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Benzene/Toluene Separation

In this tutorial you will build a complete benzene/toluene separation in DWSIM's Classic UI: a feed preheater followed by a rigorous distillation column.

What you will learn

  • How to combine a Heater and a Distillation Column into a process train
  • How to size a column for high-purity binary separation
  • How feed preheating affects column duty distribution

Prerequisites

Process Overview

Benzene (BP 80.1 °C) and toluene (BP 110.6 °C) form a nearly ideal mixture with no azeotrope. Relative volatility α ≈ 2.5, large enough that high-purity separation is possible with moderate stages.

Industrial setups preheat the feed to its bubble point before the column. Preheating reduces reboiler duty and improves separation efficiency at the feed stage.

Process Flow Diagram

graph LR
    F["Feed<br/>50% BZ / 50% TOL<br/>300 K"] --> HX["H-1<br/>Preheater<br/>370 K"]
    HX --> COL["DC-1<br/>30 stages"]
    COL -->|Distillate| D["Distillate<br/>(BZ-rich)"]
    COL -->|Bottoms| B["Bottoms<br/>(TOL-rich)"]

Key Design Parameters

Parameter Value
Compounds Benzene, Toluene
Property Package Peng-Robinson
Feed 100 mol/s, 50/50, 300 K, 1 atm
Preheater outlet 370 K
Stages 30, feed at stage 15
Reflux ratio 3.0
Bottoms flow 50 mol/s

Step-by-Step in the Classic UI

1. Set up

File > New Chemical Process Model:

  • Compounds: Benzene, Toluene
  • Property Package: Peng-Robinson

Why Peng-Robinson for BT separation?

Benzene and toluene are non-polar aromatics; PR captures their VLE accurately. Activity-coefficient models like NRTL or UNIQUAC would over-engineer the problem since this is a nearly ideal binary with no azeotrope.

2. Build the train

Drag and configure:

  1. Material Stream Feed: T=300 K, P=1 atm, molar flow=100 mol/s, Benzene=Toluene=0.5 mole frac
  2. Material Stream Preheated (empty)
  3. Heater H-1: outlet T=370 K, ΔP=0, η=100%; Inlet=Feed, Outlet=Preheated, create energy stream
  4. Material Stream Distillate (empty)
  5. Material Stream Bottoms (empty)
  6. Distillation Column DC-1:
  7. Connections: Feed=Preheated at stage 15, Distillate, Bottoms; create condenser+reboiler energy streams
  8. Configuration: 30 stages, Total Condenser, uniform pressure 1 atm
  9. Specifications: Reflux Ratio = 3.0, Bottoms Molar Flow = 50 mol/s

Why 30 stages and Reflux Ratio = 3?

BT has a relative volatility of about 2.4, requiring moderate reflux. 30 stages gives high purity (>99%) for both products. Fewer stages or lower R compromise distillate purity; higher R wastes reboiler energy.

BT separation flowsheet

3. Solve

F6 ON → Solve. The column may take several iterations.

4. Read results

  • Distillate: Benzene mole fraction > 0.95
  • Bottoms: Toluene mole fraction > 0.95
  • DC-1 Results: Condenser duty, Reboiler duty, plus Stage Profile (T from ~353 K at top to ~383 K at bottom)
  • H-1 Results: Preheater duty (smaller than reboiler duty by an order of magnitude)

Results and Validation

Variable Expected
Distillate benzene purity > 0.95
Bottoms toluene purity > 0.95
Distillate T ~353 K
Bottoms T ~383 K

Expected results

With 30 stages and reflux ratio 3.0, both products reach > 95% purity. Distillate T approaches benzene's BP (353 K), bottoms approaches toluene's (384 K).

Understanding the Results

The benzene/toluene system is the classic ideal binary distillation. Key observations:

  • Feed thermal condition: preheating to 370 K (saturated liquid) places the feed near the column's internal liquid composition at stage 15, minimizing thermodynamic mixing losses.
  • Reflux trade-off: higher reflux improves purity but increases energy consumption.
  • Stage efficiency: real trays have 60-80% efficiency, so 30 theoretical stages might require 40-50 actual trays.

Automating This Tutorial

Files in this repository

See examples/advanced/03_benzene_toluene.py in the DWSIM.Tutorials repository.

Standard sequence of dwsim.unitop.add calls for Heater and DistillationColumn.

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 "BenzeneTolueneSeparation"
- Add Benzene and Toluene as compounds; set the property package to
  "Peng-Robinson"
- Add a material stream "Feed" at 300 K and 1 atm, molar flow = 100 mol/s,
  mole fractions Benzene = 0.5, Toluene = 0.5
- Add a Heater "H-1" with outlet T = 370 K, ΔP = 0, efficiency = 100%;
  inlet = Feed, outlet = Preheated, with an energy stream
- Add a Distillation Column "DC-1" with 30 stages, total condenser,
  uniform pressure = 1 atm; feed Preheated at stage 15; specifications:
  reflux ratio = 3.0, bottoms molar flow = 50 mol/s; outlets =
  Distillate (top) and Bottoms (bottom), with condenser and reboiler
  energy streams
- Solve the flowsheet
- Report the benzene mole fraction in Distillate, the toluene mole
  fraction in Bottoms, the distillate and bottoms temperatures, and the
  condenser and reboiler duties

Exercises

  1. Reduce stages to 15 with same reflux. Can you still hit > 95% purity?
  2. Skip the preheater (feed at 300 K directly into the column). How much does the reboiler duty increase?
  3. Increase feed benzene to 80 mol%. Find a better feed stage by trying 5, 10, 15.

Further Reading

Selected references from the DWSIM technical bibliography. Click the DOI link to access each paper.

  • Henry Z. Kister. (1992). Distillation Design. McGraw-Hill Professional
  • J. D. Seader & Ernest J. Henley. (2005). Separation Process Principles. Wiley
  • R. N. Watkins. (1979). Petroleum Refinery Distillation. Gulf Publishing
  • W.L. McCabe, J. Smith & P. Harriott. (2005). Unit Operations of Chemical Engineering. McGraw-Hill Education

Next Steps

In Natural Gas Processing, you will model a multi-component dew-point control unit.