Oxygen is a naturally occurring gas comprising nearly 21% of the Earth’s atmosphere. When subjected to extreme cooling, this colorless gas transforms into a liquid state known as liquid oxygen (LOX), a cryogenic liquid. This intense cold is necessary to compress the vast volume of oxygen gas into a manageable liquid form for storage and transport.
The Specific Temperature of Liquid Oxygen
The defining characteristic of liquid oxygen is its intensely cold temperature, which is determined by its boiling point at standard atmospheric pressure. At this pressure, liquid oxygen boils at a temperature of -182.96°C. This value is commonly rounded to -183°C on the Celsius scale. This temperature translates to -297.33°F on the Fahrenheit scale. When measured on the absolute Kelvin scale, the boiling point is 90.19 K. This specific temperature is the point at which the liquid rapidly converts back into its gaseous state as it absorbs heat from the surrounding environment.
The temperature of liquid oxygen is relatively high compared to other common cryogens, such as liquid nitrogen, which boils at a much colder -196°C. This difference in boiling points is significant, as it means oxygen gas from the air can actually condense and liquefy in a vessel containing liquid nitrogen. Liquid oxygen must be maintained below its boiling point for it to remain in a liquid state, demanding specialized insulation for storage vessels to minimize heat transfer.
The Process of Creating Liquid Oxygen
Producing liquid oxygen on an industrial scale requires a complex physical process that begins with atmospheric air. The process relies on the principle that components of air have different boiling points, allowing for their separation when cooled to cryogenic temperatures. This production is primarily carried out in specialized facilities called Air Separation Units (ASUs).
The manufacturing process involves several steps:
- Air is compressed, which causes its temperature to increase significantly.
- The compressed air is cleaned to remove impurities like moisture, carbon dioxide, and hydrocarbons, which would otherwise freeze and clog the equipment at low temperatures.
- The purified, high-pressure air is cooled dramatically using heat exchangers.
- Further cooling is achieved by allowing the compressed gas to expand, a process known as the Joule-Thomson effect, until the air is cooled below its liquefaction point.
Once liquefied, the air is sent to a distillation column. Here, components, including nitrogen, argon, and oxygen, are separated through fractional distillation based on their distinct boiling points. The liquid oxygen, having the highest boiling point of the main components, is collected at the bottom of the column as the final product.
Handling and Unique Cryogenic Properties
The extreme cold of liquid oxygen dictates strict handling procedures and gives rise to its unique physical properties. Direct contact with the liquid or its cold vapor can cause instantaneous and severe frostbite, resulting in tissue damage similar to a severe burn. Even brief exposure to uninsulated equipment surfaces that are in contact with LOX can cause skin to stick and freeze instantly.
The intense cold also affects materials, causing many common substances to become brittle and lose their strength. Materials like carbon steel, plastics, and rubber can become fragile and prone to shattering or fracturing when exposed to these cryogenic temperatures. This necessitates the use of specialized alloys and non-brittle materials for storage and piping.
One of the most significant practical properties is its massive volume expansion upon warming. One liter of liquid oxygen, when vaporized and warmed to room temperature, expands to approximately 861 liters of gaseous oxygen. This dramatic expansion requires all containers and transfer lines to be equipped with pressure relief devices, as a closed system could quickly rupture from the massive pressure buildup.
Liquid oxygen also possesses two distinct visual and magnetic properties that set it apart from most other liquids. It has a clear, pale blue color, which is a result of the way oxygen molecules interact with light when in a liquid state. More unusually, liquid oxygen is strongly paramagnetic, meaning it is attracted to a magnetic field. This magnetic property is due to the presence of two unpaired electrons in the oxygen molecule, allowing the liquid to be temporarily suspended between the poles of a powerful magnet.