What is the most common and least expensive method to produce oxygen?

The most common and least expensive method to produce oxygen is fractional distillation. This process involves the separation of different gases based on their boiling points. Air is composed primarily of nitrogen and oxygen, among other gases. By cooling air to extremely low temperatures, it condenses into a liquid form. When this liquid air is gradually warmed up, the various components evaporate at different temperatures. Oxygen, which has a higher boiling point than nitrogen, can be collected separately as it boils off.

Fractional distillation is widely used because it is efficient and cost-effective for producing large quantities of oxygen, which is vital in various applications such as medical use, industrial processes, and more. This method leverages the abundant supply of atmospheric air, making it readily available and economically viable.

In contrast, the other methods mentioned are either less common or more costly. Electrolysis, for instance, is generally more expensive as it requires electrical energy to split water into hydrogen and oxygen, making it less favorable for large-scale oxygen production. Photosynthesis produces oxygen naturally but is not a controlled or efficient method for industrial purposes. Molecular sieve technology, while effective, tends to be more expensive and less scalable compared to fractional distillation for generating large volumes of oxygen.