Acetylene, the Principles of Its Generation and Use · Chapter 11The General Principles of Acetylene Generation--Acetylene Generating Apparatus (5)
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Especially when a combined washer and non-return valve follows immediately after a generator belonging to the shoot type, and the mouth of the shoot is open to the air in the plant-house, it is highly desirable that the washer shall be fitted with some arrangement of an automatic kind for preventing the water level rising much above its proper position. The liquid in a closed washer tends to rise as the apparatus remains in use, water vapour being condensed within it and liquid water, or froth of lime, being mechanically carried forward by the stream of acetylene coming from the decomposing chamber. In course of time, therefore, the vertical depth to which the gas-inlet pipe in the washer is sealed by the liquid increases; and it may well be that eventually the depth in question, plus the pressure thrown by the holder bell, may become greater than the pressure which can be set up inside the generator without danger of gas slipping under the lower edge of the shoot. Should this state of things arise, the acetylene can no longer force its way through the washer into the holder bell, but will escape from the mouth of the shoot; filling the apparatus-house with gas, and offering every opportunity for an explosion if the attendant disobeys orders and takes a naked light with him to inspect the plant.
It is indispensable that every acetylene apparatus shall be fitted with a safety-valve, or more correctly speaking a vent-pipe. The generator must have a vent-pipe in case the gas-main leading to the holder should become blocked at any time, and the acetylene which continues to be evolved in all water-to-carbide apparatus, even after the supply of water has been cut off be unable to pass away. Theoretically a non-automatic apparatus does not require a vent-pipe in its generator because all the gas enters the holder immediately, and is, or should be, unable to return through the intermediate water seal; practically such a safeguard is absolutely necessary for the reason given. The holder must have a safety-valve in case the cutting-off mechanism of the generator fails to act, and more gas passes into it than it can store. Manifestly the pressure of the gas in a water-sealed holder or in any generator fitted with a water-sealed lid cannot rise above that corresponding with the depth of water in the seal; for immediately the pressure, measured in inches of water, equals the depth of the sealing liquid, the seal will be blown out, and the gas will escape. Such an occurrence, however, as the blowing of a seal must never be possible in any item of an acetylene plant, more especially in those items that are under cover, for the danger that the issuing gas might be fired or might produce suffocation would be extremely great. Typical simple forms of vent-pipe suitable for acetylene apparatus are shown in Fig. 7. In each case the pipe marked "vent" is the so-called safety-valve; it is open at its base for the entry of gas, and open at its top for the escape of the acetylene into the atmosphere, such top being in all instances carried through the roof of the generator-house into the open air, and to a spot distant from any windows of that house or of the residence, where it can prove neither dangerous nor a nuisance by reason of its odour. At A is represented the vent-pipe of a displacement vessel, which may either be part of a displacement holder or of a generator working on the displacement principle. The vent-pipe is rigidly fixed to the apparatus. If gas is generated within the closed portion of the holder or passes through it, and if the pressure so set up remains less than that which is needed to move the water from the level l to the levels l' and l", the mouth of the pipe is under water, and acetylene cannot enter it; but immediately such an amount of gas is collected, or such pressure is produced that the interior level sinks below l", which is that of the mouth of the pipe, it becomes unsealed, and the surplus gas freely escapes. There are two minor points in connexion with this form of vent-pipe often overlooked. At the moment when the water arrives at l" in the closed half of the apparatus, its level in the interior of the vent-pipe stands at l', identical with that in the open hall of the apparatus (for the mouth of the vent-pipe and the water in the open hall of the apparatus are alike exposed to the pressure of the atmosphere only). When the water, then, descends just below l" there is an amount of water inside the pipe equal in height to the distance between l' and l"; and before the acetylene can escape, it must either force this water as a compact mass out of the upper mouth of the vent-pipe (which it is clearly not in a position to do), drive it out of the upper mouth a little at a time, or bubble through it till the water is gradually able to run downwards out of the pipe as its lower opening is more fully unsealed. In practice the acetylene partly bubbles through this water and partly drives it out of the mouth of the pipe; on some occasions temporarily yielding irregular pressures at the burners which cause them to jump, and always producing a gurgling noise in the vent- pipe which in calculated to alarm the attendant. If the pipe is too small in diameter, and especially if its lower orifice is cut off perfectly horizontal and constricted slightly, the water may refuse to escape from the bottom altogether, and the pipe will fail to perform its allotted task. It is better therefore to employ a wide tube, and to cut off its mouth obliquely, or to give its lower extremity the shape of an inverted funnel. At the half of the central divided drawing marked B (Fig. 7) is shown a precisely similar vent-pipe affixed to the bell of a rising holder, which behaves in an identical fashion when by the rising of the bell its lower end is lifted out of the water in the tank. The features described above as attendant, upon the act of unsealing of the displacement-holder vent-pipe occur here also, but to a less degree; for the water remaining in the pipe at the moment of unsealing is only that which corresponds with the vertical distance between l' and l", and in a rising holder this is only a height always equal to the pressure given by the bell. Nevertheless this form of vent-pipe produces a gurgling noise, and would be better for a trumpet-shaped mouth. A special feature of the pipe in B is that unless it is placed symmetrically about the centre of the bell its weight tends to throw the bell out of the vertical, and it may have to be supported at its upper part; conversely, if the pipe is arranged concentrically in the bell, it may be employed as part of the guiding arrangement of the bell itself. Manifestly, as the pipe must be long enough to extend through the roof of the generator-house, its weight materially increases the weight of the bell, and consequently the gas pressure in the service; this fact is not objectionable provided due allowance is made for it. So tall a vent-pipe, however, seriously raises the centre of gravity of the bell and may make it top-heavy. To work well the centre of gravity of a holder bell should be as low as possible, any necessary weighting being provided symmetrically about its circumference and close to its bottom edge. The whole length of an ascending vent-pipe need not be carried by the rising bell, because the lower portion, which must be supported by the bell, can be arranged to slide inside a wider length of pipe which is fixed to the roof of the generator-house at the point where it passes into the open air.
A refinement upon this vent-pipe is represented at C, where it is rigidly fastened to the tank of the holder, and has its internal aperture always above the level of the water in the apparatus. Rigidly fixed to the crown of the bell is a tube of wider diameter, h, which is closed at its upper end. h is always full of gas, and its mouth is normally beneath the level of the water in the seal; but when the bell rises to its highest permissible position, the mouth of h comes above the water, and communication is opened between the holder and the outer atmosphere. No water enters the vent-pipe from the holder, and therefore no gurgling or irregular pressure is produced. Another excellent arrangement of a vent-pipe, suggested by Klinger of Gumpoldskirchen, is shown at D, a drawing which has already been partly considered as a washer and water-seal. For the present purpose the main vessel and its various pipes are so dimensioned that the vertical height g to f is always appreciably greater than the gas pressure in the service or in the generator or gasholder to which it is connected. In these circumstances the gas entering at a depresses the water in the pipe below the level f to an extent equal to the pressure at which it enters that pipe--an extent normally less than the distance f to g; and therefore gas never passes into the body of the vessel, but travels away by the side tube b (which in former references to this drawing was supposed to be absent). If, however, the pressure at a exceeds that of the vertical height f to g, gas escapes at g through the water, and is then free to reach the atmosphere by means of the vent c. As before, d serves to charge the apparatus with water, and e to ensure a proper amount being added. Clearly no liquid can enter the vent-pipe in this device. Safety-valves such as are added to steam-boilers and the like, which consist of a weighted lever holding a conical valve down against its seat, are not required in acetylene apparatus, for the simpler hydraulic seals discussed above can always be fitted wherever they may be needed. It should be noticed that these vent-pipes only come into operation in emergencies, when they are required to act promptly. No economy is to be effected by making them small in diameter. For obvious reasons the vent-pipe of a holder should have a diameter equal to that of the gas-inlet tube, and the vent-pipe of a generator be equal in size to the gas-leading tube.
FROTHING IN GENERATORS.--A very annoying trouble which crops up every now and then during the evolution of acetylene consists in the production of large masses of froth within the generator. In the ordinary way, decomposition of carbide is accompanied by a species of effervescence, but the bubbles should break smartly and leave the surface of the liquid reasonably free from foam. Sometimes, however, the bubbles do not break, but a persistent "head" of considerable height is formed. Further production of gas only increases the thickness of the froth until it rises so high that it is carried forward through the gas-main into the next item of the plant. The froth disappears gradually in the pipes, but leaves in them a deposit of lime which sooner or later causes obstructions by accumulating at the angles and dips; while during its presence in the main the steady passage of gas to the holder is interrupted and the burners may even be made to jump. Manifestly the defect is chiefly, if not always, to be noticed in the working of carbide-to-water generators. The phenomenon has been examined by Mauricheau-Beaupré, who finds that frothing is not characteristic of pure carbide and that it cannot be attributed to any of the impurities normally present in commercial carbide. If, however, the carbide contains calcium chloride, frothing is liable to occur. A 0.1 per cent. solution of calcium chloride appears to yield some foam when carbide is decomposed in it, and a 1 per cent. solution to foam in a pronounced manner. In the absence of calcium chloride, the main cause of frothing seems to be the presence in the generator of new paint or tar. If a generator is taken into use before the paint in any part of it which becomes moistened by warm lime-water has had opportunity of drying thoroughly hard, frothing is certain to occur; and even if the carbide has been stored for only a short time in a tin or drum which has been freshly painted, a production of froth will follow when it is decomposed in water. The products of the polymerisation of acetylene also tend to produce frothing, but not to such an extent as the turpentine in paint and the lighter constituents of coal-tar. Carbide stored even temporarily in a newly painted tin froths on decomposition because it has absorbed among its pores some of the volatile matter given off by the paint during the process of desiccation.
THE "DRY" PROCESS OF GENERATION.--A process for generating acetylene, totally different in principle from those hitherto considered, has been introduced in this country. According to the original patents of G. J. Atkins, the process consisted in bringing small or powdered carbide into mechanical contact with some solid material containing water, the water being either mixed with the solid reagent or attached to it as water of crystallisation. Such reagents indeed were claimed as crude starch and the like, the idea being to recover a by-product of pecuniary value. Now the process seems to be known only in that particular form in which granulated carbide is treated with crystallised sodium carbonate, i.e., common washing soda. Assuming the carbide employed to be chemically pure and the reaction between it and the water of crystallisation contained in ordinary soda crystals to proceed quantitatively, the production of acetylene by the dry process should be represented by the following chemical equation:
5CaC_2 + Na_2CO_3.10H_2O = 5C_2H_2 + 5Ca(OH)_2 + Na_2CO_3.
On calculating out the molecular weights, it will be seen that 286 parts of washing soda should suffice for the decomposition of 320 parts of pure calcium carbide, or in round numbers 9 parts of soda should decompose 10 parts of carbide. In practice, however, it seems to be found that from 1 to 1.5 parts of soda are needed for every part of carbide.
The apparatus employed is a metal drum supported on a hollow horizontal spindle, one end of which is closed and carries a winch handle, and the other end of which serves to withdraw the gas generated in the plant. The drum is divided into three compartments by means of two vertical partitions so designed that when rotation proceeds in one particular direction portions of the two reagents stored in one end compartment pass into the centre compartment; whereas when rotation proceeds in the opposite direction, the material in the centre compartment is merely mixed together, partly by the revolution of the drum, partly with the assistance of a stationary agitator slung loosely from the central spindle. The other end compartment contains coke or sawdust or other dry material through which the gas passes for the removal of lime or other dust carried in suspension as it issues from the generating compartment. The gas then passes through perforations into the central spindle, one end of which is connected by a packed joint with a fixed pipe, which leads to a seal or washer containing petroleum. Approached from a theoretical standpoint, it will be seen that this method of generation entirely sacrifices the advantages otherwise accruing from the use of liquid water as a means for dissipating the heat of the chemical reaction, but on the other hand, inasmuch as the substances are both solid, the reaction presumably occurs more slowly than it would in the presence of liquid water; and moreover the fact that the water employed to act upon the carbide is in the solid state and also more or less combined with the rest of the sodium carbonate molecule, means that, per unit of weight, the water decomposed must render latent a larger amount of heat than it would were it liquid. Experiments made by one of the authors of this book tend to show that the gas evolved from carbide by the dry process contains rather less phosphorus than it might in other conditions of generation, and as a fact gas made by the dry process is ordinarily consumed without previous passage through any chemical purifying agent. It is obvious, however, that the use of the churn described above greatly increases the labour attached to the production of the gas; while it is not clear that the yield per unit weight of carbide decomposed should be as high as that obtained in wet generation. The inventor has claimed that his by-product should be valuable and saleable, apparently partly on the ground that it should contain caustic soda. Evidence, however, that a reaction between the calcium oxide or hydroxide and the sodium carbonate takes place in the prevailing conditions is not yet forthcoming, and the probabilities are that such decomposition would not occur unless the residue were largely diluted with water. [Footnote: The oldest process employed for manufacturing caustic soda consisted in mixing a solution of sodium carbonate with quick or slaked lime, and it has been well established that the causticisation of the soda will not proceed when the concentration of the liquid is greater than that corresponding with a specific gravity of about 1-10, i.e., when the liquid contains more than some 8 to 10 per cent, of sodium hydroxide.] Conversely there are some grounds for believing that the dry residue is less useful than an ordinary wet residue for horticultural purposes, and also for the production of whitewash. From a financial standpoint, the dry process suffers owing to the expense involved in the purchase of a second raw material, for which but little compensation can be discovered unless it is proved that the residue is intrinsically more valuable than common acetylene-lime and can be sold or used advantageously by the ordinary owner of an installation. The discarding of the chemical purifier at the present day is a move of which the advantage may well be overrated.
ARTIFICIAL LIGHTING OF GENERATOR SHEDS.--It has already been argued that all normal or abnormal operations in connexion with an acetylene generating plant should be carried out, if possible, by daylight; and it has been shown that on no account must a naked light ever be taken inside the house containing such a plant. It will occasionally happen, however, that the installation must be recharged or inspected after nightfall. In order to do this in safety, a double window, incapable of being opened, should be fitted in one wall of the house, as far as possible from the door, and in such a position that the light may fall on to all the necessary places. Outside this window may be suspended an ordinary hand- lantern burning oil or paraffin; or, preferably, round this window may be built a closed lantern into which some source of artificial light may be brought. If the acetylene plant has an isolated holder of considerable size, there is no reason at all why a connexion should not be made with the service-pipes, and an acetylene flame be used inside this lantern; but with generators of the automatic variety, an acetylene light is not so suitable, because of the fear that gas may not be available precisely at the moment when it is necessary to have light in the shed. It would, however, be a simple matter to erect an acetylene burner inside the lantern in such a way that when needed an oil-lamp or candle could be used instead. Artificial internal light of any kind is best avoided; the only kind permissible being an electric glow-lamp. If this is employed, it should be surrounded by a second bulb or gas-tight glass jacket, and preferably by a wire cage as well; the wires leading to it must be carefully insulated, and all switches or cut-outs (which may produce a spark) must be out of doors. The well-known Davy safety or miner's lamp is not a trustworthy instrument for use with acetylene because of (a) the low igniting-point of acetylene; (b) the high temperature of its flame; and (c) the enormous speed at which the explosive wave travels through a mixture of acetylene and air. For these reasons the metallic gauze of the Davy lamp is not so efficient a protector of the flame as it is in cases of coal-gas, methane, &c. Moreover, in practice, the Davy lamp gives a poor light, and unless in constant use is liable to be found out of order when required. It should, however, be added that modern forms of the safety lamp, in which the light is surrounded by a stout glass chimney and only sufficient gauze is used for the admission of fresh air and for the escape of the combustion products, appear quite satisfactory when employed in an atmosphere containing some free acetylene.