Industrial Extraction of Nitrogen (N)

Natural Occurrence & Major Sources

Nitrogen (N) is a non-metallic element primarily occurring in gaseous form. It does not exist in traditional “ores” like metals.

Atmospheric Nitrogen (N₂)

• Primary Source: Approximately 78% by volume of the Earth’s atmosphere is dinitrogen (N₂). This is the most significant industrial source.

Inorganic Nitrogen Compounds

• Nitrates: Occurs as naturally occurring nitrate salts, such as:
  • Chile Saltpetre: Sodium nitrate (NaNO₃)
  • Indian Saltpetre: Potassium nitrate (KNO₃)
  • These are sources for nitrogen compounds, but not typically for elemental N₂ extraction.

Organic Nitrogen Compounds

• Found in all living organisms (proteins, nucleic acids, amino acids). These are not industrial sources for elemental N₂.

Industrial Extraction of Nitrogen from Air

The industrial extraction of elemental nitrogen (N₂) focuses on its separation from atmospheric air. This process involves the liquefaction of air followed by fractional distillation. The most common method is the Linde-Frankl Process or variations thereof.

1. Preparation of Air (Pre-treatment)

Atmospheric air must be purified before liquefaction to prevent blockages due to solidification of impurities at low temperatures.

• Dust Removal: Air is filtered to remove particulate matter.
• Carbon Dioxide (CO₂) Removal: CO₂ is removed by passing the air through scrubbers containing:
  • Caustic soda (NaOH) solution: 2NaOH(aq) + CO₂(g) → Na₂CO₃(aq) + H₂O(l)
  • Or, by adsorption onto molecular sieves.
• Water Vapour (H₂O) Removal: Moisture is removed by:
  • Cooling the air to condense water.
  • Passing through desiccants like activated alumina (Al₂O₃) or silica gel (SiO₂).
  • Using molecular sieves.

2. Liquefaction of Air

The purified air is then cooled and compressed repeatedly to achieve liquefaction, relying on the Joule-Thomson effect.

• Compression: Air is compressed to high pressures (e.g., 100-200 atm).
• Cooling: The compressed air is cooled by refrigerants or by expanding previously cooled air in heat exchangers.
• Expansion (Joule-Thomson Effect): The highly compressed, cooled air is allowed to expand rapidly through an expansion valve. This sudden expansion causes a significant drop in temperature.
  • Description of Process Flow:
    1. Filtered and purified air is compressed by a compressor.
    2. The hot compressed air is cooled in heat exchangers by incoming cold gas streams.
    3. Further cooling occurs as it exchanges heat with already cold, expanding gas.
    4. The pre-cooled, high-pressure air passes through an expansion valve, where it rapidly expands and cools significantly due to the Joule-Thomson effect.
    5. A portion of the air liquefies at this stage. The unliquefied gas is sent back through heat exchangers to cool the incoming compressed air and then recycled through the compressor.
    6. The liquid air collects at the bottom of the liquefaction column.

3. Fractional Distillation of Liquid Air (Separation of Components)

Liquid air, a mixture of liquid nitrogen (boiling point -196 °C), liquid oxygen (boiling point -183 °C), and liquid argon (boiling point -186 °C), is then fractionally distilled.

• Principle: The separation is based on the difference in boiling points of the components. Nitrogen, with the lowest boiling point, vaporizes first.
• Distillation Column: A tall fractionating column is used, typically operating at low temperatures.
  • Description of Process Flow:
    1. Liquid air is introduced near the middle of a fractionating column.
    2. As the liquid descends, it is heated by ascending vapours.
    3. Nitrogen, being more volatile, vaporizes preferentially and rises to the top of the column.
    4. Oxygen, with a higher boiling point, concentrates as a liquid at the bottom of the column. Argon, having an intermediate boiling point, concentrates in a section between nitrogen and oxygen.
    5. Pure gaseous nitrogen (N₂) is collected from the top of the column.
    6. Gaseous oxygen (O₂) is collected from the bottom.
    7. Argon can be collected from an intermediate section for further purification if required.

Refining and Purification

The nitrogen obtained from the top of the fractionating column is already very pure (typically >99.5%). For even higher purity requirements, further steps can be employed:

• Further Fractional Distillation: Re-distillation of the nitrogen gas or liquid can enhance purity by removing trace oxygen or argon.
• Catalytic Deoxygenation: For ultra-high purity nitrogen (e.g., 99.999%), trace oxygen can be removed by passing the nitrogen over a catalyst (e.g., palladium) in the presence of a small amount of hydrogen, forming water: 2H₂(g) + O₂(g) → 2H₂O(g). The water is then removed by adsorption.
• Adsorption: Using molecular sieves or activated carbon to remove remaining trace impurities.

The purified nitrogen is then compressed and stored in cylinders or transported as liquid nitrogen in insulated cryogenic tanks.