1. Raw materials for the manufacturing of nitric acid
2. Process chemistry

Raw materials for the manufacturing of nitric acid

The anhydrous ammonia and process air used must be free from catalyst poisons, dust, and oil. Platinum catalysts can be poisoned by such elements as Bi, As, P, Pb, S, Si, and Sn. Fortunately, synthetic ammonia is normally of high purity unless it is accidentally contaminated. However, since air can be contaminated by dust or many other pollutants, thorough air cleaning is necessary. Location of the air intake in an area relatively free from contaminants will help. If poisoning by impure ammonia or air should arise, deep penetration may occur, leading to the formation of inactive compounds in the catalyst wires and, perhaps, to the extent of ruining the catalyst. In other instances, contamination by traces of Cr, Fe, or Ni may temporarily reduce conversion efficiency, but this can often be restored by treatment with hydrochloric acid or certain salts.

The physical properties of ammonia are available. Specifications for ammonia to be used in nitric acid processes basically limit water, oil (from lubrication), and heavy metals. The limits change from licenser to licenser and obviously influence the process yields. At the plants located in the former Soviet Union, established limits on air impurities have been given (Fertilizer Manual, page 211).

Process chemistry

The oxides of nitrogen that are of interest in nitric acid production are:

• nitrous oxide (dinitrogen monoxide). N₂O.
• nitric oxide (nitrogen monoxide). NO.
• nitrogen dioxide, NO₂.
• dinitrogen tetroxide, N₂O₄.

Of these, NO and NO₂ are of primary importance. N₂O₄ exists in equilibrium with NO₂ and is not present in significant proportions at temperatures above about 100°C, rather it acts as a transitory intermediate in low-temperature absorption of NO₂ in nitric acid. N₂O is seldom present in significant amounts but is relevant for its effect as a climate gas. A mixture of nitrogen oxides, usually NO and NO₂, is commonly referred to as NOx.

The production of weak nitric acid consists of three principal chemical steps:

• Catalytic ammonia oxidation to nitric oxide.
• Oxidation of nitric oxide to nitrogen dioxide.
• Acidic absorption of nitrogen dioxide in water.

The overall reaction (without taking into consideration side reactions) can be presented as follows:

NH₃(g) + 2O₂(g) -> HNO₃(aq) + H₂O(l) + 437 kJ/gmol

Oxidation of ammonia without a catalyst leads to the formation of only elemental nitrogen.

Various alloys and metallic oxides have been tried as catalysts, but platinum containing between 2% and 10% rhodium is usually preferred. Nitric oxide produced in the ammonia converter must be oxidised further to nitrogen dioxide by the excess air present in the reaction mixture:

2NO(g) + O₂(g) -> 2NO₂(g) + 57 kJ/gmol

When the temperature is lowered, the equilibrium of the above reaction shifts to the right. At low temperatures and in sufficient time to reach equilibrium and in the presence of excess oxygen, one can anticipate transformation of all oxides of nitrogen into dinitrogen tetroxide (N₂O₄).

During absorption HNO₂ is formed along with HNO₃:

N₂O₄ + H₂O -> HNO₃ + HNO₂

The formed HNO₂ decomposes and emits NO gas, which must again be oxidised to NO₂.

HNO₂ -> NO + H₂O

Oxidation of NO is the limiting stage of HNO₃ formation. With decreasing concentrations of nitric oxide and oxygen, the speed of oxidation decreases sharply. Absorber performance, therefore, is improved by higher pressure, lower temperature, and higher oxygen content in the gas phase. ln the ammonia converter, however, oxidation of ammonia is favoured by lower pressure.

The overall reaction of nitric acid production is strongly exothermic. Of the total heat released, more than half is released in the ammonia oxidation step at a high temperature, and economical recovery as steam or for other purposes is of high relevance. Also, part of the heat released by the subsequent oxidation of nitric oxide can be recovered at a useful temperature level. The remainder of the heat is released at a temperature, in many cases, too low for useful recovery and requires a net consumption of energy for circulation of cooling water, acid and process gas and, in some processes, for refrigeration.

References

Fertilizer Manual, edited by the United Nations Industrial Development Organization (UNIDO) and the International Fertilizer Development Center (IFDC), Kluwer Academic Publishers, 1998.

Links to related IFS Proceedings and recordings

169, (1978), Catalytic Processes in Nitric Acid Manufacture, J A Busby, G Knapton, A E R Budd

171, (1978), Industrial Heterogeneous Catalysis – Some Aspects, G C Chinchen

413, (1998), Catalyst Technology Used in Nitric Acid Production – Recent Advances, S Marret, L du Chatelier

435, (1999), Catalytic Reduction in Nitric Acid Plants of N₂O from Adipic Acid, Gerhard Kuhn, Volker Schumacher, Eckhart Wagner

582, (2006), IPPC: The BAT Reference Document (BREF) for the Manufacture of Ammonia, Acids and Fertilisers, B Serr

787, (2016), Targeting Improving Performance and Conversion Efficiency in Nitric Acid Plants, O Kay and T Buennagel

860, (2021), Identifying and Resolving Root Causes of Poor Performance in Nitric Acid Plants, J Ashcroft