Ostwald Process (Nitric Acid)

How the process works

The Ostwald process turns ammonia into nitric acid in three sequential reactions, each in its own equipment at its own optimal temperature. Step 1 wants high temperature for catalyst activity, step 2 wants low temperature for equilibrium yield, step 3 wants water and a tall absorption column. Real plants run as one heat-integrated train.

┌──────────┐      ┌──────────┐      ┌──────────┐      ┌──────────┐
│  NH₃     │  +O₂ │   NO     │  +O₂ │   NO₂    │  +H₂O│   HNO₃   │
│  (g)     │ ───► │   (g)    │ ───► │   (g)    │ ───► │   (aq)   │
└──────────┘  Pt  └──────────┘ cool └──────────┘ tower└──────────┘
              -Rh                   chamber          absorption
              gauze                 oxidation        + tail-gas
              850 °C                ~50–150 °C       ~25 °C
              ~1 ms                 minutes          residence ~30 min

Behind the scenes, this is the same nitrogen that sat as inert N₂ in the atmosphere — fixed by Haber-Bosch into NH₃, then walked up the oxidation ladder by Ostwald. Together the two processes are the chemical engine of the green revolution.

Step 1 — catalytic ammonia burning

4 NH₃(g) + 5 O₂(g)  ──Pt-Rh, 850 °C──►  4 NO(g) + 6 H₂O(g)
                                          ΔH = −907 kJ/mol

NH₃ and air (1:9 by volume, 10% NH₃) flow downward through a stack of 10–30 woven Pt-Rh gauzes, each ~80 µm wire diameter, 1024 mesh per cm². Contact time is roughly 1 millisecond — long enough to make NO, too short for NO to back-decompose to N₂. The reaction's own exothermicity holds the gauze at 850 °C; plants pre-heat with a hydrogen flame at startup, then NH₃ + O₂ feed alone sustains temperature.

Selectivity to NO is 95–98% on fresh gauzes. The remaining 2–5% goes to N₂O (greenhouse gas) and N₂ (back where the nitrogen started). Higher temperatures raise NO yield but accelerate Pt evaporation; lower ones risk coking from organic NH₃ contaminants. 850 °C is the sweet spot.

Step 2 — gas-phase NO oxidation

2 NO(g) + O₂(g)  ⇌  2 NO₂(g)        ΔH = −114 kJ/mol

Hot exit gas leaves the burner at ~850 °C. Heat exchangers pre-heat absorption-tower air and raise process steam, cooling the stream to ~150 °C and then to ~50 °C. Cooling matters because 2 NO + O₂ → 2 NO₂ is exothermic — high temperature drives it backward.

The reaction needs no catalyst. A few minutes of residence in an "oxidation drum" or the cooler tubing itself converts > 90% of NO to NO₂. The remainder gets re-oxidized by air injected later.

Step 3 — water absorption

3 NO₂(g) + H₂O(l)  ⇌  2 HNO₃(aq) + NO(g)        ΔH = −136 kJ/mol

NO₂ contacts water in a packed or sieve-tray tower. Each pass produces 2 HNO₃ per 3 NO₂ plus regenerated NO. Air injected at the bottom re-oxidizes NO back to NO₂, recycling it up the column. 30+ trays push conversion to the equilibrium ceiling.

Dual-pressure plants run absorption at 8–14 bar to push equilibrium toward HNO₃. Output sits at 50–65 wt%; the 95% concentrated-acid explosives grade comes from extractive distillation with H₂SO₄ to break the 68% azeotrope.

Worked numbers — feed to product

Take 1 tonne of NH₃ feed (58 800 mol):

• Step 1 (selectivity 96%): 0.96 × 58 800 = 56 450 mol NO produced. The other 4% becomes ~2350 mol N₂ and ~50 mol N₂O.
• Step 2 (~95% conversion): 53 600 mol NO₂.
• Step 3 (after recycle, ~98% absorption): 35 000 mol HNO₃ (3 NO₂ → 2 HNO₃, with NO recycled).
• Net HNO₃ output: 35 000 × 63 g/mol ≈ 2.2 t HNO₃ per tonne NH₃ — the standard plant ratio.
• Energy: Step 1 generates ~907 kJ/mol NH₃. After heat recovery, plants are net steam exporters by ~0.7 t per tonne HNO₃.
• Pt loss: ~150 mg per tonne HNO₃ at modern recovery rates. A 1000 tpd plant loses ~50 kg of Pt per year.

Full reaction-flow diagram

      Haber-Bosch          Ostwald (this article)
       ─────────         ─────────────────────────
  N₂  ──►  NH₃   →   NH₃ + air
                          │
                  Pt-Rh gauze, 850 °C
                          │
                          ▼
                       NO (g)  ───┐
                                  │ + O₂, cooler
                          ▼       │
                       NO₂ (g) ◄──┘
                          │
                          │  + H₂O in absorber
                          ▼
                      HNO₃ (aq)
                          │
                          ▼
            ┌─────────────┴─────────────┐
            ▼                           ▼
    NH₄NO₃ fertilizer           explosives
   (75% of output)         (TNT, RDX, ANFO,
                            rocket oxidizer)

Stages compared at a glance

	Step 1: NH₃ → NO	Step 2: NO → NO₂	Step 3: NO₂ → HNO₃	Tail-gas treatment
Equipment	Pt-Rh gauze burner	Cooler / oxidation drum	Packed absorption tower	SCR / N₂O abatement bed
Temperature	~850 °C	50–150 °C	20–40 °C	250–400 °C
Pressure	1–10 bar	1–10 bar	8–14 bar	1 bar
Residence time	~1 ms	Minutes	30+ min	0.1–1 s
Catalyst	Pt-Rh (90:10) gauze	None — gas phase	None — absorption	Fe-zeolite or Co/Ce
Conversion	95–98% to NO	~90% to NO₂	~98% absorbed (recycle)	70–90% N₂O destroyed
What kills it	Pt loss, NH₃ slip	Hot-spot re-equilibration	NO breakthrough at top	SOx poisoning

Real-world plants and history

• Wilhelm Ostwald, 1902 patent. The German chemist (1909 Nobel Prize) demonstrated the gauze burner in his Leipzig laboratory. His insight: a fast surface reaction at the right temperature beats thermodynamic equilibrium because the reverse decomposition can't keep up.
• Norsk Hydro, Notodden & Rjukan. Norway's giant Birkeland-Eyde plants on cheap hydropower were converted to Ostwald in the 1920s once cheap NH₃ from Haber-Bosch arrived.
• Modern dual-pressure plants. Yara, ThyssenKrupp, KBR license slightly different versions. Burner at low pressure (4–5 bar), absorption at high pressure (10–12 bar) — the compromise between selectivity and absorption.
• Largest single trains. Yara's Porsgrunn (Norway) and CFI's Sluiskil (Netherlands) each push past 2000 tpd. A typical greenfield plant is 500–1500 tpd.
• WWI dual-use chemistry. Germany's nitric-acid supply switched from Chilean saltpetre to synthetic in 1914–1916. The Allied blockade had cut imports; without the NH₃→HNO₃ pipeline German artillery would have run dry.
• Birkeland-Eyde, 1903. Direct N₂ + O₂ → NO via a 3000 °C electric arc. ~60 MWh per tonne HNO₃, only viable on free hydroelectricity. Survived in Norway until ~1940.

Variants and edge cases

• Mono-pressure plants. All steps at one pressure (4 bar low or 10 bar high). Cheaper capex, lower yield. Common in plants < 500 tpd.
• Concentrated nitric acid. Direct-strong-nitric (DSN) variants use cryogenic O₂ and 50% H₂O₂ to push absorption past the 68% water azeotrope without distillation.
• N₂O abatement. Secondary Fe-zeolite or Co/Ce beds downstream of the gauze decompose 2 N₂O → 2 N₂ + O₂. Modern European plants emit < 1 kg N₂O per tonne HNO₃, vs ~10 kg unmitigated.
• SCR tail-gas treatment. Selective catalytic reduction with NH₃ over V-W-Ti destroys remaining NOx in the exhaust to < 25 ppmv.
• Plasma and electrocatalytic alternatives. Active research aims to skip Haber-Bosch + Ostwald entirely by reducing N₂ + H₂O electrochemically to NO₃⁻. Faradaic efficiencies are still in single digits.

Common pitfalls and misconceptions

• "Higher gauze temperature is always better." Selectivity to NO peaks near 850 °C. Beyond ~950 °C, Pt vaporization accelerates and gauze life collapses to weeks.
• "NO₂ absorbs neatly into water." The 3 NO₂ + H₂O ⇌ 2 HNO₃ + NO equilibrium recycles a third of the nitrogen back. Without air re-injection, conversion stalls.
• "Tail gas is just NOx." It also contains N₂O, often the largest CO₂-equivalent emitter in the plant.
• "Ostwald and Haber-Bosch are independent." Almost no Ostwald plant runs alone — they sit adjacent to a Haber-Bosch plant because shipping liquid NH₃ is awkward and heat integration is natural.
• "Pt-Rh gauzes are sintered solid." They're woven wire, not pellets — high surface area, low mass-transfer resistance, and ~99.5% Pt-recoverable when retired.
• "All HNO₃ goes to fertilizer." ~75% does, mostly as ammonium nitrate. The other 25% goes to caprolactam (nylon-6), TNT, plating chemistry, and rocket-oxidizer fuming acid.