AIR SEPARATION UNIT AND METHOD FOR CRYOGENIC SEPARATION OF AIR USING A DISTILLATION COLUMN SYSTEM INCLUDING 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 present air separation unit and associated method employs a once-through kettle column reboiler, a once-through kettle column condenser, and a once-through argon condenser. The once through argon condenser is disposed within the lower pressure column where an argon-rich vapor stream is condensed against the descending liquid in the lower pressure column.

Description

TECHNICAL FIELD

The present system and method relates to separating air by cryogenic distillation, and more particularly, to an air separation unit and method that employs a higher pressure column, a lower pressure column, an intermediate pressure kettle column, and an argon column arrangement.

BACKGROUND

The conventional air separation cycle employing a higher pressure column, a lower pressure and an argon column is the standard choice for an air separation unit when the oxygen product is needed at 99.5% purity or higher, which is often referred to as ‘normal purity oxygen’ together with an argon product. However, for normal purity oxygen and argon production, a three column arrangement exhibits a significant operational deficiency, as illustrated in the McCabe-Thiele diagram of FIG. 1.

McCabe-Thiele diagrams are instructive because they illustrate the magnitude of the mass transfer driving forces in the distillation columns of an air separation unit. A McCabe-Thiele diagram for the lower pressure column is key for analysis of a cryogenic oxygen based air separation process. The space in a McCabe-Thiele diagram between the equilibrium line and the operating line is indicative of the driving force for that portion of the column. A process that has large distillation driving forces will tend to have short distillation columns (i.e. not many stages of separation), but it will also be high in power consumption. Ideally, a very efficient cryogenic distillation process will have close, and fairly constant, approaches between the operating line and the equilibrium line. A McCabe-Thiele diagram normally plots the light key component in the liquid on the x-axis and the light key component in the vapor on the y-axis. In FIG. 1, the sum of the nitrogen and argon compositions for the liquid and the vapor are plotted on each axis. Argon is the light key in the bottom section of the lower pressure column while nitrogen is the light key in the rest of the lower pressure column. By plotting the sum of argon and nitrogen fractions in FIG. 1, the entire lower pressure column can be fairly characterized.

The McCabe-Thiele diagram illustrated in FIG. 1, depicts a scenario with a conventional three column arrangement and cycle where the flow rate of elevated pressure nitrogen product is about the same flow rate of oxygen product. The result is the lower pressure column separation is challenged. As a result, the McCabe-Thiele diagram shows a virtual pinch near the top, at an x-axis value of about 0.77. The addition of liquid air to the lower pressure column relieves this virtual pinch.

A feature of most conventional three column arrangements producing normal purity oxygen is the tight approach between the equilibrium line and the operating line in the bottom section of the lower pressure column depicted in the McCabe-Thiele diagram of FIG. 1. This is because the oxygen and argon have close relative volatilities. Thus, the removal of argon from oxygen that takes place in the bottom section of the lower pressure column is the most difficult separation among the three standard components of air. Note that without sufficient vapor boil-up in the lower pressure column produced by the main condenser-reboiler at the bottom of the lower pressure column, the slope of the operating line would be lower. The resulting compositional pinch would lead to much poorer oxygen recovery and much higher power consumption to produce that oxygen.

Note, that in FIG. 1, there is a large difference between the operating line and equilibrium line in the section of the lower pressure column between the kettle liquid feed and argon column draw. This always occurs and it results from the need for the high liquid to vapor ratio (L/V) in the bottom section of the lower pressure column. Also, there is a relatively large difference between the operating line and equilibrium line in the section between the liquid air feed and the kettle liquid feed. This means that these sections of the lower pressure column have a large mass transfer driving force, which is generally unavoidable in a three column arrangement and cycle producing normal purity oxygen and argon.

Another key observation with respect to the conventional three column arrangement is that the production of high quality nitrogen reflux by the higher pressure column is limited by the equilibrium between the feed air and the kettle liquid. That is, even if the higher pressure column contains a very high number of stages, the amount of reflux generated for supply to the lower pressure column will be limited. The equilibrium between the feed air and kettle liquid necessarily means that a large amount of nitrogen escapes in the kettle liquid and cannot be converted into nitrogen reflux in conventional three column arrangements.

To address these problems, the use of an intermediate pressure column, or kettle column has been suggested. In the intermediate pressure column, the kettle liquid from the higher pressure column is further fractionated to produce additional nitrogen reflux. Examples of the intermediate pressure column are disclosed in U.S. Pat. Nos. 5,675,977; 5,657,644; 5,862,680; and 6,536,232.

U.S. Pat. No. 5,675,977 discloses the use of an intermediate pressure column for the production of low purity oxygen, where the intermediate pressure column is driven with nitrogen vapor from the higher pressure column. By diverting a portion of nitrogen vapor from the higher pressure column to the intermediate pressure column re-boiler, the loss of nitrogen boil-up in the lower pressure column is perfectly tolerable for low purity oxygen production. However, this configuration is not suitable for normal purity oxygen production, where all the available nitrogen gas from the higher pressure column must be used in the main condenser-reboiler to produce sufficient nitrogen vapor boil-up in the lower pressure column. Also note that U.S. Pat. No. 5,675,977 does not disclose the production of any argon product and therefore the disclosed cycle has limited utility due to the lack of argon production.

U.S. Pat. No. 5,657,644 discloses an air separation unit and cycle that employs a higher pressure column, a lower pressure column, an intermediate pressure column, and an argon column that is configured to produce a liquid or crude argon product, a pumped oxygen product and a low pressure nitrogen product taken from the overhead of the lower pressure column. In the four column arrangement disclosed in U.S. Pat. No. 5,657,644, a stream of kettle liquid from the higher pressure column is introduced into a lower section of the intermediate pressure column which produces an oxygen-enriched liquid bottoms and a nitrogen enriched overhead. The intermediate pressure column also includes a bottom reboiler heated by an argon-oxygen containing stream from the lower pressure column and an overhead condenser that condenses a portion of the nitrogen enriched overhead against a portion of the oxygen-enriched liquid bottoms. Another portion of the oxygen-enriched liquid bottoms drives the argon condenser disposed above the argon column while the remaining portion of the oxygen-enriched liquid bottoms is returned to the lower pressure column.

While the air separation cycle disclosed in U.S. Pat. No. 5,657,644 is economically advantageous compared to conventional three column air separation units in that there is a reduced total power consumption as well as an increased argon recovery and oxygen recovery, there is a continuing need to find further improvements to further reduce the total power consumption and provide additional product flexibility, that includes a crude argon product or a refined argon product.

Another example is disclosed in U.S. Pat. No. 6,536,232 which also discloses an air separation unit that employs a higher pressure column, a lower pressure column, an intermediate pressure column, but without an argon column. The intermediate pressure column includes a bottom reboiler heated by an argon-oxygen containing stream from the lower pressure column and an overhead condenser that condenses a portion of the nitrogen enriched overhead against a portion of the oxygen-enriched liquid bottoms. Another portion of the oxygen-enriched liquid bottoms is returned to the lower pressure column.

The air separation cycle disclosed in U.S. Pat. No. 6,536,232 differs from the air separation cycle disclosed in U.S. Pat. No. 5,657,644 mainly due to the absence of the argon column and no production of argon. As indicated above, such improved air separation cycle has limited utility because there is no argon production as well as limited nitrogen production and there is no intermediate pressure nitrogen vapor product stream.

SUMMARY

The present invention may be broadly characterized as an air separation unit comprising: (i) a main air compression arrangement configured to receive a feed air stream and compress the feed air stream in a series of main air compression stages to yield a compressed feed air stream; (ii) a pre-purification unit configured to remove contaminants and water vapor from the compressed feed air stream to yield the purified, compressed feed air stream; (iii) a main heat exchanger configured to cool the one or more streams of purified, compressed air via indirect heat exchange against one or more waste and product streams; and (iv) a distillation column system having a higher pressure column, a lower pressure column, an intermediate pressure kettle column that produces an intermediate pressure nitrogen vapor product stream and an argon column arrangement with an argon condenser disposed within the lower pressure column configured to condense an argon-rich overhead against a portion of the descending liquid in the lower pressure column.

More specifically, within the distillation column system, the higher pressure column is configured to receive one or more streams of compressed, purified air and a first reflux stream and yield a nitrogen-rich overhead and a kettle liquid. The lower pressure column is configured to receive a diverted liquid air stream and a second reflux stream and yield a low pressure product grade nitrogen overhead, an oxygen liquid at the bottom of the column, and an argon-oxygen containing side stream. A main condenser-reboiler is 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 at the bottom of the lower pressure column to yield a higher pressure nitrogen product stream, the first reflux stream and the second reflux stream. The argon column arrangement includes one or more argon columns and the once-through argon condenser.

The intermediate pressure kettle column is configured to receive the kettle liquid from the higher pressure column and yield an oxygen-rich bottoms and a nitrogen rich overhead, a portion of which is taken as an intermediate pressure nitrogen product stream. A once-through kettle column reboiler is configured to boil a portion of the descending liquid in the intermediate pressure kettle column against a first part of the argon-oxygen side stream to yield an ascending vapor stream in the kettle column and an argon-oxygen liquid stream that is returned to the lower pressure column and a once-through kettle column condenser configured to condense all or a portion of the nitrogen rich overhead of the kettle column.

In some embodiments of the above-described air separation unit and associated method of air separation, the main air compression arrangement is configured to compress the feed air stream in a series of not less than four main air compression stages to yield a compressed feed air stream at a pressure that exceeds 15 bar(a). The main heat exchanger is configured to cool one or more streams of purified, compressed air to yield at least a liquid air stream that is directed to the higher pressure column and a turbine air stream that is expanded in an excess air turbine to produce an exhaust stream. A phase separator is used to separate the exhaust stream into a liquid portion that is added to the kettle liquid and a vapor portion that is directed to the higher pressure column. In these embodiments, a portion of the vapor nitrogen from the once through kettle column condenser may be warmed in a nitrogen superheater and in the main heat exchanger to produce an intermediate pressure nitrogen product stream.

In other embodiments of the above-described air separation unit and associated method of air separation, the warm end of the air separation unit includes one or more booster compressors configured to further compress portions of the one or more purified, compressed feed air streams. The main heat exchanger is configured to cool one or more streams of purified, further compressed air streams to yield at least a liquid air stream that is directed to the higher pressure column, a booster air stream, and a turbine air stream. The turbine air stream is expanded in a lower column turbine with the resulting exhaust stream being directed to the higher pressure column.

Still other embodiments of the above-described air separation unit and associated method of air separation, an upper column turbine is used in lieu of the excess air turbine or the lower column turbine configured to expand the turbine air stream to produce an exhaust stream that is directed to the lower pressure column. Also, a cold compressor may be used in lieu of or in addition to the one or more booster compressors.

In selected embodiments, the higher pressure column is further configured to yield a dirty shelf nitrogen reflux stream. In addition, a portion of the condensed nitrogen rich overhead exiting the once-through kettle column condenser is combined or mixed with the dirty shelf nitrogen reflux stream and directed to the lower pressure column. Also, a portion of the nitrogen overhead from the higher pressure column may be warmed in the main heat exchanger to produce a higher pressure nitrogen product stream.

In certain embodiments, the present air separation units may employ a divided wall arrangement and/or an integrated kettle column condenser. One such embodiment includes an argon rejection column which may configured as a divided wall column within the lower pressure column and disposed below the once-through argon condenser. In another configuration, the intermediate pressure kettle column is configured as a divided wall column within the lower pressure column. The divided wall kettle column is preferably disposed below the once-through argon condenser and above the location where the argon-oxygen containing side stream is taken from the lower pressure column. Another configuration of the present air separation unit integrates the once-through kettle column condenser within the lower pressure column at a location above the once-through argon condenser and wherein the kettle column overhead is condensed against a portion of the descending liquid in the lower pressure column.

In all of the above-described embodiments, the diverted liquid air stream may be a synthetic liquid air stream taken from an intermediate location of the higher pressure column or may be a portion of the liquid air stream exiting the main heat exchanger. The kettle liquid from the higher pressure column is subcooled and then introduced at an intermediate location of the kettle column.

In still other embodiments of the present air separation unit and method designed to also produce an argon product, the argon column arrangement further comprises a first argon column configured to receive the second part of the argon-oxygen side stream from the lower pressure column and yield the argon-rich overhead and the oxygen-rich bottoms that is directed back to the lower pressure column and a high ratio column configured to receive a portion of a crude argon stream from the once-through argon condenser and rectify the portion of the crude argon stream to yield an argon-rich liquid and an overhead vapor. The argon column arrangement further includes a high ratio column reboiler disposed at the bottom of the high ratio column and a high ratio column condenser. In these embodiments, a portion of the argon-rich liquid at the bottom of the high ratio column is taken as liquid argon product.

BRIEF DESCRIPTION OF THE DRAWING

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

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;

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;

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;

FIG. 4 shows a schematic of the process flow diagram for an alternate embodiment of the air separation unit and associated method of air separation wherein a portion of the nitrogen overhead intermediate pressure kettle column is taken as an intermediate pressure nitrogen product stream;

FIG. 5 shows a schematic of the process flow diagram for another alternate embodiment of the present air separation unit and associated method of air separation with an argon rejection column configured as a divided wall column within the lower pressure column;

FIG. 6 shows a schematic of the process flow diagram for yet another embodiment of the present air separation unit and associated method of air separation.

FIG. 7 shows a schematic of the process flow diagram for still another embodiment of the present air separation unit and associated method of air separation.

FIG. 8 shows a schematic of the process flow diagram for yet another embodiment of the present air separation unit and associated method of air separation with the kettle column configured as a divided wall column within the lower pressure column; and.

FIG. 9 shows a schematic of the process flow diagram for yet another embodiment of the present air separation unit and associated method of air separation with the kettle column condenser integrated within the lower pressure column.

DETAILED DESCRIPTION

The present air separation unit and method for separating air by cryogenic distillation using a four column arrangement including a higher pressure column, a lower pressure column, an intermediate pressure column, and an argon column 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.

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.

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 the argon column.

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.

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.

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.

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.

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.

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.

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.

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.

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 40 which streams may be combined to yield a mixed shelf reflux stream 47. The purity of the dirty shelf nitrogen streams 78B, 46 from higher pressure column 40 and from the kettle column 70 is optimized for feed to lower pressure column 50 at or near the location where waste nitrogen is withdrawn from lower pressure column.

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.

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.

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.

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.

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.

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 2 bara to 3 bara.

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.

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.

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.

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.

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 ΔT of the argon condenser does not limit the ability to drive the kettle column before the ATs of the kettle column reboiler and kettle column condenser, while not penalizing the separation within lower pressure column, for example, by locating the argon condenser too high within the column.

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 than for the prior art.

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.

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 ΔT 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 ΔT at a given heat duty.

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 ΔT. 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.

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.

The process flow diagrams depicted in FIGS. 4-9 are 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 will not be repeated. Rather, the following discussion will focus on the differences in the process flow diagram depicted in FIGS. 4-9, when compared to the process flow diagram depicted in FIG. 3.

The embodiment shown in FIG. 4 differs from the embodiment shown in FIG. 3 in that it depicts a high air pressure (HAP) cycle. In the HAP cycle embodiment, the main air compression arrangement compresses a feed air stream in a series of not less than four main air compression stages to yield a compressed feed air stream at a pressure that exceeds 15 bar(a). The compressed feed air stream is then purified in a pre-purification unit (not shown) to remove contaminants and water vapor from the compressed feed air stream to yield the purified, compressed feed air stream 12. The purified, compressed feed air stream 12 is split into one or more streams of purified, compressed air 14, 15, 25. A first diverted stream 14 of purified, compressed feed air 14 is further compressed in booster compressor 17 and further divided into further compressed air stream 15 and turbine air stream 25. The remaining portion of purified, compressed feed air stream 12 as well as further compressed air stream 15 and the turbine air stream 25 are cooled in.the main heat exchanger 20. As is known for the HAP cycle, it is generally desirable to cool turbine air stream 25 such that turbine 27 exhaust stream 28 is two phase. Turbine 27 is configured to expand the partially cooled turbine air stream 25 to produce an exhaust stream 28. A phase separator 129 is configured to separate the exhaust stream 28 into a liquid portion 128B that is added to the kettle liquid 48 and a vapor portion 128A that is directed to higher pressure column 40.

Another difference in the embodiment of FIG. 4 is that a portion of the vapor nitrogen overhead 78D is withdrawn prior to entry into the once through kettle column condenser 75 is warmed in a nitrogen superheater 21 and in the main heat exchanger 20 to produce an intermediate pressure nitrogen product stream 178.

Use of the intermediate pressure kettle column 70 in a HAP cycle is often more advantageous than other cycles since all the feed air is compressed to a higher pressure greater than about 15 bar(a), and more preferably to a high pressure at or above 20 bar(a), an improvement in oxygen recovery attributed to the use of an intermediate pressure kettle column 70 will provide a greater percentage savings in power consumption.

Turning now to FIG. 5, the illustrated embodiment of the air separation unit 10 includes an argon rejection column 480 configured as a divided wall column within the lower pressure column 50. As shown in the drawing, the divided wall argon rejection column 480 is disposed below the once-through argon condenser 65 and is preferably an annular oriented divided wall column. This arrangement depicted in FIG. 5 is particularly advantageous when a refined argon product is not desired or required as the arrangement provides improved efficiency relative to the embodiment shown in FIG. 3. Specifically, the intermediate pressure kettle column 70 provides a similar, or possibly increased, benefit in power consumption for the arrangement having a divided wall argon rejection column configuration compared to arrangements including an argon super-stage and/or high ratio column for argon production. Such improvement in power consumption is likely achieved because the argon condenser 65 in the embodiment having a divided wall argon rejection column will naturally have a larger ΔT, due to the much lower purity argon that is condensed. This larger ΔT provides the potential to drive the intermediate pressure kettle column 70 harder and achieve a larger improvement in power consumption.

Since the rejected argon vapor stream 84 is slightly higher in pressure than the waste nitrogen stream 52 exiting the lower pressure column 50, the rejected argon vapor stream 84 can be readily combined with the waste nitrogen stream 52 at a location upstream of the main heat exchanger 20. In cases where the rejected argon vapor stream 84 is not of sufficient pressure, the rejected argon vapor stream 84 may be warmed in a separate heat exchange passage in main heat exchanger 20.

Also, because the rejected argon is not highly valued, the dirty shelf liquid nitrogen stream 46 taken from an intermediate location of the higher pressure column 40 and a portion of the condensed nitrogen rich overhead 78B exiting the once-through kettle column condenser 75 are combined or mixed with the resulting mixed stream 47 being directed to the lower pressure column 50 as a dirty shelf reflux stream. In some embodiments, particularly where the desired product nitrogen rate is relatively high, the intermediate pressure kettle column 70 may also be configured such that a portion of the nitrogen overhead from the intermediate pressure kettle column 70 is warmed in a nitrogen superheater and in the main heat exchanger and taken as an intermediate pressure or elevated pressure nitrogen product stream.

Turning now to FIG. 6, the illustrated embodiment of the air separation unit 10 includes an upper column turbine 227 and the intermediate pressure kettle column 70. The illustrated air separation unit 10 having an upper column turbine configuration is generally favored for an oxygen producing cycle when the product nitrogen rate is low, and the liquid product rates are also low. The arrangement shown in FIG. 6 is particularly advantageous as the oxygen recovery and argon recovery are relatively high and the use of the intermediate pressure kettle column 70 enables a power savings compared to an upper column turbine arrangement without the intermediate pressure kettle column.

As seen in FIG. 6, one or more booster compressors 110 and 13 are configured to further compress portions of the one or more purified, compressed feed air streams. However, the turbine air stream 225 is not further compressed but remains at a lower pressure than the other purified, compressed feed air streams. Booster compressor 110 may be the last stage of the main air compressor, of which the first stages compress the feed air prior to its entry into pre-purifier 105. The lower pressure turbine air stream 225 is partially cooled in main heat exchanger 20 and the expanded in the upper column turbine 227 to produce an exhaust stream 228 that is directed to the lower pressure column 50. The upper column turbine 227 is preferably coupled to a generator 229.

In the illustrated embodiment, a dirty shelf liquid nitrogen stream 46 taken from an intermediate location of the higher pressure column 40 and a portion of the condensed nitrogen rich overhead 78B exiting the once-through kettle column condenser 75 are combined or mixed with the resulting mixed stream 47 being directed to the lower pressure column as a dirty shelf reflux stream. Also, if needed or desired, a portion of the nitrogen overhead from the higher pressure column may optionally be taken and is warmed in the main heat exchanger to produce a higher pressure nitrogen product stream.

In this configuration power consumption is reduced because the turbine air is compressed to a lower pressure than the other purified, compressed feed air streams. Feed of the reduced pressure turbine air stream to the upper column turbine means its flow will necessarily increase to provide the required refrigeration for the air separation cycle. But, by using the intermediate pressure kettle column, the upper column turbine flow can be much larger than for air separation units without the intermediate pressure kettle column, before the penalty in oxygen recovery becomes too large.

Although not shown, it may be preferred to have two separate pre-purifier trains for a very large air separation unit. In such applications the turbine air stream may be taken from an intermediate stage of the main air compression arrangement, One pre-purifier train would operate at lower pressure and pre-purify the turbine air stream while the other train would operate at the normal pressure set by higher pressure column and would pre-purify the remaining compressed air flow.

Turning now to FIG. 7, there is shown another embodiment of the air separation unit 10 and associated air separation cycle suitable for use in scenarios where the required or desired nitrogen product rate is low. Similar to the embodiment of FIG. 6, the illustrated embodiment of FIG. 7 includes an upper column turbine 227 and the intermediate pressure kettle column 70. However, in this embodiment, one of the booster compressors is a cold compressor 115 driven by a separate motor. A similar cold compression configuration, without the intermediate pressure kettle column is disclosed in detail in United States Patent Application Publication No. 2015114037.

Like the embodiment of FIG. 6, warm booster compressors 13 shown in FIG. 7 are configured to further compress a diverted portion 14 of the purified, compressed feed air stream 12. The further compressed stream is partially cooled and then still further compressed in cold compressor 115. The cold compressed stream 116 is fully cooled in main heat exchanger 20 and directed to the higher pressure column 40. The turbine air stream 225 is not further compressed. The lower pressure turbine air stream 225 (compared to stream 116) is partially cooled in main heat exchanger 20 and the expanded in the upper column turbine 227 to produce an exhaust stream 228 that is directed to the lower pressure column 50. The upper column turbine 227 is preferably coupled to a generator 229.

Comparatively, the cold compressor arrangement will use less power than fully compressing the high pressure air in booster compressors of FIG. 6. But use of cold compression will also require increased upper column turbine flow to balance its energy addition with more refrigeration created by the upper column turbine. With the use of the intermediate pressure kettle column 70, the depicted air separation unit 10 and air separation cycle is more able to accommodate the higher upper column turbine flow without a large oxygen recovery penalty.

Also, similar to the embodiment of FIG. 6, the illustrated embodiment of FIG. 7 includes a dirty shelf liquid nitrogen stream 46 taken from an intermediate location of the higher pressure column 40 and a portion of the condensed nitrogen rich overhead 78B exiting the once-through kettle column condenser 75 that are combined or mixed. The mixed stream 47 is then directed to the lower pressure column 50 as a dirty shelf reflux stream.

Turning now to FIG. 8, the illustrated embodiment of the air separation unit 10 includes a kettle column 570 configured as a divided wall column within the lower pressure column 50, preferably at a middle section of the lower pressure column between the argon draw point and the kettle liquid feed point. As shown in the drawing, the divided wall kettle column 570 is disposed below the once-through argon condenser 65 and is preferably an annular or concentric oriented divided wall column with the kettle column stages disposed on the annulus region and stages of the lower pressure column disposed in the center or central region of the divided wall arrangement. Alternatively, the kettle column portion may be disposed in the center or central region of the divided wall arrangement while stages of the lower pressure column are disposed in the annulus region.

Turning now to FIG. 9, the illustrated embodiment of the air separation unit 10 includes a once through kettle column condenser 75 that is integrated with and disposed within the lower pressure column 50. The preferred location of the integrated kettle column condenser 75 is at another intermediate location between the diverted liquid air feed and the location where the argon-oxygen containing side stream 56 is taken from the lower pressure column 50. The kettle column condenser is configured to condense the kettle column overhead 76A against a portion of the nitrogen-rich descending liquid in the lower pressure column 50.

By locating the kettle column condenser 75 higher in the lower pressure column 50, the boiling fluid is a portion of the nitrogen-rich descending liquid in the lower pressure column 50 which contains more nitrogen than the boiling fluid used in the embodiment of FIG. 3 and is colder which means that the ΔT of the kettle column condenser 75 may be larger. To best take advantage of the larger ΔT in the kettle column condenser 75 it is best to do two things: (i) increase the driving force for the kettle column reboiler 71 and kettle column condenser 75 so that more nitrogen reflux 78D or nitrogen product 76B can be produced by the intermediate pressure kettle column 70, and (ii), decrease the operating pressure of the intermediate pressure kettle column 70 to improve its separation capability.

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.

Claims

1. An air separation unit for production of product streams from a source of purified, compressed feed air, the air separation unit comprising: a higher pressure column configured to receive one or more streams of compressed, purified air and a first reflux stream and yield a nitrogen-rich overhead and a kettle liquid;a lower pressure column configured to receive a diverted liquid air stream and a second reflux stream and yield a low pressure product grade nitrogen overhead, an oxygen liquid at the bottom of the column, and an argon-oxygen containing 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 at the bottom of the lower pressure column to yield the first reflux stream and the second reflux stream;an intermediate pressure kettle column configured to receive the kettle liquid from the higher pressure 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 descending liquid 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 the kettle column against a portion of the oxygen-rich bottoms of the intermediate pressure kettle column; andan argon column arrangement comprising one or more argon columns and a once-through 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 bottoms that is returned to the intermediate location of the lower pressure column;wherein the argon condenser is disposed within the lower pressure column at a location above the intermediate location of the lower pressure column and the argon-rich overhead is condensed against a portion of the descending liquid in the lower pressure column and/or a diverted portion of the liquid air stream to produce a crude argon stream.

2. The air separation unit of claim 1, wherein the intermediate pressure kettle column is configured to receive the kettle liquid at an intermediate location of the kettle column.

3. The air separation unit of claim 1, wherein the argon column arrangement further comprises: a first argon column configured to receive the second part of the argon-oxygen side stream from the lower pressure column and yield the argon-rich overhead and the oxygen-rich bottoms that is directed back to the lower pressure column;the once-through argon condenser is configured to receive the argon-rich overhead from the first argon column and condense the argon-rich overhead to produce a crude argon stream; anda high ratio column configured to receive a portion of the crude argon stream from the once-through argon condenser and rectify the portion of the crude argon stream to yield an argon-rich liquid and an overhead vapor;wherein a portion of the argon-rich liquid at the bottom of the high ratio column is taken as a liquid argon product.

4. The air separation unit of claim 3, wherein the argon column arrangement further comprises: a high ratio column reboiler disposed at the bottom of the high ratio column and configured for reboiling another portion of the argon-rich liquid at the bottom of the high ratio column to produce an ascending vapor stream in the high ratio column; anda high ratio column condenser configured to condense the overhead vapor from the high ratio column and return all or a portion of the condensate as a high ratio column reflux stream.

5. The air separation unit of claim 1, further comprising: a main air compression arrangement configured to receive a feed air stream and compress the feed air stream in a series of not less than four main air compression stages to yield a compressed feed air stream at a pressure that exceeds 15 bar(a);a pre-purification unit configured to remove contaminants and water vapor from the compressed feed air stream to yield the purified, compressed feed air stream;wherein the purified, compressed feed air stream is split into one or more streams of purified, compressed air;a main heat exchanger configure to cooling the one or more streams of purified, compressed air to yield at least a liquid air stream that is directed to the higher pressure column and a turbine air stream;a turbine configured to expand the turbine air stream to produce an exhaust stream; anda phase separator configured to separate the exhaust stream into a liquid portion that is added to the kettle liquid and a vapor portion that is directed to the higher pressure column.

6. The air separation unit of claim 5, further comprising a booster compressor configured to further compress a portion of the one or more purified, compressed feed air streams upstream of the excess air turbine and wherein the further compressed portion of the one or more purified, compressed feed air streams is partially cooled in the main heat exchanger to yield the turbine air stream.

7. The air separation unit of claim 5, wherein a portion of the nitrogen overhead from the intermediate pressure kettle column is warmed in a nitrogen superheater and in the main heat exchanger to produce an intermediate pressure nitrogen product stream.

8. The air separation unit of claim 1, further comprising: a main air compression arrangement configured to receive a feed air stream and compress the feed air stream in a series main air compression stages to yield a compressed feed air stream;a pre-purification unit configured to remove contaminants and water vapor from the compressed feed air stream to yield the purified, compressed feed air stream;wherein the purified, compressed feed air stream is split into one or more streams of purified, compressed air;one or more booster compressors configured to further compress portions of the one or more purified, compressed feed air streams;a main heat exchanger configured to cool the one or more streams of purified, compressed air to yield at least a liquid air stream that is directed to the higher pressure column, a booster air stream, and a turbine air stream; anda lower column turbine configured to expand the turbine air stream to produce an exhaust stream that is directed to the higher pressure column.

9. The air separation unit of claim 8, wherein: the higher pressure column is further configured to yield a dirty shelf nitrogen stream taken from an intermediate location of the higher pressure column that is directed to the lower pressure column as a dirty shelf reflux stream; anda portion of the condensed nitrogen rich overhead exiting the once-through kettle column condenser is mixed with the dirty shelf reflux stream and directed to the lower pressure column.

10. The air separation unit of claim 8, wherein a portion of the nitrogen overhead from the higher pressure column is warmed in the main heat exchanger to produce a higher pressure nitrogen product stream.

11. The air separation unit of claim 8, wherein the diverted liquid air stream is a synthetic liquid air stream taken from an intermediate location of the higher pressure column.

12. The air separation unit of claim 1, further comprising: a main air compression arrangement configured to receive a feed air stream and compress the feed air stream in a series main air compression stages to yield a compressed feed air stream;a pre-purification unit configured to remove contaminants and water vapor from the compressed feed air stream to yield the purified, compressed feed air stream;wherein the purified, compressed feed air stream is split into one or more streams of purified, compressed air;one or more booster compressors configured to further compress portions of the one or more purified, compressed feed air streams;a main heat exchanger configure to cooling the one or more streams of purified, compressed air to yield at least a liquid air stream that is directed to the higher pressure column, a booster air stream, and a turbine air stream; andan upper column turbine configured to expand the turbine air stream to produce an exhaust stream that is directed to the lower pressure column.

13. The air separation unit of claim 12, wherein: the higher pressure column is further configured to yield a dirty shelf nitrogen stream taken from an intermediate location of the higher pressure column that is directed to the lower pressure column as a dirty shelf reflux stream; anda portion of the condensed nitrogen rich overhead exiting the once-through kettle column condenser is mixed with the dirty shelf reflux stream and directed to the lower pressure column.

14. The air separation unit of claim 13, wherein a portion of the nitrogen overhead from the higher pressure column is warmed in the main heat exchanger to produce a higher pressure nitrogen product stream.

15. The air separation unit of claim 12, wherein the diverted liquid air stream is a synthetic liquid air stream taken from an intermediate location of the higher pressure column.

16. The air separation unit of claim 12, wherein the pressure of the turbine air stream is lower than the pressure of the booster air stream and the liquid air stream.

17. The air separation unit of claim 12, wherein one of the one or more booster compressors is a cold compressor.

18. The air separation unit of claim 1, wherein the one or more argon columns further comprise an argon rejection column.

19. The air separation unit of claim 18 wherein the argon rejection column is configured as a divided wall column within the lower pressure column and wherein the argon rejection column is disposed below the once-through argon condenser.

20. The air separation unit of claim 1, wherein a portion of the nitrogen overhead from the intermediate pressure kettle column is warmed in a nitrogen superheater and in the main heat exchanger to produce an intermediate pressure nitrogen product stream.

21. The air separation unit of claim 20, wherein a portion of the nitrogen overhead from the higher pressure column is warmed in the main heat exchanger to produce a higher pressure nitrogen product stream.

22. The air separation unit of claim 1, wherein the intermediate pressure kettle column is configured as a divided wall column within the lower pressure column and wherein the intermediate pressure kettle column is disposed below the once-through argon condenser and above the intermediate location of lower pressure column.

23. The air separation unit of claim 1, wherein the kettle column condenser is a once-through kettle column condenser disposed within the lower pressure column at a location above the once-through argon condenser and wherein the kettle column overhead is condensed against a portion of the descending liquid in the lower pressure column.