_Self-luminous._ _Incandescent_ 1 litre = 1.00 candle. 1 litre = 3.04 candles.
1 cubic foot = 28.4 candles. 1 cubic foot = 86.2 candles.
1 candle = 1.00 litre. 1 candle = 0.33 litre.
1 candle = 0.035 cubic foot. 1 candle = 0.012 cubic foot.
Those results may be compared with those referring to air-gas, which emits in incandescent burners from 3.0 to 12.4 candles per cubic foot according to the amount of spirit added to the air and the temperature to which the gas is exposed.
The calorific values of carburetted acetylene (Caro), and those of other gaseous fuels are:
Large Calories per _ Cubic Foot.
(Lewes) . 320 (Gand) . 403 Ordinary acetylene . . (Heil) . 365 ___ _Mean . . 363
Maximum . 680 Carburetted acetylene . . Minimum . 467 (petroleum spirit) ___ _Mean . . 573
Carburetted acetylene (50 per cent. benzol by volume) 685 Carburetted acetylene (50 per cent. alcohol by volume) 364 Coal-gas (common, unenriched) . . . . . 150 _ Maximum . 178 Air-gas, self-luminous flame Minimum . 57 ___ _Mean . . . 114 _ Maximum . 26 Air-gas, non-luminous flame Minimum . 18 ___ _Mean . . . 22
Water-gas (Strache) from coke . . . . . 71 Mond gas (from bituminous coal) . . . . . 38 Semi-water-gas from coke or anthracite . . . 36 Generator (producer) gas . . . . . . 29
Besides its relatively low upper explosive limit, carburetted acetylene exhibits a higher temperature of ignition than ordinary acetylene, which makes it appreciably safer in presence of a naked light. It also possesses a somewhat lower flame temperature and a slower speed of propagation of the explosive wave when mixed with air. These data are:
______________________________________________________________________ Explosive Temperature. Limits. Degrees C. Explosive 19 mm. Tube. Explosive _____________ __________________ Wave. Metres per Of Igni- Second. Lower. Upper. tion. Of Flame. ________________________ ______ ______ ________ _________ ____________ Acetylene (theoretical) --- --- --- 1850-2420 --- " (observed) 3.35 52.3 480 1630-2020 0.18-100 Carburetted from 2.5 10.2 582 1620 3.2 acetylene / . . to 5.4 30.0 720 1730 5.3 Carburetted acetylene 3.4 22.0 --- 1820 1.3 (benzol) . . . / Carburetted acetylene 3.1 12.0 --- 1610 1.1 (alcohol) . . . / Air-gas, self-luminous 15.0 50.0 --- 1510-1520 --- flame . . . . / Coal-gas . . . 7.9 19.1 600 --- --- ________________________ ______ ______ ________ _________ ____________
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.
Lepinay 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. Lepinay 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 Fouche, 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 (Fouche), 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.
Supposing, now, that acetylene contained in a closed vessel, either as compressed gas, as a solution in acetone, or as a liquid, were brought to explosion by spark or shock, the effects capable of production have to be considered. Berthelot and Vieille have shown that if gaseous acetylene is stored at a pressure of 11.23 kilogrammes per square centimetre, [Footnote: 1 kilo. per sq. cm. is almost identical with 1 atmosphere, or 15 lb. per sq. inch.] the pressure after explosion reaches 92.33 atmospheres on an average, which is an increase of 8.37 times the original figure; if the gas is stored at 21.13 atmospheres, the mean pressure after explosion is 213.15 atmospheres, or 10.13 times the original amount. If liquid acetylene is tested similarly, the original pressure, which must clearly be more than 21.53 atmospheres (Ansdell) at 0 C., may rise to 5564 kilos, per square centimetre, as Berthelot and Vieille observed when a steel bomb having a capacity of 49 c.c. was charged with 18 grammes of liquefied acetylene. In the case of the solution in acetone, the magnitudes of the pressures set up are of two entirely different orders according as the original pressure 20 atmospheres or somewhat less; but apart from this, they vary considerably with the extent to which the vessel is filled with the liquid, and they also depend on whether the explosion is produced in the solution or in the gas space above. Taking the lower original pressure first, viz., 10 atmospheres, when a vessel was filled with solution to 33 per cent. of its capacity, the pressure after explosion reached about 95 atmospheres if the spark was applied to the gas space; but attained 117.4 atmospheres when the spark was applied to the acetone. When the vessel was filled 56 per cent. full, the pressures after explosion reached about 89, or 155 atmospheres, according as the gas or the liquid was treated with the spark. But when the original pressure was 20 atmospheres, and the vessel was filled to 35 per cent. of its actual capacity with solution, the final pressures ranged from 303 to 568 atmospheres when the gas was fired, and from 2000 to 5100 when the spark was applied to the acetone.
Examining these figures carefully, it will be seen that the phenomena accompanying the explosion of a solution of acetylene in acetone resemble those of the explosion of compressed gaseous acetylene when the original pressure under which the solution is stored is about 10 atmospheres; but resemble those of the explosion of liquefied acetylene when the original pressure of the solution reaches 20 atmospheres, this being due to the fact that at an original pressure of 10 atmospheres the acetone itself does not explode, but, being exothermic, rather tends to decrease the severity of the explosion; whereas at an original pressure of 20 atmospheres the acetone does explode (or burn), and adds its heat of combustion to the heat evolved by the acetylene. Thus at 10 atmospheres the presence of the acetone is a source of safety; but at 20 atmospheres it becomes an extra danger.
Since sound steel cylinders may easily be constructed to boar a pressure of 250 atmospheres, but would be burst by a pressure considerably less than 5000 atmospheres, it appears that liquefied acetylene and its solution in acetone at a pressure of 20 atmospheres are quite unsafe; and it might also seem that both the solution at a pressure of 10 atmospheres and the simple gas compressed to the same limit should be safe. But there is an important difference here, in degree if not in kind, because, given a cylinder of known capacity containing (1) gaseous acetylene compressed to 10 atmospheres, or (2) containing the solution at the same pressure, if an explosion were to occur, in case (1) the whole contents would participate in the decomposition, whereas in case (2), as mentioned already, only the small quantity of gaseous acetylene above the solution would be dissociated.
It is manifest that of the three varieties of compressed acetylene now under consideration, the solution in acetone is the only one fit for general employment; but it exhibits the grave defects (_a_) that the pressure under which it is prepared must be so small that the pressure in the cylinders can never approach 20 atmospheres in the hottest weather or in the hottest situation to which they may be exposed, (_b_) that the gas does not escape smoothly enough to be convenient from large vessels unless those vessels are agitated, and (_c_) that the cylinders must always be used in a certain position with the valve at the top, lest part of the liquid should run out into the pipes. For these reasons the simple solution of acetylene in acetone has not become of industrial importance; but the processes of absorbing either the gas, or better still its solution in acetone, in porous matter have already achieved considerable success. Both methods have proved perfectly safe and trustworthy; but the combination of the acetone process with the porous matter makes the cylinders smaller per unit volume of acetylene they contain. Several varieties of solid matter appear to work satisfactorily, the only essential feature in their composition being that they shall possess a proper amount of porosity and be perfectly free from action upon the acetylene or the acetone (if present). Lime does attack acetone in time, and therefore it is not a suitable ingredient of the solid substance whenever acetylene is to be compressed in conjunction with the solvent; so that at present either a light brick earth which has a specific gravity of 0.5 is employed, or a mixture of charcoal with certain inorganic salts which has a density of 0.3, and can be introduced through a small aperture into the cylinder in a semi-fluid condition.
Both materials possess a porosity of 80 per cent., that is to say, when a cylinder is apparently filled quite full, only 20 per cent, of the space is really occupied by the solid body, the remaining 80 per cent, being available for holding the liquid or the compressed gas. If all comparisons as to degree of explosibility and effects of explosion are omitted, an analogy may be drawn between liquefied acetylene or its compressed solution in acetone and nitroglycerin, while the gas or solution of the gas absorbed in porous matter resembles dynamite.
Nitroglycerin is almost too treacherous a material to handle, but as an explosive (which in reason absorbed or dissolved acetylene is not) dynamite is safe, and even requires special arrangements to explode it.
In Paris, where the acetone process first found employment on a large scale, the company supplying portable cylinders to consumers uses large storage vessels filled, as above mentioned, apparently full of porous solid matter, and also charged to about 43 per cent, of their capacity with acetone, thus leaving about 37 per cent. of the apace for the expansion which occurs as the liquid takes up the gas. Acetylene is generated, purified, and thoroughly dried according to the usual methods; and it is then run through a double-action pump which compresses it first to a pressure of 3.5 kilos., next to a pressure of 3.5 x 3.5 = 12 kilos, per square centimetre, and finally drives it into the storage vessels.
Compression is effected in two stages, because the process is accompanied by an evolution of much heat, which might cause the gas to explode during the operation; but since the pump is fitted with two cylinders, the acetylene can be cooled after the first compression. The storage vessels then contain 100 times their apparent volume of acetylene; for as the solubility of acetylene in acetone at ordinary temperature and pressure is about 25 volumes of gas in 1 of liquid, a vessel holding 100 volumes when empty takes up 25 x 43 = 1000 volumes of acetylene roughly at atmospheric pressure; which, as the pressure is approximately 10 atmospheres, becomes 1000 x 10 = 10,000 volumes per 100 normal capacity, or 100 times the capacity of the vessel in terms of water. From these large vessels, portable cylinders of various useful dimensions, similarly loaded with porous matter and acetone, are charged simply by placing them in mutual contact, thus allowing the pressure and the surplus gas to enter the small one; a process which has the advantage of renewing the small quantity of acetone vaporised from the consumers' cylinders as the acetylene is burnt (for acetone is somewhat volatile, cf. Chapter X.), so that only the storage vessels ever need to have fresh solvent introduced.
Where it is procurable, the use of acetylene compressed in this fashion is simplicity itself; for the cylinders have only to be connected with the house service-pipes through a reducing valve of ordinary construction, set to give the pressure which the burners require. When exhausted, the bottle is simply replaced by another. Manifestly, however, the cost of compression, the interest on the value of the cylinders, and the carriage, &c., make the compressed gas more expensive per unit of volume (or light) than acetylene locally generated from carbide and water; and indeed the value of the process does not lie so much in the direction of domestic illumination as in that of the lighting, and possibly driving, of vehicles and motor-cars--more especially in the illumination of such vehicles as travel constantly, or for business purposes, over rough road surfaces and perform mostly out-and-home journeys. Nevertheless, absorbed acetylene may claim close attention for one department of household illumination, viz., the portable table-lamp; for the base of such an apparatus might easily be constructed to imitate the acetone cylinder, and it could be charged by simple connexion with a larger one at intervals. In this way the size of the lamp for a given number of candle-hours would be reduced below that of any type of actual generator, and the troubles of after-generation, always more or less experienced in holderless generators, would be entirely done away with.
Dissolved acetylene is also very useful for acetylene welding or autogenous soldering.
The advantages of compressed and absorbed acetylene depend on the small bulk and weight of the apparatus per unit of light, on the fact that no amount of agitation can affect the evolution of gas (as may happen with an ordinary acetylene generator), on the absence of any liquid which may freeze in winter, and on there being no need for skilled attention except when the cylinders are being changed. These vessels weigh between 2.5 and 3 kilos, per 1 litre capacity (normal) and since they are charged with 100 times their apparent volume of acetylene, they may be said to weigh 1 kilo, per 33 litres of available acetylene, or roughly 2 lb. per cubic foot, or, again, if half-foot burners are used, 2 lb. per 36 candle- hours. According to Fouche, if electricity obtained from lead accumulators is compared with acetylene on the basis of the weight of apparatus needed to evolve a certain quantify of light, 1 kilo, of acetylene cylinder is equal to 1.33 kilos, of lead accumulator with arc lamps, or to 4 kilos. of accumulator with glow lamps; and moreover the acetylene cylinder can be charged and discharged, broadly speaking, as quickly or as slowly as may be desired; while, it may be added, the same cylinder will serve one or more self-luminous jets, one or more incandescent burners, any number and variety of heating apparatus, simultaneously or consecutively, at any pressure which may be required.
From the aspect of space occupied, dissolved acetylene is not so concentrated a source of artificial light as calcium carbide; for 1 volume of granulated carbide is capable of omitting as much light as 4 volumes of compressed gas; although, in practice, to the 1 volume of carbide must be added that of the apparatus in which it is decomposed.
LIQUEFIED ACETYLENE.--In most civilised countries the importation, manufacture, storage, and use of liquefied acetylene, or of the gas compressed to more than a fraction of one effective atmosphere, is quite properly prohibited by law. In Great Britain this has been done by an Order in Council dated November 26, 1897, which specifies 100 inches of water column as the maximum to which compression may be pushed. Power being retained, however, to exempt from the order any method of compressing acetylene that might be proved safe, the Home Secretary issued a subsequent Order on March 28, 1898, permitting oil-gas containing not more than 20 per cent, by volume of acetylene (see below) to be compressed to a degree not exceeding 150 lb. per square inch, _i.e._, to about 10 atmospheres, provided the gases are mixed together before compression; while a third Order, dated April 10, 1901, allows the compression of acetylene into cylinders filled as completely as possible with porous matter, with or without the presence of acetone, to a pressure not exceeding 150 lb. per square inch provided the cylinders themselves have been tested by hydraulic pressure for at least ten minutes to a pressure not less than double [Footnote: In France the cylinders are tested to six times and in Russia to five times their working pressure.] that which it is intended to use, provided the solid substance is similar in every respect to the samples deposited at the Home Office, provided its porosity does not exceed 80 per cent., provided air is excluded from every part of the apparatus before the gas is compressed, provided the quantity of acetone used (if used at all) is not sufficient to fill the porosity of the solid, provided the temperature is not permitted to rise during compression, and provided compression only takes place in premises approved by H.M.'s Inspectors of Explosives.
DILUTED ACETYLENE.--Acetylene is naturally capable of admixture or dilution with any other gas or vapour; and the operation may be regarded in either of two ways; (1) as a, means of improving the burning qualities of the acetylene itself, or (2) as a means of conferring upon some other gas increased luminosity. In the early days of the acetylene industry, generation was performed in so haphazard a fashion, purification so generally omitted, and the burners were so inefficient, that it was proposed to add to the gas a comparatively small proportion of some other gaseous fluid which should be capable of making it burn without deposition of carbon while not seriously impairing its latent illuminating power. One of the first diluents suggested was carbon dioxide (carbonic acid gas), because this gas is very easy and cheap to prepare; and because it was stated that acetylene would bear an addition of 5 or even 8 per cent, of carbon dioxide and yet develop its full degree of luminosity. This last assertion requires substantiation; for it is at least a grave theoretical error to add a non-inflammable gas to a combustible one, as is seen in the lower efficiency of all flames when burning in common air in comparison with that which they exhibit in oxygen; while from the practical aspect, so harmful is carbon dioxide in an illuminating gas, that coal-gas and carburetted water-gas are frequently most rigorously freed from it, because a certain gain in illuminating power may often thus be achieved more cheaply than by direct enrichment of the gas by addition of hydrocarbons. Being prepared from chalk and any cheap mineral acid, hydrochloric by preference, in the cold, carbon dioxide is so cheap that its price in comparison with that of acetylene is almost _nil_; and therefore, on the above assumption, 105 volumes of diluted acetylene might be made essentially for the same price as 100 volumes of neat acetylene, and according to supposition emit 5 per cent. more light per unit of volume.
It is reported that several railway trains in Austria are regularly lighted with acetylene containing 0.4 to 1.0 per cent. of carbon dioxide in order to prevent deposition of carbon at the burners. The gas is prepared according to a patent process which consists in adding a certain proportion of a "carbonate" to the generator water. In the United Kingdom, also, there are several installations supplying an acetylene diluted with carbon dioxide, the gas being produced by putting into that portion of a water-to-carbide generator which lies nearest to the water- supply some solid carbonate like chalk, and using a dilute acid to attack the material. Other inventors have proposed placing a solid acid, like oxalic, in the former part of a generator and decomposing it with a carbonate solution; or they have suggested putting into the generator a mixture of a solid acid and a solid soluble carbonate, and decomposing it with plain water.
Clearly, unless the apparatus in which such mixtures as these are intended to be prepared is designed with considerable care, the amount of carbon dioxide in the gas will be liable to vary, and may fall to zero.
If any quantity of carbide present has been decomposed in the ordinary way, there will be free calcium hydroxide in the generator; and if the carbon dioxide comes into contact with this, it will be absorbed, unless sufficient acid is employed to convert the calcium carbonate (or hydroxide) into the corresponding normal salt of calcium. Similarly, during purification, a material containing any free lime would tend to remove the carbon dioxide, as would any substance which became alkaline by retaining the ammonia of the crude gas.
It cannot altogether be granted that the value of a process for diluting acetylene with carbon dioxide has been established, except in so far as the mere presence of the diluent may somewhat diminish the tendency of the acetylene to polymerise as it passes through a hot burner (_cf._ Chapter VIII.). Certainly as a fuel-gas the mixture would be less efficient, and the extra amount of carbon dioxide produced by each flame is not wholly to be ignored. Moreover, since properly generated and purified acetylene can be consumed in proper burners without trouble, all reason for introducing carbon dioxide has disappeared.
MIXTURES OF ACETYLENE AND AIR.--A further proposal for diluting acetylene was the addition to it of air. Apart from questions of explosibility, this method has the advantage over that of adding carbon dioxide that the air, though not inflammable, is, in virtue of its contained oxygen, a supporter of combustion, and is required in a flame; whereas carbon dioxide is not only not a supporter of combustion, but is actually a product thereof, and correspondingly more objectionable. According to some experiments carried out by Dufour, neat acetylene burnt under certain conditions evolved between 1.0 and 1.8 candle-power per litre- hour; a mixture of 1 volume of acetylene with 1 volume of air evolved 1.4 candle-power; a mixture of 1 volume of acetylene with 1.2 volumes of air, 2.25 candle-power; and a mixture of 1 volume of acetylene with 1.3 volumes of air, 2.70 candle-power per litre-hour of acetylene in the several mixtures. Averaging the figures, and calculating into terms of acetylene (only) burnt, Dufour found neat acetylene to develop 1.29 candle-power per litre-hour, and acetylene diluted with air to develop 1.51 candle-power. When, however, allowance is made for the cost and trouble of preparing such mixtures the advantage of the process disappears; and moreover it is accompanied by too grave risks, unless conducted on a largo scale and under most highly skilled supervision, to be fit for general employment.
Fouche, however, has since found the duty, per cubic foot of neat acetylene consumed in a twin injector burner at the most advantageous rate of 3.2 inches, to be as follows for mixtures with air in the proportions stated:
Percentage of air 0 17 27 33.5 Candles per cubic feet 38.4 36.0 32.8 26.0
At lower pressures, the duty of the acetylene when diluted appears to be relatively somewhat higher. Figures which have been published in regard to a mixture of 30 volumes of air and 70 volumes of acetylene obtained by a particular system of producing such a mixture, known as the "Molet- Boistelle," indicate that the admixture of air causes a slight increase in the illuminating duty obtained from the acetylene in burners of various sizes. The type of burner and the pressure employed in these experiments were not, however, stated. This system has been used at certain stations on the "Midi" railway in France. Nevertheless even where the admixture of air to acetylene is legally permissible, the risk of obtaining a really dangerous product and the nebulous character of the advantages attainable should preclude its adoption.
In Great Britain the manufacture, importation, storage, and use of acetylene mixed with air or oxygen, in all proportions and at all pressures, with or without the presence of other substances, is prohibited by an Order in Council dated July 1900; to which prohibition the mixture of acetylene and air that takes place in a burner or contrivance in which the mixture is intended to be burnt, and the admixture of air with acetylene that may unavoidably occur in the first use or recharging of an apparatus (usually a water-to-carbide generator), properly designed and constructed with a view to the production of pure acetylene, are the solitary exceptions.
MIXED CARBIDES.--In fact the only processes for diluting acetylene which possess real utility are that of adding vaporised petroleum spirit or benzene to the gas, as was described in Chapter X. under the name of carburetted acetylene, and one other possible method of obtaining a diluted acetylene directly from the gas-generator, to which a few words will now be devoted. [Footnote: Mixtures of acetylene with relatively large proportions of other illuminating gases, such as are referred to on subsequent pages, are also, from one aspect, forms of diluted acetylene.]
Calcium carbide is only one particular specimen of a large number of similar metallic compounds, which can be prepared in the electric furnace, or otherwise. Some of those carbides yield acetylene when treated with water, some are not attacked, some give liquid products, and some yield methane, or mixtures of methane and hydrogen. Among the latter is manganese carbide. If, then, a mixture of manganese carbide and calcium carbide is put into an ordinary acetylene generator, the gas evolved will be a mixture of acetylene with methane and hydrogen in proportions depending upon the composition of the carbide mixture. It is clear that a suitable mixture of the carbides might be made by preparing them separately and bulking the whole in the desired proportions; while since manganese carbide can be won in the electric furnace, it might be feasible to charge into such a furnace a mixture of lime, coke, and manganese oxide calculated to yield a simple mixture of the carbides or a kind of double carbide. Following the lines which have been adopted in writing the present book, it is not proposed to discuss the possibility of making mixed carbides; but it may be said in brief that Brame and Lewes have carried out several experiments in this direction, using charges of lime and coke containing (_a_) up to 20 per cent. of manganese oxide, and (_b_) more than 60 per cent. of manganese oxide. In neither case did they succeed in obtaining a material which gave a mixture of acetylene and methane when treated with water; in case (_a_) they found the gas to be practically pure acetylene, so that the carbide must have been calcium carbide only; in case (_b_) the gas was mainly methane and hydrogen, so that the carbide must have been essentially that of manganese alone. Mixed charges containing between 20 and 60 per cent. of manganese oxide remain to be studied; but whether they would give mixed carbides or no, it would be perfectly simple to mix ready-made carbides of calcium and manganese together, if any demand for a diluted acetylene should arise on a sufficiently large scale. It is, however, somewhat difficult to appreciate the benefits to be obtained from forms of diluted acetylene other than those to which reference is made later in this chapter.
There is, nevertheless, one modification of calcium carbide which, in a small but important sphere, finds a useful _role_. It has been pointed out that a carbide containing much calcium phosphide is usually objectionable, because the gas evolved from it requires extra purification, and because there is the (somewhat unlikely) possibility that the acetylene obtained from such material before purification may be spontaneously inflammable. If, now, to the usual furnace charge of lime and coke a sufficient quantity of calcium phosphate is purposely added, it is possible to win a mixture of calcium phosphide and carbide, or, as Bradley, Read, and Jacobs call it, a "carbophosphide of calcium," having the formula Ca_5C_6P_2, which yields a spontaneously inflammable mixture of acetylene, gaseous phosphine, and liquid phosphine when treated with water, and which, therefore, automatically gives a flame when brought into contact with the liquid. The value of this material will be described in Chapter XIII.
GAS-ENRICHING.--Other methods of diluting acetylene consist in adding a comparatively small proportion of it to some other gas, and may be considered rather as processes for enriching that other gas with acetylene. Provided the second gas is well chosen, such mixtures exhibit properties which render them peculiarly valuable for special purposes.
They have, usually, a far lower upper limit of explosibility than that of neat acetylene, and they admit of safe compression to an extent greatly exceeding that of acetylene itself, while they do not lose illuminating power on compression. The second characteristic is most important, and depends on the phenomena of "partial pressure," which have been referred to in Chapter VI. When a single gas is stored at atmospheric pressure, it is insensibly withstanding on all sides and in all directions a pressure of roughly 15 lb. per square inch, which is the weight of the atmosphere at sea-level; and when a mixture of two gases, X and Y, in equal volumes is similarly stored it, regarded as an entity, is also supporting a pressure of 15 lb. per square inch. But in every 1 volume of that mixture there is only half a volume of X and Y each; and, ignoring the presence of its partner, each half-volume is evenly distributed throughout a space of 1 volume. But since the volume of a gas stands in inverse ratio to the pressure under which it is stored, the half-volume of X in the 1 volume of X + Y apparently stands at a pressure of half an atmosphere, for it has expanded till it fills, from a chemical and physical aspect, the space of 1 volume: suitable tests proving that it exhibits the properties which a gas stored at a pressure of half an atmosphere should do.
Therefore, in the mixture under consideration, X and Y are both said to be at a "partial pressure" of half an atmosphere, which is manifestly 7.5 lb. per square inch. Clearly, when a gas is an entity (either an element or one single chemical compound) partial and total pressure are identical. Now, it has been shown that acetylene ceases to be a safe gas to handle when it is stored at a pressure of 2 atmospheres; but the limit of safety really occurs when the gas is stored at a _partial_ pressure of 2 atmospheres. Neat acetylene, accordingly, cannot be compressed above the mark 30 lb. shown on a pressure gauge; but diluted acetylene (if the diluent is suitable) may be compressed in safety till the partial pressure of the acetylene itself reaches 2 atmospheres. For instance, a mixture of equal volumes of X and Y (X being acetylene) contains X at a partial pressure of half the total pressure, and may therefore be compressed to (2 / 1/2 =) 4 atmospheres before X reaches the partial pressure of 2 atmospheres; and therewith the mixture is brought just to the limit of safety, any effect of Y one way or the other being neglected. Similarly, a mixture of 1 volume of acetylene with 4 volumes of Y may be safely compressed to a pressure of (2 / 1/5 =) 10 atmospheres, or, broadly, a mixture in which the percentage of acetylene is _x_ may be safely compressed to a pressure not exceeding (2 / _x_/100) atmospheres. This fact permits acetylene after proper dilution to be compressed in the same fashion as is allowable in the case of the dissolved and absorbed gas described above.
If the latent illuminating power of acetylene is not to be wasted, the diluent must not be selected without thought. Acetylene burns with a very hot flame, the luminosity of which is seriously decreased if the temperature is lowered. As mentioned in Chapter VIII., this may be done by allowing too much air to enter the flame; but it may also be effected to a certain extent by mixing with the acetylene before combustion some combustible gas or vapour which burns at a lower temperature than acetylene itself. Manifestly, therefore, the ideal diluent for acetylene is a substance which possesses as high a flame temperature as acetylene and a certain degree of intrinsic illuminating power, while the lower the flame temperature of the diluent and the less its intrinsic illuminating power, the less efficiently will the acetylene act as an enriching material. According to Love, Hempel, Wedding, and others, if acetylene is mixed with coal-gas in amounts up to 8 per cent. or thereabouts, the illuminating power of the mixture increases about 1 candle for every 1 per cent. of acetylene present: a fact which is usually expressed by saying that with coal-gas the enrichment value of acetylene is 1 candle per 1 per cent. Above 8 per cent., the enrichment value of acetylene rises, Love having found an increase in illuminating power, for each 1 per cent. of acetylene in the mixture, of 1.42 candles with 11.28 per cent. of acetylene; and of 1.54 candles with 17.62 per cent. of acetylene. Theoretically, if the illuminating power of acetylene is taken at 240 candles, its enrichment value should be (240 / 100 =) 2.4 candles per 1 per cent.; and since, in the case of coal-gas, its actual enrichment value falls seriously below this figure, it is clear that coal-gas is not an economical diluent for it. Moreover, coal-gas can be enriched by other methods much more cheaply than with acetylene. Simple ("blue") water-gas, according to Love, requires more than 10 per cent. of acetylene to be added to it before a luminous flame is produced; while a mixture of 20.3 per cent. of acetylene and 79.7 per cent. of water-gas had an illuminating power of 15.47 candles. Every addition to the proportion of acetylene when it amounted to 20 per cent. and upwards of the mixture had a very appreciable effect on the illuminating power of the latter. Thus with 27.84 per cent. of acetylene, the illuminating power of the mixture was 40.87 candles; with 38.00 per cent. of acetylene it was 73.96 candles. Acetylene would not be an economical agent to employ in order to render water-gas an illuminating gas of about the quality of coal-gas, but the economy of enrichment of water-gas by acetylene increases rapidly with the degree of enrichment demanded of it.
Carburetted water-gas which, after compression under 16 atmospheres pressure, had an illuminating power of about 17.5 candles, was enriched by additions of acetylene. 4.5 per cent. of acetylene in the mixture gave an illuminating power of 22.69 candles; 8.4 per cent., 29.54 candles; 11.21 per cent., 35.05 candles; 15.06 per cent., 42.19 candles; and 21.44 per cent., 52.61 candles. It is therefore evident that the effect of additions of acetylene on the illuminating power of carburetted water-gas is of the same order as its effect on coal-gas. The enrichment value of the acetylene increases with its proportion in the mixture; but only when the proportion becomes quite considerable, and, therefore, the gas of high illuminating power, does enrichment by acetylene become economical.
Methane (marsh-gas), owing to its comparatively high flame temperature, and to the fact that it has an intrinsic, if small, illuminating power, is a better diluent of acetylene than carbon monoxide or hydrogen, in that it preserves to a greater extent the illuminative value of the acetylene.
Actually comparisons of the effect of additions of various proportions of a richly illuminating gas, such as acetylene, on the illuminative value of a gas which has little or no inherent illuminating power, are largely vitiated by the want of any systematic method for arriving at the representative illuminative value of any illuminating gas. A statement that the illuminating power of a gas is _x_ candles is, strictly speaking, incomplete, unless it is supplemented by the information that the gas during testing was burnt (1) in a specified type of burner, and (2) either at a specified fixed rate of consumption or so as to afford a light of a certain specified intensity. There is no general agreement, even in respect of the statutory testing of the illuminating power of coal-gas supplies, as to the observance of uniform conditions of burning of the gas under test, and in regard to more highly illuminating gases there is even greater diversity of conditions. Hence figures such as those quoted above for the enrichment value of acetylene inevitably show a certain want of harmony which is in reality due to the imperfection or incompleteness of the modes of testing employed. Relatively to another, one gas appears advantageously merely in virtue of the conditions of assessing illuminating power having been more favourable to it. Therefore enrichment values, such as those given, must always be regarded as only approximately trustworthy in instituting comparisons between either different diluent gases or different enriching agents.