Thursday, January 7, 2010

Treatment of acetylene after generation

IMPURITIES IN CALCIUM CARBIDE.—The calcium carbide manufactured at the present time, even when of the best quality commercially obtainable, is by no means a chemically pure substance; it contains a large number of foreign bodies, some of which evolve gas on treatment with water. To a considerable extent this statement will probably always remain true in the future; for in order to make absolutely pure carbide it would be necessary for the manufacturer to obtain and employ perfectly pure lime, carbon, and electrodes in an electric furnace which did not suffer attack during the passage of a powerful current, or he would have to devise some process for simultaneously or subsequently removing from his carbide those impurities which were derived from his impure raw materials or from the walls of his furnace—and either of these processes would increase the cost of the finished article to a degree that could hardly be borne. Beside the impurities thus inevitably arising from the calcium carbide decomposed, however, other impurities may be added to acetylene by the action of a badly designed generator or one working on a wrong system of construction; and therefore it may be said at once that the crude gas coming from the generating plant is seldom fit for immediate consumption, while if it be required for the illumination of occupied rooms, it must invariably be submitted to a rigorous method of chemical purification.
IMPURITIES OF ACETYLENE.—Combining together what may be termed the carbide impurities and the generator impurities in crude acetylene, the foreign bodies are partly gaseous, partly liquid, and partly solid. They may render the gas dangerous from the point of view of possible explosions; they, or the products derived from them on combustion, may be harmful to health if inspired, injurious to the fittings and decorations of rooms, objectionable at the burner orifices by determining, or assisting in, the formation of solid growths which distort the flame and so reduce its illuminating power; they may give trouble in the pipes by condensing from the state of vapour in bends and dips, or by depositing, if they are already solid, in angles, &c., and so causing stoppages; or they may be merely harmful economically by acting as diluents to the acetylene and, by having little or no illuminating value of themselves, causing the gas to emit less light than it should per unit of volume consumed, more particularly, of course, when the acetylene is not burnt under the mantle. Also, not being acetylene, or isomeric therewith, they require, even if they are combustible, a different proportion of oxygen for their perfect combustion; and a good acetylene jet is only calculated to attract precisely that quantity of air to the flame which a gas having the constitution C_2H_2 demands. It will be apparent without argument that a proper system of purification is one that is competent to remove the carbide impurities from acetylene, so far as that removal is desirable or necessary; it should not be called upon to extract the generator impurities, because the proper way of dealing with them is, to the utmost possible extent, to prevent their formation. The sole exception to this rule is that of water-vapour, which invariably accompanies the best acetylene, and must be partially removed as soon as convenient. Vapour of water almost always accompanies acetylene from the generator, even when the apparatus does not belong to those systems of working where liquid water is in excess, this being due to the fact that in a generator where the carbide is in excess the temperature tends to rise until part of the water is vapourised and carried out of the decomposing chamber before it has an opportunity of reacting with the excess of carbide. The issuing gas is therefore more or less hot, and it usually comes from the generating chamber saturated with vapour, the quantity needed so to saturate it rising as the temperature of the gas increases. Practically speaking, there is little objection to the presence of water-vapour in acetylene beyond the fear of deposition of liquid in the pipes, which may accumulate till they are partially or completely choked, and may even freeze and burst them in very severe weather. Where the chemical purifiers, too, contain a solid material which accidentally or intentionally acts as a drier by removing moisture from the acetylene, it is a waste of such comparatively expensive material to allow gas to enter the purifier wetter than need be.
EXTRACTION OF MOISTURE.—In all large plants the extraction of the moisture may take place in two stages. Immediately after the generator, and before the washer if the generator requires such an apparatus to follow it, a condenser is placed. Here the gas is made to travel somewhat slowly through one or more pipes surrounded with cold air or water, or is made to travel through a space containing pipes in which cold water is circulating, the precise method of constructing the condenser being perfectly immaterial so long as the escaping gas has a temperature not appreciably exceeding that of the atmosphere. So cooled, however, the gas still contains much water-vapour, for it remains saturated therewith at the temperature to which it is reduced, and by the inevitable law of physics a further fall in temperature will be followed by a further deposition of liquid water from the acetylene. Manifestly, if the installation is so arranged that the gas can at no part of the service and on no occasion fall to a lower temperature than that at which it issues from the condenser, the removal of moisture as effected by such a condenser will be sufficient for all practical purposes; but at least in all large plants where a considerable length of main is exposed to the air, a more complete moisture extractor must be added to the plant, or water will be deposited in the pipes every cold night in the winter. It is, however, useless to put a chemical drier, or one more searching in its action than a water-cooled condenser, at so early a position in the acetylene plant, because the gas will be subsequently stored in a water- sealed holder, where it will most probably once again be saturated with moisture from the seal. When such generators are adopted as require to have a specific washer placed after them in order to remove the water- soluble impurities, e.g., those in which the gas does not actually bubble through a considerable quantity of liquid in the generating chamber itself, it is doubtful whether a separate condenser is altogether necessary, because, as the water in the washer can easily be kept at the atmospheric temperature (by means of water circulating in pipes or otherwise), the gas will be brought to the atmospheric temperature in the washer, and at that temperature it cannot carry with it more than a certain fixed proportion of moisture. The notion of partially drying a gas by causing it to pass through water may appear paradoxical, but a comprehension of physical laws will show that it is possible, and will prove efficient in practice, when due attention is given to the facts that the gas entering the washer is hot, and that it is subsequently to be stored over water in a holder.
GENERATOR IMPURITIES.—The generator impurities present in the crudest acetylene consist of oxygen and nitrogen, i.e., the main constituents of air, the various gaseous, liquid, and semi-solid bodies described in Chapter II., which are produced by the polymerising and decomposing action of heat upon the carbide, water, and acetylene in the apparatus, and, whenever the carbide is in excess in the generator, some lime in the form of a very fine dust. In all types of water-to-carbide plant, and in some automatic carbide-feed apparatus, the carbide chamber must be disconnected and opened each time a fresh charge has to be inserted; and since only about one-third of the space in the container can be filled with carbide, the remaining two-thirds are left full of air. It is easy to imagine that the carbide container of a small generator might be so large, or loaded with so small a quantity of carbide, or that the apparatus might in other respects be so badly designed, that the gas evolved might contain a sufficient proportion of air to render it liable to explode in presence of a naked light, or of a temperature superior to its inflaming-point. Were a cock, however, which should have been shut, to be carelessly left open, an escape of gas from, rather than an introduction of air into, the apparatus would follow, because the pressure in the generator is above that of the atmosphere. As is well known, roughly four-fifths by volume of the air consist of nitrogen, which is non-inflammable and accordingly devoid of danger- conferring properties; but in all flames the presence of nitrogen is harmful by absorbing much of the heat liberated, thus lowering the temperature of that flame, and reducing its illuminating power far more seriously. On the other hand, a certain quantity of air in acetylene helps to prevent burner troubles by acting as a mere diluent (albeit an inferior one to methane or marsh-gas), and therefore it has been proposed intentionally to add air to the gas before consumption, such a process being in regular use on the large scale in some places abroad. As Eitner has shown (Chapter VI.) that in a 3/4-inch pipe acetylene ceases to be explosive when mixed with less than 47.7 per cent. of air, an amount of, say, 40 per cent. or less may in theory be safely added to acetylene; but in practice the amount of air added, if any, would have to be much smaller, because the upper limit of explosibility of acetylene-air mixtures is not rigidly fixed, varying from about 50 per cent. of air when the mixture is in a small vessel, and fired electrically to about 25 per cent. of air in a large vessel approached with a flame. Moreover, safely to prepare such mixtures, after the proportion of air had been decided upon, would require the employment of some additional perfectly trustworthy automatic mechanism to the plant to draw into the apparatus a quantity of air strictly in accordance with the volume of acetylene made —a pair of meters geared together, one for the gas, the other for the air—and this would introduce extra complexity and extra expense. On the whole the idea cannot be recommended, and the action of the British Home Office in prohibiting the use of all such mixtures except those unavoidably produced in otherwise good generators, or in burners of the ordinary injector type, is perfectly justifiable. The derivation and effect of the other gaseous and liquid generator impurities in acetylene were described in Chapter II. Besides these, very hot gas has been found to contain notable amounts of hydrogen and carbon monoxide, both of which burn with non-luminous flames. The most plausible explanation of their origin has been given by Lewes, who suggests that they may be formed by the action of water-vapour upon very hot carbide or upon carbon separated therefrom as the result of previous dissociation among the gases present; the steam and the carbon reacting together at a temperature of 500° C. or thereabouts in a manner resembling that of the production of water-gas. The last generator impurity is lime dust, which is calcium oxide or hydroxide carried forward by the stream of gas in a state of extremely fine subdivision, and is liable to be produced whenever water acts rapidly upon an excess of calcium carbide. This lime occasionally appears in the alternative form of a froth in the pipes leading directly from the generating chamber; for some types of carbide-to-water apparatus, decomposing certain kinds of carbide, foam persistently when the liquid in them becomes saturated with lime, and this foam or froth is remarkably difficult to break up.
FILTERS.—It has just been stated that the purifying system added to an acetylene installation should not be called upon to remove these generator impurities; because their appearance in quantity indicates a faulty generator, which should be replaced by one of better action. On the contrary, with the exception of the gases which are permanent at atmospheric temperature—hydrogen, carbon monoxide, nitrogen, and oxygen— and which, once produced, must remain in the acetylene (lowering its illuminating value, but giving no further trouble), extraction of these generator impurities is quite simple. The dust or froth of lime will be removed in the washer where the acetylene bubbles through water—the dust itself can be extracted by merely filtering the gas through cotton-wool, felt, or the like. The least volatile liquid impurities will be removed partly in the condenser, partly in the washer, and partly by the mechanical dry-scrubbing action of the solid purifying material in the chemical purifier. To some extent the more volatile liquid bodies will be removed similarly; but a complete extraction of them demands the employment of some special washing apparatus in which the crude acetylene is compelled to bubble (in finely divided streams) through a layer of some non-volatile oil, heavy mineral lubricating oil, &c.; for though soluble in such oil, the liquid impurities are not soluble in, nor do they mix with, water; and since they are held in the acetylene as vapours, a simple passage through water, or through water-cooled pipes, does not suffice for their recovery. It will be seen that a sufficient removal of these generator impurities need throw no appreciable extra labour upon the consumer of acetylene, for he can readily select a type of generator in which their production is reduced to a minimum; while a cotton-wool or coke filter for the gas, a water washer, which is always useful in the plant if only employed as a non-return valve between the generator and the holder, and the indispensable chemical purifiers, will take out of the acetylene all the remaining generator impurities which need, and can, be extracted.
CARBIDE IMPURITIES.—Neglecting very minute amounts of carbon monoxide and hydrogen (which may perhaps come from cavities in the calcium carbide itself), as being utterly insignificant from the practical point of view, the carbide impurities of the gas fall into four main categories: those containing phosphorus, those containing sulphur, those containing silicon, and those containing gaseous ammonia. The phosphorus in the gas comes from calcium phosphide in the calcium carbide, which is attacked by water, and yields phosphoretted hydrogen (or phosphine, as it will be termed hereafter). The calcium phosphide, in its turn, is produced in the electric furnace by the action of the coke upon the phosphorus in phosphatic lime—all commercially procurable lime and some varieties of coke (or charcoal) containing phosphates to a larger or smaller extent. The sulphur in the gas comes from aluminium sulphide in the carbide, which is produced in the electric furnace by the interaction of impurities containing aluminium and sulphur (clay-like bodies, &c.) present in the lime and coke; this aluminium sulphide is attacked by water and yields sulphuretted hydrogen. Even in the absence of aluminium compounds, sulphuretted hydrogen may be found in the gases of an acetylene generator; here it probably arises from calcium sulphide, for although the latter is not decomposed by water, it gradually changes in water into calcium sulphydrate, which appears to suffer decomposition. When it exists in the gas the silicon is derived from certain silicides in the carbide; but this impurity will be dealt with by itself in a later paragraph. The ammonia arises from the action of the water upon magnesium, aluminium, or possibly calcium nitride in the calcium carbide, which are bodies also produced in the electric furnace or as the carbide is cooling. In the gas itself the ammonia exists as such; the phosphorus exists mainly as phosphine, partly as certain organic compounds containing phosphorus, the exact chemical nature of which has not yet been fully ascertained; the sulphur exists partly as sulphuretted hydrogen and partly as organic compounds analogous, in all probability, to those of phosphorus, among which Caro has found oil of mustard, and certain bodies that he regards as mercaptans. [Footnote: It will be convenient to borrow the phrase used in the coal-gas industry, calling the compounds of phosphorus other than phosphine "phosphorus compounds," and the compounds of sulphur other than sulphuretted hydrogen "sulphur compounds." The "sulphur compounds" of coal-gas, however, consist mainly of carbon bisulphide, which is certainly not the chief "sulphur compound" in acetylene, even if present to any appreciable extent.] The precise way in which these organic bodies are formed from the phosphides and sulphides of calcium carbide is not thoroughly understood; but the system of generation employed, and the temperature obtaining in the apparatus, have much to do with their production; for the proportion of the total phosphorus and sulphur found in the crude gas which exists as "compounds" tends to be greater as the generating plant yields a higher temperature. It should be noted that ammonia and sulphuretted hydrogen have one property in common which sharply distinguishes them from the sulphur "compounds," and from all the phosphorus compounds, including phosphine. Ammonia and sulphuretted hydrogen are both very soluble in water, the latter more particularly in the lime-water of an active acetylene generator; while all the other bodies referred to are completely insoluble. It follows, therefore, that a proper washing of the crude gas in water should suffice to remove all the ammonia and sulphuretted hydrogen from the acetylene; and as a matter of fact those generators in which the gas is evolved in presence of a large excess of water, and in which it has to bubble through such water, yield an acetylene practically free from ammonia, and containing nearly all the sulphur which it does contain in the state of "compounds." It must also be remembered that chemical processes which are perfectly suited to the extraction of sulphuretted hydrogen and phosphine are not necessarily adapted for the removal of the other phosphorus and sulphur compounds.
WASHERS.—In designing a washer for the extraction of ammonia and sulphuretted hydrogen it is necessary to see that the gas is brought into most intimate contact with the liquid, while yet no more pressure than can possibly be avoided is lost. Subdivision of the gas stream may be effected by fitting the mouth of the inlet-pipe with a rose having a large number of very small holes some appreciable distance apart, or by bending the pipe to a horizontal position and drilling it on its upper surface with numbers of small holes. Another method is to force the gas to travel under a series of partitions extending just below the water- level, forming the lower edges of those partitions either perfectly horizontal or with small notches like the teeth of a saw. One volume of pure water only absorbs about three volumes of sulphuretted hydrogen at atmospheric temperatures, but takes up some 600 volumes of gaseous ammonia; and as ammonia always accompanies the sulphuretted hydrogen, the latter may be said to be absorbed in the washer by a solution of ammonia, a liquid in which sulphuretted hydrogen is much more soluble. Therefore, since water only dissolves about an equal volume of acetylene, the liquid in the washer will continue to extract ammonia and sulphuretted hydrogen long after it is saturated with the hydrocarbon. For this reason, i.e., to avoid waste of acetylene by dissolution in the clean water of the washer, the plan is sometimes adopted of introducing water to the generator through the washer, so that practically the carbide is always attacked by a liquid saturated with acetylene. Provided the liquid in the generator does not become seriously heated, there is no objection to this arrangement; but if the water is heated strongly in the generator it loses much or all of its solvent properties, and the impurities may be driven back again into the washer. Clearly if the waste lime of the generator occurs as a dry or damp powder, the plan mentioned is not to be recommended; but when the waste lime is a thin cream—water being in large excess—it may be adopted. If the generator produces lime dust among the gas, and if the acetylene enters the washer through minute holes, a mechanical filter to remove the dust must be inserted between the generator and the washer, or the orifices of the leading pipe will be choked. Whenever a water-cooled condenser is employed after the generator, in which the gas does not come in contact with the water, that liquid may always be used to charge the generator. For compactness and simplicity of parts the water of the holder seal is occasionally used as the washing liquid, but unless the liquid of the seal is constantly renewed it will thus become offensive, especially if the holder is under cover, and it will also act corrosively upon the metal of the tank and bell. The water-soluble impurities in acetylene will not be removed completely by merely standing over the holder seal for a short time, and it is not good practice to pass unnecessarily impure gas into a holder. [Footnote: This is not a contradiction of what has been said in Chapter III. about the relative position of holder and chemical purifiers, because reference is now being made to ammonia and sulphuretted hydrogen only.]
HARMFULNESS OF IMPURITIES.—The reasons why the carbide impurities must be removed from acetylene before it is burned have now to be explained. From the strictly chemical point of view there are three compounds of phosphorus, all termed phosphoretted hydrogen or phosphine: a gas, PH_3; a liquid, P_2H_4; and a solid, P_4H_2. The liquid is spontaneously inflammable in presence of air; that is to say, it catches fire of itself without the assistance of spark or flame immediately it comes in contact with atmospheric oxygen; being very volatile, it is easily carried as vapour by any permanent gas. The gaseous phosphine is not actually spontaneously inflammable at temperatures below 100° C.; but it oxidises so rapidly in air, even when somewhat diluted, that the temperature may quickly rise to the point of inflammation. In the earliest days of the acetylene industry, directly it was recognised that phosphine always accompanies crude acetylene from the generator, it was believed that unless the proportion were strictly limited by decomposing only a carbide practically free from phosphides, the crude acetylene might exhibit spontaneously inflammable properties. Lewes, indeed, has found that a sample of carbide containing 1 per cent of calcium phosphide gave (probably by local decomposition—the bulk of the phosphide suffering attack first) a spontaneously inflammable gas; but when examining specimens of commercial carbide the highest amount of phosphine he discovered in the acetylene was 2.3 per cent, and this gas was not capable of self-inflammation. According to Bullier, however, acetylene must contain 80 per cent of phosphine to render it spontaneously inflammable. Berdenich has reported a case of a parcel of carbide which yielded on the average 5.1 cubic foot of acetylene per lb., producing gas which contained only 0.398 gramme of phosphorus in the form of phosphine per cubic metre (or 0.028 per cent. of phosphine) and was spontaneously inflammable. But on examination the carbide in question was found to be very irregular in composition, and some lumps produced acetylene containing a very high proportion of phosphorus and silicon compounds. No doubt the spontaneous inflammability was due to the exceptional richness of these lumps in phosphorus. As manufactured at the present day, calcium carbide ordinarily never contains an amount of phosphide sufficient to render the gas dangerous on the score of spontaneous inflammability; but should inferior material ever be put on the markets, this danger might have to be guarded against by submitting the gas evolved from it to chemical analysis. Another risk has been suggested as attending the use of acetylene contaminated with phosphine (and to a minor degree with sulphuretted hydrogen), viz., that being highly toxic, as they undoubtedly are, the gas containing them might be extremely dangerous to breathe if it escaped from the service, or from a portable lamp, unconsumed. Anticipating what will be said in a later paragraph, the worst kind of calcium carbide now manufactured will not yield a gas containing more than 0.1 per cent. by volume of sulphuretted hydrogen and 0.05 per cent. of phosphine. According to Haldane, air containing 0.07 per cent. of sulphuretted hydrogen produces fatal results on man if it is breathed for some hours, while an amount of 0.2 per cent. is fatal in 1- 1/2 minutes. Similar figures for phosphine cannot be given, because poisoning therewith is very rare or quite unknown: the cases of "phossy- jaw" in match factories being caused either by actual contact with yellow phosphorus or by inhalation of its vapour in the elemental state. However, assuming phosphine to be twice as toxic as sulphuretted hydrogen, its effect in crude acetylene of the above-mentioned composition will be equal to that of the sulphuretted hydrogen, so that in the present connexion the gas may be said to be equally toxic with a sample of air containing 0.2 per cent. of sulphuretted hydrogen, which kills in less than two minutes. But this refers only to crude acetylene undiluted with air; and being a hydrocarbon—being in fact neither oxygen nor common air—acetylene is irrespirable of itself though largely devoid of specific toxic action. Numerous investigations have been made of the amount of acetylene (apart from its impurities) which can be breathed in safety; but although these point to a probable recovery after a fairly long-continued respiration of an atmosphere charged with 30 per cent. of acetylene, the figure is not trustworthy, because toxicological experiments upon animals seldom agree with similar tests upon man. If crude acetylene were diluted with a sufficient proportion of air to remove its suffocating qualities, the percentage of specifically toxic ingredients would be reduced to a point where their action might be neglected; and short of such dilution the acetylene itself would in all probability determine pathological effects long before its impurities could set up symptoms of sulphur and phosphorus poisoning.
Ammonia is objectionable in acetylene because it corrodes brass fittings and pipes, and because it is partially converted (to what extent is uncertain) into nitrous and nitric acids as it passes through the flame. Sulphur is objectionable in acetylene because it is converted into sulphurous and sulphuric anhydrides, or their respective acids, as it passes through the flame. Phosphorus is objectionable because in similar circumstances it produces phosphoric anhydride and phosphoric acid. Each of these acids is harmful in an occupied room because they injure the decorations, helping to rot book-bindings, [Footnote: It is only fair to state that the destruction of leather bindings is commonly due to traces of sulphuric acid remaining in the leather from the production employed in preparing it, and is but seldom caused directly by the products of combustion coming from gas or oil.] tarnishing "gold-leaf" ornaments, and spoiling the colours of dyed fabrics. Each is harmful to the human system, sulphuric and phosphoric anhydrides (SO_3, and P_4O_10) acting as specific irritants to the lungs of persons predisposed to affections of the bronchial organs. Phosphorus, however, has a further harmful action: sulphuric anhydride is an invisible gas, but phosphoric anhydride is a solid body, and is produced as an extremely fine, light, white voluminous dust which causes a haze, more or less opaque, in the apartment. [Footnote: Lewes suggests that ammonia in the gas burnt may assist in the production of this haze, owing to the formation of solid ammonium salts in the state of line dust.] Immediately it comes in contact with atmospheric moisture phosphoric anhydride is converted into phosphoric acid, but this also occurs at first as a solid substance. The solidity and visibility of the phosphoric anhydride and acid are beneficial in preventing highly impure acetylene being unwittingly burnt in a room; but, on the other hand, being merely solids in suspension in the air, the combustion products of phosphorus are not so easily carried away from the room by the means provided for ventilation as are the products of the combustion of sulphur. Phosphoric anhydride is also partly deposited in the solid state at the burner orifices, perhaps actually corroding the steatite jets, and always assisting in the deposition of carbon from any polymerised hydrocarbons in the acetylene; thus helping the carbon to block up or distort those orifices. Whenever the acetylene is to be burnt on the incandescent system under a mantle of the Welsbach or other type, phosphorus, and possibly sulphur, become additionally objectionable, and rigorous extraction is necessary. As is well known, the mantle is composed of the oxides of certain "rare earths" which owe their practical value to the fact that they are non-volatile at the temperature of the gas-flame. When a gas containing phosphorus is burnt beneath such a mantle, the phosphoric anhydride attacks those oxides, partially converting them into the respective phosphates, and these bodies are less refractory. A mantle exposed to the combustion products of crude acetylene soon becomes brittle and begins to fall to pieces, occasionally showing a yellowish colour when cold. The actual advantage of burning acetylene on the incandescent system is not yet thoroughly established— in this country at all events; but it is clear that the process will not exhibit any economy (rather the reverse) unless the plant is provided with most capable chemical purifiers. Phosphorus, sulphur, and ammonia are not objectionable in crude acetylene because they confer upon the gas a nauseous odour. From a well-constructed installation no acetylene escapes unconsumed: the gas remains wholly within the pipes until it is burnt, and whatever odour it may have fails to reach the human nostrils. A house properly piped for acetylene will be no more conspicuous by its odour than a house properly piped for coal-gas. On the contrary, the fact that the carbide impurities of acetylene, which, in the absolutely pure state, is a gas of somewhat faint, hardly disagreeable, odour, do confer upon that gas a persistent and unpleasant smell, is distinctly advantageous; for, owing to that odour, a leak in the pipes, an unclosed tap, or a fault in the generating plant is instantly brought to the consumer's attention. A gas wholly devoid of odour would be extremely dangerous in a house, and would have to be scented, as is done in the case of non-carburetted water-gas when it is required for domestic purposes.
AMOUNTS OF IMPURITIES AND SCOPE OF PURIFICATION.—Partly for the reason which has just been given, and partly on the ground of expense, a complete removal of the impurities from crude acetylene is not desirable. All that need be done is to extract sufficient to deprive the gas of its injurious effects upon lungs, decorations, and burners. As it stands, however, such a statement is not sufficiently precise to be useful either to consumers of acetylene or to manufacturers of plant, and some more or less arbitrary standard must be set up in order to define the composition of "commercially pure" acetylene, as well as to gauge the efficiency of any process of purification. In all probability such limit may be reasonably taken at 0.1 milligramme of either sulphur or phosphorus (calculated as elementary bodies) per 1 litre of acetylene, i.e., 0.0-1.1 grain per cubic foot; a quantity which happens to correspond almost exactly with a percentage by weight of 0.01. Owing to the atomic weights of these substances, and the very small quantities being considered, the same limit hardly differs from that of 0.01 per cent. by weight of sulphuretted hydrogen or of phosphine—it being always recollected that the sulphur and phosphorus do not necessarily exist in the gas as simple hydrides. Keppeler, however, has suggested the higher figure of 0.15 milligramme of either sulphur or phosphorus per litre of acetylene (=0.066 grain per cubic foot) for the maximum amount of these impurities permissible in purified acetylene. He adopts this standard on the basis of the results of observations of the amounts of sulphur and phosphorus present in the gas issuing from a purifier charged with heratol at the moment when the last layer of the heratol is beginning to change colour. No limit has been given for the removal of the ammonia, partly because that impurity can more easily, and without concomitant disadvantage, be extracted entirely; and partly because it is usually removed in the washer and not in the true chemical purifier.
According to Lewes, the maximum amount of ammonia found in the acetylene coming from a dripping generator is 0.95 gramme per litre, while in carbide-to-water gas it is 0.16 gramme: 417 and 70.2 grains per cubic foot respectively. Rossel and Landriset have found 4 milligrammes (1.756 grains [Footnote: Milligrammes per litre; grains per cubic foot. It is convenient to remember that since 1 cubic foot of water weighs 62.321 x 16 - 997.14 avoirdupois ounces, grammes per litre are approximately equal to oz. per cubic foot; and grammes per cubic metre to oz. per 1000 cubic feet.]) to be the maximum in water-to-carbide gas, and none to occur in carbide-to-water acetylene. Rossel and Landriset return the minimum proportion of sulphur, calculated as H_2S, found in the gaseous state in acetylene when the carbide has not been completely flooded with water at 1.18 milligrammes per litre, or 0.52 grain per cubic foot; and the corresponding maxima at 1.9 milligrammes, or 0.84 grain. In carbide-to- water gas, the similar maxima are 0.23 milligramme or 0.1 grain. As already stated, the highest proportion of phosphine yet found in acetylene is 2.3 per cent. (Lewes), which is equal to 32.2 milligrammes of PH_3 per litre or 14.13 grains per cubic foot (Polis); but this sample dated from 1897. Eitner and Keppeler record the minimum proportion of phosphorus, calculated as PH_3, found in crude acetylene, as 0.45 milligramme per litre, and the maximum as 0.89 milligramme per litre; in English terms these figures are 0.2 and 0.4 grain per cubic foot. On an average, however, British and Continental carbide of the present day may be said to give a gas containing 0.61 milligramme of phosphorus calculated as PH_3 per litre and 0.75 milligramme of sulphur calculated as H_2S. In other units these figures are equal to 0.27 grain of PH_3 and 0.33 grain of H_2S per 1 cubic foot, or to 0.041 per cent. by volume of PH_3 and 0.052 per cent. of H_2S. Yields of phosphorus and sulphur much higher than these will be found in the journals and books, but such analytical data were usually obtained in the years 1896-99, before the manufacture of calcium carbide had reached its present degree of systematic control. A commercial specimen of carbide was seen by one of the authors as late as 1900 which gave an acetylene containing 1.12 milligramme of elementary sulphur per litre, i.e., 0.096 per cent, by volume, or 0.102 per cent, by volume of H_2S; but the phosphorus showed the low figure of 0.36 milligramme per litre (0.031 per cent, of P or 0.034 per cent, of PH_3 by volume).
The British Acetylene Association's regulations relating to carbide of calcium (vide Chap. XIV.) contain a clause to the effect that "carbide which, when properly decomposed, yields acetylene containing from all phosphorus compounds therein more than 0.05 per cent, by volume of phosphoretted hydrogen, may be refused by the buyer." This limit is equivalent to 0.74 milligramme of phosphorus calculated as PH_3 per litre. A latitude of 0.01 per cent, is, however, allowed for the analysis, so that the ultimate limit on which carbide could be rejected is: 0.06 volume per cent. of PH_3, or 0.89 milligramme of phosphorus per litre.
The existence in appreciable quantity of combined silicon as a normal impurity in acetylene seems still open to doubt. Calcium carbide frequently contains notable quantities of iron and other silicides; but although these bodies are decomposed by acids, yielding hydrogen silicide, or siliciuretted hydrogen, they are not attacked by plain water. Nevertheless Wolff and Gerard have found hydrogen silicide in crude acetylene, and Lewes looks upon it as a common impurity in small amounts. When it occurs, it is probably derived, as Vigouroux has suggested, from "alloys" of silicon with calcium, magnesium, and aluminium in the carbide. The metallic constituents of these substances would naturally be attacked by water, evolving hydrogen; and the hydrogen, in its nascent state, would probably unite with the liberated silicon to form hydrogen silicide. Many authorities, including Keppeler, have virtually denied that silicon compounds exist in crude acetylene, while the proportion 0.01 per cent. has been given by other writers as the maximum. Caro, however, has stated that the crude gas almost invariably contains silicon, sometimes in very small quantities, but often up to the limit of 0.8 per cent.; the failure of previous investigators to discover it being due to faulty analytical methods. Caro has seen one specimen of (bad) carbide which gave a spontaneously inflammable gas although it contained only traces of phosphine; its inflammability being caused by 2.1 per cent. of hydrogen silicide. Practically speaking, all the foregoing remarks made about phosphine apply equally to hydrogen silicide: it burns to solid silicon oxide (silica) at the burners, is insoluble in water, and is spontaneously inflammable when alone or only slightly diluted, but never occurs in good carbide in sufficient proportion to render the acetylene itself inflammable. According to Caro the silicon may be present both as hydrogen silicide and as silicon "compounds." A high temperature in the generator will favour the production of the latter; an apparatus in which the gas is washed well in lime-water will remove the bulk of the former. Fraenkel has found that magnesium silicide is not decomposed by water or an alkaline solution, but that dilute hydrochloric acid acts upon it and spontaneously inflammable hydrogen silicide results. If it may be assumed that the other silicides in commercial calcium carbide also behave in this manner it is plain that hydrogen silicide cannot occur in crude acetylene unless the gas is supposed to be hurried out of the generator before the alkaline water therein has had time to decompose any traces of the hydrogen silicide which is produced in the favouring conditions of high temperature sometimes prevailing. Mauricheau-Beaupré has failed to find silica in the products of combustion of acetylene from carbide of varying degrees of purity. He found, however, that a mixture of strong nitric and hydrochloric acids (aqua regia), if contaminated with traces of phosphoric acid, dissolved silica from the glass of laboratory vessels. Consequently, since phosphoric acid results from the phosphine in crude acetylene when the gas is passed through aqua regia, silica may be found on subsequently evaporating the latter. But this, silica, he found, was derived from the glass and not through the oxidation of silicon compounds in the acetylene. It is possible that some of the earlier observers of the occurrence of silicon compounds in crude acetylene may have been misled by the solution of silica from the glass vessels used in their investigations. The improbability of recognisable quantities of silicon compounds occurring in acetylene in any ordinary conditions of generation is demonstrated by a recent study by Fraenkel of the composition of the deposit produced on reflectors exposed to the products of combustion of a sample of acetylene which afforded a haze when burnt. The deposit contained 51.07 per cent. of phosphoric acid, but no silica. The gas itself contained from 0.0672 to 0.0837 per cent. by volume of phosphine.
PURIFYING MATERIALS.—When acetylene first began to be used as a domestic illuminant, most generator builders denied that there was any need for the removal of these carbide impurities from the gas, some going so far as to assert that their apparatus yielded so much purer an acetylene than other plant, where purification might be desirable, that an addition of a special purifier was wholly unnecessary. Later on the more responsible members of the trade took another view, but they attacked the problem of purification in a perfectly empirical way, either employing some purely mechanical scrubber filled with some moist or dry porous medium, or perhaps with coke or the like wetted with dilute acid, or they simply borrowed the processes adopted in the purification of coal-gas. At first sight it might appear that the more simple methods of treating coal-gas should be suitable for acetylene; since the former contains two of the impurities—sulphuretted hydrogen and ammonia—characteristic of crude acetylene. After removing the ammonia by washing with water, therefore, it was proposed to extract the sulphur by passing the acetylene through that variety of ferric hydroxide (hydrated oxide of iron) which is so serviceable in the case of coal-gas. The idea, however, was quite unsound: first, because it altogether ignores the phosphorus, which is the most objectionable impurity in acetylene, but is not present in coal- gas; secondly, because ferric hydroxide is used on gasworks to extract in a marketable form the sulphur which occurs as sulphuretted hydrogen, and true sulphuretted hydrogen need not exist in well-generated and well- washed acetylene to any appreciable extent; thirdly, because ferric hydroxide is not employed by gasmakers to remove sulphur compounds (this is done with lime), being quite incapable of extracting them, or the analogous sulphur compounds of crude acetylene.
About the same time three other processes based on somewhat better chemical knowledge were put forward. Pictet proposed leading the gas through a strong solution of calcium chloride and then through strong sulphuric acid, both maintained at a temperature of -20° to -40° C., finally washing the gas in a solution of some lead salt. Proof that such treatment would remove phosphorus to a sufficient degree is not altogether satisfactory; but apart from this the necessity of maintaining such low temperatures, far below that of the coldest winter's night, renders the idea wholly inadmissible for all domestic installations. Willgerodt suggested removing sulphuretted hydrogen by means of potassium hydroxide (caustic potash), then absorbing the phosphine in bromine water. For many reasons this process is only practicable in the laboratory. Bergé and Reychler proposed extracting both sulphuretted hydrogen and phosphine in an acid solution of mercuric chloride (corrosive sublimate). The poisonousness of this latter salt, apart from all other objections, rules such a method out.
BLEACHING POWDER.—The next idea, first patented by Smith of Aberdeen, but fully elaborated by Lunge and Cedercreutz, was to employ bleaching- powder [Footnote: Bleaching-powder is very usually called chloride of lime; but owing to the confusion which is constantly arising in the minds of persons imperfectly acquainted with chemistry between chloride of lime and chloride of calcium—two perfectly distinct bodies—the less ambiguous expression "bleaching-powder" will be adopted here.] either in the solid state or as a liquid extract. The essential constituent of bleaching-powder from the present aspect is calcium hypochlorite, which readily oxidises sulphuretted hydrogen, and more particularly phosphine, converting them into sulphuric and phosphoric acids, while the acetylene is practically unattacked. In simple purifying action the material proved satisfactory; but since high-grade commercial bleaching-powder contains some free chlorine, or some is set free from it in the purifier under the influence of the passing gas, the issuing acetylene was found to contain chlorine, free or combined; and this, burning eventually to hydrochloric acid, is hardly less harmful than the original sulphur compounds. Moreover, a mixture of acetylene, chlorine, and air is liable to catch fire of itself when exposed to bright sunlight; and therefore the use of a bleaching-powder purifier, or rather the recharging thereof, was not unattended by danger in the early days. To overcome these defects, the very natural process was adopted of diluting the bleaching-powder, such diluent also serving to increase the porosity of the material. A very unsuitable substance, however, was selected for the purpose, viz., sawdust, which is hygroscopic organic, and combustible. Owing to the exothermic chemical action between the impurities of the acetylene and the bleaching-powder, the purifying mass became heated; and thus not only were the phenomena found in a bad generator repeated in the purifying vessel, but in presence of air and light (as in emptying the purifier), the reaction proceeded so rapidly that the heat caused inflammation of the sawdust and the gas, at least on one occasion an actual fire taking place which created much alarm and did some little damage. For a time, naturally, bleaching-powder was regarded as too dangerous a material to be used for the purification of crude acetylene; but it was soon discovered that danger could be avoided by employing the substance in a proper way.
HERATOL, FRANKOLINE, ACAGINE AND PURATYLENE.—Setting aside as unworthy of attention certain compositions offered as acetylene purifying materials whose constitution has not been divulged or whose action has not been certified by respectable authority, there are now three principal chemical reagents in regular use. Those are chromic acid, cuprous chloride (sub- or proto-chloride of copper), and bleaching- powder. Chromic acid is employed in the form of a solution acidified with acetic or hydrochloric acid, which, in order to obtain the advantages (see below) attendant upon the use of a solid purifying material, is absorbed in that highly porous and inert description of silica known as infusorial earth or "kieselguhr." This substance was first recommended by Ullmann, and is termed commercially "heratol" As sold it contains somewhere about 136 grammes of chromic acid per kilo. Cuprous chloride is used as a solution in strong hydrochloric acid mixed with ferric chloride, and similarly absorbed in kieselguhr. From the name of its proposer, this composition is called "frankoline." It will be shown in Chapter VI. that the use of metallic copper in the construction of acetylene apparatus is not permissible or judicious, because the gas is liable to form therewith an explosive compound known as copper acetylide; it might seem, therefore, that the employment of a copper salt for purification courts accident. The objection is not sound, because the acetylide is not likely to be produced except in the presence of ammonia; and since frankoline is a highly acid product, the ammonia is converted into its chloride before any copper acetylide can be produced. As a special acetylene purifier, bleaching-powder exists in at least two chief modifications. In one, known as "acagine," it is mixed with 15 per cent. of lead chromate, and sometimes with about the same quantity of barium sulphate; the function of the latter being simply that of a diluent, while to the lead chromate is ascribed by its inventor (Wolff) the power of retaining any chlorine that may be set free from the bleaching-powder by the reduction of the chromic acid. The utility of the lead chromate in this direction has always appeared doubtful; and recently Keppeler has argued that it can have no effect upon the chlorine, inasmuch as in the spent purifying material the lead chromate may be found in its original condition unchanged. The second modification of bleaching-powder is designated "puratylene," and contains calcium chloride and quick or slaked lime. It is prepared by evaporating to dryness under diminished pressure solutions of its three ingredients, whereby the finished material is given a particularly porous nature.
It will be observed that both heratol and frankoline are powerfully acid, whence it follows they are capable of extracting any ammonia that may enter the purifier; but for the same reason they are liable to act corrosively upon any metallic vessel in which they are placed, and they therefore require to be held in earthenware or enamelled receivers. But since they are not liquid, the casing of the purifier can be safely constructed of steel or cast iron. Puratylene also removes ammonia by virtue of the calcium chloride in it. Acagine would probably pass the ammonia; but this is no real objection, as the latter can be extracted by a preliminary washing in water. Heratol changes, somewhat obscurely, in colour as it becomes spent, its original orange tint, due to the chromic acid, altering to a dirty green, characteristic of the reduced salts of chromium oxide. Frankoline has been asserted to be capable of regeneration or revivification, i.e., that when spent it may be rendered fit for further service by being exposed to the air for a time, as is done with gas oxide; this, however, may be true to some extent with the essential constituents of frankoline, but the process is not available with the commercial solid product. Of all these materials, heratol is the most complete purifier of acetylene, removing phosphorus and sulphur most rapidly and thoroughly, and not appreciably diminishing in speed or efficiency until its chromic acid is practically quite used up. On the other hand, heratol does act upon pure acetylene to some extent; so that purifiers containing it should be small in size and frequently recharged. In one of his experiments Keppeler found that 13 per cent. of the chromic acid in heratol was wasted by reacting with acetylene. As this waste of chromic acid involves also a corresponding loss of gas, small purifiers are preferable, because at any moment they only contain a small quantity of material capable of attacking the acetylene itself. Frankoline is very efficacious as regards the phosphorus, but it does not wholly extract the sulphur, leaving, according to Keppeler, from 0.13 to 0.20 gramme of the latter in every cubic metre of the gas. It does not attack acetylene itself; and if, owing to its free hydrochloric acid, it adds any acid vapours to the purified gas, these vapours may be easily removed by a subsequent passage through a vessel containing lime or a carbide drier. Both being essentially bleaching-powder, acagine and puratylene are alike in removing phosphorus to a satisfactory degree; but they leave some sulphur behind. Acagine evidently attacks acetylene to a slight extent, as Keppeler has found 0.2 gramme of chlorine per cubic metre in the issuing gas.
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.
ÉPURÈNE.—A purifying material to which the name of épurène has been given has been described, by Mauricheau-Beaupré, 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 épurène 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-Beaupré, suffices to double the purifying capacity of one charge of the material, while a larger proportion would achieve its continuous revivification. Épurène 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 Bergé'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.


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