Although not exploded by the application of high heat, acetylene is injured by such treatment. It is partly converted, by high heat, into other compounds, thus lessening the actual quantity of the gas, wasting it and polluting the rest by the introduction of substances which do not belong there. These compounds remain in part with the gas, causing it to burn with a persistent smoky flame and with the deposit of objectionable tarry substances. Where the gas is generated without undue rise of temperature these difficulties are avoided.
Purification of Acetylene.--Impurities in this gas are caused by impurities in the calcium carbide from which it is made or by improper methods and lack of care in generation. Impurities from the material will be considered first.
Impurities in the carbide may be further divided into two classes: those which exert no action on water and those which act with the water to throw off other gaseous products which remain in the acetylene. Those impurities which exert no action on the water consist of coke that has not been changed in the furnace and sand and some other substances which are harmless except that they increase the ash left after the acetylene has been generated.
An analysis of the gas coming from a typical generator is as follows:
Per cent
Acetylene ................................ 99.36
Oxygen ................................... .08
Nitrogen ................................. .11
Hydrogen ................................. .06
Sulphuretted Hydrogen .................... .17
Phosphoretted Hydrogen ................... .04
Ammonia .................................. .10
Silicon Hydride .......................... .03
Carbon Monoxide .......................... .01
Methane .................................. .04
The oxygen, nitrogen, hydrogen, methane and carbon monoxide are either harmless or are present in such small quantities as to be neglected. The phosphoretted hydrogen and silicon hydride are self-inflammable gases when exposed to the air, but their quantity is so very small that this possibility may be dismissed. The ammonia and sulphuretted hydrogen are almost entirely dissolved by the water used in the gas generator. The surest way to avoid impure gas is to use high-grade calcium carbide in the generator and the carbide of American manufacture is now so pure that it never causes trouble.
The first and most important purification to which the gas is subjected is its passage through the body of water in the generator as it bubbles to the top. It is then filtered through felt to remove the solid particles of lime dust and other impurities which float in the gas.
Further purification to remove the remaining ammonia, sulphuretted hydrogen and phosphorus containing compounds is accomplished by chemical means. If this is considered necessary it can be easily accomplished by readily available purifying apparatus which can be attached to any generator or inserted between the generator and torch outlets. The following mixtures have been used.
"Heratol," a solution of chromic acid or sulphuric acid absorbed in porous earth.
"Acagine," a mixture of bleaching powder with fifteen per cent of lead chromate.