Engineering lesson
Peng Robinson versus SRK on the same supported mixture
Peng–Robinson and SRK use different cubic equations and attraction functions. Compare them at the same feed, pressure, temperature, component constants and interaction assumptions before interpreting phase-split differences.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Match the thermodynamic inputs
The study uses methane through n-hexane with feed mole fractions 0.50, 0.15, 0.15, 0.10, 0.05 and 0.05. Both models use the site's pinned CoolProp component constants and zero binary interaction parameters.
The same temperature and absolute pressure are supplied to each model. Differences therefore reflect the specified EOS implementations rather than different mixtures.
Compare balances and phase properties
Vapor mole fraction, mass fraction and phase densities answer different questions. Check component closure and fugacity consistency before comparing these physical outputs.
A small equilibrium residual demonstrates numerical consistency with the selected EOS. It does not show which EOS better represents an actual petroleum sample.
Use independent data for model selection
Laboratory phase splits, densities and saturation pressures can reveal model bias. Interaction tuning and heavy-fraction characterization introduce further choices.
The recorded table is a reproducible synthetic comparison without a field accuracy ranking. Open either workflow to rerun with a supported feed and documented assumptions.
Explore the example
Recorded using both supported compositional flash engines. Independent thermo reference fixtures are separately checked by the repository's compositional regression suite.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Temperature (K) = 280. PR vapor mole fraction: 0.657564116; SRK vapor mole fraction: 0.658742945.
- PR vapor mole fraction
- SRK vapor mole fraction
Worked example
PR and SRK on an identical six-component feed
p = 2 MPa(a); methane/ethane/propane/n-butane/n-pentane/n-hexane mole fractions 0.50/0.15/0.15/0.10/0.05/0.05. CoolProp 8, kij = 0; only T changes.
Relation and unit basis
zi = (1 − β)xi + βyi; fi,liquid = fi,vapor
β is vapor mole fraction. Both models use the same pure-component constants and numerical state inputs.
1. Define the baseline
Temperature (K) = 280
p = 2 MPa(a); methane/ethane/propane/n-butane/n-pentane/n-hexane mole fractions 0.50/0.15/0.15/0.10/0.05/0.05. CoolProp 8, kij = 0; only T changes.
2. Select the relation
zi = (1 − β)xi + βyi; fi,liquid = fi,vapor
β is vapor mole fraction. Both models use the same pure-component constants and numerical state inputs.
3. Substitute the values
At 280 K and 2 MPa: PR β = 0.657564116; SRK β = 0.658742945; the complete component balances and fugacity diagnostics are saved with each solve.
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
PR vapor mole fraction = 0.657564116; SRK vapor mole fraction = 0.658742945
These are model differences, not measured errors or an accuracy ranking.
Result and interpretation
PR vapor mole fraction = 0.657564116; SRK vapor mole fraction = 0.658742945
These are model differences, not measured errors or an accuracy ranking.
Select recorded temperature states and compare PR and SRK vapor mole fractions under identical feed and kij assumptions.
Exact scenario data
Recorded using both supported compositional flash engines. Independent thermo reference fixtures are separately checked by the repository's compositional regression suite.
| Temperature (K) | PR vapor mole fraction | SRK vapor mole fraction |
|---|---|---|
| 240 | 0.482854 | 0.4870871 |
| 260 | 0.5691658 | 0.5715311 |
| 280 | 0.6575641 | 0.6587429 |
| 300 | 0.7496529 | 0.749951 |
| 320 | 0.8434581 | 0.8429687 |
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 PR and SRK agree closely, does that prove the fluid model is accurate?
Show the worked answer
No. Shared assumptions can produce shared bias. Independent measurements are needed to assess accuracy.
2. Reproduce the baseline PR vapor mole fraction without rounding intermediate values.
Show the worked answer
At 280 K and 2 MPa: PR β = 0.657564116; SRK β = 0.658742945; the complete component balances and fugacity diagnostics are saved with each solve.. PR vapor mole fraction = 0.657564116. These are model differences, not measured errors or an accuracy ranking.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Temperature (K) changes from 240 to 320. Other controls remain as stated in the premise. PR vapor mole fraction changes from 0.482854017 to 0.843458076.
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. β is vapor mole fraction. Both models use the same pure-component constants and numerical state inputs. These are model differences, not measured errors or an accuracy ranking.
Before using the result
- Match feed and pure-component constants.
- State kij assumptions.
- Inspect numerical balances before comparing properties.
These are model differences, not measured errors or an accuracy ranking.
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.