A very simple experiment will show that a notable quantity of heat is set free when calcium carbide is brought into contact with water, and by arranging the details of the apparatus in a suitable manner, the quantity of heat manifested may be measured with considerable accuracy. A lengthy description of the method of performing this operation, however, scarcely comes within the province of the present book, and it must be sufficient to say that the heat is estimated by decomposing a known weight of carbide by means of water in a small vessel surrounded on all sides by a carefully jacketed receptacle full of water and provided with a sensitive thermometer. The quantity of water contained in the outer vessel being known, and its temperature having been noted before the reaction commences, an observation of the thermometer after the decomposition is finished, and when the mercury has reached its highest point, gives data which show that the reaction between water and a known weight of calcium carbide produces heat sufficient in amount to raise a known weight of water through a known thermometric distance; and from these figures the corresponding number of large calories may easily be calculated. A determination of this quantity of heat has been made experimentally by several investigators, including Lewes, who has found that the heat evolved on decomposing 1 gramme of ordinary commercial carbide with water is 0.406 large calorie. [Footnote: Lewes returns his result as 406 calories, because he employs the "small calorie." The small calorie is the quantity of heat needed to raise 1 gramme of water 1° C.; but as there are 1000 grammes in 1 kilogramme, the large calorie is equal to 1000 small calories. In many respects the former unit is to be preferred.] As the material operated upon contained only 91.3 per cent. of true calcium carbide, he estimates the heat corresponding with the decomposition of 1 gramme of pure carbide to be 0.4446 large calorie. As, however, it is better, and more in accordance with modern practice, to quote such data in terms of the atomic or molecular weight of the substance concerned, and as the molecular weight of calcium carbide is 64, it is preferable to multiply these figures by 64, stating that, according to Lewes' researches, the heat of decomposition of "1 gramme- molecule" (i.e., 64 grammes) of a calcium carbide having a purity of 91.3 per cent. is just under 26 calories, or that of 1 gramme-molecule of pure carbide 28.454 calories. It is customary now to omit the phrase "one gramme-molecule" in giving similar figures, physicists saying simply that the heat of decomposition of calcium carbide by water when calcium hydroxide is the by-product, is 28.454 large calories.
Assuming all the necessary data known, as happens to be the case in the present instance, it is also possible to calculate theoretically the heat which should be evolved on decomposing calcium carbide by means of water. Equation (2), given on page 24, shows that of the substances taking part in the reaction 1 molecular weight of calcium carbide is decomposed, and 1 molecular weight of acetylene is formed. Of the two molecules of water, only one is decomposed, the other passing to the calcium hydroxide unchanged; and the 1 molecule of calcium hydroxide is formed by the combination of 1 atom of free calcium, 1 atom of free oxygen, and 1 molecule of water already existing as such. Calcium hydroxide and water are both exothermic substances, absorbing heat when they are decomposed, liberating it when they are formed. Acetylene is endothermic, liberating heat when it is decomposed, absorbing it when it is produced. Unfortunately there is still some doubt about the heat of formation of calcium carbide, De Forcrand returning it as -0.65 calorie, and Gin as +3.9 calories. De Forcrand's figure means, as before explained, that 64 grammes of carbide should absorb 0.65 large calorie when they are produced by the combination of 40 grammes of calcium with 24 grammes of carbon; the minus sign calling attention to the belief that calcium carbide is endothermic, heat being liberated when it suffers decomposition. On the contrary, Gin's figure expresses the idea that calcium carbide is exothermic, liberating 3.9 calories when it is produced, and absorbing them when it is decomposed. In the absence of corroborative evidence one way or the other, Gin's determination will be accepted for the ensuing calculation. In equation (2), therefore, calcium carbide is decomposed and absorbs heat; water is decomposed and absorbs heat; acetylene is produced and absorbs heat; and calcium hydroxide is produced liberating heat. On consulting the tables of thermo-chemical data given in the various text-books on physical chemistry, all the other constants needed for the present purpose will be found; and it will appear that the heat of formation of water is +69 calories, that of acetylene -58.1 calories, and that of calcium hydroxide, when 1 atom of calcium, 1 atom of oxygen, and 1 molecule of water unite together, is +160.1 calories. [Footnote: When 1 atom of calcium, 2 atoms of oxygen, and 2 atoms of hydrogen unite to form solid calcium hydroxide, the heat of formation of the latter is 229.1 (cf. infra). This value is simply 160.1 + 69.0 = 229.1; 69.0 being the heat of formation of water.] Collecting the results into the form of a balance-sheet, the effect of decomposing calcium carbide with water is this:
_Heat liberated._ | _Heat absorbed._
|
Formation of Ca(OH)_2 16O.1 | Formation of acetylene 58.1
| Decomposition of water 69.0
| Decomposition of carbide 3.9
| Balance 29.1
_____ | _____
|
Total 160.1 | Total 160.1
Therefore when 64 grammes of calcium carbide are decomposed by water, or when 18 grammes of water are decomposed by calcium carbide (the by- product in each case being calcium hydroxide or slaked lime, for the formation of which a further 18 grammes of water must be present in the second instance), 29.1 large calories are set free. It is not possible yet to determine thermo-chemical data with extreme accuracy, especially on such a material as calcium carbide, which is hardly to be procured in a state of chemical purity; and so the value 28.454 calories experimentally found by Lewes agrees very satisfactorily, considering all things, with the calculated value 29.1 calories. It is to be noticed, however, that the above calculated value has been deduced on the assumption that the calcium hydroxide is obtained as a dry powder; but as slaked lime is somewhat soluble in water, and as it evolves 3 calories in so dissolving, if sufficient water is present to take up the calcium hydroxide entirely into the liquid form (i.e., that of a solution), the amount of heat set free will be greater by those 3 calories, i.e., 32.1 large calories altogether.
THE PROCESS OF GENERATION.--Taking 28 as the number of large calories developed when 64 grammes of ordinary commercial calcium carbide are decomposed with sufficient water to leave dry solid calcium hydroxide as the by-product in acetylene generation, this quantity of heat is capable of exerting any of the following effects. It is sufficient (1) to raise 1000 grammes of water through 28° C., say from 10° C. (50° F., which is roughly the temperature of ordinary cold water) to 38° C. It is sufficient (2) to raise 64 grammes of water (a weight equal to that of the carbide decomposed) through 438° C., if that were possible. It would raise (3) 311 grammes of water through 90° C., i.e., from 10° C. to the boiling-point. If, however, instead of remaining in the liquid state, the water were converted into vapour, the same quantity of heat would suffice (4) to change 44.7 grammes of water at 10° C. into steam at 100° C.; or (5) to change 46.7 grammes of water at 10° C. into vapour at the same temperature. It is an action of the last character which takes place in acetylene generators of the most modern and usual pattern, some of the surplus water being evaporated and carried away as vapour at a comparatively low temperature with the escaping gas; for it must be remembered that although steam, as such, condenses into liquid water immediately the surrounding temperature falls below 100° C., the vapour of water remains uncondensed, even at temperatures below the freezing- point, when that vapour is distributed among some permanent gas--the precise quantity of vapour so remaining being a function of the temperature and barometric height. Thus it appears that if the heat evolved during the decomposition of calcium carbide is not otherwise consumed, it is sufficient in amount to vaporise almost exactly 3 parts by weight of water for every 4 parts of carbide attacked; but if it were expended upon some substance such as acetylene, calcium carbide, or steel, which, unlike water, could not absorb an extra amount by changing its physical state (from solid to liquid, or from liquid to gas), the heat generated during the decomposition of a given weight of carbide would suffice to raise an equal weight of the particular substance under consideration to a temperature vastly exceeding 438° C. The temperature attained, indeed, measured in Centigrade degrees, would be 438 multiplied by the quotient obtained on dividing the specific heat of water by the specific heat of the substance considered: which quotient, obviously, is the "reciprocal" of the specific heat of the said substance.
The analogy to the combustion of coal mentioned on a previous page shows that although the quantity of heat evolved during a certain chemical reaction is strictly fixed, the temperature attained is dependent on the time over which the reaction is spread, being higher as the process is more rapid. This is due to the fact that throughout the whole period of reaction heat is escaping from the mass, and passing into the atmosphere at a fairly constant speed; so that, clearly, the more slowly heat is produced, the better opportunity has it to pass away, and the less of it is left to collect in the material under consideration. During the action of an acetylene generator, there is a current of gas constantly travelling away from the carbide, there is vapour of water constantly escaping with the gas, there are the walls of the generator itself constantly exposed to the cooling action of the atmosphere, and there is either a mass of calcium carbide or of water within the generator. It is essential for good working that the temperature of both the acetylene and the carbide shall be prevented from rising to any noteworthy extent; while the amount of heat capable of being dissipated into the air through the walls of the apparatus in a given time is narrowly limited, depending upon the size and shape of the generator, and the temperature of the surrounding air. If, then, a small, suitably designed generator is working quite slowly, the loss of heat through the external walls of the apparatus may easily be rapid enough to prevent the internal temperature from rising objectionably high; but the larger the generator, and the more rapidly it is evolving gas, the less does this become possible. Since of the substances in or about a generator water is the one which has by far the largest capacity for absorbing heat, and since it is the only substance to which any necessary quantity of heat can be safely or conveniently transmitted, it follows that the larger in size an acetylene generator is, or the more rapidly that generator is made to deliver gas, the more desirable is it to use water as the means for dissipating the surplus heat, and the more necessary is it to employ an apparatus in which water is in large chemical excess at the actual place of decomposition.
The argument is sometimes advanced that an acetylene generator containing carbide in excess will work satisfactorily without exhibiting an undesirable rise in internal temperature, if the vessel holding the carbide is merely surrounded by a large quantity of cold water. The idea is that the heat evolved in that particular portion of the charge which is suffering decomposition will be communicated with sufficient speed throughout the whole mass of calcium carbide present, whence it will pass through the walls of the containing vessel into the water all round. Provided the generator is quite small, provided the carbide container is so constructed as to possess the maximum of superficial area with the minimum of cubical capacity (a geometrical form to which the sphere, and in one direction the cylinder, are diametrically opposed), and provided the walls of the container do not become coated internally or externally with a coating of lime or water scale so as to diminish in heat- transmitting power, an apparatus designed in the manner indicated is undoubtedly free from grave objection; but immediately any of those provisions is neglected, trouble is likely to ensue, for the heat will not disappear from the place of actual reaction at the necessary speed. Apparent proof that heat is not accumulating unduly in a water-jacketed carbide container even when the generator is evolving gas at a fair speed is easy to obtain; for if, as usually happens, the end of the container through which the carbide is inserted is exposed to the air, the hand may be placed upon it, and it will be found to be only slightly warm to the touch. Such a test, however, is inconclusive, and frequently misleading, because if more than a pound or two of carbide is present as an undivided mass, and if water is allowed to attack one portion of it, that particular portion may attain a high temperature while the rest is comparatively cool: and if the bulk of the carbide is comparatively cool, naturally the walls of the containing vessel themselves remain practically unheated. Three causes work together to prevent this heat being dissipated through the walls of the carbide vessel with sufficient rapidity. In the first place, calcium carbide itself is a very bad conductor of heat. So deficient in heat-conducting power is it that a lump a few inches in diameter may be raised to redness in a gas flame at one spot, and kept hot for some minutes, while the rest of the mass remains sufficiently cool to be held comfortably in the fingers. In the second place, commercial carbide exists in masses of highly irregular shape, so that when they are packed into any vessel they only touch at their angles and edges; and accordingly, even if the material were a fairly good heat conductor of itself, the air or gas present between each lump would act as an insulator, protecting the second piece from the heat generated in the first. In the third place, the calcium hydroxide produced as the by-product when calcium carbide is decomposed by water occupies considerably more space than the original carbide--usually two or three times as much space, the exact figures depending upon the conditions in which it is formed--and therefore a carbide container cannot advisedly be charged with more than one-third the quantity of solid which it is apparently capable of holding. The remaining two-thirds of the space is naturally full of air when the container is first put into the generator, but the air is displaced by acetylene as soon as gas production begins. Whether that space, however, is occupied by air, by acetylene, or by a gradually growing loose mass of slaked lime, each separate lump of hot carbide is isolated from its neighbours by a material which is also a very bad heat conductor; and the heat has but little opportunity of distributing itself evenly. Moreover, although iron or steel is a notably better conductor of heat than any of the other substances present in the carbide vessel, it is, as a metal, only a poor conductor, being considerably inferior in this respect to copper. If heat dissipation were the only point to be studied in the construction of an acetylene apparatus, far better results might be obtained by the employment of copper for the walls of the carbide container; and possibly in that case a generator of considerable size, fitted with a water- jacketed decomposing vessel, might be free from the trouble of overheating. Nevertheless it will be seen in Chapter VI. that the use of copper is not permissible for such purposes, its advantages as a good conductor of heat being neutralised by its more important defects.
When suitable precautions are not taken to remove the heat liberated in an acetylene apparatus, the temperature of the calcium carbide occasionally rises to a remarkable degree. Investigating this point, Caro has studied the phenomena of heat production in a "dipping" generator-- i.e., an apparatus in which a cage of carbide is alternately immersed in and lifted out of a vessel containing water. Using a generator designed to supply five burners, he has found a maximum recording thermometer placed in the gas space of the apparatus to give readings generally between 60° and 100° C.; but in two tests out of ten he obtained temperatures of about 160° C. To determine the actual temperature of the calcium carbide itself, he scattered amongst the carbide charge fragments of different fusible metallic alloys which were known to melt or soften at certain different temperatures. In all his ten tests the alloys melting at 120° C. were fused completely; in two tests other alloys melting at 216° and 240° C. showed signs of fusion; and in one test an alloy melting at 280° C. began to soften. Working with an experimental apparatus constructed on the "dripping" principle-- i.e., a generator in which water is allowed to fall in single drops or as a fine stream upon a mass of carbide--with the deliberate object of ascertaining the highest temperatures capable of production when calcium carbide is decomposed in this particular fashion, and employing for the measurement of the heat a Le Chatelier thermo-couple, with its sensitive wires lying among the carbide lumps, Lewes has observed a maximum temperature of 674° C. to be reached in 19 minutes when water was dripped upon 227 grammes of carbide at a speed of about 8 grammes per minute. In other experiments he used a laboratory apparatus designed upon the "dipping" principle, and found maximum temperatures, in four different trials, of 703°, 734°, 754°, and 807° C., which were reached in periods of time ranging from 12 to 17 minutes. Even allowing for the greater delicacy of the instrument adopted by Lewes for measuring the temperature in comparison with the device employed by Caro, there still remains an astonishing difference between Caro's maximum of 280° and Lewes' maximum of 807° C. The explanation of this discrepancy is to be inferred from what has just been said. The generator used by Caro was properly made of metal, was quite small in size, was properly designed with some skill to prevent overheating as much as possible, and was worked at the speed for which it was intended--in a word, it was as good an apparatus as could be made of this particular type. Lewes' generator was simply a piece of glass and metal, in which provisions to avoid overheating were absent; and therefore the wide difference between the temperatures noted does not suggest any inaccuracy of observation or experiment, but shows what can be done to assist in the dissipation of heat by careful arrangement of parts. The difference in temperature between the acetylene and the carbide in Caro's test accentuates the difficulty of gauging the heat in a carbide vessel by mere external touch, and supplies experimental proof of the previous assertions as to the low heat-conducting power of calcium carbide and of the gases of the decomposing vessel. It must not be supposed that temperatures such as Lewes has found ever occur in any commercial generator of reasonably good design and careful construction; they must be regarded rather as indications of what may happen in an acetylene apparatus when the phenomena accompanying the evolution of gas are not understood by the maker, and when all the precautions which can easily be taken to avoid excessive heating have been omitted, either by building a generator with carbide in excess too large in size, or by working it too rapidly, or more generally by adopting a system of construction unsuited to the ends in view. The fact, however, that Lewes has noted the production of a temperature of 807° C. is important; because this figure is appreciably above the point 780° C., at which acetylene decomposes into its elements in the absence of air.
Nevertheless the production of a temperature somewhat exceeding 100° C. among the lumps of carbide actually undergoing decomposition can hardly be avoided in any practical generator. Based on a suggestion in the "Report of the Committee on Acetylene Generators" which was issued by the British Home Office in 1902, Fouché has proposed that 130° C., as measured with the aid of fusible metallic rods, [Footnote: An alloy made by melting together 55 parts by weight of commercial bismuth and 45 parts of lead fuses at 127° C., and should be useful in performing the tests.] should be considered the maximum permissible temperature in any part of a generator working at full speed for a prolonged period of time. Fouché adopts this figure on the ground that 130° C. sensibly corresponds with the temperature at which a yellow substance is formed in a generator by a process of polymerisation; and, referring to French conditions, states that few actual apparatus permit the development of so high a temperature. As a matter of fact, however, a fairly high temperature among the carbide is less important than in the gas, and perhaps it would be better to say that the temperature in any part of a generator occupied by acetylene should not exceed 100° C. Fraenkel has carried out some experiments upon the temperature of the acetylene immediately after evolution in a water-to-carbide apparatus containing the carbide in a subdivided receptacle, using an apparatus now frequently described as belonging to the "drawer" system of construction. When a quantity of about 7 lb. of carbide was distributed between 7 different cells of the receptacle, each cell of which had a capacity of 25 fluid oz., and the apparatus was caused to develop acetylene at the rate of 7 cubic feet per hour, maximum thermometers placed immediately over the carbide in the different cells gave readings of from 70° to 90° C., the average maximum temperature being about 80° C. Hence the Austrian code of rules issued in 1905 governing the construction of acetylene apparatus contains a clause to the effect that the temperature in the gas space of a generator must never exceed 80° C.; whereas the corresponding Italian code contains a similar stipulation, but quotes the maximum temperature as 100° C. (vide Chapter IV.).
It is now necessary to see why the production of an excessively high temperature in an acetylene generator has to be avoided. It must be avoided, because whenever the temperature in the immediate neighbourhood of a mass of calcium carbide which is evolving acetylene under the attack of water rises materially above the boiling-point of water, one or more of three several objectionable effects is produced--(a) upon the gas generated, (b) upon the carbide decomposed, and (c) upon the general chemical reaction taking place.