Most oxygen used in the welding industry is extracted from the atmosphere by liquefaction techniques. Nitrogen can also be separated by liquefaction. In the extraction process, air may be compressed to approximately 20 MPa (3000 psig), although some
types of equipment operate at much lower pressure. The carbon dioxide and any impurities in the air are removed; the air passes through coils, and is allowed to expand to a rather low pressure. The air becomes substantially cooled during the expansion, and then it is passed back over coils, further cooling the incoming air, until liquefaction occurs. The liquid air is sprayed on a series of evaporating trays or plates in a rectifying
tower.
Nitrogen and other gases boil at lower temperatures than the oxygen and, as these gases escape from the top of the tower, high-purity liquid oxygen collects in a receiving chamber at the base. Some plants are designed to produce bulk liquid oxygen; in other
plants, gaseous oxygen is withdrawn for compression into cylinders.
Historical Background
While oxygen can be produced chemically, as in the Brinn and the Jaubert processes, the most efficient and economical means is the liquification process. In the
liquid air process, air is liquefied by means of very low temperatures and compression.
The basic idea for the separation of the elements of air by liquefaction was first suggested by Parkinson in 1892, and depends on the difference in the boiling points of the major elements constituting air, approximately -183°C (-297°F) for oxygen, and -196°C
( -320.4oF) for nitrogen. Various modifications of this idea have been developed; the principal processes were developed by Linde, Claude, Messer, Heylandt, Pictet, and Hilldebrandt. These very low temperatures are reached by external refrigeration, and
the Joule-Thompsoneffect: the fall in temperature produced when a compressed gas is allowed to expand freely through a nozzle, which results in self-cooling of the gas caused by absorption of energy (heat) during the expansion. When the oxygen and nitrogen are
allowed to boil off from the liquid air, the resulting gases are very cold. This phenomenon is used in all commercial processes to cool more incoming air for
liquefaction. This refrigeration process takes place in a heat interchanger, the principle of which was suggested by Siemens in 1857. Without the saving of power made possible by the heat interchanger, none of the liquefaction processes would be commercially practical.
In 1903 Georges Claude showed that a further cooling could be effected by allowing the compressed gas to expand and at the same time do external recoverable
work through the intermediary of an expansion technique. Applying this principle, the need for outside refrigeration was eliminated, and the initial compression requirements were decreased.
In all rectification processes, the separation is accomplished in a rectification column by means of the interaction between a descending stream of liquid and an ascending stream of vapor in direct contact with one another. As it descends, the liquid partially absorbs the constituent having the higher boiling point, and as it ascends, the vapor partially absorbs the constituent having the lower boiling point. When the mixture treated in a rectification column is liquid air, the descending liquid stream ultimately becomes almost pure oxygen, while the percentage of nitrogen in the vapor stream increases as it ascends.
In addition to its value in the production of oxygen, liquid air has a number of interesting commercial uses, such as solidifying mercury vapor in high-vacuum work; the formation of a powerful explosive by soaking charcoal cartridges in it; producing low temperatures for testing materials that are to be used at temperatures far below the freezing temperature; pulverization of various compounds for chemical analysis; purifying chemicals, and for refrigeration-ventilation.
Oxygen and Hydrogen Production by Electrolysis
In the year 1800, Nicholson and Carlisle showed that on conducting an electric current through water by immersing the two terminals of a voltaic pile into it, hydrogen was produced at one of the terminals and oxygen was produced at the other. In the commercial electrolysis of water, the water is made a conductor by the addition of alkalies or acids. The alkalies are almost entirely used commercially because they are
cost effective, and because of the resistance of a greater class of materials to their chemical action.
When an electric current is passed through an alkaline solution, the water is decomposed by a primary and secondary reaction, so that the hydrogen is liberated at the negative pole, or cathode, and the oxygen is liberated at the positive pole, or anode. The equipment and functions necessary for the decomposition of water follow:
(1)A container to hold the alkaline or acid solution or water, called the electrolyte; an anode, which is submerged in the solution and to which the current from
an outside source is led
(2) A cathode, submerged in the solution to receive the current and lead it back to its source
(3) A dividing wall to separate the gases and a means for collecting them separately, and conducting them to some desired point DC Current. The necessary current must be a direct current so that the evolution of gas will always be at the same point. It is not practical to use alternating current.
The introduction, development and use of hydrogen and oxygen for cutting steel and welding aluminum, and the large demand for hydrogen for other industrial purposes contributed further to the development of the electrolysis method of producing oxygen and hydrogen. The distinctive feature of this method is the simultaneous production of two volumes of hydrogen for every one volume of oxygen.
Modem production of hydrogen involves the steam re-forming of natural gas over a nickel catalyst. See HYDROGEN.