Brief description

The Ostwald Process is a fundamental chemical process used to produce nitric acid ($HNO_3$) from ammonia ($NH_3$). Developed by Wilhelm Ostwald at the beginning of the 20th century, it is highly linked with the Haber-Bosch process, providing the primary industrial pathway for turning atmospheric nitrogen into fertilizers and industrial chemicals.

Use / Function

Nitric Acid Production: Generates concentrated nitric acid, which is critical for making nitrogen-based agricultural fertilizers (like ammonium nitrate).
Explosives Manufacturing: Provides the essential precursor for nitrating compounds to make explosives (such as guncotton, nitroglycerin, and TNT).
Metallurgy and Chemical Synthesis: Supplies acid for metal pickling, rocket propellants, and precursor organic synthesis.
Scale: Large-scale chemical manufacturing plant working continuously.

Operating principle

The process converts ammonia ($NH_3$) into nitric acid ($HNO_3$) via catalytic oxidation in three primary steps:

Catalytic Oxidation of Ammonia: Ammonia gas is mixed with dried air and passed through a fine catalytic mesh made of platinum (often alloyed with 10% rhodium) at high temperatures (800°C–900°C) and pressure. This exothermic reaction produces nitric oxide ($NO$) and water vapor:

$$4\text{NH}_3 (g) + 5\text{O}_2 (g) \rightarrow 4\text{NO} (g) + 6\text{H}_2\text{O} (g) \quad (\Delta H = -905.2 \text{ kJ/mol})$$

Once started, the reaction is self-sustaining due to its high heat release.

Oxidation of Nitric Oxide: The gas mixture is cooled, and the nitric oxide is allowed to react with excess oxygen to form nitrogen dioxide ($NO_2$):

$$2\text{NO} (g) + \text{O}_2 (g) \rightarrow 2\text{NO}_2 (g) \quad (\Delta H = -114 \text{ kJ/mol})$$

Absorption: The nitrogen dioxide gas is absorbed in water inside a reaction tower. It reacts with water to yield dilute nitric acid ($HNO_3$) and nitric oxide ($NO$), which is recycled back to the second step:

$$3\text{NO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow 2\text{HNO}_3 (aq) + \text{NO} (g)$$

To maximize acid concentration and yield, additional oxygen is introduced to oxidize the liberated nitric oxide directly in the absorption column:

$$4\text{NO}_2 (g) + \text{O}_2 (g) + 2\text{H}_2\text{O} (l) \rightarrow 4\text{HNO}_3 (aq)$$

How to implement

1. Catalytic Reactor

Construct a pressurized vessel containing a fine mesh or gauze of Platinum.
Implement pre-heaters to raise the incoming ammonia-air mixture to roughly 200°C to initiate the reaction on the catalyst surface.
Design efficient gas-flow pathways to ensure a contact time of less than a few milliseconds, as longer exposure at high temperatures degrades the desired nitric oxide product.

2. Cooling and Oxidation Chambers

Route the hot exhaust gases through a heat exchanger (such as a steam Boiler) to quickly cool them down to below 150°C, promoting the oxidation of nitric oxide to nitrogen dioxide.
Direct the gases into a dedicated oxidation vessel where additional atmospheric air is added.

3. Absorption Tower

Build a tall absorption tower lined with acid-resistant ceramic or glass.
Introduce water at the top of the tower to trickle down over packing materials (like ceramic rings) while the nitrogen dioxide gas enters from the bottom, maximizing gas-liquid contact.
Use a Pump to recirculate the acid until the desired concentration (typically 50%–60%) is achieved.

Materials needed

Ammonia ($NH_3$): Produced via the Haber-Bosch Process.
Atmospheric Air: Dried and filtered to remove impurities that could poison the catalyst.
Catalyst: High-purity Platinum or platinum-rhodium wire mesh.
Water: Demineralized water for optimal absorption and purity.
Acid-Resistant Alloys or Ceramics: Specialized materials to handle highly corrosive nitrogen oxides and hot nitric acid.

Variants and improvements

Dual-Pressure Process: Operates the ammonia combustion step at medium pressure (for higher catalyst efficiency) and the absorption step at high pressure (to increase absorption rate and acid concentration).
Birkeland-Eyde Process: The historical alternative that used a high-energy electric arc to directly fix atmospheric nitrogen into nitric oxide. It did not require ammonia, but consumed vast amounts of electricity.

Limits and risks

Catalyst Poisoning: Impurities in the air or ammonia feed (such as sulfur or heavy metals) can “poison” the platinum catalyst, stopping the reaction.
Nitrous Fumes: Nitrogen dioxide is a highly toxic, reddish-brown gas. Leaks cause severe respiratory damage and are lethal in high concentrations.
Corrosive Degradation: Nitric acid aggressively attacks standard metals. Glass, stone, or specialized high-chromium stainless steel must be used for containment.