In making carburetted acetylene, the pressure given by the ordinary acetylene generator will be sufficient to drive the gas through the carburettor, and therefore there will be no expense involved beyond the cost of the spirit vaporised. Thus comparisons may fairly be made between ordinary and carburetted acetylene on the basis of material only, the expense of generating the original acetylene being also ignored. In Great Britain the prices of calcium carbide, petroleum spirit, and 90s benzol delivered in bulk in country places may be taken at 15£ per ton, and 1s. per gallon respectively, petroleum spirit having a specific gravity of 0.700 and benzol of 0.88. On this basis, a unit volume (100 cubic metres) of plain acetylene costs 1135d., of "petrolised" acetylene containing 66 per cent. of acetylene costs 1277d., and of "benzolised" acetylene costs 1180d. In other words, 100 volumes of plain acetylene, 90 volumes of petrolised acetylene, and 96 volumes of benzolised acetylene are of equal pecuniary value. Employing the data given in previous tables, it appears that 38.5 candles can be won from plain acetylene in a self-luminous burner, and 103 candles therefrom in an incandescent burner at the same price as 25.5-29.1 and 78-87 candles can be obtained from carburetted acetylene; whence it follows that at English prices petrolised acetylene is more expensive as an illuminant in either system of combustion than the simple gas, while benzolised acetylene, burnt under the mantle only, is more nearly equal to the simple gas from a pecuniary aspect. But considering the calorific value, it appears that for a given sum of money only 363 calories can be obtained from plain acetylene, while petrolised acetylene yields 516, and benzolised acetylene 658; so that for all heating or cooking purposes (and also for driving small motors) carburetted acetylene exhibits a notable economy. Inasmuch as the partial saturation of acetylene with any combustible vapour is an operation of extreme simplicity, requiring no power or supervision beyond the occasional recharging of the carburettor, it is manifest that the original main coming from the generator supplying any large establishment where much warming, cooking (or motor driving) might conveniently be done with the gas could be divided within the plant-house, one branch supplying all, or nearly all, the lighting burners with plain acetylene, and the other branch communicating with a carburettor, so that all, or nearly all, the warming and cooking stoves (and the motor) should be supplied with the more economical carburetted acetylene. Since any water pump or similar apparatus would be in an outhouse or basement, and the most important heating stove (the cooker) be in the kitchen, such an arrangement would be neither complicated nor involve a costly duplication of pipes.
It follows from the fact that even a trifling proportion of vapour reduces the upper limit of explosibility of mixtures of acetylene with air, that the gas may be so lightly carburetted as not appreciably to suffer in illuminating power when consumed in self-luminous jets, and yet to burn satisfactorily in incandescent burners, even if it has been generated in an apparatus which introduces some air every time the operation of recharging is performed. To carry out this idea, Caro has suggested that 5 kilos. of petroleum spirit should be added to the generator water for every 50 cubic metres of gas evolved, i.e., 1 lb. per 160 cubic feet, or, say, 1 gallon per 1000 cubic feet, or per 200 lb. of carbide decomposed. Caro proposed this addition in the case of central installations supplying a district where the majority of the consumers burnt the gas in self-luminous jets, but where a few preferred the incandescent system; but it is clearly equally suitable for employment in all private plants of sufficient magnitude.
A lowering of the upper limit of explosibility is also produced by the presence of the acetone which remains in acetylene when obtained from a cylinder holding the compressed gas (cf. Chapter XI.). According to Wolff and Caro such gas usually carries with it from 30 to 60 grammes of acetone vapour per cubic metre, i.e., 1.27 grammes per cubic foot on an average; and this amount reduces the upper limit of explosibility by about 16 per cent., so that to this extent the gas behaves more smoothly in an incandescent burner of imperfect design.
Lépinay has described some experiments on the comparative technical value of ordinary acetylene, carburetted acetylene, denatured alcohol and petroleum spirit as fuels for small explosion engines. One particular motor of 3 (French) h.p. consumed 1150 grammes of petroleum spirit per hour at full load; but when it was supplied with carburetted acetylene its consumption fell to 150 litres of acetylene and 700 grammes of spirit (specific gravity 0.680). A 1-1/4 h.p. engine running light required 48 grammes of 90 per cent. alcohol per horse-power-hour and 66 litres of acetylene; at full load it took 220 grammes of alcohol and 110 litres of acetylene. A 6 h.p. engine at full load required 62 litres of acetylene carburetted with 197 grammes of petroleum spirit per horse-power-hour (uncorrected); while a similar motor fed with low-grade Taylor fuel-gas took 1260 litres per horse-power-hour, but on an average developed the same amount of power from 73 litres when 10 per cent. of acetylene was added to the gas. Lépinay found that with pure acetylene ignition of the charge was apt to be premature; and that while the consumption of carburetted acetylene in small motors still materially exceeded the theoretical, further economics could be attained, which, coupled with the smooth and regular running of an engine fed with the carburetted gas, made carburetted acetylene distinctly the better power-gas of the two.
[CHAPTER XI]
COMPRESSED AND DISSOLVED ACETYLENE--MIXTURES WITH OTHER GASES
In all that was said in Chapters II., III., IV., and V. respecting the generation and employment of acetylene, it was assumed that the gas would be produced by the interaction of calcium carbide and water, either by the consumer himself, or in some central station delivering the acetylene throughout a neighbourhood in mains. But there are other methods of using the gas, which have now to be considered.
COMPRESSED ACETYLENE.--In the first place, like all other gases, acetylene is capable of compression, or even of conversion into the liquid state; for as a gas, the volume occupied by any given weight of it is not fixed, but varies inversely with the pressure under which it is stored. A steel cylinder, for instance, which is of such size as to hold a cubic foot of water, also holds a cubic foot of acetylene at atmospheric pressure, but holds 2 cubic feet if the gas is pumped into it to a pressure of 2 atmospheres, or 30 lb. per square inch; while by increasing the pressure to 21.53 atmospheres at 0° C. (Ansdell, Willson and Suckert) the gas is liquefied, and the vessel may then contain 1 cubic foot of liquid acetylene, which is equal to some 400 cubic feet of gaseous acetylene at normal pressure. It is clear that for many purposes acetylene so compressed or liquefied would be convenient, for if the cylinders could be procured ready charged, all troubles incidental to generation would be avoided. The method, however, is not practically permissible; because, as pointed out in Chapters II. and VI., acetylene does not safely bear compression to a point exceeding 2 atmospheres; and the liability to spontaneous dissociation or explosion in presence of spark or severe blow, which is characteristic of compressed gaseous acetylene, is greatly enhanced if compression has been pushed to the point of liquefaction.
However, two methods of retaining the portability and convenience of compressed acetylene with complete safety have been discovered. In one, due to the researches of Claude and Hess, the gas is pumped under pressure into acetone, a combustible organic liquid of high solvent power, which boils at 56° C. As the solvent capacity of most liquids for most gases rises with the pressure, a bottle partly filled with acetone may be charged with acetylene at considerable effective pressure until the vessel contains much more than its normal quantity of gas; and when the valve is opened the surplus escapes, ready for employment, leaving the acetone practically unaltered in composition or quantity, and fit to receive a fresh charge of gas. In comparison with liquefied acetylene, its solution in acetone under pressure is much safer; but since the acetone expands during absorption of gas, the bottle cannot be entirely filled with liquid, and therefore either at first, or during consumption (or both), above the level of the relatively safe solution, the cylinder contains a certain quantity of gaseous acetylene, which is compressed above its limit of safety. The other method consists in pumping acetylene under pressure into a cylinder apparently quite full of some highly porous solid matter, like charcoal, kieselguhr, unglazed brick, &c. This has the practical result that the gas is held under a high state of compression, or possibly as a liquid, in the minute crevices of the material, which are almost of insensible magnitude; or it may be regarded as stored in vessels whose diameter is less than that in which an explosive wave can be propagated (cf. Chapter VI.).
DISSOLVED ACETYLENE.--According to Fouché, the simple solution of acetylene in acetone has the same coefficient of expansion by heat as that of pure acetone, viz., 0.0015; the corresponding coefficient of liquefied acetylene is 0.007 (Fouché), or 0.00489 (Ansdell) i.e., three or five times as much. The specific gravity of liquid acetylene is 0.420 at 16.4° C. (Ansdell), or 0.528 at 20.6° C. (Willson and Suckert); while the density of acetylene dissolved in acetone is 0.71 at 15° C. (Claude). The tension of liquefied acetylene is 21.53 atmospheres at 0° C., and 39.76 atmospheres at 20.15° C. (Ansdell); 21.53 at 0° C., and 39.76 at 19.5° C. (Willson and Suckert); or 26.5 at 0° C., and 42.8 at 20.0° C. (Villard). Averaging those results, it may be said that the tension rises from 23.2 atmospheres at 0° C. to 40.77 at 20° C., which is an increment of 1/26 or 0.88 atmosphere, per 1° Centigrade; while, of course, liquefied acetylene cannot be kept at all at a temperature of 0° unless the pressure is 21 atmospheres or upwards. The solution of acetylene in acetone can be stored at any pressure above or below that of the atmosphere, and the extent to which the pressure will rise as the temperature increases depends on the original pressure. Berthelot and Vieille have shown that when (a) 301 grammes of acetone are charged with 69 grammes of acetylene, a pressure of 6.74 atmospheres at 14.0° C. rises to 10.55 atmospheres at 35.7° C.; (b) 315 grammes of acetone are charged with 118 grammes of acetylene, a pressure of 12.25 atmospheres at 14.0° C. rises to 19.46 at 36.0° C.; (c) 315 grammes of acetone are charged with 203 grammes of acetylene, a pressure of 19.98 atmospheres at 13.0° C. rises to 30.49 at 36.0° C. Therefore in (a) the increase in pressure is 0.18 atmosphere, in (b) O.33 atmosphere, and in (c) 0.46 atmosphere per 1° Centigrade within the temperature limits quoted. Taking case (b) as the normal, it follows that the increment in pressure per 1° C. is 1/37 (usually quoted as 1/30); so that, measured as a proportion of the existing pressure, the pressure in a closed vessel containing a solution of acetylene in acetone increases nearly as much (though distinctly less) for a given rise in temperature as does the pressure in a similar vessel filled with liquefied acetylene, but the absolute increase is roughly only one-third with the solution as with the liquid, because the initial pressure under which the solution is stored is only one-half, or less, that at which the liquefied gas must exist.