Industrially, nitrogen is primarily prepared through the fractional distillation of liquid air, a highly efficient cryogenic process that separates atmospheric gases based on their distinct boiling points. This method allows for the production of extremely high-purity nitrogen.
The core of this process involves supercooling an air mixture to its liquefaction point followed by distillation of its fractions at their different boiling points. This advanced technique ensures that fractional distillation will produce gases of almost hundred percent purity (99.999%), making it suitable for a vast array of industrial applications.
The Principle of Fractional Distillation
Fractional distillation works on the principle that different components in a liquid mixture will vaporize at different temperatures, corresponding to their unique boiling points. Air, being a mixture primarily of nitrogen (N₂), oxygen (O₂), and argon (Ar), can be separated because each gas turns into a liquid (and then back into a gas) at a specific very low temperature.
Here's a breakdown of the boiling points for the main components of air:
| Component | Chemical Formula | Boiling Point (°C) | Boiling Point (K) |
|---|---|---|---|
| Nitrogen | N₂ | -196 | 77 |
| Argon | Ar | -186 | 87 |
| Oxygen | O₂ | -183 | 90 |
Steps in Industrial Nitrogen Production
The preparation of industrial nitrogen via fractional distillation involves several key stages within a typical Air Separation Unit (ASU):
1. Air Pre-treatment and Compression
- Filtration: Atmospheric air is first drawn in and filtered to remove dust, particulates, and other impurities.
- Compression: The clean air is then compressed to high pressure (typically 5-10 bar).
- Purification: Compressed air passes through a pre-purification unit, often using molecular sieves, to remove unwanted components like carbon dioxide (CO₂), water vapor (H₂O), and hydrocarbons. These impurities would freeze at cryogenic temperatures, potentially blocking equipment.
2. Cooling and Liquefaction
- Heat Exchange: The purified, compressed air is then progressively cooled to extremely low temperatures using advanced heat exchangers. This cooling often involves counter-current flow with the cold product gases (nitrogen and oxygen) exiting the process.
- Expansion & Liquefaction: As the air cools, it eventually reaches its liquefaction point. This is achieved by expanding the cooled, compressed air through an expansion valve or turbine, which causes a rapid drop in temperature (Joule-Thomson effect) until the air turns into a liquid. This step directly relates to supercooling an air mixture to its liquefaction point.
3. Fractional Distillation (Separation)
- Distillation Column: The liquid air is introduced into the bottom section of a tall, insulated fractional distillation column (often a double-column system for improved efficiency).
- Vaporization and Condensation: As the liquid air gradually warms up from the bottom of the column, the components with lower boiling points begin to vaporize first.
- Nitrogen (N₂), having the lowest boiling point (-196°C), vaporizes readily and rises to the top of the column.
- Oxygen (O₂) and Argon (Ar), with higher boiling points, remain in liquid form or condense at lower levels within the column.
- Pure Nitrogen Collection: Gaseous nitrogen, now highly purified, is collected from the top of the distillation column. This separation is highly effective, as the process allows for the production of gases with almost hundred percent purity (99.999%).
4. Product Storage and Distribution
- Compression & Storage: The pure nitrogen gas is then compressed to the desired pressure for storage in high-pressure cylinders or tanks, or it can be stored as a cryogenic liquid in insulated tanks.
- Distribution: From storage, nitrogen is distributed via pipelines, tanker trucks, or cylinders to various industrial consumers.
Advantages and Applications
The industrial preparation of nitrogen by fractional distillation is a robust and highly effective method. The key advantages include:
- High Purity: As stated, it yields nitrogen of extremely high purity (99.999%), crucial for sensitive applications.
- Large Scale Production: Capable of producing vast quantities of nitrogen to meet diverse industrial demands.
- Co-production: This process also simultaneously yields high-purity oxygen and argon, making it an economically viable method for producing multiple industrial gases.
Nitrogen's inert properties and high purity make it indispensable in industries such as:
- Electronics: For inert atmospheres in semiconductor manufacturing.
- Food Packaging: To preserve freshness and extend shelf life by displacing oxygen.
- Chemical Industry: As an inert blanket, purging agent, or in chemical synthesis.
- Healthcare: As a cryogen for preserving biological samples and in cryosurgery.
