Original synthetic study
What a heavier C7 plus assumption changes in a synthetic flash
Changing a C7+ average molecular weight changes the assigned heavy-fraction model and can alter its flash result. The direction and size are conditional on the characterization, EOS and other fixed assumptions.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Keep the characterization controls explicit
The study varies the gamma heavy-fraction molecular weight while retaining specific gravity, shape parameter, feed mole fraction, pressure, temperature and interaction assumptions.
It does not add more heavy moles to the feed. Holding mole fraction fixed while increasing molecular weight changes mass weighting as well as assigned properties.
Read the result as a coupled model sensitivity
Characterization maps the changed assay input into pseudocomponent properties before flashing. The output therefore cannot be attributed to molecular weight in isolation from that mapping.
The recorded table shows actual supported workflow results rather than an assumed monotonic trend.
Use measured targets to constrain the assumption
A plausible average molecular weight range is not a calibrated fluid model. Density, saturation pressure and phase-split observations help assess whether the characterization is appropriate.
The experiment has no field accuracy ranking. It exposes one assumption so readers can decide which additional measurements would reduce uncertainty.
Explore the example
Original recorded synthetic cases solved with the site's gamma-c7-plus-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. Vapor mole fraction (dimensionless): 0.821170907; Vapor mass fraction (dimensionless): 0.54114372.
- Vapor mole fraction (dimensionless)
- Vapor mass fraction (dimensionless)
Worked example
Four-Cut Gamma 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; Conditional gamma distribution shape = 2 dimensionless; Common methane / heavy-cut interaction parameter = 0 dimensionless.
Relation and unit basis
Condition a shifted gamma distribution to 100-300 g/mol, shift=100, shape=alpha. Fit scale b to the supplied conditional mean using incomplete-gamma moments. Four quantile bins each carry 0.25 of C7+ moles; their representative molecular weights are conditional first moments, not bin midpoints. Reconstructed mean must close within 1e-7 g/mol.; 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.; Classical Peng-Robinson, standard quadratic alpha: a=sum_i sum_j xi*xj*sqrt(ai*aj)*(1-kij), b=sum_i xi*bi. The matrix is symmetric with zero diagonal. Available database coefficients and common user-assumed coefficients are distinguished; there is no volume translation or hidden parameter tuning.
C7+ measured molar-average molecular weight is varied within the supported input range. A deliberately bounded four-cut approximation over 100-300 g/mol, not a complete Whitson C7-to-C80 assay. Every cut uses the same supplied 60/60 F SG. Reject an unbracketed mean, retained gamma probability below 1e-6, Tb>=Tc or omega outside 0-1. Shape does not determine aromaticity or a measured carbon-number distribution.
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; Conditional gamma distribution shape = 2 dimensionless; Common methane / heavy-cut interaction parameter = 0 dimensionless.
2. Select the relation
Condition a shifted gamma distribution to 100-300 g/mol, shift=100, shape=alpha. Fit scale b to the supplied conditional mean using incomplete-gamma moments. Four quantile bins each carry 0.25 of C7+ moles; their representative molecular weights are conditional first moments, not bin midpoints. Reconstructed mean must close within 1e-7 g/mol.; 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.; Classical Peng-Robinson, standard quadratic alpha: a=sum_i sum_j xi*xj*sqrt(ai*aj)*(1-kij), b=sum_i xi*bi. The matrix is symmetric with zero diagonal. Available database coefficients and common user-assumed coefficients are distinguished; there is no volume translation or hidden parameter tuning.
C7+ measured molar-average molecular weight is varied within the supported input range. A deliberately bounded four-cut approximation over 100-300 g/mol, not a complete Whitson C7-to-C80 assay. Every cut uses the same supplied 60/60 F SG. Reject an unbracketed mean, retained gamma probability below 1e-6, Tb>=Tc or omega outside 0-1. Shape does not determine aromaticity or a measured carbon-number distribution.
3. Substitute the values
M = 200 g/mol; with the fixed controls above, the workflow evaluates Vapor mole fraction = 0.821170907 dimensionless; Vapor mass fraction = 0.54114372 dimensionless
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Vapor mole fraction (dimensionless) = 0.821170907; Vapor mass fraction (dimensionless) = 0.54114372
MW=100-300 g/mol, SG=0.65-0.90, shape=0.5-10; the conditional mean must also be achievable at that shape. At least two feed fractions >=0.001; C7+ is 0.004-0.99. T=180-450 K and p=0.1-10 MPa(a) are software bounds, not a validated accuracy envelope. No water, N2, CO2, H2S, solids, multiple liquids, volume shifts or density matching. Only methane/heavy-cut pairs share the user k_heavy (-0.2 to 0.4). All other pairs are zero. Properties use the pinned NeqSim Riazi-Daubert MW-SG variant plus explicit Edmister omega, not a silent Pedersen or Kesler-Lee fallback. Neither a solved flash nor a fitted parameter is proof of physical accuracy.
Result and interpretation
Vapor mole fraction (dimensionless) = 0.821170907; Vapor mass fraction (dimensionless) = 0.54114372
MW=100-300 g/mol, SG=0.65-0.90, shape=0.5-10; the conditional mean must also be achievable at that shape. At least two feed fractions >=0.001; C7+ is 0.004-0.99. T=180-450 K and p=0.1-10 MPa(a) are software bounds, not a validated accuracy envelope. No water, N2, CO2, H2S, solids, multiple liquids, volume shifts or density matching. Only methane/heavy-cut pairs share the user k_heavy (-0.2 to 0.4). All other pairs are zero. Properties use the pinned NeqSim Riazi-Daubert MW-SG variant plus explicit Edmister omega, not a silent Pedersen or Kesler-Lee fallback. Neither a solved flash nor a fitted parameter is proof of physical accuracy.
Select among recorded molecular-weight assumptions; no interpolation is labelled as a new flash solve.
Study data and reproduction
Original recorded synthetic cases solved with the site's gamma-c7-plus-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) | Vapor mole fraction (dimensionless) | Vapor mass fraction (dimensionless) |
|---|---|---|
| 160 | 0.8308345 | 0.5891119 |
| 180 | 0.8252841 | 0.5633749 |
| 200 | 0.8211709 | 0.5411437 |
| 210 | 0.8195683 | 0.5311118 |
| 220 | 0.8182105 | 0.5216835 |
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. If heavy mole fraction stays fixed while its molecular weight increases, does heavy mass fraction necessarily stay fixed?
Show the worked answer
No. Mass fractions must be recalculated from mole amounts and molecular weights.
2. Reproduce the baseline Vapor mole fraction (dimensionless) without rounding intermediate values.
Show the worked answer
M = 200 g/mol; with the fixed controls above, the workflow evaluates Vapor mole fraction = 0.821170907 dimensionless; Vapor mass fraction = 0.54114372 dimensionless. Vapor mole fraction (dimensionless) = 0.821170907. MW=100-300 g/mol, SG=0.65-0.90, shape=0.5-10; the conditional mean must also be achievable at that shape. At least two feed fractions >=0.001; C7+ is 0.004-0.99. T=180-450 K and p=0.1-10 MPa(a) are software bounds, not a validated accuracy envelope. No water, N2, CO2, H2S, solids, multiple liquids, volume shifts or density matching. Only methane/heavy-cut pairs share the user k_heavy (-0.2 to 0.4). All other pairs are zero. Properties use the pinned NeqSim Riazi-Daubert MW-SG variant plus explicit Edmister omega, not a silent Pedersen or Kesler-Lee fallback. Neither a solved flash nor a fitted parameter is proof of physical accuracy.
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 220. Other controls remain as stated in the premise. Vapor mole fraction (dimensionless) changes from 0.830834483 to 0.8182105.
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. A deliberately bounded four-cut approximation over 100-300 g/mol, not a complete Whitson C7-to-C80 assay. Every cut uses the same supplied 60/60 F SG. Reject an unbracketed mean, retained gamma probability below 1e-6, Tb>=Tc or omega outside 0-1. Shape does not determine aromaticity or a measured carbon-number distribution. MW=100-300 g/mol, SG=0.65-0.90, shape=0.5-10; the conditional mean must also be achievable at that shape. At least two feed fractions >=0.001; C7+ is 0.004-0.99. T=180-450 K and p=0.1-10 MPa(a) are software bounds, not a validated accuracy envelope. No water, N2, CO2, H2S, solids, multiple liquids, volume shifts or density matching. Only methane/heavy-cut pairs share the user k_heavy (-0.2 to 0.4). All other pairs are zero. Properties use the pinned NeqSim Riazi-Daubert MW-SG variant plus explicit Edmister omega, not a silent Pedersen or Kesler-Lee fallback. Neither a solved flash nor a fitted parameter is proof of physical accuracy.
Before using the result
- Hold mole and mass bases distinct.
- Retain the characterization mapping.
- Compare with independent PVT targets.
MW=100-300 g/mol, SG=0.65-0.90, shape=0.5-10; the conditional mean must also be achievable at that shape. At least two feed fractions >=0.001; C7+ is 0.004-0.99. T=180-450 K and p=0.1-10 MPa(a) are software bounds, not a validated accuracy envelope. No water, N2, CO2, H2S, solids, multiple liquids, volume shifts or density matching. Only methane/heavy-cut pairs share the user k_heavy (-0.2 to 0.4). All other pairs are zero. Properties use the pinned NeqSim Riazi-Daubert MW-SG variant plus explicit Edmister omega, not a silent Pedersen or Kesler-Lee fallback. Neither a solved flash nor a fitted parameter is proof of physical accuracy.
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.