How to simulate nitric acid: the ostwald process (nh3 -> no -> no2 -> hno3)
The three-stage industrial route to nitric acid, each stage a genuinely different reaction regime. First a catalytic burner oxidizes ammonia in air over a Pt-Rh gauze (4 NH₃ + 5 O₂ → 4 NO + 6 H₂O) — this is a KINETICALLY selective step, not an equilibrium one: thermodynamics actually favours N₂, and only the short contact time on the hot catalyst steers it to nitric oxide, so it's modeled as a fixed-conversion reactor (a Gibbs-minimization reactor here would wrongly predict N₂). The gas is cooled and the nitric oxide is oxidized to nitrogen dioxide (2 NO + O₂ → 2 NO₂), an exothermic step favoured by the low temperature. Finally the NO₂ is absorbed in water to form nitric acid (3 NO₂ + H₂O → 2 HNO₃ + NO) — modeled as NO₂ + water capture into the acid liquor. Cited heats of reaction (NH₃ oxidation -226.3 kJ/mol NH₃; NO oxidation -57.0 kJ/mol NO).
- 1Open the ready-made model
Open the "Nitric acid: the Ostwald process (NH3 -> NO -> NO2 -> HNO3)" model in the MaximaLabs workspace — no install, no license. It loads live on the canvas, ready to edit and run.
- 2Confirm the thermodynamics
This process is modeled with the PENG-ROBINSON property package over ammonia, oxygen, nitrogen, water, nitric_oxide, nitrogen_dioxide — already selected, so the phase equilibrium and enthalpy are physically consistent from the first run.
- 3Review the flowsheet
The flowsheet chains MIX, 2× Oxidizer, Cooler, Absorber. Every block is a real, solvable unit op you can reconfigure on the canvas.
- 4Run the simulation
Click Run. The deterministic solver converges the material and energy balances (recycles included) and fills the live stream table — the AI never invents a number.
- 5Read the results and iterate
Inspect the converged streams, tweak a spec, and re-run — or ask the AI copilot to explain a result or diagnose a failed solve in plain English.
- Thermodynamics
- PENG-ROBINSON
- Components
- ammonia, oxygen, nitrogen, water, nitric_oxide, nitrogen_dioxide
- Unit operations
- MIX2× OxidizerCoolerAbsorber
Opens live on the canvas — free, no install.
Explore the model & flowsheetModeling assumptions & limitations
What this model captures, and what it deliberately does not — from the engineers who built it.
- 1The burner and NO-oxidation conversions are fixed (representative values), not solved from kinetics; the water-absorption tower is a capture simplification (the acid-forming reaction and HNO₃ solution non-ideality are not modeled), the same posture the sulfuric-acid and carbon-capture examples use. The point is the contrast — one kinetically-controlled step next to two thermodynamically-favoured ones — which is exactly why a nitric-acid plant can't be modeled with equilibrium reactors throughout.
Frequently asked questions
- What does the Nitric acid: the Ostwald process (NH3 -> NO -> NO2 -> HNO3) model simulate?
- The three-stage industrial route to nitric acid, each stage a genuinely different reaction regime. First a catalytic burner oxidizes ammonia in air over a Pt-Rh gauze (4 NH₃ + 5 O₂ → 4 NO + 6 H₂O) — this is a KINETICALLY selective step, not an equilibrium one: thermodynamics actually favours N₂, and only the short contact time on the hot catalyst steers it to nitric oxide, so it's modeled as a fixed-conversion reactor (a Gibbs-minimization reactor here would wrongly predict N₂). The gas is cooled and the nitric oxide is oxidized to nitrogen dioxide (2 NO + O₂ → 2 NO₂), an exothermic step favoured by the low temperature. Finally the NO₂ is absorbed in water to form nitric acid (3 NO₂ + H₂O → 2 HNO₃ + NO) — modeled as NO₂ + water capture into the acid liquor. Cited heats of reaction (NH₃ oxidation -226.3 kJ/mol NH₃; NO oxidation -57.0 kJ/mol NO).
- Which thermodynamic method does it use?
- The PENG-ROBINSON property package, over ammonia, oxygen, nitrogen, water, nitric_oxide, nitrogen_dioxide — already selected. You can switch the method on the canvas before running.
- Which unit operations are in the flowsheet?
- It chains MIX, 2× Oxidizer, Cooler, Absorber. Every block is a real, solvable unit operation you can reconfigure, add to, or remove.
- Do I need to install software or buy a license?
- No. Nitric acid: the Ostwald process (NH3 -> NO -> NO2 -> HNO3) runs entirely in your browser on MaximaLabs — free, no install, no license. Open the model to load it live and run the deterministic solver.
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