PC-SAFT Polymer Solutions¶

In this tutorial you will model a polymer dissolved in a solvent with the PC-SAFT equation of state, and find the cloud point where the solution demixes into a dilute (lean) and a concentrated liquid. This is the thermodynamic foundation for every other polymer tutorial.

What you will learn

How PC-SAFT represents a polymer as a chain whose segment number grows with molar mass
How to add a built-in polymer to a flowsheet and pick its molar mass
How to detect and interpret a polymer-solution cloud point (liquid-liquid demixing)

Prerequisites

Completed the Intermediate Track
A look at the Property Packages Guide

Process Overview¶

A polymer does not behave like a small molecule: a single chain carries thousands of segments, so its mixing entropy with a solvent is tiny and small temperature changes can push a homogeneous solution to split into two liquid phases. The boundary of that two-phase region is the cloud point.

PC-SAFT captures this because it models a molecule as a chain of m spherical segments, and for a polymer m grows with the molar mass (m = (m/M)·Mn). One set of segment parameters therefore describes the whole homologous series; you only supply the molar mass.

We model polypropylene in n-pentane and locate its liquid-liquid split.

Process Flow Diagram¶

graph LR
    F["Polymer solution<br/>polypropylene + n-pentane"] --> V["V-1<br/>Flash vessel<br/>(P, T)"]
    V -->|Lean liquid| L1["Dilute phase"]
    V -->|Concentrated liquid| L2["Polymer-rich phase"]

Key Design Parameters¶

Parameter	Value
Compounds	Polypropylene, N-pentane
Property Package	PC-SAFT
Polymer molar mass	50 000 g/mol
Feed	1 kg/s, ~20 wt% polymer
Flash	isothermal, near the cloud point

Step-by-Step in the Classic UI¶

1. Set up¶

File > New Chemical Process Model:

Add the solvent N-pentane from the compound list.
Add the polymer: search the compound list for Polypropylene and add it. Built-in polymers ship with PC-SAFT segment parameters.
Property Package: PC-SAFT.

2. Set the polymer molar mass¶

Open Simulation Settings > Compounds, select Polypropylene, and set its Molar Weight to 50000 g/mol.

Why the molar mass matters

PC-SAFT reads the molar mass and turns it into a segment number. A 50 000 g/mol chain is far less soluble than a 5 000 g/mol one, so the cloud point moves with molar mass. This is the single most important input for a polymer.

3. Build the flash¶

Material Stream Solution: 1 kg/s, mass fractions n-pentane = 0.80, polypropylene = 0.20. Set T and P near the expected cloud point (start at 400 K, 30 bar).
Flash Vessel V-1.
Material Stream Lean (vapour/light outlet) and Concentrated (liquid outlet).
Connect Solution → V-1, and the two product streams.

Polymer solution flash

4. Solve and sweep temperature¶

F6 ON → Solve. Then lower the feed temperature in steps (e.g. 400 → 380 → 360 K) and re-solve. Below the cloud point the single feed liquid splits into two liquid phases with very different polymer content.

5. Inspect results¶

On the outlet streams, read the polymer mass fraction in each phase: one is nearly pure solvent (lean), the other is polymer-rich.
The temperature at which the second phase first appears is the cloud point for this composition.

Results and Validation¶

Variable	Expected
Above the cloud point	one liquid phase
Below the cloud point	two liquids: a lean (~solvent) and a concentrated (polymer-rich)
Lean-phase polymer fraction	very low

Expected results

A homogeneous solution at high temperature that demixes into a dilute and a concentrated liquid as it cools, reproducing the UCST-type behaviour of a polymer solution.

Understanding the Results¶

Small mixing entropy. One long chain contributes as little entropy as one small molecule, so the entropic drive to mix is weak. Enthalpy (segment-segment interactions) then decides miscibility, and cooling tips the balance toward demixing.
Molar mass sets the cloud point. Higher molar mass means fewer, longer chains and even less mixing entropy, so the two-phase region grows and the cloud point rises. Try it in the exercises.
One parameter set, many grades. Because m scales with molar mass, the same polypropylene parameters describe a 10 000 or a 200 000 g/mol grade; only the molar mass changes.

Automating This Tutorial¶

Files in this repository

Python script: examples/polymers/01_pcsaft_polymer_solution.py

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

See examples/polymers/01_pcsaft_polymer_solution.py in the DWSIM.Tutorials repository.

dwsim.compound.add for the solvent and the polymer, dwsim.compound.set_property to set the polymer molar mass, then dwsim.stream.add and dwsim.unitop.add for the flash.

Output may vary

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

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

- Create a flowsheet called "PolymerSolution"
- Add N-pentane and Polypropylene as compounds; set the property
  package to "PC-SAFT"
- Set the Polypropylene molar weight to 50000 g/mol
- Add a material stream "Solution" at 400 K and 30 bar, 1 kg/s, mass
  fractions n-pentane = 0.80 and Polypropylene = 0.20
- Add a Flash Vessel "V-1" with a lean outlet "Lean" and a
  concentrated outlet "Concentrated"
- Connect Solution to V-1
- Solve, then report the polymer mass fraction in each outlet at 400 K,
  380 K and 360 K

Exercises

Raise the polymer molar mass to 150 000 g/mol. Does the cloud point move up or down?
Change the solvent to n-hexane. How does the two-phase region shift?
Sweep the feed composition (10, 20, 40 wt% polymer) at fixed temperature and map where the solution is one phase or two.

Next Steps¶

Continue with Molecular-Weight Distribution to turn a single average molar mass into a real distribution of chain lengths.