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

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40

THE "DRY" PROCESS OF GENERATION.--A process for generating acetylene,
totally different in principle from those hitherto considered, has been
introduced in this country. According to the original patents of G. J.
Atkins, the process consisted in bringing small or powdered carbide into
mechanical contact with some solid material containing water, the water
being either mixed with the solid reagent or attached to it as water of
crystallisation. Such reagents indeed were claimed as crude starch and
the like, the idea being to recover a by-product of pecuniary value. Now
the process seems to be known only in that particular form in which
granulated carbide is treated with crystallised sodium carbonate,
_i.e._, common washing soda. Assuming the carbide employed to be
chemically pure and the reaction between it and the water of
crystallisation contained in ordinary soda crystals to proceed
quantitatively, the production of acetylene by the dry process should be
represented by the following chemical equation:

5CaC_2 + Na_2CO_3.10H_2O = 5C_2H_2 + 5Ca(OH)_2 + Na_2CO_3.

On calculating out the molecular weights, it will be seen that 286 parts
of washing soda should suffice for the decomposition of 320 parts of pure
calcium carbide, or in round numbers 9 parts of soda should decompose 10
parts of carbide. In practice, however, it seems to be found that from 1
to 1.5 parts of soda are needed for every part of carbide.

The apparatus employed is a metal drum supported on a hollow horizontal
spindle, one end of which is closed and carries a winch handle, and the
other end of which serves to withdraw the gas generated in the plant. The
drum is divided into three compartments by means of two vertical
partitions so designed that when rotation proceeds in one particular
direction portions of the two reagents stored in one end compartment pass
into the centre compartment; whereas when rotation proceeds in the
opposite direction, the material in the centre compartment is merely
mixed together, partly by the revolution of the drum, partly with the
assistance of a stationary agitator slung loosely from the central
spindle. The other end compartment contains coke or sawdust or other dry
material through which the gas passes for the removal of lime or other
dust carried in suspension as it issues from the generating compartment.
The gas then passes through perforations into the central spindle, one
end of which is connected by a packed joint with a fixed pipe, which
leads to a seal or washer containing petroleum. Approached from a
theoretical standpoint, it will be seen that this method of generation
entirely sacrifices the advantages otherwise accruing from the use of
liquid water as a means for dissipating the heat of the chemical
reaction, but on the other hand, inasmuch as the substances are both
solid, the reaction presumably occurs more slowly than it would in the
presence of liquid water; and moreover the fact that the water employed
to act upon the carbide is in the solid state and also more or less
combined with the rest of the sodium carbonate molecule, means that, per
unit of weight, the water decomposed must render latent a larger amount
of heat than it would were it liquid. Experiments made by one of the
authors of this book tend to show that the gas evolved from carbide by
the dry process contains rather less phosphorus than it might in other
conditions of generation, and as a fact gas made by the dry process is
ordinarily consumed without previous passage through any chemical
purifying agent. It is obvious, however, that the use of the churn
described above greatly increases the labour attached to the production
of the gas; while it is not clear that the yield per unit weight of
carbide decomposed should be as high as that obtained in wet generation.
The inventor has claimed that his by-product should be valuable and
saleable, apparently partly on the ground that it should contain caustic
soda. Evidence, however, that a reaction between the calcium oxide or
hydroxide and the sodium carbonate takes place in the prevailing
conditions is not yet forthcoming, and the probabilities are that such
decomposition would not occur unless the residue were largely diluted
with water. [Footnote: The oldest process employed for manufacturing
caustic soda consisted in mixing a solution of sodium carbonate with
quick or slaked lime, and it has been well established that the
causticisation of the soda will not proceed when the concentration of the
liquid is greater than that corresponding with a specific gravity of
about 1-10, _i.e._, when the liquid contains more than some 8 to 10
per cent, of sodium hydroxide.] Conversely there are some grounds for
believing that the dry residue is less useful than an ordinary wet
residue for horticultural purposes, and also for the production of
whitewash. From a financial standpoint, the dry process suffers owing to
the expense involved in the purchase of a second raw material, for which
but little compensation can be discovered unless it is proved that the
residue is intrinsically more valuable than common acetylene-lime and can
be sold or used advantageously by the ordinary owner of an installation.
The discarding of the chemical purifier at the present day is a move of
which the advantage may well be overrated.

ARTIFICIAL LIGHTING OF GENERATOR SHEDS.--It has already been argued that
all normal or abnormal operations in connexion with an acetylene
generating plant should be carried out, if possible, by daylight; and it
has been shown that on no account must a naked light ever be taken inside
the house containing such a plant. It will occasionally happen, however,
that the installation must be recharged or inspected after nightfall. In
order to do this in safety, a double window, incapable of being opened,
should be fitted in one wall of the house, as far as possible from the
door, and in such a position that the light may fall on to all the
necessary places. Outside this window may be suspended an ordinary hand-
lantern burning oil or paraffin; or, preferably, round this window may be
built a closed lantern into which some source of artificial light may be
brought. If the acetylene plant has an isolated holder of considerable
size, there is no reason at all why a connexion should not be made with
the service-pipes, and an acetylene flame be used inside this lantern;
but with generators of the automatic variety, an acetylene light is not
so suitable, because of the fear that gas may not be available precisely
at the moment when it is necessary to have light in the shed. It would,
however, be a simple matter to erect an acetylene burner inside the
lantern in such a way that when needed an oil-lamp or candle could be
used instead. Artificial internal light of any kind is best avoided; the
only kind permissible being an electric glow-lamp. If this is employed,
it should be surrounded by a second bulb or gas-tight glass jacket, and
preferably by a wire cage as well; the wires leading to it must be
carefully insulated, and all switches or cut-outs (which may produce a
spark) must be out of doors. The well-known Davy safety or miner's lamp
is not a trustworthy instrument for use with acetylene because of
(_a_) the low igniting-point of acetylene; (_b_) the high
temperature of its flame; and (_c_) the enormous speed at which the
explosive wave travels through a mixture of acetylene and air. For these
reasons the metallic gauze of the Davy lamp is not so efficient a
protector of the flame as it is in cases of coal-gas, methane, &c.
Moreover, in practice, the Davy lamp gives a poor light, and unless in
constant use is liable to be found out of order when required. It should,
however, be added that modern forms of the safety lamp, in which the
light is surrounded by a stout glass chimney and only sufficient gauze is
used for the admission of fresh air and for the escape of the combustion
products, appear quite satisfactory when employed in an atmosphere
containing some free acetylene.

CHAPTER IV

THE SELECTION OF AN ACETYLENE GENERATOR

In Chapter II. an attempt has been made to explain the physical and
chemical phenomena which accompany the interaction of calcium carbide and
water, and to show what features in the reaction are useful and what
inconvenient in the evolution of acetylene on a domestic or larger scale.
Similarly in Chapter III. have been described the various typical devices
which may be employed in the construction of different portions of
acetylene plant, so that the gas may be generated and stored under the
best conditions, whether it is evolved by the automatic or by the non-
automatic system. This having been done, it seemed of doubtful utility to
include in the first edition of this work a long series of illustrations
of such generators as had been placed on the markets by British, French,
German, and American makers. It would have been difficult within
reasonable limits to have reproduced diagrams of all the generators that
had been offered for sale, and absolutely impossible within the limits of
a single hand-book to picture those which had been suggested or patented.
Moreover, some generating apparatus appeared on the market ephemerally;
some was constantly being modified in detail so as to alter parts which
experience or greater knowledge had shown the makers to be in need of
alteration, while other new apparatus was constantly being brought out.
On these and other grounds it did not appear that much good purpose would
have been served by describing the particular apparatus which at that
time would have been offered to prospective purchasers. It seemed best
that the latter should estimate the value and trustworthiness of
apparatus by studying a section of it in the light of the general
principles of construction of a satisfactory generator as enunciated in
the book. While the position thus taken by the authors in 1903 would
still not be incorrect, it has been represented to them that it would
scarcely be inconsistent with it to give brief descriptions of some of
the generators which are now being sold in Great Britain and a few other
countries. Six more years' experience in the design and manufacture of
acetylene plant has enabled the older firms of manufacturers to fix upon
certain standard patterns for their apparatus, and it may confidently be
anticipated that many of these will survive a longer period. Faulty
devices and designs have been weeded out, and there are lessons of the
past as well as theoretical considerations to guide the inventor of a new
type of generator. On those grounds, therefore, an attempt has now been
made to give brief descriptions, with sectional views, of a number of the
generators now on the market in Great Britain. Moreover, as the first
edition of this book found many readers in other countries, in several of
which there is greater scope for the use of acetylene, it has been
decided to describe also a few typical or widely used foreign generators.
All the generators described must stand or fall on their merits, which
cannot be affected by any opinion expressed by the authors. In the
descriptions, which in the first instance have generally been furnished
by the manufacturers of the apparatus, no attempt has therefore been made
to appraise the particular generators, and comparisons and eulogistic
comments have been excluded. The descriptions, however, would
nevertheless have been somewhat out of place in the body of this book;
they have therefore been relegated to a special Appendix. It has, of
course, been impossible to include the generators of all even of the
English manufacturers, and doubtless many trustworthy ones have remained
unnoticed. Many firms also make other types of generators in addition to
those described. It must not be assumed that because a particular make of
generator is not mentioned it is necessarily faulty. The apparatus
described may be regarded as typical or well known, and workable, but it
is not by reason of its inclusion vouched for in any other respect by the
authors. The Appendix is intended, not to bias or modify the judgment of
the would-be purchaser of a generator, but merely to assist him in
ascertaining what generators there are now on the market.

The observations on the selection of a generator which follow, as well as
any references in other chapters to the same matter, have been made
without regard to particular apparatus of which a description may (or may
not) appear in the Appendix. With this premise, it may be stated that the
intending purchaser should regard the mechanism of a generator as shown
in a sectional view or on inspection of the apparatus itself. If the
generator is simple in construction, he should be able to understand its
method of working at a glance, and by referring it to the type
(_vide_ Chapter III.) to which it belongs, be able to appraise its
utility from a chemical and physical aspect from what has already been
said. If the generator is too complicated for ready understanding of its
mode of working, it is not unlikely to prove too complicated to behave
well in practice. Not less important than the mechanism of a generator is
good construction from the mechanical point of view, _i.e._, whether
stout metal has been employed, whether the seams and joints are well
finished, and whether the whole apparatus has been built in the workman-
like fashion which alone can give satisfaction in any kind of plant.
Bearing these points in mind, the intending purchaser may find assistance
in estimating the mechanical value of an apparatus by perusing the
remainder of this chapter, which will be devoted to elaborating at length
the so-called scientific principles underlying the construction of a
satisfactory generator, and to giving information on the mechanical and
practical points involved.

It is perhaps desirable to remark that there is scarcely any feature in
the generation of acetylene from calcium carbide and water--certainly no
important feature--which introduces into practice principles not already
known to chemists and engineers. Once the gas is set free it ranks simply
as an inflammable, moisture-laden, somewhat impure, illuminating and
heat-giving gas, which has to be dried, purified, stored, and led to the
place of combustion; it is in this respect precisely analogous to coal-
gas. Even the actual generation is only an exothermic, or heat-producing,
reaction between a solid and a liquid, in which rise of temperature and
pressure must be prevented as far as possible. Accordingly there is no
fundamental or indispensable portion of an acetylene apparatus which
lends itself to the protection of the patent laws; and even the details
(it may be said truthfully, if somewhat cynically) stand in patentability
in inverse ratio to their simplicity and utility.

During the early part of 1901 a Committee appointed by the British Home
Office, "to advise as to the conditions of safety to which acetylene
generators should conform, and to carry out tests of generators in the
market in order to ascertain how far those conform with such conditions,"
issued a circular to the trade suggesting that apparatus should be sent
them for examination. In response, forty-six British generators were
submitted for trial, and were examined in a fashion which somewhat
exceeded the instructions given to the Committee, who finally reported to
the Explosives Department of the Home Office in a Blue Book, No. Cd. 952,
which can be purchased through any bookseller. This report comprises an
appendix in which most of the apparatus are illustrated, and it includes
the result of the particular test which the Committee decided to apply.
Qualitatively the test was useful, as it was identical in all instances,
and only lacks full utility inasmuch as the trustworthiness of the
automatic mechanism applied to such generators as were intended to work
on the automatic system was not estimated. Naturally, a complete
valuation of the efficiency of automatic mechanism cannot be obtained
from one or even several tests, it demands long-continued watching; but a
general notion of reliability might have been obtained. Quantitatively,
however, the test applied by the Committee is not so free from reproach,
for, from the information given, it would appear to have been less fair
to some makers of apparatus than to others. Nevertheless the report is
valuable, and indicates the general character of the most important
apparatus which were being offered for sale in the United Kingdom in
1900-1901.

It is not possible to give a direct answer to the question as to which is
the best type of acetylene generator. There are no generators made by
responsible firms at the present time which are not safe. Some may be
easier to charge and clean than others; some require more frequent
attention than others; some have moving parts less likely to fail, when
handled carelessly, than others; some have no moving mechanism to fail.
For the illumination of a large institution or district where one man can
be fully occupied in attending to the plant, cleaning, lighting, and
extinguishing the lamps, or where other work can be found for him so as
to leave him an hour or so every day to look after the apparatus, the
hand-fed carbide-to-water generator L (Fig. 6) has many advantages, and
is probably the best of all. In smaller installations choice must be made
first between the automatic and the non-automatic principle--the
advantages most frequently lying with the latter. If a non-automatic
generator is decided upon, the hand carbide-feed or the flooded-
compartment apparatus is almost equally good; and if automatism is
desired, either a flooded-compartment machine or one of the most
trustworthy types of carbide-feed apparatus may be taken. There are
contact apparatus on the markets which appear never to have given
trouble, and those are worthy of attention. Some builders advocate their
own apparatus because the residue is solid and not a cream. If there is
any advantage in this arising from greater ease in cleaning and
recharging the generator and in disposing of the waste, that advantage is
usually neutralised by the fear that the carbide may not have been wholly
decomposed within the apparatus; and whereas any danger arising from
imperfectly spent carbide being thrown into a closed drain may be
prevented by flooding the residue with plenty of water in an open vessel,
imperfect decomposition in the generator means a deficiency in the amount
of gas evolved from a unit weight of solid taken or purchased. In fact,
setting on one side apparatus which belong to a notoriously defective
system and such as are constructed in large sizes on a system that is
only free from overheating, &c., in small sizes; setting aside all
generators which are provided with only one decomposing chamber when they
are of a capacity to require two or more smaller ones that can more
efficiently be cooled with water jackets; and setting aside any form of
plant which on examination is likely to exhibit any of the more serious
objections indicated in this and the previous chapters, there is
comparatively little to choose, from the chemical and physical points of
view, between the different types of generators now on the markets. A
selection may rather be made on mechanical grounds. The generator must be
well able to produce gas as rapidly as it will ever be required during
the longest or coldest evening; it must be so large that several more
brackets or burners can be added to the service after the installation is
complete. It must be so strong that it will bear careless handling and
the frequent rough manipulation of its parts. It must be built of stout
enough material not to rust out in a few years. Each and all of its parts
must be accessible and its exterior visible. Its pipes, both for gas and
sludge, must be of large bore (say 1 inch), and fitted at every dip with
an arrangement for withdrawing into some closed vessel the moisture, &c.,
that may condense. The number of cocks, valves, and moving parts must be
reduced to a minimum; cocks which require to be shut by hand before
recharging must give way to water-seals. It must be simple in all its
parts, and its action intelligible at a glance. It must be easy to
charge--preferably even by the sense of touch in darkness. It must be
easy to clean. The waste lime must be easily removed. It must be so
fitted with vent-pipes that the pressure can never rise above that at
which it is supposed to work. Nevertheless, a generator in which these
vent-pipes are often brought into use is badly constructed and wasteful,
and must be avoided. The water of the holder seal should be distinct from
that used for decomposing the carbide; and those apparatus where the
holder is entirely separated from the generator are preferable to such as
are built all in one, even if water-seals are fitted to prevent return of
gas. Apparatus which is supposed to be automatic should be made perfectly
automatic, the water or the carbide-feed being locked automatically
before the carbide store, the decomposing chamber, or the sludge-cock can
be opened. The generating chamber must always be in communication with
the atmosphere through a water-sealed vent-pipe, the seal of which, if
necessary, the gas can blow at any time. All apparatus should be fitted
with rising holders, the larger the better. Duplicate copies of printed
instructions should be demanded of the maker, one copy being kept in the
generator-house, and the other elsewhere for reference in emergencies.
These instructions must give simple and precise information as to what
should be done in the event of a breakdown as well as in the normal
manipulation of the plant. Technical expressions and descriptions of
parts understood only by the maker must be absent from these rules.

ADDENDUM.

BRITISH AND FOREIGN REGULATIONS FOR THE CONSTRUCTION AND INSTALLATION OF
ACETYLENE GENERATING PLANT

Dealing with the "conditions which a generator should fulfil before it
can be considered as being safe," the HOME OFFICE COMMITTEE of 1901
before mentioned write as follows:

1. The temperature in any part of the generator, when run at the maximum
rate for which it is designed, for a prolonged period, should not exceed
130 deg. C. This may be ascertained by placing short lengths of wire,
drawn from fusible metal, in those parts of the apparatus in which heat
is liable to be generated.

2. The generator should have an efficiency of not less than 90 per cent.,
which, with carbide yielding 5 cubic feet per pound, would imply a yield
of 4.5 cubic feet for each pound of carbide used.

3. The size of the pipes carrying the gas should be proportioned to the
maximum rate of generation, so that undue back pressure from throttling
may not occur.

4. The carbide should be completely decomposed in the apparatus, so that
lime sludge discharged from the generator shall not be capable of
generating more gas.

5. The pressure in any part of the apparatus, on the generator side of
the holder, should not exceed that of 20 inches of water, and on the
service side of same, or where no gasholder is provided, should not
exceed that of 5 inches of water.

6. The apparatus should give no tarry or other heavy condensation
products from the decomposition of the carbide.

7. In the use of a generator regard should be had to the danger of
stoppage of passage of the gas and resulting increase of pressure which
may arise from the freezing of the water. Where freezing may be
anticipated, steps should be taken to prevent it.

8. The apparatus should be so constructed that no lime sludge can gain
access to any pipes intended for the passage of gas or circulation of
water.

9. The use of glass gauges should be avoided as far as possible, and,
where absolutely necessary, they should be effectively protected against
breakage.

10. The air space in a generator before charging should be as small as
possible.

11. The use of copper should be avoided in such parts of the apparatus as
are liable to come in contact with acetylene.

The BRITISH ACETYLENE ASSOCIATION has drawn up the following list of
regulations which, it suggests, shall govern the construction of
generators and the installation of piping and fittings:

1. Generators shall be so constructed that, when used in accordance with
printed instructions, it shall not he possible for any undecomposed
carbide to remain in the sludge removed therefrom.

2. The limit of pressure in any part of the generator shall not exceed
that of 20 inches of water, subject to the exception that if it be shown
to the satisfaction of the Executive of the Acetylene Association that
higher pressures up to 50 inches of water are necessary in certain
generators, and are without danger, the Executive may, with the approval
of the Home Office, grant exemption for such generators, with or without
conditions.

3. The limit of pressure in service-pipes, within the house, shall not
exceed 10 inches of water.

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40