Practice and answer pack
Compositional flash teaching cases with component balances
Practice equilibrium component balances, phase amount definitions and caloric specification choices using a recorded synthetic flash with explicit engine provenance.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Begin with a component ledger
For a chosen feed amount, compute total vapor and liquid moles from β. Then multiply by each phase composition to obtain component amounts.
The sum of vapor and liquid component i must recover its feed amount. Checking only total phase moles can miss a component allocation error.
Distinguish fraction bases
The exercises separate vapor mole fraction, vapor mass fraction and phase compositions. A volume occupancy additionally requires phase densities and is not automatically a flowing holdup.
The worked case is fixed at its recorded temperature and pressure. Scaling total feed amount does not change an intensive equilibrium split under these assumptions.
Explain what was specified and solved
PT, PH and PS problems supply different constraints. The answer should identify which thermal variable is an input and which is solved.
Print the exercise sheet or download the component table. The case is synthetic and cannot establish an EOS's accuracy for a real petroleum fluid.
Explore the example
Fixed snapshot from the site's CoolProp 8 PR engine, independently compared with the thermo reference fixture. Selecting a row does not run a new flash.
Select one of 6 recorded scenarios. Intermediate states are not interpolated.
Component number (C1 to nC6) = 1. Feed component (mol): 50; Liquid component (mol): 3.32662902; Vapor component (mol): 46.673371.
- Feed component (mol)
- Liquid component (mol)
- Vapor component (mol)
Worked example
Component allocation in one recorded flash
Fixed PR case: 280 K, 2 MPa(a), total feed 100 mol, vapor mole fraction 0.657564115965. Component number selects methane through n-hexane.
Relation and unit basis
feed_i = 100zi; liquid_i = 100(1 − β)xi; vapor_i = 100βyi
Rows 1–6 are C1, C2, C3, nC4, nC5 and nC6. The equilibrium state does not change with component selection.
1. Define the baseline
Component number (C1 to nC6) = 1
Fixed PR case: 280 K, 2 MPa(a), total feed 100 mol, vapor mole fraction 0.657564115965. Component number selects methane through n-hexane.
2. Select the relation
feed_i = 100zi; liquid_i = 100(1 − β)xi; vapor_i = 100βyi
Rows 1–6 are C1, C2, C3, nC4, nC5 and nC6. The equilibrium state does not change with component selection.
3. Substitute the values
Methane feed = 100 × 0.5 = 50 mol; liquid = 100 × (1 − 0.657564116) × 0.0971460404; vapor = 100 × 0.657564116 × 0.70979194; liquid + vapor = 50 mol
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Feed component (mol) = 50; Liquid component (mol) = 3.32662902; Vapor component (mol) = 46.673371
Phase composition and phase amount have different denominators. Every component must close independently.
Result and interpretation
Feed component (mol) = 50; Liquid component (mol) = 3.32662902; Vapor component (mol) = 46.673371
Phase composition and phase amount have different denominators. Every component must close independently.
The selected component changes the displayed balance row while the recorded equilibrium state remains fixed.
Exact scenario data
Fixed snapshot from the site's CoolProp 8 PR engine, independently compared with the thermo reference fixture. Selecting a row does not run a new flash.
| Component number (C1 to nC6) | Feed component (mol) | Liquid component (mol) | Vapor component (mol) |
|---|---|---|---|
| 1 | 50 | 3.326629 | 46.67337 |
| 2 | 15 | 4.264388 | 10.73561 |
| 3 | 15 | 8.743734 | 6.256266 |
| 4 | 10 | 8.296928 | 1.703072 |
| 5 | 5 | 4.707416 | 0.2925842 |
| 6 | 5 | 4.904494 | 0.09550565 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Practice problems and answer key
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. For a fixed PT state, what happens to β if the total feed amount doubles?
Show the worked answer
It remains unchanged under the same intensive equilibrium model; all phase and component amounts double.
2. Reproduce the baseline Feed component (mol) without rounding intermediate values.
Show the worked answer
Methane feed = 100 × 0.5 = 50 mol; liquid = 100 × (1 − 0.657564116) × 0.0971460404; vapor = 100 × 0.657564116 × 0.70979194; liquid + vapor = 50 mol. Feed component (mol) = 50. Phase composition and phase amount have different denominators. Every component must close independently.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Component number (C1 to nC6) changes from 1 to 6. Other controls remain as stated in the premise. Feed component (mol) changes from 50 to 5.
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. Rows 1–6 are C1, C2, C3, nC4, nC5 and nC6. The equilibrium state does not change with component selection. Phase composition and phase amount have different denominators. Every component must close independently.
5. A 100 mol feed has β = 0.6 and vapor methane fraction y = 0.8. Calculate vapor methane amount.
Show the worked answer
100 × 0.6 × 0.8 = 48 mol. Vapor total is 60 mol; methane composition and phase fraction are different denominators.
6. Which outlet temperature is specified in a PH flash?
Show the worked answer
It is solved, not specified. Outlet pressure and enthalpy constraint determine temperature and phase state under the selected model.
Before using the result
- Check each component balance.
- Name fraction denominators.
- State the flash specification and model assumptions.
Phase composition and phase amount have different denominators. Every component must close independently.
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.