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Патент USA US3094412

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June 18, 1963
M. ARMSTRONG ETAL
3,094,402
PROCESS FOR THE SEPARATION OF ‘AIR
Filed Oct. 17, 1961
WASTE
AIR
NITROGEN
m
‘
K
m
TO
COLUMN
INVENTORS
KEANE TH CECIL 561(7)?
“J Mlcrm? 412113710110‘
A'F ORNEY
States Patent G ”
3,094,402
Patented June 18, 1963
1
2
3,094,402
erably, the expanded major and minor streams are recom_
bined prior to recti?cation.
h‘iichael Armstrong, Chertsey, and Kenneth Cecil Smith,
Carshalton, England, assignors to The British ()xygen
The relatively high pressure to‘ which the air is initially
compressed will usually be about 150 atma., the recti?ca
PRQCESS FDR THE SEPARATION OF AlR
Company Limited, a British company
Filed Oct. 17, 1961, Ser. No. 145,560
Claims priority, application Great Britain Get. 17, 1%!)
tion pressure about 5 atma. and the intermediate pressure
between about 8 and 50 atma., for example, about 18
atma. The major stream will usually comprise about
60% of the total air ‘and the minor stream remaining
40%. The optimum quantity of the major stream to be
This invention relates to the separation of air by lique 10 warmed in the main heat exchanger Will depend on the
faction and subsequent recti?cation.
intermediate pressure used. For an intermediate pressure
The separation of air into oxygen and nitrogen to
of 18 atma., about 18% should be so warmed.
produce a substantial proportion (for example, about
One example of the process of the invention will now
20%) of the oxygen output in the liquid phase is usually
be described in more detail with reference to the accom
carried out in plants in which the air is cooled while 15 panying drawing which shows diagrammatically a ?ow
6 Claims. (Cl. 62-9)
compressed and then expanded so that a considerable
degree of liquefaction takes place, the lique?ed air then
being recti?ed in a recti?cation zone. Various methods
are available by which the separation can be carried out
sheet of the process.
100 volumes of air compressed to 150 atma. and cooled
to 280° K. by any suitable means (not shown) are passed
through a precooler 1 where they are cooled to a tem
at fair thermodynamic e?'iciency, one of these being the 20 perature of 250° K. by heat exchange with a gaseous
Heylandt cycle.
nitrogen product.
In this cycle, air compressed to a relatively high pres~
sure of about 170 atma. is precooled by heat exchange
with a separated gaseous nitrogen fraction and is then
divided into two stream, a major stream consisting of
The ‘air stream leaving the precooler 1 is split, 38.7
volumes being passed through a main heat exchanger 2
as hereinafter described. The remaining 61.3 volumes of
the air are passed through an expansion engine 3 where
about 60% of the air and a minor stream consisting of
they are expanded to a pressure of 18 atma. with the
the remaining 40%. The minor stream is cooled by
passage through a main heat exchanger countercurrent
performance of external work. The temperature of the
air leaving the expansion engine 3 is 144.3" K. 10.94
to the separated gaseous nitrogen fraction on its way to
volumes of the expanded air leaving the engine 3 are
the precooler, and is then expanded to the recti?cation 30 passed through a separate path in the main heat exchanger
pressure of about 5 atma. through an expansion valve.
The major stream is expanded to the recti?cation pres
path does not pass completely through the exchanger 2
sure through an expansion engine with the performance
but is limited to an intermediate central section thereof.
2 countercurrent to the compressed air stream.
This
of external work. The two streams are then re-combined
‘The temperature of the air sub-stream passed through
and the combined stream fed to the recti?cation Zone. 35 this path rises from 144.3° K. at ‘point F to 182.0° K.
The Heylandt cycle, by recovering some of the energy
at point E. This war-med air stream is then recombined
with the remainder of the air leaving the expansion
of power ‘and is therefore more e?cient than cycles in
engine 3, the temperature of the combined stream being
which no Work is performed by the gas.
150.7° K. The combined stream is passed through a
40
In the Heylandt cycle, it is impossible to operate the
second expansion engine 4, where its pressure is reduced
present in the compressed gas, saves a certain amount
main heat exchanger in such a manner that the theoretical
to the recti?cation pressure of 5.5 atma., and its tempera
maximum quantity of energy is recovered. It has been
calculated that for maximum e?iciency the temperature
‘difference between the streams in heat exchange should
ture to 110.4” K.
be zero everywhere in the heat exchange .
It is impos
sible in practice to achieve this end, with the result that
‘a certain amount of refrigeration is Wasted.
It is an object of this invention to provide a modi?ca
tion of the Heylandt cycle in which use is made of some
of this wasted refrigeration with a consequent reduction 50
in power consumption.
According to the invention, a process for the separa
tion of air by liquefaction and subsequent recti?cation
comprises precooling air compressed to a relatively high
pressure by heat exchange with a gaseous nitrogen product,
dividing the precooled air into a major stream and a
minor stream, cooling the minor stream in a main heat
exchanger by heat exchange with the gaseous nitrogen
product passing to the precooling step, expanding the
The 38.7 volumes of the air which enter the main heat
exchanger 2 at a temperature of 250° K. emerge from it
at a temperature of 97.7° K., the pressure still being 150
atma. The intermediate temperature at points M and N
(corresponding to points E and F in the reheat section)
are l95.5° K. and 153.3” K. respectively. After leaving
the heat exchanger 2, this air stream is expanded to 5.5
atma. through an expansion valve 5.
The air stream leaving the valve 5 and that leaving
the expansion engine 4 are then combined, and the com
bined stream consisting of a mixture of liquid and vapour,
is then passed to a conventional separation column (not
shown) wherein it is separated to obtain 19.38 volumes of
liquid oxygen supercooled by 1l.9° K., 0.42 volume of
liquid ‘argon and 80.2 volumes of gaseous “waste” nitrogen
(i.e. nitrogen containing small amounts of oxygen and
cooled minor stream to recti?cation pressure, expanding
argon and very small amounts of other inert gases).
The waste nitrogen at a temperature of 84.4° K. and at
the major stream to an intermediate pressure in an expan
atmospheric pressure is passed through the main exchanger
sion machine with the performance of external work,
2 countercurrent to the compressed air stream, leaving the
warming a minor part of the expanded major stream
exchanger at a temperature of 228.2° K. The temperature
by passing it through an intermediate section of the main
at intermediate points Q and R, corresponding to points
heat exchanger countercurrent to compressed air passing 65 N and F and to points M and B respectively are 144.3 ° K.
through the heat exchanger, combining the warmed part
and 187.1° K. respectively. The waste nitrogen is then
with the remainder of the major stream, further expand
passed through the precooler 1 where its temperature
ing the re-united major stream to the recti?cation pres
rises to 278° K.
sure in an expansion machine with the performance of
The power consumption of the process will depend to
external work, and subjecting the expanded major and 70 some extent on the e?‘iciency of the expansion engines 3
minor streams to recti?cation in a recti?cation zone. Pref
and 4 but it would be impossible to obtain the same
3,094,402
4
amounts ‘of supercooled liquid oyxgen and liquid argon
from a ‘conventional Heylandt cycle with a starting pres
1. Process for "the separation of air by liquefaction and
subsequent recti?cation comprising preoooling air com
3. Process according to claim 1 wherein the major
stream comprises about 60% by volume of the total air.
4. Process according to claim 1 wherein the relatively
high pressure to which the air is initially compressed is
about 150 atrna., the recti?cation pressure is about 5 atma.
and the intermediate pressure is between about 8 and 50
pressed to a relatively high pressure by heat exchange
atma.
sure of only 150‘ atma.
I claim:
‘ with a gaseous nitrogen product, dividing the precooled
‘air into a major stream and a minor stream, cooling said
5. Process according to claim 4 wherein the interme
diate pressure is about 18 atma.
minor stream in a main heat exchanger by heat exchange 1O
6. Process according to claim 5 wherein the amount of
‘with the gaseous ‘nitrogen product passing to the pre
the major stream warmed in the main heat exchanger
cooling step, expanding the cooled minor stream to the
is about 18% by volume.
recti?cation pressure, expanding said major stream to an
intermediate pressure in an expansion machine with the
References Cited in the ?le of this patent
performance of external work, warming a minor part 15
UNITED sTATEs PATENTS
of the expanded major stream by passing it through an
intermediate secticn'of the main heat exchanger counter
Re. 19,267
Van Nuys _____________ __ Aug. 7, 1934
current to compressed air passing through said heat
967,104
Claude _______________ __ Aug. 9, 1910
exchanger, combining the warmed part with the re
1,901,389
Hazard-Plamand ______ _._ Mar. 14, 1933
mainder of said major air stream, further expanding the 20
2,078,953
Levin ________________ __ May 4, 1937
re-united major air stream to the recti?cation pressure
2,645,103
Fausek _______________ __‘ July 14, 1953
in an expansion machine with the performance of ex
ternal work, and subjecting said expanded major and
FOREIGN PATENTS
a minor streams to recti?cation in a recti?cation zone.
2. Process according to claim 1 ‘wherein said expanded 25
major and minor streams are combined prior to recti
?cation.
721,841
826,298
831,613
Germany ____________ __ June 19', 1942
Great Britain _________ __ Dec. 31, 1959
Great Britain _________ __ Mar. 30, 1960
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