Nitrogen: Basics and Applications in Process Engineering

Basics: What is nitrogen?

Nitrogen (chemical symbol N, atomic number 7) is a chemical element of the nitrogen group (15th group in the periodic table) and belongs to the category of non-metals. It occurs in nature mainly as a diatomic molecule (N₂), which makes up around 78% of the Earth's atmosphere. Molecular nitrogen is colorless, odorless and tasteless. The name “nitrogen” is derived from its ability to extinguish (suffocate) flames by displacing oxygen.

History

The importance of nitrogen was first recognized in the 18th century:

• 1771: Carl Wilhelm Scheele identified nitrogen as a component of air and described it as “foul air”.
  
• 1772: Daniel Rutherford confirmed this discovery.
  
• Antoine de Lavoisier called the gas “azote” (Greek “azotos”, hostile to life).
  
• Jean-Antoine Chaptal coined the term “nitrogène”, based on its role in saltpetre (nitrate compounds).
  
• The industrial use of nitrogen began in the 20th century with the introduction of processes such as the Haber-Bosch process for ammonia synthesis.
  

Properties of nitrogen

Physical properties

• State at room temperature: Gas
  
• Appearance: Colorless
  
• Odor and taste: Odorless and tasteless
  
• Melting point: -210 °C
  
• Boiling point: -195.8 °C
  
• Density: 1.17 g/L at 0 °C and 101.3 kPa
  
• Solubility: Slightly soluble in water (23.2 ml N₂ in 1 L water at 0 °C)
  
• Non-flammable: Molecular nitrogen can smother flames as it does not contain oxygen.
  

Chemical properties

• Inertness: Nitrogen is very stable and inert due to its strong triple bond in the N₂ molecule (bond dissociation energy: 942 kJ/mol). This requires high energy input or catalysts to break the bond.
  
• Bond types: Nitrogen prefers to form covalent bonds, e.g. in ammonia (NH₃) or amines.
  
• Electron configuration: 1s² 2s² 2p³

Liquid nitrogen

• Coolant: At -196 °C, nitrogen is liquid and is often used for cryogenic applications.
  
• Expansion: During evaporation, 1 liter of liquid nitrogen expands to about 695 liters of gaseous nitrogen.

Spectral properties

• Gas discharge: In a spectral tube, nitrogen shows a characteristic glow caused by the excitation of the molecular orbitals at high voltage.

Polymeric nitrogen

• Discovery: In 2004, researchers at the Max Planck Institute synthesized a polymeric form of nitrogen with single bonds and a cubic structure (“cubic gauche”).
  
• Properties: High instability and potential as an energy storage or explosive.
  
• New form (2020): A “black” polymeric form with two-dimensional layers, similar to graphene, which is promising for technical applications.

Isotopes of nitrogen

Nitrogen has a total of 17 known isotopes with mass numbers from ⁹N to ²⁵N, as well as some isomeric states. Of these, two isotopes are stable and exist in nature:

• ¹⁴N (99.636 % proportion in the natural composition)
  
• ¹⁵N (0.364 % proportion)
  

The unstable isotopes have different half-lives:

• ¹³N: half-life of 9.965 minutes; decays by β⁺-radiation to ¹³C.
  
• ¹⁶N: half-life of 7.13 seconds; decays to ¹⁶O by beta decay.
  
• Other unstable isotopes have half-lives in the range of seconds to milliseconds.
  

The ¹⁵N isotope, discovered in 1929, is used in biochemistry for studies of the nitrogen cycle, e.g. in arable soil or in plants. It is also used to analyze protein conversion. It can be enriched by thermal diffusion separation.

Nitrogen compounds

Nitrogen forms numerous compounds, which can be divided into inorganic and organic compounds.

Inorganic compounds

1. Ammonia (NH₃)
   • Pungent smelling, water-soluble gas.
     
   • Basis for fertilizers (e.g. urea) and ammonium compounds.
     
   • Produced using the Haber-Bosch process.
     
2. Nitrides
   • Covalent nitrides: e.g. silicon nitride, boron nitride, Dischwefeld nitride.
     
   • Metallic nitrides: e.g. titanium nitride (gold-colored protective coatings for tools).
     
   • Salt-like nitrides: e.g. lithium nitride, magnesium nitride.
     
3. Nitrogen oxides
   • Examples: Nitrogen monoxide (NO), nitrogen dioxide (NO₂), nitrous oxide (N₂O).
     
   • Formed during combustion processes, sometimes toxic and harmful to the environment.
     
   • Nitrous oxide (laughing gas) is used as an anaesthetic.
     
4. Halides
   • Examples: Nitrogen trifluoride (strong oxidizing agent, used in the semiconductor industry), iodine nitrogen (explosive).
     
5. Acids and their salts
   • Nitric acid (HNO₃): Strong acid, basis for fertilizers and explosives.
     
   • Nitrous acid (HNO₂): Unstable, decomposes when heated.
     
   • Hyposalpetrous acid (H₂N₂O₂): Decomposes at room temperature.
     
   • Hydrogen nitric acid (HN₃): Highly explosive, irritant; salts such as azides are used in initial explosives.
     
6. Further compounds
   • Hydrogen cyanide (hydrocyanic acid) and cyanides: Highly toxic.
     
   • Hydrazine (N₂H₄): Used as a fuel in space travel.

Organic compounds

1. Amino compounds
   • Amino acids, peptides and proteins.
     
   • Amines such as spermine.
     
2. Azo compounds
   • Examples: Azobenzene, azo dyes.
     
3. Nitro compounds and nitric acid esters
   • Examples: Nitroglycerine, trinitrotoluene (TNT), octanitrocubane.
     
4. Heterocycles
   • Nitrogen-containing ring systems such as pyridine, indigo.
     
5. Nucleic bases
   • Adenine, thymine, uracil.
     
6. Alkaloids
   • Examples: Morphine, caffeine.

Natural occurrence of nitrogen

Nitrogen in the air

The Earth's atmosphere consists of 78.09 percent by volume of molecular nitrogen (dinitrogen, N2). However, this is directly useless for most living beings. Only a few microorganisms can use atmospheric nitrogen by binding it and converting it into a biologically usable form.

1. Nodule bacteria
   Nodule bacteria invade the roots of legumes (e.g. peas, beans) and form a symbiosis there. They get carbohydrates from the plant and in return provide ammonium, which they produce from atmospheric nitrogen using the enzyme nitrogenase. This symbiosis makes legumes particularly valuable for enriching soil with nitrogen, e.g. in organic farming.
2. Free-living microorganisms
   Some microorganisms, such as Azotobacter and Cyanobacteria, can fix nitrogen on their own. They use this process to produce the body's own proteins. Nitrogen sequestration by these organisms contributes around 5—15 kg of nitrogen per hectare annually.
3. Electrical discharges during thunderstorms
   Thunderstorms promote natural nitrogen fixation through electrical discharges that combine nitrogen and oxygen in the air to form nitrogen oxides. These react with rainwater to form nitric acid, which penetrates the soil and is converted into nitrates. As a result, 20-25 kg of nitrogen per hectare are released into soils rich in rainfall every year.
4. Anthropogenic influences
   Through human activities such as ammonia synthesis (Haber-Bosch process), atmospheric nitrogen is technically bound and made available for fertilisers and industrial applications. Car exhaust gases also release nitrogen compounds such as nitrogen oxides, which contribute to eutrophication and environmental pollution.

Nitrogen in soil

In soil, nitrogen is mainly organically bound (over 95%), e.g. in living root mass, dead plant mass, humus and soil organisms. Only a small portion (less than 5%) is present in inorganic form, usually as ammonium, nitrate or nitrite. The total nitrogen content of a soil depends on factors such as climate, vegetation, soil type, and agricultural measures.

Nitrogen in plants

Nitrogen is essential for plants because, as a component of DNA, proteins and chlorophyll, it is crucial for growth and metabolism. Plants usually absorb nitrogen in the form of ammonium or nitrates.

Symptoms of deficiency:

• Sluggish growth
• Pale green leaves that become chlorotic on older plants and fall off prematurely
• Blooming too early (emergency bloom)

Symptoms of excess:

• Excessive growth with dark green leaves
• Delayed flower formation
• Higher vulnerability to frost and disease

Nitrogen as a mineral

Crystalline nitrogen was first discovered in diamonds from river sediments in Brazil in 2019. This allotrope, deltanitrogen (δN), crystallizes in an orthorhombic structure and is recognized as an independent mineral species by the International Mineralogical Association (IMA).

Extraction and presentation of nitrogen

Industrial extraction of nitrogen

Industrial nitrogen production is primarily carried out by fractional distillation of liquefied air in air separation plants. This process, known as the Linde process, enables a purity of up to 99.99999 %. For extremely high purities, where the impurities are below 1 ppb, additional purification steps are required. One method of removing the remaining oxygen, for example, is biological treatment with rice seedlings. Basic and detailed engineering are crucial when planning air separation plants in order to precisely implement the fractional distillation of nitrogen.

For less demanding applications, nitrogen is obtained by multi-stage adsorption and desorption on zeolites. This process is more cost-efficient and achieves purities of around 99 %. Another option is the membrane process, in which compressed air is pressed through plastic membranes. As nitrogen diffuses more slowly through the membrane than oxygen and other gases, the nitrogen content can be increased in a targeted manner. Depending on requirements, the purity can be up to 99.995 %.

Decentralized nitrogen generation

In addition to centralized plants for nitrogen production, there are also decentralized options for generating nitrogen directly on site. Companies often rely on their own nitrogen generators to flexibly control the purity, pressure and quantity of gas. This avoids transport costs and losses due to evaporation or gas bottles that are not completely emptied. When planning and developing nitrogen generators, a feasibility study can be carried out as part of pre-engineering to analyze technical and economic aspects. The most common generators are:

1. Membrane nitrogen generators: Work with plastic membranes to extract nitrogen from compressed air.
   
2. PSA (Pressure Swing Adsorption) nitrogen generators: Utilize pressure swing adsorption to achieve purities of up to 99.999% (10 ppm).

Advantages of self-production:

Producing your own nitrogen offers several advantages:

• Cost control: No transportation costs, stable price.
  
• Flexibility: Control over purity, pressure and quantity.
  
• Safety: Elimination of cryogenic storage and transportation hazards.
  
• Efficiency: No losses due to evaporation or unused residual content in gas cylinders.

Alternative methods for nitrogen recovery

There are also historical and laboratory methods for nitrogen removal:

1. Chemical removal of oxygen:
   • Binding of atmospheric oxygen to carbon with heating, followed by leaching of the resulting carbon dioxide.
     
   • Passing air over red-hot copper or through alkaline pyrogallol or sodium dithionite solutions.
     
2. Laboratory methods:
   • Decomposition of ammonium nitrite: Heating an aqueous ammonium nitrite solution produces pure nitrogen: NH₄NO₂ → 2 H₂O + N₂
     
   • Thermolysis of sodium azide: This process is used to produce spectroscopically pure nitrogen: 2 NaN₃ → 2 Na + 3 N₂

Uses and uses of nitrogen

Chemical industry

Nitrogen is used extensively in the chemical industry:

• Synthesis processes: Essential for the production of ammonia (Haber-Bosch process) and calcium cyanamide. Both products are the basis for numerous chemical applications, including fertilizers.
  
• Inertization: Nitrogen prevents unwanted reactions by excluding oxygen. This increases safety in production plants.
  
• Production of specialty chemicals: These include paints, varnishes, plastics, adhesives and release agents, which are often manufactured under nitrogen-rich conditions.

Petrochemical industry and refineries

Nitrogen is essential in petrochemicals:

• Protective gas: It protects raw materials and products from oxidation and moisture.
  
• Cleaning systems: Nitrogen is used to clean pipes, tanks and lines and to remove oxygen or moisture.
  
• Explosion protection: Inertization prevents the formation of explosive mixtures.
  
• Processing of biofuels and lubricants: Nitrogen ensures the necessary process safety.
  

Pharmaceutical industry

In pharmacy, nitrogen is used in the following areas:

• Packing and storage: Protective atmospheres made of nitrogen prevent the oxidation of active substances and ensure a longer shelf life.
• Sterile processes: Nitrogen protects sensitive products from contamination by oxygen or microorganisms.
• Temperature control: Liquid nitrogen is used to store biological samples and drugs.

Food and beverage industry

The food and beverage industry benefits significantly from nitrogen:

• Cooling and shock freezing: Liquid nitrogen ensures that food freezes quickly and keeps it fresh.
• Wrapping: Gaseous nitrogen is used as a protective gas to displace oxygen and extend the shelf life of products.
• Dispensing systems: Mixed gases of nitrogen and carbon dioxide regulate the tap pressure in beverages and minimize foam formation.

Metal processing

Nitrogen plays a crucial role in the metal industry:

• Welding processes: As a protective gas, nitrogen prevents oxidation and ensures clean welds.
• Heat treatment: Nitrogen improves the surface quality of metals and is used in hardening processes.
• Cryogenic processes: Liquid nitrogen is used to artificially age materials or to change their structure through cold working.

Water and sewage management

Water management uses nitrogen in various processes:

• Inertization: Nitrogen protects systems from corrosion caused by oxygen.
• Wastewater treatment: Nitrogen is used in process gas treatment to prevent harmful reactions.

Energy supply

Nitrogen is used in a variety of ways in the energy sector:

• Thermal oil plants and steam systems: Nitrogen is used for inerting and pressure regulation.
• Cryogenic energy storage: Liquid nitrogen helps to efficiently store energy in the form of cold.

Refrigeration and cryotechnology

Liquid nitrogen is essential in cryotechnology:

• Shock cooling: Used in the food industry to cool food quickly and safely.
• Medical applications: For storage of biological samples, such as eggs and sperm.
• Ground icing: In civil engineering, nitrogen is used to stabilize soils.

Research and analysis

High-purity nitrogen is used in laboratories and analytical processes:

• Calibration of devices: Nitrogen purifies and resets analytical instruments to zero.
• Cooling sensitive devices: Nitrogen plays a central role in cryotechnology and superconducting materials.

Other applications of nitrogen

• Aircraft tires: Due to its inertness, nitrogen prevents the risk of fires due to high temperatures.
• Explosives manufacturing: Many nitrogen compounds are used as components of explosives.
• Cryosurgery: Liquid nitrogen is used in medicine to treat warts and other skin problems.
• Nitrogen burials: Nitrogen is used in experiments to preserve biological material in an environmentally friendly way.

Which plants use nitrogen?

There are a variety of plants and project examples in which nitrogen is used to make processes more efficient, safer or more precise. Some examples of such systems are given below.

Nitrogen in food processing

• Shock freezing systems: Systems that use liquid nitrogen to freeze food in a flash. This prevents the formation of large ice crystals and maintains the quality of the products.
• Drying systems with nitrogen flushing: Avoiding oxidation when drying sensitive foods such as spices or herbs.
• Nitrogen cycle systems: Reprocessing of nitrogen from packaging processes.

Beverage and luxury food industry

• Nitrogen systems for pressure management: Used in systems that use nitrogen to control tap pressure and foam formation, e.g. in beer production.
• Carbon dioxide optimization: Systems that use nitrogen in the production of sparkling drinks to regulate CO₂ sequestration.

Medical and pharmaceutical industries

• Refrigeration systems for cryogenic storage: Planning of storage facilities that use liquid nitrogen to store temperature-sensitive substances such as vaccines or biological samples.
• Inerting systems for clean rooms: Protective atmospheres in production plants that prevent contamination by oxygen or microorganisms.

Pulp and paper industry

• Nitrogen-cooled decomposition plants: Systems for the treatment of process waste using nitrogen for cooling.
• Gas pulverization systems: Nitrogen is used to crush fiber materials under cryogenic conditions.

Waste and disposal industry

• Biogas processing plants: Use of nitrogen in the purification and optimization of biogas processes.
• Safety inertization: Systems that use nitrogen to control hazardous gases in waste processing processes.

Energy supply and storage

• Nitrogen-tight test systems: Equipment used in energy storage systems to ensure tightness.
• Liquid nitrogen cooling for energy cables: Systems for cooling high-performance cables that ensure constant power transmission.

Pet food production

• Packaging systems for protective atmospheres: Plants that use nitrogen to protect dry or moist food from oxidation.
• Freeze-drying processes: Nitrogen in drying ingredients to preserve nutrients and flavor.

Research and development

• Nitrogen laboratory plants: Laboratories that use high-purity nitrogen for the analysis and development of new products.
• Nitrogen-based pilot plants: Systems for simulating industrial production processes, in which nitrogen performs central functions.

Metal processing equipment

• Surface treatment equipment: Nitrogen as a medium in processes such as nitriding or plasma nitriding to improve material properties.
• Welding gas supply systems: Planning of systems that use nitrogen for protective gas mixtures in special welding processes.

Water management

• Nitrogen degassing systems: Systems for feeding nitrogen into water treatment plants to displace oxygen and other unwanted gases.

Safe handling of nitrogen

Risks and precautionary measures
Nitrogen is an inert gas which can displace oxygen in high concentrations. The main accident mechanism consists in the suppression of oxygen, which leads to hypoxia without those affected being able to notice this. When working with nitrogen, the following measures must be taken:

• Ventilation systems: Rooms where nitrogen is used must be equipped with powerful ventilation systems to ensure an adequate oxygen concentration.
• Warning systems: Optical and acoustic sensors that detect oxygen deficiency should be installed. These systems alert when there are dangerous oxygen concentrations.
• protective equipment: People working in nitrogen-rich areas should wear personal protection systems, such as oxygen sensors, which emit warning signals when oxygen levels fall below a critical threshold.
• Trainings: Employees must be informed, consulted and regularly trained about risks, safety protocols and how to handle emergencies.

Accident prevention

• Access controls for nitrogen-rich areas are essential. Only trained personnel should be allowed to enter them.
• Dual security barriers, such as access barriers combined with automatic sensor alerts, can minimize risks.
• Safety data sheets and technical plant documentation must be kept available at all times and read by all employees involved.

Sustainability and environmental aspects

Environmental pollution caused by nitrogen compounds
Nitrogen has a wide range of environmental effects, in particular due to improper use in agriculture, soil, water, air and biodiversity. These factors are important for process engineering plant planning and plant construction:

• Groundwater pollution: The conversion of nitrogen into nitrate can enter groundwater as a result of leaching. This increases the nitrate concentration in drinking water, which can be harmful to health.
• Surface water: Nitrogen surpluses cause eutrophication, which leads to a lack of oxygen in water bodies. This damages ecosystems and reduces biodiversity.
• Air pollution: Ammonia emissions from nitrogen-containing waste contribute to the formation of particulate matter and acid rain, which pollutes sensitive ecosystems.
• Climate change: Nitrous oxide (N₂O), a by-product of nitrogen use, is a powerful greenhouse gas with 265 times the climate impact of CO₂.

Measures to promote sustainability

• Efficient nitrogen management: Minimizing the use of nitrogen compounds in production processes.
• Waste reduction: Recycling nitrogen compounds from waste water and waste materials to reduce environmental impact.
• Energy efficiency: Integration of technologies that optimize energy consumption in nitrogen production and use.
• Monitoring and control: Implementation of systems to continuously measure and monitor nitrogen emissions to comply with environmental limits.

Regulatory requirements
The use of nitrogen is subject to strict guidelines. In the EU, measures such as the Fertilizer Ordinance (DüV) are mandatory to limit nitrogen inputs. These standards influence the planning and operation of plants that use or release nitrogen. Approval engineering therefore supports compliance with regulations and ensures the legally compliant use of nitrogen plants.

Innovations to reduce environmental impact

• Nitrogen recovery: Technologies to recycle nitrogen from exhaust gases and waste water can help reduce emissions.
• Sustainable processes: Development of new processes that convert nitrogen compounds into more environmentally friendly substances.

FAQ about nitrogen

What happens to the nitrogen we breathe in?

The majority of the nitrogen that you inhale is exhaled unchanged. Nitrogen is chemically inert for humans, which means that it is not processed or converted directly by the body in inhaled air. Your body does not use nitrogen from the air because it needs nitrogen in bound form, such as amino acids, proteins, or DNA. The inhaled nitrogen therefore only serves as a filler in the air you breathe.

What do you use nitrogen for?

Nitrogen is used in many areas. In industry, it is used to produce fertilizers, explosives and plastics. Nitrogen is used medically to preserve tissues, organs and cells, particularly in the form of liquid nitrogen. It is also used as a protective gas in the food and electronics industries to prevent oxidation. Nitrogen is also used in car tires because it provides pressure stability.

Why is nitrogen a problem?

Nitrogen becomes problematic when it is released into the environment in large quantities, particularly in the form of nitrogen compounds such as ammonia, nitrates or nitrogen oxides. They can:

• Pollute water bodies through over-fertilization (eutrophication).
• Contribute to the formation of particulate matter and ground-level ozone.
• Increase climate change with nitrous oxide (N₂O).
• Endangering biodiversity through changes in ecosystems.

Nitrogen in the form of nitrogen oxides can also cause respiratory problems in terms of health.

What do humans need nitrogen for?

You need nitrogen primarily to build vital molecules such as proteins, DNA, and RNA. These are essential for your cell functions. The nitrogen that your body uses is not absorbed from the air, but through food in the form of amino acids, which contain nitrogen-containing compounds. Plants obtain nitrogen from the soil, which also indirectly makes it indispensable for your diet.

How can nitrogen be detected?

Nitrogen can be detected both in organic and inorganically bound form:

Organically bound nitrogen:

1. Lassaigne's sample: A qualitative detection.
   
2. Kjeldahl nitrogen determination: A method for quantitative analysis.
   
3. Elemental analysis: Measures the nitrogen content in compounds.

Inorganically bound nitrogen:

1. Cross test: Specific for ammonium ions.
   
2. Ring test: Detection of nitrate ions. The sample solution is mixed with iron(II) sulphate solution and undercoated with sulphuric acid. A brown ring consisting of a nitrogen monoxide complex forms at the phase boundary.
   

The ring test is carried out in two steps:‍

1st redox reaction: 3 Fe²⁺ + NO₃- + 4 H⁺ → 3 Fe³⁺ + NO + 2 H₂O‍

2nd complex formation reaction: Fe²⁺ + NO + 5 H₂O → [Fe(H₂O)₅NO]²⁺

What are the benefits of using nitrogen?

Nitrogen offers numerous benefits:

• Inertness: Its chemical reaction inertia makes it ideal as a protective gas.
• Efficiency as a coolant: Liquid nitrogen is versatile and cost-effective.
• Increased durability: In the food industry, nitrogen protects products from spoilage.
• Versatility: It is used in a wide variety of industries and applications.

What are the challenges of using nitrogen?

Using nitrogen also poses challenges:

• Environmental impact: Nitrogen can contribute to water and air pollution, particularly through industrial applications and agriculture.
• Security risks: Liquid nitrogen and pressurized nitrogen require special precautionary measures.‍
• Costs: In some applications, such as cryogenics, operating costs can be high.