1894:

Development of Air Liquefaction and Separation

After working for ten years as CEO of the Linde Company in Wiesbaden, Carl Linde returned to the Academy of Technology in Munich in 1889. There he carried out research on a new low-temperature technology and in 1894 developed the first air liquefaction unit. To liquefy air, it must first be cooled down to around -190 degrees Celsius. This succeeds if powerfully compressed air expands, and the expansion cooling occurring is transferred to compressed, pre-cooled air. The process is repeated and the cooling produced per circuit added up until liquid air is produced and then captured in a collecting vessel.

Air liquefaction originally took 15 hours. After several improvements Linde was able to speed up the process to one hour and ultimately to fifteen minutes. This way, small units could be used for public demonstrations. In 1900 Linde’s air liquefaction machine was awarded the Grand Prix at the World’s Fair in Paris, the most highly coveted prize at the exposition.

Plant for air liquefaction at the world exhibition in Paris, France (1900)

Air liquefaction became commercially profitable when Linde, with the aid of this process, succeeded in separating the air into its individual components. With this fractionation process, which makes use of the differing boiling points of nitrogen and oxygen, a great portion of the nitrogen could be separated from the air. This way, an oxygen-rich mixture was produced with a 50% oxygen component, so called “Linde air”. This Linde air was used, for example, in a mixture with petroleum as an explosive called “Oxiliquit” in the building of the Simplon Tunnel in Switzerland in 1907. Nevertheless, it did not find as many purchasers as had been expected. A sizably larger requirement existed for pure oxygen, which, among other uses, is applied in autogenic welding and cutting in the metalworking industry.

Linde finally succeeded in fabricating pure oxygen in 1902 through the rectification process. In this process, the liquid air trickles down a tube, where it meets rising oxygen vapor. In the ongoing process of liquefaction and evaporation, in other words permanent distillation, oxygen of virtually any desired purity can be produced. The air liquefaction and separation plants could ultimately be combined into a single unit. The first production unit of this kind went into operation in 1903 in Höllriegelskreuth near Munich and was used there several years for gas extraction.

In 1903, Linde, prompted by the recommendation of scientists Adolph Frank and Nikodem Caro, altered the rectification process so that pure nitrogen could be produced. This was needed for the manufacture of fertilizers. By 1910, the company had finally developed a “two-column apparatus”, which simultaneously supplied pure oxygen and pure nitrogen at low cost. With further modified separation processes, inert gases, primarily argon, could be extracted from the air. These inert gases were used, for example, as protective gas for filling light bulbs.

To the present day, Linde has sold over 2,800 air separation plants all over the world. While the output of the plants in 1902 was around five kilograms of oxygen per hour, a modern plant, for example, such as the “Pearl” air separation plant currently under construction in Qatar, extracts 1,250,000 kilograms of oxygen in an hour.

Modern plant for air liquefaction in Mossel Bay, South Africa

You’ll find a more comprehensive chronicle of air separation here.