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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Assuming all the necessary data known, as happens to be the case in the
present instance, it is also possible to calculate theoretically the heat
which should be evolved on decomposing calcium carbide by means of water.
Equation (2), given on page 24, shows that of the substances taking part
in the reaction 1 molecular weight of calcium carbide is decomposed, and
1 molecular weight of acetylene is formed. Of the two molecules of water,
only one is decomposed, the other passing to the calcium hydroxide
unchanged; and the 1 molecule of calcium hydroxide is formed by the
combination of 1 atom of free calcium, 1 atom of free oxygen, and 1
molecule of water already existing as such. Calcium hydroxide and water
are both exothermic substances, absorbing heat when they are decomposed,
liberating it when they are formed. Acetylene is endothermic, liberating
heat when it is decomposed, absorbing it when it is produced.
Unfortunately there is still some doubt about the heat of formation of
calcium carbide, De Forcrand returning it as -0.65 calorie, and Gin as
+3.9 calories. De Forcrand's figure means, as before explained, that 64
grammes of carbide should absorb 0.65 large calorie when they are
produced by the combination of 40 grammes of calcium with 24 grammes of
carbon; the minus sign calling attention to the belief that calcium
carbide is endothermic, heat being liberated when it suffers
decomposition. On the contrary, Gin's figure expresses the idea that
calcium carbide is exothermic, liberating 3.9 calories when it is
produced, and absorbing them when it is decomposed. In the absence of
corroborative evidence one way or the other, Gin's determination will be
accepted for the ensuing calculation. In equation (2), therefore, calcium
carbide is decomposed and absorbs heat; water is decomposed and absorbs
heat; acetylene is produced and absorbs heat; and calcium hydroxide is
produced liberating heat. On consulting the tables of thermo-chemical
data given in the various text-books on physical chemistry, all the other
constants needed for the present purpose will be found; and it will
appear that the heat of formation of water is +69 calories, that of
acetylene -58.1 calories, and that of calcium hydroxide, when 1 atom of
calcium, 1 atom of oxygen, and 1 molecule of water unite together, is
+160.1 calories. [Footnote: When 1 atom of calcium, 2 atoms of oxygen,
and 2 atoms of hydrogen unite to form solid calcium hydroxide, the heat
of formation of the latter is 229.1 (cf. _infra_). This value is
simply 160.1 + 69.0 = 229.1; 69.0 being the heat of formation of water.]
Collecting the results into the form of a balance-sheet, the effect of
decomposing calcium carbide with water is this:

_Heat liberated._ | _Heat absorbed._
|
Formation of Ca(OH)_2 16O.1 | Formation of acetylene 58.1
| Decomposition of water 69.0
| Decomposition of carbide 3.9
| Balance 29.1
_____ | _____
|
Total 160.1 | Total 160.1

Therefore when 64 grammes of calcium carbide are decomposed by water, or
when 18 grammes of water are decomposed by calcium carbide (the by-
product in each case being calcium hydroxide or slaked lime, for the
formation of which a further 18 grammes of water must be present in the
second instance), 29.1 large calories are set free. It is not possible
yet to determine thermo-chemical data with extreme accuracy, especially
on such a material as calcium carbide, which is hardly to be procured in
a state of chemical purity; and so the value 28.454 calories
experimentally found by Lewes agrees very satisfactorily, considering all
things, with the calculated value 29.1 calories. It is to be noticed,
however, that the above calculated value has been deduced on the
assumption that the calcium hydroxide is obtained as a dry powder; but as
slaked lime is somewhat soluble in water, and as it evolves 3 calories in
so dissolving, if sufficient water is present to take up the calcium
hydroxide entirely into the liquid form (_i.e._, that of a
solution), the amount of heat set free will be greater by those 3
calories, _i.e._, 32.1 large calories altogether.

THE PROCESS OF GENERATION.--Taking 28 as the number of large calories
developed when 64 grammes of ordinary commercial calcium carbide are
decomposed with sufficient water to leave dry solid calcium hydroxide as
the by-product in acetylene generation, this quantity of heat is capable
of exerting any of the following effects. It is sufficient (1) to raise
1000 grammes of water through 28 deg. C., say from 10 deg. C. (50 deg. F.,
which is roughly the temperature of ordinary cold water) to 38 deg. C. It
is sufficient (2) to raise 64 grammes of water (a weight equal to that of
the carbide decomposed) through 438 deg. C., if that were possible. It would
raise (3) 311 grammes of water through 90 deg. C., _i.e._, from 10 deg. C.
to the boiling-point. If, however, instead of remaining in the liquid state,
the water were converted into vapour, the same quantity of heat would
suffice (4) to change 44.7 grammes of water at 10 deg. C. into steam at
100 deg. C.; or (5) to change 46.7 grammes of water at 10 deg. C. into
vapour at the same temperature. It is an action of the last character which
takes place in acetylene generators of the most modern and usual pattern,
some of the surplus water being evaporated and carried away as vapour at a
comparatively low temperature with the escaping gas; for it must be
remembered that although steam, as such, condenses into liquid water
immediately the surrounding temperature falls below 100 deg. C., the vapour
of water remains uncondensed, even at temperatures below the freezing-
point, when that vapour is distributed among some permanent gas--the
precise quantity of vapour so remaining being a function of the
temperature and barometric height. Thus it appears that if the heat
evolved during the decomposition of calcium carbide is not otherwise
consumed, it is sufficient in amount to vaporise almost exactly 3 parts
by weight of water for every 4 parts of carbide attacked; but if it were
expended upon some substance such as acetylene, calcium carbide, or
steel, which, unlike water, could not absorb an extra amount by changing
its physical state (from solid to liquid, or from liquid to gas), the
heat generated during the decomposition of a given weight of carbide
would suffice to raise an equal weight of the particular substance under
consideration to a temperature vastly exceeding 438 deg. C. The temperature
attained, indeed, measured in Centigrade degrees, would be 438 multiplied
by the quotient obtained on dividing the specific heat of water by the
specific heat of the substance considered: which quotient, obviously, is
the "reciprocal" of the specific heat of the said substance.

The analogy to the combustion of coal mentioned on a previous page shows
that although the quantity of heat evolved during a certain chemical
reaction is strictly fixed, the temperature attained is dependent on the
time over which the reaction is spread, being higher as the process is
more rapid. This is due to the fact that throughout the whole period of
reaction heat is escaping from the mass, and passing into the atmosphere
at a fairly constant speed; so that, clearly, the more slowly heat is
produced, the better opportunity has it to pass away, and the less of it
is left to collect in the material under consideration. During the action
of an acetylene generator, there is a current of gas constantly
travelling away from the carbide, there is vapour of water constantly
escaping with the gas, there are the walls of the generator itself
constantly exposed to the cooling action of the atmosphere, and there is
either a mass of calcium carbide or of water within the generator. It is
essential for good working that the temperature of both the acetylene and
the carbide shall be prevented from rising to any noteworthy extent;
while the amount of heat capable of being dissipated into the air through
the walls of the apparatus in a given time is narrowly limited, depending
upon the size and shape of the generator, and the temperature of the
surrounding air. If, then, a small, suitably designed generator is
working quite slowly, the loss of heat through the external walls of the
apparatus may easily be rapid enough to prevent the internal temperature
from rising objectionably high; but the larger the generator, and the
more rapidly it is evolving gas, the less does this become possible.
Since of the substances in or about a generator water is the one which
has by far the largest capacity for absorbing heat, and since it is the
only substance to which any necessary quantity of heat can be safely or
conveniently transmitted, it follows that the larger in size an acetylene
generator is, or the more rapidly that generator is made to deliver gas,
the more desirable is it to use water as the means for dissipating the
surplus heat, and the more necessary is it to employ an apparatus in
which water is in large chemical excess at the actual place of
decomposition.

The argument is sometimes advanced that an acetylene generator containing
carbide in excess will work satisfactorily without exhibiting an
undesirable rise in internal temperature, if the vessel holding the
carbide is merely surrounded by a large quantity of cold water. The idea
is that the heat evolved in that particular portion of the charge which
is suffering decomposition will be communicated with sufficient speed
throughout the whole mass of calcium carbide present, whence it will pass
through the walls of the containing vessel into the water all round.
Provided the generator is quite small, provided the carbide container is
so constructed as to possess the maximum of superficial area with the
minimum of cubical capacity (a geometrical form to which the sphere, and
in one direction the cylinder, are diametrically opposed), and provided
the walls of the container do not become coated internally or externally
with a coating of lime or water scale so as to diminish in heat-
transmitting power, an apparatus designed in the manner indicated is
undoubtedly free from grave objection; but immediately any of those
provisions is neglected, trouble is likely to ensue, for the heat will
not disappear from the place of actual reaction at the necessary speed.
Apparent proof that heat is not accumulating unduly in a water-jacketed
carbide container even when the generator is evolving gas at a fair speed
is easy to obtain; for if, as usually happens, the end of the container
through which the carbide is inserted is exposed to the air, the hand may
be placed upon it, and it will be found to be only slightly warm to the
touch. Such a test, however, is inconclusive, and frequently misleading,
because if more than a pound or two of carbide is present as an undivided
mass, and if water is allowed to attack one portion of it, that
particular portion may attain a high temperature while the rest is
comparatively cool: and if the bulk of the carbide is comparatively cool,
naturally the walls of the containing vessel themselves remain
practically unheated. Three causes work together to prevent this heat
being dissipated through the walls of the carbide vessel with sufficient
rapidity. In the first place, calcium carbide itself is a very bad
conductor of heat. So deficient in heat-conducting power is it that a
lump a few inches in diameter may be raised to redness in a gas flame at
one spot, and kept hot for some minutes, while the rest of the mass
remains sufficiently cool to be held comfortably in the fingers. In the
second place, commercial carbide exists in masses of highly irregular
shape, so that when they are packed into any vessel they only touch at
their angles and edges; and accordingly, even if the material were a
fairly good heat conductor of itself, the air or gas present between each
lump would act as an insulator, protecting the second piece from the heat
generated in the first. In the third place, the calcium hydroxide
produced as the by-product when calcium carbide is decomposed by water
occupies considerably more space than the original carbide--usually two
or three times as much space, the exact figures depending upon the
conditions in which it is formed--and therefore a carbide container
cannot advisedly be charged with more than one-third the quantity of
solid which it is apparently capable of holding. The remaining two-thirds
of the space is naturally full of air when the container is first put
into the generator, but the air is displaced by acetylene as soon as gas
production begins. Whether that space, however, is occupied by air, by
acetylene, or by a gradually growing loose mass of slaked lime, each
separate lump of hot carbide is isolated from its neighbours by a
material which is also a very bad heat conductor; and the heat has but
little opportunity of distributing itself evenly. Moreover, although iron
or steel is a notably better conductor of heat than any of the other
substances present in the carbide vessel, it is, as a metal, only a poor
conductor, being considerably inferior in this respect to copper. If heat
dissipation were the only point to be studied in the construction of an
acetylene apparatus, far better results might be obtained by the
employment of copper for the walls of the carbide container; and possibly
in that case a generator of considerable size, fitted with a water-
jacketed decomposing vessel, might be free from the trouble of
overheating. Nevertheless it will be seen in Chapter VI. that the use of
copper is not permissible for such purposes, its advantages as a good
conductor of heat being neutralised by its more important defects.

When suitable precautions are not taken to remove the heat liberated in
an acetylene apparatus, the temperature of the calcium carbide
occasionally rises to a remarkable degree. Investigating this point, Caro
has studied the phenomena of heat production in a "dipping" generator--
_i.e._, an apparatus in which a cage of carbide is alternately
immersed in and lifted out of a vessel containing water. Using a
generator designed to supply five burners, he has found a maximum
recording thermometer placed in the gas space of the apparatus to give
readings generally between 60 deg. and 100 deg. C.; but in two tests out
of ten he obtained temperatures of about 160 deg. C. To determine the
actual temperature of the calcium carbide itself, he scattered amongst the
carbide charge fragments of different fusible metallic alloys which were
known to melt or soften at certain different temperatures. In all his ten
tests the alloys melting at 120 deg. C. were fused completely; in two tests
other alloys melting at 216 deg. and 240 deg. C. showed signs of fusion;
and in one test an alloy melting at 280 deg. C. began to soften. Working
with an experimental apparatus constructed on the "dripping" principle--
_i.e._, a generator in which water is allowed to fall in single
drops or as a fine stream upon a mass of carbide--with the deliberate
object of ascertaining the highest temperatures capable of production
when calcium carbide is decomposed in this particular fashion, and
employing for the measurement of the heat a Le Chatelier thermo-couple,
with its sensitive wires lying among the carbide lumps, Lewes has
observed a maximum temperature of 674 deg. C. to be reached in 19 minutes
when water was dripped upon 227 grammes of carbide at a speed of about 8
grammes per minute. In other experiments he used a laboratory apparatus
designed upon the "dipping" principle, and found maximum temperatures, in
four different trials, of 703 deg., 734 deg., 754 deg., and 807 deg. C.,
which were reached in periods of time ranging from 12 to 17 minutes. Even
allowing for the greater delicacy of the instrument adopted by Lewes for
measuring the temperature in comparison with the device employed by Caro,
there still remains an astonishing difference between Caro's maximum of
280 deg. and Lewes' maximum of 807 deg. C. The explanation of this
discrepancy is to be inferred from what has just been said. The generator
used by Caro was properly made of metal, was quite small in size, was
properly designed with some skill to prevent overheating as much as possible,
and was worked at the speed for which it was intended--in a word, it was as
good an apparatus as could be made of this particular type. Lewes' generator
was simply a piece of glass and metal, in which provisions to avoid
overheating were absent; and therefore the wide difference between the
temperatures noted does not suggest any inaccuracy of observation or
experiment, but shows what can be done to assist in the dissipation of
heat by careful arrangement of parts. The difference in temperature
between the acetylene and the carbide in Caro's test accentuates the
difficulty of gauging the heat in a carbide vessel by mere external
touch, and supplies experimental proof of the previous assertions as to
the low heat-conducting power of calcium carbide and of the gases of the
decomposing vessel. It must not be supposed that temperatures such as
Lewes has found ever occur in any commercial generator of reasonably good
design and careful construction; they must be regarded rather as
indications of what may happen in an acetylene apparatus when the
phenomena accompanying the evolution of gas are not understood by the
maker, and when all the precautions which can easily be taken to avoid
excessive heating have been omitted, either by building a generator with
carbide in excess too large in size, or by working it too rapidly, or
more generally by adopting a system of construction unsuited to the ends
in view. The fact, however, that Lewes has noted the production of a
temperature of 807 deg. C. is important; because this figure is appreciably
above the point 780 deg. C., at which acetylene decomposes into its elements
in the absence of air.

Nevertheless the production of a temperature somewhat exceeding 100 deg. C.
among the lumps of carbide actually undergoing decomposition can hardly
be avoided in any practical generator. Based on a suggestion in the
"Report of the Committee on Acetylene Generators" which was issued by the
British Home Office in 1902, Fouche has proposed that 130 deg. C., as
measured with the aid of fusible metallic rods, [Footnote: An alloy made
by melting together 55 parts by weight of commercial bismuth and 45 parts
of lead fuses at 127 deg. C., and should be useful in performing the tests.]
should be considered the maximum permissible temperature in any part of a
generator working at full speed for a prolonged period of time. Fouche
adopts this figure on the ground that 130 deg. C. sensibly corresponds with
the temperature at which a yellow substance is formed in a generator by a
process of polymerisation; and, referring to French conditions, states
that few actual apparatus permit the development of so high a
temperature. As a matter of fact, however, a fairly high temperature
among the carbide is less important than in the gas, and perhaps it would
be better to say that the temperature in any part of a generator occupied
by acetylene should not exceed 100 deg. C. Fraenkel has carried out some
experiments upon the temperature of the acetylene immediately after
evolution in a water-to-carbide apparatus containing the carbide in a
subdivided receptacle, using an apparatus now frequently described as
belonging to the "drawer" system of construction. When a quantity of
about 7 lb. of carbide was distributed between 7 different cells of the
receptacle, each cell of which had a capacity of 25 fluid oz., and the
apparatus was caused to develop acetylene at the rate of 7 cubic feet per
hour, maximum thermometers placed immediately over the carbide in the
different cells gave readings of from 70 deg. to 90 deg. C., the average
maximum temperature being about 80 deg. C. Hence the Austrian code of
rules issued in 1905 governing the construction of acetylene apparatus
contains a clause to the effect that the temperature in the gas space of
a generator must never exceed 80 deg. C.; whereas the corresponding
Italian code contains a similar stipulation, but quotes the maximum
temperature as 100 deg. C. (_vide_ Chapter IV.).

It is now necessary to see why the production of an excessively high
temperature in an acetylene generator has to be avoided. It must be
avoided, because whenever the temperature in the immediate neighbourhood
of a mass of calcium carbide which is evolving acetylene under the attack
of water rises materially above the boiling-point of water, one or more
of three several objectionable effects is produced--(_a_) upon the
gas generated, (_b_) upon the carbide decomposed, and (_c_)
upon the general chemical reaction taking place.

It has been stated above that in moat generators when the action between
the carbide and the water is proceeding smoothly, it occurs according to
equation (2)--

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2

rather than in accordance with equation (1)--

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This is because calcium oxide, or quicklime, the by-product in (1), has
considerable affinity for water, evolving a noteworthy quantity of heat
when it combines with one molecule of water to form one molecule of
calcium hydroxide, or slaked lime, the by-product in (2). If, then, a
small amount of water is added to a large amount of calcium carbide, the
corresponding quantity of acetylene may be liberated on the lines of
equation (1), and there will remain behind a mixture of unaltered calcium
carbide, together with a certain amount of calcium oxide. Inasmuch as
both these substances possess an affinity for water (setting heat free
when they combine with it), when a further limited amount of water is
introduced into the mixture some of it will probably be attracted to the
oxide instead of to the carbide present. It is well known that at
ordinary temperatures quicklime absorbs moisture, or combines with water,
to produce slaked lime; but it is equally well known that in a furnace,
at about a red heat, slaked lime gives up water and changes into
quicklime. The reaction, in fact, between calcium oxide and water is
reversible, and whether those substances combine or dissociate is simply
a question of temperature. In other words, as the temperature rises, the
heat of hydration of calcium oxide diminishes, and calcium hydroxide
becomes constantly a less stable material. If now it should happen that
the affinity between calcium carbide and water should not diminish, or
should diminish in a lower ratio than the affinity between calcium oxide
and water as the temperature of the mass rises from one cause or other,
it is conceivable that at a certain temperature calcium carbide might be
capable of withdrawing the water of hydration from the molecule of slaked
lime, converting the latter into quicklime, and liberating one molecule
of acetylene, thus--

(3) CaC_2 + Ca_2(OH) = C_2H_2 + 2CaO.

It has been proved that a reaction of this character does occur, the
temperature necessary to determine it being given by Lewes as from 420 deg.
to 430 deg. C., which is not much more than half that which he found in a
generator having carbide in excess, albeit one of extremely bad design.
Treating this reaction in the manner previously adopted, the thermo-
chemical phenomena of equation (3) are:

_Heat liberated._ | _Heat liberated._
|
Formation of 2CaO 290.0 | Formation of acetylene 58.1
| Decomposition of Ca(OH)_2 [1] 229.1
| Decomposition of carbide 3.9
Balance 1.1 |
______ | _____
|
291.1 | 291.1

[1 Footnote: Into its elements, Ca, O_2, and H_2; _cf._ footnote,
p: 31.]

Or, since the calcium hydroxide is only dehydrated without being
entirely decomposed, and only one molecule of water is broken up, it may
be written:

Formation of CaO 145.0 | Formation of acetylene 58.1
| Decomposition of Ca(OH)_2 15.1
| Decomposition of water 69.0
Balance 1.1 | Decomposition of carbide 3.9
_____ | _____
|
146.1 | 146.1

which comes to the same thing. Putting the matter in another shape, it may
be said that the reaction between calcium carbide and water is exothermic,
evolving either 14.0 or 29.1 calories according as the byproduct is calcium
oxide or solid calcium hydroxide; and therefore either reaction proceeds
without external assistance in the cold. The reaction between carbide and
slaked lime, however, is endothermic, absorbing 1.1 calories; and therefore
it requires external assistance (presence of an elevated temperature) to
start it, or continuous introduction of heat (as from the reaction between
the rest of the carbide present and the water) to cause it to proceed. Of
itself, and were it not for the disadvantages attending the production of a
temperature remotely approaching 400 deg. C. in an acetylene generator,
which disadvantages will be explained in the following paragraphs, there is
no particular reason why reaction (3) should not be permitted to occur, for
it involves (theoretically) no loss of acetylene, and no waste of calcium
carbide. Only one specific feature of the reaction has to be remembered,
and due practical allowance made for it. The reaction represented by
equation (2) proceeds almost instantaneously when the calcium carbide is
of ordinarily good quality, and the acetylene resulting therefrom is
wholly generated within a very few minutes. Equation (3), on the
contrary, consumes much time for its completion, and the gas
corresponding with it is evolved at a gradually diminishing speed which
may cause the reaction to continue for hours--a circumstance that may be
highly inconvenient or quite immaterial according to the design of the
apparatus. When, however, it is desired to construct an automatic
acetylene generator, _i.e._, an apparatus in which the quantity of
gas liberated has to be controlled to suit the requirements of any
indefinite number of burners in use on different occasions, equation (3)
becomes a very important factor in the case. To determine the normal
reaction (No. 2) of an acetylene generator, 64 parts by weight of calcium
carbide must react with 36 parts of water to yield 26 parts by weight of
acetylene, and apparently both carbide and water are entirely consumed;
but if opportunity is given for the occurrence of reaction (3), another
64 parts by weight of carbide may be attacked, without the addition of
any more water, producing, inevitably, another 26 parts of acetylene. If,
then, water is in chemical excess in the generator, all the calcium
carbide present will be decomposed according to equation (2), and the
action will take place without delay; after a few minutes' interval the
whole of the acetylene capable of liberation will have been evolved, and
nothing further can possibly happen until another charge of carbide is
inserted in the apparatus. If, on the other hand, calcium carbide is in
chemical excess in the generator, all the water run in will be consumed
according to equation (2), and this action will again take place without
delay; but unless the temperature of the residual carbide has been kept
well below 400 deg. C., a further evolution of gas will occur which will not
cease for an indeterminate period of time, and which, by strict theory,
given the necessary conditions, might continue until a second volume of
acetylene equal to that liberated at first had been set free. In practice
this phenomenon of a secondary production of gas, which is known as
"after-generation," is regularly met with in all generators where the
carbide is in excess of the water added; but the amount of acetylene so
evolved rarely exceeds one-quarter or one-third of the main make. The
actual amount evolved and the rate of evolution depend, not merely upon
the quantity of undecomposed carbide still remaining in contact with the
damp lime, but also upon the rapidity with which carbide naturally
decomposes in presence of liquid water, and the size of the lumps. Where
"after-generation" is caused by the ascent of water vapour round lumps of
carbide, the volume of gas produced in a given interval of time is
largely governed by the temperature prevailing and the shape of the
apparatus. It is evident that even copious "after-generation" is a matter
of no consequence in any generator provided with a holder to store the
gas, assuming that by some trustworthy device the addition of water is
stopped by the time that the holder is two-thirds or three-quarters full.
In the absence of a holder, or if the holder fitted is too small to serve
its proper purpose, "aftergeneration" is extremely troublesome and
sometimes dangerous, but a full discussion of this subject must be
postponed to the next chapter.

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