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. This system demixes on heating. The solution is a single liquid at moderate temperature and splits into two liquids as it approaches the critical point of n-pentane (469.7 K). This is lower critical solution temperature (LCST) behaviour.

Process Flow Diagram¶

graph LR
    F["Polymer solution<br/>polypropylene + n-pentane"] --> V["V-1<br/>Flash vessel<br/>(P, T)"]
    V -->|Light liquid outlet| L1["Lean<br/>almost pure n-pentane"]
    V -->|Heavy liquid outlet| L2["Concentrated<br/>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, 30 bar, 440 to 460 K (cloud point near 450 K)

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. Start just below the cloud point, at 440 K and 30 bar. At 30 bar the n-pentane stays liquid all the way to its critical temperature, so no vapour forms.
Flash Vessel V-1.
Three Material Streams for the vessel outlets: Vapour on the vapour outlet, Lean on the light liquid outlet and Concentrated on the heavy liquid outlet. The solvent-rich liquid is the less dense of the two, so it leaves through the light liquid outlet. The vapour outlet carries no flow at 30 bar.
Connect Solution → V-1, and the three product streams.

The saved flowsheet has only two outlet streams

In the pre-built polymer_flash.dwxmz, the stream Lean is connected to the vapour outlet of V-1 and Concentrated to the light liquid outlet. That wiring solves while the feed is a single liquid. Once the solution splits, V-1 stops with "A second liquid phase was found" because nothing is connected to the heavy liquid outlet. Add a third stream on the heavy liquid outlet to collect the polymer-rich phase. The light liquid outlet (the stream named Concentrated in that file) then carries the solvent-rich phase.

Polymer solution flash

4. Solve and sweep temperature¶

F6 ON → Solve. Then raise the feed temperature in steps (e.g. 440 → 445 → 450 → 455 K) and re-solve, keeping it below the critical temperature of n-pentane (469.7 K). At 30 bar the solution stays a single liquid up to 445 K. At 450 K it has crossed the cloud point and the feed 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
30 bar, up to 445 K	one liquid phase
30 bar, 450 K and above	two liquids: a lean (~solvent) and a concentrated (polymer-rich)
455 K, lean phase	about 0.30 kg/s, below 0.001 wt% polymer
455 K, concentrated phase	about 0.70 kg/s, about 29 wt% polymer
Vapour	none at 30 bar

Expected results

A homogeneous solution at moderate temperature that demixes into a dilute and a concentrated liquid as it is heated toward the critical point of the solvent. This is the LCST-type behaviour that Tumakaka et al. (2002) describe with PC-SAFT for polypropylene in n-pentane.

Understanding the Results¶

Small mixing entropy. One long chain contributes as little combinatorial entropy as one small molecule, so the entropic drive to mix is weak and a modest penalty is enough to split the solution.
Why heating splits it. Close to its critical point, n-pentane expands with temperature much faster than the polymer does. Mixing the dense polymer into the expanded solvent makes the solvent contract around the chains, and that loss of entropy grows with temperature until the solution splits. Raising the pressure compresses the solvent and pushes the cloud point to a higher temperature, which is why cloud curves for these systems are often reported as cloud pressures.
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 solution splits at a lower temperature. 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
Pre-built flowsheet: examples/saved/polymer_flash.dwxmz (add the heavy liquid outlet stream described in step 3 before you heat it past the cloud point)

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 440 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 vapour outlet "Vapour", a light
  liquid outlet "Lean" and a heavy liquid outlet "Concentrated"
- Connect Solution to V-1
- Solve, then report the polymer mass fraction in each outlet at 440 K,
  450 K and 455 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.