Air separation plants, also known as Air Separation Units (ASUs), work by separating atmospheric air into its primary components, primarily nitrogen, oxygen, and argon. The main working principle behind an ASU is the separation of air via its liquefying and distilling processes. Specifically, fractional distillation is the primary separation technique employed to achieve this.
The essence of how an Air Separation Unit operates lies in converting air into a liquid state and then selectively boiling off its components. Air is a mixture of gases, each with a different boiling point. This fundamental property allows for their separation through a process called fractional distillation.
- Liquefaction: First, air is cooled to extremely low ("cryogenic") temperatures, below its boiling points, causing it to condense into a liquid.
- Distillation: Once liquid, the mixture is carefully warmed. As the temperature rises, each component gas boils off at its specific boiling point, allowing it to be collected separately.
Step-by-Step Process of an Air Separation Unit (ASU)
The operation of an air separation plant involves several critical stages, each designed to purify and separate the atmospheric air efficiently.
1. Air Compression
Atmospheric air is first drawn into the plant and compressed to a higher pressure, typically ranging from 5 to 10 bar. This initial compression increases the air's temperature, which is then cooled to prepare for subsequent purification steps.
2. Pre-Purification
Compressed air contains impurities such as water vapor, carbon dioxide (CO2), and hydrocarbons. These must be removed because they would freeze at cryogenic temperatures, potentially blocking equipment.
- Adsorption: The air is passed through a pre-purification unit (PPU), often containing molecular sieves, which adsorb water and CO2. This ensures the air entering the cryogenic section is clean and dry.
3. Cooling and Liquefaction
The purified compressed air is then cooled to extremely low temperatures (around -170°C to -190°C) using heat exchangers. This cooling process uses the cold products (nitrogen, oxygen, argon) exiting the distillation columns to pre-cool the incoming air. As the temperature drops sufficiently, the air condenses and turns into a liquid.
4. Cryogenic Distillation (Fractional Distillation)
This is the heart of the air separation process and where fractional distillation is performed. The liquid air, now a mixture of liquid oxygen (boiling point -183°C), liquid argon (boiling point -186°C), and liquid nitrogen (boiling point -196°C), is fed into tall distillation columns.
- Separation: The liquid air is introduced into a high-pressure distillation column. Nitrogen, having the lowest boiling point, vaporizes first and rises to the top of the column. Oxygen and argon, with higher boiling points, remain in the liquid phase and collect at the bottom.
- Further Purification: For higher purity and to separate argon, the liquid oxygen-rich mixture from the bottom of the high-pressure column is often transferred to a low-pressure column. An argon side-column might also be used to separate argon from the oxygen stream, followed by further purification steps.
5. Product Collection and Storage
Once separated, the individual gases (nitrogen, oxygen, argon) are collected. They can be:
- Gaseous: Piped directly to industrial consumers via pipelines.
- Liquid: Stored in cryogenic tanks for distribution via specialized transport.
- Compressed: Further compressed into gas cylinders for smaller-scale use.
Why Fractional Distillation is Key
Fractional distillation is effective because it leverages the distinct boiling points of the components in air:
| Component | Boiling Point (°C) |
|---|---|
| Nitrogen | -196 |
| Argon | -186 |
| Oxygen | -183 |
By carefully controlling temperature and pressure within the distillation columns, these differences allow for the highly efficient and precise separation of each gas, leading to products of very high purity.
Common Products and Their Applications
Air separation plants are vital for various industries due to the wide range of applications for their products:
- Oxygen (O2): Used in steel production, medical applications (respiratory therapy), welding, and chemical oxidation processes.
- Nitrogen (N2): Utilized in electronics manufacturing, food preservation (packaging), inerting chemical processes, and cryopreservation.
- Argon (Ar): Primarily used as an inert shielding gas in welding, in specialty lighting, and for creating protective atmospheres in various industrial processes.
