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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Calcium carbide is crushed by the makers into several different sizes, in
each of which all the lumps exceed a certain size and are smaller than
another size. It is necessary to find out by experiment, or from the
maker, what particular size suits the generator best, for different types
of apparatus require different sizes of carbide. Carbide cannot well be
crushed by the consumer of acetylene. It is a difficult operation, and
fraught with the production of dust which is harmful to the eyes and
throat, and if done in open vessels the carbide deteriorates in gas-
making power by its exposure to the moisture of the atmosphere. True dust
in carbide is objectionable, and practically useless for the generation
of acetylene in any form of apparatus, but carbide exceeding 1 inch in
mesh is usually sold to satisfy the suggestions of the British Acetylene
Association, which prescribes 5 per cent, of dust as the maximum. Some
grades of carbide are softer than others, and therefore tend to yield
more dust if exposed to a long journey with frequent unloadings.

There are certain varieties of ordinary carbide known as "treated
carbide," the value of which is more particularly discussed in Chapter
III. The treatment is of two kinds, or of a combination of both. In one
process the lumps are coated with a strong solution of glucose, with the
object of assisting in the removal of spent lime from their surface when
the carbide is immersed in water. Lime is comparatively much more soluble
in solutions of sugar (to which class of substances glucose belongs) than
in plain water; so that carbide treated with glucose is not so likely to
be covered with a closely adherent skin of spent lime when decomposed by
the addition of water to it. In the other process, the carbide is coated
with or immersed in some oil or grease to protect it from premature
decomposition. The latter idea, at least, fulfils its promises, and does
keep the carbide to a large extent unchanged if the lumps are exposed to
damp air, while solving certain troubles otherwise met with in some
generators (cf. Chapter III.); but both operations involve additional
expense, and since ordinary carbide can be used satisfactorily in a good
fixed generator, and can be preserved without serious deterioration by
the exercise of reasonable care, treated carbide is only to be
recommended for employment in holderless generators, of which table-lamps
are the most conspicuous forms. A third variant of plain carbide is
occasionally heard of, which is termed "scented" carbide. It is difficult
to regard this material seriously. In all probability calcium carbide is
odourless, but as it begins to evolve traces of gas immediately
atmospheric moisture reaches it, a lump of carbide has always the
unpleasant smell of crude acetylene. As the material is not to be stored
in occupied rooms, and as all odour is lost to the senses directly the
carbide is put into the generator, scented carbide may be said to be
devoid of all utility.

THE REACTION BETWEEN CARBIDE AND WATER.--The reaction which occurs when
calcium carbide and water are brought into contact belongs to the class
that chemists usually term double decompositions. Calcium carbide is a
chemical compound of the metal calcium with carbon, containing one
chemical "part," or atomic weight, of the former united to two chemical
parts, or atomic weights, of the latter; its composition expressed in
symbols being CaC_2. Similarly, water is a compound of two chemical parts
of hydrogen with one of oxygen, its formula being H_2O. When those two
substances are mixed together the hydrogen of the water leaves its
original partner, oxygen, and the carbon of the calcium carbide leaves
the calcium, uniting together to form that particular compound of
hydrogen and carbon, or hydrocarbon, which is known as acetylene, whose
formula is C_2H_2; while the residual calcium and oxygen join together to
produce calcium oxide or lime, CaO. Put into the usual form of an
equation, the reaction proceeds thus--

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This equation not only means that calcium carbide and water combine to
yield acetylene and lime, it also means that one chemical part of carbide
reacts with one chemical part of water to produce one chemical part of
acetylene and one of lime. But these four chemical parts, or molecules,
which are all equal chemically, are not equal in weight; although,
according to a common law of chemistry, they each bear a fixed proportion
to one another. Reference to the table of "Atomic Weights" contained in
any text-book of chemistry will show that while the symbol Ca is used,
for convenience, as a contraction or sign for the element calcium simply,
it bears a more important quantitative significance, for to it will be
found assigned the number 40. Against carbon will be seen the number 12;
against oxygen, 16; and against hydrogen, 1. These numbers indicate that
if the smallest weight of hydrogen ever found in a chemical compound is
called 1 as a unit of comparison, the smallest weights of calcium,
carbon, and oxygen, similarly taking part in chemical reactions are 40,
12, and 16 respectively. Thus the symbol CaC_2, comes to convoy three
separate ideas: (_a_) that the substance referred to is a compound
of calcium and carbon only, and that it is therefore a carbide of
calcium; (_b_) that it is composed of one chemical part or atom of
calcium and two atoms of carbon; and (_c_) that it contains 40 parts
by weight of calcium combined with twice twelve, or 24, parts of carbon.
It follows from (_c_) that the weight of one chemical part, now
termed a molecule as the substance is a compound, of calcium carbide is
(40 + 2 x 12) = 64. By identical methods of calculation it will be found
that the weight of one molecule of water is 18; that of acetylene, 26;
and that of lime, 56. The general equation (1) given above, therefore,
states in chemical shorthand that 64 parts by weight of calcium carbide
react with 18 parts of water to give 26 parts by weight of acetylene and
56 parts of lime; and it is very important to observe that just as there
are the same number of chemical parts, viz., 2, on each side, so there
are the same number of parts by weight, for 64 + 18 = 56 + 26 = 82. Put
into other words equation (1) shows that if 64 grammes, lb., or cwts. of
calcium carbide are treated with 18 grammes, lb., or cwts. of water, the
whole mass will be converted into acetylene and lime, and the residue
will not contain any unaltered calcium carbide or any water; whence it
may be inferred, as is the fact, that if the weights of carbide and water
originally taken do not stand to one another in the ratio 64 : 18, both
substances cannot be entirely decomposed, but a certain quantity of the
one which was in excess will be left unattacked, and that quantity will
be in exact accordance with the amount of the said excess--indifferently
whether the superabundant substance be carbide or water.

Hitherto, for the sake of simplicity, the by-product in the preparation
of acetylene has been described as calcium oxide or quicklime. It is,
however, one of the leading characteristics of this body to be
hygroscopic, or greedy of moisture; so that if it is brought into the
presence of water, either in the form of liquid or as vapour, it
immediately combines therewith to yield calcium hydroxide, or slaked
lime, whose chemical formula is Ca(OH)_2. Accordingly, in actual
practice, when calcium carbide is mixed with an excess of water, a
secondary reaction takes place over and above that indicated by equation
(1), the quicklime produced combining with one chemical part or molecule
of water, thus--

CaO + H_2O = Ca(OH)_2.

As these two actions occur simultaneously, it is more usual, and more in
agreement with the phenomena of an acetylene generator, to represent the
decomposition of calcium carbide by the combined equation--

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

By the aid of calculations analogous to those employed in the preceding
paragraph, it will be noticed that equation (2) states that 1 molecule of
calcium carbide, or 64 parts by weight, combines with 2 molecules of
water, or 36 parts by weight, to yield 1 molecule, or 26 parts by weight
of acetylene, and 1 molecule, or 74 parts by weight of calcium hydroxide
(slaked lime). Here again, if more than 36 parts of water are taken for
every 64 parts of calcium carbide, the excess of water over those 36
parts is left undecomposed; and in the same fashion, if less than 36
parts of water are taken for every 64 parts of calcium carbide, some of
the latter must remain unattacked, whilst, obviously, the amount of
acetylene liberated cannot exceed that which corresponds with the
quantity of substance suffering complete decomposition. If, for example,
the quantity of water present in a generator is more than chemically
sufficient to attack all the carbide added, however largo or small that
excess may be, no more, and, theoretically speaking, no less, acetylene
can ever be evolved than 26 parts by weight of gas for every 64 parts by
weight of calcium carbide consumed. It is, however, not correct to invert
the proposition, and to say that if the carbide is in excess of the water
added, no more, and, theoretically speaking, no less, acetylene can ever
be evolved than 26 parts by weight of gas for every 36 parts of water
consumed, as might be gathered from equation (2); because equation (1)
shows that 26 parts of acetylene may, on occasion, be produced by the
decomposition of 18 parts by weight of water. From the purely chemical
point of view this apparent anomaly is explained by the circumstance that
of the 36 parts of water present on the left-hand aide of equation (2),
only one-half, _i.e._, 18 parts by weight, are actually decomposed
into hydrogen and oxygen, the other 18 parts remaining unattacked, and
merely attaching themselves as "water of hydration" to the 56 parts of
calcium oxide in equation (1) so as to produce the 74 parts of calcium
hydroxide appearing on the right-hand side of equation (2). The matter is
perhaps rendered more intelligible by employing the old name for calcium
hydroxide or slaked lime, viz., hydrated oxide of calcium, and by writing
its formula in the corresponding form, when equation (2) becomes

CaC_2 + 2H_2O = C_2H_2 + CaO.H_2O.

It is, therefore, absolutely correct to state that if the amount of
calcium carbide present in an acetylene generator is more than chemically
sufficient to decompose all the water introduced, no more, and
theoretically speaking no less, acetylene can ever be liberated than 26
parts by weight of gas for every 18 parts by weight of water attacked.
This, it must be distinctly understood, is the condition of affairs
obtaining in the ideal acetylene generator only; since, for reasons which
will be immediately explained, when the output of gas is measured in
terms of the water decomposed, in no commercial apparatus, and indeed in
no generator which can be imagined fit for actual employment, does that
output of gas ever approach the quantitative amount; but the volume of
water used, if not actually disappearing, is always vastly in excess of
the requirements of equation (2). On the contrary, when the make of gas
is measured in terms of the calcium carbide consumed, the said make may,
and frequently does, reach 80, 90, or even 99 per cent. of what is
theoretically possible. Inasmuch as calcium carbide is the one costly
ingredient in the manufacture of acetylene, so long as it is not wasted--
so long, that is to say, as nearly the theoretical yield of gas is
obtained from it--an acetylene generator is satisfactory or efficient in
this particular; and except for the matter of solubility discussed in the
following chapter, the quantity of water consumed is of no importance
whatever.

HEAT EVOLVED IN THE REACTION.--The chemical reaction between calcium
carbide and water is accompanied by a large evolution of heat, which,
unless due precautions are taken to prevent it, raises the temperature of
the substances employed, and of the apparatus containing them, to a
serious and often inconvenient extent. This phenomenon is the most
important of all in connexion with acetylene manufacture; for upon a
proper recognition of it, and upon the character of the precautions taken
to avoid its numerous evil effects, depend the actual value and capacity
for smooth working of any acetylene generator. Just as, by an immutable
law of chemistry, a given weight of calcium carbide yields a given weight
of acetylene, and by no amount of ingenuity can be made to produce either
more or less; so, by an equally immutable law of physics, the
decomposition of a given weight of calcium carbide by water, or the
decomposition of a given weight of water by calcium carbide, yields a
perfectly definite quantity of heat--a quantity of heat which cannot be
reduced or increased by any artifice whatever. The result of a production
of heat is usually to raise the temperature of the material in which it
is produced; but this is not always the case, and indeed there is no
necessary connexion or ratio between the quantity of heat liberated in
any form of chemical reaction--of which ordinary combustion is the
commonest type--and the temperature attained by the substances concerned.
This matter has so weighty a bearing upon acetylene generation, and
appears to be so frequently misunderstood, that a couple of illustrations
may with advantage be studied. If a vessel full of cold water, and
containing also a thermometer, is placed over a lighted gas-burner, at
first the temperature of the liquid rises steadily, and there is clearly
a ratio between the size of the flame and the speed at which the mercury
mounts up the scale. Finally, however, the thermometer indicates a
certain point, viz., 100 deg. C, and the water begins to boil; yet although
the burner is untouched, and consequently, although heat must be passing
into the vessel at the same rate as before, the mercury refuses to move
as long as any liquid water is left. By the use of a gas meter it might
be shown that the same volume of gas is always consumed (_a_) in
raising the temperature of a given quantity of cold water to the boiling-
point, and another equally constant volume of gas is always consumed
(_b_) in causing the boiling water to disappear as steam. Hence, as
coal-gas is assumed for the present purpose to possess invariably the
same heating power, it appears that the same quantity of heat is always
needed to convert a given amount of cold water at a certain temperature
into steam; but inasmuch as reference to the meter would show that about
5 times the volume of gas is consumed in changing the boiling water into
steam as is used in heating the cold water to the boiling-point, it will
be evident that the temperature of the mass is raised as high by the heat
evolved during the combustion of one part of gas as it is by that
liberated on the combustion of 6 times that amount.

A further example of the difference between quantity of heat and sensible
temperature may be seen in the combustion of coal, for (say) one
hundredweight of that fuel might be consumed in a very few minutes in a
furnace fitted with a powerful blast of air, the operation might be
spread over a considerable number of hours in a domestic grate, or the
coal might be allowed to oxidise by exposure to warm air for a year or
more. In the last case the temperature might not attain that of boiling
water, in the second it would be about that of dull redness, and in the
first it would be that of dazzling whiteness; but in all three cases the
total quantity of heat produced by the time the coal was entirely
consumed would be absolutely identical. The former experiment with water
and a gas-burner, too, might easily be modified to throw light upon
another problem in acetylene generation, for it would be found that if
almost any other liquid than water were taken, less gas (_i.e._, a
smaller quantity of heat) would be required to raise a given weight of it
from a certain low to a certain high temperature than in the case of
water itself; while if it were possible similarly to treat the same
weight of iron (of which acetylene generators are constructed), or of
calcium carbide, the quantity of heat used to raise it through a given
number of thermometric degrees would hardly exceed one-tenth or one-
quarter of that needed by water itself. In technical language this
difference is due to the different specific heats of the substances
mentioned; the specific heat of a body being the relative quantity of
heat consumed in raising a certain weight of it a certain number of
degrees when the quantity of heat needed to produce the same effect on
the same weight of water is called unity. Thus, the specific heat of
water being termed 1.0, that of iron or steel is 0.1138, and that of
calcium carbide 0.247, [Footnote: This is Carlson's figure. Morel has
taken the value 0.103 in certain calculations.] both measured at
temperatures where water is a liquid. Putting the foregoing facts in
another shape, for a given rise in temperature that substance will absorb
the most heat which has the highest specific heat, and therefore, in this
respect, 1 part by weight of water will do the work of roughly 9 parts by
weight of iron, and of about 4 parts by weight of calcium carbide.

From the practical aspect what has been said amounts to this: During the
operation of an acetylene generator a large amount of heat is produced,
the quantity of which is beyond human control. It is desirable, for
various reasons, that the temperature shall be kept as low as possible.
There are three substances present to which the heat may be compelled to
transfer itself until it has opportunity to pass into the surrounding
atmosphere: the material of which the apparatus is constructed, the gas
which is in process of evolution, and whichever of the two bodies--
calcium carbide or water--is in excess in the generator. Of these, the
specific heat at constant pressure of acetylene has unfortunately not yet
been determined, but its relative capacity for absorbing heat is
undoubtedly small; moreover the gas could not be permitted to become
sufficiently hot to carry off the heat without grave disadvantages. The
specific heat of calcium carbide is also comparatively small, and there
are similar disadvantages in allowing it to become hot; moreover it is
deficient in heat-conducting power, so that heat communicated to one
portion of the mass does not extend rapidly throughout, but remains
concentrated in one spot, causing the temperature to rise objectionably.
Steel has a sufficient amount of heat-conducting power to prevent undue
concentration in one place; but, as has been stated, its specific heat is
only one-ninth that of water. Water is clearly, therefore, the proper
substance to employ for the dissipation of the heat generated, although
it is strictly speaking almost devoid of heat-conducting power; for not
only is the specific heat of water much greater than that of any other
material present, but it possesses in a high degree the faculty of
absorbing heat throughout its mass, by virtue of the action known as
convection, provided that heat is communicated to it at or near the
bottom, and not too near its upper surface. Moreover, water is a much
more valuable substance for dissipating heat than appears from the
foregoing explanation; for reference to the experiment with the gas-
burner will show that six and a quarter times as much heat can be
absorbed by a given weight of water if it is permitted to change into
steam, as if it is merely raised to the boiling-point; and since by no
urging of the gas-burner can the temperature be raised above 100 deg. C. as
long as any liquid water remains unevaporated, if an excess of water is
employed in an acetylene generator, the temperature inside can never--
except quite locally--exceed 100 deg. C., however fast the carbide be
decomposed. An indefinitely large consumption of water by evaporation in
a generator matters nothing, for the liquid may be considered of no
pecuniary value, and it can all be recovered by condensation in a
subsequent portion of the plant.

It has been said that the quantity of heat liberated when a certain
amount of carbide suffers decomposition is fixed; it remains now to
consider what that quantity is. Quantities of heat are always measured in
terms of the amount needed to raise a certain weight of water a certain
number of degrees on the thermometric scale. There are several units in
use, but the one which will be employed throughout this book is the
"Large Calorie"; a large calorie being the amount of heat absorbed in
raising 1 kilogramme of water 1 deg. C. Referring for a moment to what has
been said about specific heats, it will be apparent that if 1 large
calorie is sufficient to heat 1 kilo, of water through 1 deg. C. the same
quantity will heat 1 kilo. of steel, whose specific heat is roughly 0.11,
through (10/011) = 9 deg. C., or, which comes to the same thing, will heat 9
kilos, of steel through 1 deg. C.; and similarly, 1 large calorie will raise
4 kilos. of calcium carbide 1 deg. C. in temperature, or 1 kilo. 4 deg. C.
The fact that a definite quantity of heat is manifested when a known weight
of calcium carbide is decomposed by water is only typical; for in every
chemical process some disturbance of heat, though not necessarily of
sensible (or thermometric) character, occurs, heat being either absorbed
or set free. Moreover, if when given weights of two or more substances
unite to form a given weight of another substance, a certain quantity of
heat is set free, precisely the same amount of heat is absorbed, or
disappears, when the latter substance is decomposed to form the same
quantities of the original substances; and, _per contra_, if the
combination is attended by a disappearance of heat, exactly the same
amount is liberated when the compound is broken up into its first
constituents. Compounds are therefore of two kinds: those which absorb
heat during their preparation, and consequently liberate heat when they
are decomposed--such being termed endothermic; and those which evolve
heat during their preparation, and consequently absorb heat when they are
decomposed--such being called exothermic. If a substance absorbs heat
during its formation, it cannot be produced unless that heat is supplied
to it; and since heat, being a form of motion, is equally a form of
energy, energy must be supplied, or work must be done, before that
substance can be obtained. Conversely, if a substance evolves heat during
its formation, its component parts evolve energy when the said substance
is being produced; and therefore the mere act of combination is
accompanied by a facility for doing work, which work may be applied in
assisting some other reaction that requires heat, or may be usefully
employed in any other fashion, or wasted if necessary. Seeing that there
is a tendency in nature for the steady dissipation of energy, it follows
that an exothermic substance is stable, for it tends to remain as it is
unless heat is supplied to it, or work is done upon it; whereas,
according to its degree of endothermicity, an endothermic substance is
more or less unstable, for it is always ready to emit heat, or to do
work, as soon as an opportunity is given to it to decompose. The
theoretical and practical results of this circumstance will be elaborated
in Chapter VI., when the endothermic nature of acetylene is more fully
discussed.

A very simple experiment will show that a notable quantity of heat is set
free when calcium carbide is brought into contact with water, and by
arranging the details of the apparatus in a suitable manner, the quantity
of heat manifested may be measured with considerable accuracy. A lengthy
description of the method of performing this operation, however, scarcely
comes within the province of the present book, and it must be sufficient
to say that the heat is estimated by decomposing a known weight of
carbide by means of water in a small vessel surrounded on all sides by a
carefully jacketed receptacle full of water and provided with a sensitive
thermometer. The quantity of water contained in the outer vessel being
known, and its temperature having been noted before the reaction
commences, an observation of the thermometer after the decomposition is
finished, and when the mercury has reached its highest point, gives data
which show that the reaction between water and a known weight of calcium
carbide produces heat sufficient in amount to raise a known weight of
water through a known thermometric distance; and from these figures the
corresponding number of large calories may easily be calculated. A
determination of this quantity of heat has been made experimentally by
several investigators, including Lewes, who has found that the heat
evolved on decomposing 1 gramme of ordinary commercial carbide with water
is 0.406 large calorie. [Footnote: Lewes returns his result as 406
calories, because he employs the "small calorie." The small calorie is
the quantity of heat needed to raise 1 gramme of water 1 deg. C.; but as
there are 1000 grammes in 1 kilogramme, the large calorie is equal to
1000 small calories. In many respects the former unit is to be
preferred.] As the material operated upon contained only 91.3 per cent.
of true calcium carbide, he estimates the heat corresponding with the
decomposition of 1 gramme of pure carbide to be 0.4446 large calorie. As,
however, it is better, and more in accordance with modern practice, to
quote such data in terms of the atomic or molecular weight of the
substance concerned, and as the molecular weight of calcium carbide is
64, it is preferable to multiply these figures by 64, stating that,
according to Lewes' researches, the heat of decomposition of "1 gramme-
molecule" (_i.e._, 64 grammes) of a calcium carbide having a purity
of 91.3 per cent. is just under 26 calories, or that of 1 gramme-molecule
of pure carbide 28.454 calories. It is customary now to omit the phrase
"one gramme-molecule" in giving similar figures, physicists saying simply
that the heat of decomposition of calcium carbide by water when calcium
hydroxide is the by-product, is 28.454 large calories.

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