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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NON-AUTOMATIC CARBIDE-TO-WATER GENERATORS.--There is little to be said in
the present place about the principles underlying the construction of
non-automatic generators. Such apparatus may either be of the carbide-to-
water or the water-to-carbide type. In the former, lumps of carbide are
dropped by hand down a vertical or sloping pipe or shoot, which opens at
its lower end below the water-level of the generating chamber, and which
is fitted below its mouth with a deflector to prevent the carbide from
lodging immediately underneath that mouth. The carbide falls through the
water which stands in the shoot itself almost instantaneously, but during
its momentary descent a small quantity of gas is evolved, which produces
an unpleasant odour unless a ventilating hood is fixed above the upper
end of the tube. As the ratio of cubical contents to superficial area of
a lump is greater as the lump itself is larger, and as only the outer
surface of the lump can be attacked by the water in the shoot during its
descent, carbide for a hand-fed carbide-to-water generator should be in
fairly large masses--granulated material being wholly unsuitable--and
this quite apart from the fact that large carbide is superior to small in
gas-making capacity, inasmuch as it has not suffered the inevitable
slight deterioration while being crushed and graded to size. If carbide
is dropped too rapidly into such a generator which is not provided with a
false bottom or grid for the lumps to rest upon, the solid is apt to
descend among a mass of thick lime sludge produced at a former operation,
which lies at the bottom of the decomposing chamber; and here it may be
protected from the cooling action of fresh water to such an extent that
its surface is baked or coated with a hard layer of lime, while
overheating to a degree far exceeding the boiling-point of water may
occur locally. When, however, it falls upon a grid placed some distance
above the bottom of the water vessel, the various convection currents set
up as parts of the liquid become warm, and the mechanical agitations
produced by the upward current of gas rinse the spent lime from the
carbide, and entirely prevent overheating, unless the lumps are
excessively large in size. If the carbide charged into a hand-fed
generator is in very large lumps there is always a possibility that
overheating may occur in the centre of the masses, due to the baking of
the exterior, even if the generator is fitted with a reaction grid.
Manifestly, when carbide in lumps of reasonable size is dropped into
excess of water which is not merely a thick viscid cream of lime, the
temperature cannot possibly exceed the boiling-point--_i.e._, 100 deg.
C.--provided always the natural convection currents of the water are
properly made use of.

The defect which is, or rather which may be, characteristic of a hand-fed
carbide-to-water generator is a deficiency of gas yield due to
solubility. At atmospheric temperatures and pressure 10 volumes of water
dissolve 11 volumes of acetylene, and were the whole of the water in a
large generator run to waste often, a sensible loss of gas would ensue.
If the carbide falls nearly to the bottom of the water column, the rising
gas is forced to bubble through practically the whole of the liquid, so
that every opportunity is given it to dissolve in the manner indicated
till the liquid is completely saturated. The loss, however, is not nearly
so serious as is sometimes alleged, because (1) the water becomes heated
and so loses much of its solvent power; and (2) the generator is worked
intermittently, with sufficiently long intervals to allow the spent lime
to settle into a thick cream, and only that thick cream is run off, which
represents but a small proportion of the total water present. Moreover, a
hand-fed carbide-to-water generator will work satisfactorily with only
half a gallon [Footnote: The United States National Board of Fire
Underwriters stipulates for the presence of 1 (American) gallon of water
for every 1 lb. of carbide before such an apparatus is "permitted." This
quantity of liquid might retain nearly 4 per cent. of the total acetylene
evolved. Even this is an exaggeration; for neither her, nor in the
corresponding figure given in the text, is any allowance made for the
diminution in solvent power of the water as it becomes heated by the
reaction.] of liquid present for every 1 lb. of carbide decomposed, and
were all this water run off and a fresh quantity admitted before each
fresh introduction of carbide, the loss of acetylene by dissolution could
not exceed 2 per cent. of the total make, assuming the carbide to be
capable of yielding 5 cubic feet of gas per lb. Admitting, however, that
some loss of gas does occur in this manner, the defect is partly, if not
wholly, neutralised by the concomitant advantages of the system: (1)
granted that the generator is efficiently constructed, decomposition of
the carbide is absolutely complete, so that no loss of gas occurs in this
fashion; (2) the gas is evolved at a low temperature, so that it is
unaccompanied, by products of polymerisation, which may block the leading
pipes and must reduce the illuminating power; (3) the acetylene is not
mixed with air (as always happens at the first charging of a water-to-
carbide apparatus), which also lowers the illuminating power; and (4) the
gas is freed from two of its three chief impurities, viz., ammonia and
sulphuretted hydrogen, in the generating chamber itself. To prevent the
loss of acetylene by dissolution, carbide-to-water generators are
occasionally fitted with a reaction grid placed only just below the
water-level, so that the acetylene has no more than 1 inch or so of
liquid to bubble through. The principle is wrong, because hot water being
lighter than cold, the upper layers may be raised to the boiling-point,
and even converted into steam, while the bulk of the liquid still remains
cold; and if the water actually surrounding the carbide is changed into
vapour, nearly all control over the temperature attending the reaction is
lost.

The hand-fed carbide-to-water generator is very simple and, as already
indicated, has proved itself perhaps the best type of all for the
construction of very large installations; but the very simplicity of the
generator has caused it more than once to be built in a manner that has
not given entire satisfaction. As shown at L in Fig. 6, p. 84, the
generator essentially consists of a closed cylindrical vessel
communicating at its top with a separate rising holder. At one side as
drawn, or disposed concentrically if so preferred, is an open-mouthed
pipe or shoot (American "shute") having its lower open extremity below
the water-level. Into this shoot are dropped by hand or shovel lumps of
carbide, which fall into the water and there suffer decomposition. As the
bottom of the shoot is covered with water, which, owing to the small
effective gas pressure in the generator given by the holder, stands a few
inches higher in the shoot than in the generator, gas cannot escape from
the shoot; because before it could do so the water in the generator would
have to fall below the level of the point _a_, being either driven
out through the shoot or otherwise. Since the point _b_ of the shoot
extends further into the generator than _a_, the carbide drops
centrally, and as the bubbles of gas rise vertically, they have no
opportunity of ascending into the shoot. In practice, the generator is
fitted with a conical bottom for the collection of the lime sludge and
with a cock or other aperture at the apex of the cone for the removal of
the waste product. As it is not desirable that the carbide should be
allowed to fall directly from the shoot into the thicker portion of the
sludge within the conical part of the generator, one or more grids is
usually placed in the apparatus as shown by the dotted lines in the
sketch. It does not seem that there is any particular reason for the
employment of more than one grid, provided the size of the carbide
decomposed is suited to the generator, and provided the mesh of the grid
is suited to the size of the carbide. A great improvement, however, is
made if the grid is carried on a horizontal spindle in such a way that it
can be rocked periodically in order to assist in freeing the lumps of
carbide from the adhering particles of lime. As an alternative to the
movable grid, or even as an adjunct thereto, an agitator scraping the
conical sides of the generator may be fitted which also assists in
ensuring a reasonably complete absence of undecomposed carbide from the
sludge drawn off at intervals. A further point deserves attention. If
constructed in the ideal manner shown in Fig. 6 removal of some of the
sludge in the generator would cause the level of the liquid to descend
and, by carelessness, the level might fall below the point _a_ at
the base of the shoot. In these circumstances, if gas were unable to
return from the holder, a pressure below that of the atmosphere would be
established in the gas space of the generator and air would be drawn in
through the shoot. This air might well prove a source of danger when
generation was started again. Any one of three plans may be adopted to
prevent the introduction of air. A free path may be left on the gas-main
passing from the generator to the holder so that gas may be free to
return and so to maintain the usual positive pressure in the decomposing
vessel; the sludge may be withdrawn into some vessel so small in capacity
that the shoot cannot accidentally become unsealed; or the waterspace of
the generator may be connected with a water-tank containing a ball-valve
attached to a constant service of water be that liquid runs in as quickly
as sludge is removed, and the level remains always at the same height.
The first plan is only a palliative and has two defects. In the first
place, the omission of any non-return valve between, the generator and
the next item in the train of apparatus is objectionable of itself; in
the second place, should a very careless attendant withdraw too much
liquid, the shoot might become unsealed and the whole contents of the
holder be passed into the air of the building containing the apparatus
through the open mouth of the shoot. The second plan is perfectly sound,
but has the practical defect of increasing the labour of cleaning the
generator. The third plan is obviously the best. It can indeed be adopted
where no real constant service of water is at hand by connecting the
generator to a water reservoir of relatively large size and by making the
latter of comparatively large transverse area, in proportion to its
depth; so that the escape of even a largo volume of water from the
reservoir may not involve a large reduction in the level at which it
stands there.

The dust that always clings to lumps of carbide naturally decomposes with
extreme rapidity when the material is thrown into the shoot of a carbide-
to-water generator, and the sudden evolution of gas so produced has on
more than one occasion seriously alarmed the attendant on the plant.
Moreover, to a trifling extent the actual superficial layers of the
carbide suffer attack before the lumps reach the true interior of the
generator, and a small loss of gas thereby occurs through the open mouth
of the shoot. To remove these objections to the hand-fed generator it has
become a common practice in large installations to cause the lower end of
the shoot to dip under the level of some oil contained in an appropriate
receptacle, the carbide falling into a basket carried upon a horizontal
spindle. The basket and its support are so arranged that when a suitable
charge of carbide has been dropped into it, a partial rotation of an
external hand-wheel lifts the basket and carbide out of the oil into an
air-tight portion of the generator where the surplus oil can drain away
from the lumps. A further rotation of the hand-wheel then tips the basket
over a partition inside the apparatus, allowing the carbide to fall into
the actual decomposing chamber. This method of using oil has the
advantage of making the evolution of acetylene on a large scale appear to
proceed more quietly than usual, and also of removing the dust from the
carbide before it reaches the water of the generator. The oil itself
obviously does not enter the decomposing chamber to any appreciable
extent and therefore does not contaminate the final sludge. The whole
process accordingly lies to be favourably distinguished from those other
methods of employing oil in generators or in the treatment of carbide
which are referred to elsewhere in this book.

NON-AUTOMATIC WATER-TO-CARBIDE GENERATORS.--The only principle underlying
the satisfactory design of a non-automatic water-to-carbide generator is
to ensure the presence of water in excess at the spot where decomposition
is taking place. This may be effected by employing what is known as the
"flooded-compartment" system of construction, _i.e._, by subdividing
the total carbide charge into numerous compartments arranged either
vertically or horizontally, and admitting the water in interrupted
quantities, each more than sufficient thoroughly to decompose and
saturate the contents of one compartment, rather than in a slow, steady
stream. It would be quite easy to manage this without adopting any
mechanism of a moving kind, for the water might be stored in a tank kept
full by means of a ball-valve, and admitted to an intermediate reservoir
in a slow, continuous current, the reservoir being fitted with an
inverted syphon, on the "Tantalus-cup" principle, so that it should first
fill itself up, and then suddenly empty into the pipe leading to the
carbide container. Without this refinement, however, a water-to-carbide
generator, with subdivided charge, behaves satisfactorily as long as each
separate charge of carbide is so small that the heat evolved on its
decomposition can be conducted away from the solid through the water-
jacketed walls of the vessel, or as the latent heat of steam, with
sufficient rapidity. Still it must be remembered that a water-to-carbide
generator, with subdivided charge, does not belong to the flooded-
compartment type if the water runs in slowly and continuously: it is then
simply a "contact" apparatus, and may or may not exhibit overheating, as
well as the inevitable after-generation. All generators of the water-to-
carbide type, too, must yield a gas containing some air in the earlier
portions of their make, because the carbide containers can only be filled
one-third or one-half full of solid. Although the proportion of air so
passed into the holder may be, and usually is, far too small in amount to
render the gas explosive or dangerous in the least degree, it may well be
sufficient to reduce the illuminating power appreciably until it is swept
out of the service by the purer gas subsequently generated. Moreover, all
water-to-carbide generators are liable, as just mentioned, to produce
sufficient overheating to lower the illuminating power of the gas
whenever they are wilfully driven too fast, or when they are reputed by
their makers to be of a higher productive capacity than they actually
should be; and all water-to-carbide generators, excepting those where the
carbide is thoroughly soaked in water at some period of their operation,
are liable to waste gas by imperfect decomposition.

DEVICES TO SECURE AUTOMATIC ACTION,--The devices which are commonly
employed to render a generator automatic in action, that is to say, to
control the supply of one of the two substances required in the
intermittent evolution of gas, may be divided into two broad classes: (A)
those dependent upon the position of a rising-holder bell, and (B) those
dependent upon the gas pressure inside the apparatus. As the bell of a
rising holder descends in proportion as its gaseous contents are
exhausted, it may (A^1) be fitted with some laterally projecting pin
which, arrived at a certain position, actuates a series of rods or
levers, and either opens a cock on the water-supply pipe or releases a
mechanical carbide-feed gear, the said cock being closed again or the
feed-gear thrown out of action when the pin, rising with the bell, once
more passes a certain position, this time in its upward path. Secondly
(A^2), the bell may be made to carry a perforated receptacle containing
carbide, which is dipped into the water of the holder tank each time the
bell falls, and is lifted out of the water when it rises again. Thirdly
(A^3), by fitting inside the upper part of the bell a false interior,
conical in shape, the descent of the bell may cause the level of the
water in the holder tank to rise until it is above some lateral aperture
through which the liquid may escape into a carbide container placed
elsewhere. These three methods are represented in the annexed diagram
(Fig. 1). In Al the water-levels in the tank and bell remain always at
_l_, being higher in the tank than in the bell by a distance
corresponding with the pressure produced by the bell itself. As the bell
falls a pin _X_ moves the lever attached to the cock on the water-
pipe, and starts, or shuts off, a current passing from a store-tank or
reservoir to a decomposing vessel full of carbide. It is also possible to
make _X_ work some releasing gear which permits carbide to fall into
water--details of this arrangement are given later on. In A^1 the water
in the tank serves as a holder seal only, a separate quantity being
employed for the purposes of the chemical reaction. This arrangement has
the advantage that the holder water lasts indefinitely, except for
evaporation in hot weather, and therefore it may be prevented from
freezing by dissolving in it some suitable saline body, or by mixing with
it some suitable liquid which lowers its point of solidification. It will
be observed, too, that in A^1 the pin _X_, which derives its motive
power from the surplus weight of the falling bell, has always precisely
the same amount of work to do, viz., to overcome the friction of the plug
of the water-cock in its barrel. Hence at all times the pressure
obtaining in the service-pipe is uniform, except for a slight jerk
momentarily given each time the cock is opened or closed. When _X_
actuates a carbide-feed arrangement, the work it does may or may not vary
on different occasions, as will appear hereafter. In A^2 the bell itself
carries a perforated basket of carbide, which is submerged in the water
when the bell falls, and lifted out again when it rises. As the carbide
is thus wetted from below, the lower portion of the mass soon becomes a
layer of damp slaked lime, for although the basket is raised completely
above the water-level, much liquid adheres to the spent carbide by
capillary attraction. Hence, even when the basket is out of the water,
acetylene is being produced, and it is produced in circumstances which
prevent any control over the temperature attained. The water clinging to
the lower part of the basket is vaporised by the hot, half-spent carbide,
and the steam attacks the upper part, so that polymerisation of the gas
and baking of the carbide are inevitable. In the second place, the
pressure in the service-pipe attached to A^2 depends as before upon the
net weight of the holder bell; but here that net weight is made up of the
weight of the bell itself, that of the basket, and that of the carbide it
contains. Since the carbide is being gradually converted into damp slaked
lime, it increases in weight to an indeterminate extent as the generator
in exhausted; but since, on the other hand, some lime may be washed out
of the basket each time it is submerged, and some of the smaller
fragments of carbide may fall through the perforations, the basket tends
to decrease in weight as the generator is exhausted. Thus it happens in
A^2 that the combined weight of bell plus basket plus contents is wholly
indefinite, and the pressure in the service becomes so irregular that a
separate governor must be added to the installation before the burners
can be expected to behave properly. In the third place, the water in the
tank serves both for generation and for decomposition, and this involves
the employment of some arrangement to keep its level fairly constant lest
the bell should become unsealed, while protection from frost by saline or
liquid additions is impossible. A^2 is known popularly as a "dipping"
generator, and it will be seen to be defective mechanically and bad
chemically. In both A^1 and A^2 the bell is constructed of thin sheet-
metal, and it is cylindrical in shape; the mass of metal in it is
therefore negligible in comparison with the mass of water in the tank,
and so the level of the liquid is sensibly the same whether the bell be
high or low. In A^3 the interior of the bell is fitted with a circular
plate which cuts off its upper corners and leaves a circumferential space
_S_ triangular in vertical section. This space is always full of
air, or air and water, and has to be deducted from the available storage
capacity of the bell. Supposing the bell transparent, and viewing it from
above, its effective clear or internal diameter will be observed to be
smaller towards the top than near the bottom; or since the space _S_
is closed both against the water and against the gas, the walls of the
bell may be said to be thicker near its top. Thus it happens that as the
bell descends into the water past the lower angle of _S_, it begins
to require more space for itself in the tank, and so it displaces the
water until the levels rise. When high, as shown in the sketch marked
A^3(a), the water-level is at _l_, below the mouth of a pipe
_P_; but when low, as in A^3(b), the water is raised to the point
_l'_, which is above _P_. Water therefore flows into _P_,
whence it reaches the carbide in an attached decomposing chamber. Here
also the water in the tank is used for decomposition as well as for
sealing purposes, and its normal level must be maintained exactly at
_l_, lest the mouth of _P_ should not be covered whenever the
bell falls.

[Illustration: FIG. 1.--TYPICAL METHODS OF AUTOMATIC GENERATION
CONTROLLED BY BELL GASHOLDER.]

The devices employed to render a generator automatic which depend upon
pressure (B) are of three main varieties: (B^1) the water-level in the
decomposing chamber may be depressed by the pressure therein until its
surface falls below a stationary mass of carbide; (B^2) the level in a
water-store tank may be depressed until it falls below the mouth of a
pipe leading to the carbide vessel; (B^3) the current of water passing
down a pipe to the decomposing chamber may be interrupted by the action
of a pressure superior to the force of gravitation. These arrangements
are indicated roughly in Fig. 2. In B^1, D is a hollow cylinder closed at
all points except at the cock G and the hole E, which are always below
the level of the water in the annulus F, the latter being open to the air
at its top. D is rigidly fastened to the outer vessel F so that it cannot
move vertically, and the carbide cage is rigidly fastened to D. Normally
the water-levels are at _l_, and the liquid has access to the
carbide through perforations in the basket. Acetylene is thus produced;
but if G is shut, the gas is unable to escape, and so it presses
downwards upon the water until the liquid falls in D to the dotted line
_l"_, rising in F to the dotted line _l'_. The carbide is then
out of water, and except for after-generation, evolution of gas ceases.
On opening G more or less fully, the water more or less quickly reaches
its original position at _l_, and acetylene is again produced.
Manifestly this arrangement is identical with that of A^2 as regards the
periodical immersion of the carbide holder in the liquid; but it is even
worse than the former mechanically because there is no rising holder in
B^1, and the pressure in the service is never constant. B^2 represents
the water store of an unshown generator which works by pressure. It
consists of a vessel divided vertically by means of a partition having a
submerged hole N. One-half, H, is cloned against the atmosphere, but
communicates with the gas space of the generator through L; the other
half, K, is open to the air. M is a pipe leading water to the carbide.
When gas is being burnt as fast as, or faster than, it is being evolved,
the pressure in the generator is small, the level of the water stands at
_l_, and the mouth of M is below it. When the pressure rises by
cessation of consumption, that pressure acts through L upon the water in
H, driving it down in H and up in K till it takes the positions
_l"_, and _l'_, the mouth of M being then above the surface. It
should be observed that in the diagrams B^1 and B^3, the amount of
pressure, and the consequent alteration in level, is grossly exaggerated
to gain clearness; one inch or less in both cases may be sufficient to
start or retard evolution of acetylene. Fig. B^3 is somewhat ideal, but
indicates the principle of opposing gas pressure to a supply of water
depending upon gravitation; a method often adopted in the construction of
portable acetylene apparatus. The arrangement consists of an upper tank
containing water open to the air, and a lower vessel holding carbide
closed everywhere except at the pipe P, which leads to the burners, and
at the pipe S, which introduces water from the store-tank. If the cock at
T is closed, pressure begins to rise in the carbide holder until it is
sufficient to counterbalance the weight of the column of water in the
pipe S, when a further supply is prevented until the pressure sinks
again. This idea is simply an application of the displacement-holder
principle, and as such is defective (except for vehicular lamps) by
reason of lack of uniformity in pressure.

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