Oxygen tanks provide a concentrated supply of gas for applications ranging from medical support and deep-sea diving to industrial welding. The oxygen inside these cylinders is separated from the ambient air that surrounds us, as atmospheric air contains approximately 21% oxygen. This oxygen must be isolated and purified to a high degree before being compressed into a portable, high-pressure container. The journey from the diffuse atmosphere to a dense, localized tank involves sophisticated engineering processes that ensure both purity and efficient storage.
Extracting Oxygen from the Air
The largest volumes of high-purity oxygen are produced using cryogenic distillation, which relies on the differences in the boiling points of air’s components. Air is first compressed, cleaned of impurities like water vapor and carbon dioxide, and then cooled to extremely low temperatures. This cooling causes the air to liquefy, typically reaching temperatures well below -183°C (-297°F). The liquid air is then warmed in a distillation column, allowing the components to separate as they boil off. Nitrogen vaporizes first, followed by argon, leaving behind oxygen with a purity that can exceed 99.5% at the bottom of the column.
For smaller-scale or localized production, the Pressure Swing Adsorption (PSA) method offers a non-cryogenic alternative. This technique uses specialized materials called zeolite molecular sieves that preferentially adsorb nitrogen molecules when air is passed through them under high pressure. Oxygen passes through the sieves relatively unrestricted, resulting in a product gas that is around 95% pure oxygen. The system then rapidly drops the pressure, which releases the trapped nitrogen and regenerates the sieve material for the next cycle.
Storing Oxygen: Liquid vs. Compressed Gas
Once purified, oxygen must be prepared for efficient storage, requiring a significant reduction in volume through either extreme cooling or compression. Liquefaction is the preferred method for bulk storage and transport because it dramatically reduces the space required. Cooling oxygen to its liquid state at approximately -183°C results in a liquid that is 861 times denser than its gaseous form.
Liquid oxygen (LOX) is stored in large, vacuum-insulated vessels known as Vacuum Insulated Evaporators (VIEs) or Dewar tanks, which function like giant thermos bottles. The double-walled construction and vacuum layer minimize heat transfer from the environment, helping to maintain the necessary cryogenic temperatures. LOX is typically stored in these bulk tanks at a moderate pressure and later vaporized back into a gas before use or cylinder filling.
For portable cylinders and smaller applications, the oxygen remains a gas but is stored under tremendous pressure at ambient temperature. These high-pressure cylinders are designed to hold the gas at pressures that commonly range from 2,000 to 3,000 pounds per square inch (psi). While less dense than liquid storage, this method offers the portability and simplicity required for individual tanks used in hospitals, homes, or for diving.
The Physical Tank Filling Procedure
The filling of the final cylinder requires precise control and specialized equipment to ensure safety and efficiency. For high-pressure gas cylinders, technicians connect multiple tanks in a series to a filling station called a manifold. An oxygen booster compressor then gradually increases the pressure, forcing the purified gaseous oxygen into the cylinders until they reach their rated capacity.
This filling process must be slow and carefully monitored to manage the heat generated by compression, a phenomenon known as adiabatic heating. Rapid pressurization can cause the cylinder’s temperature to rise dangerously, potentially leading to ignition of foreign materials. Consequently, the filling rate is controlled, and the temperature is checked periodically to prevent overheating.
For liquid oxygen storage, smaller vacuum-insulated containers are filled directly from the bulk VIE tank using specialized cryogenic pumps. The liquid transfer process is controlled and often involves pre-cooling the receiving tank to minimize vaporization and maintain the ultra-cold temperature of the LOX. Once transferred, these portable liquid containers are equipped with an internal vaporizer that automatically converts the liquid back into a usable gas as it is drawn off by the user.
Final Quality Checks and Safety Standards
Before distribution, every oxygen tank must pass stringent quality and safety checks, especially for medical-grade gas. Purity testing is conducted using instruments like gas chromatographs to ensure the oxygen meets regulatory standards, such as those set by the United States Pharmacopeia (USP). This verification confirms the gas is free of harmful contaminants and has the required oxygen concentration, often 99% or higher for medical use.
Beyond gas quality, the structural integrity of the steel or aluminum cylinder itself is periodically verified through hydrostatic testing. This involves filling the tank with water and pressurizing it to 1.5 times its maximum working pressure to confirm it can safely withstand operational stress. Proper labeling is then applied to the filled tank, clearly indicating the contents, pressure, and the specific grade of oxygen, ensuring safe handling and correct usage.