PART VIII.
Purifying Apparatus, or Lime Machine.
Coal gas, even as obtained from the best species of coal, must be rendered pure before it is fit for the purpose of illumination. The gas in its crude state always contains a portion of sulphuretted hydrogen and carbonic acid; and when burnt, although its illuminating power is greater in an impure than in a pure state, it produces an oppressive and suffocating odour, which is speedily perceptible in confined places. The gaseous product evolved during its combustion, blackens paint and tarnishes metallic bodies; an impure gas besides strongly acts upon the copper branch pipes through which it is conveyed.
To obviate these defects the sulphuretted hydrogen and carbonic acid which are the cause of them must be removed, and to effect this, no method more economical and efficacious, has as yet been discovered, than to bring the gas confined under a pressure equal to a column of water, not less than eight or ten inches in height, into contact with quick-lime, diffused through water. Other means have been tried, but all of them have failed to be sufficiently efficacious or economical on a large scale.
Lime Machine originally employed for the Purification of Coal Gas.
In the lime-machine, until lately in use, the gas was made to pass in the apparatus, through passages which could not be guarded from being stopped up in the course of time by the concretion of a quantity of carbonate and hydro-sulphuret of lime, formed during the purification of the gas, so that when the stoppage occurred, a prodigious pressure was produced in the machine, in consequence of which, it was either found impossible to keep the distillatory apparatus air-tight, or if this was accomplished, a great part of the gas was forced through the purifying apparatus, without coming in contact with the lime, by driving the column of mixture of lime and water before it, and of course without being rendered fit for use, previous to its passing into the gas reservoir. This effect was unavoidable without the precaution of employing a very dilute mixture of quick-lime and water.
Numerous instances have also occurred where from the increased pressure which the gas exerted in the lime apparatus, the tar from the hydraulic main was driven up with a prodigious force through the dip pipe, P, fig. 2, [plate IV.], into the retort when the retort was opened, where it took fire to the imminent danger of the whole establishment.
The apparatus originally employed was composed of a large vessel closed on all sides to receive the gas; within this was a smaller vessel or lime trough open at top containing the quick-lime and water; and there was also a third vessel, or inverted trough into which the gas was received.
This inverted trough was open at bottom, and the edge of the open part was immersed beneath the surface of the mixture of lime and water contained in the lime-trough, so that the gas which was introduced in the last-mentioned inverted trough could not escape therefrom, except rising up through the lime and water. To facilitate this, holes or openings were made in the inverted trough near the bottom edge thereof, and beneath the surface of the purifying mixture, so that the bubbles of gas were obliged to rise up through these openings. From this construction of the machine the apertures through which the gas had to pass, were extremely liable to become stopped up, and dangerous consequences ensued.
In order to remedy in some measure the evil, a plan was adopted by Mr. Malam, for making the gas to pass in thin strata underneath a series of shelves, placed horizontally in the machine so as to expose the gas in as large a surface as possible to the contact of the lime and water, and employing the purifying mixture at the same time in a more dilute state:—this arrangement is as follows.
Fig. 4; [plate V.], represents a vertical section of the machine; it is made of cast-iron plates, rendered air-tight by screws, bolts, and iron cement. It consists of three separate chambers, a, a, a, destined to contain the mixture of quick-lime and water. At the under side of each chamber, is bolted a cylinder, h, h, h, the lower extremity of which is furnished with a large flanch, extending nearly to within the whole inner diameter of the machine.
From the bottom of each of the chambers, a, a, a, proceeds a pipe curved upwards, and communicating with a circular vessel, C, C, C, which serve for the purpose of charging the chambers, a, a, a, with the mixture of quick-lime and water, and regulating the level of the fluid within the chambers. The curved pipe likewise prevents the escape of the gas when the contents of the chambers a, a, a, are discharged.
The vessels, C, C, C, are provided with a waste pipe and stop-cock, as shown in the sketch, for discharging the contents of one chamber into the chamber placed below it, and lastly into the reservoir e.
b b, are pipes which convey the gas into the chambers, one extremity of each pipe communicates with the cylinders h, h, h, and the other with the chamber below it, and the lower pipe communicates with the valve M, so that by this means a communication is formed from the lower cylinder h, to the middle cylinder h, and from the middle to the uppermost cylinder. K, is the exit pipe which conveys the purified gas from the uppermost chamber into the reservoir destined to receive it. Through the centre of the machine passes a wrought-iron shaft, furnished with agitators or arms, to stir up the mixture of quick-lime and water. The arms are not immediately connected with the shaft, but proceed from cast-iron hydraulic cups, of the usual construction, by this means the escape of the gas is prevented, nor can the fluid pass from one chamber into another. The axis is put in motion, by wheel-work as shown in the design e, the handle for turning the shaft.
g, is a receiver to collect the condensible products. The contents of this vessel may be discharged by a hand pump being attached to the upper extremity of the pipe f, after the cap with which it is closed is removed.
The operation of this lime machine is obvious. The gas first passes into the lowermost chamber of the cylinder h, where it comes in contact with the purifying mixture and passes through the fluid to the top of the same chamber, and thence through the pipe b, into the cylinder above it which communicates with the lower chamber, where it is acted on again by the lime and water, and bubbles up through the fluid to the top of the chamber. From this compartment the gas passes into the third cylinder, where it bubbles up and passes through the lime and water; and lastly it makes its exit through the pipe K, into the gasholder or vessel destined to receive it.
When the mixture of quick-lime and water in the compartments a, a, a, of the machine, requires to be renewed, it is let off by the stop-cock at the bottom of the lowermost vessel into the reservoir e. The fluid contained in the upper chamber may be discharged into the chamber below it, and so on with the chambers below it, care being taken to close the stop-cock of the lower vessel. The machine may be recharged at the uppermost chamber with the purifying mixture. Fig. 5, exhibits the plan of the machine. b, b, b, the tubes connecting the chambers. B, the flanch of the cylinder h.
This machine has in part remedied the inconveniences stated [pages 141], [142], but the increase in the quantity of the purifying materials which the apparatus requires, is of itself productive of most serious disadvantages.
The greater accumulation of waste lime which such a practice occasions, renders it necessary that capacious reservoirs and sewers should be constructed to receive and carry off the refuse materials, and where an outlet by such means cannot be obtained, the carting away the increased quantities of waste matter adds greatly to the cost of the gas.
If attempts are made to convey the waste substances into the common sewers or drains of the neighbourhood, the proprietors of gas works are exposed to indictments for a nuisance at the suit of the inhabitants, and when the near proximity of any river or lake induces an attempt to convey the waste materials thither, the most serious injury may be done to the water, which becoming impregnated with hydrosulphuret of lime is rendered unfit not only for domestic but for many manufacturing purposes. The latter evil indeed is one which operates also in a greater or lesser degree, even when the fœtid refuse or hydrosulphuret is discharged into the common sewers, all of which ultimately empty themselves into some water course, rivulet or lake. I would here beg to suggest, that considering how rapidly the new mode of procuring light is extending throughout Britain,[37] and how much the waters of the country are liable to be contaminated, from discharging into them the noxious refuse from the process of purifying coal gas, so as to be rendered proportionably unfit for the various purposes of domestic and manufacturing economy, it is well deserving the attention of the legislature, whether such contamination ought not to be guarded against by prohibiting enactments.
[37] The Towns of Edinburgh, Glasgow, Liverpool, Bristol, Bath, Cheltenham, Birmingham, Leeds, Manchester, Exeter, Macclesfield, Kidderminster, Preston, Waterford, Rochester, Chatham and several others, have been lighted with gas within these few years.
It appears to me that it would be a wise exertion of authority, to insert in every act of Parliament granted for incorporating Gas-light Companies, a clause prohibiting the proprietors from ever conveying the waste material, or any other produce from the manufacture of coal gas, either directly or indirectly into the common sewers, drains or water courses, or into rivers and lakes adjacent. The salubrity of the water we use is of as much consequence to us, as any superior excellence or saving of cost in our light can possibly be, and we ought to take care that in profiting by an improvement which science and art have discovered, we do not unnecessarily depreciate one of those primary blessings we owe to the bounty of nature.
Lime Machine lately adopted.
In the improved purifying apparatus[38] lately brought into use, of which we shall now give an account, a shaft or axis furnished with teeth or claws, is applied within the interior of the vessel, and made to act in such a manner as to scrape out the openings or slits through which the gas has to pass every time the axis is moved round, and by which regular clearance all chance of stoppage is avoided without any augmentation of the purifying mixture.
[38] This machine has been adopted at the gas works at Chester, Birmingham, Kidderminster, Bristol, and in many other provincial Gas Establishments.
The lime trough is also made moveable on a centre or axis, in such a manner that it may readily be inverted by a lever from the outside, for the purpose of emptying its contents into the bottom of an exterior vessel, from which the waste materials may be discharged at pleasure.
With this machine we are farther enabled to employ the purifying mixture in a semi-fluid state, and consequently in a much less bulk; and after having suffered it to remain in the reservoir destined to receive it, the waste substance speedily acquires such a degree of solidity that it may be dug out with a spade and carted away in a small compass. The safety of the apparatus is therefore insured and the construction of expensive drains and sewers is rendered unnecessary. The following description will render the construction of the improved apparatus obvious.
A, A, fig. 2, [plate II.], is a rectangular four-sided prism, made of cast-iron plates, screwed together air-tight with bolts and cement. The base of the prism terminates in a rectangular four-sided pyramid placed with its apex downwards. It is surrounded by an iron stage, supported upon pillars, as shown in the design.
Within this vessel, which in fact composes only the outer case of the apparatus, is contained an oblong trough B, fig. 2, [plate II.], (it is shown in the design as if broken down), moveable upon an horizontal axis, fixed to one of its longest sides, so that by means of the wheel C, or lever communicating with the axis, and applied on the outside of the machine, the trough B, may be inverted, and its contents discharged into the exterior case, or lower part A, A, of the machine. The part B, of the machine is called the lime trough, because it is destined to contain the quick-lime and water, by means of which the crude coal gas is purified. Within this trough B, is inverted an oblong rectangular box D, closed at top and open at bottom, called the air-box, because it receives the gas to be purified.
In each of the longest sides of the box D, are perpendicular openings or slits (as shown in the design) exactly opposite to each other. Through the whole length of this box D, passes a horizontal axis furnished with as many teeth or claws as there are upright openings, through each side of the box. These claws extend a little way through the openings so that when the axis, which passes through a stuffing box, is made to revolve by means of the handle X, the ends of the claws pass through the openings and scrape them out every time the axis is turned. The claws operate first on the openings of one side of the box and then on those on the opposite side. They pass quite through and therefore keep them clear; those parts of the claws which enter into the openings are narrow in the direction of their motion, and that part of each claw which is nearest to the centre, is broad and flat, hence they act as paddles or rowers whilst they are in motion, to stir up the quick-lime and water.
Fig. 10, [plate III.], represents a transverse section of this part of the apparatus. B, B, is the lime trough; D, the air box inverted into the lime trough; the dotted circle shows the sweep of the claws when the shaft is put in motion. The darts show the course of the gas.
Fig. 10, [plate VI.], represents a plan of the machine. G, shows the inverted air-trough with its axis, and the claws or teeth fixed upon the axis. H, is the lime trough. A, the outer case of the machine; R, R, the axis, to which is affixed the wheel or lever, for inverting the trough H. L, the axis and handle to give motion to the shaft upon which the claws are fixed, for stirring up the contents of the lime trough.
The inverted air-box D, fig. 2, [plate II.], is supported within the outer case of the machine A, A, fig. 2, [plate II.], by cross bars, and the axis is put in motion by the handle X, on the outside of the machine. It is rendered air-tight by a stuffing box, and is provided with wheel-work, as shewn in the design, fig. 2, [plate II.], to communicate the motion to the axis.
The gas is brought into the air box by the pipe N, fig. 3, which proceeds from the tar vessel, fig. 3, [plate II.] The gas cannot escape out of the machine without displacing the column of fluid in the lime trough, in order to make its way through the openings or upright slits in the side of the air box D, and thus bubbling up through the mixture of quick-lime and water, the depth of which is one foot. The sulphuretted hydrogen and carbonic acid being thus made to combine with the lime, the carburetted hydrogen is left more or less pure, it is conveyed into the gas metre, by the pipe V, where it is to be measured, and from thence by the pipe W, fig. 4, into the gas-holder.
When the purifying mixture is to be removed, the workman unbolts the wheel C, fig. 2, and turns it half way round; (if the emptying of the lime trough requires more power than can conveniently be applied by means of the wheel, the axis of the trough may be worked with a tooth and pinion, a small wheel being attached to the axis of the pinion as a perpetual handle;) this motion inverts the lime trough B, and its contents become discharged into the outer case forming the inverted pyramid of the apparatus, whence the waste materials may be conveyed into the reservoir or pit Q, by drawing open the sliding valve o, fig. 2, [plate II.], or o, fig. 3, [plate VII.], added for that purpose to the discharging pipe P, fig. 2, [plate II.], or p, fig. 3, [plate VII.] To prevent the air entering into the machine when the waste lime is discharged, the lower extremity of the outlet pipe P, dips into the basin Q, fig. 2, [plate II.], which always contains a portion of the waste fluid and thus seals the extremity of the pipe P.
One side of the lime-machine is provided with two large lenses, to admit light into the interior of the apparatus, so that by means of an eye-glass fixed in a proper place, the workman is enabled to see into the interior of the apparatus. And when the machine requires to be cleaned out, the manhole as it is called, is opened for the workmen to enter into the apparatus to remove any solid incrustation of carbonate of lime, or hydrosulphuret of lime that may happen to be formed in the lime trough, or any other part of the apparatus.
The wheel C, is loaded with a counter-weight, to balance the weight of the lime trough. To bring the lime trough again into a proper position, to be re-charged with a fresh portion of the purifying mixture, the workman turns the wheel C half round, the contrary way to that which caused the trough to be turned topside-down, and the trough may then be re-filled with a fresh portion of lime and water from the reservoir R, fig. 2, [plate II.], (or R, fig. 3, [plate VII.],) containing the mixture ready prepared. Y, is a pipe to bring water from a cistern into the lime reservoir R. The prepared lime which is to supply the machine is put into the vessel R, and a sufficiency of water being mixed with it, the mixture is stirred up to the consistence of a semi-fluid mass.
T, shows the pipe furnished with a sliding valve S, for conveying the purifying mixture of quicklime and water into the lime trough from the reservoir R, which is furnished with an agitator to stir up its contents.
To give motion to the shaft for stirring up the contents of the lime trough D, the inventor of this machine (Mr. Clegg,) has happily applied the gas, to act as a power for that purpose. This operation will be explained hereafter in describing the gas metre.
The pipe N, which conveys away the purified gas, proceeds from an hydraulic valve, to cut off the communication between the gas holder and the lime machine, if occasion should require it, and to prevent the gas from passing back from the gas holder into the lime machine.
It consists of a box containing water into which dips a small pipe, by means of which the gas passes out of the lime machine, and from thence into the pipe V, communicating with the gas metre. The box is furnished with a tube curved upwards to discharge the water when it accumulates above the required height, and to prevent any quantity being thrown out of the hydraulic valve, by the concussion of the fluid in the lime trough.
One cubic foot capacity of the lime trough is sufficient to purify 1000 cubic feet of gas obtained from Newcastle coal in twenty-four hours.
Test Apparatus, for certifying the purity of coal gas, and the proper manner of working the Lime Machine.
The proper purification of the gas being a matter of essential importance, as already illustrated [page 140], it becomes of great consequence to have some ready means of ascertaining whether the workman does his duty in this respect, by keeping the lime trough D, fig. 2, [plate II.], properly charged with the requisite quantity of lime and water necessary for the purification of the gas.
For this purpose an apparatus has been adapted by Mr. Clegg to the lime machine, which serves not only to indicate the quantity of fluid contained in the machine when gas is manufactured, but which also enables the workmen to appreciate the quantity of quick-lime necessary for the purification of the gas, and to ascertain its purity. The apparatus consists of a closed cup C, fig. 23, [plate IV.], partly filled with any coloured liquid. Into this cup is cemented, air-tight, a straight glass tube a, about 21⁄2 feet long and a 1⁄4 of an inch in the bore; the lower extremity of the tube nearly touches the bottom of the cup, and is therefore sealed by the fluid. d, d, is a small copper tube, which forms a communication between the air confined above the surface of the fluid in the guage cup C, and the gas which is proceeding into the lime-trough.
The communication may be established at any part of the pipe which conveys the gas into the lime machine. When the connection is made, the fluid in the guage cup C, will be driven up into the perpendicular measuring tube a, by the pressure of the gas, to an altitude equal to a column of liquid contained in the lime-trough. It is essential that the tube a, be at least 21⁄2 feet in height, if the depth of the lime-trough is one foot, for without this precaution, the fluid will rise out of the tube in consequence of the oscillation which it suffers. By this means the overseer of the works will be enabled, by mere inspection, to know whether the workmen have charged the lime trough with the mixture of quick-lime and water to the requisite height, which should never be less than from ten to fifteen inches. Because the abstraction of the sulphuretted hydrogen and carbonic acid gas, from the carburetted hydrogen with which it is combined, is greatly facilitated by pressure, and there is no inconvenience whatever in operating under a pressure of a column of fluid of even double the height that has been stated, provided the apparatus is properly constructed. From experiments made on this subject, I am justified in stating that one half of the quantity of quick-lime that is required for the purification of coal gas in the ordinary way, is sufficient, if the column of the liquid opposed to the gas is raised to twenty inches high, nor is the evolution of the gas in any degree retarded under such a pressure.
The curved tube d d, which is cemented air-tight into the gauge cup c, has a free communication with the gas in the guage cup c. It serves to enable the workmen to form some notion of the chemical constitution of the crude gas, before it passes into the lime machine. For if the stop cock e of the tube be opened, and the descending leg a of the bended tube d be immersed in a glass containing a solution of super acetate of lead, some notion may be formed by a little practice of the quantity of lime requisite for the purification of the gas, from the quantity of (black precipitate) hydrosulphuret of lead produced. Two per cent of quick-lime to the coal employed (if Newcastle coal) is usually sufficient for the complete abstraction of all the sulphuretted hydrogen and carbonic acid, contained in the crude gas, provided the operation be carried on under a pressure of not less than a column of water twelve inches in height.
The test tube f, properly so called, may be adapted to any part of the pipe which conveys the purified gas to its place of destination. It serves to ascertain the purity of the gas, after it has been acted on by quick-lime, by suffering the gas to pass from the tube into a solution of super acetate of lead, which speedily becomes discoloured, if the gas contains sulphuretted hydrogen. The presence of carbonic acid is rendered obvious, by a white precipitate being produced when the gas is made to pass through barytic water. The precipitate, which is carbonate of barytes, effervesces with acids.
It must be obvious that the apparatus which we have now been describing does not require to be placed in the immediate vicinity of the gas light machinery. It may be arranged in the counting-house of the overseer, who, by mere inspection, can then at all times detect the slightest irregularity or insufficiency in the process thus given to the gas light manufacture, a degree of scientific controul of which few arts can boast.
The following method has been found economical and convenient, for preserving quick-lime in a ready state, fit for the purification of coal gas.
Take the lime as soon as possible after it is burnt; put it into a pit eight or ten feet long, five or six wide, and five or six deep, constructed of brick-work and level with the ground. By this pit set a wooden trough about six feet long, three feet broad, and two feet deep. The trough should have at one end a hole about six inches square, covered with an iron grating, the bars of which are a quarter of an inch distant. Let this grating be provided with a slider, which can occasionally be drawn up to uncover, or pushed down to cover, the grating. Put three or four bushels of lime at a time into the trough; throw water on it, and mix it up into a thick fluid mass with a hoe perforated with holes. When there is a good quantity of liquid, draw up the slider and let the slacked lime run into the pit. Throw more water on the remaining unslacked lime, and lastly reject those pieces which will not slack. The trough should have a small inclination and project over the pit.
After the lime thus slacked has been five or six hours in the pit, it will take the consistence of a stiff paste, which it retains for years. It should then be kept covered to keep it clean and to exclude the free contact of the air. For those who use larger quantities of lime, several pits should be constructed in preference to one larger reservoir. When the lime is wanted for use it may be dug out with a spade, and readily diluted with a sufficient quantity of water.
The quick-lime thus prepared forms a perfect homogeneous mixture. The practice of throwing lime simply slackened into the lime cistern is a wasteful and slovenly process, as will becomes obvious on examining the waste hydrosulphuret of lime discharged from the machine, which will be found to abound with lime in a concrete form, unacted on by the substances with which it was intended to combine.