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Polymer Devolatilization

Devolatilization is the finishing step of almost every polymer process: the residual solvent or monomer is stripped from the polymer so the product leaves as a clean melt. In this tutorial you will heat a polymer solution under vacuum and flash it, recovering the solvent as vapour and concentrating the polymer to a high-purity melt.

What you will learn

  • Why a polymer is non-volatile and stays in the liquid during a flash
  • How to devolatilize a polymer solution with a heater and a flash vessel
  • How the dedicated PC-SAFT polymer flash keeps the polymer feed conserved

Prerequisites

Process Overview

A polymer chain has an immeasurably low vapour pressure, so in any vapour-liquid flash it stays entirely in the liquid while the small-molecule solvent boils off. Devolatilization exploits this: heat the solution and pull a vacuum so the solvent flashes, and the polymer is left behind as a concentrated melt. DWSIM routes a polymer solution to a dedicated PC-SAFT flash that keeps the whole polymer feed in the liquid and conserves it exactly.

We devolatilize a 20 wt% poly(ethylene glycol) solution in water (dewatering), heating it under a mild vacuum.

Process Flow Diagram

graph LR
    F["Aqueous PEG<br/>20 wt%<br/>320 K, 1 atm"] --> H["H-1<br/>Heater<br/>355 K, 0.4 bar"]
    H --> V["V-1<br/>Flash vessel"]
    V -->|Vapour| W["Water vapour"]
    V -->|Liquid| P["Concentrated PEG"]

Key Design Parameters

Parameter Value
Compounds Water, Poly(ethylene glycol)
Property Package PC-SAFT (with association)
PEG molar mass 10 000 g/mol
Feed 1 kg/s, 80 wt% water / 20 wt% PEG, 320 K, 1 atm
Heater outlet 355 K, drop to 0.4 bar

Step-by-Step in the Classic UI

1. Set up

File > New Chemical Process Model:

  • Compounds: Water, Poly(ethylene glycol).
  • Property Package: PC-SAFT.
  • Set the PEG Molar Weight to 10000 g/mol in Simulation Settings > Compounds.

Why PC-SAFT with association for PEG?

Poly(ethylene glycol) hydrogen-bonds with water through its ether oxygens and end hydroxyls, so it holds water strongly. PC-SAFT's association term captures that; a cubic equation of state cannot, and would over-predict how easily the water leaves.

2. Build the devolatilizer

  1. Material Stream Aqueous-PEG: 1 kg/s, mass fractions water = 0.80, PEG = 0.20, at 320 K and 1 atm.
  2. Heater H-1: outlet temperature 355 K, pressure drop from 1 atm to 0.4 bar (a mild vacuum), efficiency 100%. Energy stream Q_heat.
  3. Material Stream Heated (empty).
  4. Flash Vessel V-1.
  5. Material Stream Water-Vapour (vapour outlet) and Concentrated-PEG (liquid outlet).
  6. Connect Aqueous-PEG → H-1 → V-1, and the two product streams.

Polymer devolatilization

3. Solve

F6 ON → Solve.

4. Inspect results

  • Water-Vapour Results: essentially pure water, no PEG.
  • Concentrated-PEG Results: the PEG mass fraction has risen from 20 wt% to about 70 wt%.
  • Check the overall mass balance: feed = vapour + concentrate.

Results and Validation

Variable Expected
Water-vapour purity > 99.9 wt% water
PEG in the vapour ≈ 0
Concentrated liquid > 50 wt% PEG
Mass balance feed = vapour + concentrate

Expected results

The water flashes off as pure vapour and the PEG is concentrated to roughly 70 wt%, with the whole polymer feed conserved in the liquid.

Understanding the Results

  • The polymer never leaves. Because the chain is non-volatile, the flash puts all of it in the liquid; only the solvent partitions. The dedicated polymer flash enforces this and conserves the polymer feed.
  • Vacuum does the work. Dropping the pressure lets the solvent flash at a lower temperature, which matters for heat-sensitive polymers. Deeper vacuum or higher temperature strips more solvent.
  • Association sets the limit. For a hydrogen-bonding polymer like PEG, the last few percent of water is held tightly; concentrating beyond ~70 wt% needs a much deeper vacuum or a second stage.

Automating This Tutorial

Files in this repository

See examples/polymers/03_polymer_devolatilization.py in the DWSIM.Tutorials repository.

dwsim.compound.add, dwsim.compound.set_property for the PEG molar mass, then dwsim.unitop.add for the heater and the flash vessel.

Output may vary

Results depend on the LLM's reasoning quality and tool-use accuracy.

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

- Create a flowsheet "PEGDewatering" with Water and Poly(ethylene
  glycol); property package PC-SAFT; set the PEG molar weight to 10000
- Add a material stream "Aqueous-PEG" at 320 K, 1 atm, 1 kg/s, mass
  fractions water = 0.80, PEG = 0.20
- Add a Heater "H-1" with outlet 355 K and outlet pressure 0.4 bar
- Add a Flash Vessel "V-1" with vapour outlet "Water-Vapour" and liquid
  outlet "Concentrated-PEG"
- Connect Aqueous-PEG to H-1 to V-1, solve, and report the water purity
  of the vapour, the PEG mass fraction of the concentrate, and the mass
  balance

Exercises

  1. Lower the flash pressure to 0.15 bar. How much further does the PEG concentrate?
  2. Replace PEG/water with polystyrene in ethylbenzene and devolatilize the melt at 470 K and 0.15 bar.
  3. Add a second devolatilizer stage (heater + flash) on the concentrate. What final purity do you reach?

Further Reading

  • R. J. Albalak (ed.). (1996). Polymer Devolatilization. Marcel Dekker
  • J. Gross & G. Sadowski. (2002). Application of the Perturbed-Chain SAFT Equation of State to Associating Systems. Industrial & Engineering Chemistry Research. doi:10.1021/ie010954d

Next Steps

Continue with the Polymerization Reactor to make the polymer in the first place.