Engineering lesson
Bubble dew and critical regions on a phase envelope
A fixed-mixture phase envelope connects bubble and dew boundaries in pressure–temperature space. Near the critical region, phase distinctions shrink and a numerical branch needs careful interpretation.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Read the branches, not just the outline
Bubble and dew branches describe incipient vapor and liquid respectively. Interior two-phase states require a flash to determine the phase amounts.
The recorded example uses the supported six-component feed and the workflow's documented PR assumptions. It is not a generic envelope for every hydrocarbon mixture.
Critical and envelope extrema differ
The highest pressure or temperature on a mixture envelope need not coincide with the critical point. Labels such as cricondenbar and cricondentherm describe different extrema.
A visually rounded join on a plot is not sufficient evidence of a critical solution. Use the workflow's branch data and diagnostics.
Keep composition fixed along the diagram
An envelope is constructed for one overall feed. A depletion process that removes phases can change overall composition and therefore change its envelope.
The explorer selects recorded boundary points. It does not interpolate a new critical point or claim that a straight segment is a solved boundary.
Explore the example
Recorded supported hydrocarbon-bubble-dew-band workflow; the full PR envelope tool is linked for continuation beyond this band.
Select one of 15 recorded scenarios. Intermediate states are not interpolated.
Temperature (K) = 285. Bubble pressure (MPa(a)): 9.58148139; Dew pressure (MPa(a)): 0.159044815.
- Bubble pressure (MPa(a))
- Dew pressure (MPa(a))
Worked example
Recorded fixed-feed bubble and dew band
Six-component feed 0.50/0.15/0.15/0.10/0.05/0.05, PR-based supported boundary workflow, 280–290 K. The complete branch record is downloadable.
Relation and unit basis
Incipient-phase boundary: phase fugacities equal, with the new phase amount approaching zero
Boundary pressure is not an interior phase fraction. The selected band does not claim to locate the critical point.
1. Define the baseline
Temperature (K) = 285
Six-component feed 0.50/0.15/0.15/0.10/0.05/0.05, PR-based supported boundary workflow, 280–290 K. The complete branch record is downloadable.
2. Select the relation
Incipient-phase boundary: phase fugacities equal, with the new phase amount approaching zero
Boundary pressure is not an interior phase fraction. The selected band does not claim to locate the critical point.
3. Substitute the values
Supply the stated overall composition and temperature band; solve the two incipient-phase branches with the documented engine. Read each pressure with its bubble or dew label.
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Bubble pressure (MPa(a)) = 9.58148139; Dew pressure (MPa(a)) = 0.159044815
Bubble and dew are different onset branches. This limited temperature band is not a full critical-region envelope.
Result and interpretation
Bubble pressure (MPa(a)) = 9.58148139; Dew pressure (MPa(a)) = 0.159044815
Bubble and dew are different onset branches. This limited temperature band is not a full critical-region envelope.
Select a recorded branch point and inspect its state labels. The full workflow produces the envelope data.
Exact scenario data
Recorded supported hydrocarbon-bubble-dew-band workflow; the full PR envelope tool is linked for continuation beyond this band.
| Temperature (K) | Bubble pressure (MPa(a)) | Dew pressure (MPa(a)) |
|---|---|---|
| 280 | 9.242807 | 0.1260187 |
| 280.7143 | 9.292321 | 0.1303525 |
| 281.4286 | 9.341466 | 0.1348097 |
| 282.1429 | 9.390238 | 0.1393929 |
| 282.8571 | 9.438631 | 0.1441052 |
| 283.5714 | 9.486639 | 0.1489493 |
| 284.2857 | 9.534258 | 0.1539282 |
| 285 | 9.581481 | 0.1590448 |
| 285.7143 | 9.628305 | 0.1643023 |
| 286.4286 | 9.674722 | 0.1697036 |
| 287.1429 | 9.720729 | 0.1752519 |
| 287.8571 | 9.766319 | 0.1809505 |
| 288.5714 | 9.811487 | 0.1868026 |
| 289.2857 | 9.856229 | 0.1928114 |
| 290 | 9.900538 | 0.1989804 |
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. Is the maximum envelope pressure necessarily the critical pressure?
Show the worked answer
No. For a mixture the envelope maximum and critical point can occur at different states.
2. Reproduce the baseline Bubble pressure (MPa(a)) without rounding intermediate values.
Show the worked answer
Supply the stated overall composition and temperature band; solve the two incipient-phase branches with the documented engine. Read each pressure with its bubble or dew label.. Bubble pressure (MPa(a)) = 9.58148139. Bubble and dew are different onset branches. This limited temperature band is not a full critical-region envelope.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Temperature (K) changes from 280 to 290. Other controls remain as stated in the premise. Bubble pressure (MPa(a)) changes from 9.24280714 to 9.90053818.
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. Boundary pressure is not an interior phase fraction. The selected band does not claim to locate the critical point. Bubble and dew are different onset branches. This limited temperature band is not a full critical-region envelope.
Before using the result
- Keep overall composition fixed.
- Distinguish branch onset from phase amount.
- Inspect critical-point diagnostics separately.
Bubble and dew are different onset branches. This limited temperature band is not a full critical-region envelope.
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.