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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WASHERS.--In designing a washer for the extraction of ammonia and
sulphuretted hydrogen it is necessary to see that the gas is brought into
most intimate contact with the liquid, while yet no more pressure than
can possibly be avoided is lost. Subdivision of the gas stream may be
effected by fitting the mouth of the inlet-pipe with a rose having a
large number of very small holes some appreciable distance apart, or by
bending the pipe to a horizontal position and drilling it on its upper
surface with numbers of small holes. Another method is to force the gas
to travel under a series of partitions extending just below the water-
level, forming the lower edges of those partitions either perfectly
horizontal or with small notches like the teeth of a saw. One volume of
pure water only absorbs about three volumes of sulphuretted hydrogen at
atmospheric temperatures, but takes up some 600 volumes of gaseous
ammonia; and as ammonia always accompanies the sulphuretted hydrogen, the
latter may be said to be absorbed in the washer by a solution of ammonia,
a liquid in which sulphuretted hydrogen is much more soluble. Therefore,
since water only dissolves about an equal volume of acetylene, the liquid
in the washer will continue to extract ammonia and sulphuretted hydrogen
long after it is saturated with the hydrocarbon. For this reason,
_i.e._, to avoid waste of acetylene by dissolution in the clean
water of the washer, the plan is sometimes adopted of introducing water
to the generator through the washer, so that practically the carbide is
always attacked by a liquid saturated with acetylene. Provided the liquid
in the generator does not become seriously heated, there is no objection
to this arrangement; but if the water is heated strongly in the generator
it loses much or all of its solvent properties, and the impurities may be
driven back again into the washer. Clearly if the waste lime of the
generator occurs as a dry or damp powder, the plan mentioned is not to be
recommended; but when the waste lime is a thin cream--water being in
large excess--it may be adopted. If the generator produces lime dust
among the gas, and if the acetylene enters the washer through minute
holes, a mechanical filter to remove the dust must be inserted between
the generator and the washer, or the orifices of the leading pipe will be
choked. Whenever a water-cooled condenser is employed after the
generator, in which the gas does not come in contact with the water, that
liquid may always be used to charge the generator. For compactness and
simplicity of parts the water of the holder seal is occasionally used as
the washing liquid, but unless the liquid of the seal is constantly
renewed it will thus become offensive, especially if the holder is under
cover, and it will also act corrosively upon the metal of the tank and
bell. The water-soluble impurities in acetylene will not be removed
completely by merely standing over the holder seal for a short time, and
it is not good practice to pass unnecessarily impure gas into a holder.
[Footnote: This is not a contradiction of what has been said in Chapter
III. about the relative position of holder and chemical purifiers,
because reference is now being made to ammonia and sulphuretted hydrogen
only.]

HARMFULNESS OF IMPURITIES.--The reasons why the carbide impurities must
be removed from acetylene before it is burned have now to be explained.
From the strictly chemical point of view there are three compounds of
phosphorus, all termed phosphoretted hydrogen or phosphine: a gas, PH_3;
a liquid, P_2H_4; and a solid, P_4H_2. The liquid is spontaneously
inflammable in presence of air; that is to say, it catches fire of itself
without the assistance of spark or flame immediately it comes in contact
with atmospheric oxygen; being very volatile, it is easily carried as
vapour by any permanent gas. The gaseous phosphine is not actually
spontaneously inflammable at temperatures below 100 deg. C.; but it oxidises
so rapidly in air, even when somewhat diluted, that the temperature may
quickly rise to the point of inflammation. In the earliest days of the
acetylene industry, directly it was recognised that phosphine always
accompanies crude acetylene from the generator, it was believed that
unless the proportion were strictly limited by decomposing only a carbide
practically free from phosphides, the crude acetylene might exhibit
spontaneously inflammable properties. Lewes, indeed, has found that a
sample of carbide containing 1 per cent of calcium phosphide gave
(probably by local decomposition--the bulk of the phosphide suffering
attack first) a spontaneously inflammable gas; but when examining
specimens of commercial carbide the highest amount of phosphine he
discovered in the acetylene was 2.3 per cent, and this gas was not
capable of self-inflammation. According to Bullier, however, acetylene
must contain 80 per cent of phosphine to render it spontaneously
inflammable. Berdenich has reported a case of a parcel of carbide which
yielded on the average 5.1 cubic foot of acetylene per lb., producing gas
which contained only 0.398 gramme of phosphorus in the form of phosphine
per cubic metre (or 0.028 per cent. of phosphine) and was spontaneously
inflammable. But on examination the carbide in question was found to be
very irregular in composition, and some lumps produced acetylene
containing a very high proportion of phosphorus and silicon compounds. No
doubt the spontaneous inflammability was due to the exceptional richness
of these lumps in phosphorus. As manufactured at the present day, calcium
carbide ordinarily never contains an amount of phosphide sufficient to
render the gas dangerous on the score of spontaneous inflammability; but
should inferior material ever be put on the markets, this danger might
have to be guarded against by submitting the gas evolved from it to
chemical analysis. Another risk has been suggested as attending the use
of acetylene contaminated with phosphine (and to a minor degree with
sulphuretted hydrogen), viz., that being highly toxic, as they
undoubtedly are, the gas containing them might be extremely dangerous to
breathe if it escaped from the service, or from a portable lamp,
unconsumed. Anticipating what will be said in a later paragraph, the
worst kind of calcium carbide now manufactured will not yield a gas
containing more than 0.1 per cent. by volume of sulphuretted hydrogen and
0.05 per cent. of phosphine. According to Haldane, air containing 0.07
per cent. of sulphuretted hydrogen produces fatal results on man if it is
breathed for some hours, while an amount of 0.2 per cent. is fatal in 1-
1/2 minutes. Similar figures for phosphine cannot be given, because
poisoning therewith is very rare or quite unknown: the cases of "phossy-
jaw" in match factories being caused either by actual contact with yellow
phosphorus or by inhalation of its vapour in the elemental state.
However, assuming phosphine to be twice as toxic as sulphuretted
hydrogen, its effect in crude acetylene of the above-mentioned
composition will be equal to that of the sulphuretted hydrogen, so that
in the present connexion the gas may be said to be equally toxic with a
sample of air containing 0.2 per cent. of sulphuretted hydrogen, which
kills in less than two minutes. But this refers only to crude acetylene
undiluted with air; and being a hydrocarbon--being in fact neither oxygen
nor common air--acetylene is irrespirable of itself though largely devoid
of specific toxic action. Numerous investigations have been made of the
amount of acetylene (apart from its impurities) which can be breathed in
safety; but although these point to a probable recovery after a fairly
long-continued respiration of an atmosphere charged with 30 per cent. of
acetylene, the figure is not trustworthy, because toxicological
experiments upon animals seldom agree with similar tests upon man. If
crude acetylene were diluted with a sufficient proportion of air to
remove its suffocating qualities, the percentage of specifically toxic
ingredients would be reduced to a point where their action might be
neglected; and short of such dilution the acetylene itself would in all
probability determine pathological effects long before its impurities
could set up symptoms of sulphur and phosphorus poisoning.

Ammonia is objectionable in acetylene because it corrodes brass fittings
and pipes, and because it is partially converted (to what extent is
uncertain) into nitrous and nitric acids as it passes through the flame.
Sulphur is objectionable in acetylene because it is converted into
sulphurous and sulphuric anhydrides, or their respective acids, as it
passes through the flame. Phosphorus is objectionable because in similar
circumstances it produces phosphoric anhydride and phosphoric acid. Each
of these acids is harmful in an occupied room because they injure the
decorations, helping to rot book-bindings, [Footnote: It is only fair to
state that the destruction of leather bindings is commonly due to traces
of sulphuric acid remaining in the leather from the production employed
in preparing it, and is but seldom caused directly by the products of
combustion coming from gas or oil.] tarnishing "gold-leaf" ornaments, and
spoiling the colours of dyed fabrics. Each is harmful to the human
system, sulphuric and phosphoric anhydrides (SO_3, and P_4O_10) acting as
specific irritants to the lungs of persons predisposed to affections of
the bronchial organs. Phosphorus, however, has a further harmful action:
sulphuric anhydride is an invisible gas, but phosphoric anhydride is a
solid body, and is produced as an extremely fine, light, white voluminous
dust which causes a haze, more or less opaque, in the apartment.
[Footnote: Lewes suggests that ammonia in the gas burnt may assist in the
production of this haze, owing to the formation of solid ammonium salts
in the state of line dust.] Immediately it comes in contact with
atmospheric moisture phosphoric anhydride is converted into phosphoric
acid, but this also occurs at first as a solid substance. The solidity
and visibility of the phosphoric anhydride and acid are beneficial in
preventing highly impure acetylene being unwittingly burnt in a room;
but, on the other hand, being merely solids in suspension in the air, the
combustion products of phosphorus are not so easily carried away from the
room by the means provided for ventilation as are the products of the
combustion of sulphur. Phosphoric anhydride is also partly deposited in
the solid state at the burner orifices, perhaps actually corroding the
steatite jets, and always assisting in the deposition of carbon from any
polymerised hydrocarbons in the acetylene; thus helping the carbon to
block up or distort those orifices. Whenever the acetylene is to be burnt
on the incandescent system under a mantle of the Welsbach or other type,
phosphorus, and possibly sulphur, become additionally objectionable, and
rigorous extraction is necessary. As is well known, the mantle is
composed of the oxides of certain "rare earths" which owe their practical
value to the fact that they are non-volatile at the temperature of the
gas-flame. When a gas containing phosphorus is burnt beneath such a
mantle, the phosphoric anhydride attacks those oxides, partially
converting them into the respective phosphates, and these bodies are less
refractory. A mantle exposed to the combustion products of crude
acetylene soon becomes brittle and begins to fall to pieces, occasionally
showing a yellowish colour when cold. The actual advantage of burning
acetylene on the incandescent system is not yet thoroughly established--
in this country at all events; but it is clear that the process will not
exhibit any economy (rather the reverse) unless the plant is provided
with most capable chemical purifiers. Phosphorus, sulphur, and ammonia
are not objectionable in crude acetylene because they confer upon the gas
a nauseous odour. From a well-constructed installation no acetylene
escapes unconsumed: the gas remains wholly within the pipes until it is
burnt, and whatever odour it may have fails to reach the human nostrils.
A house properly piped for acetylene will be no more conspicuous by its
odour than a house properly piped for coal-gas. On the contrary, the fact
that the carbide impurities of acetylene, which, in the absolutely pure
state, is a gas of somewhat faint, hardly disagreeable, odour, do confer
upon that gas a persistent and unpleasant smell, is distinctly
advantageous; for, owing to that odour, a leak in the pipes, an unclosed
tap, or a fault in the generating plant is instantly brought to the
consumer's attention. A gas wholly devoid of odour would be extremely
dangerous in a house, and would have to be scented, as is done in the
case of non-carburetted water-gas when it is required for domestic
purposes.

AMOUNTS OF IMPURITIES AND SCOPE OF PURIFICATION.--Partly for the reason
which has just been given, and partly on the ground of expense, a
complete removal of the impurities from crude acetylene is not desirable.
All that need be done is to extract sufficient to deprive the gas of its
injurious effects upon lungs, decorations, and burners. As it stands,
however, such a statement is not sufficiently precise to be useful either
to consumers of acetylene or to manufacturers of plant, and some more or
less arbitrary standard must be set up in order to define the composition
of "commercially pure" acetylene, as well as to gauge the efficiency of
any process of purification. In all probability such limit may be
reasonably taken at 0.1 milligramme of either sulphur or phosphorus
(calculated as elementary bodies) per 1 litre of acetylene, _i.e._,
0.0-1.1 grain per cubic foot; a quantity which happens to correspond
almost exactly with a percentage by weight of 0.01. Owing to the atomic
weights of these substances, and the very small quantities being
considered, the same limit hardly differs from that of 0.01 per cent. by
weight of sulphuretted hydrogen or of phosphine--it being always
recollected that the sulphur and phosphorus do not necessarily exist in
the gas as simple hydrides. Keppeler, however, has suggested the higher
figure of 0.15 milligramme of either sulphur or phosphorus per litre of
acetylene (=0.066 grain per cubic foot) for the maximum amount of these
impurities permissible in purified acetylene. He adopts this standard on
the basis of the results of observations of the amounts of sulphur and
phosphorus present in the gas issuing from a purifier charged with
heratol at the moment when the last layer of the heratol is beginning to
change colour. No limit has been given for the removal of the ammonia,
partly because that impurity can more easily, and without concomitant
disadvantage, be extracted entirely; and partly because it is usually
removed in the washer and not in the true chemical purifier.

According to Lewes, the maximum amount of ammonia found in the acetylene
coming from a dripping generator is 0.95 gramme per litre, while in
carbide-to-water gas it is 0.16 gramme: 417 and 70.2 grains per cubic
foot respectively. Rossel and Landriset have found 4 milligrammes (1.756
grains [Footnote: Milligrammes per litre; grains per cubic foot. It is
convenient to remember that since 1 cubic foot of water weighs 62.321 x
16 - 997.14 avoirdupois ounces, grammes per litre are approximately equal
to oz. per cubic foot; and grammes per cubic metre to oz. per 1000 cubic
feet.]) to be the maximum in water-to-carbide gas, and none to occur in
carbide-to-water acetylene. Rossel and Landriset return the minimum
proportion of sulphur, calculated as H_2S, found in the gaseous state in
acetylene when the carbide has not been completely flooded with water at
1.18 milligrammes per litre, or 0.52 grain per cubic foot; and the
corresponding maxima at 1.9 milligrammes, or 0.84 grain. In carbide-to-
water gas, the similar maxima are 0.23 milligramme or 0.1 grain. As
already stated, the highest proportion of phosphine yet found in
acetylene is 2.3 per cent. (Lewes), which is equal to 32.2 milligrammes
of PH_3 per litre or 14.13 grains per cubic foot (Polis); but this sample
dated from 1897. Eitner and Keppeler record the minimum proportion of
phosphorus, calculated as PH_3, found in crude acetylene, as 0.45
milligramme per litre, and the maximum as 0.89 milligramme per litre; in
English terms these figures are 0.2 and 0.4 grain per cubic foot. On an
average, however, British and Continental carbide of the present day may
be said to give a gas containing 0.61 milligramme of phosphorus
calculated as PH_3 per litre and 0.75 milligramme of sulphur calculated
as H_2S. In other units these figures are equal to 0.27 grain of PH_3 and
0.33 grain of H_2S per 1 cubic foot, or to 0.041 per cent. by volume of
PH_3 and 0.052 per cent. of H_2S. Yields of phosphorus and sulphur much
higher than these will be found in the journals and books, but such
analytical data were usually obtained in the years 1896-99, before the
manufacture of calcium carbide had reached its present degree of
systematic control. A commercial specimen of carbide was seen by one of
the authors as late as 1900 which gave an acetylene containing 1.12
milligramme of elementary sulphur per litre, i.e., 0.096 per cent, by
volume, or 0.102 per cent, by volume of H_2S; but the phosphorus showed
the low figure of 0.36 milligramme per litre (0.031 per cent, of P or
0.034 per cent, of PH_3 by volume).

The British Acetylene Association's regulations relating to carbide of
calcium (_vide_ Chap. XIV.) contain a clause to the effect that
"carbide which, when properly decomposed, yields acetylene containing
from all phosphorus compounds therein more than 0.05 per cent, by volume
of phosphoretted hydrogen, may be refused by the buyer." This limit is
equivalent to 0.74 milligramme of phosphorus calculated as PH_3 per
litre. A latitude of 0.01 per cent, is, however, allowed for the
analysis, so that the ultimate limit on which carbide could be rejected
is: 0.06 volume per cent. of PH_3, or 0.89 milligramme of phosphorus per
litre.

The existence in appreciable quantity of combined silicon as a normal
impurity in acetylene seems still open to doubt. Calcium carbide
frequently contains notable quantities of iron and other silicides; but
although these bodies are decomposed by acids, yielding hydrogen
silicide, or siliciuretted hydrogen, they are not attacked by plain
water. Nevertheless Wolff and Gerard have found hydrogen silicide in
crude acetylene, and Lewes looks upon it as a common impurity in small
amounts. When it occurs, it is probably derived, as Vigouroux has
suggested, from "alloys" of silicon with calcium, magnesium, and
aluminium in the carbide. The metallic constituents of these substances
would naturally be attacked by water, evolving hydrogen; and the
hydrogen, in its nascent state, would probably unite with the liberated
silicon to form hydrogen silicide. Many authorities, including Keppeler,
have virtually denied that silicon compounds exist in crude acetylene,
while the proportion 0.01 per cent. has been given by other writers as
the maximum. Caro, however, has stated that the crude gas almost
invariably contains silicon, sometimes in very small quantities, but
often up to the limit of 0.8 per cent.; the failure of previous
investigators to discover it being due to faulty analytical methods. Caro
has seen one specimen of (bad) carbide which gave a spontaneously
inflammable gas although it contained only traces of phosphine; its
inflammability being caused by 2.1 per cent. of hydrogen silicide.
Practically speaking, all the foregoing remarks made about phosphine
apply equally to hydrogen silicide: it burns to solid silicon oxide
(silica) at the burners, is insoluble in water, and is spontaneously
inflammable when alone or only slightly diluted, but never occurs in good
carbide in sufficient proportion to render the acetylene itself
inflammable. According to Caro the silicon may be present both as
hydrogen silicide and as silicon "compounds." A high temperature in the
generator will favour the production of the latter; an apparatus in which
the gas is washed well in lime-water will remove the bulk of the former.
Fraenkel has found that magnesium silicide is not decomposed by water or
an alkaline solution, but that dilute hydrochloric acid acts upon it and
spontaneously inflammable hydrogen silicide results. If it may be assumed
that the other silicides in commercial calcium carbide also behave in
this manner it is plain that hydrogen silicide cannot occur in crude
acetylene unless the gas is supposed to be hurried out of the generator
before the alkaline water therein has had time to decompose any traces of
the hydrogen silicide which is produced in the favouring conditions of
high temperature sometimes prevailing. Mauricheau-Beaupre has failed to
find silica in the products of combustion of acetylene from carbide of
varying degrees of purity. He found, however, that a mixture of strong
nitric and hydrochloric acids (_aqua regia_), if contaminated with
traces of phosphoric acid, dissolved silica from the glass of laboratory
vessels. Consequently, since phosphoric acid results from the phosphine
in crude acetylene when the gas is passed through aqua regia, silica may
be found on subsequently evaporating the latter. But this, silica, he
found, was derived from the glass and not through the oxidation of
silicon compounds in the acetylene. It is possible that some of the
earlier observers of the occurrence of silicon compounds in crude
acetylene may have been misled by the solution of silica from the glass
vessels used in their investigations. The improbability of recognisable
quantities of silicon compounds occurring in acetylene in any ordinary
conditions of generation is demonstrated by a recent study by Fraenkel of
the composition of the deposit produced on reflectors exposed to the
products of combustion of a sample of acetylene which afforded a haze
when burnt. The deposit contained 51.07 per cent. of phosphoric acid, but
no silica. The gas itself contained from 0.0672 to 0.0837 per cent. by
volume of phosphine.

PURIFYING MATERIALS.--When acetylene first began to be used as a domestic
illuminant, most generator builders denied that there was any need for
the removal of these carbide impurities from the gas, some going so far
as to assert that their apparatus yielded so much purer an acetylene than
other plant, where purification might be desirable, that an addition of a
special purifier was wholly unnecessary. Later on the more responsible
members of the trade took another view, but they attacked the problem of
purification in a perfectly empirical way, either employing some purely
mechanical scrubber filled with some moist or dry porous medium, or
perhaps with coke or the like wetted with dilute acid, or they simply
borrowed the processes adopted in the purification of coal-gas. At first
sight it might appear that the more simple methods of treating coal-gas
should be suitable for acetylene; since the former contains two of the
impurities--sulphuretted hydrogen and ammonia--characteristic of crude
acetylene. After removing the ammonia by washing with water, therefore,
it was proposed to extract the sulphur by passing the acetylene through
that variety of ferric hydroxide (hydrated oxide of iron) which is so
serviceable in the case of coal-gas. The idea, however, was quite
unsound: first, because it altogether ignores the phosphorus, which is
the most objectionable impurity in acetylene, but is not present in coal-
gas; secondly, because ferric hydroxide is used on gasworks to extract in
a marketable form the sulphur which occurs as sulphuretted hydrogen, and
true sulphuretted hydrogen need not exist in well-generated and well-
washed acetylene to any appreciable extent; thirdly, because ferric
hydroxide is not employed by gasmakers to remove sulphur compounds (this
is done with lime), being quite incapable of extracting them, or the
analogous sulphur compounds of crude acetylene.

About the same time three other processes based on somewhat better
chemical knowledge were put forward. Pictet proposed leading the gas
through a strong solution of calcium chloride and then through strong
sulphuric acid, both maintained at a temperature of -20 deg. to -40 deg. C.,
finally washing the gas in a solution of some lead salt. Proof that such
treatment would remove phosphorus to a sufficient degree is not
altogether satisfactory; but apart from this the necessity of maintaining
such low temperatures, far below that of the coldest winter's night,
renders the idea wholly inadmissible for all domestic installations.
Willgerodt suggested removing sulphuretted hydrogen by means of potassium
hydroxide (caustic potash), then absorbing the phosphine in bromine
water. For many reasons this process is only practicable in the
laboratory. Berge and Reychler proposed extracting both sulphuretted
hydrogen and phosphine in an acid solution of mercuric chloride
(corrosive sublimate). The poisonousness of this latter salt, apart from
all other objections, rules such a method out.

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