Polymerization Reactor¶

In this capstone tutorial you will make a polymer. The free-radical polymerization reactor solves the reaction kinetics by the method of moments, so it reports not just a conversion but the molecular weight of the polymer it produces: the number- and weight-average molar mass and the polydispersity. You will run a styrene homopolymerization, then switch to a styrene/MMA copolymer and read its composition.

What you will learn

How the polymerization reactor turns monomer and initiator into a polymer with a computed molar mass
The difference between a well-mixed (CSTR) and a plug-flow / batch (PFR) reactor, and composition drift
How a copolymer's composition follows the Mayo-Lewis model from the reactivity ratios

Prerequisites

Completed PC-SAFT Polymer Solutions and the Reaction Systems tutorial

Process Overview¶

A free-radical polymerization has four steps: an initiator decomposes into radicals, each radical grows by adding monomer (propagation), growing chains stop by combining or disproportionating (termination), and a chain can hand its radical to another molecule (transfer). The reactor solves these by the method of moments, which tracks the statistical moments of the chain-length distribution and returns Mn, Mw and the polydispersity directly, without enumerating every chain length.

For two monomers the reactor uses the terminal model: the instantaneous copolymer composition follows the Mayo-Lewis equation from the two reactivity ratios.

We polymerize styrene (using ethylbenzene as its PC-SAFT stand-in) with a soluble initiator.

Process Flow Diagram¶

graph LR
    F["Monomer + initiator"] --> R["R-1<br/>Polymerization reactor<br/>(CSTR, 90 C)"]
    R --> P["Polymer solution<br/>polymer + residual monomer"]
    Q["Q_reactor"] -.-> R

Key Design Parameters¶

Parameter	Value
Monomer	Ethylbenzene (styrene stand-in)
Initiator	N-pentane (soluble-initiator stand-in)
Polymer product	Polystyrene
Property Package	PC-SAFT
Reactor	CSTR, isothermal 90 C (363 K)
Kinetics preset	Styrene / AIBN

Why stand-in compounds?

Styrene has no shipped PC-SAFT parameters, so ethylbenzene, its saturated analogue, plays the monomer; n-pentane plays a soluble initiator. The reactor kinetics are the real AIBN-initiated styrene set, so the molar mass and polydispersity are those of actual polystyrene.

Step-by-Step in the Classic UI¶

1. Set up¶

File > New Chemical Process Model:

Compounds: Ethylbenzene, N-pentane, Polystyrene.
Property Package: PC-SAFT.

2. Add the polymerization reactor¶

From the object palette, under Reactors, drag a Polymerization Reactor onto the canvas (R-1).

Add a Material Stream Feed (1 kg/s, mass fractions ethylbenzene = 0.99, n-pentane = 0.01), a Material Stream Polymer-Solution for the product, and an Energy Stream Q_reactor. Connect Feed → R-1 → Polymer-Solution and the energy stream to the reactor.

3. Configure the reactor¶

Open the R-1 editor:

Monomer: Ethylbenzene; Initiator: N-pentane; Polymer product: Polystyrene.
Flow model: Well-mixed (CSTR); Operation mode: Isothermic; Temperature: 363.15 K; Volume: 30 m3.
Kinetics preset: Styrene / AIBN.

Polymerization reactor editor

4. Solve and read the polymer¶

F6 ON → Solve. On the R-1 Results tab, read:

Conversion of monomer to polymer
Number-average molar mass (Mn) and Weight-average molar mass (Mw)
Polydispersity index (Mw/Mn)

Results and Validation¶

Variable	Expected
Conversion	~0.6-0.8
Mn	order 10 000-100 000 g/mol
Polydispersity	~1.5 (combination termination)

Expected results

A meaningful conversion in one residence time, a number-average molar mass of order 10^4-10^5 g/mol, and a polydispersity near 1.5, the theoretical value for termination by combination with little chain transfer, which is how styrene behaves.

Going Further: copolymer and plug-flow¶

Copolymerization (Mayo-Lewis)¶

Set a Second monomer in the reactor editor (for example Toluene as an MMA stand-in) to switch on the copolymer model, and enter the two reactivity ratios (styrene/MMA: r1 = 0.52, r2 = 0.46). Feed the two monomers together. On the Results tab the reactor now reports the copolymer composition: the mole fraction of monomer 1 in the chains, which follows the Mayo-Lewis equation from the reactivity ratios.

Composition and molar mass are decoupled

The reactivity ratios set the composition; the initiator level and kinetics set the molar mass. Chain transfer and the gel effect change the molar mass but leave the composition on the Mayo-Lewis curve.

Plug-flow / batch and composition drift¶

Switch the Flow model to Plug flow / batch (PFR). A plug-flow (or batch) reactor is not perfectly mixed along its length, so as the more reactive monomer depletes the instantaneous copolymer composition drifts. The reactor reports both the instantaneous composition at the outlet and the cumulative composition averaged over all the polymer made. For a stirred tank the composition is instead pinned at the single outlet condition, so it does not drift.

Automating This Tutorial¶

Files in this repository

Python script: examples/polymers/04_polymerization_reactor.py

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

See examples/polymers/04_polymerization_reactor.py in the DWSIM.Tutorials repository. The reactor is placed with fs.Inner.AddObject(ObjectType.RCT_Polymerization, x, y, "R-1") and configured through its properties.

dwsim.unitop.add with type RCT_Polymerization, then dwsim.unitop.set_property for the monomer/initiator/polymer roles, the kinetics preset, and the operating conditions.

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 "StyrenePolymerization" with Ethylbenzene,
  N-pentane and Polystyrene; property package PC-SAFT
- Add a material stream "Feed" at 363 K, 2 bar, 1 kg/s, mass fractions
  ethylbenzene = 0.99, n-pentane = 0.01
- Add a Polymerization Reactor "R-1": monomer = Ethylbenzene, initiator
  = N-pentane, polymer product = Polystyrene, CSTR, isothermal 363 K,
  volume 30 m3, kinetics preset "Styrene / AIBN"; energy stream
  Q_reactor
- Connect Feed to R-1, solve, and report conversion, Mn, Mw and the
  polydispersity index

Exercises

Raise the initiator fraction to 4 wt%. Does the molar mass go up or down, and why?
Switch to Adiabatic operation and watch the temperature rise from the exotherm.
Turn on a second monomer with a skewed feed (1:3) and run it as a PFR to high conversion. How far does the instantaneous composition drift from the cumulative?
Enable the distribution emission so the reactor writes the polymer as a set of molar-mass cuts, then devolatilize it as in tutorial 3.

Next Steps¶

You have completed the polymer track: PC-SAFT thermodynamics, molecular-weight distributions, devolatilization, and the polymerization reactor. Combine them into a full make-and-finish flowsheet: a polymerization reactor feeding a devolatilizer that recovers the monomer and delivers a clean polymer melt.