Numerous proprietary processes for nitric acid manufacture are now available. They differ primarily in design details or selected operating conditions to suit the plant sizes, cost of raw materials, energy cost, cost of installation, and so forth, rather than in fundamental principles. The production process usually is composed of the following process units:
- Vaporisation, superheating, and filtration of anhydrous ammonia.
- Preheating, filtration, and compression of process air.
- Catalytic oxidation of ammonia.
- Cooling of nitric oxide by heat exchange with various media, e.g. process air, boiler water, tail gas.
- Oxidation of nitric oxide to higher oxides; units can be combined with various turbine drives and expanders in the form of one large single-shaft installation.
- Absorption of nitrogen oxides in water to form nitric acid.
- Bleaching of acid by additional air or other means.
- Treatment of tail gas to reduce air pollution (and to improve total plant efficiency).
- Recovery of energy in heated and compressed process gases.
- Recovery of catalysts for resale.
The various unit operations are well described in literature and in the documentation made available by licensing and engineering companies. A summary based on the state of the art in the late 1990s can be found in the Fertilizer Manual.
In the 1960s typical concentrations of NOx in the tail-gas ranged from 1,500 to 3,000 ppmv. In recent years concern about pollution control has led to laws and regulations to reduce permissible levels of NOx in tail-gas from nitric acid plants. For example, the maximum concentration in the United States in the late 1990s was equivalent to 200 ppmv of NO2 for new plants or 500 ppmv for existing plants. Requirements have become much stricter in recent years, both regarding NOx and N2O.
To meet the emission requirements that authorities in various countries have established, a variety of methods have been used. The principal methods employed to control the level of NOx in tail-gas are:
- Extended absorption.
- Nonselective catalytic reduction (NSCR) with fuel such as methane or ammonia plant purge gas.
- Selective catalytic reduction (SCR) with ammonia.
At the time of writing, 2022, abatement options are available that bring both NOx and N2O emissions down to very low levels, below 10 ppmv. Abatement efficiencies > 99 % can be achieved.
The abatement options applicable for each plant design depend on site conditions, operating conditions including final tail-gas temperature and pressure, and on integrated measures for energy recovery and emission reduction.
The reader is referred to the dedicated literature – in particular to recent IFS proceedings – and to the publications by licensing companies and from conferences such as ANNA. The BATREF LVIC-AAF document, presently under revision, is helpful.
Except in older plants operating at atmospheric pressure throughout and in very small medium- and high-pressure installations, most or all of the energy needed to drive the air compressor, or both air compressor and nitrous gas compressor, can be recovered either in the form of mechanical energy (by means of a tail-gas expansion turbine) or in the form of steam. In some instances, surplus steam for export or to produce electricity is generated.
The air compressor, steam turbine, and tail-gas expander are usually built in the form of a single, in-line unit. ln dual-pressure plants this system is also included. Tail-gas turbines usually supply 35-85% of the compression energy. The tail-gas turbine is a multistage design usually equipped with partial load valves or adjustable guide vanes to control the plant pressure.
It should be mentioned that efficient operation of the plants largely depends on efficient and reliable operation of the machines. For medium- and high-pressure oxidation, rotary compressors of the lobe, vane, centrifugal or axial-flow design are used; the latter two types are preferred because they can be combined with turbine drives and expanders as a single-shaft installation. The heat for air preheating can be provided to some extent by the heat of compression.
Precious metal recovery
During operation the surface of the catalyst is damaged by abrasion and vaporisation. Vaporisation loss dominates at the beginning of operation, but vaporisation weakens the metal structure and leads to abrasion and erosion. Platinum (and rhodium) losses are the consequence.
Platinum losses depend on the operating pressure and, in the late 1990s, were in the range of 0,05-0,4 g/t acid. Much recent research has been done to minimise these losses and the reader is referred to communications from catalyst suppliers. The ongoing research focuses both on the design of the catalysts and on plant operating conditions.
Platinum from the catalyst passes into the gas stream in the form of very fine particles, and its loss can substantially increase the production cost. Therefore, several methods of platinum recovery were developed and installed in many plants. Two types of recovery systems – catchment gauzes and mechanical filters – are usually offered.
The principle of catchment gauzes is to collect platinum at a temperature as high as possible while the main portion of the platinum loss is still in vapor form. At these temperatures, platinum atoms strike the metal surface and form an alloy with the catchment metal (for example palladium) for subsequent recovery. The system can recover up to 80% of the primary platinum losses. The catchment gauzes, which are installed at the bottom of the burner, are composed of a mesh screen and two or more metal gauzes. Catchment gauzes are returned together with the catalyst gauzes to the precious metal refining plant.
The mechanical filters, which are composed of glass wool or silica fibres, are commonly installed downstream of the catalyst where the gas temperature is below 300°C.
The corrosive behaviour of nitric acid toward metals requires the proper selection of construction material. The principal material wherever nitric acid or wet nitric oxides are present is chromium-nickel austenitic steel. The carbon content in this steel must be kept as low as possible because chromium forms carbides that are not acid resistant.
The stainless steel used in nitric acid plants to resist nitrous gases and nitric acid is standard austenitic steel of type AISI (American Iron and Steel Institute) 304 L, 321, 347 (U.S.A) or type EN (Euronorm) 1.4306, 1.4541, 1.4550 (Germany).
These stainless steels are well described and material specifications – including chemical composition, mechanical and physical properties, and information on processing and welding – are available. New stainless steels variations are being developed and are communicated, for example at ANNA conferences.
For equipment in contact with hot, higher concentration nitric acid, special alternative alloys are used (AISI 304 L – nitric grade, AISI 310 L). Alloyed steels are also used for welded parts of pumps, impellers, and rotating elements of compressors.
For equipment that handles ammonia, air, and hot, dry gases, normal carbon steel can be used. However, for safety in operation, especially during start-up and shutdown operations, nitric acid plants are often equipped with practically all stainless steel equipment. Because they are resistant to nitric acid, various fluorocarbon plastic materials are used for flanges, gaskets, and seals.
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
200, (1981), Nitric Acid Technology, J K Bradley, G Drake
206, (1982), Materials of Construction for the Nitric Acid Process, K Nutall, A R Reid
338, (1993), Cooling with a Bulk Flow Heat Exchanger,
398, (1997), Advances in Nitric Acid Manufacture, W Freitag, M Maurer
413, (1998), Catalyst Technology Used in Nitric Acid Production – Recent Advances, S Marret, L du Chatelier
416, (1998), Nitric Acid and Fertiliser Plants – Solutions to Various Engineering Problems, O von Bertele
435, (1999), Catalytic Reduction in Nitric Acid Plants of N2O from Adipic Acid, Gerhard Kuhn, Volker Schumacher, Eckhart Wagner
492, (2002), Safety Legislation and the Fertiliser Industry, K D Shah
537, (2004), Nitric Acid Production – Operational Safety, J A Hudson
539, (2004), N2O Abatement in an EU Nitric Acid Plant – A Case Study, M C E Groves, M Maurer
541, (2004), Legislation Affecting Nitric Acid Operations, K D Shah
582, (2006), IPPC: The BAT Reference Document (BREF) for the Manufacture of Ammonia, Acids and Fertilisers, B Serr
639, (2008), GHG Emissions and Energy Efficiency in European Nitrogen Fertiliser Production and Use, F Brentrup, C Pallière
743, (2014), Environmental Constraints on New Plant Construction in the USA, K Ruthardt
787, (2016), Targeting Improving Performance and Conversion Efficiency in Nitric Acid Plants, O Kay and T Buennagel
799, (2017), Sieve tray replacement in a nitric acid absorber column, D Schuler
800, (2017), NOx and SO2 abatement using gas-phase chlorine dioxide, R Richardson
860, (2021), Identifying and Resolving Root Causes of Poor Performance in Nitric Acid Plants, J Ashcroft
861, (2021), How Green Ammonia Feed and State of the Art Nitrous Oxide Abatement Contribute to Green Nitric Acid Production, D. Birke, B. Mielke
Links to external resources
The integrated pollution prevention and control reference document on Best Available Techniques for the manufacture of large volume inorganic chemicals – ammonia, acids and fertilisers (BATREF LVIC-AAF) describes in detail the processes, manufacturers, and existing challenges.
Various companies offer technologies and engineering services and the reader is advised to read the information made readily available. Such companies include, but are not limited to, CASALE, KBR-Weatherly, Stamicarbon, and Thyssenkrupp-UHDE.
The presentations made during the yearly conferences on ammonium nitrate and nitric acid (ANNA) are maintained in a library, however, this library is for conference attendees only. To access the downloads, your company must produce either ammonium nitrate or nitric acid and have attended a conference in the last 5 years.
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