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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[Illustration: FIG. 2.--TYPICAL METHODS OF AUTOMATIC GENERATION
CONTROLLED BY INTERNAL GAS PRESSURE.]

DISPLACEMENT GASHOLDERS.--An excursion may here be made for the purpose
of studying the action of a displacement holder, which in its most
elementary form is shown at C. It consists of an upright vessel open at
the top, and divided horizontally into two equal portions by a partition,
through which a pipe descends to the bottom of the lower half. At the top
of the closed lower compartment a tube is fixed, by means of which gas
can be introduced below the partition. While the cock is open to the air,
water is poured in at the open top till the lower compartment is
completely full, and the level of the liquid is at _l_. If now, gas
is driven in through the side tube, the water is forced downwards in the
lower half, up through the depending pipe till it begins to fill the
upper half of the holder, and finally the upper half is full of water and
the lower half of gas an shown by the levels _l'_ and _l"_. But
the force necessary to introduce gas into such an apparatus, which
conversely is equal to the force with which the apparatus strives to
expel its gaseous contents, measured in inches of water, is the distance
at any moment between the levels _l'_ and _l"_; and as these
are always varying, the effective pressure needed to fill the apparatus,
or the effective pressure given by the apparatus, may range from zero to
a few inches less than the total height of the whole holder. A
displacement holder, accordingly, may be used either to store a varying
quantity of gas, or to give a steady pressure just above or just below a
certain desired figure; but it will not serve both purposes. If it is
employed as a holder, it in useless as a governor or pressure regulator;
if it is used as a pressure regulator, it can only hold a certain fixed
volume of gas. The rising holder, which is shown at A^1 in Fig. 1
(neglecting the pin X, &c.) serves both purposes simultaneously; whether
nearly full or nearly empty, it gives a constant pressure--a pressure
solely dependent upon its effective weight, which may be increased by
loading its crown or decreased by supporting it on counterpoises to any
extent that may be required. As the bell of a rising holder moves, it
must be provided with suitable guides to keep its path vertical; these
guides being arranged symmetrically around its circumference and carried
by the tank walls. A fixed control rod attached to the tank over which a
tube fastened to the bell slides telescope-fashion is sometimes adopted;
but such an arrangement is in many respects less admirable than the
former.

Two other devices intended to give automatic working, which are scarcely
capable of classification among their peers, may be diagrammatically
shown in Fig. 3. The first of these (D) depends upon the movements of a
flexible diaphragm. A vessel (_a_) of any convenient size and shape
is divided into two portions by a thin sheet of metal, leather,
caoutchouc, or the like. At its centre the diaphragm is attached by some
air-tight joint to the rod _c_, which, held in position by suitable
guides, is free to move longitudinally in sympathy with the diaphragm,
and is connected at its lower extremity with a water-supply cock or a
carbide-feed gear. The tube _e_ opens at its base into the gas space
of the generator, so that the pressure below the diaphragm in _a_ is
the same as that elsewhere in the apparatus, while the pressure in
_a_ above the diaphragm is that of the atmosphere. Being flexible
and but slightly stretched, the diaphragm is normally depressed by the
weight of _c_ until it occupies the position _b_; but if the
pressure in the generator (_i.e._, in _e_) rises, it lifts the
diaphragm to somewhat about the position _b'_--the extent of
movement being, as usual, exaggerated in the sketch. The movement of the
diaphragm is accompanied by a movement of the rod _c_, which can be
employed in any desirable way. In E the bell of a rising holder of the
ordinary typo is provided with a horizontal striker which, when the bell
descends, presses against the top of a bag _g_ made of any flexible
material, such as india-rubber, and previously filled with water. Liquid
is thus ejected, and may be caused to act upon calcium carbide in some
adjacent vessel. The sketch is given because such a method of obtaining
an intermittent water-supply has at one time been seriously proposed; but
it is clearly one which cannot be recommended.

[Illustration: FIG. 3.--TYPICAL METHODS OF AUTOMATIC GENERATION
CONTROLLED BY A FLEXIBLE DIAPHRAM OR BAG.]

ACTION OF WATER-TO-CARBIDE GENERATORS.--Having by one or other of the
means described obtained a supply of water intermittent in character, it
remains to be considered how that supply may be made to approach the
carbide in the generator. Actual acetylene apparatus are so various in
kind, and merge from one type to another by such small differences, that
it is somewhat difficult to classify them in a simple and intelligible
fashion. However, it may be said that water-to-carbide generators,
_i.e._, such as employ water as the moving material, may be divided
into four categories: (F^1) water is allowed to fall as single drops or
as a fine stream upon a mass of carbide--this being the "drip" generator;
(F^2) a mass of water is made to rise round and then recede from a
stationary vessel containing carbide--this being essentially identical in
all respects save the mechanical one with the "dip" or "dipping"
generator shown in A^2, Fig. 1; (F^3) a supply of water is permitted to
rise round, or to flow upon, a stationary mass of carbide without ever
receding from the position it has once assumed--this being the "contact"
generator; and (F^4) a supply of water is admitted to a subdivided charge
of carbide in such proportion that each quantity admitted is in chemical
excess of the carbide it attacks. With the exception of F^2, which has
already been illustrated as A^2 Fig. 1, or as B^1 in Fig. 2, these
methods of decomposing carbide are represented in Figs. 4 and 5. It will
be observed that whereas in both F^1 and F^3 the liberated acetylene
passes off at the top of the apparatus, or rather from the top of the
non-subdivided charge of carbide, in F^1 the water enters at the top, and
in F^3 it enters at the bottom. Thus it happens that the mixture of
acetylene and steam, which is produced at the spot where the primary
chemical reaction is taking place, has to travel through the entire mass
of carbide present in a generator belonging to type F^3, while in F^1 the
damp gas flows directly to the exit pipe without having to penetrate the
lumps of solid. Both F^1 and F^3 exhibit after-generation caused by a
reaction between the liquid water mechanically clinging to the mass of
spent lime and the excess of carbide to an approximately equal extent;
but for the reason just mentioned, after-generation due to a reaction
between the vaporised water accompanying the acetylene first evolved and
the excess of carbide is more noticeable in F^3 than in F^1; and it is
precisely this latter description of after-generation which leads to
overheating of the most ungovernable kind. Naturally both F^1 and F^3 can
be fitted with water jackets, as is indicated by the dotted lines in the
second sketch; but unless the generating chamber in quite small and the
evolution of gas quite slow, the cooling action of the jacket will not
prove sufficient. As the water in F^1 and F^3 is not capable of backward
motion, the decomposing chambers cannot be employed as displacement
holders, as is the case in the dipping generator pictured at B^1, Fig. 2.
They must be coupled, accordingly, to a separate holder of the
displacement or, preferably, of the rising type; and, in order that the
gas evolved by after-generation may not be wasted, the automatic
mechanism must cut off the supply of water to the generator by the time
that holder is two-thirds or three-quarters full.

[Illustration: FIG. 4.--TYPICAL METHODS OF DECOMPOSING CARBIDE (WATER TO
CARBIDE).]

[Illustration: FIG. 5.--TYPICAL METHODS OF DECOMPOSING CARBIDE (WATER TO
CARBIDE).]

The diagrams G, H, and K in Figs. 4 and 5 represent three different
methods of constructing a generator which belongs either to the contact
type (F^3) if the supply of water is essentially continuous, _i.e._,
if less is admitted at each movement of the feeding mechanism than is
sufficient to submerge the carbide in each receptacle; or to the flooded-
compartment type (F') if the water enters in large quantities at a time.
In H the main carbide vessel is arranged horizontally, or nearly so, and
each partition dividing it into compartments is taller than its
predecessor, so that the whole of the solid in (1) must be decomposed,
and the compartment entirely filled with liquid before it can overflow
into (2), and so on. Since the carbide in all the later receptacles is
exposed to the water vapour produced in that one in which decomposition
is proceeding at any given moment, at least at its upper surface, some
after-generation between vapour and carbide occurs in H; but a partial
control over the temperature may be obtained by water-jacketing the
container. In G the water enters at the base and gas escapes at the top,
the carbide vessels being disposed vertically; hero, perhaps, more after-
generation of the same description occurs, as the moist gas streams round
and over the higher baskets. In K, the water enters at the top and must
completely fill basket (1) before it can run down the depending pipe into
(2); but since the gas also leaves the generator at the top, the later
carbide receptacles do not come in contact with water vapour, but are
left practically unattacked until their time arrives for decomposition by
means of liquid water. K, therefore, is the best arrangement of parts to
avoid after-generation, overheating, and polymerisation of the acetylene
whether the generator be worked as a contact or as a flooded-compartment
apparatus; but it may be freely admitted that the extent of the
overheating due to reaction between water vapour and carbide may be kept
almost negligible in either K, H, or G, provided the partitions in the
carbide container be sufficient in number--provided, that is to say, that
each compartment holds a sufficiently small quantity of carbide; and
provided that the quantity of water ultimately required to fill each
compartment is relatively so large that the temperature of the liquid
never approaches the boiling-point where vaporisation is rapid. The type
of generator indicated by K has not become very popular, but G is fairly
common, whilst H undoubtedly represents the apparatus which is most
generally adopted for use in domestic and other private installations in
the United Kingdom and the Continent of Europe. The actual generators
made according to the design shown by H usually have a carbide receptacle
designed in the form of a semi-cylindrical or rectangular vessel of steel
sliding fairly closely into an outside container, the latter being either
built within the main water space of the entire apparatus or placed
within a separate water-jacketed casing. Owing to its shape and the
sliding motion with which the carbide receptacle is put into the
container these generators are usually termed "drawer" generators. In
comparison with type G, the drawer generator H certainly exhibits a lower
rise in temperature when gas is evolved in it at a given speed and when
the carbide receptacles are constructed of similar dimensions. It is very
desirable that the whole receptacle should be subdivided into a
sufficient number of compartments and that it should be effectively
water-cooled from outside. It would also be advantageous if the water-
supply were so arranged that the generator should be a true flooded-
compartment apparatus, but experience has nevertheless shown that
generators of type H do work very well when the water admitted to the
carbide receptacle, each time the feed comes into action, is not enough
to flood the carbide in one of the compartments. Above a certain size
drawer generators are usually constructed with two or even more complete
decomposing vessels, arrangements being such that one drawer can be taken
out for cleaning, whilst the other is in operation. When this is the case
a third carbide receptacle should always be employed so that it may be
dry, lit to receive a charge of carbide, and ready to insert in the
apparatus when one of the others is withdrawn. The water-feed should
always be so disposed that the attendant can see at a glance which of the
two (or more) carbide receptacles is in action at any moment, and it
should be also so designed that the supply is automatically diverted to
the second receptacle when the first is wholly exhausted and back again
to the first (unless there are more than two) when the carbide in the
second is entirely gasified. In the sketches G, H, and K, the total space
occupied by the various carbide receptacles is represented as being
considerably smaller than the capacity of the decomposing chamber. Were
this method of construction copied in actual acetylene apparatus, the
first makes of gas would be seriously (perhaps dangerously) contaminated
with air. In practice the receptacles should fit so tightly into the
outer vessel and into one another that when loaded to the utmost extent
permissible--space being left for the swelling of the charge and for the
passage of water and gas--but little room should be left for the
retention of air in the chamber.

ACTION OF CARBIDE-TO-WATER GENERATORS.--The methods which may be adopted
to render a generator automatic when carbide is employed as the moving
material are shown at M, N, and P, in Fig. 6; but the precise devices
used in many actual apparatus are so various that it is difficult to
portray them generically. Moreover it is desirable to subdivide automatic
carbide-to-water generators, according to the size of the carbide they
are constructed to take, into two or three classes, which are termed
respectively "large carbide-feed," "small carbide-feed," and "granulated
carbide-feed" apparatus. (The generator represented at L does not really
belong to the present class, being non-automatic and fed by hand; but the
sketch is given for completeness.) M is an automatic carbide-feed
generator having its store of carbide in a hopper carried by the rising-
holder bell. The hopper is narrowed at its mouth, where it is closed by a
conical or mushroom valve _d_ supported on a rod held in suitable
guides. When the bell falls by consumption of gas, it carries the valve
and rod with it; but eventually the button at the base of _c_
strikes the bottom of the generator, or some fixed distributing plate,
and the rod can descend no further. Then, when the bell falls lower, the
mushroom _d_ rises from its seat, and carbide drops from the hopper
into the water. This type of apparatus has the defect characteristic of
A^2, Fig. 1; for the pressure in the service steadily diminishes as the
effective weight of bell plus hopper decreases by consumption of carbide.
But it has also two other defects--(1) that ordinary carbide is too
irregular in shape to fall smoothly through the narrow annular space
between the valve and its seat; (2) that water vapour penetrates into the
hopper, and liberates some gas there, while it attacks the lumps of
carbide at the orifice, producing dust or causing them to stick together,
and thus rendering the action of the feed worse than ever. Most of these
defects can be avoided by using granulated carbide, which is more uniform
in size and shape, or by employing a granulated and "treated" carbide
which has been dipped in some non-aqueous liquid to make it less
susceptible to the action of moisture. Both these plans, however, are
expensive to adopt; first, because of the actual cost of granulating or
"treating" the carbide; secondly, because the carbide deteriorates in
gas-making capacity by its inevitable exposure to air during the
granulating or "treating" process. The defects of irregularity of
pressure and possible waste of gas by evolution in the hopper may be
overcome by disposing the parts somewhat differently; making the holder
an annulus round the hopper, or making it cylindrical with the hopper
inside. In this case the hopper is supported by the main portion of the
apparatus, and does not move with the bell: the rod and valve being given
their motion in some fashion similar to that figured. Apparatus designed
in accordance with the sketch M, or with the modification just described,
are usually referred to under the name of "hopper" generators. On several
occasions trouble has arisen during their employment owing to the jamming
of the valve, a fragment of carbide rather larger than the rest of the
material lodging between the lips of the hopper and the edges of the
mushroom valve. This has been followed by a sudden descent of all the
carbide in the store into the water beneath, and the evolution of gas has
sometimes been too rapid to pass away at the necessary speed into the
holder. The trouble is rendered even more serious should the whole charge
of carbide fall at a time when, by neglect or otherwise, the body of the
generator contains much lime sludge, the decomposition then proceeding
under exceptionally bad circumstances, which lead to the production of an
excessively high temperature. Hopper generators are undoubtedly very
convenient for certain purposes, chiefly, perhaps, for the construction
of table-lamps and other small installations. Experience tends to show
that they may be employed, first, provided they are designed to take
granulated carbide--which in comparison with larger grades is much more
uniform and cylindrical in shape--and secondly, provided the quantity of
carbide in the hopper does not exceed a few pounds. The phenomenon of the
sudden unexpected descent of the carbide, popularly known as "dumping,"
can hardly be avoided with carbide larger in size than the granulated
variety; and since the results of such an accident must increase in
severity with the size of the apparatus, a limit in their capacity is
desirable.

[Illustration: FIG. 6.--TYPICAL METHODS OF DECOMPOSING CARBIDE (CARBIDE
TO WATER).]

When it is required to construct a carbide-feed generator of large size
or one belonging to the large carbide-feed pattern, it is preferable to
arrange the store in a different manner. In N the carbide is held in a
considerable number of small receptacles, two only of which are shown in
the drawing, provided with detachable lids and hinged bottoms kept shut
by suitable catches. At proper intervals of time those catches in
succession are knocked on one side by a pin, and the contents of the
vessel fall into the water. There are several methods available for
operating the pins. The rising-holder bell may be made to actuate a train
of wheels which terminate in a disc revolving horizontally on a vertical
axis somewhere just below the catches; and this wheel may bear an
eccentric pin which hits each catch as it rotates. Alternatively the
carbide boxes may be made to revolve horizontally on a vertical axis by
the movements of the bell communicated through a clutch; and thus each
box in succession may arrive at a certain position where the catch is
knocked aside by a fixed pin. The boxes, again, may revolve vertically on
a horizontal axis somewhat like a water-wheel, each box having its bottom
opened, or, by a different system of construction, being bodily upset,
when it arrives at the bottom of its circular path. In no case, however,
are the carbide receptacles carried by the bell, which is a totally
distinct part of the apparatus; and therefore in comparison with M, the
pressure given by the bell is much more uniform. Nevertheless, if the
system of carbide boxes moves at all, it becomes easier to move by
decrease in weight and consequent diminution in friction as the total
charge is exhausted; and accordingly the bell has less work to do during
the later stages of its operation. For this reason the plan actually
shown at N is preferable, since the work done by the moving pin,
_i.e._, by the descending bell, is always the same. P represents a
carbide-feed effected by a spiral screw or conveyor, which, revolved
periodically by a moving bell, draws carbide out of a hopper of any
desired size and finally drops it into a shoot communicating with a
generating chamber such as that shown in L. Here the work done by the
bell is large, as the friction against the blades of the screw and the
walls of the horizontal tube is heavy; but that amount of work must
always be essentially identical. The carbide-feed may similarly be
effected by means of some other type of conveyor instead of the spiral
screw, such as an endless band, and the friction in these cases may be
somewhat less than with the screw, but the work to be done by the bell
will always remain large, whatever type of conveyor may be adopted. A
further plan for securing a carbide-feed consists in employing some
extraneous driving power to propel a charge of carbide out of a reservoir
into the generator. Sometimes the propulsive effort is obtained from a
train of clockwork, sometimes from a separate supply of water under high
pressure. The clockwork or the water power is used either to drive a
piston travelling through the vessel containing the carbide so that the
proper quantity of material is dropped over the open mouth of a shoot, or
to upset one after another a series of carbide receptacles, or to perform
some analogous operation. In these cases the pin or other device fitted
to the acetylene apparatus itself has nothing to do beyond releasing the
mechanism in question, and therefore the work required from the bell is
but small. The propriety of employing a generator belonging to these
latter types must depend upon local conditions, _e.g._, whether the
owner of the installation has hydraulic power on a small scale (a
constant supply of water under sufficient pressure) at disposal, or
whether he does not object to the extra labour involved in the periodical
winding up of a train of clockwork.

It must be clear that all these carbide-feed arrangements have the defect
in a more or less serious degree of leaving the carbide in the main
storage vessel exposed to the attack of water vapour rising from the
decomposing chamber, for none of the valves or operating mechanism can be
made quite air-tight. Evolution of gas produced in this way does not
matter in the least, because it is easy to return the gas so liberated
into the generator or into the holder; while the extent of the action,
and the consequent production of overheating, will tend to be less than
in generators such as those shown in G and H of Figs. 4 and 5, inasmuch
as the large excess of water in the carbide-feed apparatus prevents the
liquid arriving at a temperature at which it volatilises rapidly. The
main objection to the evolution of gas in the carbide vessel of a
carbide-to-water generator depends on the danger that the smooth working
of the feed-gear may be interfered with by the formation of dust or by
the aggregation of the carbide lumps.

USE OF OIL IN GENERATORS.--Calcium carbide is a material which is only
capable of attack for the purpose of evolving acetylene by a liquid that
is essentially water, or by one that contains some water mixed with it.
Oils and the like, or even such non-aqueous liquids as absolute alcohol,
have no effect upon carbide, except that the former naturally make it
greasy and somewhat more difficult to moisten. This last property has
been found of service in acetylene generation, especially on the small
scale; for if carbide is soaked in, or given a coating of, some oil, fat,
or solid hydrocarbon like petroleum, cocoanut oil, or paraffin wax, the
substance becomes comparatively indifferent towards water vapour or the
moisture present in the air, while it still remains capable of complete,
albeit slow, decomposition by liquid water when completely immersed
therein. The fact that ordinary calcium carbide is attacked so quickly by
water is really a defect of the substance; for it is to this extreme
rapidity of reaction that the troubles of overheating are due. Now, if
the basket in the generator B^1 of Fig. 2, or, indeed, the carbide store
in any of the carbide-to-water apparatus, is filled with a carbide which
has been treated with oil or wax, as long as the water-level stands at
_l'_ and _l"_ or the carbide still remains in the hopper, it is
essentially unattacked by the vapour arising from the liquid; but
directly the basket is submerged, or the lumps fall into the water,
acetylene is produced, and produced more slowly and regularly than
otherwise. Again, oils do not mix with water, but usually float thereon,
and a mass of water covered by a thick film or layer of oil does not
evaporate appreciably. If, now, a certain quantity of oil, say lamp
paraffin or mineral lubricating oil, is poured on to the water in B^1,
Fig. 2, it moves upwards and downwards with the water. When the water
takes the position _l_, the oil is driven upwards away from the
basket of carbide, and acetylene is generated in the ordinary manner; but
when the water falls to _l"_ the oil descends also, rinses off much
of the adhering water from the carbide lumps, covers them with a greasy
film, and almost entirely stops generation till it is in turn washed off
by the next ascent of the water. Similarly, if the carbide in generators
F, G, and H (also K) has been treated with a solid or semi-solid grease,
it is practically unattacked by the stream of warm damp gas, and is only
decomposed when the liquid itself arrives in the basket. For the same
reason treated carbide can be kept for fairly long periods of time, even
in a drum with badly fitting lid, without suffering much deterioration by
the action of atmospheric moisture. The problem of acetylene generation
is accordingly simplified to a considerable degree by the use of such
treated carbide, and the advantage becomes more marked as the plant
decreases in size till a portable apparatus is reached, because the
smaller the installation the more relatively expensive or inconvenient is
a large holder for surplus gas. The one defect of the method is the extra
cost of such treated carbide; and in English conditions ordinary calcium
carbide is too expensive to permit of any additional outlay upon the
acetylene if it is to compete with petroleum or the product of a tiny
coal-gas works. The extra cost of using treated carbide falls upon the
revenue account, and is much more noticeable than that of a large holder,
which is capital expenditure. When fluid oil is employed in a generator
of type B^1, evolution of gas becomes so regular that any holder beyond
the displacement one which the apparatus itself constitutes is actually
unnecessary, though still desirable; but B^1, with or without oil, still
remains a displacement apparatus, and as such gives no constant pressure.
It must be admitted that the presence of oil so far governs the evolution
of gas that the movement of the water, and the consequent variation of
pressure, is rendered very small; still a governor or a rising holder
would be required to give the best result at the burners. One point in
connexion with the use of liquid oil must not be overlooked, viz., the
extra trouble it may give in the disposal of the residues. This matter
will be dealt with more fully in Chapter V.; here it is sufficient to say
that as the oil does not mix with the water but floats on the surface,
care has to be taken that it is not permitted to enter any open stream.
The foregoing remarks about the use of oil manifestly only apply to those
cases where it is used in quantity and where it ultimately becomes mixed
with the sludge or floats on the water in the decomposing chamber. The
employment of a limpid oil, such as paraffin, as an intermediate liquid
into which carbide is introduced on its way to the water in the
decomposing vessel of a hand-fed generator in the manner described on
page 70 is something quite different, because, except for trifling
losses, one charge of oil should last indefinitely.

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