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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RISING GASHOLDERS.--Whichever description of holder is employed in an
acetylene apparatus, the gas is always stored over, or in contact with, a
liquid that is essentially water. This introduces three subjects for
consideration: the heavy weight of a large body of liquid, the loss of
gas by dissolution in that liquid, and the protection of that liquid from
frost in the winter. The tanks of rising holders are constructed in two
different ways. In one the tank is a plain cylindrical vessel somewhat
larger in diameter than the bell which floats in it; and since there must
be nearly enough water in the tank to fill the interior of the bell when
the latter assumes its lowest position, the quantity of water is
considerable, its capacity for dissolving acetylene is large, and the
amount of any substance that may have to be added to it to lower its
freezing-point becomes so great as to be scarcely economical. All these
defects, including that of the necessity for very substantial foundations
under the holder to support its enormous weight, may be overcome by
adopting the second method of construction. It is clear that the water in
the centre of the tank is of no use,--all that is needed being a narrow
trough for the bell to work in. Large rising holders are therefore
advantageously built with a tank formed in the shape of an annulus, the
effective breadth of which is not more than 2 or 3 inches, the centre
portion being roofed over so as to prevent escape of gas. The same
principle may be retained with modified details by fitting inside a plain
cylindrical tank a "dummy" or smaller cylinder, closed by a flat or
curved top and fastened water- and air-tight to the bottom of the main
vessel. The construction of annular tanks or the insertion of a "dummy"
may be attended with difficulty if the tank is wholly or partly sunk
below the ground level, owing to the lifting force of water in the
surrounding soil. Where a steel tank is sunk, or a masonry tank is
constructed, regard must be paid, both in the design of the tank and in
the manner of construction, to the level of the underground water in the
neighbourhood, as in certain cases special precautions will be needed to
avoid trouble from the pressure of the water on the outside of the tank
until it is balanced by the pressure of the water with which the tank is
filled. So far as mere dissolution of gas is concerned, the loss may be
reduced by having a circular disc of wood, &c., a little smaller in
diameter than the boll, floating on the water of a plain tank.

EFFECT OF STORAGE IN GASHOLDER ON ACETYLENE.--It is perfectly true, as
has been stated elsewhere, that the gas coming from an acetylene
generator loses some of its illuminating power if it is stored over water
for any great length of time; such loss being given by Nichols as 94 per
cent, in five months, and having been found by one of the authors as 0.63
per cent. per day--figures which stand in fair agreement with one
another. This wastage is not due to any decomposition of the acetylene in
contact with water, but depends on the various solubilities of the
different gases which compose the product obtained from commercial
calcium carbide. Inasmuch as an acetylene evolved in the best generator
contains some foreign ingredients, and inasmuch as an inferior product
contains more (_cf._ Chapter V.), the contents of a holder are never
pure; but as those contents are principally made up of acetylene itself,
that gas stands at a higher partial pressure in the holder than the
impurities. Since acetylene is more soluble in water than any of its
diluents or impurities, sulphuretted hydrogen and ammonia excepted, and
since the solubility of all gases increases as the pressure at which they
are stored rises, the true acetylene in an acetylene holder dissolves in
the water more rapidly and comparatively more copiously than the
impurities; and thus the acetylene tends to disappear and the impurities
to become concentrated within the bell. Simultaneously at the outer part
of the seal, air is dissolved in the water; and by processes of diffusion
the air so dissolved passes through the liquid from the outside to the
inside, where it escapes into the bell, while the dissolved acetylene
similarly passes from the inside to the outside of the seal, and there
mingles with the atmosphere. Thus, the longer a certain volume of
acetylene is stored over water, the more does it become contaminated with
the constituents of the atmosphere and with the impurities originally
present in it; while as the acetylene is much more soluble than its
impurities, more gas escapes from, than enters, the holder by diffusion,
and so the bulk of stored gas gradually diminishes. However, the figures
previously given show that this action is too slow to be noticeable in
practice, for the gas is never stored for more than a few days at a time.
The action cannot be accepted as a valid argument against the employment
of a holder in acetylene plant. Such deterioration and wastage of gas may
be reduced to some extent by the use of a film of some cheap and
indifferent oil floating on the water inside an acetylene holder; the
economy being caused by the lower solubility of acetylene in oils than in
aqueous liquids not saturated with some saline material. Probably almost
any oil would answer equally well, provided it was not volatile at the
temperature of the holder, and that it did not dry or gum on standing,
_e.g._, olive oil or its substitutes; but mineral lubricating oil is
not so satisfactory. It is, however, not necessary to adopt this method
in practice, because the solvent power of the liquid in the seal can be
reduced by adding to it a saline body which simultaneously lowers its
freezing-point and makes the apparatus more trustworthy in winter.

FREEZING OF GASHOLDER SEAL.--The danger attendant upon the congelation of
the seal in an acetylene holder is very real, not so much because of the
fear that the apparatus may be burst, which is hardly to be expected, as
because the bell will be firmly fixed in a certain position by the ice,
and the whole establishment lighted by the gas will be left in darkness.
In these circumstances, hurried and perhaps injudicious attempts may be
made to thaw the seal by putting red-hot bars into it or by lighting
fires under it, or the generator-house may be thoughtlessly entered with
a naked light at a time when the apparatus is possibly in disorder
through the loss of storage-room for the gas it is evolving. Should a
seal ever freeze, it must be thawed only by the application of boiling
water; and the plant-house must be entered, if daylight has passed, in
perfect darkness or with the assistance of an outside lamp whining
through a closed window. [Footnote: By "closed window" is to be
understood one incapable of being opened, fitted with one or two
thicknesses of stout glass well puttied in, and placed in a wall of the
house as far as possible from the door.] There are two ways of preventing
the seal from freezing. In all large installations the generator-house
will be fitted with a warm-water heating apparatus to protect the portion
of the plant where the carbide is decomposed, and if the holder is also
inside the same building it will naturally be safe. If it is outside, one
of the flow-pipes from the warming apparatus should be led into and round
the lowest part of the seal, care being taken to watch for, or to provide
automatic arrangements for making good, loss of water by evaporation. If
the holder is at a distance from the generator-house, or if for any other
reason it cannot easily be brought into the warming circuit, the seal can
be protected in another way; for unlike the water in the generator, the
water in the holder-seal will perform its functions equally well however
much it be reduced in temperature, always providing it is maintained in
the liquid condition. There are numerous substances which dissolve in, or
mix with, water, and yield solutions or liquids that do not solidify
until their temperature falls far below that of the natural freezing-
point. Assuming that those substances in solution do not attack the
acetylene, nor the metal of which the holder is built, and are not too
expensive, choice may be made between them at will. Strictly speaking the
cost of using them is small, because unless the tank is leaky they last
indefinitely, not evaporating with the water as it is vaporised into the
gas or into the air. The water-seal of a holder standing within the
generator-house may eventually become so offensive to the nostrils that
the liquid has to be renewed; but when this happens it is due to the
accumulation in the water of the water-soluble impurities of the crude
acetylene. If, as should be done, the gas is passed through a washer or
condenser containing much water before it enters the holder the
sulphuretted hydrogen and ammonia will be extracted, and the seal will
not acquire an obnoxious odour for a very long time.

Four principal substances have been proposed for lowering the freezing-
point of the water in an acetylene-holder seal; common salt (sodium
chloride), calcium chloride (not chloride of lime), alcohol (methylated
spirit), and glycerin. A 10 per cent. solution of common salt has a
specific gravity of 1.0734, and does not solidify above -6 deg. C. or 21.2 deg.
F.; a 15 per cent. solution has a density of 1.111, and freezes at -10 deg.
C. or 14 deg. F. Common salt, however, is not to be recommended, as its
solutions always corrode iron and steel vessels more or less quickly.
Alcohol, in its English denatured form of methylated spirit, is still
somewhat expensive to use, but it has the advantage of not increasing the
viscosity of the water; so that a frost-proof mixture of alcohol and
water will flow as readily through minute tubes choked with needle-
valves, or through felt and the like, or along wicks, as will plain
water. For this reason, and for the practically identical one that it is
quite free from dirt or insoluble matter, diluted spirit is specially
suitable for the protection of the water in cyclists' acetylene lamps,
[Footnote: As will appear in Chapter XIII., there is usually no holder in
a vehicular acetylene lamp, all the water being employed eventually for
the purpose of decomposing the carbide. This does not affect the present
question. Dilute alcohol does not attack calcium carbide so energetically
as pure water, because it stands midway between pure water and pure
alcohol, which is inert. The attack, however, of the carbide is as
complete as that of pure water, and the slower speed thereof is a
manifest advantage in any holderless apparatus.] where strict economy is
less important than smooth working. For domestic and larger installations
it is not indicated. As between calcium chloride and glycerin there is
little to choose; the former will be somewhat cheaper, but the latter
will not be prohibitively expensive if the high-grade pure glycerins of
the pharmacist are avoided. The following tables show the amount of each
substance which must be dissolved in water to obtain a liquid of definite
solidifying point. The data relating to alcohol were obtained by Pictet,
and those for calcium chloride by Pickering. The latter are materially
different from figures given by other investigators, and perhaps it would
be safer to make due allowance for this difference. In Germany the
Acetylene Association advocates a 17 per cent. solution of calcium
chloride, to which Frank ascribes a specific gravity of 1.134, and a
freezing-point of -8 deg. C. or 17.6 deg. F.

_Freezing-Points of Dilute Alcohol._
_________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Alcohol. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 4.8 | 0.9916 | -2.0 | +28.4 |
| 11.3 | 0.9824 | 5.0 | 23.0 |
| 16.4 | 0.9761 | 7.5 | 18.5 |
| 18.8 | 0.9732 | 9.4 | 15.1 |
| 20.3 | 0.9712 | 10.6 | 12.9 |
| 22.1 | 0.9689 | 12.2 | 10.0 |
| 24.2 | 0.9662 | 14.0 | 6.8 |
| 26.7 | 0.9627 | 16.0 | 3.2 |
| 29.9 | 0.9578 | 18.9 | -2.0 |
|_______________|___________________|__________|__________|

_Freezing-Points of Dilute Glycerin._
_________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Glycerin. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 10 | 1.024 | -1.0 | +30.2 |
| 20 | 1.051 | 2.5 | 27.5 |
| 30 | 1.075 | 6.0 | 21.2 |
| 40 | 1.105 | 17.5 | 0.5 |
| 50 | 1.127 | 31.3 | -24.3 |
|_______________|___________________|__________|__________|

_Freezing-Points of Calcium Chloride Solutions._
_________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| CaCl_2. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 6 | 1.05 | -3.0 | +26.6 |
| 8 | 1.067 | 4.3 | 24.3 |
| 10 | 1.985 | 5.9 | 21.4 |
| 12 | 1.103 | 7.7 | 18.1 |
| 14 | 1.121 | 9.8 | 14.4 |
| 16 | 1.140 | 12.2 | 10.0 |
| 18 | 1.159 | 15.2 | 4.6 |
| 20 | 1.170 | 18.6 | -1.5 |
|_______________|___________________|__________|__________|

Calcium chloride will probably be procured in the solid state, but it can
be purchased as a concentrated solution, being sold under the name of
"calcidum" [Footnote: This proprietary German article is a liquid which
begins to solidify at -42 deg. C. (-43.6 deg. F.), and is completely solid
at -56 deg. C. (-69) deg. F.). Diluted with one-third its volume of water,
it freezes between -20 deg. and -28 deg. C. (-4 deg. and-l8.4 deg. F.). The
makers recommend that it should be mixed with an equal volume of water.
Another material known as "Gefrierschutzfluessigkeit" and made by the
Floersheim chemical works, freezes at -35 deg. C. (-3 deg. F.). Diluted
with one-quarter its volume of water, it solidifies at -18 deg. C.
(-0.4 deg. F.); with equal parts of water it freezes at -12 deg. C.
(10.4 deg. F.). A third product, called "calcidum oxychlorid," has been
found by Caro and Saulmann to be an impure 35 per cent. solution of calcium
chloride. Not one of these is suitable for addition to the water used in
the generating chamber of an acetylene apparatus, the reasons for this
having already been mentioned.] for the protection of gasholder seals.
Glycerin itself resembles a strong solution of calcium chloride in being a
viscid, oily-looking liquid; and both are so much heavier than water that
they will not mix with further quantities unless they are thoroughly
agitated therewith. Either may be poured through water, or have water
floated upon it, without any appreciable admixture taking place; and
therefore in first adding them to the seal great care must be taken that
they are uniformly distributed throughout the liquid. If the whole contents
of the seal cannot conveniently be run into an open vessel in which the
mixing can be performed, the sealing water must be drawn off a little at a
time and a corresponding quantity of the protective reagent added to it.
Care must be taken also that motives of economy do not lead to excessive
dilution of the reagent; the seal must be competent to remain liquid under
the prolonged influence of the most severe frost ever known to occur in the
neighbourhood where the plant is situated. If the holder is placed out of
doors in an exposed spot where heavy rains may fall on the top of the
bell, or where snow may collect there and melt, the water is apt to run
down into the seal, diluting the upper layers until they lose the frost-
resisting power they originally had. This danger may be prevented by
erecting a sloping roof over the bell crown, or by stirring up the seal
and adding more preservative whenever it has been diluted with rain
water. Quite small holders would probably always be placed inside the
generator-house, where their seals may be protected by the same means as
are applied to the generator itself. It need hardly be said that all
remarks about the dangers incidental to the freezing of holder seals and
the methods for obviating them refer equally to every item in the
acetylene plant which contains water or is fitted with a water-sealed
cover; only the water which is actually used for decomposing the calcium
carbide cannot be protected from frost by the addition of calcium
chloride or glycerin--that water must be kept from falling to its natural
freezing-point. From Mauricheau-Beaupre's experiments, referred to on
page 106, it would appear that a further reason for avoiding an addition
of calcium chloride to the water used for decomposing carbide should lie
in the danger of causing a troublesome production of froth within the
generator.

It will be convenient to digress here for the purpose of considering how
the generators of an acetylene apparatus themselves should be protected
from frost; but it may be said at the outset that it is impossible to lay
down any fixed rules applicable to all cases, since local conditions,
such as climate, available resources, dimensions, and exposed or
protected position of the plant-house vary so largely in different
situations. In all important installations every item of the plant,
except the holder, will be collected in one or two rooms of a single
building constructed of brick or other incombustible material. Assuming
that long-continued frost reigns at times in the neighbourhood, the whole
of such a building, with the exception of one apartment used as a carbide
store only, is judiciously fitted with a heating arrangement like those
employed in conservatories or hothouses; a system of pipes in which warm
water is kept circulating being run round the walls of each chamber near
the floor. The boiler, heated with coke, paraffin, or even acetylene,
must naturally be placed in a separate room of the apparatus-house having
no direct (indoor) communication with the rooms containing the
generators, purifiers, &c. Instead of coils of pipe, "radiators" of the
usual commercial patterns may be adopted; but the immediate source of
heat should be steam, or preferably hot water, and not hot air or
combustion products from the stove. In exposed situations, where the
holder is out of doors, one branch of the flow-pipe should enter and
travel round the seal as previously suggested. Most large country
residences are already provided with suitable heating apparatus for
warming the greenhouses, and part of the heat may be capable of diversion
into the acetylene generator-shed if the latter is erected in a
convenient spot. In fact, if any existing hot-water warming appliances
are already at hand, and if they are powerful enough to do a little more
work, it may be well to put the generator-building in such a position
that it can be efficiently supplied with artificial warmth from those
boilers; for any extra length of main necessary to lead the gas into the
residence from a distant generator will cost less on the revenue account
than the fuel required to feed a special heating arrangement. In smaller
installations, especially such as are to be found in mild climates, it
may be possible to render the apparatus-house sufficiently frost-proof
without artificial heat by building it partly underground, fitting it
with a double skylight in place of a window for the entrance of daylight,
and banking up its walls all round with thick layers of earth. The house
must have a door, however, which must open outwards and easily, so that
no obstacle may prevent a hurried exit in emergencies. Such a door can
hardly be made very thick or double without rendering it heavy and
difficult to open; and the single door will be scarcely capable of
protecting the interior if the frost is severe and prolonged.
Ventilators, too, must be provided to allow of the escape of any gas that
may accidentally issue from the plant during recharging, &c.; and some
aperture in the roof will be required for the passage of the vent pipe or
pipes, which, in certain types of apparatus, move upwards and downwards
with the bell of the holder. These openings manifestly afford facilities
for the entry of cold air, so that although this method of protecting
generator-houses has proved efficient in many places, it can only be
considered inferior to the plan of installing a proper heating
arrangement. Occasionally, where local regulations do not forbid, the
entire generator-house may be built as a "lean-to" against some brick
wall which happens to be kept constantly warm, say by having a furnace or
a large kitchen stove on its other side.

In less complicated installations, where there are only two distinct
items in the plant to be protected from frost--generator and holder--or
where generator and holder are combined into one piece of apparatus,
other methods of warming become possible. As the reaction between calcium
carbide and water evolves much heat, the most obvious way of preventing
the plant from freezing is to economise that heat, _i.e._, to retain
as much of it as is necessary within the apparatus. Such a process,
clearly, is only available if the plant is suitable in external form, is
practically self-contained, and comprises no isolated vessels containing
an aqueous liquid. It is indicated, therefore, rather for carbide-to-
water generators, or for water-to-carbide apparatus in which the carbide
chambers are situated inside the main water reservoir--any apparatus, in
fact, where much water is present and where it is all together in one
receptacle. Moreover, the method of heat economy is suited for
application to automatic generators rather than to those belonging to the
opposite system, because automatic apparatus will be generating gas, and
consequently evolving heat, every evening till late at night--just at the
time when frost begins to be severe. A non-automatic generator will
usually be at work only in the mornings, and its store of heat will
accordingly be much more difficult to retain till nightfall. With the
object of storing up the heat evolved in the generator, it must be
covered with some material possessed of the lowest heat-conducting power
possible; and the proper positions for that material in order of
decreasing importance are the top, sides, and bottom of the plant. The
generator may either be covered with a thick layer of straw, carpet,
flannel, or the like, as is done in the protection of exposed water-
pipes; or it may be provided with a jacket filled with some liquid. In
view of the advisability of not having any organic or combustible
material near the generator, the solid substances just mentioned may
preferably be replaced by one of those partially inorganic compositions
sold for "lagging" steam-pipes and engine-cylinders, such as "Fossil
meal." Indeed, the exact nature of the lagging matters comparatively
little, because the active substance in retaining the heat in the
acetylene generator or the steam-pipe is the air entangled in the pores
of the lagging; and therefore the value of any particular material
depends mainly on its exhibiting a high degree of porosity. The idea of
fitting a water jacket round an acetylene generator is not altogether
good, but it may be greatly improved upon by putting into the jacket a
strong solution of some cheap saline body which has the property of
separating from its aqueous solution in the form of crystals containing
water of crystallisation, and of evolving much heat in so separating.
This method of storing much heat in a small space where a fire cannot be
lighted is in common use on some railways, where passengers' foot-warmers
are filled with a strong solution of sodium acetate. When sodium acetate
is dissolved in water it manifestly exists in the liquid state, and it is
presumably present in its anhydrous condition (i.e., not combined with
water of crystallisation). The common crystals are solid, and contain 3
molecules of water of crystallisation--also clearly in the solid state.
Now, the reaction

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