Acetylene, the Principles of Its Generation and Use · Chapter 9The General Principles of Acetylene Generation--Acetylene Generating Apparatus (3)
This chapter is not translated yet. You are reading the original.
Two other devices intended to give automatic working, which are scarcely capable of classification among their peers, may be diagrammatically shown in Fig. 3. The first of these (D) depends upon the movements of a flexible diaphragm. A vessel (a) of any convenient size and shape is divided into two portions by a thin sheet of metal, leather, caoutchouc, or the like. At its centre the diaphragm is attached by some air-tight joint to the rod c, which, held in position by suitable guides, is free to move longitudinally in sympathy with the diaphragm, and is connected at its lower extremity with a water-supply cock or a carbide-feed gear. The tube e opens at its base into the gas space of the generator, so that the pressure below the diaphragm in a is the same as that elsewhere in the apparatus, while the pressure in a above the diaphragm is that of the atmosphere. Being flexible and but slightly stretched, the diaphragm is normally depressed by the weight of c until it occupies the position b; but if the pressure in the generator (i.e., in e) rises, it lifts the diaphragm to somewhat about the position b'--the extent of movement being, as usual, exaggerated in the sketch. The movement of the diaphragm is accompanied by a movement of the rod c, which can be employed in any desirable way. In E the bell of a rising holder of the ordinary typo is provided with a horizontal striker which, when the bell descends, presses against the top of a bag g made of any flexible material, such as india-rubber, and previously filled with water. Liquid is thus ejected, and may be caused to act upon calcium carbide in some adjacent vessel. The sketch is given because such a method of obtaining an intermittent water-supply has at one time been seriously proposed; but it is clearly one which cannot be recommended.
ACTION OF WATER-TO-CARBIDE GENERATORS.--Having by one or other of the means described obtained a supply of water intermittent in character, it remains to be considered how that supply may be made to approach the carbide in the generator. Actual acetylene apparatus are so various in kind, and merge from one type to another by such small differences, that it is somewhat difficult to classify them in a simple and intelligible fashion. However, it may be said that water-to-carbide generators, i.e., such as employ water as the moving material, may be divided into four categories: (F^1) water is allowed to fall as single drops or as a fine stream upon a mass of carbide--this being the "drip" generator; (F^2) a mass of water is made to rise round and then recede from a stationary vessel containing carbide--this being essentially identical in all respects save the mechanical one with the "dip" or "dipping" generator shown in A^2, Fig. 1; (F^3) a supply of water is permitted to rise round, or to flow upon, a stationary mass of carbide without ever receding from the position it has once assumed--this being the "contact" generator; and (F^4) a supply of water is admitted to a subdivided charge of carbide in such proportion that each quantity admitted is in chemical excess of the carbide it attacks. With the exception of F^2, which has already been illustrated as A^2 Fig. 1, or as B^1 in Fig. 2, these methods of decomposing carbide are represented in Figs. 4 and 5. It will be observed that whereas in both F^1 and F^3 the liberated acetylene passes off at the top of the apparatus, or rather from the top of the non-subdivided charge of carbide, in F^1 the water enters at the top, and in F^3 it enters at the bottom. Thus it happens that the mixture of acetylene and steam, which is produced at the spot where the primary chemical reaction is taking place, has to travel through the entire mass of carbide present in a generator belonging to type F^3, while in F^1 the damp gas flows directly to the exit pipe without having to penetrate the lumps of solid. Both F^1 and F^3 exhibit after-generation caused by a reaction between the liquid water mechanically clinging to the mass of spent lime and the excess of carbide to an approximately equal extent; but for the reason just mentioned, after-generation due to a reaction between the vaporised water accompanying the acetylene first evolved and the excess of carbide is more noticeable in F^3 than in F^1; and it is precisely this latter description of after-generation which leads to overheating of the most ungovernable kind. Naturally both F^1 and F^3 can be fitted with water jackets, as is indicated by the dotted lines in the second sketch; but unless the generating chamber in quite small and the evolution of gas quite slow, the cooling action of the jacket will not prove sufficient. As the water in F^1 and F^3 is not capable of backward motion, the decomposing chambers cannot be employed as displacement holders, as is the case in the dipping generator pictured at B^1, Fig. 2. They must be coupled, accordingly, to a separate holder of the displacement or, preferably, of the rising type; and, in order that the gas evolved by after-generation may not be wasted, the automatic mechanism must cut off the supply of water to the generator by the time that holder is two-thirds or three-quarters full.
The diagrams G, H, and K in Figs. 4 and 5 represent three different methods of constructing a generator which belongs either to the contact type (F^3) if the supply of water is essentially continuous, i.e., if less is admitted at each movement of the feeding mechanism than is sufficient to submerge the carbide in each receptacle; or to the flooded- compartment type (F') if the water enters in large quantities at a time. In H the main carbide vessel is arranged horizontally, or nearly so, and each partition dividing it into compartments is taller than its predecessor, so that the whole of the solid in (1) must be decomposed, and the compartment entirely filled with liquid before it can overflow into (2), and so on. Since the carbide in all the later receptacles is exposed to the water vapour produced in that one in which decomposition is proceeding at any given moment, at least at its upper surface, some after-generation between vapour and carbide occurs in H; but a partial control over the temperature may be obtained by water-jacketing the container. In G the water enters at the base and gas escapes at the top, the carbide vessels being disposed vertically; hero, perhaps, more after- generation of the same description occurs, as the moist gas streams round and over the higher baskets. In K, the water enters at the top and must completely fill basket (1) before it can run down the depending pipe into (2); but since the gas also leaves the generator at the top, the later carbide receptacles do not come in contact with water vapour, but are left practically unattacked until their time arrives for decomposition by means of liquid water. K, therefore, is the best arrangement of parts to avoid after-generation, overheating, and polymerisation of the acetylene whether the generator be worked as a contact or as a flooded-compartment apparatus; but it may be freely admitted that the extent of the overheating due to reaction between water vapour and carbide may be kept almost negligible in either K, H, or G, provided the partitions in the carbide container be sufficient in number--provided, that is to say, that each compartment holds a sufficiently small quantity of carbide; and provided that the quantity of water ultimately required to fill each compartment is relatively so large that the temperature of the liquid never approaches the boiling-point where vaporisation is rapid. The type of generator indicated by K has not become very popular, but G is fairly common, whilst H undoubtedly represents the apparatus which is most generally adopted for use in domestic and other private installations in the United Kingdom and the Continent of Europe. The actual generators made according to the design shown by H usually have a carbide receptacle designed in the form of a semi-cylindrical or rectangular vessel of steel sliding fairly closely into an outside container, the latter being either built within the main water space of the entire apparatus or placed within a separate water-jacketed casing. Owing to its shape and the sliding motion with which the carbide receptacle is put into the container these generators are usually termed "drawer" generators. In comparison with type G, the drawer generator H certainly exhibits a lower rise in temperature when gas is evolved in it at a given speed and when the carbide receptacles are constructed of similar dimensions. It is very desirable that the whole receptacle should be subdivided into a sufficient number of compartments and that it should be effectively water-cooled from outside. It would also be advantageous if the water- supply were so arranged that the generator should be a true flooded- compartment apparatus, but experience has nevertheless shown that generators of type H do work very well when the water admitted to the carbide receptacle, each time the feed comes into action, is not enough to flood the carbide in one of the compartments. Above a certain size drawer generators are usually constructed with two or even more complete decomposing vessels, arrangements being such that one drawer can be taken out for cleaning, whilst the other is in operation. When this is the case a third carbide receptacle should always be employed so that it may be dry, lit to receive a charge of carbide, and ready to insert in the apparatus when one of the others is withdrawn. The water-feed should always be so disposed that the attendant can see at a glance which of the two (or more) carbide receptacles is in action at any moment, and it should be also so designed that the supply is automatically diverted to the second receptacle when the first is wholly exhausted and back again to the first (unless there are more than two) when the carbide in the second is entirely gasified. In the sketches G, H, and K, the total space occupied by the various carbide receptacles is represented as being considerably smaller than the capacity of the decomposing chamber. Were this method of construction copied in actual acetylene apparatus, the first makes of gas would be seriously (perhaps dangerously) contaminated with air. In practice the receptacles should fit so tightly into the outer vessel and into one another that when loaded to the utmost extent permissible--space being left for the swelling of the charge and for the passage of water and gas--but little room should be left for the retention of air in the chamber.
ACTION OF CARBIDE-TO-WATER GENERATORS.--The methods which may be adopted to render a generator automatic when carbide is employed as the moving material are shown at M, N, and P, in Fig. 6; but the precise devices used in many actual apparatus are so various that it is difficult to portray them generically. Moreover it is desirable to subdivide automatic carbide-to-water generators, according to the size of the carbide they are constructed to take, into two or three classes, which are termed respectively "large carbide-feed," "small carbide-feed," and "granulated carbide-feed" apparatus. (The generator represented at L does not really belong to the present class, being non-automatic and fed by hand; but the sketch is given for completeness.) M is an automatic carbide-feed generator having its store of carbide in a hopper carried by the rising- holder bell. The hopper is narrowed at its mouth, where it is closed by a conical or mushroom valve d supported on a rod held in suitable guides. When the bell falls by consumption of gas, it carries the valve and rod with it; but eventually the button at the base of c strikes the bottom of the generator, or some fixed distributing plate, and the rod can descend no further. Then, when the bell falls lower, the mushroom d rises from its seat, and carbide drops from the hopper into the water. This type of apparatus has the defect characteristic of A^2, Fig. 1; for the pressure in the service steadily diminishes as the effective weight of bell plus hopper decreases by consumption of carbide. But it has also two other defects--(1) that ordinary carbide is too irregular in shape to fall smoothly through the narrow annular space between the valve and its seat; (2) that water vapour penetrates into the hopper, and liberates some gas there, while it attacks the lumps of carbide at the orifice, producing dust or causing them to stick together, and thus rendering the action of the feed worse than ever. Most of these defects can be avoided by using granulated carbide, which is more uniform in size and shape, or by employing a granulated and "treated" carbide which has been dipped in some non-aqueous liquid to make it less susceptible to the action of moisture. Both these plans, however, are expensive to adopt; first, because of the actual cost of granulating or "treating" the carbide; secondly, because the carbide deteriorates in gas-making capacity by its inevitable exposure to air during the granulating or "treating" process. The defects of irregularity of pressure and possible waste of gas by evolution in the hopper may be overcome by disposing the parts somewhat differently; making the holder an annulus round the hopper, or making it cylindrical with the hopper inside. In this case the hopper is supported by the main portion of the apparatus, and does not move with the bell: the rod and valve being given their motion in some fashion similar to that figured. Apparatus designed in accordance with the sketch M, or with the modification just described, are usually referred to under the name of "hopper" generators. On several occasions trouble has arisen during their employment owing to the jamming of the valve, a fragment of carbide rather larger than the rest of the material lodging between the lips of the hopper and the edges of the mushroom valve. This has been followed by a sudden descent of all the carbide in the store into the water beneath, and the evolution of gas has sometimes been too rapid to pass away at the necessary speed into the holder. The trouble is rendered even more serious should the whole charge of carbide fall at a time when, by neglect or otherwise, the body of the generator contains much lime sludge, the decomposition then proceeding under exceptionally bad circumstances, which lead to the production of an excessively high temperature. Hopper generators are undoubtedly very convenient for certain purposes, chiefly, perhaps, for the construction of table-lamps and other small installations. Experience tends to show that they may be employed, first, provided they are designed to take granulated carbide--which in comparison with larger grades is much more uniform and cylindrical in shape--and secondly, provided the quantity of carbide in the hopper does not exceed a few pounds. The phenomenon of the sudden unexpected descent of the carbide, popularly known as "dumping," can hardly be avoided with carbide larger in size than the granulated variety; and since the results of such an accident must increase in severity with the size of the apparatus, a limit in their capacity is desirable.
When it is required to construct a carbide-feed generator of large size or one belonging to the large carbide-feed pattern, it is preferable to arrange the store in a different manner. In N the carbide is held in a considerable number of small receptacles, two only of which are shown in the drawing, provided with detachable lids and hinged bottoms kept shut by suitable catches. At proper intervals of time those catches in succession are knocked on one side by a pin, and the contents of the vessel fall into the water. There are several methods available for operating the pins. The rising-holder bell may be made to actuate a train of wheels which terminate in a disc revolving horizontally on a vertical axis somewhere just below the catches; and this wheel may bear an eccentric pin which hits each catch as it rotates. Alternatively the carbide boxes may be made to revolve horizontally on a vertical axis by the movements of the bell communicated through a clutch; and thus each box in succession may arrive at a certain position where the catch is knocked aside by a fixed pin. The boxes, again, may revolve vertically on a horizontal axis somewhat like a water-wheel, each box having its bottom opened, or, by a different system of construction, being bodily upset, when it arrives at the bottom of its circular path. In no case, however, are the carbide receptacles carried by the bell, which is a totally distinct part of the apparatus; and therefore in comparison with M, the pressure given by the bell is much more uniform. Nevertheless, if the system of carbide boxes moves at all, it becomes easier to move by decrease in weight and consequent diminution in friction as the total charge is exhausted; and accordingly the bell has less work to do during the later stages of its operation. For this reason the plan actually shown at N is preferable, since the work done by the moving pin, i.e., by the descending bell, is always the same. P represents a carbide-feed effected by a spiral screw or conveyor, which, revolved periodically by a moving bell, draws carbide out of a hopper of any desired size and finally drops it into a shoot communicating with a generating chamber such as that shown in L. Here the work done by the bell is large, as the friction against the blades of the screw and the walls of the horizontal tube is heavy; but that amount of work must always be essentially identical. The carbide-feed may similarly be effected by means of some other type of conveyor instead of the spiral screw, such as an endless band, and the friction in these cases may be somewhat less than with the screw, but the work to be done by the bell will always remain large, whatever type of conveyor may be adopted. A further plan for securing a carbide-feed consists in employing some extraneous driving power to propel a charge of carbide out of a reservoir into the generator. Sometimes the propulsive effort is obtained from a train of clockwork, sometimes from a separate supply of water under high pressure. The clockwork or the water power is used either to drive a piston travelling through the vessel containing the carbide so that the proper quantity of material is dropped over the open mouth of a shoot, or to upset one after another a series of carbide receptacles, or to perform some analogous operation. In these cases the pin or other device fitted to the acetylene apparatus itself has nothing to do beyond releasing the mechanism in question, and therefore the work required from the bell is but small. The propriety of employing a generator belonging to these latter types must depend upon local conditions, e.g., whether the owner of the installation has hydraulic power on a small scale (a constant supply of water under sufficient pressure) at disposal, or whether he does not object to the extra labour involved in the periodical winding up of a train of clockwork.
It must be clear that all these carbide-feed arrangements have the defect in a more or less serious degree of leaving the carbide in the main storage vessel exposed to the attack of water vapour rising from the decomposing chamber, for none of the valves or operating mechanism can be made quite air-tight. Evolution of gas produced in this way does not matter in the least, because it is easy to return the gas so liberated into the generator or into the holder; while the extent of the action, and the consequent production of overheating, will tend to be less than in generators such as those shown in G and H of Figs. 4 and 5, inasmuch as the large excess of water in the carbide-feed apparatus prevents the liquid arriving at a temperature at which it volatilises rapidly. The main objection to the evolution of gas in the carbide vessel of a carbide-to-water generator depends on the danger that the smooth working of the feed-gear may be interfered with by the formation of dust or by the aggregation of the carbide lumps.
USE OF OIL IN GENERATORS.--Calcium carbide is a material which is only capable of attack for the purpose of evolving acetylene by a liquid that is essentially water, or by one that contains some water mixed with it. Oils and the like, or even such non-aqueous liquids as absolute alcohol, have no effect upon carbide, except that the former naturally make it greasy and somewhat more difficult to moisten. This last property has been found of service in acetylene generation, especially on the small scale; for if carbide is soaked in, or given a coating of, some oil, fat, or solid hydrocarbon like petroleum, cocoanut oil, or paraffin wax, the substance becomes comparatively indifferent towards water vapour or the moisture present in the air, while it still remains capable of complete, albeit slow, decomposition by liquid water when completely immersed therein. The fact that ordinary calcium carbide is attacked so quickly by water is really a defect of the substance; for it is to this extreme rapidity of reaction that the troubles of overheating are due. Now, if the basket in the generator B^1 of Fig. 2, or, indeed, the carbide store in any of the carbide-to-water apparatus, is filled with a carbide which has been treated with oil or wax, as long as the water-level stands at l' and l" or the carbide still remains in the hopper, it is essentially unattacked by the vapour arising from the liquid; but directly the basket is submerged, or the lumps fall into the water, acetylene is produced, and produced more slowly and regularly than otherwise. Again, oils do not mix with water, but usually float thereon, and a mass of water covered by a thick film or layer of oil does not evaporate appreciably. If, now, a certain quantity of oil, say lamp paraffin or mineral lubricating oil, is poured on to the water in B^1, Fig. 2, it moves upwards and downwards with the water. When the water takes the position l, the oil is driven upwards away from the basket of carbide, and acetylene is generated in the ordinary manner; but when the water falls to l" the oil descends also, rinses off much of the adhering water from the carbide lumps, covers them with a greasy film, and almost entirely stops generation till it is in turn washed off by the next ascent of the water. Similarly, if the carbide in generators F, G, and H (also K) has been treated with a solid or semi-solid grease, it is practically unattacked by the stream of warm damp gas, and is only decomposed when the liquid itself arrives in the basket. For the same reason treated carbide can be kept for fairly long periods of time, even in a drum with badly fitting lid, without suffering much deterioration by the action of atmospheric moisture. The problem of acetylene generation is accordingly simplified to a considerable degree by the use of such treated carbide, and the advantage becomes more marked as the plant decreases in size till a portable apparatus is reached, because the smaller the installation the more relatively expensive or inconvenient is a large holder for surplus gas. The one defect of the method is the extra cost of such treated carbide; and in English conditions ordinary calcium carbide is too expensive to permit of any additional outlay upon the acetylene if it is to compete with petroleum or the product of a tiny coal-gas works. The extra cost of using treated carbide falls upon the revenue account, and is much more noticeable than that of a large holder, which is capital expenditure. When fluid oil is employed in a generator of type B^1, evolution of gas becomes so regular that any holder beyond the displacement one which the apparatus itself constitutes is actually unnecessary, though still desirable; but B^1, with or without oil, still remains a displacement apparatus, and as such gives no constant pressure. It must be admitted that the presence of oil so far governs the evolution of gas that the movement of the water, and the consequent variation of pressure, is rendered very small; still a governor or a rising holder would be required to give the best result at the burners. One point in connexion with the use of liquid oil must not be overlooked, viz., the extra trouble it may give in the disposal of the residues. This matter will be dealt with more fully in Chapter V.; here it is sufficient to say that as the oil does not mix with the water but floats on the surface, care has to be taken that it is not permitted to enter any open stream. The foregoing remarks about the use of oil manifestly only apply to those cases where it is used in quantity and where it ultimately becomes mixed with the sludge or floats on the water in the decomposing chamber. The employment of a limpid oil, such as paraffin, as an intermediate liquid into which carbide is introduced on its way to the water in the decomposing vessel of a hand-fed generator in the manner described on page 70 is something quite different, because, except for trifling losses, one charge of oil should last indefinitely.
RISING GASHOLDERS.--Whichever description of holder is employed in an acetylene apparatus, the gas is always stored over, or in contact with, a liquid that is essentially water. This introduces three subjects for consideration: the heavy weight of a large body of liquid, the loss of gas by dissolution in that liquid, and the protection of that liquid from frost in the winter. The tanks of rising holders are constructed in two different ways. In one the tank is a plain cylindrical vessel somewhat larger in diameter than the bell which floats in it; and since there must be nearly enough water in the tank to fill the interior of the bell when the latter assumes its lowest position, the quantity of water is considerable, its capacity for dissolving acetylene is large, and the amount of any substance that may have to be added to it to lower its freezing-point becomes so great as to be scarcely economical. All these defects, including that of the necessity for very substantial foundations under the holder to support its enormous weight, may be overcome by adopting the second method of construction. It is clear that the water in the centre of the tank is of no use,--all that is needed being a narrow trough for the bell to work in. Large rising holders are therefore advantageously built with a tank formed in the shape of an annulus, the effective breadth of which is not more than 2 or 3 inches, the centre portion being roofed over so as to prevent escape of gas. The same principle may be retained with modified details by fitting inside a plain cylindrical tank a "dummy" or smaller cylinder, closed by a flat or curved top and fastened water- and air-tight to the bottom of the main vessel. The construction of annular tanks or the insertion of a "dummy" may be attended with difficulty if the tank is wholly or partly sunk below the ground level, owing to the lifting force of water in the surrounding soil. Where a steel tank is sunk, or a masonry tank is constructed, regard must be paid, both in the design of the tank and in the manner of construction, to the level of the underground water in the neighbourhood, as in certain cases special precautions will be needed to avoid trouble from the pressure of the water on the outside of the tank until it is balanced by the pressure of the water with which the tank is filled. So far as mere dissolution of gas is concerned, the loss may be reduced by having a circular disc of wood, &c., a little smaller in diameter than the boll, floating on the water of a plain tank.
EFFECT OF STORAGE IN GASHOLDER ON ACETYLENE.--It is perfectly true, as has been stated elsewhere, that the gas coming from an acetylene generator loses some of its illuminating power if it is stored over water for any great length of time; such loss being given by Nichols as 94 per cent, in five months, and having been found by one of the authors as 0.63 per cent. per day--figures which stand in fair agreement with one another. This wastage is not due to any decomposition of the acetylene in contact with water, but depends on the various solubilities of the different gases which compose the product obtained from commercial calcium carbide. Inasmuch as an acetylene evolved in the best generator contains some foreign ingredients, and inasmuch as an inferior product contains more (cf. Chapter V.), the contents of a holder are never pure; but as those contents are principally made up of acetylene itself, that gas stands at a higher partial pressure in the holder than the impurities. Since acetylene is more soluble in water than any of its diluents or impurities, sulphuretted hydrogen and ammonia excepted, and since the solubility of all gases increases as the pressure at which they are stored rises, the true acetylene in an acetylene holder dissolves in the water more rapidly and comparatively more copiously than the impurities; and thus the acetylene tends to disappear and the impurities to become concentrated within the bell. Simultaneously at the outer part of the seal, air is dissolved in the water; and by processes of diffusion the air so dissolved passes through the liquid from the outside to the inside, where it escapes into the bell, while the dissolved acetylene similarly passes from the inside to the outside of the seal, and there mingles with the atmosphere. Thus, the longer a certain volume of acetylene is stored over water, the more does it become contaminated with the constituents of the atmosphere and with the impurities originally present in it; while as the acetylene is much more soluble than its impurities, more gas escapes from, than enters, the holder by diffusion, and so the bulk of stored gas gradually diminishes. However, the figures previously given show that this action is too slow to be noticeable in practice, for the gas is never stored for more than a few days at a time. The action cannot be accepted as a valid argument against the employment of a holder in acetylene plant. Such deterioration and wastage of gas may be reduced to some extent by the use of a film of some cheap and indifferent oil floating on the water inside an acetylene holder; the economy being caused by the lower solubility of acetylene in oils than in aqueous liquids not saturated with some saline material. Probably almost any oil would answer equally well, provided it was not volatile at the temperature of the holder, and that it did not dry or gum on standing, e.g., olive oil or its substitutes; but mineral lubricating oil is not so satisfactory. It is, however, not necessary to adopt this method in practice, because the solvent power of the liquid in the seal can be reduced by adding to it a saline body which simultaneously lowers its freezing-point and makes the apparatus more trustworthy in winter.