U.S. patent number 3,754,406 [Application Number 05/124,253] was granted by the patent office on 1973-08-28 for the production of oxygen.
This patent grant is currently assigned to Air Products and Chemicals, Inc.. Invention is credited to Rodney John Allam.
United States Patent |
3,754,406 |
Allam |
August 28, 1973 |
THE PRODUCTION OF OXYGEN
Abstract
A plant for the production of low purity oxygen, in which a low
pressure stream of incoming air is cooled against outgoing gas
streams and fed into a high pressure fractionating column, and a
high pressure stream of incoming air is cooled against outgoing gas
streams, partially condensed against boiling liquid oxygen product
in a product vaporizer, and separated into gas and liquid streams,
the liquid stream being sub-cooled and expanded into a low pressure
fractionating column while a major part of the gas stream is
re-heated and expanded to provide plant refrigeration. Crude liquid
oxygen from the bottom of the high pressure column is cooled
against waste outgoing nitrogen from the low pressure column and
then admitted to the low pressure column after first being used to
liquefy some of the nitrogen from the high pressure column in an
external reboiler/condenser. Liquid oxygen product from the low
pressure column is pumped to a higher pressure before being passed
through the sub-cooler and the product vaporizer. The remainder of
the high pressure column nitrogen is liquefied in a second external
reboiler/condenser by the separated high pressure liquid feed on
its way to the low pressure column. The liquefied high pressure
column nitrogen is used as reflux for the two columns, that for the
low pressure column being cooled against outgoing waste nitrogen.
The expander exhaust is likewise cooled against outgoing waste
nitrogen before admission to the low pressure column.
Inventors: |
Allam; Rodney John (Guildford,
Surrey, EN) |
Assignee: |
Air Products and Chemicals,
Inc. (Allentown, PA)
|
Family
ID: |
10007366 |
Appl.
No.: |
05/124,253 |
Filed: |
March 15, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Mar 16, 1970 [GB] |
|
|
12,586/70 |
|
Current U.S.
Class: |
62/646;
62/654 |
Current CPC
Class: |
F25J
3/04303 (20130101); F25J 3/0486 (20130101); F25J
3/04218 (20130101); F25J 3/04884 (20130101); F25J
3/04424 (20130101); F25J 3/04206 (20130101); F25J
3/0409 (20130101); F25J 2205/24 (20130101); F25J
2245/40 (20130101); F25J 2205/60 (20130101); F25J
2250/50 (20130101); F25J 2250/40 (20130101); F25J
2245/50 (20130101); F25J 2235/02 (20130101); F25J
2250/02 (20130101); F25J 2235/50 (20130101); F25J
2205/02 (20130101); F25J 2250/20 (20130101) |
Current International
Class: |
F25J
3/04 (20060101); F25j 003/02 (); F25j 003/04 () |
Field of
Search: |
;62/23,24,27,28,29,30,41,38,39,13-15 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Purcell; A. F.
Claims
What I claim is:
1. Plant for the production of low purity oxygen by the
fractionation of air, comprising a low pressure compressor
compressing a first air feed stream in a first feed line, a high
pressure compressor compressing a second air feed stream in a
second feed line, a first heat exchanger in which said first feed
stream from said low pressure compressor is cooled against outgoing
gas streams, a high pressure fractionating column receiving the
cooled first feed stream from said first heat exchanger as feed, a
second heat exchanger in which said second feed stream from said
high pressure compressor is cooled against outgoing gas streams, a
product vaporizer in which said cooled second feed stream from said
second heat exchanger is partially condensed against boiling oxygen
product, a first separator receiving said partially condensed
second feed stream from said product vaporizer and separating it
into a liquid fraction stream, a first sub-cooler receiving said
liquid fraction stream from said first separator and cooling it
against outgoing liquid oxygen, a low pressure fractionating column
receiving said cooled liquid fraction stream from said sub-cooler
as feed after flashing of said cooled liquid fraction stream down
to low pressure column pressure, an outgoing liquid product line
delivering liquid oxygen from the bottom of said low pressure
column to said product vaporizer via said sub-cooler, and an
outgoing waste nitrogen line delivering nitrogen gas from the top
of said low pressure column to said first and second heat
exchangers.
2. Plant according to claim 1, further comprising passages in said
second heat exchanger receiving a major proportion of said vapour
fraction stream from said first separator and reheating said major
proportion of said vapour fraction stream, an expander receiving
said reheated major proportion of said vapour fraction stream from
said passages and expanding it to provide plant refrigeration, and
an expander exhaust line delivering the exhaust from said expander
to said low pressure column as feed.
3. Plant according to claim 2, further comprising a nitrogen
superheater in which a minor proportion of said vapour fraction
stream from said first separator is condensed against outgoing
waste nitrogen in said waste nitrogen line, the condensed minor
proportion of said vapour fraction stream from said nitrogen
superheater being combined with said cooled liquid fraction stream
from said sub-cooler for delivery to said low pressure column.
4. Plant according to claim 1, further comprising a first external
reboiler/condenser receiving crude liquid oxygen from the bottom of
said high pressure column after it has been flashed down to low
pressure column pressure, and partially vaporizing it against a
condensing nitrogen stream from the top of said high pressure
column, the partially vaporized crude liquid oxygen being then fed
to the bottom of the low pressure column.
5. Plant according to claim 4, further comprising a second external
reboiler/condenser in which another portion of tne nitrogen from
the top of the high pressure column is condensed against the
boiling cooled liquid fraction stream from said sub-cooler before
said fraction stream enters the low pressure column, the condensed
nitrogen from said reboiler/condensers providing reflux streams for
both columns.
6. Plant according to claim 5, further comprising a second
sub-cooler in which the nitrogen reflux stream for the low pressure
column is sub-cooled against the waste nitrogen stream from the top
of that column.
7. Plant according to claim 2, wherein said expander exhaust line
includes a cooler in which the expander exhaust is cooled against
the waste nitrogen from the top of the low pressure column.
8. Plant according to claim 4, further comprising a cooler in which
the crude oxygen from the bottom of said high pressure column is
cooled against the waste nitrogen from the top of the low pressure
column before entering said first reboiler/condenser.
9. Plant according to claim 1, further comprising a pump in said
outgoing liquid product line which pumps the liquid oxygen to a
higher pressure before it enters said product vaporizer.
10. Plant according to claim 1, wherein passages in said product
vaporizer conduct said cooled second feed stream through said
product vaporizer in a downward flow direction, said feed stream
entering at a point part way down from the top of said
vaporizer.
11. Plant according to claim 10, further comprising a bleed line
withdrawing a small liquid bleed from said liquid fraction stream
leaving said separator and delivering it to said product vaporizer
passages at the top of said vaporizer.
12. Plant according to claim 1, further comprising a second
separator receiving the product oxygen from said product vaporizer
and separating it into a vaporized product fraction and a liquid
product fraction, a line returning said liquid product fraction to
said product vaporizer, a gaseous product line delivering said
vaporized product fraction to said first and second heat
exchangers, and a bleed line delivering a small amount of said
liquid product fraction to said low pressure column.
13. Plant according to claim 5, comprising further separators
respectively receiving the crude liquid oxygen from said first
reboiler/condenser and said feed liquid fraction stream from said
second reboiler/condenser, each of said further separators having a
liquid delivery line returning separated liquid to the respective
reboiler/condenser, a gas delivery line feeding separated vapour to
said low pressure volumn, and a bleed line delivering a small
portion of separated liquid to said low pressure column.
Description
This invention relates to the production of oxygen, especially low
purity oxygen.
There are a number of well established industrial processes using
large quantities of air which may become more efficient when the
air is oxygen enriched. For example, in the steel industry there is
an interest in the use of oxygen enrichment of the air supply to
blast furnaces. Oxygen enrichment may also be economically
benficial in glass melting furnaces, the fire refining of copper
and numerous processes associated with the petrochemical and
petroleum cracking industries.
The oxygen from the air separation unit is often mixed with varying
quantities of air to produce an enriched air stream which may
contain from 25 % to 35 % oxygen. The purity of the oxygen produced
from the air separation plant determines the amount of air which
must be added to produce a final enriched air stream. The overall
cost of producing the enriched air stream is thus a function of the
cost of oxygen from the air separation unit, the cost of
compressing the oxygen to the enriched air delivery pressure and
the cost of compressing the air which is mixed with the oxygen to
the enriched air delivery pressure.
The pressure at which the enriched air is used is usually less than
50 psia, for example, when oxygen enriched air is blown into a
blast furnace producing iron, or in the fire refining of copper. It
is an object of this invention to provide a means of producing an
oxygen stream containing up to 70 % of oxygen at a pressure which
is high enough for use directly without the need to compress the
oxygen gas in an oxygen compressor.
According to the present invention, low purity oxygen is obtained
from the fractionation of air in a plant operating according to a
split pressure cycle, with a low pressure air feed passing, after
cooling, to a high pressure fractionating column, and a high
pressure feed passing, after cooling, to a product vaporizer in
which it is partially condensed against boiling oxygen product, the
liquid fraction then being sub-cooled and flashed down to low
pressure column pressure for admission to a low pressure
fractionating column.
A major portion of the vapour fraction of the high pressure feed
may be reheated and expanded to provide plant refrigeration, and a
minor portion condensed against waste nitrogen and combined with
the sub-cooled liquid fraction.
An important preferred feature of the cycle is that a first
external reboiler/condenser supplies the vapour boil-up for the low
pressure column when crude liquid from the base of the high
pressure column vaporizes against condensing nitrogen, and a second
external reboiler/condenser condenses the remaining nitrogen from
the high pressure column against a boiling liquid air stream.
A liquid oxygen stream can be withdrawn from the base of the low
pressure distillation column and passed through a pump where its
pressure is raised to the required oxygen product delivery
pressure, and the oxygen product can be vaporized in a
reboiler-condenser with heat transferred from the condensing high
pressure air stream.
Such a plant can produce oxygen product at up to 70 % oxygen
content. The pressure of oxygen is fixed by the pressure of the
high pressure air stream, for example, 50 psia oxygen can be
produced with a high pressure air pressure of 109 psia entering the
cold box. The limit of 70 % oxygen purity is fixed by the
equilibrium between liquid and vapour at the bottom tray of the low
pressure distillation column and the mass balance around the base
of the low pressure column. The use of the vaporized crude oxygen
stream as the vapour feed to the base of the low pressure column
sets an upper limit of about 70 % oxygen on the liquid collecting
in the sump of the low pressure column.
A low purity oxygen plant in accordance with the invention will now
be described by way of example and with reference to the
accompanying drawings, in which:
FIG. 1 is a flow diagram of the complete plant; and,
FIG. 2 illustrates a modification of the plant of FIG. 1.
Referring to FIG. 1, air enters the plant at two pressure levels
from a low pressure compressor K101 and a high pressure compressor
K102. Just over half of the total plant air, the low pressure air,
is delivered at 54.3 psia. It is cooled in low pressure reversing
heat exchanger cores E101 and E102 and passed directly to a high
pressure fractionating column C101.
The reversing heat exchangers are a well known means of cooling
down the feed stream to a low temperature plant against warming
product streams and at the same time causing high boiling point
impurities to be removed from the cooling stream and sublimed into
one of the low pressure warming streams by switching the cooling
stream and the particular warming stream passages at regular
intervals. This switching of the two streams between identical sets
of passages in the heat exchanger is accomplished by means of
switch valves at the warm end of the heat exchanger and self-acting
check valves at the cold end of the heat exchanger. In the present
plant the air and waste nitrogen streams are switched at regular
intervals to remove water and carbon dioxide from the air
streams.
The rest of the plant air, the high pressure air, is delivered at
109.0 psia. It is cooled in high pressure reversing heat exchanger
cores E103 and E104 and enters a product vapourizer E109 where it
is partially condensed against boiling product liquid oxygen. The
resulting two-phase air mixture passes into a phase separator C110.
The liquid air fraction, about 75 % of the total high pressure air
stream, is passed through a sub-cooler E108 where it is cooled
against the product liquid oxygen stream then flashed down to low
pressure column pressure and fed into a separator C107. The liquid
portion of the stream from the separator C107 passes through an
external reboiler/condenser E106 in which about 88 % of the stream
is vaporized against condensing nitrogen from the top of the high
pressure column C101. The liquid and vapour air fractions from the
separator C107 then enter a low pressure fractionating column C102.
The operating pressure of the low pressure column is 21.5 psia at
the base.
Crude liquid oxygen containing 44 % oxygen, withdrawn from the
bottom of the high pressure column C101, is sub-cooled against
waste nitrogen from the top of the low pressure column in a main
sub-cooler E.105. Hydrocarbons are then removed in adsorbers C105
and the crude liquid is flashed down to low pressure column
pressure and fed into a separator C108. The liquid stream from the
separator C108 is mostly vaporized in an external
reboiler/condenser E107 against condensing nitrogen from the top of
the high pressure column, and the whole of the flow from the
separator C108 enters the base of the low pressure column C102. A
small liquid purge stream is taken from the separator C108 to the
sump of the column C102, by-passing the reboiler/condenser E107, to
prevent possible hydrocarbon buildup.
The operating pressure of the high pressure column C101 is 48.3
psia at the top. Nitrogen gas leaving the top of the high pressure
column C101 is totally condensed in the external
reboiler/condensers E106 and E.107 and part of it provides the
reflux for the high pressure column C101. The remaining part is
sub-cooled against waste nitrogen from the top of the low pressure
column C102 in the main sub-cooler E105 and flashed down to low
pressure column pressure to provide the reflux for the low pressure
column C102.
Returning to the partial condensation of the high pressure air
stream in the product vaporizer E109, about 25 % of the total high
pressure air stream leaves the separator C110 as vapour. 90 % of
this vapour fraction is used as a reheat stream for the high and
low pressure heat exchanger cold cores E102 and E104. This method
of controlling the temperature difference at the cold ends of the
reversing heat exchangers E102 and E104 ensures that the solid
carbon dioxide deposited from the air stream on to the surface of
the heat exchangers is sublimed into the waste nitrogen stream when
the exchangers are switched. The remaining 10 % of vapour is
condensed in a nitrogen superheater E110 against that fraction of
waste nitrogen from the sub-cooler E105 which leaves the plant
through the high pressure reversing heat exchangers E103 and E104.
The liquid air stream leaving the superheater E110 rejoins the main
liquid air stream from the sub-cooler E108. The function of the
nitrogen superheater E110 is to warm the outgoing nitrogen
sufficiently to ensure that the high pressure air feed is not
liquefied in the cold cores E104 of the high pressure reversing
heat exchanger. This is necessary for proper removal of CO.sub.2 in
the reversing heat exchangers E103 and E104.
The fraction of the high pressure air which has been reheated in
the reversing heat exchangers is expanded through a turbo-expander
K103 and provides the plant refrigeration requirement. A by-pass is
available to pass more high pressure air through the turbine should
additional refrigeration be required. The expander exhaust is
cooled in the main sub-cooler E105 and enters the low pressure
column C102 slightly superheated.
Outgoing waste nitrogen from the low pressure column C102 is warmed
against liquid nitrogen from the high pressure column, the crude
O.sub.2 liquid from the high pressure column, and the
turbo-expander exhaust stream, in the main sub-cooler E105. It
leaves the plant via the high and low pressure reversing heat
exchangers E101 to E104.
Liquid containing 68 % oxygen is withdrawn from the low pressure
column sump and raised in pressure by a pump G101 to 55 psia. The
total stream is passed through hydrocarbon adsorbers C106, warmed
to its bubble point against liquid air in the sub-cooler E108 and
fed to a separator C109. A small liquid bleed from this separator
is flashed down to low pressure column pressure and returned to the
low pressure column to prevent possible hydrocarbon buildup. The
remaining liquid oxygen is totally vaporized against condensing
high pressure air in the product vaporizer E109. This, the product
stream, passes through the reversing heat exchangers E101-E104, and
leaves the plant at 50 psia.
The concentration of CO.sub.2 in the high pressure air leaving the
high pressure reversing heat exchanger E104 is about 0.3 ppm, and
in addition, there may be some solid CO.sub.2 carryover from the
heat exchanger E104. This CO.sub.2 would tend to deposit in the air
channels at the top end of the product vaporizer E109 where no
liquefaction takes place unless special methods were used to
prevent this. Several different methods are available to prevent
this deposition.
1. The high pressure air may be fed into the vaporizer E109 at a
point part way down its length as indicated in FIG. 1. This
arrangement leaves some free surface area above the feed point
where condensation will take place providing a falling liquid film
to wash away any CO.sub.2 deposited on the surface area below or
near the feed point.
2. Another method is to pump a small liquid bleed from the bottom
of the separator C110 to the top of the vaporizer E109 by a pump as
shown in FIG. 2. This arrangement will also ensure that the
incoming air meets a falling liquid film to dissolve and wash away
any CO.sub.2 deposition.
3. A further method is to pass the high pressure air stream leaving
the high pressure reversing heat exchangers through carbon dioxide
absorbers. These will also remove any hydrocarbons in the
stream.
The CO.sub.2 concentration in the vapour fraction leaving the
separator C110 is so low that there is no risk of CO.sub.2
deposition in the expander K103 or in the main sub-cooler E105.
Also, the vapour leaving the separator is at its dewpoint and will
immediately condense in the nitrogen superheater E110 giving no
deposition problem in that unit.
It will be seen that two pairs of hydrocarbon absorbers are
provided, one C105 to remove hydrocarbons from the crude liquid
oxygen stream from the high pressure column, and the other C106 to
remove hydrocarbons from the product liquid before it enters the
vaporizer. The external reboiler/condenser E107 in which the crude
oxygen is totally vaporized, is protected from the build-up of
residual hydrocarbons by providing the small liquid bleed to the
low pressure column from the separator C108. The other external
reboiler/condenser E106 only partly vaporizes the liquid air stream
and the constant liquid off-take prevents hydrocarbon build-up in
that unit. The small liquid bleed from the separator C109 back to
the low pressure column prevents build-up of residual hydrocarbons
in the product vaporizer E109.
The plant described has been designed to eliminate problems of
CO.sub.2 deposition and hydrocarbon build-up which are often
critical in low purity oxygen plants.
Variations on the design described above may be considered which
use other means than reversing heat exchangers such as plate-fin
heat exchangers, to cool the air and remove water and carbon
dioxide impurities. These would include regenerators filled with
stones or other types of packing and having air sidestreams instead
of reheat streams for temperature difference control.
* * * * *