_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.