Acetylene, The Principles Of Its Generation And Use

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A lowering of the upper limit of explosibility is also produced by the
presence of the acetone which remains in acetylene when obtained from a
cylinder holding the compressed gas (_cf._ Chapter XI.). According
to Wolff and Caro such gas usually carries with it from 30 to 60 grammes
of acetone vapour per cubic metre, _i.e._, 1.27 grammes per cubic
foot on an average; and this amount reduces the upper limit of
explosibility by about 16 per cent., so that to this extent the gas
behaves more smoothly in an incandescent burner of imperfect design.

Lepinay has described some experiments on the comparative technical value
of ordinary acetylene, carburetted acetylene, denatured alcohol and
petroleum spirit as fuels for small explosion engines. One particular
motor of 3 (French) h.p. consumed 1150 grammes of petroleum spirit per
hour at full load; but when it was supplied with carburetted acetylene
its consumption fell to 150 litres of acetylene and 700 grammes of spirit
(specific gravity 0.680). A 1-1/4 h.p. engine running light required 48
grammes of 90 per cent. alcohol per horse-power-hour and 66 litres of
acetylene; at full load it took 220 grammes of alcohol and 110 litres of
acetylene. A 6 h.p. engine at full load required 62 litres of acetylene
carburetted with 197 grammes of petroleum spirit per horse-power-hour
(uncorrected); while a similar motor fed with low-grade Taylor fuel-gas
took 1260 litres per horse-power-hour, but on an average developed the
same amount of power from 73 litres when 10 per cent. of acetylene was
added to the gas. Lepinay found that with pure acetylene ignition of the
charge was apt to be premature; and that while the consumption of
carburetted acetylene in small motors still materially exceeded the
theoretical, further economics could be attained, which, coupled with the
smooth and regular running of an engine fed with the carburetted gas,
made carburetted acetylene distinctly the better power-gas of the two.

CHAPTER XI

COMPRESSED AND DISSOLVED ACETYLENE--MIXTURES WITH OTHER GASES

In all that was said in Chapters II., III., IV., and V. respecting the
generation and employment of acetylene, it was assumed that the gas would
be produced by the interaction of calcium carbide and water, either by
the consumer himself, or in some central station delivering the acetylene
throughout a neighbourhood in mains. But there are other methods of using
the gas, which have now to be considered.

COMPRESSED ACETYLENE.--In the first place, like all other gases,
acetylene is capable of compression, or even of conversion into the
liquid state; for as a gas, the volume occupied by any given weight of it
is not fixed, but varies inversely with the pressure under which it is
stored. A steel cylinder, for instance, which is of such size as to hold
a cubic foot of water, also holds a cubic foot of acetylene at
atmospheric pressure, but holds 2 cubic feet if the gas is pumped into it
to a pressure of 2 atmospheres, or 30 lb. per square inch; while by
increasing the pressure to 21.53 atmospheres at 0 deg. C. (Ansdell, Willson
and Suckert) the gas is liquefied, and the vessel may then contain 1
cubic foot of liquid acetylene, which is equal to some 400 cubic feet of
gaseous acetylene at normal pressure. It is clear that for many purposes
acetylene so compressed or liquefied would be convenient, for if the
cylinders could be procured ready charged, all troubles incidental to
generation would be avoided. The method, however, is not practically
permissible; because, as pointed out in Chapters II. and VI., acetylene
does not safely bear compression to a point exceeding 2 atmospheres; and
the liability to spontaneous dissociation or explosion in presence of
spark or severe blow, which is characteristic of compressed gaseous
acetylene, is greatly enhanced if compression has been pushed to the
point of liquefaction.

However, two methods of retaining the portability and convenience of
compressed acetylene with complete safety have been discovered. In one,
due to the researches of Claude and Hess, the gas is pumped under
pressure into acetone, a combustible organic liquid of high solvent power,
which boils at 56 deg. C. As the solvent capacity of most liquids for
most gases rises with the pressure, a bottle partly filled with acetone
may be charged with acetylene at considerable effective pressure until
the vessel contains much more than its normal quantity of gas; and when
the valve is opened the surplus escapes, ready for employment, leaving
the acetone practically unaltered in composition or quantity, and fit to
receive a fresh charge of gas. In comparison with liquefied acetylene,
its solution in acetone under pressure is much safer; but since the
acetone expands during absorption of gas, the bottle cannot be entirely
filled with liquid, and therefore either at first, or during consumption
(or both), above the level of the relatively safe solution, the cylinder
contains a certain quantity of gaseous acetylene, which is compressed
above its limit of safety. The other method consists in pumping acetylene
under pressure into a cylinder apparently quite full of some highly
porous solid matter, like charcoal, kieselguhr, unglazed brick, &c. This
has the practical result that the gas is held under a high state of
compression, or possibly as a liquid, in the minute crevices of the
material, which are almost of insensible magnitude; or it may be regarded
as stored in vessels whose diameter is less than that in which an
explosive wave can be propagated (_cf._ Chapter VI.).

DISSOLVED ACETYLENE.--According to Fouche, the simple solution of
acetylene in acetone has the same coefficient of expansion by heat as
that of pure acetone, viz., 0.0015; the corresponding coefficient of
liquefied acetylene is 0.007 (Fouche), or 0.00489 (Ansdell) _i.e._,
three or five times as much. The specific gravity of liquid acetylene is
0.420 at 16.4 deg. C. (Ansdell), or 0.528 at 20.6 deg. C. (Willson and
Suckert); while the density of acetylene dissolved in acetone is 0.71 at 15
deg. C. (Claude). The tension of liquefied acetylene is 21.53 atmospheres at
0 deg. C., and 39.76 atmospheres at 20.15 deg. C. (Ansdell); 21.53 at 0 deg.
C., and 39.76 at 19.5 deg. C. (Willson and Suckert); or 26.5 at 0 deg. C.,
and 42.8 at 20.0 deg. C. (Villard). Averaging those results, it may be said
that the tension rises from 23.2 atmospheres at 0 deg. C. to 40.77 at 20 deg.
C., which is an increment of 1/26 or 0.88 atmosphere, per 1 deg. Centigrade;
while, of course, liquefied acetylene cannot be kept at all at a temperature
of 0 deg. unless the pressure is 21 atmospheres or upwards. The solution of
acetylene in acetone can be stored at any pressure above or below that of
the atmosphere, and the extent to which the pressure will rise as the
temperature increases depends on the original pressure. Berthelot and
Vieille have shown that when (_a_) 301 grammes of acetone are
charged with 69 grammes of acetylene, a pressure of 6.74 atmospheres at
14.0 deg. C. rises to 10.55 atmospheres at 35.7 deg. C.; (_b_) 315 grammes
of acetone are charged with 118 grammes of acetylene, a pressure of 12.25
atmospheres at 14.0 deg. C. rises to 19.46 at 36.0 deg. C.; (_c_) 315
grammes of acetone are charged with 203 grammes of acetylene, a pressure
of 19.98 atmospheres at 13.0 deg. C. rises to 30.49 at 36.0 deg. C.
Therefore in (_a_) the increase in pressure is 0.18 atmosphere, in (_b_)
O.33 atmosphere, and in (_c_) 0.46 atmosphere per 1 deg. Centigrade
within the temperature limits quoted. Taking case (_b_) as the
normal, it follows that the increment in pressure per 1 deg. C. is 1/37
(usually quoted as 1/30); so that, measured as a proportion of the
existing pressure, the pressure in a closed vessel containing a solution
of acetylene in acetone increases nearly as much (though distinctly less)
for a given rise in temperature as does the pressure in a similar vessel
filled with liquefied acetylene, but the absolute increase is roughly
only one-third with the solution as with the liquid, because the initial
pressure under which the solution is stored is only one-half, or less,
that at which the liquefied gas must exist.

Supposing, now, that acetylene contained in a closed vessel, either as
compressed gas, as a solution in acetone, or as a liquid, were brought to
explosion by spark or shock, the effects capable of production have to be
considered. Berthelot and Vieille have shown that if gaseous acetylene is
stored at a pressure of 11.23 kilogrammes per square centimetre,
[Footnote: 1 kilo. per sq. cm. is almost identical with 1 atmosphere, or
15 lb. per sq. inch.] the pressure after explosion reaches 92.33
atmospheres on an average, which is an increase of 8.37 times the
original figure; if the gas is stored at 21.13 atmospheres, the mean
pressure after explosion is 213.15 atmospheres, or 10.13 times the
original amount. If liquid acetylene is tested similarly, the original
pressure, which must clearly be more than 21.53 atmospheres (Ansdell) at
0 deg. C., may rise to 5564 kilos, per square centimetre, as Berthelot and
Vieille observed when a steel bomb having a capacity of 49 c.c. was
charged with 18 grammes of liquefied acetylene. In the case of the
solution in acetone, the magnitudes of the pressures set up are of two
entirely different orders according as the original pressure 20
atmospheres or somewhat less; but apart from this, they vary considerably
with the extent to which the vessel is filled with the liquid, and they
also depend on whether the explosion is produced in the solution or in
the gas space above. Taking the lower original pressure first, viz., 10
atmospheres, when a vessel was filled with solution to 33 per cent. of
its capacity, the pressure after explosion reached about 95 atmospheres
if the spark was applied to the gas space; but attained 117.4 atmospheres
when the spark was applied to the acetone. When the vessel was filled 56
per cent. full, the pressures after explosion reached about 89, or 155
atmospheres, according as the gas or the liquid was treated with the
spark. But when the original pressure was 20 atmospheres, and the vessel
was filled to 35 per cent. of its actual capacity with solution, the
final pressures ranged from 303 to 568 atmospheres when the gas was
fired, and from 2000 to 5100 when the spark was applied to the acetone.
Examining these figures carefully, it will be seen that the phenomena
accompanying the explosion of a solution of acetylene in acetone resemble
those of the explosion of compressed gaseous acetylene when the original
pressure under which the solution is stored is about 10 atmospheres; but
resemble those of the explosion of liquefied acetylene when the original
pressure of the solution reaches 20 atmospheres, this being due to the
fact that at an original pressure of 10 atmospheres the acetone itself
does not explode, but, being exothermic, rather tends to decrease the
severity of the explosion; whereas at an original pressure of 20
atmospheres the acetone does explode (or burn), and adds its heat of
combustion to the heat evolved by the acetylene. Thus at 10 atmospheres
the presence of the acetone is a source of safety; but at 20 atmospheres
it becomes an extra danger.

Since sound steel cylinders may easily be constructed to boar a pressure
of 250 atmospheres, but would be burst by a pressure considerably less
than 5000 atmospheres, it appears that liquefied acetylene and its
solution in acetone at a pressure of 20 atmospheres are quite unsafe; and
it might also seem that both the solution at a pressure of 10 atmospheres
and the simple gas compressed to the same limit should be safe. But there
is an important difference here, in degree if not in kind, because, given
a cylinder of known capacity containing (1) gaseous acetylene compressed
to 10 atmospheres, or (2) containing the solution at the same pressure,
if an explosion were to occur, in case (1) the whole contents would
participate in the decomposition, whereas in case (2), as mentioned
already, only the small quantity of gaseous acetylene above the solution
would be dissociated.

It is manifest that of the three varieties of compressed acetylene now
under consideration, the solution in acetone is the only one fit for
general employment; but it exhibits the grave defects (_a_) that the
pressure under which it is prepared must be so small that the pressure in
the cylinders can never approach 20 atmospheres in the hottest weather or
in the hottest situation to which they may be exposed, (_b_) that
the gas does not escape smoothly enough to be convenient from large
vessels unless those vessels are agitated, and (_c_) that the
cylinders must always be used in a certain position with the valve at the
top, lest part of the liquid should run out into the pipes. For these
reasons the simple solution of acetylene in acetone has not become of
industrial importance; but the processes of absorbing either the gas, or
better still its solution in acetone, in porous matter have already
achieved considerable success. Both methods have proved perfectly safe
and trustworthy; but the combination of the acetone process with the
porous matter makes the cylinders smaller per unit volume of acetylene
they contain. Several varieties of solid matter appear to work
satisfactorily, the only essential feature in their composition being
that they shall possess a proper amount of porosity and be perfectly free
from action upon the acetylene or the acetone (if present). Lime does
attack acetone in time, and therefore it is not a suitable ingredient of
the solid substance whenever acetylene is to be compressed in conjunction
with the solvent; so that at present either a light brick earth which has
a specific gravity of 0.5 is employed, or a mixture of charcoal with
certain inorganic salts which has a density of 0.3, and can be introduced
through a small aperture into the cylinder in a semi-fluid condition.
Both materials possess a porosity of 80 per cent., that is to say, when a
cylinder is apparently filled quite full, only 20 per cent, of the space
is really occupied by the solid body, the remaining 80 per cent, being
available for holding the liquid or the compressed gas. If all
comparisons as to degree of explosibility and effects of explosion are
omitted, an analogy may be drawn between liquefied acetylene or its
compressed solution in acetone and nitroglycerin, while the gas or
solution of the gas absorbed in porous matter resembles dynamite.
Nitroglycerin is almost too treacherous a material to handle, but as an
explosive (which in reason absorbed or dissolved acetylene is not)
dynamite is safe, and even requires special arrangements to explode it.

In Paris, where the acetone process first found employment on a large
scale, the company supplying portable cylinders to consumers uses large
storage vessels filled, as above mentioned, apparently full of porous
solid matter, and also charged to about 43 per cent, of their capacity
with acetone, thus leaving about 37 per cent. of the apace for the
expansion which occurs as the liquid takes up the gas. Acetylene is
generated, purified, and thoroughly dried according to the usual methods;
and it is then run through a double-action pump which compresses it first
to a pressure of 3.5 kilos., next to a pressure of 3.5 x 3.5 = 12 kilos,
per square centimetre, and finally drives it into the storage vessels.
Compression is effected in two stages, because the process is accompanied
by an evolution of much heat, which might cause the gas to explode during
the operation; but since the pump is fitted with two cylinders, the
acetylene can be cooled after the first compression. The storage vessels
then contain 100 times their apparent volume of acetylene; for as the
solubility of acetylene in acetone at ordinary temperature and pressure
is about 25 volumes of gas in 1 of liquid, a vessel holding 100 volumes
when empty takes up 25 x 43 = 1000 volumes of acetylene roughly at
atmospheric pressure; which, as the pressure is approximately 10
atmospheres, becomes 1000 x 10 = 10,000 volumes per 100 normal capacity,
or 100 times the capacity of the vessel in terms of water. From these
large vessels, portable cylinders of various useful dimensions, similarly
loaded with porous matter and acetone, are charged simply by placing them
in mutual contact, thus allowing the pressure and the surplus gas to
enter the small one; a process which has the advantage of renewing the
small quantity of acetone vaporised from the consumers' cylinders as the
acetylene is burnt (for acetone is somewhat volatile, cf. Chapter X.), so
that only the storage vessels ever need to have fresh solvent introduced.

Where it is procurable, the use of acetylene compressed in this fashion
is simplicity itself; for the cylinders have only to be connected with
the house service-pipes through a reducing valve of ordinary
construction, set to give the pressure which the burners require. When
exhausted, the bottle is simply replaced by another. Manifestly, however,
the cost of compression, the interest on the value of the cylinders, and
the carriage, &c., make the compressed gas more expensive per unit of
volume (or light) than acetylene locally generated from carbide and
water; and indeed the value of the process does not lie so much in the
direction of domestic illumination as in that of the lighting, and
possibly driving, of vehicles and motor-cars--more especially in the
illumination of such vehicles as travel constantly, or for business
purposes, over rough road surfaces and perform mostly out-and-home
journeys. Nevertheless, absorbed acetylene may claim close attention for
one department of household illumination, viz., the portable table-lamp;
for the base of such an apparatus might easily be constructed to imitate
the acetone cylinder, and it could be charged by simple connexion with a
larger one at intervals. In this way the size of the lamp for a given
number of candle-hours would be reduced below that of any type of actual
generator, and the troubles of after-generation, always more or less
experienced in holderless generators, would be entirely done away with.
Dissolved acetylene is also very useful for acetylene welding or
autogenous soldering.

The advantages of compressed and absorbed acetylene depend on the small
bulk and weight of the apparatus per unit of light, on the fact that no
amount of agitation can affect the evolution of gas (as may happen with
an ordinary acetylene generator), on the absence of any liquid which may
freeze in winter, and on there being no need for skilled attention except
when the cylinders are being changed. These vessels weigh between 2.5 and
3 kilos, per 1 litre capacity (normal) and since they are charged with
100 times their apparent volume of acetylene, they may be said to weigh 1
kilo, per 33 litres of available acetylene, or roughly 2 lb. per cubic
foot, or, again, if half-foot burners are used, 2 lb. per 36 candle-
hours. According to Fouche, if electricity obtained from lead
accumulators is compared with acetylene on the basis of the weight of
apparatus needed to evolve a certain quantify of light, 1 kilo, of
acetylene cylinder is equal to 1.33 kilos, of lead accumulator with arc
lamps, or to 4 kilos. of accumulator with glow lamps; and moreover the
acetylene cylinder can be charged and discharged, broadly speaking, as
quickly or as slowly as may be desired; while, it may be added, the same
cylinder will serve one or more self-luminous jets, one or more
incandescent burners, any number and variety of heating apparatus,
simultaneously or consecutively, at any pressure which may be required.
From the aspect of space occupied, dissolved acetylene is not so
concentrated a source of artificial light as calcium carbide; for 1
volume of granulated carbide is capable of omitting as much light as 4
volumes of compressed gas; although, in practice, to the 1 volume of
carbide must be added that of the apparatus in which it is decomposed.

LIQUEFIED ACETYLENE.--In most civilised countries the importation,
manufacture, storage, and use of liquefied acetylene, or of the gas
compressed to more than a fraction of one effective atmosphere, is quite
properly prohibited by law. In Great Britain this has been done by an
Order in Council dated November 26, 1897, which specifies 100 inches of
water column as the maximum to which compression may be pushed. Power
being retained, however, to exempt from the order any method of
compressing acetylene that might be proved safe, the Home Secretary
issued a subsequent Order on March 28, 1898, permitting oil-gas
containing not more than 20 per cent, by volume of acetylene (see below)
to be compressed to a degree not exceeding 150 lb. per square inch,
_i.e._, to about 10 atmospheres, provided the gases are mixed
together before compression; while a third Order, dated April 10, 1901,
allows the compression of acetylene into cylinders filled as completely
as possible with porous matter, with or without the presence of acetone,
to a pressure not exceeding 150 lb. per square inch provided the
cylinders themselves have been tested by hydraulic pressure for at least
ten minutes to a pressure not less than double [Footnote: In France the
cylinders are tested to six times and in Russia to five times their
working pressure.] that which it is intended to use, provided the solid
substance is similar in every respect to the samples deposited at the
Home Office, provided its porosity does not exceed 80 per cent., provided
air is excluded from every part of the apparatus before the gas is
compressed, provided the quantity of acetone used (if used at all) is not
sufficient to fill the porosity of the solid, provided the temperature is
not permitted to rise during compression, and provided compression only
takes place in premises approved by H.M.'s Inspectors of Explosives.

DILUTED ACETYLENE.--Acetylene is naturally capable of admixture or
dilution with any other gas or vapour; and the operation may be regarded
in either of two ways; (1) as a, means of improving the burning qualities
of the acetylene itself, or (2) as a means of conferring upon some other
gas increased luminosity. In the early days of the acetylene industry,
generation was performed in so haphazard a fashion, purification so
generally omitted, and the burners were so inefficient, that it was
proposed to add to the gas a comparatively small proportion of some other
gaseous fluid which should be capable of making it burn without
deposition of carbon while not seriously impairing its latent
illuminating power. One of the first diluents suggested was carbon
dioxide (carbonic acid gas), because this gas is very easy and cheap to
prepare; and because it was stated that acetylene would bear an addition
of 5 or even 8 per cent, of carbon dioxide and yet develop its full
degree of luminosity. This last assertion requires substantiation; for it
is at least a grave theoretical error to add a non-inflammable gas to a
combustible one, as is seen in the lower efficiency of all flames when
burning in common air in comparison with that which they exhibit in
oxygen; while from the practical aspect, so harmful is carbon dioxide in
an illuminating gas, that coal-gas and carburetted water-gas are
frequently most rigorously freed from it, because a certain gain in
illuminating power may often thus be achieved more cheaply than by direct
enrichment of the gas by addition of hydrocarbons. Being prepared from
chalk and any cheap mineral acid, hydrochloric by preference, in the
cold, carbon dioxide is so cheap that its price in comparison with that
of acetylene is almost _nil_; and therefore, on the above
assumption, 105 volumes of diluted acetylene might be made essentially
for the same price as 100 volumes of neat acetylene, and according to
supposition emit 5 per cent. more light per unit of volume.

It is reported that several railway trains in Austria are regularly
lighted with acetylene containing 0.4 to 1.0 per cent. of carbon dioxide
in order to prevent deposition of carbon at the burners. The gas is
prepared according to a patent process which consists in adding a certain
proportion of a "carbonate" to the generator water. In the United
Kingdom, also, there are several installations supplying an acetylene
diluted with carbon dioxide, the gas being produced by putting into that
portion of a water-to-carbide generator which lies nearest to the water-
supply some solid carbonate like chalk, and using a dilute acid to attack
the material. Other inventors have proposed placing a solid acid, like
oxalic, in the former part of a generator and decomposing it with a
carbonate solution; or they have suggested putting into the generator a
mixture of a solid acid and a solid soluble carbonate, and decomposing it
with plain water.

Clearly, unless the apparatus in which such mixtures as these are
intended to be prepared is designed with considerable care, the amount of
carbon dioxide in the gas will be liable to vary, and may fall to zero.
If any quantity of carbide present has been decomposed in the ordinary
way, there will be free calcium hydroxide in the generator; and if the
carbon dioxide comes into contact with this, it will be absorbed, unless
sufficient acid is employed to convert the calcium carbonate (or
hydroxide) into the corresponding normal salt of calcium. Similarly,
during purification, a material containing any free lime would tend to
remove the carbon dioxide, as would any substance which became alkaline
by retaining the ammonia of the crude gas.

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