In large industrial air separation plants atmospheric air is filtered, compressed, and cooled. The contaminants (primarily moisture, carbon dioxide and heavy hydrocarbons) are removed and are either frozen out in a reversing heat exchanger or absorbed by a molecular sieve. The air is then separated in a four column system.

The columns are trayed distillation columns with vapor rising through holes in the trays as liquid travels across them, and then down to the next tray. As the vapor rises, it contains a higher percentage nitrogen, which is the lower boiling point constituent. As the liquid descends the column, it becomes richer in the higher boiling point constituents of air: oxygen and argon.

The first column produces nearly pure (99.9999%) nitrogen vapor, and oxygen rich (40%) liquid. The second column produces nearly pure oxygen liquid at the bottom, and nearly pure nitrogen vapor at the top. A side stream which is rich in argon is removed from the second column. The sidestream is removed at a point where the vapor rising through the trays is about 7 - 15% argon, with the balance being oxygen.

This stream is then distilled in a third column which produces a product stream which is about 98% argon and 2% oxygen. The remaining oxygen is removed in a catalytic oxidizer; then any trace amounts of nitrogen are removed in a fourth distillation column.

In plants which are designed to produce large amounts of cryogenic liquids, a freestanding nitrogen liquefier is usually used to liquefy the nitrogen vapor produced off of the distillation columns. This liquefier consists of a large compressor and a pair of matched expansion turbines which drive booster compressors. Nitrogen is circulated and compressed by the compressor, and then by the booster compressors. The heat of compression is removed in heat exchangers by cooling water. The high pressure nitrogen is then expanded through the turbines which remove energy from the high pressure stream by driving the booster compressors. This lowers the nitrogen to its liquefaction temperature.

The plant is designed to be operated by a single operator from the main control room which looks over the facility. A distributed control system collects process data throughout the plant and sends the information via coaxial cable to the control room. There, the information is displayed on an operator interface consisting of two CRTs and keyboards where the operator and a supervisory control monitor, control, and optimize the process.

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