The annual production of nitric acid in Europe was in 2000 about 20 Mio tonnes 100% nitric acid and the world total production is estimated to be 50 Mio tonnes. EFMA (2000) states that 78 plants are in operation within the European Community in 1992, from which 45 were dual pressure plants (mostly medium p./high p.) and 33 single pressure plants (22 medium p./medium p. and 11 high pressure/high pressure). According to EMEP/CORINAIR (2001) all new plants are built for pressure ranges above 4 bar.
The production of nitric acid is based on the Ostwald Process. Initially, ammonia is oxidised catalyti-cally. Nitrous gases are obtained, which are converted into nitric acid together with oxygen and water. All plants for the production of nitric acid are currently based on the same basic chemical operations:
- Oxidation of ammonia with air to give nitric oxide (NO)
- Oxidation of the nitric oxide to nitrogen dioxide (NO2) and absorption in water resulting in a solution of nitric acid (HNO3)
Two kinds of nitric acid are currently produced: weak nitric acid (50-70% acid in water) and concentrated nitric acid (about 98%).
Process technologies for weak nitric acid production differ mainly with regard to working pressure levels. There are two types of nitric acid plants, single pressure plants and dual pressure plants. In the single pressure plant, the oxidation and absorption takes place at the same pressure. In the dual pressure plant absorption takes place at a higher pressure than the oxidation stage. The oxidation and absorption steps can be classified according to EFMA (2000) and UNEP (1998) as:
- low pressure (below 1.7 bar)
- medium pressure (between 1.7 and 6.5 bar)
- high pressure (between 6.5 and 13 bar)
There is no concordance in the different literature sources about what should be considered low, medium and high pressures.
Weak nitric acid is produced by the Ostwald process: The overall reaction can be described as:
12NH3 + 21O2 → 8HNO3 + 14H2O + 4NO (0)
According the Ostwald process, ammonia is oxidised with air on platinum/rhodium alloy catalysts in the oxidation section of nitric acid plants. Nitric oxide and water are formed in this process according to the main equation:
4NH3 + 5O2 → 4NO + 6H2O (1)
Simultaneously nitrous oxide, nitrogen and water are formed as well, in accordance with the following equations:
4NH3 + 3O2 → 2N2 + 6H2O (2)
4NH3 + 4O2 → 2N2O + 6H2O (3)
Some of the platinum and rhodium vaporises during the reaction process and in most cases a recovery system is installed. The enthalpy of the hot reaction gases is used to produce steam and/or to preheat the waste gas. The heated waste gas is discharged to the atmosphere through a gas turbine for energy recovery. The combustion gas after this heat transfer for energy recovery has a temperature of 100 to 200 °C, depending on the process and it is then further cooled with water. The water produced in reactions (1) to (3) is then condensed in a cooler condenser and transferred to the absorption column.
The nitric oxide is oxidised to nitrogen dioxide as the combustion gases are cooled, according the equation:
2NO + O2 → 2NO2 (4)
For this purpose, secondary air is added to the gas mixture obtained from the ammonia oxidation to increase the oxygen content to such a level that the waste gas leaving the plant has a normal oxygen content between 2 and 4% by volume. The absorber is operated with a counter-current flow of water. The absorption of the nitrogen dioxide and the reaction to nitric acid and nitric oxide take place simul-taneously in the gaseous and liquid phases according to equations (4) and (5). These reactions depend on pressure and temperature to a large extent and are favoured by high pressure and lower temperature.
3NO2 + H2O → NO + 2HNO3 (5)
Reaction (5) is exothermic and continuous cooling is therefore required within the absorber. As the conversion of NO to NO2 is favoured by low temperature, this reaction will take place significantly until the gases leave the absorption column. The nitric acid produced in the absorber contains dissolved nitrogen oxides and is then bleached by secondary air.
Process integrated emission reduction:
Reduction facilities are normally an integral part of nitric acid plants. The two most common techniques used in order to control NOx emissions are extended absorption with water and catalytic reduction (non-selective or selective).
Extended absorption implies the treatment of the waste gas either with water in an additional absorp-tion column. Nox emissions are reduced due to a better absorption efficiency.
In the non-selective catalytic reduction process the waste gas reacts with a reduction agent (hydrogen and/or hydrocarbons e.g. natural gas, waste gas from ammonia plants or naphtha) by passing through a catalyst which contains platinum, rhodium or palladium. The reduction product is N2, however the utilisation of hydrocarbons has the disadvantage of high fuel consumption as well secondary emis-sions, such as CO, CO2 and hydrocarbons in a non-converted or partially converted state. In the selective reduction process the reduction agent (ammonia) reacts with nitric oxides to form nitrogen and water. The most common catalysts used are vanadium pentoxide, platinum, iron/chromium oxides mixtures and zeolites.
Concentrated nitric acid:
Weak nitric acid is also starting material in the indirect process for the production of concentrated nitric acid. This process implies the concentration of common weak nitric acid based on extractive dis-tillation and rectification with sulphuric acid or magnesium nitrate. The other process in use for the manufacturing of concentrated nitric acid is the direct process. By this, first nitrogen dioxide (NO2) is produced as described above for weak nitric acid. This NO2 is then absorbed in highly concentrated acid, distillated condensed and finally converted into highly concentrated nitric acid (about 98% w/w) at high pressure by adding a mixture of water and pure oxygen.
For this study the production of weak nitric acid with the Ostwald process is considered. According to EFMA (2000) the typical plants considered for this study are the single pressure plant and the dual pressure (medium pressure/high pressure) plant. These are the processes most often used in the European industry. Except for some very old plants, single pressure plants operate at medium or high pressure and dual pressure plants operate at medium pressure for the oxidation stage and high pressure for the absorption. In literature there are different ranges for the concentration of weak nitric acid: 50-65% acid in water according to EFMA (2000), up to 69.2% to Wiesenberger (2001) and 50 to 70% according to Patyk (1997). In the database ecoinvent a concentration of 50% acid in water is considered.
In regard to to the production capacity in the manufacturing plants, EFMA (2000) considers 1000 t/d a typical capacity for new plants. This is the value adopted for the study. UNEP (1998) considers 1000 to 2000 t/d typical capacities and Wiesenberger (2001) quotes an average capacity for the Aus-trian plants of 660 t/d.
References:
Joint EMEP/CORINAIR (2001) Atmospheric Emission Inventory Guidebook, Third Edition. Copenhagen: European Environment Agency. http://reports.eea.eu.int/technical_report_2001_3/en/group04.pdf
European Fertiliser Manufacturers´ Association (2000) Production of Nitric Acid. Best Available Techniques for Pollution Prevention and Control in the European Sulphuric Acid and Fertiliser Industries. Booklet N°2. Brussel.
United Nations Environment Programme (UNEP) Industry and the Environment, United Nations Industrial Development Organization (UNIDO) and International Fertiliser Association (IFA). (1998) Mineral Fertiliser Production and the Envi-ronment. Part 1. The Fertiliser Industry's Manufacturing Processes and Environ-mental Issues. United Nations Publications, New York.
Wiesenberger H. (2001) State-of-the-art for the Production of Nitric Acid with regard to the IPPC Directive. Monographien; Band 150. Wien.
Sukalac K. (2002) International Fertiliser Industry Association (IFA) Personal communication on 30.09.2002