Engineering lesson
Bubble point versus dew point for petroleum fluids
At a specified temperature, bubble pressure marks the onset of vapor from liquid; dew pressure marks the onset of liquid from vapor. Mixture composition and the path through state space determine which boundary is encountered.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Read a phase boundary as an incipient phase
Near a bubble boundary, the vapor amount tends toward zero while the parent phase is liquid. Near a dew boundary, liquid is incipient in a vapor parent.
The compositions of the parent and incipient phases generally differ. A bubble-point result is not the same as a 50/50 phase split.
A mixture can have a phase envelope
The two boundaries form branches in pressure–temperature space for a fixed overall mixture. Near critical regions the phases become difficult to distinguish and numerical continuation can require care.
The linked bubble/dew workflow uses its documented hydrocarbon model. Do not treat a fixed-composition envelope as a history of a reservoir whose overall composition changes through production.
Use a full flash inside the envelope
A boundary calculation identifies onset; an interior flash determines phase amounts and compositions at specified P and T. These solve related but different questions.
The example displays recorded boundary data from the supported workflow rather than redrawing a generic envelope and assigning it invented pressure values.
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.
The panel samples a documented fixed-feed boundary calculation. It is not a universal phase diagram for petroleum fluids.
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. At a bubble point, is the vapor fraction generally one half?
Show the worked answer
No. The vapor phase is incipient, so its amount approaches zero on the liquid-side boundary.
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
- State overall composition.
- Distinguish onset from interior split.
- Check the numerical critical-region scope.
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 · Related method 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.