Manufacture of nitric acid

The manufacture of nitric acid is a three-stage process each having distinct reaction:

a) Ammonia oxidation

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4NH3 + 5O2 4NO + 6H2O ?H = – 900 kJmol-1

b) Nitric oxide oxidation

2NO + O2 2NO2 ?H = – 115 kJmol-1

2NO2 N2O4 ?H = – 58 kJmol-1

c) Dinitrogen tetroxide absorption

3N2O4 + 2H2O 4HNO3 + 2NO ?H = – 103 kJmol-1

Industrial Production of Nitric Acid

Nitric acid is produced by 2 methods.

1. The first method involves oxidation, condensation, and absorption and yields a weak nitric acid with concentrations ranging from 30 to 70 percent nitric acid.

2. The second method combines dehydrating, bleaching, condensing, and absorption and produces a high-strength nitric acid from a weak nitric acid. High-strength nitric acid contains more than 90 percent nitric acid.

Weak Nitric Acid Production

This is done by the high-temperature catalytic oxidation of ammonia as shown in Figure. This process consists of 3 steps. Each step corresponds to a distinct chemical reaction

* Ammonia oxidation.

* Nitric oxide oxidation.

* Absorption.

.1. Ammonia Oxidation:

A mixture of Ammonia and air is oxidized at a temperature of 1380 to 1470 F while it passes through a catalytic converter.

4NH3 + 5O2 4NO + 6H2O ?H = – 900 kJmol-1

The catalyst used is made of 90 percent platinum and 10 percent rhodium gauze.

Electron micrograph of platinum/rhodium catalyst

Effect of Temperature:

The oxidation of ammonia to nitric oxide (NO) is an exothermic reaction and yields 93 to 98 percent. A higher catalyst temperature increases NO production whereas lower catalyst temperatures produce nitrogen (N) and nitrous oxide (NO). Nitric oxide is a pollutant whereas nitrous oxide is a global warming gas.

2. Nitric Oxide Oxidation –

The nitric oxide formed during the ammonia oxidation must be oxidized. This is achieved by passing the stream through a cooler/condenser and cooled to 100 F or less at pressures up to 116 pounds per square inch absolute (psia).

The nitric oxide reacts non-catalytically with residual oxygen to form nitrogen dioxide (NO) 2 and its liquid dimer, nitrogen tetroxide:

2NO + O2 2NO2

This reaction is highly dependent on temperature and pressure dependent. Maximum production of NO in minimum reaction time is achieved at low temperatures and high pressures.

3. Absorption:

The mixture is pumped into the bottom of the absorption tower, while liquid dinitrogen tetroxide is added at a higher point. Deionized process water enters the top of the column. Both liquids flow countercurrent to the nitrogen dioxide/dimer gas mixture. Oxidation takes place in the free space between the trays, while absorption occurs on the trays. The exothermic reaction occurs as follows:

3N2O4 + 2H2O 4HNO3 + 2NO ?H = – 103 kJmol-1

A secondary air stream is introduced into the column to re-oxidize the NO that is formed in Reaction. This secondary air also removes NO from the product acid. This results in yield of 30 to 70 percent nitric acid, which depend upon factors such as temperature, pressure, number of absorption stages, and concentration of nitrogen oxides entering the absorber.

High-Strength Nitric Acid Production:

* A high-strength nitric acid (98 to 99 percent concentration) can be obtained by concentrating the weak nitric acid using extractive distillation. The distillation process is carried out in the presence of a dehydrating agent (Concentrated sulfuric acid 60 %).

* The nitric acid concentration process consists of feeding strong sulfuric acid and 55 to 65 percent nitric acid to the top of a packed dehydrating column at atmospheric pressure.

* The acid mixture flows downward, countercurrent to ascending vapors. The process concentrates nitric acid as 99 percent vapor, containing a small amount of NO and oxygen (O) resulting from dissociation of nitric acid.

* The concentrated acid is then condensed while the oxygen and oxides of nitrogen (NO) are separated as byproducts. These byproducts then flow to an absorption column where the nitric oxide mixes with auxiliary air to form NO, which is recovered as weak nitric acid. Emissions from this process are relatively minor. A small absorber can be used to recover NO.

Flow Diagram – High-strength nitric acid productions.

Uses of Nitric Acid:

1. To manufacture ammonium nitrate a nitrogenous fertilisers.

2. Several explosives are made using nitric acid, including TNT (trinitrotoluene), nitroglycerine and components of “semtex”.

3. The manufacture of nylon requires hexanedioic (adipic) acid, produced using nitric acid.

4. Many processes with metals such as electro plating require the surface to be “pickled” using nitric acid.

5. Nitric acid is also used in gold and silver separation.

Hazards of Nitric Acid:

* Nitric acid is corrosive and can cause severe burns to all parts of the body.

* Its vapors are corrosive to the respiratory tract and may cause pulmonary Edema, which may be fatal.

* It may produce severe burns to skin.

* It is highly corrosive to all parts of the body.

* Liquid splashes may cause serious damage to eyes.

* On ingestion nitric acid will immediately cause corrosion and damage to the gastro-intestinal tract.

* Repeated exposure to high levels produces adverse effects on lung and teeth.

* Nitric acid is harmful to aquatic life.

Critical Evaluation of the manufacturing process:

Large-scale manufacturing involves various issues such as cost and economies, safety and environmental issues. The environmental impact of the process and the byproducts has to be weighed carefully and newer more environmental friendly processes have to be designed.

Running the process at optimum temperature and pressure using an appropriate catalyst can bring down the cost of the manufacturing. This is evident from both Haber process as well as for the synthesis of Nitric acid.

It is important that emission levels are checked under permissible limits for any chemical process. Various products and by products shall be handled carefully with adequate safety measures adopted while manufacturing as well as during transport and storage.

The benefits of Haber process and Manufacturing process of nitric acid far outweigh the hazards and therefore must be continued, however new research should be carried out to make the process as smooth, easy, economic and hazard free as possible.

References:

Nitric Acid Manufacturing: Retrieved from www on 18th Jan 2006 URL: www.epa.gov/ttn/chief/ap42/ch08/final/c08s08.pdf

Nitric Acid Manufacturing: Retrieved from www on 18th Jan 2006 URL: www.airqa.com/applications/GasmetNitricAcid.pdf