Fractional distillation of air.
Many gases are obtained in commercial quantities through a process known as fractional distillation of liquefied air. Air is filtered to remove dust and other solids, water vapour and carbon dioxide. Water vapour and carbon dioxide are removed because they solidify at low temperatures and this would block the flow of liquid air through the pumps and pipes. As the dry air free of carbon dioxide is compressed to 200 times atmospheric pressure it becomes warm. Heat is removed by a network of pipes carrying liquid nitrogen. The cold, compressed air is then allowed to expand rapidly thus cooling even further to the point where most of the air is liquefied.
The gas mixture is then purified by fractional distillation. Dense oxygen is captured as liquid as it sinks while nitrogen gas escapes from the top and is collected.