Industrial Production of Nitric Acid

Overview of Industrial Nitric Acid Production

In industry, nitric acid is produced almost exclusively by the Ostwald process, which converts ammonia (largely obtained by the Haber–Bosch process) into nitric acid using a sequence of catalytic and absorption steps. The process is a central link between synthetic ammonia and nitrogen-containing fertilizers and explosives.

Key characteristics of the industrial process:

• Feedstock: gaseous ammonia and air (oxygen)
• Main products: nitric acid of about 55–68% by mass (more concentrated acid is produced by further concentration steps)
• By-products and issues: formation of nitrogen oxides ($\text{NO}$, $\text{NO}_2$, $\text{N}_2\text{O}$), heat release, and gas cleaning requirements

The Ostwald Process: Reaction Sequence

The Ostwald process converts ammonia to nitric acid via nitrogen monoxide and nitrogen dioxide as intermediate products. Overall, the formation of nitric acid from ammonia and oxygen can be summarized in three reaction stages.

1. Catalytic Oxidation of Ammonia

Ammonia is first oxidized to nitrogen monoxide on a platinum–rhodium catalyst:

$$
\text{4 NH}_3(g) + \text{5 O}_2(g) \rightarrow \text{4 NO}(g) + \text{6 H}_2\text{O}(g) \quad \Delta H < 0
$$

Specific industrial aspects:

• Strongly exothermic reaction
• Typical temperatures: about $800{-}950^\circ\text{C}$
• Typical pressures: around $4{-}10\,\text{bar}$ (depending on plant design)
• Catalyst: fine platinum–rhodium gauze, often multiple layers
• Ammonia–air mixture: carefully controlled (usually 9–11% $\text{NH}_3$ in air) to avoid explosive mixtures and to achieve high selectivity to $\text{NO}$

Side reactions:

• Formation of nitrogen and nitrous oxide:
  $$
  \text{4 NH}_3 + \text{3 O}_2 \rightarrow \text{2 N}_2 + \text{6 H}_2\text{O}
  $$
  $$
  \text{4 NH}_3 + \text{4 O}_2 \rightarrow \text{2 N}_2\text{O} + \text{6 H}_2\text{O}
  $$

These side reactions reduce yield and generate undesired gases ($\text{N}_2\text{O}$ is a greenhouse gas). Operating conditions and catalyst design are therefore chosen to maximize the fraction of ammonia converted to $\text{NO}$ (typically >95%).

2. Oxidation of Nitrogen Monoxide to Nitrogen Dioxide

The nitrogen monoxide formed in step 1 is further oxidized in the gas phase:

$$
\text{2 NO}(g) + \text{O}_2(g) \rightarrow \text{2 NO}_2(g) \quad \Delta H < 0
$$

Industrial aspects:

• Occurs spontaneously at moderate temperatures (usually $30{-}150^\circ\text{C}$)
• Reaction is favored at higher pressures and lower temperatures
• Gas mixture after the catalyst is cooled and passed through oxidation towers or ducts with sufficient residence time to approach equilibrium between $\text{NO}$ and $\text{NO}_2$

Because the next stage requires $\text{NO}_2$ (or its dimer $\text{N}_2\text{O}_4$) to form nitric acid, the process aims to shift the equilibrium:

$$
\text{2 NO}_2 \rightleftharpoons \text{N}_2\text{O}_4
$$

towards $\text{NO}_2/\text{N}_2\text{O}_4$ by appropriate pressure and temperature control.

3. Absorption of Nitrogen Dioxide in Water

Nitrogen dioxide is finally absorbed in water to form nitric acid. The simplified overall reaction for this stage is:

$$
\text{3 NO}_2(g) + \text{H}_2\text{O}(l) \rightarrow \text{2 HNO}_3(aq) + \text{NO}(g)
$$

Key industrial aspects:

• Absorption occurs in a tall absorption tower (packed or plate column) with downward-flowing water and upward-flowing process gas
• The $\text{NO}$ formed is not wasted: it is re-oxidized to $\text{NO}_2$ and re-enters the absorption cycle, increasing the overall yield:
  $$
  \text{2 NO}(g) + \text{O}_2(g) \rightarrow \text{2 NO}_2(g)
  $$

Combining the main steps gives an overall stoichiometric equation for nitric acid formation:

$$
\text{NH}_3(g) + \text{2 O}_2(g) \rightarrow \text{HNO}_3(aq) + \text{H}_2\text{O}(l)
$$

However, industrially the process is not a single-step reaction but a sequence with recycle of $\text{NO}$ within the absorption section.

Process Flow and Equipment

Main Process Sections

A modern nitric acid plant using the Ostwald process typically includes:

1. Feed preparation
• Vaporization and preheating of ammonia
• Filtration, drying, and compression of air
• Precise mixing of ammonia and air in controlled ratios
2. Ammonia oxidation section
• Catalytic reactor containing platinum–rhodium gauze
• High-temperature combustion-like conditions
• Hot gas leaving the catalyst contains mainly $\text{NO}$, $\text{H}_2\text{O}$, $\text{N}_2$, $\text{O}_2$, and small amounts of $\text{N}_2\text{O}$
3. Heat recovery
• The very hot reaction gas ($\sim900^\circ\text{C}$) is cooled in waste-heat boilers
• Steam production for use elsewhere in the plant or in other units
• Further cooling in gas coolers before absorption
4. Oxidation and absorption section
• Ducts or small towers where $\text{NO}$ is oxidized to $\text{NO}_2$
• Absorption tower (tray or packed column), where water (or dilute nitric acid) flows downward and gas flows upward
• Formation of nitric acid solutions typically in the range $50{-}68\%$ by mass
5. Product handling and concentration
• For many uses (e.g. fertilizers) ~60–68% nitric acid is sufficient
• For more concentrated nitric acid (up to ~98%), further concentration is needed, usually by:
• Distillation with dehydrating agents (e.g. sulfuric acid)
• Special processes under reduced pressure and with nitrogen oxide recycling
6. Tail gas treatment
• Residual $\text{NO}_x$ in the off-gas must be minimized before venting to the atmosphere
• Treated in tail gas treatment units, commonly:
• Selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) using ammonia or urea
• Additional absorption or catalytic oxidation stages

Operating Pressure Variants

Nitric acid plants are often classified by operating pressure:

• Low-pressure processes
• $\sim1{-}5\,\text{bar}$
• Higher selectivity to $\text{NO}$
• Lower power consumption for compression
• Lower absorption efficiency (lower $\text{HNO}_3$ concentration without extra measures)
• Medium- and high-pressure processes
• Up to $\sim8{-}12\,\text{bar}$, sometimes higher
• Improve absorption efficiency and allow higher acid concentrations directly from the absorption tower
• Require more robust equipment and higher compression energy
• Often chosen to match integrated plant designs and energy recovery concepts

Catalyst and Materials Considerations

Platinum–Rhodium Catalyst

The catalytic gauze is crucial in the first reaction step:

• Composition: typically ~90–95% Pt and 5–10% Rh
• Form: woven gauze sheets, several stacked layers
• Operating life: limited by high-temperature corrosion and platinum loss (volatilization as PtO$_2$ and mechanical losses)
• Platinum recovery:
• Special catcher gauzes or ceramic supports downstream to capture volatilized platinum
• Economic incentive due to high catalyst cost

The catalyst must:

• Provide high activity to allow high space velocity (high throughput)
• Give high selectivity to $\text{NO}$ over $\text{N}_2$ and $\text{N}_2\text{O}$
• Withstand high temperatures and corrosive, oxidizing conditions

Construction Materials

Key considerations:

• High-temperature corrosion resistance in the burner and waste-heat boiler
• Resistance to nitric acid and nitrogen oxides in the absorption section
• Common materials:
• Stainless steels with good corrosion resistance in oxidizing acid media
• Special alloys or linings for highly corrosive sections
• Nonmetallic packing materials (e.g. acid-resistant ceramics, PTFE, certain plastics) in absorption towers, depending on temperature and acid concentration

Process Control and Safety Aspects

Mixture Control and Explosion Prevention

The ammonia–air mixture must remain outside the explosion range:

• Typical industrial operating range: 9–11 vol% $\text{NH}_3$ in air
• Mixed just before entering the burner or catalytic reactor
• Continuous monitoring of:
• Flow rates of ammonia and air
• Oxygen content in the mixture and in the exhaust
• Temperatures before and after the catalyst

Explosion risks are minimized by:

• Design that avoids accumulation of unreacted ammonia
• Automatic shutdown and purge systems
• Multiple layers of instrumentation and safety interlocks

Temperature Management

The oxidation of ammonia is strongly exothermic:

• Temperature must be high enough for good kinetics but not so high that:
• The catalyst sinters and loses surface area
• Undesired side reactions increase excessively
• Temperature control via:
• Proper preheating of reactants
• Heat removal in waste-heat boilers
• Control of feed concentration and flow rates

Sudden changes in feeds can cause temperature excursions; thus, modern plants employ advanced control systems and gradual ramping of operating conditions.

Handling of Nitrogen Oxides and Nitric Acid

Hazards:

• $\text{NO}_2$ is toxic and corrosive
• Nitric acid is a strong oxidizing acid, causing severe burns and promoting fires in contact with organic materials

Industrial safety measures:

• Gas-tight equipment and piping with appropriate pressure ratings
• Gas detection systems for $\text{NO}_x$
• Ventilation and controlled vent systems
• Corrosion monitoring and maintenance schedules
• Personal protective equipment (PPE) and emergency procedures for leaks or spills

Environmental Aspects and Tail Gas Cleaning

Emissions of Nitrogen Oxides

Without treatment, the off-gas from the absorption section contains:

• Unreacted $\text{NO}$ and $\text{NO}_2$
• Possibly $\text{N}_2\text{O}$ from the ammonia oxidation stage
• Residual oxygen and nitrogen

$\text{NO}_x$ contributes to:

• Smog formation
• Acid rain
• Health problems (respiratory irritation)

Therefore, regulations strictly limit stack emissions of $\text{NO}_x$ and increasingly also of $\text{N}_2\text{O}$.

Tail Gas Treatment Methods

Common methods include:

1. Selective Catalytic Reduction (SCR)
• Reaction (for $\text{NO}$+$\text{NO}_2$):
  $$
  4 \text{ NO} + 4 \text{ NH}_3 + \text{O}_2 \rightarrow 4 \text{ N}_2 + 6 \text{ H}_2\text{O}
  $$
• Catalysts: typically based on $\text{V}_2\text{O}_5$, zeolites, or other metal oxides
• Ammonia or urea is injected as a reducing agent
2. Selective Non-Catalytic Reduction (SNCR)
• Similar chemistry, but without catalyst at higher temperatures
• Lower efficiency and less commonly used in modern nitric acid plants compared to SCR
3. Additional absorption or catalytic oxidation
• Enhanced oxidation of $\text{NO}$ to $\text{NO}_2$ followed by further absorption
• Sometimes used in conjunction with other methods

For $\text{N}_2\text{O}$:

• Dedicated $\text{N}_2\text{O}$ decomposition catalysts may be installed in:
• The catalyst gauze system (primary measures)
• Downstream reactors (secondary or tertiary measures)
• Typical reaction:
  $$
  2 \text{ N}_2\text{O} \rightarrow 2 \text{ N}_2 + \text{O}_2
  $$

Energy Integration

Because the Ostwald process is highly exothermic, modern plants integrate energy recovery:

• High-pressure steam generation in waste-heat boilers
• Use of generated steam for:
• Driving turbines (process compressors, electricity generation)
• Supplying heat to other process units in the chemical complex

This improves overall energy efficiency and reduces the environmental footprint per unit of nitric acid produced.

Product Grades and Uses

Concentrations and Purity Levels

Industrial nitric acid is produced in several typical grades:

• Weak nitric acid
• About 50–68% $\text{HNO}_3$ by mass
• Direct product of the absorption tower at medium- or high-pressure plants
• Main bulk product for fertilizers
• Concentrated nitric acid
• About 98% $\text{HNO}_3$
• Obtained by distillation and dehydration of weaker acid
• Used where high oxidizing strength and low water content are required (e.g. some nitration reactions, specialty chemicals)
• Fuming nitric acid
• Contains significant dissolved nitrogen oxides ($\text{NO}_2$)
• Highly reactive; used in specific niche applications (e.g. certain propellants)
• Typically produced in specialized plants or units, not in standard bulk nitric acid plants

Purity requirements differ by application:

• Fertilizer-grade nitric acid: moderate purity is sufficient
• Electronic or laboratory-grade: impurities must be tightly controlled; produced by additional purification steps

Main Industrial Uses

Nitric acid serves primarily as an intermediate in large-scale chemical production:

• Fertilizers
• Production of ammonium nitrate and other nitrate fertilizers
• Explosives
• Nitration of organic compounds (e.g. glycerol, toluene) to give energetic materials
• Organic synthesis
• Nitration of aromatic and aliphatic compounds
• Oxidizing agent in some processes
• Metallurgy
• Pickling and cleaning of metal surfaces
• Production of some metal nitrates

The scale of nitric acid production in a chemical complex is often determined by fertilizer demand and integrated with ammonia production (Haber–Bosch) and other downstream units (e.g. ammonium nitrate, nitro compounds).

Process Variants and Modern Developments

Integrated Ammonia–Nitric Acid–Fertilizer Complexes

To minimize energy consumption and transportation needs, many plants are integrated:

• Ammonia from a nearby Haber–Bosch unit is fed directly to the Ostwald plant
• Nitric acid from the Ostwald plant is fed directly to:
• Ammonium nitrate production
• NPK (nitrogen–phosphorus–potassium) fertilizer production

Heat and steam generated in the nitric acid plant are used:

• To drive compressors
• In other process units within the same site

Advanced Catalysts and Process Intensification

Areas of improvement:

• Gauze designs with reduced platinum losses
• Catalysts that minimize $\text{N}_2\text{O}$ formation
• Better mass-transfer performance in absorption towers
• Process automation and optimization to maintain optimal operating conditions

New technologies may include:

• Alternative reactor designs for improved mixing and selectivity
• Enhanced tail gas treatment processes
• Digital control systems with advanced process control and predictive maintenance

Summary of Key Industrial Features

• Nitric acid is produced almost exclusively by the Ostwald process, starting from ammonia and air.
• The process comprises:
1. Catalytic oxidation of ammonia to $\text{NO}$ on Pt–Rh gauze
2. Gas-phase oxidation of $\text{NO}$ to $\text{NO}_2$
3. Absorption of $\text{NO}_2$ in water to yield $\text{HNO}_3$ with recycle of $\text{NO}$
• The process is highly exothermic; energy is recovered as steam and used within the plant.
• Platinum-based catalysts and appropriate construction materials are essential for high efficiency and durability.
• Environmental control focuses on minimizing emissions of $\text{NO}_x$ and $\text{N}_2\text{O}$ via tail gas treatment and improved catalyst systems.
• Nitric acid is a central intermediate in fertilizer and explosive manufacture, making its industrial production a cornerstone of modern large-scale chemistry.