Although some of these materials attack acetylene slightly, and some leave sulphur in the purified gas, they may be all considered reasonably efficient from the practical point of view; for the loss of true acetylene is too small to be noticeable, and the quantity of sulphur not extracted too trifling to be harmful or inconvenient. They may be valued, accordingly, mainly by their price, proper allowance being made for the quantity of gas purified per unit weight of substance taken. This quantity of gas must naturally vary with the proportion of phosphorus and sulphur in the crude acetylene; but on an average the composition of unpurified gas is what has already been given above, and so the figures obtained by Keppeler in his investigation of the subject may be accepted. In the annexed table these are given in two forms: (1) the number of litres of gas purified by 1 kilogramme of the substance, (2) the number of cubic feet purified per lb. It should be noted that the volumes of gas refer to a laboratory degree of purification; in practice they may all be increased by 10 or possibly 20 per cent.
_________ Litres Cubic Feet per Kilogramme. per Lb. ___ ____ ___ Heratol 5,000 80 Frankoline 9,000 144 Puratylene 10,000 160 Acagine 13,000 208 ___ ____ ___
Another method of using dry bleaching-powder has been proposed by Pfeiffer. He suggests incorporating it with a solution of some lead salt, so that the latter may increase the capacity of the calcium hypochlorite to remove sulphur. Analytical details as to the efficiency of this process have not been given. During 1901 and 1902 Bullier and Maquenne patented a substance made by mixing bleaching-powder with sodium sulphate, whereby a double decomposition occurs, sodium hypochlorite, which is equally efficient with calcium hypochlorite as a purifying material, being produced together with calcium sulphate, which, being identical with plaster of Paris, sets into a solid mass with the excess of water present, and is claimed to render the whole more porous. This process seemed open to objection, because Blagden had shown that a solution of sodium hypochlorite was not a suitable purifying reagent in practice, since it was much more liable to add chlorine to the gas than calcium hypochlorite. The question how a solidified modification of sodium hypochlorite would behave in this respect has been investigated by Keppeler, who found that the Bullier and Maquenne material imparted more chlorine to the gas which had traversed it than other hypochlorite purifying agents, and that the partly foul material was liable to cause violent explosions. About the same time Rossel and Landriset pointed out that purification might be easily effected in all generators of the carbide-to-water pattern by adding to the water of the generator itself a quantity of bleaching-powder equivalent to 5 to 20 grammes for every 1 kilogramme of carbide decomposed, claiming that owing to the large amount of liquid present, which is usually some 4 litres per kilogramme of carbide (0.4 gallon per lb.), no nitrogen chloride could be produced, and that owing to the dissolved lime in the generator, chlorine could not be added to the gas. The process is characterised by extreme simplicity, no separate purifier being needed, but it has been found that an introduction of bleaching-powder in the solid condition is liable to cause an explosive combination of acetylene and chlorine, while the use of a solution is attended by certain disadvantages. Granjon has proposed impregnating a suitable variety of wood charcoal with chlorine, with or without an addition of bleaching-powder; then grinding the product to powder, and converting it into a solid porous mass by the aid of cement. The material is claimed to last longer than ordinary hypochlorite mixtures, and not to add chlorine to the acetylene.
SUBSIDIARY PURIFYING MATERIALS.—Among minor reagents suggested as purifying substances for acetylene may be mentioned potassium permanganate, barium peroxide, potassium bichromate, sodium plumbate and arsenious oxide. According to Benz the first two do not remove the sulphuretted hydrogen completely, and oxidise the acetylene to some extent; while potassium bichromate leaves some sulphur and phosphorus behind in the gas. Sodium plumbate has been suggested by Morel, but it is a question whether its action on the impurities would not be too violent and whether it would be free from action on the acetylene itself. The use of arsenious oxide dissolved in a strong acid, and the solution absorbed in pumice or kieselguhr has been protected by G. F. Jaubert. The phosphine is said to combine with the arsenic to form an insoluble brownish compound. In 1902 Javal patented a mixture of 1 part of potassium permanganate, 5 of "sulphuric acid," and 1 of water absorbed in 4 parts of infusorial earth. The acid constantly neutralised by the ammonia of the crude gas is as constantly replaced by fresh acid formed by the oxidation of the sulphuretted hydrogen; and this free acid, acting upon the permanganate, liberates manganese peroxide, which is claimed to destroy the phosphorus and sulphur compounds present in the crude acetylene.
EPURENE.—A purifying material to which the name of epurene has been given has been described, by Mauricheau-Beaupre, as consisting of a mixture of ferric chloride and ferric oxide in the proportion of 2 molecules, or 650 parts, of the former with one molecule, or 160 parts, of the latter, together with a suitable quantity of infusorial earth. In the course of preparation, however, 0.1 to 0.2 per cent. of mercuric chloride is introduced into the material. This mercuric chloride is said to form an additive compound with the phosphine of the crude acetylene, which compound is decomposed by the ferric chloride, and the mercuric chloride recovered. The latter therefore is supposed to act only as a carrier of the phosphine to the ferric chloride and oxide, by which it is oxidised according to the equation:
8Fe_2Cl_6 + 4Fe_2O_3 + 3PH_3 = 12Fe_2Cl_4 + 3H_3PO_4.
Thus the ultimate products are phosphoric acid and ferrous chloride, which on exposure to air is oxidised to ferric chloride and oxide. It is said that this revivification of the fouled or spent epurene takes place in from 20 to 48 hours when it is spread in the open in thin layers, or it may be partially or wholly revivified in situ by adding a small proportion of air to the crude acetylene as it enters the purifier. The addition of 1 to 2 per cent. of air, according to Mauricheau-Beaupre, suffices to double the purifying capacity of one charge of the material, while a larger proportion would achieve its continuous revivification. Epurene is said to purify 10,000 to 11,000 litres of crude acetylene per kilogramme, or, say, 160 to 176 cubic feet per pound, when the acetylene contains on the average 0.05 per cent, by volume of phosphine.
For employment in all acetylene installations smaller than those which serve complete villages, a solid purifying material is preferable to a liquid one. This is partly due to the extreme difficulty of subdividing a stream of gas so that it shall pass through a single mass of liquid in small enough bubbles for the impurities to be removed by the time the gas arrives at the surface. This time cannot be prolonged without increasing the depth of liquid in the vessel, and the greater the depth of liquid, the more pressure is consumed in forcing the gas through it. Perfect purification by means of fluid reagents unattended by too great a consumption of pressure is only to be effected by a mechanical scrubber such as is used on coal-gas works, wherein, by the agency of external power, the gas comes in contact with large numbers of solid surfaces kept constantly wetted; or by the adoption of a tall tower filled with porous matter or hollow balls over which a continuous or intermittent stream of the liquid purifying reagent is made to trickle, and neither of these devices is exactly suited to the requirements of a domestic acetylene installation. When a solid material having a proper degree of porosity or aggregation is selected, the stream of gas passing through it is broken up most thoroughly, and by employing several separate layers of such material, every portion of the gas is exposed equally to the action of the chemical reagent by the time the gas emerges from the vessel. The amount of pressure so consumed is less than that in a liquid purifier where much fluid is present; but, on the other hand, the loss of pressure is absolutely constant at all times in a liquid purifier, provided the head of liquid is maintained at the same point. A badly chosen solid purifying agent may exhibit excessive pressure absorption as it becomes partly spent. A solid purifier, moreover, has the advantage that it may simultaneously act as a drier for the gas; a liquid purifier, in which the fluid is mainly water, obviously cannot behave in a similar fashion For thorough purification it is necessary that the gas shall actually stream through the solid material; a mere passage over its surface is neither efficient nor economical of material.
DISPOSITION OF PURIFYING MATERIAL.—Although much has been written, and some exaggerated claims made, about the maximum, volume of acetylene a certain variety of purifying material will treat, little has been said about the method in which such a material should be employed to obtain the best results. If 1 lb. of a certain substance will purify 200 cubic feet of normal crude acetylene, that weight is sufficient to treat the gas evolved from 40 lb. of carbide; but it will only do so provided it is so disposed in the purifier that the gas does not pass through it at too high a speed, and that it is capable of complete exhaustion. In the coal- gas industry it is usually assumed that four layers of purifying material, each having a superficial area of 1 square foot, are the minimum necessary for the treatment of 100 cubic feet of gas per hour, irrespective of the nature of the purifying material and of the impurity it is intended to extract. If there is any sound basis for this generalization, it should apply equally to the purification of acetylene, because there is no particular reason to imagine that the removal of phosphine by a proper substance should occur at an appreciably different speed from the removal of carbon dioxide, sulphuretted hydrogen, and carbon bisulphide by lime, ferric oxide, and sulphided lime respectively, Using the coal gas figures, then, for every 10 cubic feet of acetylene generated per hour, a superficial area of (4 x 144 / 10) 57.6 square inches of purifying material is required. In the course of Keppeler's research upon different purifying materials it is shown that 400 grammes of heratol, 360 grammes of frankoline, 250 grammes of acagine, and 230 grammes of puratylene each occupy a space of 500 cubic centimetres when loosely loaded into a purifying vessel, and from these data, the following table has been calculated:
Weight Weight Cubic Inches per Gallon per Cubic Foot Occupied in Lbs. in Lbs. per Lb. Water 10.0 62.321 27.73 Heratol 8.0 49.86 31.63 Frankoline 7.2 41.87 38.21 Acagine 6.0 31.16 55.16 Puratylene 4.6 28.67 60.28
As regards the minimum weight of material required, data have been given by Pfleger for use with puratylene. He states that 1 Kilogramme of that substance should be present for every 100 litres of crude acetylene evolved per hour, 4 kilogrammes being the smallest quantity put into the purifier. In English units these figures are 1 lb. per 1.5 cubic feet per hour, with 9 lb. as a minimum, which is competent to treat 1.1 cubic feet of gas per hour. Thus it appears that for the purification of the gas coming from any generator evolving up to 14 cubic feet of acetylene per hour a weight of 9 lb of puratylene must be charged into the purifier, which will occupy (60.28 / 9) 542 cubic inches of space; and it must be so spread out as to present a total superficial area of (4 x 144 x 14 / 100) 80.6 square inches to the passing gas. It follows, therefore, that the material should be piled to a depth of (542 / 80.6) 6.7 inches on a support having an area of 80.6 square inches; but inasmuch as such a depth is somewhat large for a small vessel, and as several layers are better than one, it would be preferable to spread out these 540 cubic inches of substance on several supports in such a fashion that a total surface of 80.6 square inches or upwards should be exhibited. These figures may obviously be manipulated in a variety of ways for the design of a purifying vessel; but, to give an example, if the ordinary cylindrical shape be adopted with four circular grids, each having a clear diameter of 8 inches (i.e., an area of 50.3 square inches), and if the material is loaded to a depth of 3 inches on each, there would be a total volume of (50.3 x 3 x 4) = 604 cubic inches of puratylene in the vessel, and it would present a total area of (50.3 x 4) = 201 square inches to the acetylene. At Keppeler's estimation such an amount of puratylene should weigh roughly 10 lb., and should suffice for the purification of the gas obtained from 320 lb. of ordinary carbide; while, applying the coal-gas rule, the total area of 201 square inches should render such a vessel equal to the purification of acetylene passing through it at a speed not exceeding (201 / 5.76) = 35 cubic feet per hour. Remembering that it is minimum area in square inches of purifying material that must govern the speed at which acetylene may be passed through a purifier, irrespective probably of the composition of the material; while it is the weight of material which governs the ultimate capacity of the vessel in terms of cubic feet of acetylene or pounds of carbide capable of purification, these data, coupled with Keppeler's efficiency table, afford means for calculating the dimensions of the purifying vessel to be affixed to an installation of any desired number of burners. There is but little to say about the design of the vessel from the mechanical aspect. A circular horizontal section is more likely to make for thorough exhaustion of the material. The grids should be capable of being lifted out for cleaning. The lid may be made tight either by a clamp and rubber or leather washer, or by a liquid seal. If the purifying material is not hygroscopic, water, calcium chloride solution, or dilute glycerin may be used for sealing purposes; but if the material, or any part of it, does absorb water, the liquid in the seal should be some non-aqueous fluid like lubricating oil. Clamped lids are more suitable for small purifiers, sealed lids for large vessels. Care must be taken that condensation products cannot collect in the purifying vessel. If a separate drying material is employed in the same purifier the space it takes must be considered separately from that needed by the active chemical reagent. When emptying a foul purifier it should be recollected that the material may be corrosive, and being saturated with acetylene is likely to catch fire in presence of a light.
Purifiers charged with heratol are stated, however, to admit of a more rapid flow of the gas through them than that stated above for puratylene. The ordinary allowance is 1 lb. of heratol for every cubic foot per hour of acetylene passing, with a minimum charge of 7 lb. of the material. As the quantity of material in the purifier is increased, however, the flow of gas per hour may be proportionately increased, e.g., a purifier charged with 132 lb. of heratol should purify 144 cubic feet of acetylene per hour.
In the systematic purification of acetylene, the practical question arises as to how the attendant is to tell when his purifiers approach exhaustion and need recharging; for if it is undesirable to pass crude gas into the service, it is equally undesirable to waste so comparatively expensive a material as a purifying reagent. In Chapter XIV. it will be shown that there are chemical methods of testing for the presence, or determining the proportion, of phosphorus and sulphur in acetylene; but these are not suitable for employment by the ordinary gas-maker. Heil has stated that the purity of the gas may be judged by an inspection of its atmospheric flame as given by a Bunsen burner. Pure acetylene gives a perfectly transparent moderately dark blue flame, which has an inner cone of a pale yellowish green colour; while the impure gas yields a longer flame of an opaque orange-red tint with a bluish red inner zone. It should be noted, however, that particles of lime dust in the gas may cause the atmospheric flame to be reddish or yellowish (by presence of calcium or sodium) quite apart from ordinary impurities; and for various other reasons this appearance of the non-luminous flame is scarcely to be relied upon. The simplest means of ascertaining definitely whether a purifier is sufficiently active consists in the use of the test-papers prepared by E. Merck of Darmstadt according to G. Keppeler's prescription. These papers, cut to a convenient size, are put up in small books from which they may be torn one at a time. In order to test whether gas is sufficiently purified, one of the papers is moistened with hydrochloric acid of 10 per cent. strength, and the gas issuing from a pet-cock or burner orifice is allowed to impinge on the moistened part. The original black or dark grey colour of the paper is changed to white if the gas contains a notable amount of impurity, but remains unchanged if the gas is adequately purified. The paper consists of a specially prepared black porous paper which has been dipped in a solution of mercuric chloride (corrosive sublimate) and dried. Moistening the paper with hydrochloric acid provides in a convenient form for application Berge's solution for the detection of phosphine (vide Chapter XIV.). The Keppeler test-papers turn white when the gas contains either ammonia, phosphine, siliciuretted hydrogen, sulphuretted hydrogen or organic sulphur compounds, but with carbon disulphide the change is slow. Thus the paper serves as a test for all the impurities likely to occur in acetylene. The sensitiveness of the test is such that gas containing about 0.15 milligramme of sulphur, and the same amount of phosphorus, per litre (= 0.0655 grain per cubic foot) imparts in five minutes a distinct white mark to the moistened part of the paper, while gas containing 0.05 milligramme of sulphur per litre (= 0.022 grain per cubic foot) gives in two minutes a dull white mark visible only by careful inspection. If, therefore, a distinct white mark appears on moistened Keppeler paper when it is exposed for five minutes to a jet of acetylene, the latter is inadequately purified. If the gas has passed through a purifier, this test indicates that the material is not efficient, and that the purifier needs recharging. The moistening of the Keppeler paper with hydrochloric acid before use is essential, because if not acidified the paper is marked by acetylene itself. The books of Keppeler papers are put up in a case which also contains a bottle of acid for moistening them as required and are obtainable wholesale of E. Merek, 16 Jewry Street, London, E.C., and retail of the usual dealers in chemicals. If Keppeler's test-papers are not available, the purifier should be recharged as a matter of routine as soon as a given quantity of carbide—proportioned to the purifying capacity of the charge of purifying material—has been used since the last recharging. Thus the purifier may conveniently contain enough material to purify the gas evolved from two drums of carbide, in which case it would need recharging when every second drum of carbide is opened.
REGULATIONS AS TO PURIFICATION.—The British Acetylene Association has issued the following set of regulations as to purifying material and purifiers for acetylene:
Efficient purifying material and purifiers shall comply with the following requirements:
(1) The purifying material shall remove phosphorus and sulphur compounds to a commercially satisfactory degree; i.e., not to a greater degree than will allow easy detection of escaping gas through its odour.
(2) The purifying material shall not yield any products capable of corroding the gas-mains or fittings.
(3) The purifying material shall, if possible, be efficient as a drying agent, but the Association does not consider this an absolute necessity.
(4) The purifying material shall not, under working conditions, be capable of forming explosive compounds or mixtures. It is understood, naturally, that this condition does not apply to the unavoidable mixture of acetylene and air formed when recharging the purifier.
(5) The apparatus containing the purifying material shall be simple in construction, and capable of being recharged by an inexperienced person without trouble. It shall be so designed as to bring the gas into proper contact with the material.
(6) The containers in purifiers shall be made of such materials as are not dangerously affected by the respective purifying materials used.
(7) No purifier shall be sold without a card of instructions suitable or hanging up in some convenient place. Such instructions shall be of the most detailed nature, and shall not presuppose any expert knowledge whatever on the part of the operator.
Reference also to the abstracts of the official regulations as to acetylene installations in foreign countries given in Chapter IV. will show that they contain brief rules as to purifiers.
DRYING.—It has been stated in Chapter III. that the proper position for the chemical purifiers of an acetylene plant is after the holder; and they therefore form the last items in the installation unless a "station" governor and meter are fitted. It is therefore possible to use them also to remove the moisture in the gas, if a material hygroscopic in nature is employed to charge them. This should be true more particularly with puratylene, which contains a notable proportion of the very hygroscopic body calcium chloride. If a separate drier is desirable, there are two methods of charging it. It may be filled either with some hygroscopic substance such as porous calcium chloride or quicklime in very coarse powder, which retains the water by combining with it; or the gas may be led through a vessel loaded with calcium carbide, which will manifestly hold all the moisture, replacing it by an equivalent quantity of (unpurified) acetylene. The objection is sometimes urged against this latter method, that it restores to the gas the nauseous odour and the otherwise harmful impurities it had more or less completely lost in the purifiers; but as regards the first point, a nauseous odour is not, as has previously been shown, objectionable in itself, and as regards the second, the amount of impurities added by a carbide drier, being strictly limited by the proportion of moisture in the damp gas, is too small to be noticeable at the burners or elsewhere. As is the case with purification, absolute removal of moisture is not called for; all that is needed is to extract so much that the gas shall never reach its saturation-point in the inaccessible parts of the service during the coldest winter's night. Any accessible length of main specially exposed to cold may be safeguarded by itself; being given a steady fall to a certain point (preferably in a frost-free situation), and there provided with a collecting-box from which the deposited liquid can be removed periodically with a pump or otherwise.
FILTRATION.—The gas issuing from the purifier or drier is very liable to hold in suspension fine dust derived from the purifying or drying material used. It is essential that thin dust should be abstracted before the gas reaches the burners, otherwise it will choke the orifices and prevent them functioning properly. Consequently the gas should pass through a sufficient layer of filtering material after it has traversed the purifying material (and drier if one is used). This filtering material may be put either as a final layer in the purifier (or drier), or in a separate vessel known as a filter. Among filtering materials in common use may be named cotton-wool, fine canvas or gauze, felt and asbestos-wool. The gas must be fairly well dried before it enters the filter, otherwise the latter will become choked with deposited moisture, and obstruct the passage of the gas.
Having now described the various items which go to form a well-designed acetylene installation, it may be useful to recapitulate briefly, with the object of showing the order in which they should be placed. From the generator the gas passes into a condenser to cool it and to remove any tarry products and large quantities of water. Next it enters a washing apparatus filled with water to extract water-soluble impurities. If the generator is of the carbide-to-water pattern, the condenser may be omitted, and the washer is only required to retain any lime froth and to act as a water-seal or non-return valve. If the generator does not wash the gas, the washer must be large enough to act efficiently as such, and between it and the condenser should be put a mechanical filter to extract any dust. From the washer the acetylene travels to the holder. From the holder it passes through one or two purifiers, and from there travels to the drier and filter. If the holder does not throw a constant pressure, or if the purifier and drier are liable to cause irregularities, a governor or pressure regulator must be added after the drier. The acetylene is then ready to enter the service; but a station meter (the last item in the plant) is useful as giving a means of detecting any leak in the delivery-pipes and in checking the make of gas from the amount of carbide consumed. If the gas is required for the supply of a district, a station meter becomes quite necessary, because the public lamps will be fed with gas at a contract rate, and without the meter there would be no control over the volume of acetylene they consume. Where the gas finally leaves the generating-house, or where it enters the residence, a full-way stopcock should be put on the main.
GENERATOR RESIDUES.—According to the type of generator employed the waste product removed therefrom may vary from a dry or moist powder to a thin cream or milk of lime. Any waste product which is quite liquid in its consistency must be completely decomposed and free from particles of calcium carbide of sensible magnitude; in the case of more solid residues, the less fluid they are the greater is the improbability (or the less is the evidence) that the carbide has been wholly spent within the apparatus. Imperfect decomposition of the carbide inside the generator not only means an obvious loss of economy, but its presence among the residues makes a careful handling of them essential to avoid accident owing to a subsequent liberation of acetylene in some unsuitable, and perhaps closed, situation. A residue which is not conspicuously saturated with water must be taken out of the generator- house into the open air and there flooded with water, being left in some uncovered receptacle for a sufficient time to ensure all the acetylene being given off. A residue which is liquid enough to flow should be run directly from the draw-off cock of the generator through a closed pipe to the outside; where, if it does not discharge into an open conduit, the waste-pipe must be trapped, and a ventilating shaft provided so that no gas can blow back into the generator-house.
DISPOSAL OF RESIDUES.—These residues have now to be disposed of. In some circumstances they can be put to a useful purpose, as will be explained in Chapter XII.; otherwise, and always perhaps on the small scale— certainly always if the generator overheats the gas and yields tar among the spent lime—they must be thrown into a convenient place. It should be remembered that although methods of precipitating sewage by adding lime, or lime water, to it have frequently been used, they have not proved satisfactory, partly because the sludge so obtained is peculiarly objectionable in odour, and partly because an excess of lime yields an effluent containing dissolved lime, which among other disadvantages is harmful to fish. The plan of running the liquid residues of acetylene manufacture into any local sewerage system which may be found in the neighbourhood of the consumer's premises, therefore, is very convenient to the consumer; but is liable to produce complaints if the sewage is afterwards treated chemically, or if its effluent is passed untreated into a highly preserved river; and the same remark applies in a lesser degree if the residues are run into a private cesspool the liquid contents of which automatically flow away into a stream. If, however, the cesspool empties itself of liquid matter by filtration or percolation through earth, there can be no objection to using it to hold the lime sludge, except in so far as it will require more frequent emptying. On the whole, perhaps the best method of disposing of these residues is to run them into some open pit, allowing the liquid to disappear by evaporation and percolation, finally burying the solid in some spot where it will be out of the way. When a large carbide-to-water generator is worked systematically so as to avoid more loss of acetylene by solution in the excess of liquid than is absolutely necessary, the liquid residues coming from it will be collected in some ventilated closed tank where they can settle quietly. The clear lime-water will then be pumped back into the generator for further use, and the almost solid sludge will be ready to be carried to the pit where it is to be buried. Special care must be taken in disposing of the residues from a generator in which oil is used to control evolution of gas. Such oil floats on the aqueous liquid; and a very few drops spread for an incredible distance as an exceedingly thin film, causing those brilliant rainbow-like colours which are sometimes imagined to be a sign of decomposing organic matter. The liquid portions of these residues must be led through a pit fitted with a depending partition projecting below the level at which the water is constantly maintained; all the oil then collects on the first side of the partition, only water passing underneath, and the oil may be withdrawn and thrown away at intervals.
THE CHEMICAL AND PHYSICAL PROPERTIES OF ACETYLENE
It will only be necessary for the purpose of this book to indicate the more important chemical and physical properties of acetylene, and, in particular, those which have any bearing on the application of acetylene for lighting purposes. Moreover, it has been found convenient to discuss fully in other chapters certain properties of acetylene, and in regard to such properties the reader is referred to the chapters mentioned.
PHYSICAL PROPERTIES.—Acetylene is a gas at ordinary temperatures, colourless, and, when pure, having a not unpleasant, so-called "ethereal" odour. Its density, or specific gravity, referred to air as unity, has been found experimentally by Leduc to be 0.9056. It is customary to adopt the value 0.91 for calculations into which the density of the gas enters (vide Chapter VII.). The density of a gas is important not only for the determination of the size of mains needed to convey it at a given rate of flow under a given pressure, as explained in Chapter VII., but also because the volume of gas which will pass through small orifices in a given time depends on its density. According to Graham's well-known law of the effusion of gases, the velocity with which a gas effuses varies directly as the square root of the difference of pressure on the two sides of the opening, and inversely as the square root of the density of the gas. Hence it follows that the volume of gas which escapes through a porous pipe, an imperfect joint, or a burner orifice is, provided the pressure in the gas-pipe is the same, a function of the square root of the density of the gas. Hence this density has to be taken into consideration in the construction of burners, i.e., a burner required to pass a gas of high density must have a larger orifice than one for a gas of low density, if the rate of flow of gas is to be the same under the same pressure. This, however, is a question for the burner manufacturers, who already make special burners for gases of different densities, and it need not trouble the consumer of acetylene, who should always use burners devised for the consumption of that gas. But the Law of effusion indicates that the volume of acetylene which can escape from a leaky supply-pipe will be less than the volume of a gas of lower density, e.g., coal-gas, if the pressure in the pipe is the same for both. This implies that on an extensive distributing system, in which for practical reasons leakage is not wholly avoidable, the loss of gas through leakage will be less for acetylene than for coal-gas, given the same distributing pressure. If v = the loss of acetylene from a distributing system and v' = the loss of coal-gas from a similar system worked at the same pressure, both losses being expressed in volumes (cubic feet) per hour, and the coal-gas being assumed to have a density of 0.04, then
(1) (v/v') = (0.40 / 0.91)^(1/2) = 0.663
or, v = 0.663v',
which signifies that the loss of acetylene by leakage under the same conditions of pressure, &c., will be only 0.663 times that of the loss of coal-gas. In practice, however, the pressures at which the gases are usually sent through mains are not identical, being greater in the case of acetylene than in that of coal-gas. Formula (1) therefore requires correction whenever the pressures are different, and calling the pressure at which the acetylene exists in the main p, and the corresponding pressure of the coal-gas p', the relative losses by leakage are—
(2) (v/v') = (0.40 / 0.91)^(1/2) x (p/p')^(1/2)
v = 0.663v' x (p/p')^(1/2)
It will be evident that whenever the value of the fraction (p/p')^(1/2), is less than 1.5, i.e., whenever the pressure of the acetylene does not exceed double that of the coal-gas present in pipes of given porosity or unsoundness, the loss of acetylene will be less than that of coal-gas. This is important, especially in the case of large village acetylene installations, where after a time it would be impossible to avoid some imperfect joints, fractured pipes, &c., throughout the extensive distributing mains. The same loss of gas by leakage would represent a far higher pecuniary value with acetylene than with coal-gas, because the former must always be more costly per unit of volume than the latter. Hence it is important to recognise that the rate of leakage, coeteris paribus, is less with acetylene, and it is also important to observe the economical advantage, at least in terms of gas or calcium carbide, of sending the acetylene into the mains at as low a pressure as is compatible with the length of those mains and the character of the consumers' burners. As follows from what will be said in Chapter VII., a high initial pressure makes for economy in the prime cost of, and in the expense of laying, the mains, by enabling the diameter of those mains to be diminished; but the purchase and erection of the distributing system are capital expenses, while a constant expenditure upon carbide to meet loss by leakage falls upon revenue.
The critical temperature of acetylene, i.e., the temperature below which an abrupt change from the gaseous to the liquid state takes place if the pressure is sufficiently high, is 37 deg. C., and the critical pressure, i.e., the pressure under which that change takes place at that temperature, is nearly 68 atmospheres. Below the critical temperature, a lower pressure than this effects liquefaction of the gas, i.e., at 13.5 deg. C. a pressure of 32.77 atmospheres, at 0 deg. C., 21.53 atmospheres (Ansdell, cf. Chapter XI.). These data are of comparatively little practical importance, owing to the fact that, as explained in Chapter XI., liquefied acetylene cannot be safely utilised.
The mean coefficient of expansion of gaseous acetylene between 0 deg. C. and 100 deg. C., is, under constant pressure, 0.003738; under constant volume, 0.003724. This means that, if the pressure is constant, 0.003738 represents the increase in volume of a given mass of gaseous acetylene when its temperature is raised one degree (C.), divided by the volume of the same mass at 0 deg. C. The coefficients of expansion of air are: under constant pressure, 0.003671; under constant volume, 0.003665; and those of the simple gases (nitrogen, hydrogen, oxygen) are very nearly the same. Strictly speaking the table given in Chapter XIV., for facilitating the correction of the volume of gas measured over water, is not quite correct for acetylene, owing to the difference in the coefficients of expansion of acetylene and the simple gases for which the table was drawn up, but practically no appreciable error can ensue from its use. It is, however, for the correction of volumes of gases measured at different temperatures to one (normal) temperature, and, broadly, for determining the change of volume which a given mass of the gas will undergo with change of temperature, that the coefficient of expansion of a gas becomes an important factor industrially.
Ansdell has found the density of liquid acetylene to range from 0.460 at -7 deg. C. to 0.364 at +35.8 deg. C., being 0.451 at 0 deg. C. Taking the volume of the liquid at -7 deg. as unity, it becomes 1.264 at 35.8 deg., and thence Ansdell infers that the mean coefficient of expansion per degree is 0.00489 deg. for the total range of pressure." Assuming that the liquid was under the same pressure at the two temperatures, the coefficient of expansion per degree Centigrade would be 0.00605, which agrees more nearly with the figure 0.007 which is quoted, by Fouche As mentioned before, data referring to liquid (i.e., liquefied) acetylene are of no practical importance, because the substance is too dangerous to use. They are, however, interesting in so far as they indicate the differences in properties between acetylene converted into the liquid state by great pressure, and acetylene dissolved in acetone under less pressure; which differences make the solution fit for employment. It may be observed that as the solution of acetylene in acetone is a liquid, the acetylene must exist therein as a liquid; it is, in fact, liquid acetylene in a state of dilution, the diluent being an exothermic and comparatively stable body.
The specific heat of acetylene is given by M. A. Morel at 0.310, though he has not stated by whom the value was determined. For the purpose of a calculation in Chapter III. the specific heat at constant pressure was assumed to be 0.25, which, in the absence of precise information, appears somewhat more probable as an approximation to the truth. The ratio (k or Cp/Cv ) of the specific heat at constant pressure to that at constant volume has been found by Maneuvrier and Fournier to be 1.26; but they did not measure the specific heat itself. [Footnote: The ratio 1.26 k or (Cp/Cv) has been given in many text-books as the value of the specific heat of acetylene, whereas this value should obviously be only about one-fourth or one-fifth of 1.26.
By employing the ordinary gas laws it is possible approximately to calculate the specific heat of acetylene from Maneuvrier and Fournier's ratio. Taking the molecular weight of acetylene as 26, we have
26 Cp - 26 Cv = 2 cal.,
Cp = 1.26 Cv.
From this it follows that C_p, _i.e._, the specific heat at constant pressure of acetylene, should be 0.373.] It will be seen that this value for _k_ differs considerably from the corresponding ratio in the case of air and many common gases, where it is usually 1.41; the figure approaches more closely that given for nitrous oxide. For the specific heat of calcium carbide Carlson quotes the following figures:
0 deg. 1000 deg. 1500 deg. 2000 deg. 2500 deg. 3000 deg. 3500 deg. 0.247 0.271 0.296 0.325 0.344 0.363 0.381
The molecular volume of acetylene is 0.8132 (oxygen = 1).
According to the international atomic weights adopted in 1908, the molecular weight of acetylene is 26.016 if O = 16; in round numbers, as ordinarily used, it is 26. Employing the latest data for the weight of 1 litre of dry hydrogen and of dry normal air containing 0.04 per cent. of carbon dioxide at a temperature of 0 deg. C. and a barometric pressure of 760 mm. in the latitude of London, viz., 0.089916 and 1.29395 grammes respectively (Castell-Evans), it now becomes possible to give the weight of a known volume of dry or moist acetylene as measured under stated conditions with some degree of accuracy. Using 26.016 as the molecular weight of the gas (O = 16), 1 litre of dry acetylene at 0 deg. C. and 760 mm. weighs 1.16963 grammes, or 1 gramme measures 0.854973 litre. From this it follows that the theoretical specific gravity of the gas at 0 deg./0 deg. C. is 0.9039 (air = 1), a figure which may be compared with Leduc's experimental value of 0.9056. Taking as the coefficient of expansion at constant pressure the figure already given, viz., 0.003738, the weights and measures of dry and moist acetylene observed under British conditions (60 deg. F. and 30 inches of mercury) become approximately:
Dry. Saturated. 1 litre . . . 1.108 grm. . . 1.102 grm. 1 gramme . . . 0.902 litre. . . 0.907 litre. 1000 cubic feet . 69.18 lb. . . . 68.83 lb.
It should be remembered that unless the gas has been passed through a chemical drier, it is always saturated with aqueous vapour, the amount of water present being governed by the temperature and pressure. The 1 litre of moist acetylene which weighs 1.102 gramme at 60 deg. F. and 30 inches of mercury, contains 0.013 gramme of water vapour; and therefore the weight of dry acetylene in the 1 litre of moist gas is 1.089 gramme. Similarly, the 68.83 pounds which constitute the weight of 1000 cubic feet of moist acetylene, as measured under British standard conditions, are composed of almost exactly 68 pounds of dry acetylene and 0.83 pound of water vapour. The data required in calculating the mass of vapour in a known volume of a saturated gas at any observed temperature and pressure, i.e., in reducing the figures to those which represent the dry gas at any other (standard) temperature and pressure, will be found in the text-books of physical chemistry. It is necessary to recollect that since coal-gas is measured wet, the factors given in the table quoted in Chapter XIV. from the "Notification of the Gas Referees" simply serve to convert the volume of a wet gas observed under stated conditions to the equivalent volume of the same wet gas at the standard conditions mentioned.
HEAT OF COMBUSTION, &C—Based on Berthelot and Matignon's value for the heat of combustion which is given on a subsequent page, viz., 315.7 large calories per molecular weight of 26.016 grammes, the calorific power of acetylene under different conditions is shown in the following table:
Dry. Dry. Saturated. 0 deg. C. & 760 mm. 60 deg. F & 30 ins. 60 deg. F. & 30 ins.
1 gramme 12.14 cals. 12.14 cals. 12.0 cals. 1 litre 14.l9 " 13.45 " 13.22 " 1 cubic foot 40.19 " 380.8 " 374.4 "
The figures in the last column refer to the dry acetylene in the gas, no correction having been made for the heat absorbed by the water vapour present. As will appear in Chapter X., the average of actual determinations of the calorific value of ordinary acetylene is 363 large calories or 1440 B.Th.U. per cubic foot. The temperature of ignition of acetylene has been generally stated to be about 480 deg. C. V. Meyer and Muench in 1893 found that a mixture of acetylene and oxygen ignited between 509 deg. and 515 deg. C. Recent (1909) investigations by H. B. Dixon and H. F. Coward show, however, that the ignition temperature in neat oxygen is between 416 deg. and 440 deg. (mean 428 deg. C.) and in air between 406 deg. and 440 deg., with a mean of 429 deg. C. The corresponding mean temperature of ignition found by the same investigators for other gases are: hydrogen, 585 deg.; carbon monoxide, moist 664 deg., dry 692 deg.; ethylene, in oxygen 510 deg., in air 543 deg.; and methane, in oxygen between 550 deg. and 700 deg., and in air, between 650 deg. and 750 deg. C.
Numerous experiments have been performed to determine the temperature of the acetylene flame. According to an exhaustive research by L. Nichols, when the gas burns in air it attains a maximum temperature of 1900 deg. C. +- 20 deg., which is 120 deg. higher than the temperature he found by a similar method of observation for the coal-gas flame (fish-tail burner). Le Chatelier had previously assigned to the acetylene flame a temperature between 2100 deg. and 2400 deg., while Lewes had found for the dark zone 459 deg., for the luminous zone 1410 deg., and for the tip 1517 deg. C, Fery and Mahler have also made measurements of the temperatures afforded by acetylene and other fuels, some of their results being quoted below. Fery employed his optical method of estimating the temperature, Mahler a process devised by Mallard and Le Chatelier. Mahler's figures all relate to flames supplied with air at a temperature of 0 deg. C. and a constant pressure of 760 mm.
Hydrogen . . . . . . . . . . . 1900 1960 Carbon monoxide . . . . . . . . . 2100 Methane . . . . . . . . . . . 1850 Coal-gas (luminous) . . . . . . . . 1712 " (atmospheric, with deficient supply of air) . 1812 1950 " (atmospheric, with full supply of air) . . 1871 Water-gas . . . . . . . . . . 2000 Oxy-coal-gas blowpipe . . . . . . . 2200 Oxy-hydrogen blowpipe . . . . . . . 2420 Acetylene . . . . . . . . . . 2548 2350 Alcohol . . . . . . . . . . . 1705 1700 Alcohol (in Denayrouze Bunsen) . . . . . 1862 Alcohol and petrol in equal parts . . . . 2053 Crude petroleum (American) . . . . . . 2000 Petroleum spirit " . . . . . . . 1920 Petroleum oil " . . . . . . . 1660
Catani has published the following determinations of the temperature yielded by acetylene when burnt with cold and hot air and also with oxygen:
Acetylene and cold air . . . . . . 2568 deg. C. " air at 500 deg. C . . . . 2780 deg. C. " air at 1000 deg. C . . . . 3000 deg. C. " oxygen . . . . . . 4160 deg. C.
EXPLOSIVE LIMITS.—The range of explosibility of mixtures of acetylene and air has been determined by various observers. Eitner's figures for the lower and upper explosive limits, when the mixture, at 62.6 deg. F., is in a tube 19 mm. in diameter, and contains 1.9 per cent. of aqueous vapour, are 3.35 and 52.3 per cent. of acetylene (cf. Chapter X.). In this case the mixture was fired by electric spark. In wider vessels, the upper explosive limit, when the mixture was fired by a Bunsen flame, was found to be as high as 75 per cent. of acetylene. Eitner also found that when 13 of the 21 volumes of oxygen in air are displaced by carbon dioxide, a mixture of such "carbon dioxide air" with acetylene is inexplosive in all proportions. Also that when carbon dioxide is added to a mixture of acetylene and air, an explosion no longer occurs when the carbon dioxide amounts to 46 volumes or more to every 54 volumes of air, whatever may be the proportion of acetylene in the mixture. [Footnote: According to Caro, if acetylene is added to a mixture composed of 55 per cent. by volume of air and 45 per cent. of carbon dioxide, the whole is only explosive when the proportion of acetylene lies between 5.0 and 5.8 per cent. Caro has also quoted the effect of various inflammable vapours upon the explosive limits of acetylene, his results being referred to in Chapter X.] These figures are valuable in connexion with the prevention of the formation of explosive mixtures of air and acetylene when new mains or plant are being brought into operation (cf. Chapter VII.). Eitner has also shown, by direct investigation on mixtures of other combustible gases and air, that the range of explosibility is greatly reduced by increase in the proportion of aqueous vapour present. As the proportion of aqueous vapour in gas standing over water increases with the temperature the range of explosibility of mixtures of a combustible gas and air is naturally and automatically reduced when the temperature rises, provided the mixture is in contact with water. Thus at 17.0 deg. C., mixtures of hydrogen, air, and aqueous vapour containing from 9.3 to 65.0 per cent, of hydrogen are explosive, whereas at 78.1 deg. C., provided the mixture is saturated with aqueous vapour, explosion occurs only when the percentage of hydrogen in the mixture is between 11.2 and 21.9. The range of explosibility of mixtures of acetylene and air is similarly reduced by the addition of aqueous vapour (though the exact figures have not been experimentally ascertained); and hence it follows that when the temperature in an acetylene generator in which water is in excess, or in a gasholder, rises, the risk of explosion, if air is mixed with the gas, is automatically reduced with the rise in temperature by reason of the higher proportion of aqueous vapour which the gas will retain at the higher temperature. This fact is alluded to in Chapter II. Acetone vapour also acts similarly in lowering the upper explosive limit of acetylene (cf. Chapter XI.).
It may perhaps be well to indicate briefly the practical significance of the range of explosibility of a mixture of air and a combustible gas, such as acetylene. The lower explosive limit is the lowest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in less than that percentage explosion is impossible. The upper explosive limit is the highest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in more than that percentage explosion is impossible. Mixtures, however, in which the percentage of combustible gas lies between these two limits will explode when a light or spark is applied to them; and the comprehensive term "range of explosibility" is used to cover all lying between the two explosive limits. If, then, a naked light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is below the lower limit of explosibility, the gas will not take fire, but the light will continue to burn, deriving its necessary oxygen from the excess of air present. On the other hand, if a light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is above the upper limit of explosibility, the light will be extinguished, and within the vessel the gaseous mixture will not burn; but it may burn at the open mouth of the vessel as it comes in contact with the surrounding air, until by diffusion, &c., sufficient air has entered the vessel to form, with the remaining gas, a mixture lying within the explosive limits, when an explosion will occur. Again, if a gaseous mixture containing less of its combustible constituent than is necessary to attain the lower explosive limit escapes from an open-ended pipe and a light is applied to it, the mixture will not burn as a useful compact flame (if, indeed, it fires at all); if the mixture contains more of its combustible constituent than is required to attain the upper explosive limit, that mixture will burn quietly at the mouth of the pipe and will be free from any tendency to fire back into the pipe—assuming, of course, that the gaseous mixture within the pipe is constantly travelling towards the open end. If, however, a gaseous mixture containing a proportion of its combustible constituent which lies between the lower and the upper explosive limit of that constituent escapes from an open- ended pipe and a light is applied, the mixture will fire and the flame will pass back into the pipe, there to produce an explosion, unless the orifice of the said pipe is so small as to prevent the explosive wave passing (as is the case with a proper acetylene burner), or unless the pipe itself is so narrow as appreciably to alter the range of explosibility by lowering the upper explosive limit from its normal value.
By far the most potent factor in altering the range of explosibility of any gas when mixed with air is the diameter of the vessel containing or delivering such mixture. Le Chatelier has investigated this point in the case of acetylene, and his values are reproduced overleaf; they are comparable among themselves, although it will be observed that his absolute results differ somewhat from those obtained by Eitner which are quoted later:
Explosive Limits of Acetylene mixed with Air.—(Le Chatelier.)
Explosive Limits. Diameter of Tube Range of in Millimetres. Explosibility. Lower. Upper. Per Cent. Per Cent. Per Cent. 40 2.9 64 61.1 30 3.1 62 58.9 20 3.5 55 51.5 6 4.0 40 36.0 4 4.5 25 20.5 2 5.0 15 10.0 0.8 7.7 10 2.3 0.5 ... ... ...
Thus it appears that past an orifice or constriction 0.5 mm. in diameter no explosion of acetylene can proceed, whatever may be the proportions between the gas and the air in the mixture present.
With every gas the explosive limits and the range of explosibility are also influenced by various circumstances, such as the manner of ignition, the pressure, and other minor conditions; but the following figures for mixtures of air and different combustible gases were obtained by Eitner under similar conditions, and are therefore strictly comparable one with another. The conditions were that the mixture was contained in a tube 19 mm. (3/4-inch) wide, was at about 60 deg. to 65 deg. F., was saturated with aqueous vapour, and was fired by electric spark.
Table giving the Percentage by volume of Combustible Gas in a Mixture of that Gas and Air corresponding with the Explosive Limits of such a Mixture.—(Eitner.)
Description of Lower Upper Difference between the Combustible Gas. Explosive Explosive Lower and Upper Limits, Limit. Limit. showing the range covered by the Explosive Mixtures. Per Cent. Per Cent. Per Cent. Carbon monoxide 16.50 74.95 58.45 Hydrogen 9.45 66.40 57.95 Water-gas (uncarburetted) 12.40 66.75 54.35 ACETYLENE 3.35 52.30 48.95 Coal-gas 7.90 19.10 11.20 Ethylene 4.10 14.60 10.50 Methane 6.10 12.80 6.70 Benzene (vapour) 2.65 6.50 3.85 Pentane " 2.40 4.90 2.50 Benzoline " 2.40 4.90 2.50
These figures are of great practical significance. They indicate that a mixture of acetylene and air becomes explosive (i.e., will explode if a light is applied to it) when only 3.35 per cent. of the mixture is acetylene, while a similar mixture of coal-gas and air is not explosive until the coal-gas reaches 7.9 per cent. of the mixture. And again, air may be added to coal-gas, and it does not become explosive until the coal-gas is reduced to 19.1 per cent. of the mixture, while, on the contrary, if air is added to acetylene, the mixture becomes explosive as soon as the acetylene has fallen to 52.3 per cent. Hence the immense importance of taking precautions to avoid, on the one hand, the escape of acetylene into the air of a room, and, on the other hand, the admixture of air with the acetylene in any vessel containing it or any pipe through which it passes. These precautions are far more essential with acetylene than with coal-gas. The table shows further how great is the danger of explosion if benzene, benzoline, or other similar highly volatile hydrocarbons [Footnote: The nomenclature of the different volatile spirits is apt to be very confusing. "Benzene" is the proper name for the most volatile hydrocarbon derived from coal-tar, whose formula is C6H6. Commercially, benzene is often known as "benzol" or "benzole"; but it would be generally advantageous if those latter words were only used to mean imperfectly rectified benzene, i.e., mixtures of benzene with toluene, &c., such as are more explicitly understood by the terms "90.s benzol" and "50.s benzol." "Gasoline," "carburine," "petroleum ether," "benzine," "benzoline," "petrol," and "petroleum spirit" all refer to more or less volatile (the most volatile being mentioned first) and more or less thoroughly rectified products obtained from petroleum. They are mixtures of different hydrocarbons, the greater part of them having the general chemical formula CnH2n+2 where n = 5 or more. None of them is a definite chemical compound as is benzene; when n = 5 only the product is pentane. These hydrocarbons are known to chemists as "paraffins," "naphthenes" being occasionally met with; while a certain proportion of unsaturated hydrocarbons is also present in most petroleum spirits. The hydrocarbons of coal-tar are "aromatic hydrocarbons," their generic formula being CnH2^n-6, where n is never less than 6.] are allowed to vaporise in a room in which a light may be introduced. Less of the vapour of these hydrocarbons than of acetylene in the air of a room brings the mixture to the lower explosive limit, and therewith subjects it to the risk of explosion. This tact militates strongly against the use of such hydrocarbons within a house, or against the use of air-gas, which, as explained in Chapter I., is air more or less saturated with the vapour of volatile hydrocarbons. Conversely, a combustible gas, such as acetylene, may be safely "carburetted" by these hydrocarbons in a properly constructed apparatus set up outside the dwelling-house, as explained in Chapter X., because there would be no air (as in air-gas) in the pipes, &c., and a relatively large escape of carburetted acetylene would be required to produce an explosive atmosphere in a room. Moreover, the odour of the acetylene itself would render the detection of a leak far easier with carburetted acetylene than with air-gas.
N. Teclu has investigated the explosive limits of mixtures of air with certain combustible gases somewhat in the same manner as Eitner, viz.: by firing the mixture in an eudiometer tube by means of an electric spark. He worked, however, with the mixture dry instead of saturated with aqueous vapour, which doubtless helps to account for the difference between his and Eitner's results.
Table giving the Percentages by volume of Combustible Gas in a Dehydrated Mixture of that Gas and Air between which the Explosive Limits of such a Mixture lie.—(Teclu).
Lower Explosive Limit. Upper Explosive Limit. Description of Combustible Gas. Per Cent. of Gas. Per Cent. of Gas. ACETYLENE 1.53-1.77 57.95-58.65 Hydrogen 9.73-9.96 62.75-63.58 Coal-gas 4.36-4.82 23.35-23.63 Methane 3.20-3.67 7.46- 7.88
Experiments have been made at Lechbruch in Bavaria to ascertain directly the smallest proportion of acetylene which renders the air of a room explosive. Ignition was effected by the flame resulting when a pad of cotton-wool impregnated with benzoline or potassium chlorate was fired by an electrically heated wire. The room in which most of the tests were made was 8 ft. 10 in. long, 6 ft. 7 in. wide, and 6 ft. 8 in. high, and had two windows. When acetylene was generated in this room in normal conditions of natural ventilation through the walls, the volume generated could amount to 3 per cent. of the air-space of the room without explosion ensuing on ignition of the wool, provided time elapsed for equable diffusion, which, moreover, was rapidly attained. Further, it was found that when the whole of the acetylene which 2 kilogrammes or 4.4 lb. of carbide (the maximum permissible charge in many countries for a portable lamp for indoor use) will yield was liberated in a room, a destructive explosion could not ensue on ignition provided the air-space exceeded 40 cubic metres or 1410 cubic feet, or, if the evolved gas were uniformly diffused, 24 cubic metres or 850 cubic feet. When the walls of the room were rendered impervious to air and gas, and acetylene was liberated, and allowed time for diffusion, in the air of the room, an explosion was observed with a proportion of only 2-1/2 per cent. of acetylene in the air.
Solubility of Acetylene in Various Liquids.
Volumes of Tem- Acetylene Solvent. perature. dissolved by Authority. 100 Vols. of Solvent. Degs. C Acetone . . . . 15 2500 Claude and Hess " . . . . 50 1250 " Acetic acid; alcohol . 18 600 Berthelot Benzoline; chloroform . 18 400 " Paraffin oil . . . 0 103.3 E. Muller " . . . 18 150 Berthelot Olive oil . . . . 48 Fuchs and Schiff Carbon bisulphide . . 18 100 Berthelot " tetrachloride . 0 25 Nieuwland Water (at 4 65 atmospheres pressure) . . 0 160 Villard " (at 755 mm. pressure) 12 118 Berthelot " (760 mm. pressure) . 12 106.6 E. Mueller " " . 15 110 Lewes " " . 18 100 Berthelot " " . 100 E. Davy (in 1836) " " . 19.5 97.5 E. Mueller Milk of lime: about 10 grammes of calcium hy- 5 112 Hammerschmidt droxide per 100 c.c. . and Sandmann " " " 10 95 " " " " 20 75 " " " " 50 38 " " " " 70 20 " " " " 90 6 " Solution of common salt,5% 19 67.9 " (sodium chloride) " 25 47.7 " " 20% 19 29.6 " " " 25 12.6 " "(nearly saturated, 26%) . . 15 20.6 " "(saturated, sp. gr. 1-21) . . 0 22.0 E. Mueller " " " 12 21.0 " " " " 18 20.4 " Solution of calcium Hammerschmidt chloride (saturated) . 15 6.0 and Sandmann Berge and Reychler's re- agent . . . . 95 Nieuwland
SOLUBILITY.—Acetylene is readily soluble in many liquids. It is desirable, on the one hand, as indicated in Chapter III., that the liquid in the seals of gasholders, &c., should be one in which acetylene is soluble to the smallest degree practically attainable; while, on the other hand, liquids in which acetylene is soluble in a very high degree are valuable agents for its storage in the liquid state. Hence it is important to know the extent of the solubility of acetylene in a number of liquids. The tabular statement (p. 179) gives the most trustworthy information in regard to the solubilities under the normal atmospheric pressure of 760 mm. or thereabouts.
The strength of milk of lime quoted in the above table was obtained by carefully allowing 50 grammes of carbide to interact with 550 c.c. of water at 5 deg. C. A higher degree of concentration of the milk of lime was found by Hammerschmidt and Sandmann to cause a slight decrease in the amount of acetylene held in solution by it. Hammerschmidt and Sandmann's figures, however, do not agree well with others obtained by Caro, who has also determined the solubility of acetylene in lime-water, using first, a clear saturated lime-water prepared at 20 deg. C. and secondly, a milk of lime obtained by slaking 10 grammes of quicklime in 100 c.c. of water. As before, the figures relate to the volumes of acetylene dissolved at atmospheric pressure by 100 volumes of the stated liquid.
_________ Temperature. Lime-water. Milk of Lime. ___ ___ ____ Degs C. 0 146.2 152.6 5 138.5 15 122.8 134.8 50 43.9 62.6 90 6.2 9.2 ___ ___ ____
Figures showing the solubility of acetylene in plain water at different temperatures have been published in Landolt-Boernstein's Physico- Chemical Tables. These are reproduced below. The "Coefficient of Absorption" is the volume of the gas, measured at 0 deg. C. and a barometric height of 760 mm. taken up by one volume of water, at the stated temperature, when the gas pressure on the surface, apart from the vapour pressure of the water itself, is 760 mm. The "Solubility" is the weight of acetylene in grammes taken up by 100 grammes of water at the stated temperature, when the total pressure on the surface, including that of the vapour pressure of the water, is 760 mm.
Temperature. Coefficient of Solubility. Absorption. Degs. C. 0 1.73 0.20 1 1.68 0.19 2 1.63 0.19 3 1.58 0.18 4 1.53 0.18 5 1.49 0.17 6 1.45 0.17 7 1.41 0.16 8 1.37 0.16 9 1.34 0.15 10 1.31 0.15 11 1.27 0.15 12 1.24 0.14 13 1.21 0.14 14 1.18 0.14 15 1.15 0.13 16 1.13 0.13 17 1.10 0.13 18 1.08 0.12 19 1.05 0.12 20 1.03 0.12 21 1.01 0.12 22 0.99 0.11 23 0.97 0.11 24 0.95 0.11 25 0.93 0.11 26 0.91 0.10 27 0.89 0.10 28 0.87 0.10 29 0.85 0.10 30 0.84 0.09
Advantage is taken, as explained in Chapter XI., of the high degree of solubility of acetylene in acetone, to employ a solution of the gas in that liquid when acetylene is wanted in a portable condition. The solubility increases very rapidly with the pressure, so that under a pressure of twelve atmospheres acetone dissolves about 300 times its original volume of the gas, while the solubility also increases greatly with a reduction in the temperature, until at -80 deg. C. acetone takes up 2000 times its volume of acetylene under the ordinary atmospheric pressure. Further details of the valuable qualities of acetone as a solvent of acetylene are given in Chapter XI., but it may here be remarked that the successful utilisation of the solvent power of acetone depends to a very large extent on the absolute freedom from moisture of both the acetylene and the acetone, so that acetone of 99 per cent. strength is now used as the solvent.
Turning to the other end of the scale of solubility, the most valuable liquids for serving as seals of gasholders, &c., are readily discernible. Far superior to all others is a saturated solution of calcium chloride, and this should be selected as the confining liquid whenever it is important to avoid dissolution of acetylene in the liquid as far as may be. Brine comes next in order of merit for this purpose, but it is objectionable on account of its corrosive action on metals. Olive oil should, according to Fuchs and Schiff, be of service where a saline liquid is undesirable; mineral oil seems useless. Were they concordant, the figures for milk of lime would be particularly useful, because this material is naturally the confining liquid in the generating chambers of carbide-to-water apparatus, and because the temperature of the liquid rises through the heat evolved during the generation of the gas (vide Chapters II. and III.). It will be seen that these figures would afford a means of calculating the maximum possible loss of gas by dissolution when a known volume of sludge is run off from a carbide-to- water generator at about any possible temperature.
According to Garelli and Falciola, the depression in the freezing-point of water caused by the saturation of that liquid with acetylene is 0.08 deg. C., the corresponding figure for benzene in place of water being 1.40 deg. C. These figures indicate that 100 parts by weight of water should dissolve 0.1118 part by weight of acetylene at 0 deg. C., and that 100 parts of benzene should dissolve about 0.687 part of acetylene at 5 deg. C. In other words, 100 volumes of water at the freezing-point should dissolve 95 volumes of acetylene, and 100 volumes of benzene dissolve some 653 volumes of the gas. The figure calculated for water in this way is lower than that which might be expected from the direct determinations at other temperatures already referred to; that for benzene may be compared with Berthelot's value of 400 volumes at 18 deg. C. Other measurements of the solubility of acetylene in water at 0 deg. C. have given the figure 0.1162 per cent. by weight.
TOXICITY.—Many experiments have been made to determine to what extent acetylene exercises a toxic action on animals breathing air containing a large proportion of it; but they have given somewhat inconclusive results, owing probably to varying proportions of impurities in the samples of acetylene used. The sulphuretted hydrogen and phosphine which are found in acetylene as ordinarily prepared are such powerful toxic agents that they would always, in cases of "acetylene" poisoning, be largely instrumental in bringing about the effects observed. Acetylene per se would appear to have but a small toxic action; for the principal toxic ingredient in coal-gas is carbon monoxide, which does not occur in sensible quantity in acetylene as obtained from calcium carbide. The colour of blood is changed by inhalation of acetylene to a bright cherry-red, just as in cases of poisoning by carbon monoxide; but this is due to a more dissolution of the gas in the haemoglobin of the blood, so that there is much more hope of recovery for a subject of acetylene poisoning than for one of coal-gas poisoning. Practically the risk of poisoning by acetylene, after it has been purified by one of the ordinary means, is nil. The toxic action of the impurities of crude acetylene is discussed in Chapter V.
Acetylene is an "endothermic" compound, as has been mentioned in Chapter II., where the meaning of the expression endothermic is explained. It has there been indicated that by reason of its endothermic nature it is unsafe to have acetylene at either a temperature of 780 deg. C. and upwards, or at a pressure of two atmospheres absolute, or higher. If that temperature or that pressure is exceeded, dissociation (i.e., decomposition into its elements), if initiated at any spot, will extend through the whole mass of acetylene. In this sense, acetylene at or above 780 deg. C., or at two or more atmospheres pressure, is explosive in the absence of air or oxygen, and it is thereby distinguished from the majority of other combustible gases, such as the components of coal-gas. But if, by dilution with another gas, the partial pressure of the acetylene is reduced, then the mixture may be subjected to a higher pressure than that of two atmospheres without acquiring explosiveness, as is fully shown in Chapter XI. Thus it becomes possible safely to compress mixtures of acetylene and oil-gas or coal-gas, whereas unadmixed acetylene cannot be safely kept under a pressure of two atmospheres absolute or more. In a series of experiments carried out by Dupre on behalf of the British Home Office, and described in the Report on Explosives for 1897, samples of moist acetylene, free from air, but apparently not purified by any chemical process, were exposed to the influence of a bright red-hot wire. When the gas was held in the containing vessel at the atmospheric pressure then obtaining, viz., 30.34 inches (771 mm.) of mercury, no explosion occurred. When the pressure was raised to 45.34 inches (1150 mm.), no explosion occurred; but when the pressure was further raised to 59.34 inches (1505 mm., or very nearly two atmospheres absolute) the acetylene exploded, or dissociated into its elements.
Acetylene readily polymerises when heated, as has been stated in Chapter II., where the meaning of the term "polymerisation" has been explained. The effects of the products of the polymerisation of acetylene on the flame produced when the gas is burnt at the ordinary acetylene burners have been stated in Chapter VIII., where the reasons therefor have been indicated. The chief primary product of the polymerisation of acetylene by heat appears to be benzene. But there are also produced, in some cases by secondary changes, ethylene, methane, naphthalene, styrolene, anthracene, and homologues of several of these hydrocarbons, while carbon and hydrogen are separated. The production of these bodies by the action of heat on acetylene is attended by a reduction of the illuminative value of the gas, while owing to the change in the proportion of air required for combustion (see Chapter VIII.), the burners devised for the consumption of acetylene fail to consume properly the mixture of gases formed by polymerisation from the acetylene. It is difficult to compare the illuminative value of the several bodies, as they cannot all be consumed economically without admixture, but the following table indicates approximately the maximum illuminative value obtainable from them either by combustion alone or in admixture with some non- illuminating or feebly-illuminating gas:
Candles per Cubic Foot (say) Acetylene C2H2 50 Hydrogen H2 0 Methane CH4 1 Ethane C2H6 7 Propane C3H8 11 Pentane C5H12 (vapour) 35 Hexane C6H14 " 45 Ethylene C2H4 20 Propylene C3H6 25 Benzene C6H6 (vapour) 200 Toluene C7H8 " 250 Naphthalene C10H8 " 400
It appears from this table that, with the exception of the three hydrocarbons last named, no substance likely to be formed by the action of heat on acetylene has nearly so high an illuminative value—volume for volume—as acetylene itself. The richly illuminating vapours of benzene and naphthalene (and homologues) cannot practically add to the illuminative value of acetylene, because of the difficulty of consuming them without smoke, unless they are diluted with a large proportion of feebly- or non-illuminating gas, such as methane or hydrogen. The practical effect of carburetting acetylene with hydrocarbon vapours will be shown in Chapter X. to be disastrous so far as the illuminating efficiency of the gas is concerned. Hence it appears that no conceivable products of the polymerisation of acetylene by heat can result in its illuminative value being improved—even presupposing that the burners could consume the polymers properly—while practically a considerable deterioration of its value must ensue.
The heat of combustion of acetylene was found by J. Thomson to be 310.57 large calories per gramme-molecule, and by Berthelot to be 321.00 calories. The latest determination, however, made by Berthelot and Matignon shows it to be 315.7 calories at constant pressure. Taking the heat of formation of carbon dioxide from diamond carbon at constant pressure as 94.3 calories (Berthelot and Matignon), which is equal to 97.3 calories from amorphous carbon, and the heat of formation of liquid water as 69 calories; this value for the heat of combustion of acetylene makes its heat of formation to be 94.3 x 2 + 69 - 315.7 = -58.1 large calories per gramme-molecule (26 grammes) from diamond carbon, or -52.1 from amorphous carbon. It will be noticed that the heat of combustion of acetylene is greater than the combined heats of combustion of its constituents; which proves that heat has been absorbed in the union of the hydrogen and carbon in the molecule, or that acetylene is endothermic, as elsewhere explained. These calculations, and others given in Chapter IX., will perhaps be rendered more intelligible by the following table of thermochemical phenomena:
___________ Reaction. Diamond Amorphous Carbon. Carbon. ______ __ ___ __ (1) C (solid) + O . . . 26.1 29.1 ... (2) C (solid) + O_2 . . . 94.3 97.3 ... (3) CO + O (2 - 1) . . . ... ... 68.2 (4) Conversion of solid carbon into gas (3 - 1) . . . 42.1 39.1 ... (5) C (gas) + O (1 + 4) . . ... ... 68.2 (6) Conversion of amorphous carbon to diamond . . ... ... 3.0 (7) C_2 + H_2 . . . . -58.1 -52.1 ... (8) C_2H_2 + 2-1/2O_2 . . ... ... 315.7 ______ __ ___ __
W. G. Mixter has determined the heat of combustion of acetylene to be 312.9 calories at constant volume, and 313.8 at constant pressure. Using Berthelot and Matignon's data given above for amorphous carbon, this represents the heat of formation to be -50.2 (Mixter himself calculates it as -51.4) calories. By causing compressed acetylene to dissociate under the influence of an electric spark, Mixter measured its heat of formation as -53.3 calories. His corresponding heats of combustion of ethylene are 344.6 calories (constant volume) and 345.8 (constant pressure); for its heat of formation he deduces a value -7.8, and experimentally found one of about -10.6 (constant pressure).
THE ACETYLENE FLAME.—It has been stated in Chapter I. that acetylene burnt in self-luminous burners gives a whiter light than that afforded by any other artificial illuminant, because the proportion of the various spectrum colours in the light most nearly resembles the corresponding proportion found in the direct rays of the sun. Calling the amount of monochromatic light belonging to each of the five main spectrum colours present in the sun's rays unity in succession, and comparing the amount with that present in the light obtained from electricity, coal-gas, and acetylene, Muensterberg has given the following table for the composition of the several lights mentioned:
____________ Electricity Coal-Gas Acetylene ___ ___ ___ __ Colour in With Spectrum. Arc. Incan- Lumin- Incan- Alone. 3 per Sun- descent. ous. descent. Cent. light. Air. __ _ __ __ __ __ __ __ Red 2.09 1.48 4.07 0.37 1.83 1.03 1 Yellow 1.00 1.00 1.00 0.90 1.02 1.02 1 Green 0.99 0.62 0.47 4.30 0.76 0.71 1 Blue 0.87 0.91 1.27 0.74 1.94 1.46 1 Violet 1.08 0.17 0.15 0.83 1.07 1.07 1 Ultra- Violet 1.21 ... ... ... ... ... 1 __ _ __ __ __ __ __ __
These figures lack something in explicitness; but they indicate the greater uniformity of the acetylene light in its proportion of rays of different wave-lengths. It does not possess the high proportion of green of the Welsbach flame, or the high proportion of red of the luminous gas- flame. It is interesting to note the large amount of blue and violet light in the acetylene flame, for these are the colours which are chiefly concerned in photography; and it is to their prominence that acetylene has been found to be so very actinic. It is also interesting to note that an addition of air to acetylene tends to make the light even more like that of the sun by reducing the proportion of red and blue rays to nearer the normal figure.
H. Erdmann has made somewhat similar calculation, comparing the light of acetylene with that of the Hefner (amyl acetate) lamp, and with coal-gas consumed in an Argand and an incandescent burner. Consecutively taking the radiation of the acetylene flame as unity for each of the spectrum colours, his results are:
Coal-Gas Colour in Wave-Lengths, Spectrum uu Hefner Light Argand Incandescent Red 650 1.45 1.34 1.03 Orange 610 1.22 1.13 1.00 Yellow 590 1.00 1.00 1.00 Green 550 0.87 0.93 0.86 Blue 490 0.72 1.27 0.92 Violet 470 0.77 1.35 1.73
B. Heise has investigated the light of different flames, including acetylene, by a heterochromatic photometric method; but his results varied greatly according to the pressure at which the acetylene was supplied to the burner and the type of burner used. Petroleum affords light closely resembling in colour the Argand coal-gas flame; and electric glow-lamps, unless overrun and thereby quickly worn out, give very similar light, though with a somewhat greater preponderance of radiation in the red and yellow.
Percent of Total Light. Energy manifested Observer. as Light. Candle, spermaceti . . 2.1 Thomsen " paraffin . . . 1.53 Rogers Moderator lamp . . . 2.6 Thomsen Coal-gas . . . . . 1.97 Thomsen " . . . . . 2.40 Langley " batswing . . . 1.28 Rogers " Argand . . . 1.61 Rogers " incandesce . . 2 to 7 Stebbins Electric glow-lamp . . about 6 Merritt " " . . 5.5 Abney and Festing Lime light (new) . . . 14 Orehore " (old) . . . 8.4 Orehore Electric arc . . . . 10.4 Tyndall; Nakano " . . . . 8 to 13 Marks Magnesium light . . . 12.5 Rogers Acetylene . . . . 10.5 Stewart and Hoxie " (No. 0 slit burner 11.35 Neuberg " (No. 00000 . . Bray fishtail) 13.8 Neuberg " (No. 3 duplex) . 14.7 Neuberg Geissler tube . . . 32.0 Staub