MANUFACTURE] GAS The liquid products of the destructive distillation of coal are tar and ammoniacal liquor. Tar derived from ordinary bituminous coal is a black, somewhat viscid liquid, varying in specific Liquid products. gravity from 11 to 1.2. The ultimate composition of tar made in the London Gas Works is approximately as follows: Carbon Hydrogen Nitrogen Sulphur Oxygen. 77.53 6.33 1.03 0.61 14.50 100.00 These elements in tar are built up into an enormous number of compounds (see COAL TAR), and its value as a by-product may be gathered from the fact that on fractional distillation it yields (1) benzene and its homologues, from which aniline, the source of most of the coal-tar colours, can be derived; (2) carbolic acid, from which picric acid, used as a dye, a powerful explosive, and to give the bitter flavour to some kinds of beer, is made, also many most valuable disinfectants; (3) naphthalene, used for disinfecting, and also as the "Albo-carbon employed in an enriching burner for gas; (4) pitch, extensively used in path-making, from which such bodies as anthracene and saccharin can be extracted. The second liquid product of the destructive distillation of coal is the ammoniacal or gas liquor, which consists of water containing ammonia salts in solution, partly condensed from the hot gas, and partly added to wash the gas in the scrubbers. It contains, as its principal constituents, ammonia, partly combined with carbonic acid and sulphuretted hydrogen to form compounds which are gas, and partly decomposed on boiling, with evolution of ammonia combined with stronger acids to form compounds which require to be acted upon by a strong alkali before the ammonia contained in them can be liberated: The ammonia in the first class of compounds is technically spoken of as "free"; that present in the latter as "fixed." The following analysis by L. T. Wright will give an idea of the relative quantities in which these compounds exist in the liquor: Grammes per litre. 3.03 39.16 485 | M.P.E.Berthelot came to the conclusion that the illuminating value of the Paris coal gas was almost entirely due to benzene vapour. But here again another mistaken idea arose, owing to a faulty method of estimating the benzene, and there is no doubt that methane is one of the most important of the hydrocarbons present, when the gas is burnt in such a way as to evolve from it the proper illuminating power, whilst the benzene vapour, small as the quantity is, comes next in importance and the ethylene last. It is the combined action of the hydrocarbons which gives the effect, not any one of them acting alone. The series of operations connected with the manufacture and distribution of coal gas embraces the processes of distillation, condensation, exhaustion, wet purification by washing and scrubbing, dry purification, measuring, storing and distribution to the mains whence the consumer's supply is drawn. Site of gas works. The choice of a site for a gas works is necessarily governed by local circumstances; but it is a necessity that there should be a ready means of transport available, and for this reason the works should be built upon the banks of a navigable river or canal, and should have a convenient railway siding. By this means coal may be delivered direct to the store or retorthouse, and in the same way residual products may be removed. The fact that considerable area is required and that the works do not improve the neighbourhood are important conditions, and although economy of space should be considered, arrangements should be such as to allow of extension. In the case of a works whose daily make of gas exceeds four to five million cub. ft., it is usual to divide the works into units, there being an efficiency limit to the size of apparatus employed. Under these conditions the gas is dealt with in separate streams, which mix when the holder is reached. From the accompanying ground plan of a works (fig. 4) River. From a scientific point of view, the term "free" is absolutely incorrect, and in using it the fact must be clearly borne in mind that in this case it merely stands for ammonia, which can be liberated on simply boiling the liquor. The gas which is obtained by the destructive distillation of coal, and which we employ as our chief illuminant, is not a definite com products. pound, but a mechanical mixture of several gases, some Gaseous of which are reduced to the lowest limit, in order to develop as fully as possible the light-giving properties of the most important constituents of the gas. The following analysis gives a fair idea of the composition of an average sample of gas made from coal, purified but without enrichment: Hydrogen Unsaturated hydrocarbons 52.22 These constituents may be divided into—(a) light-yielding hydrocarbons, (b) combustible diluents and (c) impurities. The hydrocarbons, upon which the luminosity of the flame entirely depends, are divided in the analysis into two groups, saturated and unsaturated, according to their behaviour with a solution of bromine in potassium bromide, which has the power of absorbing those termed unsaturated," but does not affect in diffused daylight the gaseous members of the " saturated "series of hydrocarbons. They may be separated in a similar way by concentrated sulphuric acid, which has the same absorbent effect on the one class, and not on the other. The chief unsaturated hydrocarbons present in coal gas are: ethylene, CH, butylene, C.H,, acetylene, CH,, benzene, CH, and naphthalene, CH, and the saturated hydrocarbons consist chiefly of methane, CH, and ethane, C2H. The light-giving power of coal gas is undoubtedly entirely due to the hydrocarbons. The idea held up to about 1890 was that the illuminating value depended upon the amount of ethylene present. This, however, is manifestly incorrect, as, if it were true, 4% of ethylene mixed with 96% of a combustible diluent such as hydrogen should give 16- to 17-candle gas, whereas a mixture of 10% of ethylene and 90% of hydrogen is devoid of luminosity. In 1876 Offices FIG. 4.-Plan of Works. Retorts. it will be possible to gain an idea of the order in which the operations The retorts in which the coal is carbonized are almost universally in gas manufacture are carried out and the arrangement of the plant. ended retort, which was about 9 ft. in length, has given made of fire-clay, and in all but small country works the old singleway to a more economical construction known as doubles, double-ended, or "through" retorts. These are from 18 to 22 ft. long, and as it is found inconvenient to produce this length in one piece, they are manufactured in three sections, the jointing together of which demands great care. The two outer pieces are swelled at one end to take an iron mouthpiece. The cross sections generally employed for retorts are known as "D-shaped," "oval" and "round" (fig. 5). The "D" form is mostly adopted owing to its power of retaining its shape after long exposure to heat, and the large amount of heating surface it presents at its base. The life of this retort is about thirty working months. A cast iron mouthpiece and lid is bolted to the exterior end of each retort, socket end to receive the the mouthpiece carrying a The ascension pipe, through which the gas passes_on retorts are heated exterleaving the retort. nally and are set in an arch, the construction depending upon the number of retorts, twelve. The arch and its which varies from three to retorts is termed a bed or FIG. 5.-Cross Section of Retorts. setting, and a row of beds constitutes a bench. It is usual to have a separate furnace for each setting, the retorts resting The heating of the retorts is carried out either by the "direct upon walls built transversely in the furnace. or by the "regenerative" system, the latter affording firing" marked advantages over the former method, which is now becoming For the purpose of discharging the coke from the retort either extinct. In the regenerative system of firing, a mixture of carbon compressed air or hydraulic machinery is employed, a rake being monoxide and nitrogen is produced by passing air through incan-made to enter the retort and withdraw the coke on returning. With descent gas coke in a generator placed below the bench of retorts, this method it is necessary that the rake should enter and discharge and the heating value of the gases so produced is increased in most several times before the retort is clear, and thus the use of a telescopic cases by the admixture of a small proportion of steam with the ram worked by hydraulic power, which pushes the coke before it primary air supply, the steam being decomposed by contact with and discharges it at the other end, is an advantage. As much as the red-hot coke in the generator into water gas, a mixture of carbon one-third on each ton of coal carbonized is saved by the use of monoxide and hydrogen (see FUEL: Gaseous). The gases so formed machinery in the retort-house. Taking into account the original vary in proportion with the temperature of the generator and the cost of such machines, and the unavoidable wear and tear upon the amount of steam, but generally contain 32 to 38% of combustible retorts brought about by using labour-saving appliances, and the gas, the remainder being the residual nitrogen of the air and carbon fact that the coke-dust is very detrimental to the machinery, it is dioxide. These gases enter the combustion chamber around the retorts clear that the suggestion of setting the retorts at an incline in order at a high temperature, and are there supplied with sufficient air to to facilitate the work presented great inducements to the gas manager. complete their combustion, this secondary air supply being heated by The object aimed at in thus setting retorts is to allow gravity to the hot products of combustion on their way to the exit flue. This play the part of charging and discharging the coal and coke, the method of firing results in the saving of about one-third the weight retorts being inclined at an angle to suit the slip of the class of coal of coke used in the old form of furnace per ton of coal carbonized, used; this angle is between 28° and 34° The coal, previously and enables higher temperatures to be obtained, the heat being also elevated to hoppers, is dropped into the feeding chambers, which are more equally distributed. so arranged that they can travel from end to end of the retorthouse and feed the coal into the retorts. When the retort is to be charged, an iron stop or barrier is placed in the lower mouthpiece, and the door closed. The shoot is placed in the upper mouthpiece, and the stop or door, which retains the coal in the chamber, is released; the coal is then discharged into the retort, and rushing down the incline, is arrested by the barrier, and banks up, forming a continuous backing to the coal following. By experience with the class of coal used and the adjustment of the stops in the shoot, the charge can be run into the retort to form an even layer of any desired depth. For the withdrawal of the residual coke at the end of the carbonization, the lower mouthpiece door is opened, the barrier removed and the coke in the lower part of the retort is "tickled or gently stirred with an iron rod to overcome a slight adhesion to the retort; the entire mass then readily discharges itself. Guides are placed in front of the retort to direct its course to the coke hoppers or conveyer below, and to prevent scattering of the hot material. This system shows a greater economy in the cost of carbonizing the coal, but the large outlay and the wear and tear of the mechanical appliances involved have so far prevented its very general adoption. There are a great number of methods of applying the regenerative principle which vary only in detail. Fig. 6 gives an idea of the general arrangement. The furnace A is built of fire-brick, coke is charged at the top through the iron door B, and near the bottom are placed fire bars C, upon which the fuel lies. The primary air necessary for the partial combustion of the coke to" producer "gas enters between these bars. The gases are conducted from the furnace to the combustion chamber E through the nostrils D D, and the secondary air is E FIG. 6.-Regenerative Setting. admitted at the inlet F a little above, this air having been already heated by traversing the setting. Complete combustion takes place at this point with the production of intense heat, the gases on rising are baffled in order to circulate them in every direction round the retorts, and upon arriving at the top of the setting they are conducted down a hollow chamber communicating with the main flue and shaft. The amount of draft which is necessary to carry out the circulation of the gases and to draw in the adequate amount of air is regulated by dampers placed in the main flue. By analysis of the " producer and "spent gases this amount can be readily gauged. Retorts are set in either the horizontal, inclined or vertical position, and the advantages of the one over the other is a question upon which almost every gas engineer has his own views. The introduction of labour-saving appliances into gas works has rendered the difficult work of charging and discharging horizontal retorts comparatively simple. Formerly it was the Charging practice to carry out such operations entirely by hand, and men charging the retorts either by means of shovel or drawing. hand-scoop, and the coke produced being withdrawn with hand rakes. Now, however, only the smaller gas works adhere to this system, and this work is done by machinery driven by either compressed air, hydraulic or electric power. In the first two cases a scoop, filled with coal from an overhead hopper carried by the travelling machine, is made to enter the retort and is turned over; the operation is then repeated, but this time the scoop is turned over in the opposite direction, the coal thus assuming such a position that as much of its under surface as possible is exposed to the heated side of the retort. With "through" retorts charging machines feed the retorts at both ends, the scoop, which has a capacity of about 1 cwt., entering and discharging its contents twice at each end, so that the total charge is about 6 cwt., which is allowed from four to six hours to distil off according to the quality of the gas required. The machines charge simultaneously at each end, so that the lids of the retorts may be shut immediately the coal enters. The charging machines travel on lines in front of the retort bench, and the power is transmitted by connexions made with flexible hose. A device of more recent introduction is an electrically-driven charging machine, in which the centrifugal force created by a fly-wheel revolving at high speed is applied to drive coal into the retort. If the velocity is sufficiently high the coal may be carried the whole length of a 20-ft. retort, the coal following banking up until an even layer is formed throughout the length of the retort. The vertical retort was one of the first forms experimented with by Murdoch, but owing to the difficulty of withdrawing the coke, the low illuminating power of the gas made in it, and the damage to the retort itself, due to the swelling of the charge during distillation, it was quickly abandoned. About the beginning of the 20th century, however, the experiments of Messrs Settle and Padfield at Exeter, Messrs Woodall and Duckham at Bournemouth, and Dr of the vertical retort again came to the front, and several systems Bueb in Germany showed such encouraging results that the idea were proposed and tried. The cause of the failure of Murdoch's original vertical retort was undoubtedly that it was completely filled with coal during charging, with the result that the gas liberated from the lower portions of the retort had to pass through a deep bed of red-hot coke, which, by over-baking the gas, destroyed the illuminating hydrocarbons. There is no doubt that the question of rapidly removing the gas, as soon as it is properly formed, from the influence of the highly-heated walls of the retort and residual coke, is one of the most important in gas manufacture. In the case of horizontal retorts the space between the top of the coal and the retort is of necessity considerable in order to permit the introduction of the scoop and rake; the gas has therefore a free channel to travel along, but has too much contact with the highly heated surface of the retort before it leaves the mouthpiece. In the case of inclined retorts this disadvantage is somewhat reduced, but with vertical retorts the ideal conditions can be more nearly approached. The heating as well as the illuminating value of the gas per unit volume is lowered by over-baking, and Dr Bueb gives the following figures as to the heating value of gas obtained from the same coal but by different methods of carbonization: Vertical Retorts, 604 British thermal units per cub. ft. "1 584 " " Of the existing forms of vertical retort it remains a matter to be decided whether the coal should be charged in bulk to the retort or whether it should be introduced in small quantities at regular and short intervals; by this latter means (the characteristic feature of the Settle-Padfield process) a continuous layer of coal is in process of carbonization on the top, whilst the gas escapes without contact with the mass of red-hot coke, a considerable increase in volume and value in the gas and a much denser coke being the result. From the retort the gas passes by the ascension pipe to the hy draulic main (fig. 7). This is a long reservoir placed in a horizontal position and supported by columns upon the top of the Hydraulic retort stack, and through it is maintained a slow but constant flow of water, the level of which is kept uniform. malo. The ascension pipe dips about 2 in. into the liquid, and so makes a seal that allows of any retort being charged singly without the risk of the gas produced from the other retorts in the bench escaping through the open retort. Coal gas, being a mixture of gases and vapours of liquids having very varying boiling points, must necessarily undergo physical changes when the temperature is lowered Vapours of liquids of high boiling point will be condensed more quickly than those having lower boiling points, but condensation of each vapour will take place in a definite ratio with the decrease of temperature, the rate being dependent upon the boiling point of the liquid from which it is formed. The result is that from the time the gaseous mixture leaves the retort it begins to deposit condensation products owing to the decrease in temperature. Condensation takes place in the ascension pipe, in the arch piece leading to the hydraulic main, and to a still greater extent in the hydraulic main itself where the gas has to pass through water. Ascension pipes give trouble unless they are frequently cleared by an instrument called an "auger," whilst the arch pipe is fitted with hand holes through which it may be easily cleared in case of stoppage. The most soluble of the constituents of crude coal gas is ammonia, 780 volumes of which are soluble in one volume of water at normal temperature and pressure, and the water in the hydraulic main absorbs a considerable quantity of this compound from the gas and FIG. 7-Hydraulic although the liquor is well agitated by the gas helps to form the ammoniacal liquor, whilst, bubbling through it, a partial separation of tar from liquor is effected by gravitation. The liquor is run off at a constant rate from the hydraulic main to the store tank, and the gas passes from the top of the hydraulic main to the foul main. tion. Main. The gas as it leaves the hydraulic main is still at a temperature of from 130° to 150° F., and should now be reduced as nearly as Condensa possible to the temperature of the surrounding atmosphere. The operation of efficient condensing is not by any means as simple as might be supposed. The tar and liquor when condensed have a dissolving action on various valuable light-giving constituents of the gas, which in the ordinary way would not be deposited by the lowering of temperature, and for this reason the heavy tar, and especially that produced in the hydraulic main, should come in contact with the gas as little as possible, and condensation should take place slowly. The main difficulty which the condenser ought to overcome and upon which its efficiency should depend is the removal of naphthalene; this compound, which is present in the gas, condenses on cooling to a solid which crystallizes out in the form of white flakes, and the trouble caused by pipe stoppages in the works as well as in the district supplied is very considerable. The higher the heat of carbonization the more naphthalene appears to be produced, and gas managers of to-day find the removal of naphthalene from the gas a difficult problem to solve. It was for some time debated as to whether naphthalene added materially to the illuminating value of the gas, and whether an endeavour should be made to carry it to the point of combustion; but it is now acknowledged that it is a troublesome impurity, and that the sooner it is extracted the better. Gas leaves the retorts saturated with naphthalene, and its capacity for holding that impurity seems to be augmented by the presence of water vapour. The condenser, by effecting the condensation of water vapour, also brings about the deposition of solid naphthalene, apart from that which naturally condenses owing to reduction of temperature. Condensers are either air-cooled or water-cooled, or both. In the former case the gas traverses pipes exposed to the atmosphere and so placed that the resulting products of condensation may be collected at the lowest point. Water is a more efficient cooling medium than air, owing to its high specific heat, and the degree of cooling may be more easily regulated by its use. In water-cooled condensers it is usual to arrange that the water passes through a large number of small pipes contained in a larger one through which the gas flows, and as it constantly happened that condenser pipes became choked by naphthalene, the so-called reversible condenser, in which the stream of gas may be altered from time to time and the walls of the pipes cleaned by pumping tar over them, is a decided advance. The solubility of naphthalene by various oils has led some engineers to put in naphthalene washers, in which gas is brought into contact with a heavy tar oil or certain fractions distilled from it, the latter being previously mixed with some volatile hydrocarbon to replace in the gas those illuminating vapours which the oil dissolves out; and by fractional distillation of the washing oil the naphthalene and volatile hydrocarbons are afterwards recovered. The exhauster is practically a rotary gas pump which serves the purpose of drawing the gas from the hydraulic main through the Exhauster, condensers, and then forcing it through the purifying vessels to the holder. Moreover, by putting the retorts under a slight vacuum, the amount of gas produced is increased by about 12%, and is of better quality, owing to its leaving the heated retort more quickly. A horizontal compound steam-engine is usually employed to drive the exhauster. At this point in the manufacturing process the gas has already undergone some important changes in its composition, but there yet remain impurities which must be removed, these being ammonia, sulphuretted hydrogen, carbon disulphide and carbon dioxide. Ammonia is of considerable marketable value, and even in places where the local Gas Act does not prescribe that it shall be removed, it is extracted. Sulphuretted hydrogen is a noxious impurity, and its complete removal from the gas is usually imposed by parliament. As nearly as possible all the carbon dioxide is extracted, but most gas companies are now exempt from having to purify the gas from sulphur compounds other than sulphuretted hydrogen. Cyanogen compounds also are present in the gas, and in large works, where the total quantity is sufficient, their extraction is effected for the production of either prussiate or cyanide of soda. Atkinson Butterfield gives the composition of the gas at this point to be about per cent. by vol. from 42 Hydrogen Methane . "39 "10 to 53 99 4.5 Sulphuretted hydrogen Ammonia Cyanogen Carbon disulphide Naphthalene Carbon dioxide. Nitrogen Washers. It happens that ammonia, being a strong base, will effect the extraction of a certain proportion of such compounds as sulphuretted hydrogen, carbon dioxide and hydrocyanic acid, and the gas is now washed with water and ammoniacal liquor. The process is termed washing or scrubbing, and is carried out in various forms of apparatus, the efficiency of which is dependent upon the amount of contact the apparatus allows between the finely divided gas and water in a unit area and the facility with which it may be cleared out. The "Livesey "washer, a well-known type, is a rectangular cast iron vessel. The gas enters in the centre, and to make its escape again it has to pass into long wrought iron diameter. A constant flow of liquor is regulated through the washer, inverted troughs through perforations one-twentieth of an inch in and the gas, in order to pass through the perforations, drives the liquor up into the troughs. The liquor foams up owing to agitation by the finely divided streams of gas, and is brought into close contact with it. Two or three of these washers are connected in serics according to the quantity of gas to be dealt with. The final washing for ammonia is effected in an apparatus termed a" scrubber," which is a cylindrical tower packed with boards in. thick by 11 in. broad, placed on end and close together; Scrubbers. water is caused to flow down over the surface of these boards, the object being to break up the gas as much as possible and bring it into close contact with the water. In this wet purifying apparatus the gas is almost wholly freed from ammonia and from part of the sulphuretted hydrogen, whilst carbon dioxide and carbon disulphide are also partially extracted. FIG. 9.-Purifier Grid. When the gas had to be purified from carbon disulphide as well as from sulphuretted hydrogen, slaked lime was employed for the removal of carbon dioxide and the greater quantity of the sulphur compounds, whilst a catch box or purifier of oxide of iron served to remove the last traces of sulphuretted hydrogen. Not fewer than four lime purifiers were employed, and as the one which was first in the series became exhausted, i.e. began to show signs of allowing carbon dioxide to pass through it unabsorbed, it was filled with fresh slaked lime and made the last of the series, the one which was second becoming first, and this procedure went on continuously. This operation was necessitated by the fact that carbon dioxide has the power of breaking up the sulphur compounds formed by the lime, so that until all carbon dioxide is absorbed with the formation of calcium carbonate, the withdrawal of sulphuretted hydrogen cannot proceed, whilst since it is calcium sulphide formed by the absorption of sulphuretted hydrogen by the slaked lime that absorbs the vapour of carbon disulphide, purification from the latter can only be accomplished after the necessary calcium sulphide has been formed. The foul gas leaving the scrubbers contains, as a general average, 30 grains of sulphuretted hydrogen, 40 grains of carbon disulphide and 200 grains of carbon dioxide per 100 cub. ft. On entering the first purifier, which contains calcium thiocarbonate and other combinations of calcium and sulphur in small quantity, the sulphuretted hydrogen and disulphide vapour have practically no action upon the material, but the carbon dioxide immediately attacks the calcium thiocarbonate, forming calcium carbonate with the production of carbon disulphide vapour, which is carried over with the gas into the second box. In the connexion between the first and the second box the gas is found to contain 500 grains of sulphuretted hydrogen and 80 grains of carbon disulphide per 100 cub. ft., but no trace of carbon dioxide. In the second box the formation of calcium thiocarbonate takes place by the action of carbon disulphide upon the calcium sulphide with the liberation of sulphuretted hydrogen, which is carried over to the third purifier. The gas in the connecting pipe between the second and third purifier will be found to contain 400 grains of sulphuretted hydrogen and 20 grains of carbon disulphide. The contents of the third box, being mostly composed of slaked lime, take up sulphuretted hydrogen forming calcium sulphide, and practically remove the remaining impurities, the outlet gas showing 20 grains of sulphuretted hydrogen and 8 grains of carbon disulphide per 100 cub. ft., whilst the catch box of oxide of iron then removes all traces of sulphuretted hydrogen. It will be noticed that in the earlier stages the quantity of sulphur impurities is actually increased between the purifiers-in fact, the greater amount of sulphiding procures the ready removal of the carbon disulphide, but it is the carbon dioxide in the gas that is the disturbing element, inasmuch as it decomposes the combinations of sulphur and calcium; consequently it is a paramount object in this system to prevent this latter impurity finding its way through the first box of the series. The finding of any traces of carbon dioxide in the gas between the first two boxes is generally the signal for a new clean purifier being put into action, and the first one shut off, emptied and recharged with fresh lime, the impregnated material being sometimes sold for dressing certain soils. The action of oxide of iron, which has now partly replaced the lime purification, depends on its power of combining with sulphuretted hydrogen to form sulphide of iron. Such is the affinity of the oxide for this impurity that it may contain from 50 to 60% by weight of free sulphur after revivification and still remain active. Upon removing the material from the vessel and exposing it to the atmosphere the sulphide of iron undergoes a revivifying process, the oxygen of the air displacing the sulphur from the sulphide as free sulphur, and with moisture converting the iron into hydrated oxide of iron. This revivification can be carried on a number of times until the material when dry contains about 50% of free sulphur and even occasionally 60% and over; it is then sold to manufacturers of sulphuric acid to be used in the sulphur kilns instead of pyrites (see SULPHURIC ACID). Apart from the by-products coke, coke-breeze, tar and retort carbon, which are sold direct, gas companies are now in many cases preparing from their spent purifying material pure chemical products which are in great demand. The most important of these is sulphate of ammonia, which is used for agricultural purposes as a manure, and is obtained by passing ammonia into sulphuric acid and crystallizing out the ammonium sulphate produced. To do this, saturated ammoniacal liquor is decomposed by lime in the presence of steam, and the freed ammonia is passed into strong sulphuric acid, the saturated solution of ammonium sulphate being carefully crystallized. The market value of the salt varies, but an average figure is £12 per ton, whilst the average yield is about 24 lb of salt per ton of coal carbonized. In large works the sulphuric acid is usually manufactured on the spot from the spent oxide, so that the sulphuretted hydrogen, which in the gas is considered an undesirable impurity, plays a valuable part in the manufacture of an important by-product. from the spent oxide or from ammoniacal liquor, and some large gas Cyanogen compounds are extracted either direct from the gas, works now produce sodium cyanide, this being one of the latest developments in the gas chemical industry. a gasometer), which may be either single lift, i.e. a simple bell inThe purified gas now passes to a gasholder (sometimes known as verted in a tank of water, or may be constructed on the telescopic principle, in which case much ground space is Gasholder. saved, as a holder of much greater capacity can be contained in the same-sized tank. The tank for the gasholder is usually made by FIG. 10. Gasholder. excavating a circular reservoir somewhat larger in diameter than the proposed holder. A banking is allowed to remain in the centre, as shown in fig. 10, which is known as the " dumpling," this arrange ment not only saving work and water, but acting as a support for the king post of a trussed holder when the holder is empty. The tank must be water-tight, and the precaution necessary to be taken in order to ensure this is dependent upon the nature of the soil: it is usual, however, for the tanks to be lined with concrete. Where the conditions of soil are very bad, steel tanks are built above ground, but the cost of these is much greater. The holder is made of sheet iron riveted together, the thickness depending upon the size of the holder. The telescopic form consists of two or more lifts which slide in one another, and may be described as a single lift holder encircled by other cylinders of slightly larger diameter, but of about the same length. Fig. 10 shows the general construction. Gas on entering at A causes the top lift to rise; the bottom of this lift being turned up all round to form a cup, whilst the top of the next lift is turned down to form a so-called grip, the two interlock (see fig. 11), forming what is known as the hydraulic cup. Under these conditions the cup will necessarily be filled with water, and a seal will be formed, preventing the escape of gas. A guide framing is built round the holder, and guide rollers are fixed at various intervals round the grips of each lift, whilst at the bottom of the cup guide rollers are also fixed (fig. 11). In the year 1892 the largest existing gasholder was built at the East Greenwich works of the South Metropolitan Gas Company; it has six lifts, its diameter is 293 ft., and when filled with gas stands 180 ft. high. The capacity for gas is 12 million cub. ft. The governor consists usually of a bell floating in a cast iron tank partially filled with water, and is in fact a small gas- Governor. holder, from the centre of which is suspended a conical valve controlling the gas inlet and closing it as the bell fills. Any deviation in pressure will cause the floating bell to be lifted or lowered, and the size of the inlet will be decreased or increased, thus regulating the flow. FIG. 11.-Cup and Grip. The fact that coal gas of an illuminating power of from 14 to 16 candles can be made from the ordinary gas coal at a fairly low rate. while every candle power added to the gas increases the cost in as enormous and rapidly growing ratio, has, from the earliest days of the gas industry, caused the attention of inventors to be turned to the enrichment of coal gas. Formerly cannel coal was used for producing a very rich gas which could be mixed with the ordinary gas, thereby enriching it, but as the supply meat. became limited and the price prohibitive, other methods Enrich were from time to time advocated to replace its use in the enrich- 2. Mixing with the coal gas oil gas, obtained by decomposing crude oils by heat. 3. The carburetting of low-power gas by impregnating it with the vapours of volatile hydrocarbons. 4. Mixing the coal gas with water gas, which has been highly carburetted by passing it with the vapours of various hydrocarbons through superheaters in order to give permanency to the hydrocarbon gases. Very many attempts have been made to utilize tar for Enrichthe production and enrichment of gas, and to do this ment by two methods may be adopted: tar. (a) Condensing the tar in the ordinary way, and afterwards using the whole or portions of it for cracking into a permanent gas. (b) Cracking the tar vapours before condensation by passing the gas and vapours through superheaters. has gone on for three hours, the rich portions of coal have distilled The fundamental objections to oil gas for the enrichment of coal gas are, first, that its manufacture is a slow process, requiring as much plant and space for retorting as coal gas; and, secondly, that although on a small scale it can be made to mix perfectly with coal gas and water gas, great difficulties are found in doing this on the large scale, because in spite of the fact that theoretically gases of such widely different specific gravities ought to form a perfect mixture by diffusion, layering of the gas is very apt to take place in the holder, and thus there is an increased liability to wide variations in the illuminating value of the gas sent out. Earichment by volatile The wonderful carburetting power of benzol vapour is well known, a large proportion of the total illuminating power of coal gas being due to the presence of a minute trace of its vapour carried in suspension. For many years the price of benzol has been falling, owing to the large quantities produced in the coke ovens, and at its present price it is by far the cheapest enriching material that can be obtained. Hence at many gas-works where it is found necessary to do so it is used in various forms of carburettor, in which it is volatilized and its vapour used for enriching coal gas up to the requisite illuminating power. hydrocarbons. Earich If the first method be adopted, the trouble which presents itself is that the tar contains a high percentage of pitch, which tends rapidly to choke and clog up all the pipes. A partly successful attempt to make use of certain portions of the liquid products of distillation of coal before condensation by the second method was the Dinsmore process, in which the coal gas and vapours which, if allowed to cool, would form tar, were made to pass through a heated chamber, and a certain proportion of otherwise condensible hydrocarbons was thus converted into permanent gases. Even with a poor class of coal it was claimed that 9800 cub. ft. of 20- to 21-candle gas could be made by this process, whereas by the ordinary process 9000 cub. ft. of 15-candle gas would have been produced. This process, although strongly advocated by the gas engineer who experimented with it, was never a commercial success. The final solution of the question of enrichment of gas by hydrocarbons derived from tar may be arrived at by a process which prevents the formation of part of the tar during the carbonization of the coal, or by the process devised by C. B. Tully and now in use at Truro, in One of the most generally adopted methods of enrichment now which tar is injected into the incandescent fuel in a water-gas gener- is by means of carburetted water gas mixed with poor coal gas. ator and enriches the water gas with methane and other hydro-When steam acts upon carbon at a high temperature the carbons, the resulting pitch and carbon being filtered off by the resultant action may be looked upon as giving a mixture ment by column of coke through which the gas passes. of equal volumes of hydrogen and carbon monoxide, both carburetted The earliest attempts at enrichment by oil gas consisted in spray- of which are inflammable but non-luminous gases. This water gas. ing oil upon the red hot mass in the retort during carbonization; water gas is then carburetted, ie, rendered luminous by but experience soon showed that this was not an econo- passing it through chambers in which oils are decomposed by heat, Enrich. mical method of working, and that it was far better to the mixture being made so as to give an illuminating value of 22 meat by decompose the liquid hydrocarbon in the presence of the to 25 candles. This, mixed with the poor coal gas, brings up its oil gas. diluents which are to mingle with it and act as its carrier, illuminating value to the required limit. Coke or anthracite is since, if this were done, a higher temperature could be employed heated to incandescence by an air blast in a generator lined with and more of the heavier portions of the oil converted into gas, with-fire-brick, and the heated products of combustion as they leave the out at the same time breaking down the gaseous hydrocarbons generator and enter the superheaters are supplied with more air, too much. In carburetting poor coal gas with hydrocarbons from which causes the combustion of carbon monoxide present in the mineral oil it must be borne in mind that, as coal is undergoing producer gas and heats up the fire-brick baffles with which the superdistillation, a rich gas is given off in the earlier stages, but towards heater is filled. When the necessary temperature of the fuel and the end of the operation the gas is very poor in illuminants, the superheater has been reached, the air blast is cut off, and steam is methane disappearing with the other hydrocarbons, and the increase blown through the generator, forming water gas, which meets the in hydrogen being very marked. Lewis T. Wright employed a coal enriching oil at the top of the first superheater, called the carburettor, requiring six hours for its distillation, and took samples of the gas and carries the vapours with it through the main superheaters, at different periods of the time. On analysis these yielded the where the fixing of the hydrocarbons takes place. The chief advan following results:tage of this apparatus is that a low temperature can be used for Time after beginning Distillation. 5 hours O.II 1.50 67.12 3 hours 25 minutes. 6.12 22.58 Unsaturated 10.62 5.98 3.04 1.79 0.78 This may be regarded as a fair example of the changes which take | place in the quality of the gas during the distillation of the coal. In carburetting such a gas by injecting mineral oil into the retort, many of the products of the decomposition of the oil being vapours, it would be wasteful to do so for the first two hours, as a rich gas is being given off which has not the power of carrying in suspension a much larger quantity of hydrocarbon vapours without being supersaturated with them. Consequently, to make it carry any further quantity in a condition not easily deposited, the oil would have to be completely decomposed into permanent gases, and the temperature necessary to do this would seriously affect the quality of the gas given off by the coal. When, however, the distillation fixing owing to the enormous surface for superheating, and thus to a great extent the deposition of carbon is avoided. This form of apparatus has been very generally adopted in Great Britain as well as in America, and practically all carburetted water-gas plants are founded upon the same set of actions. Important factors in the use of carburetted water gas for enrichment are that it can be made with enormous rapidity and with a minimum of labour; and not only is the requisite increase in illuminating power secured, but the volume of the enriched gas is increased by the bulk of carburetted water gas added, which in ordinary English practice amounts to from 25 to 50%. The public at first strongly opposed its introduction on the ground of the poisonous properties of the carbon monoxide, which is present in it to the extent of about 28 to 30%. Still when this comes to be diluted with 60 to 75% of ordinary coal gas, containing as a rule only 4 to 6% of carbon monoxide, the percentage of poisonous monoxide in the mixture falls to below 16%, which experience has shown to be a fairly safe limit. A rise in the price of oil suitable for carburetting has caused the gas industry to consider other methods by which the volume of gas obtainable from coal can be increased by admixture with blue or nonluminous water gas. In Germany, at several important gas-works, non-luminous water gas is passed into the foul main or through |
