Ammonia Synthesis (Haber-Bosch)

In this tutorial you will build a simplified Haber-Bosch ammonia synthesis loop in DWSIM's Classic UI: a single-pass train of compression, preheating, equilibrium reaction, cooling, and gas-liquid separation.

What you will learn

• How to build a multi-unit industrial flowsheet from feed to product
• How to define an equilibrium reaction with a temperature-dependent Keq
• How to verify atomic balances (H and N conservation)

Prerequisites

• Completed Reaction Systems and Refrigeration Cycle

Process Overview

Haber-Bosch: N2 + 3H2 → 2NH3 (exothermic, equilibrium-limited). Industrial conditions: 100-300 bar, 400-500 °C.

In this simplified single-pass model, we compress a 3:1 H2:N2 feed to 200 bar, heat to 700 K, react, cool to 250 K to condense liquid NH3, and separate gas from liquid.

Process Flow Diagram

graph LR
    F["Feed<br/>3H2 + 1N2<br/>30 bar"] --> C["C-1<br/>Compressor<br/>200 bar"]
    C --> H["H-1<br/>Heater<br/>700 K"]
    H --> R["R-1<br/>Eq. Reactor"]
    R --> CL["CL-1<br/>Cooler<br/>250 K"]
    CL --> V["V-1<br/>Separator"]
    V -->|Gas| PG["Purge Gas"]
    V -->|Liquid| NH3["Liquid NH3"]

Key Design Parameters

Parameter	Value
Compounds	Hydrogen, Nitrogen, Ammonia
Property Package	Peng-Robinson
Feed	3 H2 + 1 N2 mol/s, 300 K, 30 bar
Compressor outlet	200 bar, 75% efficiency
Reactor T	700 K (isothermal)
Cooler outlet	250 K
ln(Keq)	11000/T - 25.0

Step-by-Step in the Classic UI

1. Set up

File > New Chemical Process Model:

• Compounds: Hydrogen, Nitrogen, Ammonia
• Property Package: Peng-Robinson

Why Peng-Robinson at 200 bar?

PR handles high-pressure non-ideality and the supercritical-like behavior of light gases. Activity-coefficient models would be inappropriate at this pressure range; cubic EOS is the right tool for high-pressure gas-phase systems.

2. Define the equilibrium reaction

Edit > Simulation Settings > Reactions → Add New Reaction → Equilibrium type:

• Name: R_NH3
• Stoichiometry: Nitrogen = -1, Hydrogen = -3, Ammonia = +2
• Base compound: Ammonia
• Phase: Vapor, Basis: Activity
• ln(Keq): 11000/T - 25.0

Create a Reaction Set NH3Set and add the reaction to it.

Why an Equilibrium Reactor?

The Haber-Bosch reaction is severely equilibrium-limited at industrial conditions. A Conversion Reactor would force an arbitrary extent; the Equilibrium Reactor uses thermodynamics (Keq vs T) to determine the maximum NH3 yield achievable.

Why high pressure favors NH3?

By Le Chatelier's principle, the reaction goes from 4 mol of gas to 2 mol; high P pushes equilibrium toward fewer moles (NH3). Industrial loops run at 150-300 bar despite the compression cost, precisely for this reason.

3. Build the train (drag-and-drop)

Drag the following from the Object Palette in this order, renaming each:

1. Material Stream Feed (T=300 K, P=30 bar, total molar flow=4 mol/s, mole fractions: H2=0.75, N2=0.25, NH3=0)
2. Compressor C-1 (Adiabatic, outlet P=200 bar, η=75%)
3. Material Stream Comp-Out (empty)
4. Heater H-1 (outlet T=700 K, ΔP=0, η=100%)
5. Material Stream Hot (empty)
6. Equilibrium Reactor R-1 (Isothermal mode, Reaction Set = NH3Set, ΔP=0)
7. Material Stream Rx-Out (empty)
8. Cooler CL-1 (outlet T=250 K, ΔP=0, η=100%)
9. Material Stream Cold (empty)
10. Separator Vessel V-1
11. Material Stream Purge-Gas (empty)
12. Material Stream NH3-Liquid (empty)

Plus three energy streams: W_comp, Q_rx, Q_cool (auto-create from each unit's Energy connection).

4. Wire the connections

Open each unit operation's Object Editor and set its Connections panel:

• C-1: Inlet=Feed, Outlet=Comp-Out, Energy=W_comp
• H-1: Inlet=Comp-Out, Outlet=Hot
• R-1: Inlet=Hot, Outlet 1 (vapor)=Rx-Out, Outlet 2 (liquid)=Rx-Liq (create empty), Energy=Q_rx
• CL-1: Inlet=Rx-Out, Outlet=Cold, Energy=Q_cool
• V-1: Inlet=Cold, Vapor Outlet=Purge-Gas, Liquid Outlet=NH3-Liquid

5. Solve

F6 ON → Solve.

6. Inspect results

• NH3-Liquid stream: ammonia mole fraction should be > 0.50 (enriched)
• Purge-Gas stream: predominantly H2 and N2
• Energy streams: W_comp shows compression power, Q_rx and Q_cool show heat removal

Verify atomic balances using the Mass and Energy Balance Summary under Flowsheet Analysis menu.

Results and Validation

Variable	Expected
N2 conversion	5 - 99%
NH3 in liquid product	> 50 mol%
NH3 in purge gas	< 50 mol%
H atomic balance	Within 0.5%
N atomic balance	Within 0.5%

Expected results

Equilibrium-limited NH3 production at 700 K. Cooling to 250 K condenses NH3, separating it from unreacted H2/N2.

Understanding the Results

• Le Chatelier: high pressure (200 bar) shifts equilibrium toward products (4 mol → 2 mol)
• Temperature trade-off: lower T favors equilibrium but catalyst needs ~400-500 °C
• Separation by condensation: NH3 (BP -33 °C) condenses at 250 K / 200 bar; H2 (-253 °C) and N2 (-196 °C) remain gaseous

In a real plant, the purge gas would be recycled back to the compressor (with a small purge to remove inerts), pushing overall conversion above 95%.

Automating This Tutorial

Files in this repository

• Python script: examples/advanced/02_ammonia_synthesis.py
• Pre-built flowsheet: examples/saved/ammonia_synthesis.dwxmz

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

See examples/advanced/02_ammonia_synthesis.py in the DWSIM.Tutorials repository.

Build using dwsim.reaction.define_equilibrium, dwsim.unitop.add for each operation, dwsim.unitop.connect, and dwsim.solve.run.

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 "AmmoniaSynthesis"
- Add Hydrogen, Nitrogen and Ammonia as compounds; set the property
  package to "Peng-Robinson"
- Define an equilibrium reaction "R_NH3" with stoichiometry
  N2 = -1, H2 = -3, NH3 = +2; base compound = Ammonia, phase = Vapor,
  basis = Activity, ln(Keq) = 11000/T - 25.0; add it to a new reaction
  set "NH3Set"
- Add a material stream "Feed" at 300 K and 30 bar with total molar flow
  = 4 mol/s and mole fractions H2 = 0.75, N2 = 0.25, NH3 = 0
- Add a Compressor "C-1" (adiabatic, outlet P = 200 bar, efficiency = 75%)
  with energy stream W_comp
- Add a Heater "H-1" with outlet T = 700 K, ΔP = 0, efficiency = 100%
- Add an Equilibrium Reactor "R-1" in isothermal mode using reaction set
  NH3Set, ΔP = 0, with energy stream Q_rx
- Add a Cooler "CL-1" with outlet T = 250 K, ΔP = 0, efficiency = 100%,
  energy stream Q_cool
- Add a Separator Vessel "V-1" with vapor outlet "Purge-Gas" and liquid
  outlet "NH3-Liquid"
- Connect: Feed → C-1 → H-1 → R-1 → CL-1 → V-1
- Solve the flowsheet
- Report N2 conversion, the NH3 mole fraction in NH3-Liquid and in
  Purge-Gas, and the energy flows W_comp, Q_rx and Q_cool

Exercises

1. Lower reactor T to 500 K. How does conversion change?
2. Reduce compression to 50 bar. Equilibrium impact?
3. Add a recycle loop (Recycle logical operation) connecting Purge-Gas back to a Mixer at the feed.

Further Reading

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

• Kai Hou & Ronald Hughes. (2001). The Kinetics of Methane Steam Reforming over a Ni/α-Al₂O₃ Catalyst. Chemical Engineering Journal. doi:10.1016/S1385-8947(00)00367-3
• Arthur L. Kohl & Richard B. Nielsen. (1997). Gas Purification. Gulf Publishing
• A. K. Myers & A. L. Myers. (1986). Numerical Solution of Chemical Equilibria with Simultaneous Reactions. J. Chem. Phys.
• Ding-Yu Peng & Donald B. Robinson. (1976). A New Two-Constant Equation of State. Industrial & Engineering Chemistry Fundamentals. doi:10.1021/i160057a011

Next Steps

In Benzene/Toluene Separation, you will build a complete distillation train.