Phase Envelope¶

In this tutorial you will generate a pressure-temperature (PT) phase envelope for a natural gas mixture using DWSIM's Classic UI Utilities menu. The phase envelope shows bubble-point and dew-point curves, the critical point, and the regions where the mixture exists as liquid, vapor, or two phases.

What you will learn

What a phase envelope is and why it matters for process design
How to access the Phase Envelope utility from the Utilities menu
How to read bubble-point, dew-point, cricondentherm, and cricondenbar from the chart
How to export the envelope data for external analysis

Prerequisites

Completed Mixer Basics
Concept of phase boundaries

Process Overview¶

A phase envelope is essential for natural gas processing, pipeline design, and any application where the phase state at given T and P is critical. Pipeline operators must keep gas single-phase (above the dew point) to avoid liquid dropout and slug flow.

For pure components, the phase boundary is a single curve (the vapor pressure curve). For mixtures, bubble and dew curves enclose a two-phase region with the critical point inside.

Key Design Parameters¶

Parameter	Value
Compounds	Methane (80%), Ethane (10%), Propane (5%), n-Butane (3%), n-Pentane (2%)
Property Package	Peng-Robinson
Composition basis	Molar

Step-by-Step in the Classic UI¶

1. Set up the simulation¶

File > New Chemical Process Model:

Compounds: Methane, Ethane, Propane, N-butane, N-pentane
Property Package: Peng-Robinson
Click Finish

Why Peng-Robinson?

Peng-Robinson is the industry standard for hydrocarbon mixtures, light gases, and high-pressure equilibria. As a cubic EOS it is fast to evaluate and gives reliable bubble/dew curves and critical points for natural gas and petroleum fractions. Avoid PR for highly polar or hydrogen-bonding systems (water-alcohol, electrolytes), where activity-coefficient models are more appropriate.

2. Create a material stream with the gas composition¶

Drag a Material Stream, name it NatGas. The stream conditions are arbitrary for envelope generation; what matters is composition.

In the Object Editor:

T = 300 K, P = 1 atm, Molar Flow = 1 mol/s
Composition (Mole Fraction):
- Methane: 0.80
- Ethane: 0.10
- Propane: 0.05
- N-butane: 0.03
- N-pentane: 0.02

Press Enter, then F6 ON → Solve to make sure the stream is calculated.

3. Open the Phase Envelope utility¶

In the FlowsheetForm menu: Utilities > Phase Envelope.

A dialog opens letting you choose:

Source stream: select NatGas from the dropdown
Property Package: defaults to the stream's package; can override here
Pressure range: leave at default (e.g., 1 to 200 bar) or customize
Temperature range: leave at default

Click Calculate.

Phase Envelope utility setup

4. View the chart¶

The utility computes the bubble and dew curves and displays them on a PT chart. Key points are labeled:

Bubble curve (left side, increasing pressure with temperature)
Dew curve (right side, decreasing then increasing)
Critical point at the top of the loop
Cricondentherm (rightmost point - max T)
Cricondenbar (topmost point - max P)

Phase envelope for natural gas

5. Read specific values¶

Hover the mouse over a point on either curve to see the exact T and P. Use the Lookup panel (if visible) or the Export Data button to save the curve points to CSV/clipboard.

You can also right-click the chart for options like Add Hydrate Curve (if the package supports it), Save Image, or Copy to Clipboard.

6. Save and revisit¶

Save the simulation (File > Save). The phase envelope chart is associated with the stream and can be reopened from the same Utilities menu.

To compare envelopes, you can create additional streams with different compositions, calculate envelopes for each, and overlay them in a single chart (right-click chart → Add Series).

Results and Validation¶

Variable	Expected Range
Critical temperature	190 - 220 K
Critical pressure	45 - 55 bar
Cricondentherm	250 - 300 K
Cricondenbar	55 - 70 bar

Expected results

The envelope should be a closed curve. For this methane-rich gas, the critical temperature is well below ambient, meaning the gas is supercritical at typical pipeline conditions (300 K, 70 bar).

Understanding the Results¶

The phase envelope reveals key design constraints:

Cricondentherm: max temperature at which liquid can exist
Cricondenbar: max pressure at which vapor can exist
Pipeline design: operating point must stay outside the envelope (single-phase vapor) to avoid liquid dropout

Why bubble and dew curves?

The bubble curve is computed by tracking the locus of points where the first vapor bubble appears under a (P, T) sweep; the dew curve tracks where the first liquid drop forms. The two curves meet at the critical point. Together they bound the two-phase region of the multi-component mixture and tell you, for any (T, P), whether the system is liquid, vapor, or two-phase.

Peng-Robinson is the standard EOS for hydrocarbon mixtures and gives reliable phase envelopes for natural gas.

Automating This Tutorial¶

Files in this repository

Python script: examples/intermediate/05_phase_envelope.py

FluentAPI (Python)MCP Server (JSON-RPC)LLM Prompt

from DWSIM.Automation.FluentAPI import Flowsheet, PropertyPackages, Q

fs = (Flowsheet.Create("PhaseEnvelopeTutorial")
      .WithCompounds("Methane", "Ethane", "Propane",
                     "N-butane", "N-pentane")
      .WithPropertyPackage(PropertyPackages.PengRobinson))

gas = (fs.AddMaterialStream("NatGas")
       .At(Q.Kelvin(300.0), Q.Pascal(101325.0))
       .WithMolarFlow(1.0.MolPerSecond())
       .SetCompoundMolarFlow("Methane", 0.80)
       .SetCompoundMolarFlow("Ethane", 0.10)
       .SetCompoundMolarFlow("Propane", 0.05)
       .SetCompoundMolarFlow("N-butane", 0.03)
       .SetCompoundMolarFlow("N-pentane", 0.02))

fs.Solve()
envelope = gas.GeneratePhaseEnvelope()
print(f"Critical T: {envelope.CriticalTemperatureK:.1f} K")
print(f"Critical P: {envelope.CriticalPressurePa/1e5:.2f} bar")

{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{
  "name":"dwsim.stream.phase_envelope",
  "arguments":{"flowsheet_id":"<ID>","name":"NatGas"}
}}

Output may vary

Results depend on the LLM's reasoning quality and tool-use accuracy. Always verify the simulation matches your intent before relying on the numbers.

Use DWSIM (via the MCP server) to build the following simulation:

- Create a flowsheet called "PhaseEnvelopeTutorial"
- Add Methane, Ethane, Propane, N-butane and N-pentane as
  compounds; set the property package to "PengRobinson"
- Add a material stream named "NatGas" at 300 K and 101325 Pa
  with a molar flow of 1 mol/s and mole-fraction composition
  Methane = 0.80, Ethane = 0.10, Propane = 0.05, N-butane = 0.03,
  N-pentane = 0.02
- Solve the flowsheet
- Generate the phase envelope of the NatGas stream
- Report the critical temperature (K) and the critical pressure
  (bar) of the mixture

Exercises

Add 1% CO2 and 1% N2 to the mixture (reduce methane to 78%). How does the envelope shape change?
Increase the pentane content to 10% (reduce methane to 72%). How does the cricondentherm shift?
Generate envelopes for a lean gas (95% methane, 5% ethane) and a rich gas (70% methane + heavier). Compare the two-phase regions side by side.

Next Steps¶

You have completed the intermediate track in the Classic UI! You can now model distillation columns, heat exchangers, reactors, recycle loops, and phase envelopes.

Continue to the Advanced Track for complete industrial process simulations, starting with Refrigeration Cycle.