Industrial Preparation of Nitric Acid from Oxidation of NH3 by Ostwald’s Process

Industrial Preparation of Nitric Acid from Oxidation of NH3 by Ostwald’s Process

Introduction and Raw Materials

Nitric acid formerly called as aqua fortis (meaning strong water) is manufactured by two methods namely Birkland-Eyed process and Ostwald’s process. Now a days nitric acid is preferably manufactured commercially (i.e. on large scale) through catalytic oxidation of ammonia by Ostwald’s Process. The following are raw materials used in the process:

Ammonia ------------------------------- (to get NO)

Air or Oxygen -------- ----------------- (for oxidation)

Water ------------------------------------ (for absorption)

Platinum-Rhodium Gauze ------------ (Catalyst)

Basic Principle

The process is based on catalytic air oxidation of ammonia to nitric oxide which is further oxidized to nitrogen dioxide which on absorption in water disproportionate to give nitric acid. Thus process involves three steps:

Plant Used and Details of Process

The process is carried out in a large plant, which has three main parts involving three steps.

1. Oxidation of ammonia into NO in Catalyst chamber/ ammonia oxidation converter.

2. Oxidation of NO into NO2 in Oxidation chamber.

3. Disproportionation of NO2 into HNO3 in Absorption towers.

(1)  Oxidation of ammonia into NO in Ammonia-Oxidation Converter/Catalyst Chamber

A mixture of ammonia gas and excess of air in the ratio of 1:8 (10) by volumes is passed over platinum-rhodium gauze (90% Pt +10 Rh%) used as catalyst at a temperature of 600°C–700°C in the catalyst chamber where 96% of ammonia is oxidized catalytically to nitric oxide (NO is an important neurotransmitter in human body. It causes the muscles that line blood vessels to relax, thus allowing an increased passage of blood). Reaction is complete in 0.0003 second.

The oxidation of NH₃ to NO is reversible, exothermic and proceeds with increase in volume of products. Le-Chatelier’s Principle suggests following conditions for the maximum yield of NO from NH₃:

(2) Oxidation of NO into NO₂ in Oxidation or Cooling Tower

Nitric oxide (NO) gas formed is first cooled down quickly by passing through iron pipes of coolers (boilers or heat exchanger) up to 150°C and then it is mixed with excess of air in oxidation chamber where nitric oxide is oxidized by air to nitrogen dioxide (nitrogen peroxide).

(3).  Disproportionation of NO₂ into HNO₃ in Absorption Towers

Nitrogen dioxide (NO₂) produced is directly dissolved in water (sprayed from above) in the absorption tower packed with quartz or acid-proof stones (to increase the surface area) to produce nitric acid which is 60% concentrated liberating nitric oxide gas which is recycled to get more nitric acid. (The 60% concentrated nitric acid can be concentrated further up to 98% by passing over concentrated H₂SO₄). This is a disproportionation or auto-redox reaction in which nitrogen of NO₂ is both oxidized (to HNO₃) and reduced (to NO).

4.   Preparation of concentrated Nitric Acid

Commercial 68% concentrated HNO₃ is obtained by simple reduced pressure (vacuum) distillation of dilute HNO₃. (Pure nitric acid is colourless liquid. The commercial concentrated nitric acid often has a yellow brown colour. The colour is due to NO₂ produced by a slight amount of decomposition of nitric acid). 98% concentrated HNO₃ is obtained by distilling dilute HNO₃ with concentrated H₂SO₄ which holds back the water. 100% concentrated HNO₃ is obtained by strongly cooling 98% HNO₃, when pure acid is deposited as colourless crystals at –41.6°C. These crystals are melted to 100% pure concentrated nitric acid.