Industrial Production of Oxygen (O)

Oxygen (O) is a non-metallic element and a gaseous component at standard temperature and pressure. Its industrial production does not involve metallurgical processes like the extraction of metals from ores. Instead, it is produced from readily available sources such as air and water.

Natural Occurrence

Oxygen is the most abundant element in the Earth’s crust by mass, forming compounds, and the second most abundant gas in the atmosphere by volume.

• Atmosphere: Approximately 21% by volume as diatomic oxygen (O₂).
• Hydrosphere: A major component of water (H₂O).
• Lithosphere: Present in numerous oxide minerals (e.g., silicates, carbonates, iron oxides). However, these are not considered “ores” for oxygen extraction in the metallurgical sense.

Industrial Production Methods

The primary industrial methods for obtaining oxygen are:

1. Fractional Distillation of Liquid Air (Cryogenic Air Separation)

This is the most common and economical method for large-scale production of high-purity oxygen, nitrogen, and argon.

Process Steps:

1. Air Pre-treatment:

   • Atmospheric air is drawn in and filtered to remove dust particles.
   • It is then compressed to high pressure (typically 5-10 MPa).
   • The compressed air is passed through adsorbers containing molecular sieves or activated alumina to remove carbon dioxide (CO₂) and water vapor (H₂O). These impurities would freeze at cryogenic temperatures, blocking equipment.

2. Air Liquefaction:

   • The purified, compressed air is then cooled significantly by passing it through heat exchangers, often using the cold products (nitrogen and oxygen) from the distillation process.
   • Further cooling and expansion (e.g., through a Joule-Thomson expander or expansion turbine) lead to the liquefaction of air. Liquid air is primarily a mixture of liquid nitrogen (boiling point -196 °C) and liquid oxygen (boiling point -183 °C).

3. Fractional Distillation:

   • The liquid air is fed into a fractional distillation column, typically a double column system (high-pressure and low-pressure columns).
   • High-Pressure Column: Liquid air enters this column. Nitrogen, with a lower boiling point (-196 °C), vaporizes more readily and rises, while oxygen, with a higher boiling point (-183 °C), tends to remain in the liquid phase and collects at the bottom. An argon-rich fraction can be drawn off in the middle.
   • Low-Pressure Column: The partially separated streams from the high-pressure column are further separated in the low-pressure column. Liquid oxygen (typically 99.5% pure) collects at the bottom, while gaseous nitrogen (99.99% pure) exits from the top. Gaseous oxygen can also be withdrawn from the top of the low-pressure column after further purification if required.

Purity and Applications:

• Oxygen produced by this method can achieve purity levels exceeding 99.5%.
• Used in steelmaking, chemical synthesis, medical applications, welding, and rocketry.

2. Electrolysis of Water

This method is suitable for smaller-scale production or where high purity is required, and electricity is abundant and cheap.

Principle:

Water (H₂O) is decomposed into hydrogen gas (H₂) and oxygen gas (O₂) by passing an electric current through it.

$2H_2O(l) \xrightarrow{\text{Electrical energy}} 2H_2(g) + O_2(g)$

Process Setup:

• An electrolytic cell typically consists of two inert electrodes (e.g., platinum, nickel, or stainless steel) immersed in an aqueous electrolyte.
• The electrolyte is often a solution of an acid (e.g., sulfuric acid) or a base (e.g., potassium hydroxide) to increase the electrical conductivity of water.

Electrode Reactions:

• At the Anode (Positive Electrode): Oxidation occurs, and oxygen gas is produced. $2H_2O(l) \longrightarrow O_2(g) + 4H^+(aq) + 4e^-$ (in acidic medium) or $4OH^-(aq) \longrightarrow O_2(g) + 2H_2O(l) + 4e^-$ (in basic medium)

• At the Cathode (Negative Electrode): Reduction occurs, and hydrogen gas is produced. $4H^+(aq) + 4e^- \longrightarrow 2H_2(g)$ (in acidic medium) or $4H_2O(l) + 4e^- \longrightarrow 2H_2(g) + 4OH^-(aq)$ (in basic medium)

Purity and Applications:

• Oxygen produced by electrolysis is typically very pure (up to 99.999%), making it suitable for specialized applications where high purity is critical.
• Used in medical and pharmaceutical industries, laboratories, and as a component in fuel cells.

Storage and Transportation

Industrial oxygen is stored and transported in various forms:

• Compressed Gas: In high-pressure steel cylinders (for gaseous oxygen, O₂) at pressures up to 200 bar.
• Liquid Oxygen (LOX): In insulated cryogenic tanks (dewars) for large-scale storage and transportation, as liquid oxygen occupies significantly less volume than gaseous oxygen.
• On-site Generation: For very large consumers, oxygen generation plants (cryogenic or PSA/VPSA) are often built directly at the user’s facility.