June, 1922
T H E JOURNAL OF INDUSTRlAL A N D ENGINEERING CHEMISTRY
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539
Purification of Gases from Methane' By G. R. Fondat and H. N. Van Aernem RESEARCH LABORATORY, GENERAL ELECTRIC Co., SCHENECTADY, NEW YORK
In the following paper is. described a method for the removal of methane from inert gases to be used in incandescent lamps. The method inuolves the decomposition of methane info carbon and hydrogen by passage through finely divided nickel at low temperature (about 500" C.), with subsequent removal of the hydrogen by passage through copper oxide at 500". The passage of a gas containing 1.5 per cent methane, for example, through nickel at 500" reduces the content to 0.24 per cenf,and a passage through a second series lowers i f to 0.OI per cenf. The advantage in the use of nickel rather than copper oxide alone is that a lower temperature is possible. Under the same conditions of femperature and flow an equal oolume of copper oxide gioes less than one-eighth the decomposition caused by nickel. While a mixture of nickel and copper oxide will give equally good results, it has the disadvantage that the charge,must be renewed upon exhaustion of the copper oxide. Oxidation of the exhausted mass leads to oxidation of the nickel at as low a temperature as 275' C. Nickel oxide in such a mixture is not catalytic and the mass is no more eficient than copper oxide alone. I f has been established by experiment that the constant for fhe reaction obtained by Mayer and Altmayer holds fairly closely for the reaction at the low partial pressure of 0.03 atmosphere.
ONTROL of the hydrocarbon content is one of the essential features in the preparation of gas for use in incandescent gas-filled lamps. Whether nitrogen or argon, it must be free from such substances as attack the tungsten filament or lead to discoloration of the lamp. Of the common impurities hydrogen may lead to destruction of the filament and blackening of the bulb through the formation of moisture from whatever oxides .are present, and carbon monoxide and dioxide cause a less pronounced discoloration, and attack the filament slightly. All of these three gases are, however, readily removable. Hydrocarbons are much more objectionable impurities. They decompose in contact with a glowing tungsten filament and the carbon liberated is absorbed by the tungsten as carbide, which in very small amounts renders the filament extremely brittle. To remove them from the gas is furthermore a difficult matter. I n the usual procedure, which consists in passage of the gas over copper oxide, such a high temperature is required that early deterioration of the enclosing tube is likely to take place, as well as a sintering of the copper oxide, which makes purification still more difficult. It was with the aim of facilitating the purification that some experiments were made with nickel as a catalyst for its decomposition, a reaction which it was thought might proceed readily at a lower temperature than is possible in its oxidation by copper oxide. Such a reaction was studied for methane a t atmospheric pressure by Mayer and A l t m a ~ e r . ~They developed a thermodynamic equation for expressing the extent of the decomposition a t various temperahres, based on determinations of the equilibrium constant in the equation
C
CH4=C f 2Hz.
This equilibrium bears on the present problem, for methane ia formed when other members of the hydrocarbon series are heated and it is the most resistant to chemical attack. Its decomposition by nickel seemed t o be particularly ap1
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Received February 9, 1922. Research Chemist, Research Laboratory, G. E. C. Ber., 40 (1907),2134.
propriate here because of the low partial pressure which methane would have as an impurity in a neutral gas. The reaction leads to an increase in volume, resulting in a shift in the equilibrium toward greater decomposition as the pressure of the methane is reduced. If the equation given by Mayer and Altmayer holds a t these low partial pressures, it promises a ready purification by passage of the gas through nickel at a low temperature. The hydrogen liberated could be removed by copper oxide, likewise a t low temperature.
EXPERIMENTAL With the aim, therefore, of investigating this equation, an apparatus similar to that of Mayer and Altmayer was made up and nitrogen containing about 2' per cent methane was passed through the nickel catalyzer and the equilibrium in the outcoming gases determined. Methane was made by the aluminium carbide and water reaction and diluted with nitrogen. On its way to the catalyst chamber it was further purified by passing over copper and copper oxide a t 280" C. to remove traces of hydrogen, carbon monoxide, and oxygen, and then through soda lime, phosphorous pentoxide, and sulfuric acid. The nickel catalyzer was obtained as a finely divided powder on asbestos feathers, soaked in nickel nitrate and heated in hydrogen. Three cc. were prepared in a quartz tube, and placed inside an alundum tube, heated electrically by two separate windings, one occupying the middle, and the other placed on the ends to compensate for cooling. The adjustment of a uniform temperature, as well as its maintenance, were secured by exploring with a small thermocouple. The whole was packed in alumina. This catalytic chamber was followed by a copper oxide furnace at 850" C., in which the hydrogen liberated by the dissociation of methane over the nickel was burned to water. In addition the undissociated methane, stable a t equilibrium, was completely oxidized to water and carbon dioxide. The composition of the original mixture was found by passing through phosphorus pentoxide the gas which came from the copper oxide furnace, and weighing the water, which corresponded to the original content of methane. This proved to be 2.23 per cent. The composition of the gas a t equilibrium was determined from the carbon dioxide formed in the copper oxide furnace by passing the gas through 0.2 N barium hydroxide solution. The consumption of 0.2 N barium hydroxide, as given in the table, yields then the partial pressure of methane stable at equilibrium. The partial pressure of hydrogen was calculated from the amount of methane decomposed by the catalyst. As one molecule of methane yields two molecules of hydrogen the latter is equal to twice the difference between the original methane content and that stable a t equilibrium. Because of the fact that carbon was deposited in the nickel and might contaminate the catalyst, only short runs were made. I n order to measure accurately the small amounts of gas involved the emerging gas was passed into a large buret, 4 em. in diameter and 1.5 m. long, calibrated for volume, and filled with water. A manometer was attached to aid in maintaining the pressure at atmospheric as the water flowed out. The thermodynamic equation of Mayer and Altmayer has the following form:
