[0001] The objective of the invention is to efficiently and cost-effectively generate two
different oxygen purities from the same air separation plant, when only a small amount
of high purity oxygen is required. It also provides a way to make small amounts of
crude argon product if required on a plant which makes predominantly low purity oxygen.
This is consistent with the requirements of new grass-roots steel mills based on the
COREX steel-making process.
[0002] In the past there were two commonly used alternatives to make two different oxygen
purities from the same facility. One was to build two independent cryogenic trains
- one for each purity. This is capital intensive and complicated. The other was to
design the entire plant for the high purity, requiring a main air compressor discharge
pressure at a pressure consistent with high purity oxygen. This is not energy efficient,
since much of the air only needs to be raised to a lower pressure consistent with
generating low purity oxygen.
[0003] A number of efficient plants for the production of low purity oxygen are known in
the literature. US-A-4,702,757; US-A-4,704,148; and US-A-4,936,099 describe a number
of very efficient process cycles employing multiple reboiler condensers. However,
none of these cycles involve the coproduction of a portion of oxygen product at a
purity greater than 95%.
[0004] US-A-5,515,833 describes a cycle in which a portion of expanded gas from a compander
is used to reboil the bottom reboiler of a dual-reboiler low pressure ("LP") column.
In this cycle, one double-column assembly is used for the two products. All air feed
to the process is compressed to a pressure sufficient to allow the expander exhaust
to reboil the high purity oxygen in the bottom reboiler of the LP column. A high pressure
nitrogen stream from the top of the high pressure ("HP") column feeds an intermediate
reboiler which is at a location above the low purity oxygen withdrawal location. This
makes the process inefficient in that very high recoveries of oxygen and, especially,
nitrogen cannot be achieved because a large quantity of air needs to be condensed
in the bottom reboiler of the LP column to provide the boilup for both the high purity
and the low purity gaseous oxygen products and also all the vapour stream for the
distillation above the low purity oxygen withdrawal location. The large quantity of
air which needs to be condensed in the bottom reboiler of the LP column decreases
the amount of available liquid nitrogen reflux and negatively impacts oxygen recovery.
Further, the larger portion of air needed at higher pressure to be condensed against
pure oxygen increases energy consumption and thereby wastes compression energy. This
process also forces the LP column to be sized for the entire air feed, leading to
potential manufacturing and transportation difficulties.
[0005] US-A-5,515,833 also shows an argon sidearm column connected to the main column between
the two oxygen products withdrawal locations. This feed location is necessary to eliminate
nitrogen from the sidearm column feed. However, it provides a sidearm column feed
with a very low argon concentration (less than 4% Ar). This makes this distillation
much more difficult than where the sidearm column feed is in the 9-14% Ar range. Due
to low argon concentration in the feed to the sidearm column, for a given oxygen recovery,
the argon recovery will be poor.
[0006] Clearly there is a need for more efficient cycles to produce high purity as well
as low purity oxygen with high efficiency and ease of operability.
[0007] The present invention relates to cryogenic distillation of a stream containing nitrogen
and oxygen to efficiently produce oxygen at at least two levels of purity. The first
product is a low-purity oxygen stream containing less than 97% oxygen (but generally
greater than 80% oxygen) and the second product is a high purity oxygen product stream
containing more than 97% oxygen, preferably more than 99.5% oxygen. The high efficiency
is achieved by taking a high efficiency process cycle for the production of low purity
oxygen and modifying it according to the invention. The high efficiency process cycle
consists at least a distillation column where feed stream is distilled to produce
low purity oxygen from the bottom and a nitrogen-rich stream from the top. The bottom
of this column has a reboiler where a suitable process stream is condensed to provide
boilup to the distillation column. According to the invention, a liquid stream having
an oxygen concentration at least equal to that of said feed stream is withdrawn either
from the bottom of this first column (at the location of the bottom reboiler) or from
a point which is some separation stages above the withdrawal location of the low purity
oxygen and fed to the top of a sideleg column. The bottom of the sideleg column is
boiled by a suitable process fluid and high-purity oxygen product is withdrawn from
the bottom of this sideleg column. The vapour from top of the sideleg column is returned
to the first column, preferably at same separation stage from where the liquid feed
stream was withdrawn.
[0008] When coproduction of argon is desired, an argon sidearm column is attached at a proper
location of the sideleg column producing high-purity oxygen, i.e., a vapour feed from
an intermediate location of the sideleg column is fed to the argon sidearm column
to produce argon from the top of this column and the liquid stream from the bottom
of this column is returned back to the sideleg column.
[0009] The advantage of this method is that boilup at the high temperature of the high purity
oxygen is kept at minimum. A much larger amount of heat is provided at the location
of the production of low purity oxygen. This leads to substantial energy savings.
[0010] In its broadest aspect, the present invention provides a process for cryogenic distillation
of a compressed feed air stream in a distillation system comprising a low purity column
producing a low purity (less than 97%) oxygen product stream and a nitrogen-rich stream,
said column having a bottom reboiler in which a suitable first process stream is condensed
to provide boilup to the column, characterized in that an oxygen-rich stream having
an oxygen concentration at least equal to that of the feed to the low purity column
is withdrawn from the distillation system and rectified in a high purity column whose
bottoms reboiler heat is provided by condensation of a suitable second process stream
to provide a high-purity (more than 97%) oxygen product stream, said second process
stream being at a higher pressure than said first process stream.
[0011] Usually, the low purity oxygen product stream will contain more than 70% oxygen and
preferably at least 90% oxygen. The high purity oxygen product stream preferably contains
at least 99.5% oxygen.
[0012] In a presently preferred embodiment of the invention,the distillation system comprises
a high pressure column and a low pressure column and:
(a) at least a portion of the compressed air feed is fed to the high pressure column
in which the air feed is rectified into a high pressure nitrogen overhead and a high
pressure crude liquid oxygen bottoms;
(b) at least a portion of the high pressure crude liquid oxygen bottoms is fed to
the low pressure column in which the high pressure crude liquid oxygen bottoms is
rectified into a low pressure nitrogen overhead and a low pressure liquid oxygen bottoms;
(c) at least a portion of the high pressure nitrogen overhead is condensed and at
least a portion of the condensed high pressure nitrogen overhead is returned to the
high pressure column as reflux;
(d) at least a portion of the low pressure liquid oxygen bottoms is boiled by a bottom
reboiler in which a suitable first process stream is condensed;
(e) the low purity oxygen product stream is withdrawn from the low pressure column;
(f) the oxygen-rich stream having an oxygen concentration at least equal to that of
the high pressure crude liquid oxygen bottoms is withdrawn from the distillation system
and fed to a high purity column in which it is rectified into an oxygen-lean overhead
vapour and a high-purity liquid oxygen bottoms;
(g) at least a portion of the high purity liquid oxygen bottoms is boiled by a bottom
reboiler by condensation of a suitable second process stream compressed to a pressure
higher than that of the first process stream boiling the low pressure column;
(h) the high purity oxygen product stream is withdrawn from the high purity column.
[0013] The process stream providing bottom reboil to the high purity column is a portion
of the compressed feed air which has been further compressed. At least a portion of
this compressed feed air portion can be is fed to the high pressure column and/or
to the low pressure column.
[0014] Alternatively, the process stream providing bottom reboil to the high purity column
can be at least a portion of the high pressure nitrogen overhead which has been further
compressed.
[0015] The oxygen-rich stream can be withdrawn from the low pressure column, usually the
sump thereof, and, preferably, the oxygen-lean overhead vapour is returned to the
low pressure column at substantially the same location as that at which the oxygen-rich
stream was withdrawn.
[0016] Alternatively, the oxygen-rich stream can be withdrawn from the high pressure column,
suitably as a portion of the high pressure crude liquid oxygen bottoms. A vapour stream
can be withdrawn from the low pressure column at a location below the feed of the
high pressure crude liquid oxygen bottoms thereto and fed to the high purity column
at a location below the feed of the oxygen-rich stream thereto.
[0017] The process stream providing bottom reboil to the low pressure column can be at least
a portion of the high pressure nitrogen overhead or a portion of the compressed feed
air. When it is a portion of the compressed air feed, at least a portion of the resultant
condensed fed can be fed to the high pressure column and/or to the low pressure column.
If the high pressure nitrogen overhead is not condensed to provide low pressure column
reboil, at least a portion thereof can be condensed at an intermediate location in
the low pressure column.
[0018] As mentioned previously, an argon rich vapour stream withdrawn from an intermediate
location of the high purity column can be separated in an argon column to produce
an argon product stream and a liquid argon-depleted stream which is returned to the
high purity column.
[0019] When high pressure nitrogen overhead is condensed at an intermediate location in
the low pressure column, the oxygen-rich stream suitably is withdrawn from the low
pressure column at a location above said intermediate location and an argon rich vapour
stream withdrawn from an intermediate location of the high purity column is separated
in an argon column to produce an argon product stream and a liquid argon-depleted
stream which is returned to the high purity column.
[0020] Argon overhead from an argon column can be condensed by boiling a portion of the
high pressure crude liquid oxygen bottoms and the vaporized high pressure crude liquid
oxygen bottoms fed to the low pressure column and/or to the high purity column.
[0021] A portion of the compressed air feed can be fed to the low pressure column or a portion
of the compressed air feed further compressed to a higher pressure than the main compressed
air feed portion to the high pressure column and at least a portion of the further
compressed feed air portion fed to the high pressure column and/or to the low pressure
column.
[0022] At least a portion of the condensed high pressure nitrogen overhead can be fed to
the low pressure column as reflux. Alternatively, all of the condensed high pressure
nitrogen overhead can be fed to the high pressure column as reflux and the low pressure
column refluxed with a sidestream withdrawn from the high pressure column. A portion
of the nitrogen rich sidestream can be fed to the high purity column.
[0023] If not all of the high pressure nitrogen overhead is condensed, the remaining portion
can be recovered as product.
[0024] The low purity oxygen product stream can be boiled against the portion of the compressed
air feed fed to the high pressure column to at least partially vaporize the compressed
air feed portion.
[0025] The low purity oxygen product stream and high purity oxygen product stream can be
withdrawn from the low pressure column and high purity column respectively as liquids
and pumped prior to heat exchange with the main compressed air feed to generate pressurized
oxygen products.
[0026] The present invention also provides an apparatus for producing low purity oxygen
and high purity oxygen products by the cryogenic distillation of a compressed air
feed by the process of Claim 1, said apparatus comprising:
(i) a distillation system for rectifying the compressed air feed and comprising a
low purity column for producing a low purity (less than 97%) oxygen product stream
and a nitrogen-rich stream;
(ii) a bottom reboiler in said low purity column for condensing a suitable first process
stream to provide boilup to the low purity column;
(iii) means for supplying said first process stream to said reboiler; and
(iv) means for withdrawing the low purity oxygen product stream from the apparatus,
characterized in that the apparatus further includes
(v) a high purity column for rectifying an oxygen rich stream having an oxygen concentration
at least equal to that of the feed to provide a high-purity (more than 97%) oxygen
product stream;
(vi) means for withdrawing said oxygen-rich stream from the distillation system and
feeding it to the high purity column;
(vii) a bottom reboiler in said high purity column for condensing a suitable second
process stream to provide boilup to the high purity column;
(viii) means for supplying said second process stream to said high purity column reboiler
at a higher pressure than said second process stream is supplied to the low purity
column reboiler; and
(ix) means for withdrawing the high purity oxygen product stream from the apparatus.
[0027] In a presently preferred embodiment of the apparatus aspect the distillation system
comprises:
(1) a high pressure column for rectifying at least a portion of the compressed air
feed into a high pressure nitrogen overhead and a high pressure crude liquid oxygen
bottoms;
(2) a low pressure column for rectifying the high pressure crude liquid oxygen bottoms
into a low pressure nitrogen overhead and a low pressure liquid oxygen bottoms;
(3) means for feeding at least a portion of the high pressure crude liquid oxygen
bottoms to the low pressure column;
(4) means for condensing at least a portion of the high pressure nitrogen overhead
and returning at least a portion of the condensed high pressure nitrogen overhead
to the high pressure column as reflux;
(5) a bottom reboiler in the low pressure column for condensing a suitable first process
stream to provide boilup to the low pressure column;
(6) means for supplying said first process stream to said reboiler;
(7) means for withdrawing the low purity oxygen product stream from the apparatus;
(8) a high purity column for rectifying an oxygen rich stream into an oxygen-lean
overhead vapour and a high-purity liquid oxygen bottoms;
(9) means for withdrawing the oxygen-rich stream from the distillation system and
feeding it to the high purity column;
(10) a bottom reboiler in said high purity column for condensing a suitable second
process stream to provide boilup to the high purity column;
(11) means for supplying said second process stream to said high purity column reboiler
at a higher pressure than said second process stream is supplied to the low pressure
column reboiler; and
(12) means for withdrawing the high purity oxygen product stream from the apparatus.
[0028] The following is a description by way of example only and with reference to the accompanying
drawings of presently preferred embodiments of the invention. In the drawings:-
Figure 1 shows a first (basic) embodiment of the present invention;
Figure 2 shows a second embodiment of the present invention in which a LOX boil vaporizer
is used for the low purity oxygen product;
Figure 3 shows a third embodiment of the present invention in which liquid pumps are
used to generate pressurized oxygen and nitrogen products;
Figure 4 shows a fourth embodiment of the present invention which is a simplification
of the embodiment of Figure 2 in which the LOX boil vaporizer is omitted;
Figure 5 shows a fifth embodiment of the present invention which is a modification
of the embodiment of Figure 2 incorporating a crude argon sidearm column;
Figure 6 shows a sixth embodiment of the present invention which is a modification
of the embodiment of Figure 3 incorporating a crude argon sidearm column and has the
sideleg column decoupled from the LP column.
Figure 7 shows a seventh embodiment of the present invention which is a modification
of the embodiment of Figure 6 in which the LP column is coupled to both the sidearm
column and the sideleg column.
[0029] In all figures, the same reference numerals are used to identify the same or equivalent
components.
[0030] The basics of the invention are described below with reference to Figure 1. Briefly
stated, a system comprising a HP column (15), a LP column (25) and a sideleg column
(23) is used. Low purity oxygen product (95% GOX) is obtained from the bottom of the
LP column (25). High purity oxygen product (99.5% GOX) is obtained from the bottom
of the sideleg column (23), in which a liquid stream (35) from the bottom of the LP
column (25) is distilled and boiled by a portion (21) of the feed air (10) which has
been boosted in pressure. Vapour (37) from the top of the sideleg column (23) is returned
to the bottom of the LP column (25).
[0031] In more detail, clean, dry compressed air (10) from a front-end purification system
(1) is split into three streams (11, 12, 13). A first feed air stream (11) is fed
directly to a main heat exchanger (14), where it is cooled to near its dew point temperature,
and then (11') to the HP column (15). A second feed air stream (12) is fed via the
compressor end (16) of a compander (17) to the main heat exchanger (14) from which
it is withdrawn as a side stream (18) and fed via the expander end (19) of the compander
(17) to the LP column (25). The compander (17) generates refrigeration for the cycle.
A third feed air stream (13) is fed via a low pressure booster air compressor (20)
to the main heat exchanger (14) where it is cooled to near its dew point. The resultant
cooled stream (21) is fed to a reboiler (22) in the bottom of the high purity or sideleg
column (23) to provide reboil to that column. The liquified air (24) from this reboiler
(22) is fed to the HP column (15).
[0032] Alternately, some or all of the liquified air (24) can be subcooled and fed to the
LP column (25) rather than to the HP column (15).
[0033] The HP column (15) provides an initial distillation of air to generate a liquid oxygen-enriched
(crude LOX) stream (26) at the bottom of the column and a HP gaseous nitrogen stream
(27) at the top of the column. The crude LOX stream (26) has an oxygen content usually
greater than 30% and more often greater than 35%. This stream (26) is subcooled against
a LP gaseous nitrogen stream (28) from the top of the LP column (25) in a subcooler
(29) and is then reduced in pressure and fed to an intermediate location of the LP
column (25). The warmed LP gaseous nitrogen stream (28') is further warmed in the
main heat exchanger (14) before being vented to atmosphere (WASTE) or recovered as
a coproduct stream.
[0034] At least the majority (30) of the HP gaseous nitrogen (27) from the top of the HP
column (15) is fed to a reboiler (31) in the bottom of the LP column (25). The condensed
nitrogen stream from this reboiler (31) is split to provide two substreams (32, 33).
One substream (32) is used to reflux the HP column (15) and the other substream (33)
is cooled in the subcooler (29) and then used to reflux the LP column (25).
[0035] Part (34) of the gaseous nitrogen stream (27) withdrawn from the top of the HP column
(15) is warmed in the main heat exchanger (14) for recovery as product (GAN). If no
product nitrogen is required, all of the withdrawn HP nitrogen stream (27) can be
fed to the reboiler (31). However, if large amounts of product nitrogen are required,
it is typically more efficient to take this from the LP column (25).
[0036] If very high purity (ppm levels of oxygen) nitrogen product is required, it may be
more efficient to reflux the LP column (25) with impure liquid nitrogen. In this case,
all of the condensed nitrogen stream from the reboiler (31) is fed to the top of the
HP column (15) and the LP column (25) is refluxed with a side stream (not shown) withdrawn
from the HP column (15) several stages below the top and then cooled in the subcooler
(29) prior to feed to the LP column (25).
[0037] A low purity liquid oxygen stream (35) is withdrawn from the sump of the LP column
(25) and fed to the top of the sideleg column (23). The low purity liquid oxygen is
distilled and boiled against the cooled third feed air stream (21) fed to the reboiler
(22). A high purity gaseous oxygen stream (36) is removed from above the sump of the
sideleg column (23) and fed to the main heat exchanger (14) from where it is recovered
as product (99.5% GOX). An oxygen-lean overhead vapour stream (37) is returned from
the sideleg column (23) to the LP column (25) just above the sump thereof. Since liquid
feed (35) for the sideleg column (23) is withdrawn from a location of the LP column
(25) which is below the crude LOX feed (26'), the concentration of oxygen in the liquid
feed (35) is greater than that in the crude LOX feed (26').
[0038] A low purity oxygen product stream (38) is withdrawn from the bottom of the LP column
(25) and fed to the main heat exchanger (14) from where it is recovered as product
(95% GOX).
[0039] In Figure 1, a separate booster (20) is provided to compress the third feed air stream
(13) used to provide reboil in heat exchanger (22) to the bottom of the sideleg column
(23). However, this feed air stream (13) could be boosted in the compressor end (16)
of the compander system (17).
[0040] The embodiment of Figure 2 differs from that of Figure 1 in that the low purity oxygen
stream (38) is withdrawn from the LP column (25) as a liquid stream and is boiled
against a portion (215) of the first cooled feed air stream (11) in a LOX boil vaporizer
(210); the reboiler (31) is relocated to an intermediate location in the LP column
(25); and a further reboiler (211) fed by a cooled feed air stream (212) is located
in the bottom of the LP column (25). Only the differences in the two systems will
be described.
[0041] A minor portion of the cooled first feed air stream (11') is withdrawn as a substream
(212) and is fed to the bottom reboiler (211) in the LP column (25). The condensed
air (213) from this reboiler (211) is fed to an intermediate location of the HP column
(15).
[0042] The remaining (larger) portion (215) of the cooled first feed air stream (11') is
partially condensed in the LOX boil vaporizer (210) by heat exchange with the low
purity liquid oxygen product (38) from the LP column (25). The resultant two-phase
feed air stream (214) is then fed to the bottom of the HP column (15).
[0043] If required, the condensed air (213) can be fed to the HP column (15) at the same
place as the two-phase air feed (214) instead of to the intermediate location. This
arrangement is simpler but less efficient than feed to the midpoint.
[0044] Some or all of the liquified air from the air-fed reboilers (22, 211) can be cooled
in subcooler (29) and fed to the LP column (25) instead of to the HP column (15).
[0045] At least the major portion of the HP gaseous nitrogen stream (27) from the top of
the HP column is fed to the reboiler (31) which, relative to the system of Figure
1, has been relocated at an intermediate location in the LP column (25).
[0046] The low purity liquid oxygen stream (38) withdrawn from the sump of the LP column
(25), increased in pressure slightly by its own static head, is fed to the LOX boil
vaporizer (210). The oxygen stream (38) is boiled by the cooled first air stream (215)
and then warmed in the main heat exchanger (14) for recovery as product (95% GOX).
The LOX boil vaporizer (210) increases the pressure of the low purity oxygen vapour
thus reducing compression power.
[0047] The embodiment of Figure 3 differs from that of Figure 2 in that the second feed
air stream (12) is boosted in pressure by a compressor (310); the exhaust (311) from
the expander end (19) of compander (17) is fed to the HP column (15) instead of to
the LP column (25); additional air feed (312/313, 312/314) is provided to the LP column
(25) and/or to the HP column (15); the non-reflux portion (33) of the condensed HP
nitrogen stream (315) is withdrawn as eventually gaseous product (GAN) instead of
being fed to the LP column (25); the LOX boil vaporizer is omitted; the high purity
oxygen product (36) stream is withdrawn from the sideleg column (23) as liquid; the
low purity oxygen stream (38), high purity oxygen stream (36) and nitrogen product
stream (33) are pumped using liquid pumps (316, 317, 318) to generate pressurized
oxygen and nitrogen products; and the LP column (25) is refluxed with a side stream
(319) from the HP column (15). Only the differences in the two systems will be described.
[0048] A portion (215) of the cooled first feed air stream (11') remaining after withdrawal
of the substream (212) is fed directly to the bottom of the HP column (15).
[0049] The second feed air stream (12) is compressed in a high pressure booster air compressor
(310) and split into two substreams (312, 320). The larger of the substreams (312),
containing the majority of the compressed air, is fed to the main heat exchanger (14)
where it is condensed to vaporize the liquid products. As shown in Figure 3, part
of the liquified air (314) can be fed to the HP column (15) and the remainder (313)
cooled in the subcooler (29) and fed to the LP column (25). However, all of the liquified
air could be fed to the HP column (15) or to the LP column (25).
[0050] The smaller substream (320) of the air from the booster compressor (310) is fed to
the compressor end (16) of the compander (17) and, after partial cooling in the main
heat exchanger (14) is fed to the expander end (19) of the compander (17) to generate
refrigeration for the plant. The expander exhaust (311) is combined with the cooled
first feed air stream portion (215) fed to the bottom of the HP column (15). The compander
feed can be taken as a sidestream from an inter-stage of the booster compressor (310)
instead of from the booster compressor discharge as shown in Figure 3.
[0051] It may be preferred to simply expand the second feed air substream (320) instead
of companding it. In that case, the compander (17) is omitted and the partially cooled
substream (320) is fed from the main heat exchanger (14) to an expander replacing
the expander end (19) of the compander (17).
[0052] It also may be advantageous to use a common booster compressor to replace the two
booster compressors (20, 310). In this case, the compressed third feed air stream
(13) required to reboil the sideleg column (23) can be withdrawn as a sidestream from
the common booster compressor or from the common booster compressor discharge product.
[0053] A side stream (319) of impure reflux is withdrawn from the HP column (15) several
stages below the top, cooled in the subcooler (29) and fed (319') to the top of the
LP column (25).
[0054] The HP nitrogen stream (27) withdrawn from the top of the HP column (15) is condensed
in the intermediate reboiler (31) and the condensed HP nitrogen stream divided into
a reflux substream (32) and a product stream (33). The product stream (33) is pumped
by a liquid pump (318) prior to vaporization in the main heat exchanger (14) for collection
as gaseous nitrogen product (GAN).
[0055] The low purity liquid oxygen stream (38) withdrawn from the sump of the LP column
(25) also is pumped by a liquid pump (316) prior to vaporization in the main heat
exchanger (14) for collection as low purity gaseous oxygen product (95% GOX).
[0056] Similarly, the high purity liquid oxygen stream (36) from the sideleg column (23)
is pumped by liquid pump (317) prior to vaporization in the main heat exchanger (14)
for collection as high purity gaseous oxygen product (99.5% GOX)
[0057] In the embodiment of Figure 3, it is not necessary to recover all of the products
(GAN, 95% GOX, 99.5% GOX) via liquid pumps and vaporization in the main heat exchanger
(14). Any combination of liquid and gaseous products from the columns is allowed.
[0058] The embodiment of Figure 4 is a simplification of that of Figure 2 in which the LOX
boil vaporizer (210) is omitted and the (larger) portion (215) of the cooled first
feed air stream (11') is fed directly to the bottom of the HP column (15). The low
purity oxygen product stream (438) is withdrawn from the sump of the LP column (25)
as gas instead of as liquid.
[0059] The embodiment of Figure 5 differs from that of Figure 2 in that the feed to and
return from the sideleg column (23) is at a location of the LP column (25) above the
intermediate reboiler (31) and an argon sidearm column (510) has been added. Only
the differences in the two systems will be described.
[0060] In Figure 5, the feed stream (35) to the sideleg column (23), instead of being withdrawn
from the sump of the LP column (25) as in Figure 2, is withdrawn from the middle of
that column, above the intermediate reboiler (31). Alternatively, the feed stream
(35) to the sideleg column (23) could be withdrawn at a location below the intermediate
reboiler (31) but above the sump of the LP column (25). The vapour stream (37) from
the top of the sideleg column (23) is returned preferably to this same point in the
LP column (25). High purity oxygen product stream (36) is withdrawn from the bottom
of the column (23) either as gas (as in Figure 2) or as liquid (as in Figure 3).
[0061] An argon rich vapour sidestream (511) is withdrawn from the middle of the sideleg
column (23) at a point where there is low nitrogen content in the column vapour. The
nitrogen concentration in the argon rich vapor sidestream (511) usually is less than
1%, preferably less than 0.5% and especially less than 100 ppm. The argon rich sidestream
(511) is distilled further in a crude argon or sidearm column (510). The product from
this column is mostly argon and can contain as much a 4% or as little as 1 ppm of
oxygen, depending on the number of stages in the column. The sidestream (511) can
be further purified if necessary with any suitable purifier. An argon depleted liquid
stream (512) from the bottom of the sidearm column (510) is returned to the middle
of the sideleg column (23) preferably at the same point where the vapour sidestream
(511) was withdrawn. The argon at the top of the sidearm column (510) is condensed
by boiling a portion (513) of the subcooled crude LOX in a reboiler (514). The vaporized
crude LOX (crude GOX, 515) is fed to a suitable point in the LP column (25). A portion
of the argon stream from the top of the sidearm column (510) is recovered as an argon
product stream. Preferably, said argon product stream is a portion (520) of the condensed
stream from the reboiler (514).
[0062] The embodiment of Figure 6 differs from that of Figure 3 in that the sideleg column
(23) is decoupled from the LP column (25) and an argon sidearm column (510) has been
added. Only the differences in the two systems will be described.
[0063] In Figure 6, the feed to the sideleg column, instead of being withdrawn from the
LP column (25), is provided by feeding to the top of the sideleg column (23) a portion
(610) of the subcooled nitrogen rich impure reflux (319) and feeding to an intermediate
location of the sideleg column (23) a subcooled portion (611) of the crude LOX reflux
(26) to the LP column (25).
[0064] The vapour stream (612) from the top of sideleg column (23) is oxygen depleted waste
and is mixed with the oxygen depleted waste (28) from the top of the LP column (25).
[0065] High purity oxygen product (36) is withdrawn from the bottom of the sideleg column
(23) either as liquid (as in Figure 3) or as gas (as in Figure 2).
[0066] An argon rich vapour sidestream (511) is withdrawn from the middle of the sideleg
column (23) at a point where there is low nitrogen content in the column vapor. The
nitrogen concentration in the argon rich vapor sidestream (511) usually is less than
1%, preferably less than 0.5% and especially less than 100 ppm. The argon rich sidestream
(511) is distilled further in the sidearm column (510). The product from this column
is mostly argon and can contain as much a 4% or as little as 1 ppm of oxygen, depending
on the number of stages in the column. The sidestream (511) can be further purified
if necessary with any suitable purifier. An argon depleted liquid stream (512) from
the bottom of the sidearm column (510) is returned to the middle of the sideleg column
(23) preferably at the same point where the vapour sidestream (511) was withdrawn.
The argon at the top of the sidearm column (510) is condensed by boiling a portion
(513) of the subcooled crude LOX in a reboiler (514). The vaporized crude LOX (crude
GOX, 613) is fed to a suitable point in the sideleg column (23). A portion (520) of
the condensed argon stream from the reboiler (514) is recovered as an argon product
stream.
[0067] In some other variations of the process in Figure 6, a vapour stream can be withdrawn
from a stage below the crude LOX feed (26) to the LP column (25) and fed to the sideleg
column (23) at a location which is below the feed point of the vaporized crude LOX
(613).
[0068] The embodiment of Figure 7 differs from that of Figure 6 in that the vaporized crude
LOX (713) from the top of the sidearm column (510) is fed to the LP column (25) and
a vapour stream (711) withdrawn from the LP column (25) from a location below this
feed is fed to the sideleg column (23).
[0069] In all of the embodiments described above, it is possible to use nitrogen to reboil
the sideleg column instead of air. For example, the nitrogen product stream (GAN)
is compressed in the LP booster compressor (20) instead of the third feed air stream
(13). After cooling to cryogenic temperatures in the main heat exchanger (14), the
compressed nitrogen product stream is fed to the reboiler (22) in the sideleg column
(23). The resultant condensed nitrogen can be fed to any suitable point of the HP
column (15).
[0070] In all of the embodiments, the pressure of the supply air from the front end (1)
is only as high as it needs to be to boil low purity oxygen. This is consistent with
the low pressure of a dual-reboiler cycle. Only the portion of the air necessary to
make the sideleg oxygen is compressed to the pressure required to boil high purity
oxygen. This reduces the compression power over cycles where the entire air feed is
used to make high purity oxygen and compared with the cycle of US-A-5,515,833.
[0071] The new cycle of the invention also permits of reduction in size of the LP column
(25). If the sideleg column (23) is decoupled as in Figure 6, the overall diameter
of the LP column (25) is smaller because it is not purifying the entire air stream.
It also permits of reduction in the number of stages which must be of a large diameter,
since the large main column (15, 25) no longer has to make high purity oxygen.
[0072] By taking the feed (511) to the sidearm column (510) from the midpoint of the sideleg
column (23), this feed is rich in argon (typically 6-22%, preferably 9-15%, Ar). This
not only simplifies and shortens the sidearm column over the cycle of US-A-5,515,833
which must start with a feed severely depleted in argon (typically less than 4% Ar),
but provides much higher recoveries of argon.
1. A process for cryogenic distillation of a compressed feed air stream (10) in a distillation
system (15, 25) comprising a low purity column (25) producing a low purity (less than
97%) oxygen product stream (38) and a nitrogen-rich stream (28), said column (25)
having a bottom reboiler (31; 211) in which a suitable first process stream (30; 212)
is condensed to provide boilup to the column (25), characterized in that an oxygen-rich
stream (35; 611) having an oxygen concentration at least equal to that of the feed
(26') to the low purity column (25) is withdrawn from the distillation system (15,
25) and rectified in a high purity column (23) whose bottoms reboiler heat is provided
by condensation of a suitable second process stream (21) to provide a high-purity
(more than 97%) oxygen product stream (36), said second process stream (21) being
at a higher pressure than said first process stream (30; 212).
2. A process as claimed in Claim 1, wherein said distillation system comprises a high
pressure column (15) and a low pressure column (25) and wherein:
(a) at least a portion (11) of the compressed air feed (10) is fed to the high pressure
column (15) in which the air feed (11) is rectified into a high pressure nitrogen
overhead (27) and a high pressure crude liquid oxygen bottoms (26);
(b) at least a portion of the high pressure crude liquid oxygen bottoms (26) is fed
to the low pressure column (25) in which the high pressure crude liquid oxygen bottoms
(26) is rectified into a low pressure nitrogen overhead (28) and a low pressure liquid
oxygen bottoms;
(c) at least a portion (30) of the high pressure nitrogen overhead (27) is condensed
(31) and at least a portion (32) of the condensed high pressure nitrogen overhead
is returned to the high pressure column (15) as reflux;
(d) at least a portion of the low pressure liquid oxygen bottoms is boiled by a bottom
reboiler (31; 211) in which a suitable first process stream (30; 212) is condensed;
(e) the low purity oxygen product stream (38) is withdrawn from the low pressure column
(25);
(f) the oxygen-rich stream (26; 35) having an oxygen concentration at least equal
to that of the high pressure crude liquid oxygen bottoms (26) is withdrawn from the
distillation system (15, 25) and fed to a high purity column (23) in which it is rectified
into an oxygen-lean overhead vapour (37) and a high-purity liquid oxygen bottoms;
(g) at least a portion of the high purity liquid oxygen bottoms is boiled by a bottom
reboiler (22) by condensation of a suitable second process stream (21) compressed
to a pressure higher than that of the first process stream (30; 212) boiling the low
pressure column (15);
(h) the high purity oxygen product stream (36) is withdrawn from the high purity column
(23).
3. A process as claimed in Claim 2, wherein the process stream providing bottom reboil
(22) to the high purity column (23) is a portion (21) of the compressed feed air (10)
which has been further compressed (20).
4. A process as claimed in Claim 2, wherein the process stream providing bottom reboil
(22) to the high purity column (23) is at least a portion of the high pressure nitrogen
overhead (27) which has been further compressed (20).
5. A process as claimed in any one of Claims 2 to 4, wherein the oxygen-rich stream (35)
is withdrawn from the low pressure column (25).
6. A process as claimed in Claim 5, wherein the oxygen-lean overhead vapour (37) is returned
to the low pressure column (25) at substantially the same location as that at which
the oxygen-rich stream (35) was withdrawn.
7. A process as claimed in Claim 5 or Claim 6, wherein the oxygen-rich stream (35) is
withdrawn from the sump of the low pressure column (25).
8. A process as claimed in any one of Claims 2 to 4, wherein the oxygen-rich stream (611)
is a portion of the high pressure crude liquid oxygen bottoms (26).
9. A process as claimed in Claim 8, wherein a vapour stream (711) is withdrawn from the
low pressure column (25) at a location below the feed of the high pressure crude liquid
oxygen bottoms (26) thereto and is fed to the high purity column (23) at a location
below the feed of the oxygen-rich stream (611) thereto.
10. A process as claimed in any one of Claims 2 to 9, wherein the process stream providing
bottom reboil (211) to the low pressure column is a portion (212) of the compressed
feed air (10) and the condensation of at least a portion (30) of the high pressure
nitrogen overhead (27) is in a reboiler (31) at an intermediate location in the low
pressure column (25).
11. A process as claimed in Claim 10, wherein the oxygen-rich stream (35) is withdrawn
from the low pressure column (25) at a location above said intermediate reboiler (31)
and an argon rich vapour stream (511) withdrawn from an intermediate location of the
high purity column (23) is separated in an argon column (510) to produce an argon
product stream and a liquid argon-depleted stream (512) which is returned to the high
purity column (23).
12. A process as claimed in any one of Claims 2 to 11 comprising a distillation system
as defined in Claim 2, wherein all of the condensed high pressure nitrogen overhead
(27) is fed (30) to the high pressure column as reflux and the low pressure column
(25) is refluxed with a sidestream (319) withdrawn from the high pressure column (15).
13. A process as claimed in Claim 12, wherein a portion (610) of the nitrogen rich sidestream
(319) is fed to the high purity column (23).
14. An apparatus for producing low purity oxygen and high purity oxygen products by the
cryogenic distillation of a compressed air feed (10) by the process of Claim 1, said
apparatus comprising:
(i) a distillation system (15, 25) for rectifying the compressed air feed (10) and
comprising a low purity column (25) for producing a low purity (less than 97%) oxygen
product stream (38) and a nitrogen-rich stream (28);
(ii) a bottom reboiler (31; 211) in said low purity column (25) for condensing a suitable
first process stream to provide boilup to the low purity column (25);
(iii) means (27, 30; 11, 212) for supplying said first process stream to said reboiler;
and
(iv) means (38) for withdrawing the low purity oxygen product stream from the apparatus,
characterized in that the apparatus further includes
(v) a high purity column (23) for rectifying an oxygen rich stream having an oxygen
concentration at least equal to that of the feed (26') to the low purity column (25)
to provide a high-purity (more than 97%) oxygen product stream;
(vi) means (35; 26, 611) for withdrawing said oxygen-rich stream from the distillation
system (15, 25) and feeding it to the high purity column (23);
(vii) a bottom reboiler (22) in said high purity column (23) for condensing a suitable
second process stream to provide boilup to the high purity column (23);
(viii) means (20, 21) for supplying said second process stream to said high purity
column reboiler (22) at a higher pressure than said first process stream is supplied
to the low purity column reboiler (31; 211); and
(ix) means (36) for withdrawing the high purity oxygen product stream from the apparatus.
15. An apparatus as claimed in Claim 14, wherein said distillation system comprises:
(1) a high pressure column (15) for rectifying at least a portion of the compressed
air feed (10) into a high pressure nitrogen overhead and a high pressure crude liquid
oxygen bottoms;
(2) a low pressure column (25) for rectifying the high pressure crude liquid oxygen
bottoms into a low pressure nitrogen overhead and a low pressure liquid oxygen bottoms;
(3) means (26, 29) for feeding at least a portion of the high pressure crude liquid
oxygen bottoms to the low pressure column (25);
(4) means (27, 31, 32) for condensing at least a portion of the high pressure nitrogen
overhead and returning at least a portion of the condensed high pressure nitrogen
overhead to the high pressure column (15) as reflux;
(5) a bottom reboiler (31; 211) in the low pressure column (25) for condensing a suitable
first process stream to provide boilup to the low pressure column (25);
(6) means (27, 30; 11, 212) for supplying said first process stream to said reboiler
(31, 211);
(7) means for withdrawing the low purity oxygen product stream (38) from the apparatus;
(8) a high purity column (23) for rectifying an oxygen rich stream into an oxygen-lean
overhead vapour (37) and a high-purity liquid oxygen bottoms;
(9) means (26; 35) for withdrawing the oxygen-rich stream from the distillation system
(15, 25) and feeding it to the high purity column (23);
(10) a bottom reboiler (22) in said high purity column (23) for condensing a suitable
second process stream to provide boilup to the high purity column (23);
(11) means (20, 21) for supplying said second process stream to said high purity column
reboiler (22) at a higher pressure than said first process stream is supplied to the
low pressure column reboiler (31; 211); and
(12) means (36) for withdrawing the high purity oxygen product stream from the apparatus.
16. An apparatus as claimed in Claim 15 including the components required to carry out
an individual process as claimed in any one of Claims 3 to 13.