Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the recovery of ammonia from
underground storage caverns, for example caverns made in natural
salt deposits.
Because of the seasonal demand for ammonia and ammonia
products, for example in the fertilizer industry, manufacturing
capacity during off-seasons exceeds the rate of consumption. It
is economic to produce excess ammonia at these times and store
this excess for use during times of peak demand. Although under-
ground storage of ammonia is known, this method of storage has
not generally gained favour over use of above ground storage
tanks, some of which are large enough to hold 40,000 tons of
ammonia. Although many precautions are taken to provide safe
above ground storage of ammonia, such as storage in remote areas
on stable ground, there is a hazard of accidental release of
large quantities of ammonia to the atmosphere. Underground ;
storage reduces this hazard substantially. However, known pro-
cesses for the recovery of ammonia from underground caverns
have disadvantages in their need of elaborate heat exchange and
gas purification means. Some of the problems arise because the
underground caverns are made from natural salt deposits and the
salt contaminates the ammonia, particularly when it is withdrawn
from the cavern in liquid form.
In United States patent No. 2,732,334 ~Pollack),
gaseous ammonia is withdrawn from the region above the surface
of liquefied ammonia stored in an underground salt cavern. The
withdrawn gaseous ammonia is compressed without liquefying, and
is returned to the cavern where it is passed through a heat
exchange conduit immersed in the liquefied ammonia. The gaseous
ammonia condenses in the immersed conduit, giving up heat which
evaporates some of the stored liquid. The condensed ammonia
is then pumped by a submerged pump from the immersed con~duit
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to an above ground outlet product line. This process therefore
requires the presence of a heat exchange conduit and a pump sub-
merged in the liquefied ammonia in the cavern.
In one embodiment of the process described in United
States patent No. 2,713,775 (Cottle), an inert liquid, which is
denser than ammonia and has a higher boiling point, such as
pentane, is added to a salt cavern containing liquefied ammonia
to cover heat exchanger piping and a pump. Ammonia gas is removed
` from above the liquid ammonia, compressed and passed through the
submerged piping. ~eat given up to the pentane by the ammonia
-~ gas is, in turn, given up to the layer of liquid ammonia above -
it, some of which is thereby evaporated. The ammonia gas in -
the submerged piping condenses as it loses heat to the pentane
and is pumped to the surface, where it is separated from any en-
trained pentane liquid. In another embodiment, liquid ammonia
is removed from the cavern and passed through an evaporator in
which a pentane layer is used to separate dissolved salt. The
complications involved by the use of an inert liquid such as
pentane are readily apparent.
~ 20 In the process described in United States patent No.
;; 2,878,165 (Cottle),ammonia gas is pumped into a salt cavern to
move salt-containing liquid ammonia into an above ground puri-
~i fication system where salt is separated. United States patent
No. 2,901,403 (Adams) provides a process in which an inert gas, ~ -
such as off-gas from ammonia synthesis,is introduced into a
salt cavern containing liquid ammonia and dissolved salt with
sufficient force to lift the liquid into an above ground separa- :-
-tor,where the inert gas is separated and recycled. Ammonia is
then distilled from the salt solution. Again, the complicated
3 nature of these processes is readily apparent.
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- It is therefore an object of the invention to provide
a process for recovering ammonia from an underground cavern,
such as a salt cavern, in a relatively uncontaminated state
and with a less comp]icated withdrawal system than those pre-
viously known.
~; According to the present invention, gaseous ammonia
is heated to a temperature below that at which decomposition
t~- occurs. The heated gaseous ammonia is then fed into the lique-
fied ammonia in the cavern and released in the liquefied ammonia
to cause conversion of some of the liquid ammonia into gaseous
form. Ammonia gas thus formed is then withdrawn from the cavern.
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Such a process avoids the complications of the prior art and,
~0~ at the same time, enables ammonia gas of reasonable purity to
-~ be recovered from a cavern such as a salt cavern.
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The ammonia gas may be heated in any convenient
manner, for example by a simple gas-fired heater or by passage
through a heat exchanger provided with cycling heated fluid.
The heated gaseous ammonia is preferably sparged into the liquid
ammonia near the centre of the cavern, away from the walls,
and at sufficient depth to get good heat exchange by circulating
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~`~ the liquid ammonia.
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~ Ammonia gas withdrawn from the cavern is preferably
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passed through a mist eliminator which removes entrained liquid
~ droplets, such as droplets of ammonia-salt solution if a salt
-. cavern is used. Better droplet removal is obtained if a low
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'`t gas velocity is maintained in the mist eliminator. Ammonia
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~j~ gas containing less than 100 parts per million chloride may be
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*~ obtained in this way. A fine wire mesh, for example, stainless
steel, mist eliminator may be used. Such gas purity is accept-
able for direct application of the ammonia to soil as a fertilizer
or in such chemical processes as the manufacture of urea. Some
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further purification may be necessary on the entry of the
ammonia into a plant for the synthesis of nitric acid.
Advantageously, a portion of the ammonia gas with-
drawn from the cavern is supplied to the heating step, and
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another portion of the ammonia gas withdrawn from the cavern is
utilized elsewhere for whatever purpose it is required.
In the heating step, the gaseous ammonia is prefer-
-` ably heated to a temperature between about 260C. and about
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460C-
One embodiment of the present invention will now be -`
described, by way of example, with reference to the accompany-
ing drawing, which shows a schematic view of an underground
salt cavern in which ammonia is stored, together with means
; for recovering the ammonia therefrom.
`- Referring to the accompanying drawing, a sealed under-
ground salt cavern 10 is filled by means of supply pipe 12 with
liquid ammonia 14 to a depth preferably no~ exceeding 200 feet,
~ lea~ing a space 16 above the liquid ammonia for accumulation of
`~ gaseous ammonia. Stored ammonia 14 is kept at about 116 psig
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~ pressure at a ground temperature of about 18C. Gaseous ammonia
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is withdrawn through pipe 18, which passes through mist elimi-
nator 20 for removal of entrained droplets of ammonia-salt solu-
tion, accumulated liquid being withdrawn through drain 22. A
recycle portion passes to pump 24, while a product portion passes
to pump 26. The product portion passes through pipe 28 to a
cooler 30, where cooling water entering through inlet 32 lowers
the temperature from about 65C. to about 38C., and thence ~`
~-~ passes through line 34 to a consuming operation.
` The recycle portion passes through pipe 36 at about
212 psig pressure to heat exchanger 38, in which cycling, tempera-
ture-controlled fluid entering through inlet 40 heats the gaseous
ammonia to a temperature below that at which decomposition occurs.
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~eated gaseous ammonia is then returned through pipe 42 to the
~; storage cavern where it is released in the liquid ammonia 14
from a sparger 44. The sparger 44 is preferably located near
the centre of the cavern, away from its side walls, to minimize
side wall erosion as the gaseous ammonia is mixed with th~ fluid
ammonia, the sparger 44 being immersed at a depth that permits
circulation of enough of the liquid to ensure good heat exchange.
The gaseous ammonia thus produced rises to space 16 for with-
drawal through pipe 18.
Depending on the temperature of the heated gaseous
ammonia entering the cavern through pipe 42, about 30 to 45 per
~` cent of the evaporated ammonia can be recovered as product. In
~ one specific example, with gas heated to 427C., 234,000 pounds
per hour of gaseous ammonia were divided into a recovery stream
of 100,000 pounds per hour or 43 per cent of the evaporated
ammonia, and a recycle stream of 134,000 pounds per hour or
57 per cent of the evaporated ammonia. In another specific
~; example, with heating to 296C., 300,000 pounds per hour of
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gaseous ammonia were divided into recovery and recycle streams
of 100,000 and 200,000 pounds per hour or 33 per cent and 67
per cent, respectively. A simple mist eliminator in which drop-
- lets were retained on wire gauze decreased entrainment of
-; chloride in the gaseous ammonia to less than 100 parts per
million.
The described embodiment and specific examples ade-
quately illustrate the simplicity and efficiency of the inven- -
~ tion. Various other embodiments within the scope of the inven-
qj tion will be apparent to the man skilled in the art, the scope
of the invention being defined in the appended claims.
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