Young, then of Manchester, to a dense petroleum which issued from the
crevices of the coal in a Derbyshire mine. It was soon found that this
substance yielded a distillation—a pale yellow oil—which, on cooling,
deposited solid paraffin. Mr. Young, recognizing the importance of this
discovery, had an establishment at once erected on the spot, and the
work of extracting paraffin was carried on until the supply of the
petroleum had become nearly exhausted. Forced to seek for other sources
of paraffin, Mr. Young was fortunate enough, after many trials, to
discover that a species of bituminous coal, which occurs at Boghead,
near Bathgate, in the county of Linlithgow, yielded by distillation
annually large quantities of paraffin. In 1850 he procured a patent for
“treating bituminous coals to obtain paraffin, and oil containing
paraffin, therefrom.” This method consisted in distilling the coal in an
iron retort, gradually heated up to low redness, and kept at that
temperature until the volatile products ceased to come off. Under this
patent, Mr. Young developed the manufacture of paraffin into a new and
important branch of industry. The oil which first comes over in the
distillation of the Boghead mineral is largely used for illuminating
purposes under a variety of names besides that of _paraffin oil_, which
term is, we believe, chiefly applied to a less volatile portion,
extensively used for lubricating machinery, and consisting of liquid
hydro-carbons of the same percentage composition as solid paraffin,
which substance it also holds in solution. Mr. Young’s process consisted
in placing the mineral in a retort encased in brickwork—an arrangement
which caused the temperature of the retort to be more uniform than if
the heat of the furnace had been applied to it directly. The retorts
were placed vertically, and they were fed with the mineral by a hopper
at the top. The products of the distillation passed through a worm tube
surrounded by cold water into a cooled receiver. The result of the first
distillation was a crude oily matter, differing from tar in being
lighter than water, and in not drying-up when exposed to the air. This
crude oil was then several times alternately treated with sulphuric acid
and caustic potash, and distilled; and when about two-thirds of the oil
had been separated from the rest, as an oil for burning and lubricating
purposes, the residue yielded paraffin, or “paraffin wax,” as it is
sometimes called. It is estimated that in Scotland no less than 800,000
tons of shale are annually distilled for mineral hydro-carbons, with a
consumption of 500,000 tons of fuel. It is believed that about
25,000,000 gallons of crude oil are thus obtained, and from this 350,000
gallons of illuminating oil, 10,000 tons of lubricating oil, and 5,800
tons of solid paraffin are produced. Among the products exhibited in the
International Exhibition of 1862, was a block of beautifully translucent
paraffin, of nearly half a ton weight.
Paraffin is also obtained on the continent by distilling a variety of
coal termed _lignite_. The tar which comes over is distilled, until
nothing but coke remains. The condensed products are then treated with
caustic soda, in order to remove carbolic acid and other substances.
After washing with water, the oils are treated with sulphuric acid, in
order to remove basic substances. The oil is again washed, and is then
rectified by another distillation. The products which successively come
over are, if necessary, separated by being collected in different
vessels; but sometimes they are mixed together, and sent into the market
as illuminating oils under various names, such as “photogen,” “solar
oil,” &c. Oils having a specific gravity about 0·9 are collected apart,
and are placed in tanks in a very cool place. In the course of a few
weeks the solid paraffin, which is dissolved in the other hydro-carbons,
crystallizes out. The liquid oils are drawn off, and the crude paraffin,
which is of a dark colour, is freed from adhering oil by a centrifugal
machine, and afterwards by pressure applied by hydraulic power. It then
undergoes several other processes of purification before it is obtained
as a colourless translucent solid.
Several thousand tons of paraffin are annually consumed for making
candles, which is the most important application of the material. The
variation in the fusing-points of different specimens is doubtless due
to admixtures in greater or less proportion of other more easily fusible
hydro-carbons. It was on account of the imperfect separation of these
that the candles first made from paraffin were so liable to soften and
bend, and felt greasy to the touch. Paraffin for candle-making is
sometimes mixed with a certain proportion of other substances, such as
palmitic acid, &c. Among the patented applications of paraffin are the
lining of beer-barrels, and the preserving of fruits, jams, and meat.
Some kinds of paraffin are also used in the manufacture of matches.
Liebig once expressed a wish that coal-gas might be obtained in a solid
form: “It would certainly be esteemed one of the greatest discoveries of
the age if any one could succeed in condensing coal-gas into a white,
dry, odourless substance, portable and capable of being placed in a
candlestick or burned in a lamp.” Now, it is curious that paraffin has
nearly the same composition as good coal-gas: it burns with a bright and
smokeless flame, and beautiful candles are formed of it, which burn like
those made of the finest wax. When the fused paraffin first assumes the
solid form, it is transparent like glass; and if it could be retained in
that condition, we might have the pleasing novelty of transparent
candles. But the particles seek to arrange themselves in crystalline
forms, and the substance soon takes on its white semi-opaque appearance.
The great richness of the Boghead mineral in paraffin, which appears to
exist in it ready formed, prevented for many years any successful
competition by the working of other sources of supply. But paraffin is
an abundant constituent of Rangoon petroleum, and considerable
quantities may be obtained by distilling peat, and other fossil
substances. All petroleums and paraffins are, in fact, mixtures of a
number of hydro-carbons, which in many cases cannot be entirely
separated from each other. The accidents which have from time to time
occurred with some of these combustibles, and have caused legislative
enactments with regard to them, are due to the imperfect removal by
distillation of the more volatile bodies, which rise in vapour at
ordinary temperatures. Explosions of the hydro-carbons can occur only
under the same circumstances as with coal-gas; that is to say, the
application of a flame to a mixture of the vapour with atmospheric air.
[Illustration:
FIG. 346.—_View of the City of London Gas-works._
]
COAL-GAS.
When coal is burning in a common fire, we may see jets of smoky gas
issuing from the pieces of coal before they become red hot. This vapour,
coming in contact with flame in another part of the fire, may often be
observed to ignite, thus supplying an instance of gas-lighting in its
most elementary form. In the ordinary fire the air has free access, and
the inflammable gases and vapours continue to burn with flames more or
less bright, and when these have ceased the carbonaceous portion
continues afterward to glow until nearly the whole has been consumed,
except the solid residue which we call the ashes. These ashes in general
contain a portion of unconsumed carbon, mixed with what is chemically
_the ash_, namely, certain incombustible salts, constituting the white
part of the ashes. If, however, we heat the coal in a vessel which
prevents access of air, and allows the gases to escape, the coal is
decomposed much in the same way as when it is burnt in the open fire;
but the products formed are no longer burnt, the supply of oxygen being
cut off. Every one knows the familiar experiment of filling the bowl of
a common clay tobacco-pipe with powdered coal, then covering it with a
dab of clay, and placing it in a fire. The gas which soon comes from the
stem of the pipe does not take fire unless a light be applied, when it
may be seen to burn with a bright flame, and after the flow of gas has
ceased, nearly the whole of the carbon of the coal will be found
unconsumed in the bowl of the pipe. This simple experiment illustrates
perfectly the first step in the manufacture of coal-gas, namely, the
process of heating coal to redness in closed vessels, by which operation
the substances originally contained in coal are destroyed, and their
elements enter into new combinations.
These elements are few in number; for, except the very small portion
which remains as incombustible white ash, coal is constituted of carbon,
hydrogen, oxygen, nitrogen, and a little sulphur. All the varied and
interesting products obtained by the destructive distillation of coal
are combinations of two or more of these four or five elements.
Illuminating gas is far from being the only product when coal is heated
without access of air; for of the numerous substances volatized at the
red heat of the gas-retort a great number are not only incapable of
affording light, but liable to generate noxious compounds when burnt.
Besides this there are numerous bodies which, though leaving the retort
in the gaseous form, immediately assume the liquid or solid state at
ordinary temperatures. All such substances must be separated before
permanent gases are obtained fit for illuminating purposes and capable
of being carried through pipes to distant places. Thus an important part
of the apparatus for gas manufacture consists in arrangements for
separating the condensable bodies, and for removing useless or injurious
gases from the remainder.
[Illustration:
FIG. 347.—_Section of Gas-making Apparatus._
]
The products resulting from the destructive distillation of the coal
may, therefore, be classified as—_a_, solids left behind in the retort;
_b_, solids and liquids condensed by cooling the vapours which issue
from the retort; _c_, coal-gas—a mixture of gases from which certain
useless and noxious constituents must be removed. Fig. 347 is intended
to give a diagrammatic view of the apparatus employed in the generation,
purification, and storage of gas, the various parts being shown in
section. A is the furnace containing several retorts, of which B is one.
From each retort a tube, _d_, rises vertically, and curving downward
like an inverted U, it enters a long horizontal cylinder, _f_; half
filled with water, beneath the surface of which the open end of the
recurved tube dips. The cylinder containing water passes horizontally
along the whole range of furnaces in the gas-works, and is known as the
_hydraulic main_. It is here, then, the tar and the moisture first
condense, and the pipe is always kept half full of these liquids, so
that the ends of the pipes, _d_, from the retorts, dipping beneath its
surface, form traps or water-valves, which allow any retort to be opened
without permitting the gas to escape. As the tar accumulates in the
hydraulic main, it flows over through a pipe, _g_, leading downwards
into the _tar-well_, H. The gases take the same course; but while the
tar flows down the vertical tube, R, the gases pass on through _j_ into
the condensers or refrigerators. Gas cannot escape from the open end of
the tube, for it is always closed by the liquids—tar and ammoniacal
liquor—which accumulate and flow over the top of the open inner vessel
into the cistern, S, from which they are drawn off from time to time by
the stop-cock, I. Although when the gas has arrived at this cistern much
of its tar and ammoniacal vapours have been condensed, a portion is
still retained by reason of the high temperature of the gas; and this
has to be removed before it is permitted to enter the purifier. This is
the object of passing the gas through the series of pipes, _j j_,
forming the _condenser_. These are kept cool by the large surface they
expose to the air, and, when necessary, cold water from the cistern, K,
may be made to flow over them. The tar and other liquids condense in the
iron chest, T, which is so divided by partitions as to compel the gas to
pass through the whole series of tubes; and as the liquid accumulates,
it also overflows into the tar-well. The cooled gas then enters the
purifier, L L, in which are layers of slaked lime placed on a number of
shelves. By contact with the extensive surface of slaked lime the gas
has its sulphuretted hydrogen, carbonic acid, and some other impurities,
removed; and it then, through the tube _n_, enters the gasholder, in
which it is stored up for use.
Hydrated oxide of iron is now much used for purifying coal-gas. The
oxide is mixed with sawdust, and placed in layers 10 in. thick. Sulphide
of iron and water is formed; and when the mixture has ceased to absorb
any more, it is removed and exposed to a current of air; the hydrated
oxide is thus reproduced and sulphur set free. The process may be
repeated many times in succession, until the absorbent power is impaired
by the accumulation of sulphur.
The gasholder—or “gasometer,” as it is often improperly named—is an
immense cylindrical bell, made of wrought iron plates, and inverted in a
tank of water, in which it rises or falls. It is counterpoised by
weights attached to chains passing over pulleys, so as to press the gas
with a small force in order to drive it along the main, which
communicates with the pipes supplying it to the various consumers. The
pressure impelling the gas through the mains does not in general exceed
that of a column of water two or three inches high.
It will be necessary, after this slight outline describing the essential
parts of the apparatus, to enter more fully into the details of the
several parts.
The retorts are constructed of wrought iron, cast iron, or earthenware,
and in shape are cylindrical, with a diameter of 12 in. to 18 in., or
more, and a length of 6 ft. to 10 ft. Though sometimes circular in
section, other forms are commonly used—such as the elliptical, and
especially the ⌓-shaped. The retorts are closed except at the mouth-end,
Fig. 348, from the top of which rises the stand-pipe, A, which has
usually a diameter of about 5 in. When the charge has been introduced,
the mouth is closed by a plate of iron, B, closely and securely applied
by means of a screw, C, as shown in the figure—a perfectly tight joint
being obtained by a luting of lime mortar spread on the part of the lid
which comes into contact with the mouth of the retort. The retorts are
always set horizontally in the furnace—each furnace usually including a
set of five retorts. The charge of coals is introduced on a tray of
sheet iron adapted to the size of the retort, which, when properly
pushed in, is inverted so as turn out the contents, and then withdrawn.
The time required to completely expel the volatile constituents from the
charge in a gas retort varies very much, because there are great
diversities in the composition of the different kinds of coal employed.
Some varieties of coal, such as cannel, are easily decomposed, and the
operation may be complete in about three hours; while other kinds may
require double that time. The quantity of gas procurable from a given
weight of coal also varies according to the kind of coal made use of.
Thus, while a hundredweight of cannel may give 430 cubic feet of gas,
the same weight of Newcastle coals will yield but 370 cubic feet. The
nature of the gases given off from a retort will be different at the
different stages of the operation.
The scene presented by the retort-house of a large gas manufactory, when
viewed at night, is a singular spectacle. The strange lurid gleams which
shoot out amid the general darkness as the retorts are opened to
withdraw the coke, and the black forms of the workmen partially
illuminated by the glare, or flitting like dark shadows across it, form
a picture which might engage the pencil of a Rembrandt. In Fig. 348_a_
is depicted the retort-house at the Imperial Gas Works, King’s Cross.
Here the retorts are arranged in several tiers—the coal being brought,
and the coke withdrawn, by the aid of an iron carriage running on rails
parallel to the line of furnaces.
[Illustration:
FIG. 348.—_The Retort._
]
In the process of heating, a proper regulation of the temperature is of
the highest importance. It is found that when the retorts are heated to
bright cherry-red, the best results are obtained. At a lower temperature
a larger quantity of condensable vapours are given off, which collect in
the gasholders and distributing pipes as solid or liquid, and occasion
much inconvenience, while the quantity of gas obtained is decreased. On
the other hand, if the temperature be too high, some of the gases are
decomposed, and the quantity of carbon contained in the product is so
much diminished as seriously to impair the illuminating power. Again,
every second the gases after their production remain in the red-hot
retort diminishes their light-giving value; for those hydro-carbons on
which the luminiferous power of the gas depends, are then liable to
partial decomposition; a portion of their carbon is deposited on the
walls of the retort in a dense layer, gradually choking it up, while the
liberated hydrogen does not add to the illuminating but to the heating
constituents of the gas. A plan has been patented by Mr. White, of
Manchester, for rapidly removing the illuminating gases from the retort
by sweeping them out by means of a current of what has been termed
“water gas.” This water gas is produced by causing steam to pass over
heated coke, and is a mixture of carbonic acid, carbonic oxide, and
hydrogen. Though only two of these are combustible gases—and even they
do not yield light by their combustion, and, by adding to the bulk of
the gas, serve rather to dilute it—yet it has been found that in some
cases twice the amount of light is obtainable by White’s process than
the same weight of coal supplies when treated in the ordinary manner.
[Illustration:
FIG. 348_a_.—_Retort House of the Imperial Gas-Works, King’s Cross,
London._
]
The hydraulic main, as already mentioned, being kept half full of tar
into which the lower ends of the dip-pipes descend, prevents the gas
from escaping through the stand-pipes when the lid of a retort is
removed for the introduction of a fresh charge. The hydraulic main is
from 12 to 18 in. diameter, and the dip pipes pass into it by gas-tight
joints. Various forms of purifiers are in use besides the simple one
already mentioned. Some of these have arrangements for agitating the gas
with a purifying liquid by mechanical means, the motion being supplied
by a steam engine.
The gasholder, as it sinks in the water of the cistern, presses with
less force on the contained gas, and unless this inequality of pressure
were counteracted there would be very unequal velocities in the flow of
gas from the burner. The equality of pressure is obtained by making the
weight of the chains by which the gasholder is suspended equal to half
the weight the gasholder loses in the same length of its motion.
Gasholders are also constructed without chains or counterpoises, as
these are found to be unnecessary where the height of the gasholder does
not exceed half its width. In such cases, especially when the vessel is
very large, the difference of pressure at the highest and lowest
position is quite inconsiderable, and nothing more is necessary than
that upright guides or pillars be placed to preserve the vertical motion
of the vessel. Another improvement, which enables a lofty gasholder to
be used without increasing the depth of the tank, consists in forming
the gasholder of several cylinders, which slide in and out of one
another like the draw-tubes of a telescope. Each cylinder has a groove
formed by turning up the iron inside the rim, and at the top of the next
cylinder the edge is turned outwards so as to drop in the groove or
channel, which thus forms a gas-tight joint, for it is of course filled
with water as it rises. The pressure is, however, more accurately
regulated by an apparatus called the _governor_, through which the gas
passes in before it enters the mains. The construction and action of the
regulator will be understood from Fig. 349, where A represents a kind of
miniature gasholder, inverted in the cistern, B. From the centre of the
interior of the bell hangs a cone, C, within the contracted orifice of
the inlet-pipe. If this cone be drawn up, the size of the orifice, D, is
reduced, and, on the other hand, by its descent it enlarges the opening
through which the gas passes outward. By properly adjusting the weights
of the counterpoise, E, such a position of the cone may be found that
the gas passes into the mains at an assigned pressure. Suppose, now,
that from any cause the pressure of gas in F increases, that pressure
acting upon the inverted bell, A, causes it to rise and carry with it
the cone, which, by narrowing the orifice of the outlet, checks the flow
of gas. Similarly, a decrease of pressure in the mains would be followed
by the descent of the cone, and consequently freer egress of gas. In
hilly towns it is necessary to fix regulators of this kind at certain
heights in order to equalize the pressure. It is found that a difference
of 30 ft. in level affects the pressure of gas in the same main to about
the same amount as would a column of water one-fifth of an inch high,
the pressure being least at the lowest point.
[Illustration:
FIG. 349.—_The Gas Governor._
]
Coal-gas is a mixture of several gases, and these may be classified as,
first, the light-giving gases, or those which burn with a luminous
flame; secondly, gases which burn with a non-luminous flame, and which
therefore contribute to the _heat_, and not to the light, of a
gas-flame, and have the effect of diluting the gas; third, gases and
vapours which are properly termed impurities, as they are either
incombustible or by their combustion give rise to injurious products. Of
the first kind the principal is olefiant gas, a gas which burns with a
brilliant white flame without smoke. It is a compound of hydrogen and
carbon, six parts by weight of carbon being combined with one part by
weight of hydrogen. Besides olefiant gas other gaseous hydro-carbons are
found in smaller quantities. These contain a larger proportion of carbon
than olefiant gas. The second class contains hydrogen, light carburetted
hydrogen, and carbonic oxide. Hydrogen is one element of water, of which
it forms one-ninth of the weight. It burns with a flame giving
singularly little light, but having intensely heating power; in fact,
one of the brightest lights we can produce is obtained by allowing the
flame of burning hydrogen to heat a piece of lime. Light carburetted
hydrogen, like olefiant gas, is a compound of hydrogen and carbon, but
the proportion of carbon to hydrogen is only half what it is in olefiant
gas, namely, three parts to one. This gas enters largely into the
composition of coal-gas, and occurs naturally in the coal seams, being,
in fact, the dreaded _fire-damp_ of the miner. It is much lighter than
olefiant gas, for while that gas is of nearly the same specific gravity
as atmospheric air, light carburetted hydrogen is only a little more
than half that specific gravity. It is this ingredient of coal-gas which
renders it so light as to be available for inflating balloons. It burns
with either a bluish or a slightly yellow flame, yielding hardly any
light. Olefiant gas and the other luminiferous hydro-carbons, when
exposed to a bright red heat, split up for the most part into this gas
and carbon. This explains the importance of rapidly removing the gas
from the retort in which it is generated, a point which has been
referred to above. Carbonic oxide is a gas which one may often see
burning with a pale blue flame above the glowing embers of a common
fire, the flame giving, however, little light. It is a compound of
carbon and oxygen, containing only one-half the quantity of oxygen which
its carbon is capable of uniting with, and therefore ready to unite with
another proportion, which it does in burning, carbonic acid being the
product.