Nitric acid is essential for fertilizers, explosives, and metal treatment. Over 75% goes into ammonium nitrate. Understanding its production is key to reducing emissions, improving efficiency, and aligning with environmental and safety regulations.
Introduction
Nitric acid (HNO3) is one of the most critical inorganic acids for the chemical industry. It is a major precursor for the manufacture of fertilizers, explosives, dyes, and specialty chemicals. Global nitric acid production of more than 75% is consumed for the production of ammonium nitrate—a vital ingredient in nitrogen fertilizers. Nitric acid also has widespread application in metal treatment, pharmaceuticals, and aerospace applications because of its high oxidizing nature.
Comprehending the production process of nitric acid is essential for producers and those involved in raw material supply chains and environmental adherence. The process directly affects cost-effectiveness, greenhouse gas emissions (especially nitrous oxide), and energy integration. With increasing environmental governance and decarbonization efforts, extensive insight into nitric acid production aids in enhanced process optimization, safety regulation, and research into cleaner technologies.
Overview of the Production Process
The majority of industrial production of nitric acid is through the Ostwald process, which is a continuous catalytic oxidation route. Ammonia (NH3) is oxidised with air on a platinum-rhodium catalyst to produce nitric oxide (NO), which is then oxidised to give nitrogen dioxide (NO2). This is dissolved in water to give nitric acid.
This is carried out in a continuous operation to maximize throughput and energy recovery. The three basic steps are:
Operating conditions are severe, at temperatures of approximately 850–900°C and pressures of 5–10 atm. Dilute nitric acid (55–68% HNO3) is normal product, but concentrated solutions can be distilled from it. Heat released upon oxidation is used for the generation of steam, making the process partially energy self-sufficient. Batch production is used for specialized or lab-scale applications, whereas continuous mode predominates because of improved heat integration and decreased energy loss.
Figure 1 Production Process of Nitric Acid
Raw Materials and Input Requirements
The production of nitric acid relies on a small set of highly purified inputs. These include:
• Ammonia (NH3): The principal feedstock, usually derived from natural gas via steam reforming. Ammonia purity must exceed 99.9% to avoid catalyst fouling.
• Air: Supplies the oxygen required for oxidation reactions. It must be clean and moisture-free.
• Water: Used in the absorption stage to convert NO2 to nitric acid.
• Catalysts: Platinum-rhodium gauze is used to accelerate ammonia oxidation.
Other requirements for the process are control system compressed air, steam, and cooling water. Catalyst longevity and efficiency depend upon the absence of sulphur and chlorine impurities in the feed stream. Utilities such as heat exchangers and high-pressure compressors are essential to maintain temperature and pressure control during the reaction phases.
Major Production Routes
• Single Pressure Ostwald Process:
The classical method where ammonia is oxidized and absorbed under a single pressure. It is suitable for small to mid-scale plants with low capital expenditure.
• Dual Pressure Process:
Ammonia is oxidized at high pressure while absorption occurs at a lower pressure, improving nitric acid concentration and energy efficiency. This method is widely adopted in modern plants, especially in Europe and North America.
• Low Pressure Process:
Operates entirely at near-atmospheric pressure and is typically found in older plants. While simpler, it is less efficient and offers limited heat recovery.
Green Alternatives and Regional Trends:
With global efforts to reduce emissions, plasma-assisted and electrochemical oxidation technologies are being considered. Green ammonia made from renewable hydrogen is also being used in pilot programs. In Europe, with carbon taxes rising, plants retro-fit dual-pressure units with nitrous oxide abatement to reduce environmental liabilities.
Equipment and Technology Used
The production of nitric acid involves robust, high-performance equipment capable of handling extreme thermal and chemical stress.
Core Equipment Includes:
• Ammonia Vaporizers & Mixers: Convert and mix ammonia with air prior to oxidation.
• Catalytic Reactors: Contain platinum-rhodium gauze for high-temperature ammonia oxidation.
• Heat Recovery Units: Capture thermal energy for generating process steam.
• Absorption Towers: Typically, packed columns where NO2 dissolves in water to form HNO3.
• Tail Gas Treatment Units: Reduce harmful emissions such as NOx and N2O.
• Control Systems: Advanced distributed control systems (DCS) manage reaction parameters in real time.
Modern plants integrate heat exchangers and energy recovery turbines to optimize efficiency. Predictive maintenance tools, AI-driven monitoring, and digital twins are increasingly used to improve process reliability and reduce unplanned downtime.
Environmental and Safety Considerations
Nitric acid production is inherently associated with emissions of greenhouse gases and toxic substances. The primary environmental concern is nitrous oxide (N2O), a by-product of the oxidation process and a potent greenhouse gas.
Emission Profile:
• N2O: High global warming potential (~300x CO2); formed during catalytic oxidation.
• NOx (NO & NO2): Contribute to acid rain and ozone formation.
• Thermal emissions and effluents: From cooling water, blowdown, and unreacted gases.
Mitigation Measures:
• N2O Abatement Units: Catalytic decomposition systems reduce N2O emissions by up to 95%.
• Selective Catalytic Reduction (SCR): Reduces NOx in the tail gas.
• Effluent Treatment Plants (ETP): Neutralize acidic waste streams prior to discharge.
Regulatory Compliance:
• Europe: Plants fall under REACH and EU ETS guidelines with strict N2O emission quotas.
• United States: Subject to EPA's Clean Air Act and OSHA’s PSM requirements.
• Asia-Pacific: Countries like Japan and South Korea are aligning with international environmental protocols, while others are adopting standards gradually.
Regular risk assessments, emission monitoring, and employee training are critical to maintaining compliance and ensuring operational safety.
Conclusion and Future Innovations
Nitric acid continues to be a foundational chemical in the global industrial ecosystem. However, its production process is under increasing scrutiny due to its energy intensity and environmental impact. To remain competitive, manufacturers are investing in process intensification, emission abatement, and green ammonia integration.
Innovations on the horizon include:
• Electrochemical Nitric Acid Synthesis: Direct conversion of air and water under mild conditions.
• Green Ammonia Feedstock: Using hydrogen from electrolysis to reduce fossil dependency.
• Advanced Catalyst Design: To minimize N2O formation and improve reaction selectivity.
• Modular Plants: Flexible, small-scale units for localized production with lower environmental risk.
As sustainability becomes a strategic priority, future nitric acid production will need to strike a balance between efficiency, safety, and environmental responsibility.
