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Page 24
In all these processes the action of the gas is impeded by the bulky
presence of its fellow constituent of air, nitrogen. We may say, for
instance, in homely phrase, that whenever a fire burns there are four
volumes of nitrogen tending to extinguish it for every volume of
oxygen supporting its combustion, and to the same degree the nitrogen
interferes with all other processes of atmospheric oxidation, of which
most metallurgical operations may be given as instances. If, then, it
has become possible to remove this diluent gas simply and cheaply in
order to give the oxygen free play in its various applications, we are
doubtless on the eve of a revolution among some of the most extensive
and familiar of the world's industries.
A series of chemical reactions has long been known by means of which
oxygen could be separated out of air in the laboratory, and at various
times processes based on these reactions have been patented for the
production of oxygen on a large scale. Until recently, however, none
of these methods gave sufficiently satisfactory results. The simplest
and perhaps the best of them was based on the fact first noticed by
Boussingault, that when baryta (BaO) is heated to low redness in a
current of air, it takes up oxygen and becomes barium dioxide
(BaO_{2}), and that this dioxide at a higher temperature is
reconverted into free oxygen and baryta, the latter being ready for
use again. For many years it was assumed, however, by chemists that
this ideally simple reaction was inapplicable on a commercial scale,
owing to the gradual loss of power to absorb oxygen which was always
found to take place in the baryta after a certain number of
operations. About eight years ago Messrs. A. & L. Brin, who had
studied chemistry under Boussingault, undertook experiments with the
view of determining why the baryta lost its power of absorbing oxygen.
They found that it was owing to molecular and physical changes caused
in it by impurities in the air used and by the high temperature
employed for decomposing the dioxide. They discovered that by heating
the dioxide in a partial vacuum the temperature necessary to drive off
its oxygen was much reduced. They also found that by supplying the air
to the baryta under a moderate pressure, its absorption of oxygen was
greatly assisted. Under these conditions, and by carefully purifying
the air before use, they found that it became possible to use the
baryta an indefinite number of times. Thus the process became
practically, as it was theoretically, continuous.
After securing patent protection for their process, Messrs. Brin
erected a small producer in Paris, and successfully worked it for
nearly three years without finding a renewal of the original charge of
baryta once necessary. This producer was exhibited at the Inventions
Exhibition in London, in 1885. Subsequently an English company was
formed, and in the autumn of last year Brin's Oxygen Company began
operations in Horseferry Road, Westminster, where a large and complete
demonstration plant was erected, and the work commenced of developing
the production and application of oxygen in the industrial world.
[Illustration: APPARATUS FOR MAKING OXYGEN.]
We give herewith details of the plant now working at Westminster. It
is exceedingly simple. On the left of the side elevation and plan are
shown the retorts, on the right is an arrangement of pumps for
alternately supplying air under pressure and exhausting the oxygen
from the retorts. As is shown in the plan, two sets of apparatus are
worked side by side at Westminster, the seventy-two retorts shown in
the drawings being divided into two systems of thirty-six. Each system
is fed by the two pumps on the corresponding side of the boiler. Each
set of retorts consists of six rows of six retorts each, one row above
the other. They are heated by a small Wilson's producer, so that the
attendant can easily regulate the supply of heat and obtain complete
control over the temperature of the retorts. The retorts, A, are made
of wrought iron and are about 10 ft long and 8 in. diameter.
Experience, however, goes to prove that there is a limit to the
diameter of the retorts beyond which the results become less
satisfactory. This limit is probably somewhat under 8 in. Each retort
is closely packed with baryta in lumps about the size of a walnut. The
baryta is a heavy grayish porous substance prepared by carefully
igniting the nitrate of barium; and of this each retort having the
above dimensions holds about 125 lb. The retorts so charged are closed
at each end by a gun metal lid riveted on so as to be air tight. From
the center of each lid a bent gun metal pipe, B, connects each retort
with the next of its series, so that air introduced into the end
retort of any row may pass through the whole series of six retorts.
Suppose now that the operations are to commence.
The retorts are first heated to a temperature of about 600� C. or
faint redness, then the air pumps, C C, are started. Air is drawn by
them through the purifier, D, where it is freed from carbon dioxide
and moisture by the layers of quicklime and caustic soda with which
the purifier is charged. The air is then forced along the pipe, E,
into the small air vessel, F, which acts as a sort of cushion to
prevent the baryta in the retorts being disturbed by the pulsation of
the pumps. From this vessel the air passes by the pipe, G, and is
distributed in the retorts as rapidly as possible at such a pressure
that the nitrogen which passes out unabsorbed at the outlet registers
about 15 lb. to the square inch. With the baryta so disposed in the
retorts as to present as large a superficies as possible to the action
of the air, it is found that in 1� to 2 hours--during which time about
12,000 cub. ft of air have been passed through the retorts--the gas at
the outlet fails to extinguish a glowing chip, indicating that oxygen
is no longer being absorbed. The pumping now ceases, and the
temperature of the retorts is raised to about 800� C. The workman is
able to judge the temperature with sufficient accuracy by means of the
small inspection holes, H, fitted with panes of mica, through which
the color of the heat in the furnace can be distinguished. The pumps
are now reversed and the process of exhaustion begins. At Westminster
the pressure in the retorts is reduced to about 1� in. of mercury. In
this partial vacuum the oxygen is given off rapidly, and if forced by
the pumps through another pipe and away into an ordinary gas holder,
where it is stored for use. With powerful pumps such as are used in
the plant under notice the whole of the oxygen can be drawn off in an
hour, and from one charge a yield of about 2,000 cub. ft. is obtained.
With a less perfect vacuum the time is longer--even as much as four
hours. The whole operation of charging and exhausting the retorts can
be completed in from three to four hours. As soon as the evolution of
oxygen is finished, the doors, K, and ventilators, L, may be opened
and the retorts cooled for recharging.
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