AIR SEPARATION UNIT AND METHOD FOR PRODUCTION OF HIGH PURITY NITROGEN PRODUCT USING A DISTILLATION COLUMN SYSTEM WITH AN INTERMEDIATE PRESSURE KETTLE COLUMN

Abstract

An air separation unit and associated method for separating air by cryogenic distillation using a distillation column system including a higher pressure column, a lower pressure column, an intermediate pressure kettle column, and an argon column arrangement is provided. The disclosed air separation unit and method is particularly suited for production of high purity nitrogen for electronics applications and includes nitrogen recycle circuit necessary to attain the higher purity nitrogen products. In addition to the intermediate pressure kettle column, the present air separation unit and associated method employs a once-through argon condenser, preferably disposed within the lower pressure column as well as a once-through kettle column reboiler, a once-through kettle column condenser.

Claims

1. An air separation unit for production of oxygen, nitrogen and argon from a purified, compressed feed air stream, the air separation unit comprising: a main heat exchanger configure to cool the purified, compressed feed air stream via indirect heat exchange against one or more product streams, a recycle stream and a waste stream to yield a liquid air stream; a higher pressure column configured to receive the liquid air steam, a nitrogen recycle stream, and a first reflux stream and yield a nitrogen-rich overhead, a kettle liquid, and a dirty shelf nitrogen stream; a lower pressure column configured to receive the dirty shelf nitrogen stream from the higher pressure column as a second reflux stream, and yield a nitrogen overhead stream, a nitrogen waste stream, an oxygen liquid bottoms, an oxygen waste stream, and an argon-oxygen side stream; a main condenser-reboiler disposed in the lower pressure column and configured for thermally coupling the higher pressure column and the lower pressure column by liquefying at least a portion of the nitrogen-rich overhead from the higher pressure column against the oxygen liquid bottoms in the lower pressure column to yield the first reflux stream and the oxygen waste stream; an intermediate pressure kettle column configured to receive the kettle liquid from the higher pressure column at an intermediate location of the kettle rectification column and yield an oxygen-rich bottoms and a nitrogen rich overhead; a once-through kettle column reboiler configured to boil a portion of the oxygen-rich bottoms in the intermediate pressure kettle column against a first part of the argon-oxygen side stream to yield an ascending vapor stream in the intermediate pressure kettle column and an argon-oxygen liquid stream that is returned to an intermediate location of the lower pressure column; a once through kettle column condenser configured to condense all or a portion of the nitrogen rich overhead of intermediate pressure kettle column against a portion of the oxygen-rich bottoms of the intermediate pressure kettle column; and an argon column arrangement comprising one or more argon columns and an argon condenser, the argon column is configured to receive a second part of the argon-oxygen side stream from the lower pressure column and yield an argon-rich overhead and an oxygen-rich liquid stream that is returned to the intermediate location of the lower pressure column; a nitrogen recycle circuit configured to recycle all or a portion of the nitrogen overhead from the lower pressure column to the higher pressure column as the nitrogen recycle stream; and wherein a portion of the nitrogen-rich overhead from the higher pressure column is warmed in the main heat exchanger and taken as a higher purity nitrogen product stream and a portion of the oxygen liquid bottoms is taken as an oxygen product stream.

2. The air separation unit of claim 1, wherein the nitrogen recycle circuit further comprises: a nitrogen superheater configured to warm the nitrogen overhead from the lower pressure column wherein the warmed stream is further warmed in the main heat exchanger to yield a warmed nitrogen overhead stream; a recycle compressor configured to compress the warmed nitrogen overhead stream to yield a compressed recycle stream; and wherein the compressed recycle stream is cooled in the main heat exchanger and then directed to the higher pressure column.

3. The air separation unit of claim 1, wherein the once through kettle column condenser is configured to condense a first portion of the nitrogen rich overhead of the intermediate pressure kettle column against a first portion of the oxygen-rich bottoms of the intermediate pressure kettle column and wherein a second portion of the nitrogen rich overhead of the intermediate pressure kettle column is taken as a lower pressure nitrogen product stream.

4. The air separation unit of claim 3, wherein a first portion of the condensate from the once through kettle column condenser is directed as reflux to the intermediate pressure kettle column and a second portion of the condensate from the once through kettle column condenser is directed to the lower pressure column as a third reflux stream.

5. (canceled)

6. The air separation unit of claim 2, wherein: the waste nitrogen from the lower pressure column is warmed in the superheater and subsequently mixed with the oxygen waste stream to form a mixed waste stream; the mixed waste stream is warmed in the main heat exchanger to yield a warmed waste stream.

7. The air separation unit of claim 6, further comprising: a waste turbine configured to expand the warmed waste stream to produce a waste exhaust stream; and wherein the waste exhaust stream is warmed in the main heat exchanger to provide supplemental refrigeration.

8. The air separation unit of claim 1, wherein a portion of a liquid nitrogen stream exiting the main condenser-reboiler is taken as a liquid nitrogen product stream.

9. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] It is believed that the claimed invention will be better understood when taken in connection with the accompanying drawings in which:

[0024] FIG. 1 depicts a McCabe-Thiele diagram for a conventional three column arrangement and cycle known in the prior art where the flow rate of elevated pressure nitrogen product is about the same flow rate of oxygen product;

[0025] FIG. 2 depicts a McCabe-Thiele diagram for an embodiment of the present air separation unit and method comprising a four column arrangement including a higher pressure column, a lower pressure column, an intermediate pressure column, and an argon column;

[0026] FIG. 3 shows a schematic of the process flow diagram for an air separation unit having a distillation column system that includes an intermediate pressure kettle column; and

[0027] FIG. 4 shows a schematic of the process flow diagram for an embodiment of the air separation unit and associated method of air separation configured to produce a high purity nitrogen product stream,

DETAILED DESCRIPTION

[0028] The disclosed air separation unit and method for separating air by cryogenic distillation using a four column arrangement shown in FIG. 3 includes a higher pressure column, a lower pressure column, an intermediate pressure column, and an argon column arrangement and is particularly suited for production of normal purity oxygen, argon and one or more nitrogen products where the nitrogen production rate of elevated pressure nitrogen gas product and/or liquid nitrogen product exceeds 50% of the total normal purity oxygen production rate.

[0029] By generating additional nitrogen reflux and/or supplemental elevated pressure (i.e. intermediate pressure) nitrogen product from the intermediate pressure column, or kettle column, the present air separation unit system and method enables higher oxygen recovery, higher argon recovery with improved efficiency and reduce power consumption relative to conventional three column arrangements and many of the prior art intermediate column arrangements. On the McCabe-Thiele diagram of FIG. 1 for the conventional three column arrangement, the tight pinch in the upper portion of the lower pressure column is relieved by taking advantage of the excess distillation driving force in selected sections of the lower pressure column. In addition, one may also realize certain benefits, including an efficiency benefit in applications when the elevated pressure nitrogen gas product and/or liquid nitrogen product production rate is below 50% of the total oxygen production rate.

[0030] FIG. 2 illustrates the McCabe-Thiele diagram for an embodiment of the present four column arrangement and cycle. As indicated above, the space between the equilibrium line 102 and the operating line 103 is indicative of the driving force for that portion of the column. Note that the mass transfer driving force in the section of the lower pressure column between the kettle liquid feed and argon column draw is reduced. The reduction in the mass transfer driving force in the section of the lower pressure column between the kettle liquid feed and argon column draw is achieved by driving the reboiler of the intermediate pressure column or kettle column with vapor from the base of this section of the lower pressure column, which is preferably the same source as the vapor that is fed from the lower pressure column to argon column.

[0031] A key principle or characteristic of the present four column arrangement and associated air separation cycle is that: (1) the mass transfer driving force in the top section of the lower pressure column is increased by taking advantage of the excess driving force in the section between the kettle liquid feed and argon column draw; and (2) the mass transfer driving force of the bottom section of the lower pressure column is not reduced. In addition, the use of present four column arrangement and cycle reduces or avoids the need to draw product nitrogen from the top of the lower pressure column due to its ability to produce supplemental nitrogen product and/or nitrogen reflux from the intermediate pressure column.

[0032] Turning now to FIG. 3, there is shown an air separation unit 10 that comprises a warm end arrangement and a cold end arrangement that includes one or more heat exchangers and a distillation column system 30. As discussed in more detail below, the one or more heat exchangers preferably include at least a main heat exchanger 20 and a nitrogen superheater 21 or subcooler.

[0033] The warm-end arrangement is configured for conditioning a feed air stream for separation into its constituent components, namely argon, oxygen, and nitrogen, The warm-end arrangement receives a feed air stream, compresses the feed air stream in a series of main air compression stages and purifies the compressed air stream in a pre-purification unit to produce a compressed and purified air stream 12.

[0034] A first main portion of the compressed and purified air stream 12 is directed to the main heat exchanger 20 where it is cooled to temperatures suitable for rectification in the distillation column system 30 and exits the main heat exchanger as a cooled, compressed and purified stream 22. A second portion of the compressed and purified air stream 14 is further compressed in a first booster compressor 13 and cooled in aftercooler. A part of the further compressed second portion of the compressed and purified air stream is still further compressed in a second booster compressor 15 and cooled in aftercooler to yield a booster air stream 16 that is also directed to the main heat exchanger 20. The booster air stream 16 is cooled in main heat exchanger 20 to yield a liquid air stream 26 that is directed to the higher pressure column 40 of the distillation column system 30.

[0035] The remaining part of the further compressed first portion of the compressed and purified air stream is diverted as stream 18 that is further compressed in another booster compressor 17 to yield a turbine air stream 25 that is then partially cooled in main heat exchanger 20. The partially cooled stream is the expanded in turbine 27 yielding an exhaust stream 28 that is also directed to the higher pressure column 40 of the distillation column system 30. Note the exhaust stream 28 may be combined with the cooled, compressed and purified air stream 22.

[0036] The first portion of the compressed and purified air stream 12 as well as the turbine air stream 18 and booster air stream 16 are cooled in the main heat exchanger 20 via indirect heat exchange with a plurality of streams from the distillation column system 30 including: a clean shelf nitrogen stream 44; a pumped liquid oxygen stream 53; a pumped high pressure gaseous nitrogen stream 68; a waste nitrogen stream 59; and a low pressure gaseous nitrogen stream 52. The warmed streams exit the main heat exchanger 20 as: a product grade gaseous nitrogen stream 144; a product grade gaseous oxygen stream 153; a product grade high pressure gaseous nitrogen stream 168; a warmed waste nitrogen stream 159; and a product grade low pressure gaseous nitrogen stream 152.

[0037] The illustrated distillation column system 30 comprises: a higher pressure column 40, an intermediate pressure column or kettle column 70; a lower pressure column 50; an integrated argon condenser 65; an argon column 80; and a high ratio column 90.

[0038] The higher pressure column 40 configured to receive one or more streams of compressed, purified air including the liquid air stream 26, the cooled, compressed and purified air stream 22, as well as the exhaust stream 28 together with a reflux stream and yields a nitrogen-rich overhead 42, a clean shelf vapor stream 44, a dirty shelf nitrogen stream 46, a kettle liquid 48, and a synthetic liquid air stream 45 taken from an intermediate location of the higher pressure column 40.

[0039] The lower pressure column 50 is configured to receive the synthetic liquid air stream 45, an oxygen-rich bottoms 83 and or more reflux streams or other streams to yield a low pressure product grade nitrogen overhead 52, an oxygen liquid 51 at the bottom of the column to be taken as a liquid oxygen stream 53, and an argon-oxygen containing side stream 56 taken from an intermediate location of the lower pressure column 50. A portion of the liquid oxygen stream 53 may be taken as a liquid oxygen product 154 while the majority of the liquid oxygen stream 53 is pumped via pump 55 and vaporized in the main heat exchanger to produce the gaseous oxygen product 153. The one or more reflux streams introduced into the lower pressure column 50 preferably include stream 78B from the kettle column condenser 75 and the dirty shelf nitrogen stream 46 from the higher pressure column which streams may be combined to yield a mixed shelf reflux stream 47. The purity of the dirty shelf nitrogen streams 78B, 46 from the higher pressure column 40 and from the kettle column 70 is optimized for feed to the lower pressure column 50 at or near a location where waste nitrogen is withdrawn from lower pressure column.

[0040] The lower pressure column 50 also houses a main condenser-reboiler configured for thermally coupling higher pressure column 40 and lower pressure column 50 by liquefying at least a portion of the nitrogen-rich overhead 42 from the higher pressure column 40 against the oxygen liquid 51 at the bottom of the lower pressure column 50 to yield a nitrogen reflux stream 62 directed to the higher pressure column 40 and another nitrogen stream 61, a portion of which is directed as reflux stream 63 to the top of the lower pressure column 50. Another portion of nitrogen stream 61 is preferably taken as high pressure product grade nitrogen stream 68 that is pumped via pump 67 and directed to the main heat exchanger 20 while the remaining portion of nitrogen stream 61 is preferably taken as a liquid nitrogen product stream 164.

[0041] A once-through argon condenser is also disposed within the lower pressure column 50 at a location above the intermediate location of the lower pressure column. The argon condenser is configured to condense an argon-rich overhead taken from the argon column against all or a portion of the descending liquid in the lower pressure column 50 including feed streams 79, 97, 98, and optionally a diverted portion 66 of the liquid air stream 45 to produce a crude argon stream 69, a portion of which is a reflux stream 81 for the argon column 80.

[0042] In some cases, it may be preferred to use the diverted portion of the subcooled synthetic liquid air 66 to further increase the temperature driving force of the argon condenser 65. Doing this introduces this liquid air to a non-ideal location within the lower pressure column, resulting in a small penalty in argon recovery. Unlike the prior art systems and methods, using a small, diverted portion of the subcooled synthetic liquid air 66 to drive the argon condenser 65 in the present air separation unit and method can enable a further increase in the driving force of the kettle column reboiler 71 and kettle column condenser 75. When this is desirable, it results in a further increase in oxygen recovery and a further reduction in power consumption.

[0043] An intermediate pressure column or kettle column 70 is configured to receive the kettle liquid 48 from the higher pressure column 40 and yield an oxygen-rich kettle bottoms 72 and a nitrogen rich kettle overhead 76A. The kettle liquid 48 is preferably subcooled in the nitrogen superheater 21 and routed through the high ratio column reboiler 95. Preferably, the subcooled kettle liquid 48 is then introduced into the kettle column 70, preferably at an intermediate location of the kettle column several stages above the bottom section. Operatively associated with the kettle column 70 is a once-through kettle column reboiler 71 and a once-through kettle column condenser 75. The once-through kettle column reboiler 71 is configured to boil a portion of the descending liquid in the kettle column 70 against a first part 58 of the argon-oxygen side stream 56 to yield an ascending vapor stream in the kettle column 70 and an argon-oxygen liquid stream 77 that is returned at or near the intermediate location of the lower pressure column 50. For that reason, the kettle column 70 is spatially disposed preferably above the intermediate location of lower pressure column 50 so that the return liquid from the kettle column reboiler and the transferred kettle can be fed to the lower pressure column 50 by gravity. Also, unlike some of the prior art disclosures related to intermediate pressure columns, none of the synthetic liquid air or liquid air feed is directed to the kettle column.

[0044] The once-through kettle column condenser 75 is configured to condense all or a portion of the nitrogen rich kettle overhead 76A of the kettle column 70 against a first major portion 73 of the oxygen-rich kettle bottoms 72 of the kettle column 70 to yield a nitrogen reflux stream 78A for the kettle column 70, a shelf nitrogen liquid stream 78B and a boil-off vapor stream or transferred kettle stream 79 that is returned to the lower pressure column 50. The first major portion 73 of the oxygen-rich kettle bottoms 72 is let down in pressure and then fed to the Kettle Column condenser 75. The remaining or second minor portion 74 of the oxygen-rich kettle bottoms 72 of the kettle column 70 is also let down in pressure and preferably directed to a high ratio column condenser 96.

[0045] In the disclosed embodiment of FIG. 3, the kettle column 70 preferably has between 15 stages and 30 stages of separation and can use either structured packing or trays, although structured packing is preferred. When using only between 15 stages and 30 stages of separation, the shelf nitrogen stream 78B taken from the kettle column 70 is of a lower purity and referred to as a dirty shelf nitrogen stream. In this embodiment, the pressure of the intermediate pressure column or kettle column 70 is set by the temperature differences of the kettle column condenser 75 and kettle column reboiler 71, typically in the range of about 2 bara to 3 bara.

[0046] As indicated above, the kettle column of FIG. 3 produces dirty shelf liquid nitrogen reflux. The dirty shelf nitrogen stream 78B augments the dirty shelf reflux stream 46 produced by the higher pressure column 40 and forms the mixed reflux stream 47 for the lower pressure column 50. The dirty shelf liquid configuration of FIG. 3 maximizes the power savings of the air separation unit 10 but sacrifices some argon recovery compared to other configurations that use a higher purity nitrogen stream or clean shelf nitrogen stream from the kettle column having more than 30 stages of separation.

[0047] The argon column 80 is configured to receive a second part 57 of the argon-oxygen side stream 56 from the lower pressure column 50 and yield an argon-rich overhead 82 that is directed to the once through argon condenser 65 and an oxygen-rich bottoms 83 that is returned at or near the intermediate location of the lower pressure column 70.

[0048] The high ratio column 90 is configured to receive a portion 84 of the crude argon stream 69 from the once-through argon condenser 65 and rectify the portion 84 of the crude argon stream 69 to yield an argon-rich liquid 94 and an overhead vapor 92. A portion of the argon-rich liquid 94 at the bottom of the high ratio column 90 is taken as liquid argon product 194. Associated with the high ratio column 90 is a high ratio reboiler 95 and a high ratio column condenser 96. The high ratio column reboiler 95 is disposed at the bottom of the high ratio column 90 and configured for reboiling another portion of the argon-rich liquid at the bottom of the high ratio column 90 against a stream of the kettle liquid 48 to produce an ascending vapor stream in the high ratio column 90. The high ratio column condenser 96 is configured to condense the overhead vapor 92 from the high ratio column 90 and return all or a portion of the condensate as a high ratio column reflux stream 97. All or a portion of the high ratio column condenser boil-off vapor 98 as well as a portion of the excess condensing media 99 is be returned to the lower pressure column 50. Together the argon column 80, the high ratio column 90, the once-through argon condenser 65, the high ratio column reboiler 95, and the high ratio column condenser 96 make up an argon column arrangement.

[0049] It is essential that the argon condenser 65 is once-through on the boiling side and this feature provides a large advantage over the prior art disclosures of four column arrangements, due in part, to the impure boiling stream. A once-through up-flow configuration for the argon condenser 65 is preferred due to its lower cost and simplicity, although a once-through downflow configuration for the argon condenser 65 would also provide an advantage.

[0050] A key difference between the present air separation unit and associated methods and those disclosed in the prior art references related to four column arrangements with an intermediate pressure column, is the argon condenser 65 is an integrated unit disposed within the lower pressure column and therefore not directly coupled to the kettle column. By locating the Argon condenser within the lower pressure column, the boiling flow through the argon condenser is much greater, and by locating the argon condenser at the optimal location in the lower pressure column, the composition of the boiling stream (i.e. descending liquid) can be higher in nitrogen content. The optimal location for the argon condenser within the lower pressure column is such that the T of the argon condenser does not limit the ability to drive the kettle column before the Ts of the kettle column reboiler and kettle column condenser, while not penalizing the separation within the lower pressure column, for example, by locating the argon condenser too high within the column.

[0051] The benefit of using the once-through kettle column reboiler and kettle column condenser, as well as the integrated once-through argon condenser is large. This configuration naturally increases the temperature differences of each device due to the lower purity boiling streams compared to a pool boiling (i.e. thermosyphon) configurations for such devices. But rather than designing and operating the air separation cycle with large temperature differences, which would make the kettle column reboiler, kettle column condenser, and argon condenser smaller, and save some capital cost, it is far better to use these larger temperature driving forces to dramatically increase the capacity of the intermediate pressure column or kettle column. Increasing the capacity of the kettle column results in much more liquid reflux production (or product nitrogen generation) from the kettle column, and a much larger advantage for its use. The greater production of nitrogen from the kettle column means that the oxygen-rich kettle bottoms 72 is richer in oxygen. This results in reduced temperature differences in the kettle column reboiler, kettle column condenser, and argon condenser. Ultimately, size and minimum temperature difference design constraints of these devices limit the capacity of the kettle column, but at a much greater magnitude in this configuration than for the prior art.

[0052] The transferred kettle stream 79 exiting the kettle column condenser 75 is fed to the lower pressure column 50. The transferred kettle stream 79 is likely a two phase stream and it may be transferred to the lower pressure column as a two phase stream or it may be separated in a phase separator (not shown) before transferring to the lower pressure column 50. If a phase separator is used, then the kettle column condenser 75 may be contained within the phase separator. In the illustrated embodiment, the transferred kettle stream 79 is preferably fed to the lower pressure column 50 just above the location of the integrated once-through argon condenser 65. Within the lower pressure column 50 the liquid portion of the transferred kettle stream 79 is combined with the downflowing liquid in the lower pressure column 50.

[0053] Similar to the argon condenser, it is essential that both the kettle column reboiler and kettle column condenser are once-through on the boiling side, A once-through up-flow configuration of the kettle column reboiler as well as the kettle column condenser is preferred. A once-through downflow configuration for the kettle column reboiler and kettle column condenser would provide some additional advantage but will be more costly and would probably not be justified in most cases. Due to the large feed liquid flow to the boiling side of the once-through kettle column reboiler, the vapor fraction of the exiting fluid is low. This means that the once-through up-flow or once through downflow configurations can be used safely. With the once through up-flow or downflow kettle column reboiler, the vapor fraction of its outlet is minimized which enables the kettle column reboiler to operate at high duty with an appropriate T so that its size is reasonable, and within safe operating criteria with its walls being sufficiently wetted. For the kettle column condenser and argon condenser similar points can be made. They handle the feed of large liquid flow rates. As a result, the vapor fraction exiting is low. This enables safe operation and maximizes the T at a given heat duty.

[0054] Since the boiling fluid within the kettle column reboiler is very impure, a once-through kettle column reboiler provides a large benefit. If a pool boiler (i.e., thermosyphon reboiler) were used instead, the boiling flow would be significantly higher in oxygen concentration which would decrease its T. The resulting penalty would be a greatly reduced ability to drive the kettle column, with much less production of nitrogen. The kettle column reboiler is preferably driven by the same vapor source from the lower pressure column that feeds the argon column. Also, the liquid returned from the kettle column condenser is fed to the same general location in the lower pressure column as the vapor source.

[0055] Unlike some of the prior art disclosures related to four column arrangements with an intermediate pressure column which splits the oxygen-rich kettle bottoms between the kettle column condenser and the argon condenser severely limiting the kettle column capacity, none of the oxygen-rich kettle bottoms 72 in the present air separation unit and method are directed to the argon condenser 65. Rather, the oxygen-rich kettle bottoms 72 are supplied mainly to the kettle column condenser except for a very minor takeoff that is directed to the high ratio column condenser, if it is used.

[0056] In some respects, the process flow diagrams depicted in FIG. 4 is somewhat similar to the process flow diagram of FIG. 3 described above, and for sake of brevity, much of the descriptions of the detailed arrangements of the intermediate pressure kettle column will not be repeated. Rather, the following discussion will focus on the key features of the high purity nitrogen and argon producing air separation unit 10 shown in FIG. 4 and the differences in the process flow diagram of the depicted in FIG. 4 when compared to the process flow diagram of the air separation unit depicted in FIG. 3.

[0057] As indicated above, there is a need for enhancements to high purity nitrogen and argon producing air separation units with nitrogen recycle in order to potentially reduce power costs while concurrently improving argon recovery and ensuring sufficient availability of higher purity nitrogen product streams. One such improvement is the use of an intermediate pressure kettle column 70 to further fractionate the kettle liquid 48 from the higher pressure column 40 which improves the available nitrogen reflux 78B to the lower pressure column 50. With the additional nitrogen reflux 78B from the intermediate pressure kettle column 70, the rate of nitrogen recycle can be reduced, saving power costs. While the intermediate pressure kettle column 70 represents an additional capital cost compared to the prior art three column air separation units with a nitrogen recycle circuit, this is partially off-set by the capital cost reduction in the reduction in other equipment including the size of the main heat exchanger 20, the main condenser-reboiler 60, the higher pressure column 40, and the lower pressure column 50 there will be cost savings in much of the other equipment due to the reduced rate of nitrogen recycle.

[0058] Turning now to FIG. 4, there is shown an embodiment of a high purity nitrogen and argon producing air separation unit 10 with nitrogen recycle. The key differences between this embodiment and the embodiment shown in FIG. 3 is the nitrogen recycle circuit, production of ultra-high purity nitrogen from the higher pressure column, oxygen production, and supplemental refrigeration using a waste expander configured to expand the waste nitrogen and waste oxygen streams.

[0059] As shown in FIG. 4, the nitrogen recycle circuit takes the nitrogen overhead 52 from the lower pressure column 50 and warms that stream in the nitrogen superheater 21 and the main heat exchanger 20. The warmed nitrogen stream is then compressed in a nitrogen recycle compressor 350, aftercooled in an aftercooler. The compressed recycle stream 352 is further cooled in the main heat exchanger 20 before being directed to the higher pressure column 40. By continuous recycling of the nitrogen overhead 52 from the top of the lower pressure column 50 to the higher pressure column 40, the purity of the nitrogen overhead 42 from the higher pressure column 40 is increased to the point where a portion of the nitrogen overhead 42 from the higher pressure column 40 can be taken as an ultra-high purity nitrogen vapor stream 44 that is warmed in the main heat exchanger 20 to yield an ultra-high purity nitrogen product stream 144.

[0060] FIG. 4 also shows an oxygen liquid stream 51 taken from the bottom of the lower pressure column 50 to a storage tank 151. Liquid oxygen 154 may be taken directly from the storage tank 151 or an oxygen stream 53 may be pumped from the storage tank 151 via pump 155 to the main heat exchanger 20 where the oxygen is vaporized to form a gaseous oxygen product stream 153. The disclosed air separation unit 10 and associated cycle also uses supplemental refrigeration from a waste expansion circuit, which is configured to receive a waste nitrogen stream 59 from the lower pressure column 50 several stages below the nitrogen overhead as well as a waste oxygen stream 355 taken from a lower section of the lower pressure column 50. The waste nitrogen stream 59 is warmed in the nitrogen superheater 21 with the resulting warmed waste nitrogen stream 359 combined with and waste oxygen stream 355 to form a mixed waste stream 351 that is warmed in the main heat exchanger 20 and then expanded in a waste expander 337 to generate a waste exhaust stream 338 that is warmed in the main heat exchanger 20 to yield the necessary refrigeration for the cycle. The warmed waste exhaust stream 340 is vented or used for other purposes within the plant or facility. The waste expander 337 is shown operatively coupled to a generator 339.

[0061] Similar to the air separation unit in FIG. 3, the mass transfer driving forces in portions of the lower pressure column of the air separation unit 10 depicted in FIG. 4 are unnecessarily high and one can exploit this feature by using vapor stream 58 originating from an intermediate location of the lower pressure column 50 to produce vapor in the bottom of the intermediate pressure kettle column 70 via the kettle column reboiler 71 and by using the liquid bottoms 72, 73 from the intermediate pressure kettle column 70 to condense the nitrogen overhead 76A from the intermediate pressure kettle column 70 in the kettle column condenser 75 to produce liquid nitrogen streams 78A,78B.

[0062] Also, similar to the embodiment discussed with reference to FIG. 3, the kettle column reboiler 71 and kettle column condenser 75 in the embodiment of FIG. 4 are once through devices. Since the boiling stream in the kettle column reboiler 71 and the kettle column condenser 75 are of mixed purity, once through designs lead to much larger ATs than arrangements using pool boilers. The best way to benefit from this is to drive the kettle column reboiler 71 and kettle column condenser 75 to a maximum level, limited by the T or other sizing criteria. This maximizes the benefit realized by the intermediate pressure kettle column 70. Once through up-flow devices are usually preferred for the kettle column reboiler 71 and the kettle column condenser 75. Once through downflow devices may provide slightly more benefit, but usually the cost to build them is considerably higher. The vapor fraction on boiling side is quite low in the kettle column reboiler 71 and kettle column condenser 75 which allows safe operation of kettle column reboiler 71 and kettle column condenser 75 as the devices will not approach unsafe conditions such as boiling to dryness.

[0063] Although not shown, a portion of the nitrogen overhead 76A from the intermediate pressure kettle column 70 may be taken as an elevated pressure nitrogen vapor product stream. Taking a portion of the nitrogen overhead from the intermediate pressure kettle column as an elevated pressure nitrogen vapor product stream enhances the recovery benefit of the intermediate pressure kettle column and enables possible further reduction of the nitrogen recycle flow. In other embodiments, the distillation column system of the air separation unit may further include a fifth distillation column, namely an ultra-high purity oxygen column (not shown) where production of an ultra-high purity (UHP) oxygen product stream is required.

[0064] While the present invention has been described with reference to a preferred embodiment or embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present invention as set forth in the appended claims.