning a split, independently re-evaluate the cubic pressure equation and log fugacity coefficients at both returned phase densities. Enforce component material balance, equal fugacity, composition sums and the molar-volume lever rule. A stable single-phase result has feed composition and bulk density only: no absent-phase compositions or vapor quality are invented. The pinned native cubic phase flag is not used to assign a gas/liquid label because it mislabels some single-phase gas-density states.
  5. The engine is downloaded from this site's versioned static assets only when a flash is requested, then calculations run locally. Saved work and file backups retain canonical pressure, temperature and every feed fraction; reopening recalculates against the current version. CSV preserves the component table and canonical inputs, while PDF includes phase compositions, closure diagnostics, assumptions and sources. There is no account, uploaded composition or remote calculation service.

Worked check

The synthetic 280 K, 2 MPa feed has mole fractions 0.50 methane, 0.15 ethane, 0.15 propane, 0.10 n-butane, 0.05 n-pentane and 0.05 n-hexane. Peng-Robinson predicts beta about 0.657564, with a smaller vapor mass fraction because the vapor is enriched in lighter molecules. These are uncalibrated demonstration inputs, not a measured separator test.

Synthetic examples for reproducibility, not measured field results.

Assumptions and limitations

  • Screening PT vapor-liquid equilibrium for mixtures of C1, C2, C3, nC4, nC5 and nC6 only, with conventional quadratic attraction/linear covolume mixing and kij=0. No isobutane/isopentane, C7+ pseudocomponents, water, CO2, H2S, nitrogen, brine, hydrates, wax, solids, adsorption or multiple liquid phases. Zero kij is a disclosed assumption, not a verified interaction fit for any particular sample.
  • Software bounds are 180-450 K and 0.1-10 MPa absolute, not a universal EOS accuracy envelope. Reject splits with vapor mole fraction below 1e-6 or above 1-1e-6, liquid/vapor molar-density ratio below 1.1, or maximum composition separation below 1e-4. These conservative numerical gates exclude vanishing phases and poorly separated near-critical roots; they do not prove distance from every critical point. No bubble/dew-point or complete phase-envelope tracing is performed.
  • Native stability search is limited to the implemented vapor-liquid problem; it is not proof of global stability against arbitrary additional phases. Reject component-balance error above 2e-7, log-fugacity mismatch above 2e-5 or relative EOS-pressure error above 2e-6. Numerical closure is not experimental accuracy. No enthalpy/entropy flash, Joule-Thomson valve model, separator train, heat transfer, well pressure traverse, slip/holdup or transient reservoir simulation is included.

Implemented equations

PR cubic equation of state

p=RTv−b−a(T)v2+2bv−b2a=∑i∑jcicjaiaj(1−kij)b=∑icibi,kij=0\begin{aligned}p&=\frac{RT}{v-b}-\frac{a(T)}{v^2+2bv-b^2}\\a&=\sum_i\sum_j c_i c_j\sqrt{a_i a_j}(1-k_{ij})\\b&=\sum_i c_i b_i,\quad k_{ij}=0\end{aligned}

Fugacity equilibrium and component conservation

xiφiL=yiφiVzi=(1−β)xi+βyiKi=yi/xi0=∑izi(Ki−1)1+β(Ki−1)\begin{aligned}x_i\varphi_i^L&=y_i\varphi_i^V\\z_i&=(1-\beta)x_i+\beta y_i\\K_i&=y_i/x_i\\0&=\sum_i\frac{z_i(K_i-1)}{1+\beta(K_i-1)}\end{aligned}

Mole fraction is not mass fraction

Mz=∑iziMiwV=β∑iyiMiMzvˉ=(1−β)vL+βvVρbulk=Mz/vˉ\begin{aligned}M_z&=\sum_i z_i M_i\\w_V&=\frac{\beta\sum_i y_iM_i}{M_z}\\\bar v&=(1-\beta)v_L+\beta v_V\\\rho_{\rm bulk}&=M_z/\bar v\end{aligned}

Sources and maintenance

Published by PetroCalcHub Team. Implementation and source checks: . This does not claim independent expert review.

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