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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Patents have been taken out by Schwander for the preparation of a mixture
of acetylene, air, and vaporised petroleum spirit. A current of naturally
damp, or artificially moistened, air is led over or through a mass of
calcium carbide, whereby the moisture is replaced by an equivalent
quantity of acetylene; and this mixture of acetylene and air is
carburetted by passing it through a vessel of petroleum spirit in the
manner adopted with air-gas. No details as to the composition,
illuminating power, and calorific values of the gas so made have been
published. It would clearly tend to be of highly indefinite constitution
and might range between what would be virtually inferior carburetted
acetylene, and a low-grade air-gas. It is also doubtful whether the
combustion of such gas would not be accompanied by too grave risks to
render the process useful.

CHAPTER XII

SUNDRY USES

There are sundry uses for acetylene, and to some extent for carbide,
which are not included in what has been said in previous chapters of this
book; and to them a few words may be devoted.

In orchards and market gardens enormous damage is frequently done to the
crops by the ravages of caterpillars of numerous species. These
caterpillars cannot be caught by hand, and hitherto it has proved
exceedingly difficult to cope with them. However, when they have changed
into the perfect state, the corresponding butterflies and moths, like
most other winged insects, are strongly attracted by a bright light. As
acetylene can easily be burnt in a portable apparatus, and as the burners
can be supplied with gas at such comparatively high pressure that the
flames are capable of withstanding sharp gusts of wind even when not
protected by glass, the brilliant light given by acetylene forms an
excellent method of destroying the insects before they have had time to
lay their eggs. Two methods of using the light have been tried with
astonishing success: in one a naked flame is supported within some
receptacle, such as a barrel with one end knocked out, the interior of
which is painted heavily with treacle; in the other the flame is
supported over an open dish filled with some cheap heavy oil (or perhaps
treacle would do equally well). In the first case the insects are
attracted by the light and are caught by the adhesive surfaces; in the
second they are attracted and singed, and then drowned in, or caught by,
the liquid. Either a well-made, powerful, vehicular lamp with its bull's-
eye (if any) removed could be used for this purpose, or a portable
generator of any kind might be connected with the burner through a
flexible tube. It is necessary that the lights should be lit just before
dusk when the weather is fine and the nights dark, and for some twenty
evenings in June or July, exactly at the period of the year when the
perfect insects are coming into existence. In some of the vineyards of
Beaujolais, in France, where great havoc has been wrought by the pyralid,
a set of 10-candle-power lamps were put up during July 1901, at distances
of 150 yards apart, using generators containing 6 oz. of carbide, and
dishes filled with water and petroleum 18 or 20 inches in diameter. In
eighteen nights, some twenty lamps being employed, the total catch of
insects was 170,000, or an average of 3200 per lamp per night. At French
prices, the cost is reported to have been 8 centimes per night, or 32
centimes per hectare (2.5 acres). In Germany, where school children are
occasionally paid for destroying noxious moths, two acetylene lamps
burning for twelve evenings succeeded in catching twice as many insects
as the whole juvenile population of a village during August 1902. A
similar process has been recommended for the destruction of the malarial
mosquito, and should prove of great service to mankind in infected
districts. The superiority of acetylene in respect of brilliancy and
portability will at once suggest its employment as the illuminant in the
"light" moth-traps which entomologists use for entrapping moths. In these
traps, the insects, attracted by the light, flutter down panes of glass,
so inclined that ultimate escape is improbable; while they are protected
from injury through contact with the flame by moans of an intervening
sheet of glass.

Methods of spraying with carbide dust have been found useful in treating
mildew in vines; while a process of burying small quantities of carbide
at the roots has proved highly efficacious in exterminating phylloxera in
the French and Spanish vineyards. It was originally believed that the
impurities of the slowly formed acetylene, the phosphine in particular,
acted as toxic agents upon the phylloxera; and therefore carbide
containing an extra amount of decomposable phosphides was specially
manufactured for the vine-growers. But more recently it has been argued,
with some show of reason, that the acetylene itself plays a part in the
process, the effects produced being said to be too great to be ascribed
wholly to the phosphine. It is well known that many hydrocarbon vapours,
such as the vapour of benzene or of naphthalene, have a highly toxic
action on low organisms, and the destructive effect of acetylene on
phylloxera may be akin to this action.

As gaseous acetylene will bear a certain amount of pressure in safety--a
pressure falling somewhat short of one effective atmosphere--and as
pressure naturally rises in a generating apparatus where calcium carbide
reacts with water, it becomes possible to use this pressure as a source
of energy for several purposes. The pressure of the gas may, in fact, be
employed either to force a stream of liquid through a pipe, or to propel
certain mechanism. An apparatus has been constructed in France on the
lines of some portable fire-extinguishing appliances in which the
pressure set up by the evolution of acetylene in a closed space produces
a spray of water charged with lime and gas under the pressure obtaining;
the liquid being thrown over growing vines or other plants in order to
destroy parasitic and other forms of life. The apparatus consists of a
metal cylinder fitted with straps so that it can be carried by man or
beast. At one end it has an attachment for a flexible pipe, at the other
end a perforated basket for carbide introduced and withdrawn through a
"man-hole" that can be tightly closed. The cylinder is filled with water
to a point just below the bottom of the basket when the basket is
uppermost; the carbide charge is then inserted, and the cover fastened
down. As long as the cylinder is carried in the same position, no
reaction between the carbide and the water occurs, and consequently no
pressure arises; but on inverting the vessel, the carbide is wetted, and
acetylene is liberated in the interior. On opening the cock on the outlet
pipe, a stream of liquid issues and may be directed as required. By
charging the cylinder in the first place with a solution of copper
sulphate, the liquid ejected becomes a solution and suspension of copper
and calcium salts and hydroxides, resembling "Bordeaux mixture," and may
be employed as such. In addition, it is saturated with acetylene which
adds to its value as a germicide.

The effective gas pressure set up in a closed generator has also been
employed in Italy to drive a gas-turbine, and so to produce motion. The
plant has been designed for use in lighthouses where acetylene is burnt,
and where a revolving or flashing light is required. The gas outlet from
a suitably arranged generator communicates with the inlet of a gas-
turbine, and the outlet of the turbine is connected to a pipe leading to
the acetylene burners. The motion of the turbine is employed to rotate
screens, coloured glasses, or any desired optical arrangements round the
flames; or, in other situations, periodically to open and close a cock on
the gas-main leading to the burners. In the latter case, a pilot flame
fed separately is always alight, and serves to ignite the gas issuing
from the main burners when the cock is opened.

Another use for acetylene, which is only dependent upon a suitably
lowered price for carbide to become of some importance, consists in the
preparation of a black pigment to replace ordinary lampblack. One method
for this purpose has been elaborated by Hubou. Acetylene is prepared from
carbide smalls or good carbide, according to price, and the gas is pumped
into small steel cylinders to a pressure of 2 atmospheres. An electric
spark is then passed, and the gas, standing at its limit of safety,
immediately dissociates, yielding a quantitative amount of hydrogen and
free carbon. The hydrogen is drawn off, collected in holders, and used
for any convenient purpose; the carbon is withdrawn from the vessel, and
is ready for sale. At present the pigment is much too expensive, at least
in British conditions, to be available in the manufacture of black paint;
but its price would justify its employment in the preparation of the best
grades of printers' ink. One of the authors has examined an average
sample and has found it fully equal in every way to blacks, such as those
termed "spirit blacks," which fetch a price considerably above their real
value. It has a pure black cast of tint, is free from greasy matter, and
can therefore easily be ground into water, or into linseed oil without
interfering with the drying properties of the latter. Acetylene black has
also been tried in calico printing, and has given far better results in
tone and strength than other blacks per unit weight of pigment. It may be
added that the actual yield of pigment from creosote oils, the commonest
raw material for the preparation of lampblack ("vegetable black"), seldom
exceeds 20 or 25 per cent., although the oil itself contains some 80 per
cent, of carbon. The yield from acetylene is clearly about 90 per cent.,
or from calcium carbide nearly 37.5 per cent, of the original weight.

An objection urged against the Hubou process is that only small
quantities of the gas can be treated with the spark at one time; if the
cylinders are too large, it is stated, tarry by-products are formed. A
second method of preparing lampblack (or graphite) from acetylene is that
devised by Frank, and depends on utilising the reactions between carbon
monoxide or dioxide and acetylene or calcium carbide, which have already
been sketched in Chapter VI. When acetylene is employed, the yield is
pure carbon, for the only by-product is water vapour; but if the carbide
process is adopted, the carbon remains mixed with calcium oxide. Possibly
such a material as Frank's carbide process would give, viz., 36 parts by
weight of carbon mixed with 56 parts of quicklime or 60 parts of carbon
mixed with 112 parts of quicklime, might answer the purpose of a pigment
in some black paints where the amount of ash left on ignition is not
subject to specification. Naturally, however, the lime might be washed
away from the carbon by treatment with hydrochloric acid; but the cost of
such a purifying operation would probably render the residual pigment too
expensive to be of much service except (conceivably) in the manufacture
of certain grades of printers' ink, for which purpose it might compete
with the carbon obtainable by the Hubou process already referred to.

Acetylene tetrachloride, or tetrachlorethane, C_2H_2Cl_4, is now produced
for sale as a solvent for chlorine, sulphur, phosphorus, and organic
substances such as fats. It may be obtained by the direct combination of
acetylene and chlorine as explained in Chapter VI., but the liability of
the reaction to take place with explosive violence would preclude the
direct application of it on a commercial scale. Processes free from such
risk have now, however, been devised for the production of
tetrachlorethane. One patented by the Salzbergwerk Neu-Stassfurt consists
in passing acetylene into a mixture of finely divided iron and chloride
of sulphur. The iron acts as a catalytic. The liquid is kept cool, and as
soon as the acetylene passes through unabsorbed, its introduction is
stopped and chlorine is passed in. Acetylene and chlorine are then passed
in alternately until the liquid finally is saturated with acetylene. The
tetrachlorethane, boiling at 147 deg. C., is then distilled off, and the
residual sulphur is reconverted to the chloride for use again in the
process. A similar process in which the chlorine is used in excess is
applicable also to the production of hexachlorethane.

Dependent upon price, again, are several uses for calcium carbide as a
metallurgical or reducing reagent; but as those are uses for carbide only
as distinguished from acetylene, they do not fall within the purview of
the present book.

When discussing, in Chapter III., methods for disposing of the lime
sludge coming from an acetylene generator, it was stated that on occasion
a use could be found for this material. If the carbide has been entirely
decomposed in an apparatus free from overheating, the waste lime is
recovered as a solid mass or as a cream of lime practically pure white in
colour. Sometimes, however, as explained in Chapter II., the lime sludge
is of a bluish grey tint, even in cases where the carbide decomposed was
of good quality and there was no overheating in the generator. Such
discoloration is of little moment for most of the uses to which the
sludge may be put. The residue withdrawn from a carbide-to-water
generator is usually quite fluid; but when allowed to rest in a suitable
pit or tank, it settles down to a semi-solid or pasty mass which contains
on a rough average 47 per cent. of water and 53 per cent. of solid
matter, the amount of lime present, calculated as calcium oxide, being
about 40 per cent. Since 64 parts by weight of pure calcium carbide yield
74 parts of dry calcium hydroxide, it may be said that 1 part of ordinary
commercial carbide should yield approximately 1.1 parts of dry residue,
or 2.1 parts of a sludge containing 47 per cent. of moisture; and sludge
of this character has been stated by Vogel to weigh about 22.5 cwt. per
cubic yard.

Experience has shown that those pasty carbide residues can be employed
very satisfactorily, and to the best advantage from the maker's point of
view, by builders and decorators for the preparation of ordinary mortar
or lime-wash. The mortar made from acetylene lime has been found equal in
strength and other properties to mortar compounded from fresh slaked
lime; while the distemper prepared by diluting the sludge has been used
most successfully in all places where a lime-wash is required,
_e.g._, on fruit-trees, on cattle-pens, farm-buildings, factories,
and the "offices" of a residence. Many of the village installations
abroad sell their sludge to builders for the above-mentioned purposes at
such a price that their revenue accounts are materially benefited by the
additional income. The sludge is also found serviceable for softening the
feed-water of steam boilers by the common liming process; although it has
been stated that the material contains certain impurities--notably "fatty
matter"--which becomes hydrolysed by the steam, yielding fatty acids that
act corrosively upon the boiler-plates. This assertion would appear to
require substantiation, but a patent has been taken out for a process of
drying the sludge at a temperature of 150 deg. to 200 deg. C. in order to
remove the harmful matter by the action of the steam evolved. So purified,
it is claimed, the lime becomes fit for treating any hard potable or
boiler-feed water. It is very doubtful, however, whether the intrinsic value
of acetylene lime is such in comparison with the price of fresh lime that,
with whatever object in view, it would bear the cost of any method of
artificial drying if obtained from the generators in a pasty state.

When, on the other hand, the residue is naturally dry, or nearly so, it
is exactly equal to an equivalent quantity of quick or slaked lime as a
dressing for soil. In this last connexion, however, it must be remembered
that only certain soils are improved by an addition of lime in any shape,
and therefore carbide residues must not be used blindly; but if analysis
indicates that a particular plot of ground would derive benefit from an
application of lime, acetylene lime is precisely as good as any other
description. Naturally a residue containing unspent carbide, or
contaminated with tarry matter, is essentially valueless (except as
mentioned below); while it must not be forgotten that a solid residue if
it is exposed to air, or a pasty residue if not kept under water, will
lose many of its useful properties, because it will be partially
converted into calcium carbonate or chalk.

Nevertheless, in some respects, the residue from a good acetylene
generator is a more valuable material, agriculturally speaking, than pure
lime. It contains a certain amount of sulphur, &c., and it therefore
somewhat resembles the spent or gas lime of the coal-gas industry. This
sulphur, together, no doubt, with the traces of acetylene clinging to it,
renders the residue a valuable material for killing the worms and vermin
which tend to infest heavily manured and under-cultivated soil. Acetylene
lime has been found efficacious in exterminating the "finger-and-toe" of
carrots, the "peach-curl" of peach-trees, and in preventing cabbages from
being "clubbed." It may be applied to the ground alone, or after
admixture with some soil or stable manure. The residue may also be
employed, either alone or mixed with some agglomerate, in the
construction of garden paths and the like.

If the residues are suitably diluted with water and boiled with (say)
twice their original weight of flowers of sulphur, the product consists
of a mixture of various compounds of calcium and sulphur, or calcium
sulphides--which remain partly in solution and partly in the solid state.
This material, used either as a liquid spray or as a moist dressing, has
been said to prove a useful garden insecticide and weed-killer.

There are also numerous applications of the acetylene light, each of much
value, but involving no new principle which need be noticed. The light is
so actinic, or rich in rays acting upon silver salts, that it is
peculiarly useful to the photographer, either for portraiture or for his
various positive printing operations. Acetylene is very convenient for
optical lantern work on the small scale, or where the oxy-hydrogen or
oxy-coal-gas light cannot be used. Its intensity and small size make its
self-luminous flame preferable on optical grounds to the oil-lamp or the
coal-gas mantle; but the illuminating surface is nevertheless too large
to give the best results behind such condensers as have been carefully
worked to suit a source of light scarcely exceeding the dimensions of a
point. For lantern displays on very large screens, or for the projection
of a powerful beam of light to great distances in one direction (as in
night signalling, &c.), the acetylene blowpipe fed with pure oxygen, or
with air containing more than its normal proportion of oxygen, which is
discussed in Chapter IX., is specially valuable, more particularly if the
ordinary cylinder of lime is replaced by one of magnesia, zirconia, or
other highly refractory oxide.

CHAPTER XIII

PORTABLE ACETYLENE LAMPS AND PLANT

It will be apparent from what has been said in past chapters that the
construction of a satisfactory generator for portable purposes must be a
problem of considerable complexity. A fixed acetylene installation tends
to work the more smoothly, and the gas evolved therefrom to burn the more
pleasantly, the more technically perfect the various subsidiary items of
the plant are; that is to say, the more thoroughly the acetylene is
purified, dried, and delivered at a strictly constant pressure to the
burners and stoves. Moreover, the efficient behaviour of the generator
itself will depend more upon the mechanical excellence and solidity of
its construction than (with one or two exceptions) upon the precise
system to which it belongs. And, lastly, the installation will, broadly
speaking, work the better, the larger the holder is in proportion to the
demands ever made upon it; while that holder will perform the whole duty
of a gasholder more effectually if it belongs to the rising variety than
if it is a displacement holder. All these requirements of a good
acetylene apparatus have to be sacrificed to a greater or less extent in
portable generators; and since the sacrifice becomes more serious as the
generator is made smaller and lighter in weight, it may be said in
general terms that the smaller a portable (or, indeed, other) acetylene
apparatus is, the less complete or permanent satisfaction will it give
its user. Again, small portable apparatus are only needed to develop
intensities of light insignificant in comparison with those which may
easily be won from acetylene on a larger scale; they are therefore fitted
with smaller burners, and those burners are not merely small in terms of
consumption and illuminating power, but not infrequently are very badly
constructed, and are relatively deficient in economy or duty. Thus any
comparisons which may be made on lines similar to those adopted in
Chapter I., or between unit weights, volumes, or monetary equivalents of
calcium carbide, paraffin, candles, and colza oil, become utterly
incorrect if the carbide is only decomposed in a small portable generator
fitted with an inefficient jet; first, because the latent illuminating
power of the acetylene evolved is largely wasted; secondly, because any
gas produced over and above that capable of instant combustion must be
blown off from a vent-pipe; and thirdly, because the carbide itself tends
to be imperfectly decomposed, either through a defect in the construction
of the lamp, or through the brief and interrupted requirements of the
consumer.

In several important respects portable acetylene apparatus may be divided
into two classes from a practical point of view. There is the portable
table or stand lamp intended for use in an occupied room, and there is
the hand or supported lamp intended for the illumination of vehicles or
open-air spaces. Economy apart, no difficulty arises from imperfect
combustion or escape of unburnt gas from an outdoor lamp, but in a room
the presence of unburnt acetylene must always be offensive even if it is
not dangerous; while the combustion products of the impurities--and in a
portable generator acetylene cannot be chemically purified--are highly
objectionable. It is simply a matter of good design to render any form of
portable apparatus safe against explosion (employment of proper carbide
being assumed), for one or more vent-pipes can always be inserted in the
proper places; but from an indoor lamp those vent-pipes cannot be made to
discharge into a place of safety, while, as stated before, a generator in
which the vent-pipes come into action with any frequency is but an
extravagant piece of apparatus for the decomposition of so costly a
material as calcium carbide. Looked at from one aspect the holder of a
fixed apparatus is merely an economical substitute for the wasteful vent-
pipe, because it is a place in which acetylene can be held in reserve
whenever the make exceeds the consumption in speed. It is perhaps
possible to conceive of a large table acetylene lamp fitted with a water-
sealed rising holder; but for vehicular purposes the displacement holder
is practically the only one available, and in small apparatus it becomes
too minute in size to be of much service as a store for the gas produced
by after-generation. Other forms of holder have been suggested by
inventors, such as a collapsible bag of india-rubber or the like; but
rubber is too porous, weak, and perishable a material to be altogether
suitable. If it is possible, by bringing carbide and water into mutual
contact in predetermined quantities, to produce gas at a uniform rate,
and at one which corresponds with the requirements of the burner, in a
small apparatus--and experience has shown it to be possible within
moderately satisfactory limits--it is manifest that the holder is only
needed to take up the gas of after-generation; and in Chapters II. and
III. it was pointed out that after-generation only occurs when water is
brought into contact with an excess of carbide. If, then, the opposite
system of construction is adopted, and carbide is fed into water
mechanically, no after-generation can take place; and provided the make
of gas can be controlled in a small carbide-feed generator as accurately
as is possible in a small water-to-carbide generator, the carbide-feed
principle will exhibit even greater advantages in portable apparatus than
it does in plant of domestic size. Naturally almost every variety of
carbide-feeding gear, especially when small, requires or prefers
granulated (or granulated and "treated") carbide; and granulated carbide
must inevitably be considerably more expensive per unit of light evolved
than the large material, but probably in the application to which the
average portable acetylene apparatus is likely to be put, strict economy
is not of first consequence. In portable acetylene generators of the
carbide-feed type, the supply is generally governed by the movements of a
mushroom-headed or conical valve at the mouth of a conical carbide
vessel; such movements occurring in sympathy with the alterations in
level of the water in the decomposing chamber, which is essentially a
small displacement holder also, or being produced by the contraction of a
flexible chamber through which the gas passes on its way to the burner.
So far as it is safe to speak definitely on a matter of this kind, the
carbide-feed device appears to work satisfactorily in a stationary
(_e.g._, table) lamp; but it is highly questionable whether it could
be applied to a vehicular apparatus exposed to any sensible amount of
vibration. The device is satisfactory on the table of an occupied room so
far, be it understood, as any small portable generators can be: it has no
holder, but since no after-generation occurs, no holder is needed; still
the combustion products contaminate the room with all the sulphur and
phosphorus of the crude acetylene.

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