Acetylene, The Principles Of Its Generation And Use

F >> F. H. Leeds and W. J. Atkinson Butterfield >> Acetylene, The Principles Of Its Generation And Use

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In less critical situations in an acetylene plant, such as the partitions
of a carbide container, &c., where the collapse of the seam or joint
would not be followed by any of the effects previously suggested, there
is less cause for prohibiting the use of unfortified solder; but even
here, two or three rivets, just sufficient to hold the metal in position
if the solder should give way, are advisedly put into all apparatus. In
other portions of an acetylene installation where a merely soldered joint
is exposed to warm damp gas which is in process of cooling, instead of
being bathed in hard water, an equal, though totally dissimilar, danger
is courted. The main constituent of such solders that are capable of
being applied with the bitt is lead; lead is distinctly soluble in soft
or pure water; and the water which separates by condensation out of a
warm damp gas is absolutely soft, for it has been distilled. If
condensation takes place at or near a soldered joint in such a way that
water trickles over the solder, by slow degrees the metallic lead will be
dissolved and removed, and eventually a time will come when the joint is
no longer tight to gas. In fact, if an acetylene installation is of more
than very small dimensions, _e.g._, when it is intended to supply
any building as large as, or larger than, the average country residence,
if it is to give satisfaction to both constructor and purchaser by being
quite trustworthy and, possessed of a due lease of life, say ten or
fifteen years, it must be built of stouter materials than the light
sheets which alone are suitable for manipulation with the soldering-iron
or for bending in the ordinary type of metal press. Sound cast-iron,
heavy sheet-metal, or light boiler-plate is the proper substance of which
to construct all the important parts of a generator, and the joints in
wrought metal must be riveted and caulked or soldered autogeneously as
mentioned above. So built, the installation becomes much more costly to
lay down than an apparatus composed of tinplate, zinc, or thin galvanised
iron, but it will prove more economical in the long run. It is not too
much to say that if ignorant and short-sighted makers in the earliest
days of the acetylene industry had not recommended and supplied to their
customers lightly built apparatus which has in many instances already
begun to give trouble, to need repairs, and to fail by thorough
corrosion--apparatus which frequently had nothing but cheapness in its
favour--the use of the gas would have spread more rapidly than it has
done, and the public would not now be hearing of partial or complete
failures of acetylene installations. Each of these failures, whether
accompanied by explosions and injury to persons or not, acts more
powerfully to restrain a possible new customer from adopting the
acetylene light, than several wholly successful plants urge him to take
it up; for the average member of the public is not in a position to
distinguish properly between the collapse of a certain generator owing to
defective design or construction (which reflects no discredit upon the
gas itself), and the failure of acetylene to show in practice those
advantages that have been ascribed to it. One peculiar and noteworthy
feature of acetylene, often overlooked, is that the apparatus is
constructed by men who may have been accustomed to gas-making plant all
their lives, and who may understand by mere habit how to superintend a
chemical operation; but the same apparatus is used by persons who
generally have no special acquaintance with such subjects, and who, very
possibly, have not even burnt coal-gas at any period of their lives.
Hence it happens that when some thoughtless action on the part of the
country attendant of an acetylene apparatus is followed by an escape of
gas from the generator, and by an accumulation of that gas in the house
where the plant is situated, or when, in disregard of rules, he takes a
naked light into the house and an explosion follows, the builder
dismisses the episode as a piece of stupidity or wilful misbehaviour for
which he can in nowise be held morally responsible; whereas the builder
himself is to blame for designing an apparatus from which an escape of
gas can be accompanied by sensible risks to property or life. However
unpalatable this assertion may be, its truth cannot be controverted;
because, short of criminal intention or insanity on the part of the
attendant, it is in the first place a mere matter of knowledge and skill
so to construct an acetylene plant that an escape of gas is extremely
unlikely, even when the apparatus is opened for recharging, or when it is
manipulated wrongly; and in the second place, it is easy so to arrange
the plant that any disturbance of its functions which may occur shall be
followed by an immediate removal of the surplus gas into a place of
complete safety outside and above the generator-house.

GENERATION AT LOW TEMPERATURES.--In all that has been said hitherto about
the reaction between calcium carbide and water being instantaneous, it
has been assumed that the two substances are brought together at or about
the usual temperature of an occupied room, _i.e._, 15 degrees C. If,
however, the temperature is materially lower than this, the speed of the
reaction falls off, until at -5 degrees C., supposing the water still to
remain liquid, evolution of acetylene practically ceases. Even at the
freezing-point of pure water gas is produced but slowly; and if a lump of
carbide is thrown on to a block of ice, decomposition proceeds so gently
that the liberated acetylene may be ignited to form a kind of torch,
while heat is generated with insufficient rapidity to cause the carbide
to sink into the block. This fact has very important bearings upon the
manipulation of an acetylene generator in winter time. It is evident that
unless precautions are taken those portions of an apparatus which contain
water are liable to freeze on a cold night; because, even if the
generator has been at work producing gas (and consequently evolving heat)
till late in the evening, the surplus heat stored in the plant may escape
into the atmosphere long before more acetylene has to be made, and
obviously while frost is still reigning in the neighbourhood. If the
water freezes in the water store, in the pipes leading therefrom, in the
holder seal, or in the actual decomposing chamber, a fresh batch of gas
is either totally incapable of production, because the water cannot be
brought into contact with the calcium carbide in the apparatus, or it can
only be generated with excessive slowness because the carbide introduced
falls on to solid ice. Theoretically, too, there is a possibility that
some portion of the apparatus--a pipe in particular--may be burst by the
freezing, owing to the irresistible force with which water expands when
it changes into the solid condition. Probably this last contingency,
clearly accompanied as it would be by grave risk, is somewhat remote, all
the plant being constructed of elastic material; but in practice even a
simple interference with the functions of a generator by freezing,
ideally of no special moment, is highly dangerous, because of the great
likelihood that hurried and wholly improper attempts to thaw it will be
made by the attendant. As it has been well known for many years that the
solidifying point of water can be lowered to almost any degree below
normal freezing by dissolving in it certain salts in definite
proportions, one of the first methods suggested for preventing the
formation of ice in an acetylene generator was to employ such a salt,
using, in fact, for the decomposition of the carbide some saline solution
which remains liquid below the minimum night temperature of the winter
season. Such a process, however, has proved unsuitable for the purpose in
view; and the explanation of that fact is found in what has just been
stated: the "water" of the generator may admittedly be safely maintained
in the fluid state, but from so cold a liquid acetylene will not be
generated smoothly, if at all. Moreover, were it not so, a process of
this character is unnecessarily expensive, although suitable salts are
very cheap, for the water of the generator is constantly being consumed,
[Footnote: It has already been said that most generators "consume" a much
larger volume of water than the amount corresponding with the chemical
reaction involved: the excess of water passing into the sludge or by-
product. Thus a considerable quantity of any anti-freezing agent must be
thrown aside each time the apparatus is cleaned out or its fluid contents
are run off.] and as constantly needs renewal; which means that a fresh
batch of salt would be required every time the apparatus was recharged,
so long as frost existed or might be expected. A somewhat different
condition obtains in the holder of an acetylene installation. Here,
whenever the holder is a separate item in the plant, not constituting a
portion of the generating apparatus, the water which forms the seal of a
rising holder, or which fills half the space of a displacement holder,
lasts indefinitely; and it behaves equally well, whatever its temperature
may be, so long as it retains a fluid state. This matter will be
discussed with greater detail at the end of Chapter III. At present the
point to be insisted on is that the temperature in any constituent of an
acetylene installation which contains water must not be permitted to fall
to the freezing-point; while the water actually used for decomposition
must be kept well above that temperature.

GENERATION AT HIGH TEMPERATURES.--At temperatures largely exceeding those
of the atmosphere, the reaction between calcium carbide and water tends
to become irregular; while at a red heat steam acts very slowly upon
carbide, evolving a mixture of acetylene and hydrogen in place of pure
acetylene. But since at pressures which do not materially exceed that of
the atmosphere, water changes into vapour at 100 deg. C., above that
temperature there can be no question of a reaction between carbide and
liquid water. Moreover, as has been pointed out, steam or water vapour
will continue to exist as such at temperatures even as low as the
freezing-point so long as the vapour is suspended among the particles of
a permanent gas. Between calcium carbide and water vapour a double
decomposition occurs chemically identical with that between carbide and
liquid water; but the physical effect of the reaction and its practical
bearings are considerably modified. The quantity of heat liberated when
30 parts by weight of steam react with 64 parts of calcium carbide should
be essentially unaltered from that evolved when the reagent is in the
liquid state; but the temperature likely to be attained when the speed of
reaction remains the same as before will be considerably higher for two
conspicuous reasons. In the first place, the specific heat of steam in is
only 0.48, while that of liquid water is 1.0. Hence, the quantity of heat
which is sufficient to raise the temperature of a given weight of liquid
water through _n_ thermometric degrees, will raise the temperature
of the same weight of water vapour through rather more than 2 _n_
degrees. In the second place, that relatively large quantity of heat
which in the case of liquid water merely changes the liquid into a
vapour, becoming "latent" or otherwise unrecognisable, and which, as
already shown, forms roughly five-sixths of the total heat needed to
convert cold water into steam, has no analogue if the water has
previously been vaporised by other means; and therefore the whole of the
heat supplied to water vapour raises its sensible temperature, as
indicated by the thermometer. Thus it appears that, except for the
sufficient amount of cooling that can be applied to a large vessel
containing carbide by surrounding it with a water jacket, there is no way
of governing its temperature satisfactorily if water vapour is allowed to
act upon a mass of carbide--assuming, of course, that the reaction
proceeds at any moderate speed, _e.g._, at a rate much above that
required to supply one or two burners with gas.

The decomposition which with perfect chemical accuracy has been stated to
occur quantitatively between 36 parts by weight, of water and 64 parts of
calcium carbide scarcely ever takes place in so simple a fashion in an
actual generator. Owing to the heat developed when carbide is in excess,
about half the water is converted into vapour; and so the reaction
proceeds in two stages: half the water added reacting with the carbide as
a liquid, the other half, in a state of vapour, afterwards reacting
similarly, [Footnote: This secondary reaction is manifestly only another
variety of the phenomenon known as "after-generation" (cf. _ante_).
After-generation is possible between calcium carbide and mechanically
damp slaked lime, between carbide and damp gas, or between carbide and
calcium hydroxide, as opportunity shall serve. In all cases the carbide
must be in excess.] or hardly reacting at all, as the case may be.
Suppose a vessel, A B, somewhat cylindrical in shape, is charged with
carbide, and that water is admitted at the end called A. Suppose now (1)
that the exit for gas is at the opposite end, B. As the lumps near A are
attacked by half the liquid introduced, while the other half is changed
into steam, a current, of acetylene and water vapour travels over the
charge lying between the decomposing spot and the end B. During its
passage the second half of the water, as vapour, reacts with the excess
of carbide, the first make of acetylene being dried, and more gas being
produced. Thus a second quantity of heat is developed, equal by theory to
that previously evolved; but a second elevation in temperature, far more
serious, and far less under control, than the former also occurs; and
this is easily sufficient to determine some of those undesirable effects
already described. Digressing for a moment, it may be admitted that the
desiccation of the acetylene produced in this manner is beneficial, even
necessary; but the advantages of drying the gas at this period of its
treatment are outweighed by the concomitant disadvantages and by the
later inevitable remoistening thereof. Suppose now (2) that both the
water inlet and the gas exit of the carbide cylinder are at the same end,
A. Again half the added water, as liquid, reacts with the carbide it
first encounters, but the hot stream of damp gas is not permitted to
travel over the rest of the lumps extending towards B: it is forced to
return upon its steps, leaving B practically untouched. The gas
accordingly escapes from the cylinder at A still loaded with water
vapour, and for a given weight of water introduced much less acetylene is
evolved than in the former case. The gas, too, needs drying somewhere
else in the plant; but these defects are preferable to the apparent
superiority of the first process because overheating is, or can be, more
thoroughly guarded against.

PRESSURE IN GENERATORS.--Inasmuch as acetylene is prone to dissociate or
decompose into its elements spontaneously whenever its pressure reaches 2
atmospheres or 30 lb. per square inch, as well as when its temperature at
atmospheric pressure attains 780 deg. C., no pressure approaching that of 2
atmospheres is permissible in the generator. A due observance of this
rule, however, unlike a proper maintenance of a low temperature in an
acetylene apparatus, is perfectly easy to arrange for. The only reason
for having an appreciable positive pressure in any form of generating
plant is that the gas may be compelled to travel through the pipes and to
escape from the burner orifices; and since the plant is only installed to
serve the burners, that pressure which best suits the burners must be
thrown by the generator or its holder. Therefore the highest pressure it
is ever requisite to employ in a generator is a pressure sufficient
(_a_) to lift the gasholder bell, or to raise the water in a
displacement holder, (_b_) to drive the gas through the various
subsidiary items in the plant, such as washers and purifiers, (_c_)
to overcome the friction in the service-pipes, [Footnote: This friction
manifestly causes a loss of pressure, _i.e._, a fall in pressure, as
a gas travels along a pipe; and, as will be shown in Chapter VII., it is
the fall in pressure in a pipe rather than the initial pressure at which
a gas enters a pipe that governs the volume of gas passing through that
pipe. The proper behaviour and economic working of a burner (acetylene or
other, luminous or incandescent) naturally depend upon the pressure in
the pipe to which the burner is immediately attached being exactly suited
to the design of that burner, and have nothing to do with the fall in
pressure occurring in the delivery pipes. It is therefore necessary to
keep entirely separate the ideas of proper burner pressure and of maximum
desirable fall in pressure within the service due to friction.] and
(d) to give at the points of combustion a pressure which is
required by the particular burners adopted. In all except village or
district installations, (_c_) may be virtually neglected. When the
holder has a rising bell, (_a_) represents only an inch or so of
water; but if a displacement holder is employed the pressure needed to
work it is entirely indeterminate, being governed by the size and shape
of the said holder. It will be argued in Chapter III. that a rising
holder is always preferable to one constructed on the displacement
principle. The pressure (d) at the burners may be taken at 4
inches of water as a maximum, the precise figure being dependent upon the
kind of burners--luminous, incandescent, boiling, &c.--attached to the
main. The pressure (_b_) also varies according to circumstances, but
averages 2 or 3 inches. Thus a pressure in the generator exceeding that
of the atmosphere by some 12 inches of water--_i.e._, by about 7
oz., or less than half a pound per square inch--is amply sufficient for
every kind of installation, the less meritorious generators with
displacement holders only excepted. This pressure, it should be noted, is
the net or effective pressure, the pressure with which the gas raises the
liquid in a water-gauge glass out of the level while the opposite end of
the water column is exposed to the atmosphere. The absolute pressure in a
vessel containing gas at an effective pressure of 12 inches of water is 7
oz. plus the normal, insensible pressure of the atmosphere itself--say
15-1/4 lb. per square inch. The liquid in a barometer which measures the
pressure of the atmosphere stands at a height of 30 inches only, because
that liquid is mercury, 13.6 times as heavy as water. Were it filled with
water the barometer would stand at (30 X 13.6) = 408 inches, or 34 feet,
approximately. Gas pressures are always measured in inches of water
column, because expressed either as pounds per square inch or as inches
of mercury, the figures would be so small as to give decimals of unwieldy
length.

It would of course be perfectly safe so to arrange an acetylene plant
that the pressure in the generating chamber should reach the 100 inches
of water first laid down by the Home Office authorities as the maximum
allowable. There is, however, no appreciable advantage to be gained by so
doing, or by exceeding that pressure which feeds the burners best. Any
higher original pressure involves the use of a governor at the exit of
the plant, and a governor is a costly and somewhat troublesome piece of
apparatus that can be dispensed with in most single installations by a
proper employment of a well-balanced rising holder.

CHAPTER III

THE GENERAL PRINCIPLES OF ACETYLENE GENERATION--ACETYLENE GENERATING
APPARATUS

Inasmuch as acetylene is produced by the mere interaction of calcium
carbide and water, that is to say, by simply bringing those two
substances in the cold into mutual contact within a suitable closed
space, and inasmuch as calcium carbide can always be purchased by the
consumer in a condition perfectly fit for immediate decomposition, the
preparation of the gas, at least from the theoretical aspect, is
characterised by extreme simplicity. A cylinder of glass or metal, closed
at one end and open at the other, filled with water, and inverted in a
larger vessel containing the same liquid, may be charged almost
instantaneously with acetylene by dropping into the basin a lump of
carbide, which sinks to the bottom, begins to decompose, and evolves a
rapid current of gas, displacing the water originally held in the
inverted cylinder or "bell." If a very minute hole is drilled in the top
of the floating bell, acetylene at once escapes in a steady stream, being
driven out by the pressure of the cylinder, the surplus weight of which
causes it to descend into the water of the basin as rapidly as gas issues
from the orifice. As a laboratory experiment, and provided the bell has
been most carefully freed from atmospheric air in the first instance,
this escaping gas may be set light to with a match, and will burn with a
more or loss satisfactory flame of high illuminating power. Such is an
acetylene generator stripped of all desirable or undesirable adjuncts,
and reduced to its most elementary form; but it is needless to say that
so simple an apparatus would not in any way fulfil the requirements of
everyday practice.

Owing to the inequality of the seasons, and to the irregular nature of
the demand for artificial light and heat in all households, the capacity
of the plant installed for the service of any institution or district
must be amply sufficient to meet the consumption of the longest winter
evening--for, as will be shown in the proper place, attempts to make an
acetylene generator evolve gas more quickly than it is designed to do are
fraught with many objections--while the operation of the plant, must be
under such thorough control that not only can a sudden and unexpected
demand for gas be met without delay, but also that a sudden and
unexpected interruption or cessation of the demand shall not be followed
by any disturbance in the working of the apparatus. Since, on the one
hand, acetylene is produced in large volumes immediately calcium carbide
is wetted with water, so that the gas may be burnt within a minute or two
of its first evolution; and, on the other, that acetylene once prepared
can be stored without trouble or appreciable waste for reasonable periods
of time in a water-sealed gasholder closely resembling, in everything but
size, the holders employed on coal-gas works; it follows that there are
two ways of bringing the output of the plant into accord with the
consumption of the burners. It is possible to make the gas only as and
when it is required, or it is possible in the space of an hour or so,
during the most convenient part of the day, to prepare sufficient to last
an entire evening, storing it in a gasholder till the moment arrives for
its combustion. It is clear that an apparatus needing human attention
throughout the whole period of activity would be intolerable in the case
of small installations, and would only be permissible in the case of
larger ones if the district supplied with gas was populous enough to
justify the regular employment of two men at least in or about the
generating station. But with the conditions obtaining in such a country
as Great Britain, and in other lands where coal is equally cheap and
accessible, if a neighbourhood was as thickly populated as has been
suggested, it would be preferable on various grounds to lay down a coal-
gas or electricity works; for, as has been shown in the first chapter,
unless a very material fall in the price of calcium carbide should take
place--a fall which at present is not to be expected--acetylene can only
be considered a suitable and economical illuminant and heating agent for
such places as cannot be provided cheaply with coal-gas or electric
current. To meet this objection, acetylene generators have been invented
in which, broadly speaking, gas is only produced when it is required,
control of the chemical reaction devolving upon some mechanical
arrangement. There are, therefore, two radically different types of
acetylene apparatus to be met with, known respectively as "automatic" and
"non-automatic" generators. In a non-automatic generator the whole of the
calcium carbide put into the apparatus is more or less rapidly
decomposed, and the entire volume of gas evolved from it is collected in
a holder, there to await the moment of consumption. In an automatic
apparatus, by means of certain devices which will be discussed in their
proper place, the act of turning on a burner-tap causes some acetylene to
be produced, and the act of turning it off brings the reaction to an end,
thus obviating the necessity for storage. That, at any rate, is the
logical definition of the two fundamentally different kinds of generator:
in automatic apparatus the decomposition of the carbide is periodically
interrupted in such fashion as more or less accurately to synchronise
with the consumption of gas; in the non-automatic variety decomposition
proceeds without a break until the carbide vessels are empty.
Unfortunately a somewhat different interpretation of these two words has
found frequent acceptance, a generator being denominated non-automatic or
automatic according as the holder attached to it is or is not large
enough to store the whole of the acetylene which the charge of carbide is
capable of producing if it is decomposed all at once. Apart from the fact
that a holder, though desirable, is not an absolutely indispensable part
of an acetylene plant, the definition just quoted was sufficiently free
from objection in the earliest days of the industry; but now efficient
commercial generators are to be met with which become either automatic or
non-automatic according to the manner of working them, while some would
be termed non-automatic which comprise mechanism of a conspicuously self-
acting kind.

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