ACETYLENE

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ACETYLENE
In 1862 a chemist, Woehler, announced the discovery of the preparation
of
acetylene gas from calcium carbide, which he had made by heating to a
high temperature a mixture of charcoal with an alloy of zinc and
calcium.
His product would decompose water and yield the gas. For nearly thirty
years these substances were neglected, with the result that acetylene
was
practically unknown, and up to 1892 an acetylene flame was seen by very
few persons and its possibilities were not dreamed of. With the
development of the modern electric furnace the possibility of calcium
carbide as a commercial product became known.

In the above year, Thomas L. Willson, an electrical engineer of Spray
North Carolina, was experimenting in an attempt to prepare metallic
calcium, for which purpose he employed an electric furnace operating on
a
mixture of lime and coal tar with about ninety-five horse power. The
result was a molten mass which became hard and brittle when cool. This
apparently useless product was discarded and thrown in a nearby stream,
when, to the astonishment of onlookers, a large volume of gas was
immediately liberated, which, when ignited, burned with a bright and
smoky flame and gave off quantities of soot. The solid material proved
to
be calcium carbide and the gas acetylene.

Thus, through the incidental study of a by-product, and as the result of
an accident, the possibilities in carbide were made known, and in the
spring of 1895 the first factory in the world for the production of this
substance was established by the Willson Aluminum Company.
When water and calcium carbide are brought together an action takes
place
which results in the formation of acetylene gas and slaked lime.

CARBIDE
Calcium carbide is a chemical combination of the elements carbon and
calcium, being dark brown, black or gray with sometimes a blue or red
tinge. It looks like stone and will only burn when heated with oxygen.
Calcium carbide may be preserved for any length of time if protected
from
the air, but the ordinary moisture in the atmosphere gradually affects
it
until nothing remains but slaked lime. It always possesses a penetrating
odor, which is not due to the carbide itself but to the fact that it is
being constantly affected by moisture and producing small quantities of
acetylene gas.

This material is not readily dissolved by liquids, but if allowed to
come
in contact with water, a decomposition takes place with the evolution of
large quantities of gas. Carbide is not affected by shock, jarring or
age.

A pound of absolutely pure carbide will yield five and one-half cubic
feet of acetylene. Absolute purity cannot be attained commercially, and
in practice good carbide will produce from four and one-half to five
cubic feet for each pound used.

Carbide is prepared by fusing lime and carbon in the electric furnace
under a heat in excess of 6,000 degrees Fahrenheit. These materials are
among the most difficult to melt that are known. Lime is so infusible
that it is frequently employed for the materials of crucibles in which
the highest melting metals are fused, and for the pencils in the calcium
light because it will stand extremely high temperatures.

Carbon is the material employed in the manufacture of arc light
electrodes and other electrical appliances that must stand extreme heat.
Yet these two substances are forced into combination in the manufacture
of calcium carbide. It is the excessively high temperature attainable in
the electric furnace that causes this combination and not any effect of
the electricity other than the heat produced.

A mixture of ground coke and lime is introduced into the furnace through
which an electric arc has been drawn. The materials unite and form an
ingot of very pure carbide surrounded by a crust of less purity.

The poorer crust is rejected in breaking up the mass into lumps which
are
graded according to their size. The largest size is 2 by 3-1/2 inches
and
is called "lump," a medium size is 1/2 by 2 inches and is called "egg,"
an intermediate size for certain types of generators is 3/8 by 1-1/4
inches and called "nut," and the finely crushed pieces for use in still
other types of generators are 1/12 by 1/4 inch in size and are called
"quarter." Instructions as to the size best suited to different
generators are furnished by the makers of those instruments.

These sizes are packed in air-tight sheet steel drums containing 100
pounds each. The Union Carbide Company of Chicago and New York,
operating
under patents, manufactures and distributes the supply of calcium
carbide
for the entire United States. Plants for this manufacture are
established
at Niagara Falls, New York, and Sault Ste. Marie, Michigan. This company
maintains a system of warehouses in more than one hundred and ten
cities,
where large stocks of all sizes are carried.

The National Board of Fire Underwriters gives the following rules for
the
storage of carbide:
Calcium carbide in quantities not to exceed six hundred pounds may be
stored, when contained in approved metal packages not to exceed one
hundred pounds each, inside insured property, provided that the place of
storage be dry, waterproof and well ventilated and also provided that
all
but one of the packages in any one building shall be sealed and that
seals shall not be broken so long as there is carbide in excess of one
pound in any other unsealed package in the building.

Calcium carbide in quantities in excess of six hundred pounds must be
stored above ground in detached buildings, used exclusively for the
storage of calcium carbide, in approved metal packages, and such
buildings shall be constructed to be dry, waterproof and well
ventilated.
Properties of Acetylene.--This gas is composed of twenty-four parts of
carbon and two parts of hydrogen by weight and is classed with natural
gas, petroleum, etc., as one of the hydrocarbons. This gas contains the
highest percentage of carbon known to exist in any combination of this
form and it may therefore be considered as gaseous carbon. Carbon is the
fuel that is used in all forms of combustion and is present in all fuels
from whatever source or in whatever form. Acetylene is therefore the
most
powerful of all fuel gases and is able to give to the torch flame in
welding the highest temperature of any flame.

Acetylene is a colorless and tasteless gas, possessed of a peculiar and
penetrating odor. The least trace in the air of a room is easily
noticed,
and if this odor is detected about an apparatus in operation, it is
certain to indicate a leakage of gas through faulty piping, open valves,
broken hose or otherwise. This leakage must be prevented before
proceeding with the work to be done.

All gases which burn in air will, when mixed with air previous to
ignition, produce more or less violent explosions, if fired. To this
rule acetylene is no exception. One measure of acetylene and twelve
and one-half of air are required for complete combustion; this is
therefore the proportion for the most perfect explosion. This is not the
only possible mixture that will explode, for all proportions from three
to thirty per cent of acetylene in air will explode with more or less
force if ignited.

The igniting point of acetylene is lower than that of coal gas, being
about 900 degrees Fahrenheit as against eleven hundred degrees for coal
gas. The gas issuing from a torch will ignite if allowed to play on the
tip of a lighted cigar.

It is still further true that acetylene, at some pressures, greater than
normal, has under most favorable conditions for the effect, been found
to
explode; yet it may be stated with perfect confidence that under no
circumstances has anyone ever secured an explosion in it when subjected
to pressures not exceeding fifteen pounds to the square inch.

Although not exploded by the application of high heat, acetylene is
injured by such treatment. It is partly converted, by high heat, into
other compounds, thus lessening the actual quantity of the gas, wasting
it and polluting the rest by the introduction of substances which do not
belong there. These compounds remain in part with the gas, causing it to
burn with a persistent smoky flame and with the deposit of objectionable
tarry substances. Where the gas is generated without undue rise of
temperature these difficulties are avoided.

Purification of Acetylene.--Impurities in this gas are caused by
impurities in the calcium carbide from which it is made or by improper
methods and lack of care in generation. Impurities from the material
will
be considered first.

Impurities in the carbide may be further divided into two classes: those
which exert no action on water and those which act with the water to
throw off other gaseous products which remain in the acetylene. Those
impurities which exert no action on the water consist of coke that has
not been changed in the furnace and sand and some other substances which
are harmless except that they increase the ash left after the acetylene
has been generated.

An analysis of the gas coming from a typical generator is as follows:

Per cent
Acetylene ................................ 99.36
Oxygen ................................... .08
Nitrogen ................................. .11
Hydrogen ................................. .06
Sulphuretted Hydrogen .................... .17
Phosphoretted Hydrogen ................... .04
Ammonia .................................. .10
Silicon Hydride .......................... .03
Carbon Monoxide .......................... .01
Methane .................................. .04

The oxygen, nitrogen, hydrogen, methane and carbon monoxide are either
harmless or are present in such small quantities as to be neglected. The
phosphoretted hydrogen and silicon hydride are self-inflammable gases
when exposed to the air, but their quantity is so very small that this
possibility may be dismissed. The ammonia and sulphuretted hydrogen are
almost entirely dissolved by the water used in the gas generator. The
surest way to avoid impure gas is to use high-grade calcium carbide in
the generator and the carbide of American manufacture is now so pure
that
it never causes trouble.

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Warmly,
Pat Mitchell