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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BLEACHING POWDER.--The next idea, first patented by Smith of Aberdeen,
but fully elaborated by Lunge and Cedercreutz, was to employ bleaching-
powder [Footnote: Bleaching-powder is very usually called chloride of
lime; but owing to the confusion which is constantly arising in the minds
of persons imperfectly acquainted with chemistry between chloride of lime
and chloride of calcium--two perfectly distinct bodies--the less
ambiguous expression "bleaching-powder" will be adopted here.] either in
the solid state or as a liquid extract. The essential constituent of
bleaching-powder from the present aspect is calcium hypochlorite, which
readily oxidises sulphuretted hydrogen, and more particularly phosphine,
converting them into sulphuric and phosphoric acids, while the acetylene
is practically unattacked. In simple purifying action the material proved
satisfactory; but since high-grade commercial bleaching-powder contains
some free chlorine, or some is set free from it in the purifier under the
influence of the passing gas, the issuing acetylene was found to contain
chlorine, free or combined; and this, burning eventually to hydrochloric
acid, is hardly less harmful than the original sulphur compounds.
Moreover, a mixture of acetylene, chlorine, and air is liable to catch
fire of itself when exposed to bright sunlight; and therefore the use of
a bleaching-powder purifier, or rather the recharging thereof, was not
unattended by danger in the early days. To overcome these defects, the
very natural process was adopted of diluting the bleaching-powder, such
diluent also serving to increase the porosity of the material. A very
unsuitable substance, however, was selected for the purpose, viz.,
sawdust, which is hygroscopic organic, and combustible. Owing to the
exothermic chemical action between the impurities of the acetylene and
the bleaching-powder, the purifying mass became heated; and thus not only
were the phenomena found in a bad generator repeated in the purifying
vessel, but in presence of air and light (as in emptying the purifier),
the reaction proceeded so rapidly that the heat caused inflammation of
the sawdust and the gas, at least on one occasion an actual fire taking
place which created much alarm and did some little damage. For a time,
naturally, bleaching-powder was regarded as too dangerous a material to
be used for the purification of crude acetylene; but it was soon
discovered that danger could be avoided by employing the substance in a
proper way.

HERATOL, FRANKOLINE, ACAGINE AND PURATYLENE.--Setting aside as unworthy
of attention certain compositions offered as acetylene purifying
materials whose constitution has not been divulged or whose action has
not been certified by respectable authority, there are now three
principal chemical reagents in regular use. Those are chromic acid,
cuprous chloride (sub- or proto-chloride of copper), and bleaching-
powder. Chromic acid is employed in the form of a solution acidified with
acetic or hydrochloric acid, which, in order to obtain the advantages
(_see_ below) attendant upon the use of a solid purifying material,
is absorbed in that highly porous and inert description of silica known
as infusorial earth or "kieselguhr." This substance was first recommended
by Ullmann, and is termed commercially "heratol" As sold it contains
somewhere about 136 grammes of chromic acid per kilo. Cuprous chloride is
used as a solution in strong hydrochloric acid mixed with ferric
chloride, and similarly absorbed in kieselguhr. From the name of its
proposer, this composition is called "frankoline." It will be shown in
Chapter VI. that the use of metallic copper in the construction of
acetylene apparatus is not permissible or judicious, because the gas is
liable to form therewith an explosive compound known as copper acetylide;
it might seem, therefore, that the employment of a copper salt for
purification courts accident. The objection is not sound, because the
acetylide is not likely to be produced except in the presence of ammonia;
and since frankoline is a highly acid product, the ammonia is converted
into its chloride before any copper acetylide can be produced. As a
special acetylene purifier, bleaching-powder exists in at least two chief
modifications. In one, known as "acagine," it is mixed with 15 per cent.
of lead chromate, and sometimes with about the same quantity of barium
sulphate; the function of the latter being simply that of a diluent,
while to the lead chromate is ascribed by its inventor (Wolff) the power
of retaining any chlorine that may be set free from the bleaching-powder
by the reduction of the chromic acid. The utility of the lead chromate in
this direction has always appeared doubtful; and recently Keppeler has
argued that it can have no effect upon the chlorine, inasmuch as in the
spent purifying material the lead chromate may be found in its original
condition unchanged. The second modification of bleaching-powder is
designated "puratylene," and contains calcium chloride and quick or
slaked lime. It is prepared by evaporating to dryness under diminished
pressure solutions of its three ingredients, whereby the finished
material is given a particularly porous nature.

It will be observed that both heratol and frankoline are powerfully acid,
whence it follows they are capable of extracting any ammonia that may
enter the purifier; but for the same reason they are liable to act
corrosively upon any metallic vessel in which they are placed, and they
therefore require to be held in earthenware or enamelled receivers. But
since they are not liquid, the casing of the purifier can be safely
constructed of steel or cast iron. Puratylene also removes ammonia by
virtue of the calcium chloride in it. Acagine would probably pass the
ammonia; but this is no real objection, as the latter can be extracted by
a preliminary washing in water. Heratol changes, somewhat obscurely, in
colour as it becomes spent, its original orange tint, due to the chromic
acid, altering to a dirty green, characteristic of the reduced salts of
chromium oxide. Frankoline has been asserted to be capable of
regeneration or revivification, _i.e._, that when spent it may be
rendered fit for further service by being exposed to the air for a time,
as is done with gas oxide; this, however, may be true to some extent with
the essential constituents of frankoline, but the process is not
available with the commercial solid product. Of all these materials,
heratol is the most complete purifier of acetylene, removing phosphorus
and sulphur most rapidly and thoroughly, and not appreciably diminishing
in speed or efficiency until its chromic acid is practically quite used
up. On the other hand, heratol does act upon pure acetylene to some
extent; so that purifiers containing it should be small in size and
frequently recharged. In one of his experiments Keppeler found that 13
per cent. of the chromic acid in heratol was wasted by reacting with
acetylene. As this waste of chromic acid involves also a corresponding
loss of gas, small purifiers are preferable, because at any moment they
only contain a small quantity of material capable of attacking the
acetylene itself. Frankoline is very efficacious as regards the
phosphorus, but it does not wholly extract the sulphur, leaving,
according to Keppeler, from 0.13 to 0.20 gramme of the latter in every
cubic metre of the gas. It does not attack acetylene itself; and if,
owing to its free hydrochloric acid, it adds any acid vapours to the
purified gas, these vapours may be easily removed by a subsequent passage
through a vessel containing lime or a carbide drier. Both being
essentially bleaching-powder, acagine and puratylene are alike in
removing phosphorus to a satisfactory degree; but they leave some sulphur
behind. Acagine evidently attacks acetylene to a slight extent, as
Keppeler has found 0.2 gramme of chlorine per cubic metre in the issuing
gas.

Although some of these materials attack acetylene slightly, and some
leave sulphur in the purified gas, they may be all considered reasonably
efficient from the practical point of view; for the loss of true
acetylene is too small to be noticeable, and the quantity of sulphur not
extracted too trifling to be harmful or inconvenient. They may be valued,
accordingly, mainly by their price, proper allowance being made for the
quantity of gas purified per unit weight of substance taken. This
quantity of gas must naturally vary with the proportion of phosphorus and
sulphur in the crude acetylene; but on an average the composition of
unpurified gas is what has already been given above, and so the figures
obtained by Keppeler in his investigation of the subject may be accepted.
In the annexed table these are given in two forms: (1) the number of
litres of gas purified by 1 kilogramme of the substance, (2) the number
of cubic feet purified per lb. It should be noted that the volumes of gas
refer to a laboratory degree of purification; in practice they may all be
increased by 10 or possibly 20 per cent.

_________________________________________________
| | | |
| | Litres | Cubic Feet |
| | per Kilogramme. | per Lb. |
|______________|___________________|______________|
| | | |
| Heratol | 5,000 | 80 |
| Frankoline | 9,000 | 144 |
| Puratylene | 10,000 | 160 |
| Acagine | 13,000 | 208 |
|______________|___________________|______________|

Another method of using dry bleaching-powder has been proposed by
Pfeiffer. He suggests incorporating it with a solution of some lead salt,
so that the latter may increase the capacity of the calcium hypochlorite
to remove sulphur. Analytical details as to the efficiency of this
process have not been given. During 1901 and 1902 Bullier and Maquenne
patented a substance made by mixing bleaching-powder with sodium
sulphate, whereby a double decomposition occurs, sodium hypochlorite,
which is equally efficient with calcium hypochlorite as a purifying
material, being produced together with calcium sulphate, which, being
identical with plaster of Paris, sets into a solid mass with the excess
of water present, and is claimed to render the whole more porous. This
process seemed open to objection, because Blagden had shown that a
solution of sodium hypochlorite was not a suitable purifying reagent in
practice, since it was much more liable to add chlorine to the gas than
calcium hypochlorite. The question how a solidified modification of
sodium hypochlorite would behave in this respect has been investigated by
Keppeler, who found that the Bullier and Maquenne material imparted more
chlorine to the gas which had traversed it than other hypochlorite
purifying agents, and that the partly foul material was liable to cause
violent explosions. About the same time Rossel and Landriset pointed out
that purification might be easily effected in all generators of the
carbide-to-water pattern by adding to the water of the generator itself a
quantity of bleaching-powder equivalent to 5 to 20 grammes for every 1
kilogramme of carbide decomposed, claiming that owing to the large amount
of liquid present, which is usually some 4 litres per kilogramme of
carbide (0.4 gallon per lb.), no nitrogen chloride could be produced, and
that owing to the dissolved lime in the generator, chlorine could not be
added to the gas. The process is characterised by extreme simplicity, no
separate purifier being needed, but it has been found that an
introduction of bleaching-powder in the solid condition is liable to
cause an explosive combination of acetylene and chlorine, while the use
of a solution is attended by certain disadvantages. Granjon has proposed
impregnating a suitable variety of wood charcoal with chlorine, with or
without an addition of bleaching-powder; then grinding the product to
powder, and converting it into a solid porous mass by the aid of cement.
The material is claimed to last longer than ordinary hypochlorite
mixtures, and not to add chlorine to the acetylene.

SUBSIDIARY PURIFYING MATERIALS.--Among minor reagents suggested as
purifying substances for acetylene may be mentioned potassium
permanganate, barium peroxide, potassium bichromate, sodium plumbate and
arsenious oxide. According to Benz the first two do not remove the
sulphuretted hydrogen completely, and oxidise the acetylene to some
extent; while potassium bichromate leaves some sulphur and phosphorus
behind in the gas. Sodium plumbate has been suggested by Morel, but it is
a question whether its action on the impurities would not be too violent
and whether it would be free from action on the acetylene itself. The use
of arsenious oxide dissolved in a strong acid, and the solution absorbed
in pumice or kieselguhr has been protected by G. F. Jaubert. The
phosphine is said to combine with the arsenic to form an insoluble
brownish compound. In 1902 Javal patented a mixture of 1 part of
potassium permanganate, 5 of "sulphuric acid," and 1 of water absorbed in
4 parts of infusorial earth. The acid constantly neutralised by the
ammonia of the crude gas is as constantly replaced by fresh acid formed
by the oxidation of the sulphuretted hydrogen; and this free acid, acting
upon the permanganate, liberates manganese peroxide, which is claimed to
destroy the phosphorus and sulphur compounds present in the crude
acetylene.

EPURENE.--A purifying material to which the name of epurene has been
given has been described, by Mauricheau-Beaupre, as consisting of a
mixture of ferric chloride and ferric oxide in the proportion of 2
molecules, or 650 parts, of the former with one molecule, or 160 parts,
of the latter, together with a suitable quantity of infusorial earth. In
the course of preparation, however, 0.1 to 0.2 per cent. of mercuric
chloride is introduced into the material. This mercuric chloride is said
to form an additive compound with the phosphine of the crude acetylene,
which compound is decomposed by the ferric chloride, and the mercuric
chloride recovered. The latter therefore is supposed to act only as a
carrier of the phosphine to the ferric chloride and oxide, by which it is
oxidised according to the equation:

8Fe_2Cl_6 + 4Fe_2O_3 + 3PH_3 = 12Fe_2Cl_4 + 3H_3PO_4.

Thus the ultimate products are phosphoric acid and ferrous chloride,
which on exposure to air is oxidised to ferric chloride and oxide. It is
said that this revivification of the fouled or spent epurene takes place
in from 20 to 48 hours when it is spread in the open in thin layers, or
it may be partially or wholly revivified _in situ_ by adding a small
proportion of air to the crude acetylene as it enters the purifier. The
addition of 1 to 2 per cent. of air, according to Mauricheau-Beaupre,
suffices to double the purifying capacity of one charge of the material,
while a larger proportion would achieve its continuous revivification.
Epurene is said to purify 10,000 to 11,000 litres of crude acetylene per
kilogramme, or, say, 160 to 176 cubic feet per pound, when the acetylene
contains on the average 0.05 per cent, by volume of phosphine.

For employment in all acetylene installations smaller than those which
serve complete villages, a solid purifying material is preferable to a
liquid one. This is partly due to the extreme difficulty of subdividing a
stream of gas so that it shall pass through a single mass of liquid in
small enough bubbles for the impurities to be removed by the time the gas
arrives at the surface. This time cannot be prolonged without increasing
the depth of liquid in the vessel, and the greater the depth of liquid,
the more pressure is consumed in forcing the gas through it. Perfect
purification by means of fluid reagents unattended by too great a
consumption of pressure is only to be effected by a mechanical scrubber
such as is used on coal-gas works, wherein, by the agency of external
power, the gas comes in contact with large numbers of solid surfaces kept
constantly wetted; or by the adoption of a tall tower filled with porous
matter or hollow balls over which a continuous or intermittent stream of
the liquid purifying reagent is made to trickle, and neither of these
devices is exactly suited to the requirements of a domestic acetylene
installation. When a solid material having a proper degree of porosity or
aggregation is selected, the stream of gas passing through it is broken
up most thoroughly, and by employing several separate layers of such
material, every portion of the gas is exposed equally to the action of
the chemical reagent by the time the gas emerges from the vessel. The
amount of pressure so consumed is less than that in a liquid purifier
where much fluid is present; but, on the other hand, the loss of pressure
is absolutely constant at all times in a liquid purifier, provided the
head of liquid is maintained at the same point. A badly chosen solid
purifying agent may exhibit excessive pressure absorption as it becomes
partly spent. A solid purifier, moreover, has the advantage that it may
simultaneously act as a drier for the gas; a liquid purifier, in which
the fluid is mainly water, obviously cannot behave in a similar fashion
For thorough purification it is necessary that the gas shall actually
stream through the solid material; a mere passage over its surface is
neither efficient nor economical of material.

DISPOSITION OF PURIFYING MATERIAL.--Although much has been written, and
some exaggerated claims made, about the maximum, volume of acetylene a
certain variety of purifying material will treat, little has been said
about the method in which such a material should be employed to obtain
the best results. If 1 lb. of a certain substance will purify 200 cubic
feet of normal crude acetylene, that weight is sufficient to treat the
gas evolved from 40 lb. of carbide; but it will only do so provided it is
so disposed in the purifier that the gas does not pass through it at too
high a speed, and that it is capable of complete exhaustion. In the coal-
gas industry it is usually assumed that four layers of purifying
material, each having a superficial area of 1 square foot, are the
minimum necessary for the treatment of 100 cubic feet of gas per hour,
irrespective of the nature of the purifying material and of the impurity
it is intended to extract. If there is any sound basis for this
generalization, it should apply equally to the purification of acetylene,
because there is no particular reason to imagine that the removal of
phosphine by a proper substance should occur at an appreciably different
speed from the removal of carbon dioxide, sulphuretted hydrogen, and
carbon bisulphide by lime, ferric oxide, and sulphided lime respectively,
Using the coal gas figures, then, for every 10 cubic feet of acetylene
generated per hour, a superficial area of (4 x 144 / 10) 57.6 square
inches of purifying material is required. In the course of Keppeler's
research upon different purifying materials it is shown that 400 grammes
of heratol, 360 grammes of frankoline, 250 grammes of acagine, and 230
grammes of puratylene each occupy a space of 500 cubic centimetres when
loosely loaded into a purifying vessel, and from these data, the
following table has been calculated:

__________________________________________________________
| | | | |
| | Weight | Weight | Cubic Inches |
| | per Gallon | per Cubic Foot | Occupied |
| | in Lbs. | in Lbs. | per Lb. |
|_____________|____________|________________|______________|
| | | | |
| Water | 10.0 | 62.321 | 27.73 |
| Heratol | 8.0 | 49.86 | 31.63 |
| Frankoline | 7.2 | 41.87 | 38.21 |
| Acagine | 6.0 | 31.16 | 55.16 |
| Puratylene | 4.6 | 28.67 | 60.28 |
|_____________|____________|________________|______________|

As regards the minimum weight of material required, data have been given
by Pfleger for use with puratylene. He states that 1 Kilogramme of that
substance should be present for every 100 litres of crude acetylene
evolved per hour, 4 kilogrammes being the smallest quantity put into the
purifier. In English units these figures are 1 lb. per 1.5 cubic feet per
hour, with 9 lb. as a minimum, which is competent to treat 1.1 cubic feet
of gas per hour. Thus it appears that for the purification of the gas
coming from any generator evolving up to 14 cubic feet of acetylene per
hour a weight of 9 lb of puratylene must be charged into the purifier,
which will occupy (60.28 / 9) 542 cubic inches of space; and it must be
so spread out as to present a total superficial area of (4 x 144 x 14 /
100) 80.6 square inches to the passing gas. It follows, therefore, that
the material should be piled to a depth of (542 / 80.6) 6.7 inches on a
support having an area of 80.6 square inches; but inasmuch as such a
depth is somewhat large for a small vessel, and as several layers are
better than one, it would be preferable to spread out these 540 cubic
inches of substance on several supports in such a fashion that a total
surface of 80.6 square inches or upwards should be exhibited. These
figures may obviously be manipulated in a variety of ways for the design
of a purifying vessel; but, to give an example, if the ordinary
cylindrical shape be adopted with four circular grids, each having a
clear diameter of 8 inches (_i.e._, an area of 50.3 square inches),
and if the material is loaded to a depth of 3 inches on each, there would
be a total volume of (50.3 x 3 x 4) = 604 cubic inches of puratylene in
the vessel, and it would present a total area of (50.3 x 4) = 201 square
inches to the acetylene. At Keppeler's estimation such an amount of
puratylene should weigh roughly 10 lb., and should suffice for the
purification of the gas obtained from 320 lb. of ordinary carbide; while,
applying the coal-gas rule, the total area of 201 square inches should
render such a vessel equal to the purification of acetylene passing
through it at a speed not exceeding (201 / 5.76) = 35 cubic feet per
hour. Remembering that it is minimum area in square inches of purifying
material that must govern the speed at which acetylene may be passed
through a purifier, irrespective probably of the composition of the
material; while it is the weight of material which governs the ultimate
capacity of the vessel in terms of cubic feet of acetylene or pounds of
carbide capable of purification, these data, coupled with Keppeler's
efficiency table, afford means for calculating the dimensions of the
purifying vessel to be affixed to an installation of any desired number
of burners. There is but little to say about the design of the vessel
from the mechanical aspect. A circular horizontal section is more likely
to make for thorough exhaustion of the material. The grids should be
capable of being lifted out for cleaning. The lid may be made tight
either by a clamp and rubber or leather washer, or by a liquid seal. If
the purifying material is not hygroscopic, water, calcium chloride
solution, or dilute glycerin may be used for sealing purposes; but if the
material, or any part of it, does absorb water, the liquid in the seal
should be some non-aqueous fluid like lubricating oil. Clamped lids are
more suitable for small purifiers, sealed lids for large vessels. Care
must be taken that condensation products cannot collect in the purifying
vessel. If a separate drying material is employed in the same purifier
the space it takes must be considered separately from that needed by the
active chemical reagent. When emptying a foul purifier it should be
recollected that the material may be corrosive, and being saturated with
acetylene is likely to catch fire in presence of a light.

Purifiers charged with heratol are stated, however, to admit of a more
rapid flow of the gas through them than that stated above for puratylene.
The ordinary allowance is 1 lb. of heratol for every cubic foot per hour
of acetylene passing, with a minimum charge of 7 lb. of the material. As
the quantity of material in the purifier is increased, however, the flow
of gas per hour may be proportionately increased, _e.g._, a purifier
charged with 132 lb. of heratol should purify 144 cubic feet of acetylene
per hour.

In the systematic purification of acetylene, the practical question
arises as to how the attendant is to tell when his purifiers approach
exhaustion and need recharging; for if it is undesirable to pass crude
gas into the service, it is equally undesirable to waste so comparatively
expensive a material as a purifying reagent. In Chapter XIV. it will be
shown that there are chemical methods of testing for the presence, or
determining the proportion, of phosphorus and sulphur in acetylene; but
these are not suitable for employment by the ordinary gas-maker. Heil has
stated that the purity of the gas may be judged by an inspection of its
atmospheric flame as given by a Bunsen burner. Pure acetylene gives a
perfectly transparent moderately dark blue flame, which has an inner cone
of a pale yellowish green colour; while the impure gas yields a longer
flame of an opaque orange-red tint with a bluish red inner zone. It
should be noted, however, that particles of lime dust in the gas may
cause the atmospheric flame to be reddish or yellowish (by presence of
calcium or sodium) quite apart from ordinary impurities; and for various
other reasons this appearance of the non-luminous flame is scarcely to be
relied upon. The simplest means of ascertaining definitely whether a
purifier is sufficiently active consists in the use of the test-papers
prepared by E. Merck of Darmstadt according to G. Keppeler's
prescription. These papers, cut to a convenient size, are put up in small
books from which they may be torn one at a time. In order to test whether
gas is sufficiently purified, one of the papers is moistened with
hydrochloric acid of 10 per cent. strength, and the gas issuing from a
pet-cock or burner orifice is allowed to impinge on the moistened part.
The original black or dark grey colour of the paper is changed to white
if the gas contains a notable amount of impurity, but remains unchanged
if the gas is adequately purified. The paper consists of a specially
prepared black porous paper which has been dipped in a solution of
mercuric chloride (corrosive sublimate) and dried. Moistening the paper
with hydrochloric acid provides in a convenient form for application
Berge's solution for the detection of phosphine (_vide_ Chapter
XIV.). The Keppeler test-papers turn white when the gas contains either
ammonia, phosphine, siliciuretted hydrogen, sulphuretted hydrogen or
organic sulphur compounds, but with carbon disulphide the change is slow.
Thus the paper serves as a test for all the impurities likely to occur in
acetylene. The sensitiveness of the test is such that gas containing
about 0.15 milligramme of sulphur, and the same amount of phosphorus, per
litre (= 0.0655 grain per cubic foot) imparts in five minutes a distinct
white mark to the moistened part of the paper, while gas containing 0.05
milligramme of sulphur per litre (= 0.022 grain per cubic foot) gives in
two minutes a dull white mark visible only by careful inspection. If,
therefore, a distinct white mark appears on moistened Keppeler paper when
it is exposed for five minutes to a jet of acetylene, the latter is
inadequately purified. If the gas has passed through a purifier, this
test indicates that the material is not efficient, and that the purifier
needs recharging. The moistening of the Keppeler paper with hydrochloric
acid before use is essential, because if not acidified the paper is
marked by acetylene itself. The books of Keppeler papers are put up in a
case which also contains a bottle of acid for moistening them as required
and are obtainable wholesale of E. Merek, 16 Jewry Street, London, E.C.,
and retail of the usual dealers in chemicals. If Keppeler's test-papers
are not available, the purifier should be recharged as a matter of
routine as soon as a given quantity of carbide--proportioned to the
purifying capacity of the charge of purifying material--has been used
since the last recharging. Thus the purifier may conveniently contain
enough material to purify the gas evolved from two drums of carbide, in
which case it would need recharging when every second drum of carbide is
opened.

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