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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Acetylene Blowpipes--The design of a satisfactory blowpipe for use with
acetylene had at first proved a matter of some difficulty, since the jet,
like that of an ordinary self-luminous burner, usually exhibited a
tendency to become choked with carbonaceous growths. But when acetylene
had become available for various purposes at considerable pressure, after
compression into porous matter as described in Chapter XI, the troubles
were soon overcome; and a new form of blowpipe was constructed in which
acetylene was consumed under pressure in conjunction with oxygen. The
temperature given by this apparatus exceeds that of the familiar oxy-
hydrogen blowpipe, because the actual combustible material is carbon
instead of hydrogen. When 2 atoms of hydrogen unite with 1 of oxygen to
form 1 molecule of gaseous water, about 59 large calories are evolved,
and when 1 atom of solid amorphous carbon unites with 2 atoms of oxygen
to form 1 molecule of carbon dioxide, 97.3 calories are evolved. In both
cases, however, the heat attainable is limited by the fact that at
certain temperatures hydrogen and oxygen refuse to combine to form water,
and carbon and oxygen refuse to form carbon dioxide--in other words,
water vapour and carbon dioxide dissociate and absorb heat in the process
at certain moderately elevated temperatures. But when 1 atom of solid
amorphous carbon unites with 1 atom of oxygen to form carbon monoxide,
29.1 [Footnote: Cf. Chapter VI., page 185.] large calories are produced,
and carbon monoxide is capable of existence at much higher temperatures
than either carbon dioxide or water vapour. In any gaseous hydrocarbon,
again, the carbon exists in the gaseous state, and when 1 atom of the
hypothetical gaseous carbon combines with 1 atom of oxygen to produce 1
molecule of carbon monoxide, 68.2 large calories are evolved. Thus while
solid amorphous carbon emits more heat than a chemically equivalent
quantity of hydrogen provided it is enabled to combine with its higher
proportion of oxygen, it emits less if only carbon monoxide is formed;
but a higher temperature can be attained in the latter case, because the
carbon monoxide is more permanent or stable. Gaseous carbon, on the other
hand, emits more heat than an equivalent quantity of hydrogen, [Footnote:
In a blowpipe flame hydrogen can only burn to gaseous, not liquid,
water.] even when it is only converted into the monoxide. In other words,
a gaseous fuel which consists of hydrogen alone can only yield that
temperature as a maximum at which the speed of the dissociation of the
water vapour reaches that of the oxidation of the hydrogen; and were
carbon dioxide the only oxide of carbon, a similar state of affairs would
be ultimately reached in the flame of a carbonaceous gas. But since in
the latter case the carbon dioxide does not tend to dissociate
completely, but only to lose one atom of oxygen, above the limiting
temperature for the formation of carbon dioxide, carbon monoxide is still
produced, because there is less dissociating force opposed to its
formation. Thus at ordinary temperatures the heat of combustion of
acetylene is 315.7 calories; but at temperatures where water vapour and
carbon dioxide no longer exist, there is lost to that quantity of 315.7
calories the heat of combustion of hydrogen (69.0) and twice that of
carbon monoxide (68.2 x 2 = 136.4); so that above those critical
temperatures, the heat of combustion of acetylene is only 315.7 - (69.0 +
136.4) = 110.3. [Footnote: When the heat of combustion of acetylene is
quoted as 315.7 calories, it is understood that the water formed is
condensed into the liquid state. If the water remains gaseous, as it must
do in a flame, the heat of formation is reduced by about 10 calories.
This does not affect the above calculation, because the heat of
combustion of hydrogen when the water remains gaseous is similarly 10
calories less than 69, _i.e._, 59, as mentioned above in the text.
Deleting the heat of liquefaction of water, the calculation referred to
becomes 305.7 - (59.0 + l36.4) = 110.3 as before.] This value of 110.3
calories is clearly made up of the heat of formation of acetylene itself,
and twice the heat of conversion of carbon into carbon monoxide,
_i.e._, for diamond carbon, 58.1 + 26.1 x 2 = 110.3; or for
amorphous carbon, 52.1 + 29.1 x 2 = 110.3. From the foregoing
considerations, it may be inferred that the acetylene-oxygen blowpipe can
be regarded as a device for burning gaseous carbon in oxygen; but were it
possible to obtain carbon in the state of gas and so to lead it into a
blowpipe, the acetylene apparatus should still be more powerful, because
in it the temperature would be raised, not only by the heat of formation
of carbon monoxide, but also by the heat attendant upon the dissociation
of the acetylene which yields the carbon.

Acetylene requires 2.5 volumes of oxygen to burn it completely; but in
the construction of an acetylene-oxygen blowpipe the proportion of oxygen
is kept below this figure, viz., at 1.1 to 1.8 volumes, so that the
deficiency is left to be made up from the surrounding air. Thus at the
jet of the blowpipe the acetylene dissociates and its carbon is oxidised,
at first no doubt to carbon monoxide only, but afterwards to carbon
dioxide; and round the flame of the gaseous carbon is a comparatively
cool, though absolutely very hot jacket of hydrogen burning to water
vapour in a mixture of oxygen and air, which protects the inner zone from
loss of heat. As just explained, theoretical grounds support the
conclusions at which Fouche has arrived, viz., that the temperature of
the acetylene-oxygen blowpipe flame is above that at which hydrogen will
combine with oxygen to form water, and that it can only be exceeded by
those found in a powerful electric furnace. As the hydrogen dissociated
from the acetylene remains temporarily in the free state, the flame of
the acetylene blowpipe, possesses strong reducing powers; and this,
coupled probably with an intensity of heat which is practically otherwise
unattainable, except by the aid of a high-tension electric current,
should make the acetylene-oxygen blowpipe a most useful piece of
apparatus for a large variety of metallurgical, chemical, and physical
operations. In Fouche's earliest attempts to design an acetylene
blowpipe, the gas was first saturated with a combustible vapour, such as
that of petroleum spirit or ether, and the mixture was consumed with a
blast of oxygen in an ordinary coal-gas blow-pipe. The apparatus worked
fairly well, but gave a flame of varying character; it was capable of
fusing iron, raised a pencil of lime to a more brilliant degree of
incandescence than the eth-oxygen burner, and did not deposit carbon at
the jet. The matter, however, was not pursued, as the blowpipe fed with
undiluted acetylene took its place. The second apparatus constructed by
Fouche was the high-pressure blowpipe, the theoretical aspect of which
has already been studied. In this, acetylene passing through a water-seal
from a cylinder where it is stored as a solution in acetone (_cf._
Chapter XI.), and oxygen coming from another cylinder, are each allowed
to enter the blowpipe at a pressure of 118 to 157 inches of water column
(_i.e._, 8.7 to 11.6 inches of mercury; 4.2 to 5.7 lb. per square
inch, or 0.3 to 0.4 atmosphere). The gases mix in a chamber tightly
packed with porous matter such as that which is employed in the original
acetylene reservoir, and finally issue from a jet having a diameter of 1
millimetre at the necessary speed of 100 to 150 metres per second.
Finding, however, that the need for having the acetylene under pressure
somewhat limited the sphere of usefulness of his apparatus, Fouche
finally designed a low-pressure blowpipe, in which only the oxygen
requires to be in a state of compression, while the acetylene is drawn
directly from any generator of the ordinary pattern that does not yield a
gas contaminated with air. The oxygen passes through a reducing valve to
lower the pressure under which it stands in the cylinder to that of 1 or
1.5 effective atmosphere, this amount being necessary to inject the
acetylene and to give the previously mentioned speed of escape from the
blowpipe orifice. The acetylene is led through a system of long narrow
tubes to prevent it firing-back.

AUTOGENOUS SOLDERING AND WELDING.--The blowpipe is suitable for the
welding and for the autogenous soldering or "burning" of wrought or cast
iron, steel, or copper. An apparatus consuming from 600 to 1000 litres of
acetylene per hour yields a flame whose inner zone is 10 to 15
millimetres long, and 3 to 4 millimetres in diameter; it is sufficiently
powerful to burn iron sheets 8 to 9 millimetres thick. By increasing the
supply of acetylene in proportion to that of the oxygen, the tip of the
inner zone becomes strongly luminous, and the flame then tends to
carburise iron; when the gases are so adjusted that this tip just
disappears, the flame is at its best for heating iron and steel. The
consumption of acetylene is about 75 litres per hour for each millimetre
of thickness in the sheet treated, and the normal consumption of oxygen
is 1.7 times as much; a joint 6 metres long can be burnt in 1 millimetre
plate per hour, and one of 1.5 metres in 10 millimetre plate. In certain
cases it is found economical to raise the metal to dull redness by other
means, say with a portable forge of the usual description, or with a
blowpipe consuming coal-gas and air. There are other forms of low-
pressure blowpipe besides the Fouche, in some of which the oxygen also is
supplied at low pressure. Apart from the use of cylinders of dissolved
acetylene, which are extremely convenient and practically indispensable
when the blowpipe has to be applied in confined spaces (as in repairing
propeller shafts on ships _in situ_), acetylene generators are now
made by several firms in a convenient transportable form for providing
the gas for use in welding or autogenous soldering. It is generally
supposed that the metal used as solder in soldering iron or steel by this
method must be iron containing only a trifling proportion of carbon (such
as Swedish iron), because the carbon of the acetylene carburises the
metal, which is heated in the oxy-acetylene flame, and would thereby make
ordinary steel too rich in carbon. But the extent to which the metal used
is carburised in the flame depends, as has already been indicated, on the
proper adjustment of the proportion of oxygen to acetylene. Oxy-acetylene
autogenous soldering or welding is applicable to a great variety of work,
among which may be mentioned repairs to shafts, locomotive frames,
cylinders, and to joints in ships' frames, pipes, boilers, and rails. The
use of the process is rapidly extending in engineering works generally.
Generators for acetylene soldering or welding must be of ample size to
meet the quickly fluctuating demands on them and must be provided with
water-seals, and a washer or scrubber and filter capable of arresting all
impurities held mechanically in the crude gas, and with a safety vent-
pipe terminating in the open at a distance from the work in hand. The
generator must be of a type which affords as little after-generation as
possible, and should not need recharging while the blowpipe is in use.
There should be a main tap on the pipe between the generator and the
blowpipe. It does not appear conclusively established that the gas
consumed should have been chemically purified, but a purifier of ample
size and charged with efficient material is undoubtedly beneficial. The
blowpipe must be designed so that it remains sufficiently cool to prevent
polymerisation of the acetylene and deposition of the resultant particles
of carbon or soot within it.

It is important to remember that if a diluent gas, such as nitrogen, is
present, the superior calorific power of acetylene over nearly all gases
should avail to keep the temperature of the flame more nearly up to the
temperature at which hydrogen and oxygen cease to combine. Hence a
blowpipe fed with air and acetylene would give a higher temperature than
any ordinary (atmospheric) coal-gas blowpipe, just as, as has been
explained in Chapter VI., an ordinary acetylene flame has a higher
temperature than a coal-gas flame. It is likely that a blowpipe fed with
"Linde-air" (oxygen diluted with less nitrogen than in the atmosphere)
and acetylene would give as high a limelight effect as the oxy-hydrogen
or oxy-coal-gas blowpipe.

CHAPTER X

CARBURETTED ACETYLENE

Now that atmospheric or Bunsen burners for the consumption of acetylene
for use in lighting by the incandescent system and in heating have been
so much improved that they seem to be within measurable reach of a state
of perfection, there appears to be but little use at the present time for
a modified or diluted acetylene which formerly seemed likely to be
valuable for heating and certain other purposes. Nevertheless, the facts
relating to this so-called carburetted acetylene are in no way traversed
by its failure to establish itself as an active competitor with simple
acetylene for heating purposes, and since it is conceivable that the
advantages which from the theoretical standpoint the carburetted gas
undoubtedly possesses in certain directions may ultimately lead to its
practical utilisation for special purposes, it has been deemed expedient
to continue to give in this work an account of the principles underlying
the production and application of carburetted acetylene.

It has already been explained that acetylene is comparatively a less
efficient heating agent than it is an illuminating material, because, per
unit of volume, its calorific power is not so much greater than that of
coal-gas as is its illuminating capacity. It has also been shown that the
high upper explosive limit of mixtures of acetylene and air--a limit so
much higher than the corresponding figure with coal-gas and other gaseous
fuels--renders its employment in atmospheric burners (either for lighting
or for heating) somewhat troublesome, or dependent upon considerable
skill in the design of the apparatus. If, therefore, either the upper
explosive limit of acetylene could be reduced, or its calorific value
increased (or both), by mixing with it some other gas or vapour which
should not seriously affect its price and convenience as a self-luminous
illuminant, acetylene would compare more favourably with coal-gas in its
ready applicability to the most various purposes. Such a method has been
suggested by Heil, and has been found successful on the Continent. It
consists in adding to the acetylene a certain proportion of the vapour of
a volatile hydrocarbon, so as to prepare what is called "carburetted
acetylene." In all respects the method of making carburetted acetylene is
identical with that of making "air-gas," which was outlined in Chapter
I., viz., the acetylene coming from an ordinary generating plant is led
over or through a mass of petroleum spirit, or other similar product, in
a vessel which exposes the proper amount of superficial area to the
passing gas. In all respects save one the character of the product is
similar to that of air-gas, _i.e._, it is a mixture of a permanent
gas with a vapour; the vapour may possibly condense in part within the
mains if they are exposed to a falling temperature, and if the product is
to be led any considerable distance, deposition of liquid may occur
(conceivably followed by blockage of the mains) unless the proportion of
vapour added to the gas is kept below a point governed by local climatic
and similar conditions. But in one most important respect carburetted
acetylene is totally different from air-gas: partial precipitation of
spirit from air-gas removes more or less of the solitary useful
constituent of the material, reducing its practical value, and causing
the residue to approach or overpass its lower explosive limit (_cf._
Chapter I.); partial removal of spirit from carburetted acetylene only
means a partial reconversion of the material into ordinary acetylene,
increasing its natural illuminating power, lowering its calorific
intensity somewhat, and causing the residue to have almost its primary
high upper explosive limit, but essentially leaving its lower explosive
limit unchanged. Thus while air-gas may conceivably become inefficient
for every purpose if supplied from any distance in very cold weather, and
may even pass into a dangerous explosive within the mains; carburetted
acetylene can never become explosive, can only lose part of its special
heating value, and will actually increase in illuminating power.

It is manifest that, like air-gas, carburetted acetylene is of somewhat
indefinite composition, for the proportion of vapour, and the chemical
nature of that vapour, may vary. 100 litres of acetylene will take up 40
grammes of petroleum spirit to yield 110 litres of carburetted acetylene
evidently containing 9 per cent. of vapour, or 100 litres of acetylene
may be made to absorb as much as 250 grammes of spirit yielding 200
litres of carburetted acetylene containing 50 per cent. of vapour; while
the petroleum spirit may be replaced, if prices are suitable, by benzol
or denatured alcohol.

The illuminating power of acetylene carburetted with petroleum spirit has
been examined by Caro, whose average figures, worked out in British
units, are:

ILLUMINATING POWER OF CARBURETTED ACETYLENE.
HALF-FOOT BURNERS.

_Self-luminous._ | _Incandescent_
1 litre = 1.00 candle. | 1 litre = 3.04 candles.
1 cubic foot = 28.4 candles. | 1 cubic foot = 86.2 candles.
1 candle = 1.00 litre. | 1 candle = 0.33 litre.
1 candle = 0.035 cubic foot. | 1 candle = 0.012 cubic foot.

Those results may be compared with those referring to air-gas, which
emits in incandescent burners from 3.0 to 12.4 candles per cubic foot
according to the amount of spirit added to the air and the temperature to
which the gas is exposed.

The calorific values of carburetted acetylene (Caro), and those of other
gaseous fuels are:

Large Calories per
_ Cubic Foot.
| (Lewes) . 320
| (Gand) . 403
Ordinary acetylene . . | (Heil) . 365
| ___
|_Mean . . 363

| Maximum . 680
Carburetted acetylene . . | Minimum . 467
(petroleum spirit) | ___
|_Mean . . 573

Carburetted acetylene (50 per cent. benzol by volume) 685
Carburetted acetylene (50 per cent. alcohol by volume) 364
Coal-gas (common, unenriched) . . . . . 150
_
| Maximum . 178
Air-gas, self-luminous flame | Minimum . 57
| ___
|_Mean . . . 114
_
| Maximum . 26
Air-gas, non-luminous flame | Minimum . 18
| ___
|_Mean . . . 22

Water-gas (Strache) from coke . . . . . 71
Mond gas (from bituminous coal) . . . . . 38
Semi-water-gas from coke or anthracite . . . 36
Generator (producer) gas . . . . . . 29

Besides its relatively low upper explosive limit, carburetted acetylene
exhibits a higher temperature of ignition than ordinary acetylene, which
makes it appreciably safer in presence of a naked light. It also
possesses a somewhat lower flame temperature and a slower speed of
propagation of the explosive wave when mixed with air. These data are:

______________________________________________________________________
| | | | |
| | Explosive | Temperature. | |
| | Limits. | Degrees C. | Explosive |
| |19 mm. Tube. | | Explosive |
| |_____________|__________________| Wave. |
| | | | | | Metres per |
| | | |Of Igni-| | Second. |
| |Lower.|Upper.| tion. |Of Flame.| |
|________________________|______|______|________|_________|____________|
| | | | | | |
| Acetylene (theoretical)| --- | --- | --- |1850-2420| --- |
| " (observed) | 3.35 | 52.3 | 480 |1630-2020| 0.18-100 |
| Carburetted \ from | 2.5 | 10.2 | 582 | 1620 | 3.2 |
| acetylene / . . to | 5.4 | 30.0 | 720 | 1730 | 5.3 |
| Carburetted acetylene\ | 3.4 | 22.0 | --- | 1820 | 1.3 |
| (benzol) . . . / | | | | | |
| Carburetted acetylene\ | 3.1 | 12.0 | --- | 1610 | 1.1 |
| (alcohol) . . . / | | | | | |
| Air-gas, self-luminous\|15.0 | 50.0 | --- |1510-1520| --- |
| flame . . . . /| | | | | |
| Coal-gas . . . | 7.9 | 19.1 | 600 | --- | --- |
|________________________|______|______|________|_________|____________|

In making carburetted acetylene, the pressure given by the ordinary
acetylene generator will be sufficient to drive the gas through the
carburettor, and therefore there will be no expense involved beyond the
cost of the spirit vaporised. Thus comparisons may fairly be made between
ordinary and carburetted acetylene on the basis of material only, the
expense of generating the original acetylene being also ignored. In Great
Britain the prices of calcium carbide, petroleum spirit, and 90s benzol
delivered in bulk in country places may be taken at 15L per ton, and
1s. per gallon respectively, petroleum spirit having a specific
gravity of 0.700 and benzol of 0.88. On this basis, a unit volume (100
cubic metres) of plain acetylene costs 1135d., of "petrolised"
acetylene containing 66 per cent. of acetylene costs 1277d., and
of "benzolised" acetylene costs 1180d. In other words, 100 volumes
of plain acetylene, 90 volumes of petrolised acetylene, and 96 volumes of
benzolised acetylene are of equal pecuniary value. Employing the data
given in previous tables, it appears that 38.5 candles can be won from
plain acetylene in a self-luminous burner, and 103 candles therefrom in
an incandescent burner at the same price as 25.5-29.1 and 78-87 candles
can be obtained from carburetted acetylene; whence it follows that at
English prices petrolised acetylene is more expensive as an illuminant in
either system of combustion than the simple gas, while benzolised
acetylene, burnt under the mantle only, is more nearly equal to the
simple gas from a pecuniary aspect. But considering the calorific value,
it appears that for a given sum of money only 363 calories can be
obtained from plain acetylene, while petrolised acetylene yields 516, and
benzolised acetylene 658; so that for all heating or cooking purposes
(and also for driving small motors) carburetted acetylene exhibits a
notable economy. Inasmuch as the partial saturation of acetylene with any
combustible vapour is an operation of extreme simplicity, requiring no
power or supervision beyond the occasional recharging of the carburettor,
it is manifest that the original main coming from the generator supplying
any large establishment where much warming, cooking (or motor driving)
might conveniently be done with the gas could be divided within the
plant-house, one branch supplying all, or nearly all, the lighting
burners with plain acetylene, and the other branch communicating with a
carburettor, so that all, or nearly all, the warming and cooking stoves
(and the motor) should be supplied with the more economical carburetted
acetylene. Since any water pump or similar apparatus would be in an
outhouse or basement, and the most important heating stove (the cooker)
be in the kitchen, such an arrangement would be neither complicated nor
involve a costly duplication of pipes.

It follows from the fact that even a trifling proportion of vapour
reduces the upper limit of explosibility of mixtures of acetylene with
air, that the gas may be so lightly carburetted as not appreciably to
suffer in illuminating power when consumed in self-luminous jets, and yet
to burn satisfactorily in incandescent burners, even if it has been
generated in an apparatus which introduces some air every time the
operation of recharging is performed. To carry out this idea, Caro has
suggested that 5 kilos. of petroleum spirit should be added to the
generator water for every 50 cubic metres of gas evolved, _i.e._, 1
lb. per 160 cubic feet, or, say, 1 gallon per 1000 cubic feet, or per 200
lb. of carbide decomposed. Caro proposed this addition in the case of
central installations supplying a district where the majority of the
consumers burnt the gas in self-luminous jets, but where a few preferred
the incandescent system; but it is clearly equally suitable for
employment in all private plants of sufficient magnitude.

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