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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NaC_2H_3O_2 + 3H_2O = NaC_2H_3O_2.3H_2O

(anhydrous acetate) (crystals)

evolves 4.37 calories (Berthelot), or 1.46 calorie for each molecule of
water; and whereas 1 kilo. of water only evolves 1 large calorie of heat
as its temperature falls 1 deg. C., 18 grammes of water (1 gramme-molecule)
evolve l.46 large calorie when they enter into combination with anhydrous
sodium acetate to assist in forming crystals--and this 1.46 calorie may
either be permitted to warm the mass of crystals, or made to do useful work
by raising the temperature of some adjacent substance. Sodium acetate
crystals dissolve in 3.9 parts by weight of water at 6 deg. C. (43 deg. F.)
or in 2.4 parts at 37 deg. C. (99 deg. F.). If, then, a jacket round an
acetylene apparatus is filled with a warm solution of sodium acetate
crystals in (say) 3 parts by weight of water, the liquid will crystallise
when it reaches some temperature between 99 deg. and 43 deg. F.; but when
the generator comes into action, the heat liberated will change the mass of
crystals into a liquid without raising its sensible temperature to
anything like the extent that would happen were the jacket full of simple
water. Not being particularly warm to the touch, the liquefied product in
the jacket will not lose much heat by radiation, &c., into the
surrounding air; but when the water in the generator falls again (after
evolution of acetylene ceases) the contents of the jacket will also cool,
and finally will begin to crystallise once more, passing a large amount
of low-temperature heat into the water of the generator, and safely
maintaining it for long periods of time at a temperature suitable for the
further evolution of gas. Like the liquid in the seal of an isolated
gasholder, the liquid in such a jacket will last indefinitely; and
therefore the cost of the sodium acetate in negligible.

Another method of keeping warm the water in any part of an acetylene
installation consists in piling round the apparatus a heap of fresh
stable manure, which, as is well known, emits much heat as it rots. Where
horses are kept, such a process may be said to cost nothing. It has the
advantage over methods of lagging or jacketing that the manure can be
thrown over any pipe, water-seal, washing apparatus, &c., even if the
plant is constructed in several separate items. Unfortunately the ammonia
and the volatile organic compounds which are produced during the natural
decomposition of stable manure tend seriously to corrode iron and steel,
and therefore this method of protecting an apparatus from frost should
only be employed temporarily in times of emergency.

CORROSION IN APPARATUS.--All natural water is a solution of oxygen and
may be regarded also as a weak solution of the hypothetical carbonic
acid. It therefore causes iron to rust more or less quickly; and since no
paint is absolutely waterproof, especially if it has been applied to a
surface already coated locally with spots of rust, iron and steel cannot
be perfectly protected by its aid. More particularly at a few inches
above and below the normal level of the water in a holder, therefore, the
metal soon begins to exhibit symptoms of corrosion which may eventually
proceed until the iron is eaten away or becomes porous. One method of
prolonging the life of such apparatus is to give it fresh coats of paint
periodically; but unless the old layers are removed where they have
cracked or blistered, and the rust underneath is entirely scraped off
(which is practically impossible), the new paint films will not last very
long. Another more elegant process for preserving any metal like iron
which is constantly exposed to the attack of a corrosive liquid, and
which is readily applicable to acetylene holders and their tanks, depends
on the principle of galvanic action. When two metals in good electrical
contact are immersed in some liquid that is capable of attacking both,
only that metal will be attacked which is the more electro-positive, or
which (the same thing in other words) is the more readily attacked by the
liquid, evolving the more heat during its dissolution. As long as this
action is proceeding, as long, that is, as some of the more electro-
positive material is present, the less electro-positive material will not
suffer. All that has to be done, therefore, to protect the walls of an
acetylene-holder tank and the sides of its bell is to hang in the seal,
supported by a copper wire fastened to the tank walls by a trustworthy
electrical joint (soldering or riveting it), a plate or rod of some more
electro-positive metal, renewing that plate or rod before it is entirely
eaten away. [Footnote: Contact between the bell and the rod may be
established by means of a flexible metallic wire; or a separate rod might
be used for the bell itself.] If the iron is bare or coated with lead
(paint may be overlooked), the plate may be zinc; if the iron is
galvanised, _i.e._, coated with zinc, the plate may be aluminium or
an alloy of aluminium and zinc. The joint between the copper wire and the
zinc or aluminium plate should naturally be above the water-level. The
foregoing remarks should be read in conjunction with what was said in
Chapter II., about the undesirability of employing a soft solder
containing lead in the construction of an acetylene generator. Here it is
proposed intentionally to set up a galvanic couple to prevent corrosion;
there, with the same object in view, the avoidances of galvanic action is
counselled. The reason for this difference is self-evident; here a
foreign metal is brought into electrical contact with the apparatus in
order that the latter may be made electro-negative; but when a joint is
soldered with lead, the metal of the generator is unintentionally made
electro-positive. Here the plant is protected by the preferential
corrosion of a cheap and renewable rod; in the former case the plant is
encouraged to rust by the unnecessary presence of an improperly selected
metal.

OTHER ITEMS IN GENERATING PLANT.--It has been explained in Chapter II.
that the reaction between calcium carbide and water is very tumultuous in
character, and that it occurs with great rapidity. Clearly, therefore,
the gas comes away from the generator in rushes, passing into the next
item of the plant at great speed for a time, and then ceasing altogether.
The methods necessarily adopted for purifying the crude gas are treated
of in Chapter V.; but it is manifest now that no purifying material can
prove efficient unless the acetylene passes through it at a uniform rate,
and at one which is as slow as other conditions permit. For this reason
the proper position of the holder in an acetylene installation is before
the purifier, and immediately after the condenser or washer which adjoins
the generator. By this method of design the holder is filled up
irregularly, the gas passing into it sometimes at full speed, sometimes
at an imperceptible rate; but if the holder is well balanced and guided
this is a matter of no consequence. Out of the holder, on the other hand,
the gas issues at a rate which is dependent upon the number and capacity
of the burners in operation at any moment; and in ordinary conditions
this rate is so much more uniform during the whole of an evening than the
rate at which the gas is evolved from the carbide, that a purifier placed
after the holder is given a far better opportunity of extracting the
impurities from the acetylene than it would have were it situated before
the holder, as is invariably the case on coal-gas works.

For many reasons, such as capacity for isolation when being recharged or
repaired, it is highly desirable that each item in an acetylene plant
shall be separated, or capable of separation, from its neighbours; and
this observation applies with great force to the holder and the
decomposing vessel of the generator. In all large plants each vessel
should be fitted with a stopcock at its inlet and, if necessary, one at
its outlet, being provided also with a by-pass so that it can be thrown
out of action without interfering with the rest of the installation. In
the best practice the more important vessels, such as the purifiers, will
be in duplicate, so that unpurified gas need not be passed into the
service while a solitary purifier is being charged afresh. In smaller
plants, where less skilled labour will probably be bestowed on the
apparatus, and where hand-worked cocks are likely to be neglected or
misused, some more, automatic arrangement for isolating each item is
desirable. There are two automatic devices which may be employed for the
purposes in view, the non-return valve and the water-seal. The non-return
valve is simply a mushroom or ball valve without handle, lifted off its
seat by gas passing from underneath whenever the pressure of the gas
exceeds the weight of the valve, but falling back on to its seat and
closing the pipe when the pressure decreases or when pressure above is
greater than that below. The apparatus works perfectly with a clean gas
or liquid which is not corrosive; but having regard to the possible
presence of tarry products, lime dust, or sludge, condensed water loaded
with soluble impurities, &c., in the acetylene, a non-return valve is not
the best device to adopt, for both it and the hand-worked cock or screw-
down valve are liable to stick and give trouble. The best arrangement in
all respects, especially between the generator and the holder, is a
water-seal. A water-seal in made by leading the mouth of a pipe
delivering gas under the level of water in a suitable receptacle, so that
the issuing gas has to bubble through the liquid. Gas cannot pass
backwards through the pipe until it has first driven so much liquid
before it that the level in the seal has fallen below the pipe's mouth;
and if the end of the pipe is vertical more pressure than can possibly be
produced in the apparatus is necessary to effect this. Omitting the side
tube _b_, one variety of water-seal is shown at D in Fig. 7 on page
103. The water being at the level _l_, gas enters at _a_ and
bubbles through it, escaping from the apparatus at _c_. It cannot
return from _c_ to _a_ without driving the water out of the
vessel till its level falls from _f_ to _g_; and since the area
of the vessel is much greater than that of the pipe, so great a fall in
the vessel would involve a far greater rise in _a_. It is clear that
such a device, besides acting as a non-return valve, also fulfils two
other useful functions: it serves to collect and retain all the liquid
matter that may be condensed in the pipe _a_ from the spot at which
it was originally level or was given a fall to the seal, as well as that
condensing in _c_ as far as the spot where _c_ dips again; and
it equally acts as a washer to the gas, especially if the orifice
_g_ of the gas-inlet pipe is not left with a plain mouth as
represented in the figure, but terminates in a large number of small
holes, the pipe being then preferably prolonged horizontally, with minute
holes in it so as to distribute the gas throughout the entire vessel.
Such an apparatus requires very little attention. It may with advantage
be provided with the automatic arrangement for setting the water-level
shown at _d_ and _e_. _d_ is a tunnel tube extending
almost to the bottom of the vessel, and _e_ is a curved run-off pipe
of the form shown. The lower part of the upper curve in _e_ is above
the level _f_, being higher than _f_ by a distance equal to
that of the gas pressure in the pipes; and therefore when water is poured
into the funnel it fills the vessel till the internal level reaches
_f_, when the surplus overflows of itself. The operation thus not
only adjusts the quantity of water present to the desired level so that
_a_ cannot become unsealed, but it also renews the liquid when it
has become foul and nearly saturated with dissolved and condensed
impurities from the acetylene. It would be a desirable refinement to give
the bottom of the vessel a slope to the mouth of _e_, or to some
other spot where a large-bore draw-off cock could be fitted for the
purpose of extracting any sludge of lime, &c., that may collect. By
having such a water-seal, or one simpler in construction, between the
generator and the holder, the former may be safely opened at any time for
repairs, inspection, or the insertion of a fresh charge of carbide while
the holder is full of gas, and the delivery of acetylene to the burners
at a specified pressure will not be interrupted. If a cock worked by hand
were employed for the separation of the holder from the generator, and
the attendant were to forget to close it, part or all of the acetylene in
the holder would escape from the generator when it was opened or
disconnected.

Especially when a combined washer and non-return valve follows
immediately after a generator belonging to the shoot type, and the mouth
of the shoot is open to the air in the plant-house, it is highly
desirable that the washer shall be fitted with some arrangement of an
automatic kind for preventing the water level rising much above its
proper position. The liquid in a closed washer tends to rise as the
apparatus remains in use, water vapour being condensed within it and
liquid water, or froth of lime, being mechanically carried forward by the
stream of acetylene coming from the decomposing chamber. In course of
time, therefore, the vertical depth to which the gas-inlet pipe in the
washer is sealed by the liquid increases; and it may well be that
eventually the depth in question, plus the pressure thrown by the holder
bell, may become greater than the pressure which can be set up inside the
generator without danger of gas slipping under the lower edge of the
shoot. Should this state of things arise, the acetylene can no longer
force its way through the washer into the holder bell, but will escape
from the mouth of the shoot; filling the apparatus-house with gas, and
offering every opportunity for an explosion if the attendant disobeys
orders and takes a naked light with him to inspect the plant.

It is indispensable that every acetylene apparatus shall be fitted with a
safety-valve, or more correctly speaking a vent-pipe. The generator must
have a vent-pipe in case the gas-main leading to the holder should become
blocked at any time, and the acetylene which continues to be evolved in
all water-to-carbide apparatus, even after the supply of water has been
cut off be unable to pass away. Theoretically a non-automatic apparatus
does not require a vent-pipe in its generator because all the gas enters
the holder immediately, and is, or should be, unable to return through
the intermediate water seal; practically such a safeguard is absolutely
necessary for the reason given. The holder must have a safety-valve in
case the cutting-off mechanism of the generator fails to act, and more
gas passes into it than it can store. Manifestly the pressure of the gas
in a water-sealed holder or in any generator fitted with a water-sealed
lid cannot rise above that corresponding with the depth of water in the
seal; for immediately the pressure, measured in inches of water, equals
the depth of the sealing liquid, the seal will be blown out, and the gas
will escape. Such an occurrence, however, as the blowing of a seal must
never be possible in any item of an acetylene plant, more especially in
those items that are under cover, for the danger that the issuing gas
might be fired or might produce suffocation would be extremely great.
Typical simple forms of vent-pipe suitable for acetylene apparatus are
shown in Fig. 7. In each case the pipe marked "vent" is the so-called
safety-valve; it is open at its base for the entry of gas, and open at
its top for the escape of the acetylene into the atmosphere, such top
being in all instances carried through the roof of the generator-house
into the open air, and to a spot distant from any windows of that house
or of the residence, where it can prove neither dangerous nor a nuisance
by reason of its odour. At A is represented the vent-pipe of a
displacement vessel, which may either be part of a displacement holder or
of a generator working on the displacement principle. The vent-pipe is
rigidly fixed to the apparatus. If gas is generated within the closed
portion of the holder or passes through it, and if the pressure so set up
remains less than that which is needed to move the water from the level
_l_ to the levels _l'_ and _l"_, the mouth of the pipe is
under water, and acetylene cannot enter it; but immediately such an
amount of gas is collected, or such pressure is produced that the
interior level sinks below _l"_, which is that of the mouth of the
pipe, it becomes unsealed, and the surplus gas freely escapes. There are
two minor points in connexion with this form of vent-pipe often
overlooked. At the moment when the water arrives at _l"_ in the
closed half of the apparatus, its level in the interior of the vent-pipe
stands at _l'_, identical with that in the open hall of the
apparatus (for the mouth of the vent-pipe and the water in the open hall
of the apparatus are alike exposed to the pressure of the atmosphere
only). When the water, then, descends just below _l"_ there is an
amount of water inside the pipe equal in height to the distance between
_l'_ and _l"_; and before the acetylene can escape, it must
either force this water as a compact mass out of the upper mouth of the
vent-pipe (which it is clearly not in a position to do), drive it out of
the upper mouth a little at a time, or bubble through it till the water
is gradually able to run downwards out of the pipe as its lower opening
is more fully unsealed. In practice the acetylene partly bubbles through
this water and partly drives it out of the mouth of the pipe; on some
occasions temporarily yielding irregular pressures at the burners which
cause them to jump, and always producing a gurgling noise in the vent-
pipe which in calculated to alarm the attendant. If the pipe is too small
in diameter, and especially if its lower orifice is cut off perfectly
horizontal and constricted slightly, the water may refuse to escape from
the bottom altogether, and the pipe will fail to perform its allotted
task. It is better therefore to employ a wide tube, and to cut off its
mouth obliquely, or to give its lower extremity the shape of an inverted
funnel. At the half of the central divided drawing marked B (Fig. 7) is
shown a precisely similar vent-pipe affixed to the bell of a rising
holder, which behaves in an identical fashion when by the rising of the
bell its lower end is lifted out of the water in the tank. The features
described above as attendant, upon the act of unsealing of the
displacement-holder vent-pipe occur here also, but to a less degree; for
the water remaining in the pipe at the moment of unsealing is only that
which corresponds with the vertical distance between _l'_ and
_l"_, and in a rising holder this is only a height always equal to
the pressure given by the bell. Nevertheless this form of vent-pipe
produces a gurgling noise, and would be better for a trumpet-shaped
mouth. A special feature of the pipe in B is that unless it is placed
symmetrically about the centre of the bell its weight tends to throw the
bell out of the vertical, and it may have to be supported at its upper
part; conversely, if the pipe is arranged concentrically in the bell, it
may be employed as part of the guiding arrangement of the bell itself.
Manifestly, as the pipe must be long enough to extend through the roof of
the generator-house, its weight materially increases the weight of the
bell, and consequently the gas pressure in the service; this fact is not
objectionable provided due allowance is made for it. So tall a vent-pipe,
however, seriously raises the centre of gravity of the bell and may make
it top-heavy. To work well the centre of gravity of a holder bell should
be as low as possible, any necessary weighting being provided
symmetrically about its circumference and close to its bottom edge. The
whole length of an ascending vent-pipe need not be carried by the rising
bell, because the lower portion, which must be supported by the bell, can
be arranged to slide inside a wider length of pipe which is fixed to the
roof of the generator-house at the point where it passes into the open
air.

[Illustration: FIG. 7.--TYPICAL FORMS OF VENT-PIPES OR SAFETY-VALVES.]

A refinement upon this vent-pipe is represented at C, where it is rigidly
fastened to the tank of the holder, and has its internal aperture always
above the level of the water in the apparatus. Rigidly fixed to the crown
of the bell is a tube of wider diameter, _h_, which is closed at its
upper end. _h_ is always full of gas, and its mouth is normally
beneath the level of the water in the seal; but when the bell rises to
its highest permissible position, the mouth of _h_ comes above the
water, and communication is opened between the holder and the outer
atmosphere. No water enters the vent-pipe from the holder, and therefore
no gurgling or irregular pressure is produced. Another excellent
arrangement of a vent-pipe, suggested by Klinger of Gumpoldskirchen, is
shown at D, a drawing which has already been partly considered as a
washer and water-seal. For the present purpose the main vessel and its
various pipes are so dimensioned that the vertical height _g_ to
_f_ is always appreciably greater than the gas pressure in the
service or in the generator or gasholder to which it is connected. In
these circumstances the gas entering at _a_ depresses the water in
the pipe below the level _f_ to an extent equal to the pressure at
which it enters that pipe--an extent normally less than the distance
_f_ to _g_; and therefore gas never passes into the body of the
vessel, but travels away by the side tube _b_ (which in former
references to this drawing was supposed to be absent). If, however, the
pressure at _a_ exceeds that of the vertical height _f_ to
_g_, gas escapes at _g_ through the water, and is then free to
reach the atmosphere by means of the vent _c_. As before, _d_
serves to charge the apparatus with water, and _e_ to ensure a
proper amount being added. Clearly no liquid can enter the vent-pipe in
this device. Safety-valves such as are added to steam-boilers and the
like, which consist of a weighted lever holding a conical valve down
against its seat, are not required in acetylene apparatus, for the
simpler hydraulic seals discussed above can always be fitted wherever
they may be needed. It should be noticed that these vent-pipes only come
into operation in emergencies, when they are required to act promptly. No
economy is to be effected by making them small in diameter. For obvious
reasons the vent-pipe of a holder should have a diameter equal to that of
the gas-inlet tube, and the vent-pipe of a generator be equal in size to
the gas-leading tube.

FROTHING IN GENERATORS.--A very annoying trouble which crops up every now
and then during the evolution of acetylene consists in the production of
large masses of froth within the generator. In the ordinary way,
decomposition of carbide is accompanied by a species of effervescence,
but the bubbles should break smartly and leave the surface of the liquid
reasonably free from foam. Sometimes, however, the bubbles do not break,
but a persistent "head" of considerable height is formed. Further
production of gas only increases the thickness of the froth until it
rises so high that it is carried forward through the gas-main into the
next item of the plant. The froth disappears gradually in the pipes, but
leaves in them a deposit of lime which sooner or later causes
obstructions by accumulating at the angles and dips; while during its
presence in the main the steady passage of gas to the holder is
interrupted and the burners may even be made to jump. Manifestly the
defect is chiefly, if not always, to be noticed in the working of
carbide-to-water generators. The phenomenon has been examined by
Mauricheau-Beaupre, who finds that frothing is not characteristic of pure
carbide and that it cannot be attributed to any of the impurities
normally present in commercial carbide. If, however, the carbide contains
calcium chloride, frothing is liable to occur. A 0.1 per cent. solution
of calcium chloride appears to yield some foam when carbide is decomposed
in it, and a 1 per cent. solution to foam in a pronounced manner. In the
absence of calcium chloride, the main cause of frothing seems to be the
presence in the generator of new paint or tar. If a generator is taken
into use before the paint in any part of it which becomes moistened by
warm lime-water has had opportunity of drying thoroughly hard, frothing
is certain to occur; and even if the carbide has been stored for only a
short time in a tin or drum which has been freshly painted, a production
of froth will follow when it is decomposed in water. The products of the
polymerisation of acetylene also tend to produce frothing, but not to
such an extent as the turpentine in paint and the lighter constituents of
coal-tar. Carbide stored even temporarily in a newly painted tin froths
on decomposition because it has absorbed among its pores some of the
volatile matter given off by the paint during the process of desiccation.

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