Acetylene, The Principles Of Its Generation And Use

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CHEMICAL PROPERTIES.--It is unnecessary for the purpose of this work to
give an exhaustive account of the general chemical reactions of acetylene
with other bodies, but a few of the more important must be referred to.
Since the gases are liable to unite spontaneously when brought into
contact, the reactions between, acetylene and chlorine require attention,
first, because of the accidents that have occurred when using bleaching-
powder (_see_ Chapter V.) as a purifying material for the crude gas;
secondly, because it has been proposed to manufacture one of the products
of the combination, viz., acetylene tetrachloride, on a large scale, and
to employ it as a detergent in place of carbon tetrachloride or carbon
disulphide. Acetylene forms two addition products with chlorine,
C_2H_2Cl_2, and C_2H_2Cl_4. These are known as acetylene dichloride and
tetrachloride respectively, or more systematically as dichlorethylene and
tetrachlorethane. One or both of the chlorides is apt to be produced when
acetylene comes into contact with free chlorine, and the reaction
sometimes proceeds with explosive violence. The earliest writers, such as
E. Davy, Woehler, and Berthelot, stated that an addition of chlorine to
acetylene was invariably followed by an explosion, unless the mixture was
protected from light; whilst later investigators thought the two gases
could be safely mixed if they were both pure, or if air was absent. Owing
to the conflicting nature of the statements made, Nieuwland determined in
1905 to study the problem afresh; and the annexed account is chiefly
based on his experiments, which, however, still fail satisfactorily to
elucidate all the phenomena observed. According to Nieuwland's results,
the behaviour of mixtures of acetylene and chlorine appears capricious,
for sometimes the gases unite quietly, although sometimes they explode.
Acetylene and chlorine react quite quietly in the dark and at low
temperatures; and neither a moderate increase in temperature, nor the
admission of diffused daylight, nor the introduction of small volumes of
air, is necessarily followed by an explosion. Doubtless the presence of
either light, air, or warmth increases the probability of an explosive
reaction, while it becomes more probable still in their joint presence;
but in given conditions the reaction may suddenly change from a gentle
formation of addition products to a violent formation of substitution
products without any warning or manifest cause. When the gases merely
unite quietly, tetrachlorethane, or acetylene tetrachloride, is produced
thus:

C_2H_2 + 2Cl_2 = C_2H_2Cl_4;

but when the reaction is violent some hexachlorethane is formed,
presumably thus:

2C_2H_2 + 5Cl_2 = 4HCl + C_2 + C_2Cl_6.

The heat evolved by the decomposition of the acetylene by the formation
of the hydrochloric acid in the last equation is then propagated amongst
the rest of the gaseous mixture, accelerating the action, and causing the
acetylene to react with the chlorine to form more hydrochloric acid and
free carbon thus;

C_2H_2 + Cl_2 = 2HCl + C_2.

It is evident that these results do not altogether explain the mechanism
of the reactions involved. Possibly the formation of substitution
products and the consequent occurrence of an explosion is brought about
by some foreign substance which acts as a catalytic agent. Such substance
may conceivably be one of the impurities in crude acetylene, or the solid
matter of a bleaching-powder purifying material. The experiments at least
indicate the direction in which safety may be sought when bleaching-
powder is employed to purify the crude gas, viz., dilution of the powder
with an inert material, absence of air from the gas, and avoidance of
bright sunlight in the place where a spent purifier is being emptied.
Unfortunately Nieuwland did not investigate the action on acetylene of
hypochlorites, which are presumably the active ingredients in bleaching-
powder. As will appear in due course, processes have been devised and
patented to eliminate all danger from the reaction between acetylene and
chlorine for the purpose of making tetrachlorethane in quantity.

Acetylene combines with hydrogen in the presence of platinum black, and
ethylene and then ethane result. It was hoped at one time that this
reaction would lead to the manufacture of alcohol from acetylene being
achieved on a commercial basis; but it was found that it did not proceed
with sufficient smoothness for the process to succeed, and a number of
higher or condensation products were formed at the same time. It has been
shown by Erdmann that the cost of production of alcohol from acetylene
through this reaction must prove prohibitive, and he has indicated
another reaction which he considered more promising. This is the
conversion of acetylene by means of dilute sulphuric acid (3 volumes of
concentrated acid to 7 volumes of water), preferably in the presence of
mercuric oxide, to acetaldehyde. The yield, however, was not
satisfactory, and the process does not appear to have passed beyond the
laboratory stage.

It has also been proposed to utilise the readiness with which acetylene
polymerises on heating to form benzene, for the production of benzene
commercially; but the relative prices of acetylene and benzene would have
to be greatly changed from those now obtaining to make such a scheme
successful. Acetylene also lends itself to the synthesis of phenol or
carbolic acid. If the dry gas is passed slowly into fuming sulphuric
acid, a sulpho-derivative results, of which the potash salt may be thrown
down by means of alcohol. This salt has the formula C_2H_4O_2,S_2O_6K_2,
and on heating it with caustic potash in an atmosphere of hydrogen,
decomposing with excess of sulphuric acid, and distilling, phenol results
and may be isolated. The product is, however, generally much contaminated
with carbon, and the process, which was devised by Berthelot, does not
appear to have been pursued commercially. Berthelot has also investigated
the action of ordinary concentrated sulphuric acid on acetylene, and
obtained various sulphonic derivatives. Schroeter has made similar
investigations on the action of strongly fuming sulphuric acid on
acetylene. These investigations have not yet acquired any commercial
significance.

If a mixture of acetylene with either of the oxides of carbon is led
through a red-hot tube, or if a similar mixture is submitted to the
action of electric sparks when confined within a closed vessel at some
pressure, a decomposition occurs, the whole of the carbon is liberated in
the free state, while the hydrogen and oxygen combine to form water.
Analogous reactions take place when either oxide of carbon is led over
calcium carbide heated to a temperature of 200 deg. or 250 deg. C., the
second product in this case being calcium oxide. The equations
representing these actions are:

C_2H_2 + CO = H_2O + 3C

2C_2H_2 + CO_2 = 2H_2O + 5C

CaC_2 + CO = CaO + 3C

2CaC_2 + CO_2 = 2CaO + 5C

By urging the temperature, or by increasing the pressure at which the
gases are led over the carbide, the free carbon appears in the graphitic
condition; at lower temperatures and pressures, it is separated in the
amorphous state. These reactions are utilised in Frank's process for
preparing a carbon pigment or an artificial graphite (_cf._ Chapter
XII.).

Parallel decompositions occur between carbon bisulphide and either
acetylene or calcium carbide, all the carbon of both substances being
eliminated, while the by-product is either sulphuretted hydrogen or
calcium (penta) sulphide. Other organic bodies containing sulphur are
decomposed in the same fashion, and it has been suggested by Ditz that if
carbide could be obtained at a suitable price, the process might be made
useful in removing sulphur (_i.e._, carbon bisulphide and thiophen)
from crude benzol, in purifying the natural petroleum oil which contains
sulphur, and possibly in removing "sulphur compounds" from coal-gas.

COMPOUNDS WITH COPPER. By far the most important chemical reactions of
acetylene in connexion with its use as an illuminant or fuel are those
which it undergoes with certain metals, notably copper. It is known that
if acetylene comes in contact with copper or with one of its salts, in
certain conditions a compound is produced which, at least when dry, is
highly explosive, and will detonate either when warmed or when struck or
gently rubbed. The precise mechanism of the reaction, or reactions,
between acetylene and copper (or its compounds), and also the character
of the product, or products, obtained have been studied by numerous
investigators; but their results have been inconclusive and sometimes
rather contradictory, so that it can hardly be said that the conditions
which determine or preclude the formation of an explosive compound and
the composition of the explosive compound are yet known with certainty.
Copper is a metal which yields two series of compounds, cuprous and
cupric salts, the latter of which contain half the quantity of metal per
unit of acid constituent that is found in the former. It should follow,
therefore, that there are two compounds of copper with carbon, or copper
carbides: cuprous carbide, Cu_2C_2, and cupric carbide, CuC_2. Acetylene
reacts at ordinary temperatures with an ammoniacal solution of any cupric
salt, forming a black cupric compound of uncertain constitution which
explodes between 50 deg. and 70 deg. C. It is decomposed by dilute acids,
yielding some polymerised substances. At more elevated temperatures other
cupric compounds are produced which also give evidence of polymerisation.
Cuprous carbide or acetylide is the reddish brown amorphous precipitate
which is the ultimate product obtained when acetylene is led into an
ammoniacal solution of cuprous chloride. This body is decomposed by
hydrochloric acid, yielding acetylene; but of itself it is, in all
probability, not explosive. Cuprous carbide, however, is very unstable
and prone to oxidation; so that, given the opportunity, it combines with
oxygen or hydrogen, or both, until it produces the copper acetylide, or
acetylene-copper, which is explosive--a body to which Blochmann's formula
C_2H_2Cu_2O is generally ascribed. Thus it should happen that the exact
nature of the copper acetylene compound may vary according to the
conditions in which it has been formed, from a substance that is not
explosive at all at first, to one that is violently explosive; and the
degree of explosiveness should depend on the greater exposure of the
compound to air and moisture, or the larger amount of oxygen and moisture
in the acetylene during its contact with the copper or copper salt. For
instance, Mai has found that freshly made copper acetylide can be heated
to 60 deg. C. or higher without explosion; but that if the compound is
exposed to air for a few hours it explodes on warming, while if warmed
with oxygen it explodes on contact with acetylene. It is said by Mai and
by Caro to absorb acetylene when both substances are dry, becoming so hot
as to explode spontaneously. Freund and Mai have also observed that when
copper acetylide which has been dried in contact with air for four or
five hours at a temperature of 50 deg. or 60 deg. C. is allowed to explode
in the presence of a current of acetylene, an explosion accompanied by light
takes place; but it is always local and is not communicated to the gas,
whether the latter is crude or pure. In contact with neutral or acid
solutions of cuprous salts acetylene yields various double compounds
differing in colour and crystallising power; but according to Chavastelon
and to Caro they are all devoid of explosive properties. Sometimes a
yellowish red precipitate is produced in solutions of copper salts
containing free acid, but the deposit is not copper acetylide, and is
more likely to be, at least in part, a copper phosphide--especially if
the gas is crude. Hence acid solutions or preparations of copper salts
may safely be used for the purification of acetylene, as is done in the
case of frankoline, mentioned in Chapter V. It is clear that the amount
of free acid in such a material is much more than sufficient to
neutralise all the ammonia which may accompany the crude acetylene into
the purifier until the material is exhausted in other respects; and
moreover, in the best practice, the gas would have been washed quite or
nearly free from ammonia before entering the purifier.

From a practical aspect the possible interaction of acetylene and
metallic copper has been investigated by Gerdes and by Grittner, whose
results, again, are somewhat contradictory. Gerdes exposed neat acetylene
and mixtures of acetylene with oil-gas and coal-gas to a pressure of nine
or ten atmospheres for ten months at ordinary summer and winter
temperatures in vessels made of copper and various alloys. Those metals
and alloys which resisted oxidation in air resisted the attack of the
gases, but the more corrodible substances were attacked superficially;
although in no instance could an explosive body be detected, nor could an
explosion be produced by heating or hammering. In further experiments the
acetylene contained ammonia and moisture and Gerdes found that where
corrosion took place it was due exclusively to the ammonia, no explosive
compounds being produced even then. Grittner investigated the question by
leading acetylene for months through pipes containing copper gauze. His
conclusions are that a copper acetylide is always produced if impure
acetylene is allowed to pass through neutral or ammoniacal solutions of
copper; that dry acetylene containing all its natural impurities except
ammonia acts to an equal extent on copper and its alloys, yielding the
explosive compound; that pure and dry gas does not act upon copper or its
alloys, although it is possible that an explosive compound may be
produced after a great length of time. Grittner has asserted that an
explosive compound may be produced when acetylene is brought into contact
with such alloys of copper as ordinary brass containing 64.66 per cent.
of copper, or red brass containing 74.46 per cent. of copper, 20.67 per
cent. of zinc, and 4.64 per cent. of tin; whereas none is obtained when
the metal is either "alpaca" containing 64.44 per cent. of copper, 18.79
per cent. of nickel, and 16.33 per cent. of zinc, or britannia metal
composed of 91.7 per cent. of copper and 8.3 per cent. of tin. Caro has
found that when pure dry acetylene is led for nine months over sheets or
filings of copper, brass containing 63.2 per cent. of copper, red brass
containing 73.8 per cent., so-called "alpaca-metal" containing 65.3 per
cent., and britannia metal containing 90.2 per cent. of copper, no action
whatever takes place at ordinary temperatures; if the gas is moist very
small quantities of copper acetylide are produced in six months, whatever
metal is tested, but the yield does not increase appreciably afterwards.
At high temperatures condensation occurs between acetylene and copper or
its alloys, but explosive bodies are not formed.

Grittner's statement that crude acetylene, with or without ammonia, acts
upon alloys of copper as well as upon copper itself, has thus been
corroborated by Caro; but experience renders it tolerably certain that
brass (and presumably gun-metal) is not appreciably attacked in practical
conditions. Gerdes' failure to obtain an explosive compound in any
circumstances may very possibly be explained by the entire absence of any
oxygen from his cylinders and gases, so that any copper carbide produced
remained unoxidised. Grittner's gas was derived, at least partially, from
a public acetylene supply, and is quite likely to have been contaminated
with air in sufficient quantity to oxidise the original copper compound,
and to convert it into the explosive modification.

For the foregoing reasons the use of unalloyed copper in the construction
of acetylene generators or in the subsidiary items of the plant, as well
as in burner fittings, is forbidden by statute or some quasi-legal
enactment in most countries, and in others the metal has been abandoned
for one of its alloys, or for iron or steel, as the case may be.
Grittner's experiments mentioned above, however, probably explain why
even alloys of copper are forbidden in Hungary. (_Cf._ Chapter IV.,
page 127.)

When acetylene is passed over finely divided copper or iron (obtained by
reduction of the oxide by hydrogen) heated to from 130 deg. C. to 250 deg.
C., the gas is more or less completely decomposed, and various products,
among which hydrogen predominates, result. Ethane and ethylene are
undoubtedly formed, and certain homologues of them and of acetylene, as
well as benzene and a high molecular hydrocarbon (C_7H_6)_n termed
"cuprene," have been found by different investigators. Nearly the same
hydrocarbons, and others constituting a mixture approximating in
composition to some natural petroleums, are produced when acetylene is
passed over heated nickel (or certain other metals) obtained by the
reduction of the finely divided oxide. These observations are at present
of no technical importance, but are interesting scientifically because
they have led up to the promulgation of a new theory of the origin of
petroleum, which, however, has not yet found universal acceptance.

CHAPTER VII

MAINS AND SERVICE-PIPES--SUBSIDIARY APPARATUS

The process by which acetylene is produced, and the methods employed for
purifying it and rendering it fit for consumption in dwelling-rooms,
having been dealt with in the preceding pages, the present chapter will
be devoted to a brief account of those items in the plant which lie
between the purifier outlet and the actual burner, including the meter,
governor, and pressure gauge; the proper sizes of pipe for acetylene;
methods of laying it, joint-making, quality of fittings, &c.; while
finally a few words will be said about the precautions necessary when
bringing a new system of pipes into use for the first time.

THE METER.--A meter is required either to control the working of a
complete acetylene installation or to measure the volume of gas passing
through one particular pipe, as when a number of consumers are supplied
through separate services under agreement from a central supply plant.
The control which may be afforded by the inclusion of a meter in the
equipment of a domestic acetylene generating plant is valuable, but in
practice will seldom be exercised. The meter records check the yield of
gas from the carbide consumed in a simple and trustworthy manner, and
also serve to indicate when the material in the purifier is likely to be
approaching exhaustion. The meter may also be used experimentally to
check the soundness of the service-pipes or the consumption of a
particular burner or group of burners. Altogether it may be regarded as a
useful adjunct to a domestic lighting plant, provided full advantage is
taken of it. If, however, there is no intention to pay systematic
attention to the records of the meter, it is best to omit it from such an
installation, and so save its initial cost and the slight loss of
pressure which its use involves on the gas passing through it. A domestic
acetylene lighting plant can be managed quite satisfactorily without a
meter, and as a multiplication of parts is undesirable in an apparatus
which will usually be tended by someone not versed in technical
operations, it is on the whole better to omit the meter in such an
installation. Where the plant is supervised by a technical man, a meter
may advisedly be included in the equipment. Its proper position in the
train of apparatus is immediately after the purifier. A meter must not be
used for unpurified or imperfectly purified acetylene, because the
impurities attack the internal metallic parts and ultimately destroy
them. The supply of acetylene to various consumers from a central
generating station entails the fixing of a meter on each consumer's
service-pipe, so that the quantity consumed by each may be charged for
accordingly, just as in the case of public coal-gas supplies.

There are two types of gas-meter in common use, either of which may,
without essential alteration, be employed for measuring the volume of
acetylene passing through a pipe. It is unnecessary to refer here at
length to their internal mechanism, because their manufacture by other
than firms of professed meter-makers is out of the question, and the user
will be justified in accepting the mechanism as trustworthy and durable.
Meters can always be had stamped with the seal of a local authority or
other body having duly appointed inspectors under the Sales of Gas Act,
and the presence of such a stamp on a meter implies that it has been
officially examined and found to register quantities accurately, or not
varying beyond 2 per cent. in favour of the seller, or 3 per cent, in
favour of the consumer. [Footnote: It may be remarked that when a meter--
wet or dry--begins to register incorrectly by reason of old age or want
of adjustment, its error is very often in the direction that benefits the
customer, _i.e._, more gas passes through it than the dials record.]
Hence a "stamped" meter may be regarded for practical purposes as
affording a correct register of the quantities of gas passing through it.

Except that the use of unalloyed copper in any part of the meter where it
may come in contact with the gas must be wholly avoided, for the reason
that copper is inadmissible in acetylene apparatus (_see_ Chapter
VI.), the meters ordinarily employed for coal-gas serve quite well for
acetylene. Obviously, however, since so very much less acetylene than
coal-gas is consumed per burner, comparatively small meters only will be
required even for large installations of acetylene lighting. This fact is
now recognised by meter-makers, and meters of all suitable sizes can be
obtained. It is desirable, if an ordinary coal-gas meter is being bought
for use with acetylene, to have it subjected to a somewhat more rigorous
test for soundness than is customary before "stamping" but the makers
would readily be able to carry out this additional test.

The two types of gas-meter are known as "wet" and "dry." The case of the
wet meter is about hall-filled with water or other liquid, the level of
which has to be maintained nearly constant. Several ingenious devices are
in use for securing this constancy of level over a more or less extended
period, but the necessity for occasional inspection and adjustment of the
water-level, coupled with the stoppage of the passage of gas in the event
of the water becoming frozen, are serious objections to the employment of
the wet meter in many situations. The trouble of freezing may be avoided
by substituting for the simple water an aqueous solution of glycerin, or
mixture of glycerin with water, suitable strengths for which may be
deduced from the table relating to the use of glycerin in holder seals
given at the close of Chapter III. The dry meter, on the other hand, is
very convenient, because it is not obstructed by the effects of frost,
and because it acts for years without requiring attention. It is not
susceptible of adjustment for measuring with so high a degree of accuracy
as a good wet meter, but its indications are sufficiently correct to fall
well within the legalised deviations already mentioned. Such errors,
perhaps, are somewhat large for so costly and powerful a gas as
acetylene, and they would be better reduced; but it is not so very often
that a dry meter reaches its limit of inaccuracy. Whether wet or dry, the
meter should be fixed in a place where the temperature is tolerably
uniform, otherwise the volumes registered at different times will not
bear the same ratio to the mass of gas (or volume at normal temperature),
and the registrations will be misleading unless troublesome corrections
to compensate for changes of temperature are applied.

THE GOVERNOR, which can be dispensed with in most ordinary domestic
acetylene lighting installations provided with a good gasholder of the
rising-bell type, is designed to deliver the acetylene to a service-pipe
at a uniform pressure, identical with that under which the burners
develop their maximum illuminating efficiency. It must therefore both
cheek the pressure anterior to it whenever that is above the determined
limit to which it is set, and deliver to the efferent service-pipe
acetylene at a constant pressure whether all or any number of the burners
down to one only are in use. Moreover, when the pressure anterior to the
governor falls to or below the determined limit, the governor should
offer no resistance--entailing a loss of pressure to the passage of the
acetylene. These conditions, which a perfect governor should fulfil, are
not absolutely met by any simple apparatus at present in use, but so far
as practical utility is concerned service governors which are readily
obtainable are sufficiently good. They are broadly of two types, viz.,
those having a bell floating in a mercury seal, and those having a
diaphragm of gas-tight leather or similar material, either the bell or
the diaphragm being raised by the pressure of the gas. The action is
essentially the same in both cases: the bell or the diaphragm is so
weighted that when the pressure of the gas exceeds the predetermined
limit the diaphragm or bell is lifted, and, through an attached rod and
valve, brings about a partial closure of the orifice by which the gas
flows into the bell or the diaphragm chamber. The valve of the governor,
therefore, automatically throttles the gas-way more or less according to
the difference in pressure before and after the apparatus, until at any
moment the gas-way is just sufficient in area to pass the quantity of gas
which any indefinite number of burners require at their fixed working
pressure; passing it always at that fixed working pressure irrespective
of the number of burners, and maintaining it constant irrespective of the
amount of pressure anterior to the governor, or of any variations in that
anterior pressure. In most patterns of service governor weights may be
added when it is desired to increase the pressure of the effluent gas. It
is necessary, in ordering a governor for an acetylene-supply, to state
the maximum number of cubic feet per hour it will be required to pass,
and approximately the pressure at which it will be required to deliver
the gas to the service-pipe. This will usually be between 3 and 5 inches
(instead of about 1 inch in the case of coal-gas), and if the anterior
pressure is likely to exceed 10 inches, this fact should be stated also.
The mercury-seal governors are usually the more trustworthy and durable,
but they are more costly than those with leather diaphragms. The seal
should have twice or thrice the depth it usually has for coal-gas. The
governor should be placed where it is readily accessible to the man in
charge of the installation, but where it will not be interfered with by
irresponsible persons. In large installations, where a number of separate
buildings receive service-pipes from one long main, each service-pipe
should be provided with a governor.

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