Engineering lesson
Characterizing a C7 plus fraction for a flash model
A C7+ assay fraction is a lump of many compounds. Characterization assigns a supported pseudocomponent or distribution from measured fraction properties and explicit correlations; it does not identify a unique molecular composition.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Separate measured inputs from assigned properties
The site's single-cut workflow uses plus-fraction molecular weight and specific gravity with its documented property correlations. Critical properties and acentric factor are assigned model quantities.
The gamma workflow distributes the heavy fraction across multiple cuts under additional assumptions. These are different characterization choices.
Retain the fraction basis
Mole fraction, molecular weight and specific gravity must describe the same assay fraction and reference basis. Replacing a mass fraction with a mole fraction changes the mixture inventory.
The explorer changes plus-fraction molecular weight while retaining the other selected assumptions. It is a sensitivity experiment, not a reconstructed assay.
Validate against suitable PVT measurements
Saturation pressure, density, phase fractions and other measured targets help assess a characterization. One matched target may leave other properties biased.
Document correlations, bounds, interaction parameters and any calibration. A successful flash only establishes a result for the assigned fluid model.
Explore the example
Original recorded synthetic cases solved with the site's c7-plus-characterization-flash engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
C7+ measured molar-average molecular weight (g/mol) = 200. C7+ estimated normal boiling point (K): 563.300322; C7+ estimated critical temperature (K): 732.228116.
- C7+ estimated normal boiling point (K)
- C7+ estimated critical temperature (K)
Worked example
C7+ Characterization and PR Flash Calculator
Fixed controls: Equilibrium absolute pressure = 2 MPa(a); Equilibrium absolute temperature = 350 K; C7+ specific gravity (60/60 F) = 0.85 dimensionless; Methane feed mole fraction = 0.45 dimensionless; Ethane feed mole fraction = 0.15 dimensionless; Propane feed mole fraction = 0.15 dimensionless; n-Butane feed mole fraction = 0.1 dimensionless; n-Pentane feed mole fraction = 0.05 dimensionless; n-Hexane feed mole fraction = 0.03 dimensionless; C7+ feed mole fraction = 0.07 dimensionless.
Relation and unit basis
MW-SG variant: Tb=97.58*M^0.3323*SG^0.04609 K. Tc=(5/9)*554.4*exp(-1.3478e-4*M-0.61641*SG)*M^0.2998*SG^1.0555 K. M is g/mol, SG dimensionless; no Pedersen fallback.; Pc(bar)=0.068947*45203*exp(-0.0018078*M-0.3084*SG)*M^-0.8063*SG^1.6015. Retain NeqSim's rounded conversion coefficient for reference reproducibility, then convert bar to Pa exactly.; Edmister omega=(3/7)*log10(Pc(Pa)/101325)/(Tc/Tb-1)-1. This is NeqSim calcAcentricFactor2, not its Kesler-Lee default. Watson Kw=(1.8*Tb(K))^(1/3)/SG uses Rankine.; PR: p=R*T/(v-b)-a(T)/(v*v+2*b*v-b*b). v is molar volume. For each phase c=x or y, a=sum_i sum_j ci*cj*sqrt(ai*aj)*(1-kij), b=sum_i ci*bi. All kij=0; standard PR alpha(T) and pinned CoolProp cubic component constants are used, without volume translation.; At common T and p, xi*phiL_i=yi*phiV_i. zi=(1-beta)*xi+beta*yi; Ki=yi/xi and sum[zi*(Ki-1)/(1+beta*(Ki-1))]=0. CoolProp performs stability analysis and phase-split iteration; Wilson K-factors are initial guesses, not the final equilibrium solution.
C7+ measured molar-average molecular weight is varied within the supported input range. 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.
1. Define the baseline
C7+ measured molar-average molecular weight (g/mol) = 200
Fixed controls: Equilibrium absolute pressure = 2 MPa(a); Equilibrium absolute temperature = 350 K; C7+ specific gravity (60/60 F) = 0.85 dimensionless; Methane feed mole fraction = 0.45 dimensionless; Ethane feed mole fraction = 0.15 dimensionless; Propane feed mole fraction = 0.15 dimensionless; n-Butane feed mole fraction = 0.1 dimensionless; n-Pentane feed mole fraction = 0.05 dimensionless; n-Hexane feed mole fraction = 0.03 dimensionless; C7+ feed mole fraction = 0.07 dimensionless.
2. Select the relation
MW-SG variant: Tb=97.58*M^0.3323*SG^0.04609 K. Tc=(5/9)*554.4*exp(-1.3478e-4*M-0.61641*SG)*M^0.2998*SG^1.0555 K. M is g/mol, SG dimensionless; no Pedersen fallback.; Pc(bar)=0.068947*45203*exp(-0.0018078*M-0.3084*SG)*M^-0.8063*SG^1.6015. Retain NeqSim's rounded conversion coefficient for reference reproducibility, then convert bar to Pa exactly.; Edmister omega=(3/7)*log10(Pc(Pa)/101325)/(Tc/Tb-1)-1. This is NeqSim calcAcentricFactor2, not its Kesler-Lee default. Watson Kw=(1.8*Tb(K))^(1/3)/SG uses Rankine.; PR: p=R*T/(v-b)-a(T)/(v*v+2*b*v-b*b). v is molar volume. For each phase c=x or y, a=sum_i sum_j ci*cj*sqrt(ai*aj)*(1-kij), b=sum_i ci*bi. All kij=0; standard PR alpha(T) and pinned CoolProp cubic component constants are used, without volume translation.; At common T and p, xi*phiL_i=yi*phiV_i. zi=(1-beta)*xi+beta*yi; Ki=yi/xi and sum[zi*(Ki-1)/(1+beta*(Ki-1))]=0. CoolProp performs stability analysis and phase-split iteration; Wilson K-factors are initial guesses, not the final equilibrium solution.
C7+ measured molar-average molecular weight is varied within the supported input range. 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.
3. Substitute the values
M = 200 g/mol; with the fixed controls above, the workflow evaluates C7+ estimated normal boiling point = 563.300322 K; C7+ estimated critical temperature = 732.228116 K
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
C7+ estimated normal boiling point (K) = 563.300322; C7+ estimated critical temperature (K) = 732.228116
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.
Result and interpretation
C7+ estimated normal boiling point (K) = 563.300322; C7+ estimated critical temperature (K) = 732.228116
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.
Only the specified heavy-fraction molecular weight changes. The assigned component properties are model dependent.
Exact scenario data
Original recorded synthetic cases solved with the site's c7-plus-characterization-flash engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
| C7+ measured molar-average molecular weight (g/mol) | C7+ estimated normal boiling point (K) | C7+ estimated critical temperature (K) |
|---|---|---|
| 160 | 523.0423 | 688.5479 |
| 180 | 543.9197 | 711.3756 |
| 200 | 563.3003 | 732.2281 |
| 220 | 581.4265 | 751.4243 |
| 240 | 598.4832 | 769.2076 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. Does one C7+ molecular weight uniquely determine all heavy molecules?
Show the worked answer
No. Many distributions can share the same average molecular weight; characterization requires additional assumptions and data.
2. Reproduce the baseline C7+ estimated normal boiling point (K) without rounding intermediate values.
Show the worked answer
M = 200 g/mol; with the fixed controls above, the workflow evaluates C7+ estimated normal boiling point = 563.300322 K; C7+ estimated critical temperature = 732.228116 K. C7+ estimated normal boiling point (K) = 563.300322. 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.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
C7+ measured molar-average molecular weight (g/mol) changes from 160 to 240. Other controls remain as stated in the premise. C7+ estimated normal boiling point (K) changes from 523.042287 to 598.48315.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. C7+ measured molar-average molecular weight is varied within the supported input range. 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.
Before using the result
- Match assay fraction and mole/mass basis.
- Separate measured from correlated properties.
- Use independent PVT targets.
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.
Continue learning
Start with the broader workflow guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.