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C7+ Characterization and PR Flash Calculator

Estimate one C7+ pseudocomponent from measured molecular weight and specific gravity, then calculate its Peng-Robinson equilibrium split with C1-C6.

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.

Characterizing a C7 plus fraction for a flash model

Calculation sequence

  1. Use laboratory molar-average molecular weight and petroleum specific gravity for the C7+ fraction, not the entire recombined feed. Enter the six light-component mole fractions and the measured C7+ mole fraction on one consistent molar basis. All seven must sum to one without arbitrary normalization. The uncharacterized material is represented as one lump; normal hexane remains a separate real component.
  2. Estimate boiling temperature, critical temperature and critical pressure with the pinned NeqSim Riazi-Daubert MW-SG variant. This implementation uses molecular weight in g/mol, not kg/mol. Its rounded pressure conversion is retained for reproducibility. Select the explicit Edmister acentric approximation (calcAcentricFactor2), not the different Kesler-Lee routine. A known boiling point, molecular distribution or measured saturation pressure is not fitted or inferred from additional hidden data.
  3. Construct a distinct native cubic pseudocomponent with these properties, register it with CoolProp and solve the common-pressure/common-temperature PR equilibrium with C1-C6. Do not substitute hexane or a fixed normal alkane for the entire plus fraction. Re-evaluate PR pressure/fugacity using the actual estimated properties, then check material, molar-volume and mass-conversion identities just as for the light-component tools.
  4. The output separates estimated C7+ properties from the resulting equilibrium split. The seventh composition row and C7+ plot tick refer to this particular single lump, not to an individually identified molecule. Save/reopen and file backups retain M, SG and every fraction; rebuilding the pseudocomponent must reproduce the result. CSV retains all seven component allocations; the PDF also includes the characterization inputs.

Worked check

The synthetic feed at 350 K and 2 MPa has C1-C6 fractions 0.45/0.15/0.15/0.10/0.05/0.03 plus 0.07 C7+. For M=200 g/mol and SG=0.85, estimated Tc is 732.228116 K, Pc is 1.796624 MPa and Edmister omega is 0.784573. The equilibrium vapor mole fraction is about 0.823939. This is not a measured assay or evidence that one lump reproduces an actual C7+ distribution; SG informs the correlation but does not force the EOS liquid density to match a measured density.

Synthetic examples for reproducibility, not measured field results.

Assumptions and limitations

  • One light-to-medium C7+ lump only: M=100-300 g/mol and SG=0.65-0.90. This is a deliberately restricted correlation use, not a universal heavy-oil/asphaltene characterization. Reject Tb>=Tc or omega outside 0-1. No fallback to Pedersen, multicut/gamma splitting, carbon-number distribution, volume translation, sulfur/aromatic calibration or measured-PVT matching.
  • PT screening at 180-450 K and 0.1-10 MPa absolute, kij=0 for every pair including the heavy lump. The usual composition, fugacity, pressure, volume and near-critical/vanishing-phase gates remain enforced. No water, sour components, multiple liquids, wax, hydrate or solid stability. Supply petroleum specific gravity at 60/60 F, not density or operating-temperature specific gravity; no density-basis conversion is performed.
  • No ideal-gas heat capacity or enthalpy model is invented for C7+. This tool is not permitted in the C1-C6 energy flashes. Critical properties alone cannot establish a heavy-end caloric model. Native registrations are reused by exact M/SG input and capped at 128 distinct characterizations per loaded engine; reload before exceeding that bound. No composition is uploaded.

Implemented equations

MW-SG normal boiling and critical temperatures

Tb=97.58M0.3323SG0.04609Tc=59(554.4)eETM0.2998SG1.0555ET=−1.3478×10−4M−0.61641SG\begin{aligned}T_b&=97.58M^{0.3323}SG^{0.04609}\\T_c&=\tfrac59(554.4)e^{E_T}M^{0.2998}SG^{1.0555}\\E_T&=-1.3478\times10^{-4}M-0.61641SG\end{aligned}

Estimated critical pressure

Pc[bar]=0.068947(45203)eEP⋅M−0.8063SG1.6015EP=−0.0018078M−0.3084SG\begin{aligned}P_c[\mathrm{bar}]&=0.068947(45203)e^{E_P}\\&\quad\cdot M^{-0.8063}SG^{1.6015}\\E_P&=-0.0018078M-0.3084SG\end{aligned}

Edmister acentric estimate and Watson factor

ω=37log⁡10(Pc/101325)Tc/Tb−1−1KW=(1.8Tb)1/3/SG\begin{aligned}\omega&=\frac37\frac{\log_{10}(P_c/101325)}{T_c/T_b-1}-1\\K_W&=(1.8T_b)^{1/3}/SG\end{aligned}

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}

Sources and maintenance

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

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Related tools and references

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