TRY : Organic). If subjected to a pressure of 61.6 atmos. at the critical temperature of 36.5° C, it suddenly becomes a col ourless transparent liquid. At lower temperatures it becomes liquid under less pressure, e.g., at 15° C under slightly less than 38 atmos., and at —82° C under one atmos. On a slight further reduction of temperature, the liquid contracts considerably, passing to a crystalline solid.
The characteristic and unpleas ant smell generally associated with acetylene is not due to this gas at all, but to traces of impurities in the calcium carbide from which it is made in all ordinary cases (see below).
The density of pure gaseous acetylene is 0.95 (air= 1), whereas that of marsh gas (methane) is 0•55, and of town gas (coal-gas, usually with some water gas) between 0.40 and 0.55. Its higher density signifies that acetylene requires pipes and nozzles of larger bore to pass the same volume of gas without • greater pressure, but this disadvantage is offset by the fact that a given heating or lighting effect is obtainable from a considerably smaller volume of acetylene than of either of the other gases named. One litre of dry acetylene at o° C and under 76omm. pressure weighs 1.1613 gram; one cu.ft. of acetylene, saturated with water vapour at 6o° F and 3oin. pressure, weighs 0•06751b. Its mean coefficient of expansion between o° and ioo° C is 0.003738 (air 0.003671).
The density of liquid acetylene is 0.613 at —8o° C, 0.455 at o° C and 0.397 at 3o° C; the coefficient of expansion is thus exceptionally high-0.00224 for each degree. The figures also show that liquid acetylene is one of the lightest, of liquids. It is of interest only from the scientific standpoint, because it behaves as an explosive when subjected to heat or when exposed to a spark, and its manufacture and sale are generally prohibited.
Water at i 5° C dissolves 1•15 times its volume of gaseous acetylene at atmospheric pressure; at o° C, 1.73 times, and at 3o° C, 0.84 times. Whereas this degree of solubility in water is considerably higher than that of other fuel gases, it is not such as to preclude the use of water as a confining liquid in acetylene generating plant and gasholders. In special cases in which it is desirable to reduce the diffusion of acetylene through the seals of such plant, fresh water is replaced by a saturated solution of calcium chloride, which at 15° C dissolves only 0.06 of its volume of acetylene, or by strong brine. A saturated solution of common salt at 15° C dissolves 0.21 of its volume of acetylene. These saline solutions freeze at considerably lower temperatures than fresh water, and consequently gasholders in which they are employed as the confining liquid may be used without protection from mild frosts. On the other hand, they shorten the life of the plant, their corrosive action on iron and other metals being greater than that of water. A water solution of glycerine is an alternative which is non-corrosive and does not affect metals.
Acetylene dissolves more freely in most organic liquids than in water. At ordinary temperature and pressure one volume of each of the following liquids dissolves the number of volumes of acetylene shown: Vols. Vols.
Carbon disulphide . . i•o Chloroform . . . . 4•o Water . . . . 1.15 Amyl alcohol . . . 3.5 Lime water patd. at 20°C 1.23 Absolute ethyl alcohol . 6•o Paraffin oil . . . 1•5 Acetic acid . . . . 6•o Benzene . . . . 4.0 Acetone . . . . 25•0 "Dissolved Acetylene."—The relatively large solubility in acetone was first observed by G. Claude and A. Hess in 1897, who stated in the Comptes Rendus of that year that the solubility was also very nearly proportional to the pressure, being 12 X 25=-300 vols. at a pressure of 12 atmos. They pointed out that the co efficient of expansion of the solution in acetone was very much lower than that of liquid acetylene, and moreover, the solution did not explode when a wire in it was raised to bright redness. Their observations led ultimately to the development of the present method of supplying acetylene in portable steel cylinders, in which it is safely stored under pressure as a solution in acetone. Acetone increases in volume by 4.4% for each atmosphere pres sure at which it is saturated with acetylene. Consequently acetone in which acetylene is dissolved under an absolute pressure of 16 atmos. occupies 1-1- (i6 X 0.044)=1.684 times its original volume. This pressure, viz., 15 atmos. above the normal, is that to which steel cylinders packed with porous substance and containing acetone, are permitted by law to be charged with acetylene in Great Britain. The cylinders must first have been tested and found to show no permanent stretch after being subjected for 15 minutes to four times the charging pressure. Similar regulations are en forced in other countries in regard to the supply of acetylene in steel cylinders. The cylinders are now usually seamless or solid drawn. In the U.S., acetylene cylinders are constructed with welded or biased heads or tops.
Acetylene dissolved under pressure in acetone in steel cylinders is known commercially as "dissolved acetylene." Its production has become a considerable industry in most countries, on account of the convenience to users of a portable supply of acetylene, with relief from the provision and supervision of generating and purifying plant. Since the volume of acetone varies with the amount of acetylene dissolved in it, it is obvious that the cylinders in use must be only partially filled with the liquid, the remaining space being occupied by compressed gaseous acetylene. Such com pressed free acetylene would be liable to explode if the cylinder were exposed to great heat or shock, but this risk is avoided by first filling the cylinder with inert porous material, the interstices in which will be sufficiently small to prevent any explosion which may happen to be initiated at some point therein extending beyond that point. The porous material in this respect acts similarly to the fine wire gauze of a miner's safety lamp, which prevents the flame within the lamp igniting explosive gas outside it. Various porous materials have been used for this purpose, e.g., dried clay, porous brick, infusorial earth, Kieselguhr and charcoal, singly or in admixture with or without a binder. In Great Britain the porous material preferred is the vegetable fibre, kapok, the resilient .properties of which cause it to be evenly distributed, and free from open spaces of such a size that, should an explosion occur, it would assume dangerous magnitude. Kapok has the dual ad vantage of providing spaces between its fibres, and of having natural capillaries within the latter.
Current practice in Great Britain is to pack the cylinder uni formly with kapok until the solid matter of the latter occupies one-fifth of the capacity of the cylinder, thus leaving four-fifths as interstices in the kapok packing. Then acetone is run in to fill a further two-fifths; leaving two-fifths of the internal volume of the cylinder for expansion of the acetone on charging with acetylene, and any subsequent exposure to heat. Cylinders of dissolved acetylene thus prepared are convenient and safe.
Uses.—Acetylene requires 2.5 times its volume of oxygen to effect its complete combustion ; consequently all the oxygen pres ent in 11.95 times its volume of air is used up when acetylene is burned in open jets. Such jets are employed as lights in portable lamps, buoys, road signals, isolated premises, etc., but the con sumption of acetylene for lighting is small compared with its use in the industrial welding and cutting of metals, for which purposes it is consumed in a blow-pipe to which a supply of un diluted oxygen is given from a steel cylinder containing that gas under pressure. Acetylene is an "endothermic" compound, which means that heat has been absorbed in its formation ; and this heat is given out again, in addition to the heat of combustion of its constituent carbon and hydrogen, when acetylene is burned. On this account, acetylene affords a higher flame temperature than other fuel gases. Its calorific value lies, according to different observers, between 11.8 and 12.2 cals. per grm. Taking Berthelot and Matignon's figure of 12.13, the calorific value of one litre of acetylene at o° C and 76omm., dry, is 54-086 calories, and that of one cu.ft. at 60° F, 3oin., and saturated, is 1,474 B.Th.U. (British standard conditions). For comparison with the latter figure, it should be mentioned that town gas in Great Britain averages about 500 B.Th.U., and natural gas in U.S.A. nearly 1,000 B.Th.U.
Disadvantages.—Acetylene in itself has very slight toxic properties, but certain of the impurities, such as phosphine, which occur in it before purification, are highly toxic. Never theless, even crude acetylene may be inhaled with less risk than ordinary town gas.
A mixture of acetylene with air will explode from a light or spark, according to F. Clowes, unless the acetylene forms less than 3% or more than 82% of the mixture. Other observers give a more limited range to the explosive mixture, but it is certainly very much wider in similar conditions than for coal gas or natural gas, and greater care should be exercised to avoid both escapes of acetylene and naked lights in the proximity of leaks. It must be emphasized, however, that acetylene at ordinary pressures cannot be made to explode in the absence of air, oxygen or chlorine, and in this respect it is similar to coal gas and other fuel gases. When, however, it is compressed to more than 2 atmos., a spark or red-hot wire will effect its dissociation, which will spread through the whole, and the heat liberated by reason of its endo thermic character will give rise to all the effects of an explosion. On this account in most countries acetylene may not be corn pressed to more than 2 atmos., unless it is within a porous material, as described above.
or 375.6 litres at 15° C, 76omm., and saturated per kg. of CaC2.
Commercial calcium carbide contains upwards of 20% of inert material, and is ordinarily guaranteed to give a standard yield of 4.80 cu.ft., at 6o° F, 3oin., and saturated, per lb., or the equivalent figure of 300 litres, at 15 '5° C, 76omm. and saturated, per kg. This standard represents fairly closely the average yield obtainable from commercial carbide in 1925-27, though occasionally the inert material in the carbide falls as low as io%, and on the other hand sometimes reaches as much as 3o%. Pure calcium carbide has a specific gravity of 2.22, and resembles fine-grained granite or hard limestone in appearance and hardness. The commercial product is broken and screened to lumps of convenient size for different uses, and is packed for transport in air-tight steel drums of so and lookg. capacity. Since it evolves acetylene slowly when exposed to damp air, and rapidly when wetted, calcium carbide is everywhere subject to special regulations for transport by sea and land, and for storage.
Calcium carbide is manufactured by heating together in the electric furnace a mixture of lime and anthracite containing about 4o% of the latter. Both materials must be as free as possible from impurities, and especially from compounds of phosphorus, sulphur or silicon, which render the resultant carbide liable to produce acetylene containing phosphine, hydrogen sulphide and silicon hydride. Carbide is sold generally under a guarantee that the gas it yields shall not contain more than 0.05% by volume of phosphine, or 0.15% of hydrogen sulphide. These impurities are inimical to the satisfactory use of the acetylene for either lighting or welding, and it is necessary to pass the gas before it is used through a vessel containing a purifying agent which will abstract them. Purifying agents which will abstract phosphine completely and economically, without acting upon the acetylene, have been sought in many directions, but only three classes of materials have found practical application. The principal active constituents of these materials are :—(r) chlorine liberated from a hypochlorite as used in "Acagin," "Puratylene" and "Klinger's Material"; (2) oxygen derived from an acid solution of chromic acid absorbed by infusorial earth, as in "Heratol," or from an ill-defined iron-silicic-acid compound, as in "Carburylene"; and (3) copper, as an acid solution of cuprous chloride. absorbed in infusorial earth, as in "Frankolin." All these materials, when properly prepared, may be regarded as effective in that 'lb. will abstract the impurities from 8o to 200 cu.ft. of acetylene pro duced from carbide of the normal degree of purity. It is obvious, however, that the purifying material will be exhausted less quickly the smaller are the proportions of phosphine and hydrogen sul phide in the crude gas, and consequently there is an initial ad vantage in calcium carbide which is relatively free from phos phorus and sulphur compounds.
Acetylene is obtained from calcium carbide by bringing the latter in contact with water, and innumerable forms of "gen erators" have been devised with the object of effecting and con trolling the reaction between the carbide and the water in such a manner that the acetylene shall be liberated at the desired rate. The reaction, represented by the formula is one in which heat is set free. Thus, 'lb. of pure calcium carbide decomposed with excess of water liberates about 4,380 B.Th.U. (or one kg. liberates 2,433kg.-calories). These figures become more significant if it is borne in mind that 4,38o B.Th.U. represents the heat imparted to 2.92gals. of water in raising it from 62° F to its boiling point. Except on the very small scale of a bicycle lamp, therefore, no acetylene generator can depend on atmospheric cooling for the dissipation of the liberated heat. Generators must, in practice, be so designed that the heat liberated therein shall be imparted to water, which shall be available in sufficient volume not to be raised to the boiling point. The water may be so disposed or circulated that it gives up its heat rapidly to the atmosphere.
In small generators, water commonly drips on to the carbide, at a rate controlled by a hand-operated valve and, or by the re striction of the free egress of the evolved gas. In large gen erators, however, the carbide either is dropped into a large excess of water, or is intermittently submerged by a local excess of water. Provision should be made for the sludge of slaked lime, which is the residue of the decomposition of carbide, to settle down out of close contact with carbide which has not already been decomposed. The sludge is, in most instances, withdrawn or run off at intervals in a semi-fluid condition. The rapid decomposi tion of a large charge of carbide may raise particles of metal inside the generator to a temperature above that at which acetylene ignites in air, and it is important that generators, liable to be re charged at short intervals, should be so constructed that air cannot enter the heated interior during discharging and recharging.
The harmfulness of overheating in generators, through inade quate cooling, lies in the fact that acetylene is a compound which is transformed, when heated to temperatures not much above that of boiling water, into polymerides or other hydrocarbon compounds, such as benzene, which have the same or nearly the same percentage composition, but are not gaseous at ordinary temperatures, or have not the same value as illuminating or heat ing gases. The fact that acetylene is prone to polymerization (q.v.), and to enter into combination, at comparatively low tem peratures, makes it a very valuable medium for the manufacture of a number of organic chemicals such as acetaldehyde (q.v.), acetic acid (q.v.) and acetone (q.v.).
Special forms of blow-pipes are used to produce the oxy acetylene flame for the welding and cutting of steel and other metals, for which purposes acetylene finds its principal outlet.