System and method for cryogenic air separation using four distillation columns including one medium pressure column

Abstract

A system and method for separating air by cryogenic distillation using a four-column arrangement including a high-pressure column, a low-pressure column, a medium-pressure column, and an argon column is provided. The disclosed systems and methods are particularly suitable for the production of standard purity oxygen and employ a once-through kettle column reboiler, once-through kettle column condenser, and once-through argon condenser. The direct flow argon condenser is disposed within the low pressure column in which a vapor stream rich in argon is condensed by contacting with a downstream liquid within the low pressure column.

Description

Technical Field

[0001] The system and method of the present invention relates to separating air by cryogenic distillation, and more particularly, to a system and method using a high pressure column, a low pressure column, an intermediate pressure column and an argon column. Background Art

[0002] When an oxygen product with a purity of 99.5% or higher and an argon product are required, a conventional air separation cycle using a high pressure column, a low pressure column, and an argon column is the standard choice for air separation units. This purity of oxygen product is often referred to as "standard purity oxygen." However, to produce standard purity oxygen and argon, a three-column apparatus exhibits significant operational drawbacks, such as Figure 1 The McCabe-Thiele diagram is shown in FIG.

[0003] McCabe-Thiele diagrams are helpful because these diagrams show the magnitude of the driving force for mass transfer in the distillation columns of an air separation unit. The McCabe-Thiele diagram for the low pressure column is key to analyzing cryogenic oxygen based air separation processes. The space between the equilibrium line and the operating line in the McCabe-Thiele diagram represents the driving force for that portion of the column. Processes with large distillation driving forces tend to have short distillation columns (i.e., without many separation stages), but power consumption is also high. Ideally, a very efficient cryogenic distillation process would have a close and fairly constant approach between the operating line and the equilibrium line. The McCabe-Thiele diagram typically plots the light key components in the liquid on the x-axis and the light key components in the vapor on the y-axis. In Figure 1 In the , the sum of the nitrogen and argon compositions of the liquid and vapor is plotted on each axis. Argon is the light key in the bottom section of the low pressure column, while nitrogen is the light key in the remainder of the low pressure column. Figure 1 By plotting the sum of the argon and nitrogen fractions in , the entire low-pressure column can be clearly characterized.

[0004] Figure 1 The McCabe-Thiele diagram shown in depicts a scenario for a conventional three-column plant and cycle, where the flow rate of the high-pressure nitrogen product is approximately the same as the flow rate of the oxygen product. The result is that the low-pressure column separation is challenged. As a result, the McCabe-Thiele diagram shows a virtual pinch near the top, with an x-axis value of approximately 0.77. Adding liquid air to the low-pressure column alleviates this virtual pinch.

[0005] Most conventional three-column plants for producing standard purity oxygen are characterized by Figure 1The McCabe-Thiele diagram depicts the close proximity between the equilibrium line and the operating line in the bottom portion of the low-pressure column. This is because oxygen and argon have close relative volatilities. Therefore, the removal of argon from oxygen in the bottom portion of the low-pressure column is the most difficult separation of the three standard components of air. Note that if there is not enough vapor boiling in the low-pressure column produced by the main condenser-reboiler at the bottom of the low-pressure column, the slope of the operating line will be lower. The resulting composition clamping will result in poorer oxygen recovery and higher power consumption to produce such oxygen.

[0006] Note that Figure 1 In the example, there is a large difference between the operating line and the equilibrium line in the section of the low pressure column between the kettle liquid feed and the argon column draw. This always happens and is due to the need for a higher liquid to vapor ratio (L / V) in the bottom section of the low pressure column. In addition, there is a relatively large difference between the operating line and the equilibrium line in the section between the liquid air feed and the kettle liquid feed. This indicates that these sections of the low pressure column have a large mass transfer driving force, which is generally unavoidable in a three column arrangement and cycle that produces standard purity oxygen and argon.

[0007] Another key observation about the conventional three-column arrangement is that the generation of high quality nitrogen reflux through the high pressure column is limited by the balance between feed air and kettle liquid. That is, even if the high pressure column contains a very large number of stages, the amount of reflux generated to supply the low pressure column will be limited. The balance between 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 a conventional three-column arrangement.

[0008] To solve these problems, it is suggested to use a medium pressure column or kettle column. In the medium pressure column, the kettle liquid from the high pressure column is further fractionated to produce additional nitrogen reflux. Examples of medium pressure columns are disclosed in U.S. Patent Nos. 5,675,977; 5,657,644; 5,862,680; and 6,536,232.

[0009] U.S. Patent No. 5,675,977 discloses the production of low purity oxygen using a medium pressure column, wherein the medium pressure column is driven by nitrogen vapor from a high pressure column. Nitrogen boil-off losses in the low pressure column are quite tolerable for low purity oxygen production by diverting a portion of the nitrogen vapor from the high pressure column to the medium pressure column reboiler. However, this configuration is not suitable for standard purity oxygen production, wherein all available nitrogen from the high pressure column must be used in the main condenser-reboiler to produce sufficient nitrogen vapor boil-off in the low pressure column. It is also noted that U.S. Patent 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.

[0010] U.S. Patent No. 5,657,644 discloses an air separation unit and cycle, using a high pressure column, a low pressure column, an intermediate pressure column and an argon column, the argon column being 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 low pressure column. In the four-column device disclosed in U.S. Patent No. 5,657,644, a kettle liquid stream from the high pressure column is introduced into the lower portion of the intermediate pressure column, which produces a liquid bottoms product rich in oxygen and an overhead product rich in nitrogen. The intermediate pressure column also includes a bottom reboiler heated by an argon-oxygen containing stream from the low pressure column and an overhead condenser that condenses a portion of the overhead product rich in nitrogen by contact with a portion of the liquid bottoms product rich in oxygen. Another portion of the liquid bottoms product rich in oxygen drives the argon condenser disposed above the argon column, while the remainder of the liquid bottoms product rich in oxygen is returned to the low pressure column.

[0011] Although the air separation cycle disclosed in U.S. Pat. No. 5,657,644 is economically advantageous compared to a conventional three-tower air separation unit because of reduced overall power consumption and increased argon and oxygen recoveries, there remains a need to find further improvements to further reduce overall power consumption and provide additional product flexibility, including a crude argon product or a refined argon product.

[0012] Another example is disclosed in U.S. Pat. No. 6,536,232, which also discloses an air separation unit that employs a high pressure column, a low 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 low pressure column and an overhead condenser that condenses a portion of the nitrogen-rich overhead product by contact with a portion of the oxygen-rich liquid bottom product. Another portion of the oxygen-rich liquid bottom product is returned to the low pressure column.

[0013] 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 primarily in that there is no argon column and no argon is produced. As described above, this modified air separation cycle has limited utility because no argon is produced and nitrogen production is limited. Summary of the invention

[0014] The present invention features an air separation unit comprising: (i) a main air compression device configured to receive a feed air stream and compress the feed air stream in a series of main air compression stages to produce a compressed feed air stream; (ii) a pre-purification unit configured to remove contaminants and water vapor from the compressed feed air stream to produce a purified compressed feed air stream that can be divided into one or more purified compressed air streams; (iii) one or more heat exchangers, the one or more heat exchangers including a main heat exchanger configured to cool the one or more purified compressed air streams via indirect heat exchange with an oxygen product stream and one or more nitrogen streams; and (iv) a distillation column system comprising a high pressure column, a low pressure column, an intermediate pressure kettle distillation column, and an argon column. The distillation column system also includes a direct current kettle column reboiler, a direct current kettle column condenser, and a direct current argon condenser. The once-through argon condenser is disposed within the low pressure column, wherein an argon-rich vapor stream is condensed by contact with descending liquid within the low pressure column.

[0015] More specifically, the distillation column system includes: (v) a high-pressure column, the high-pressure column being configured to receive one or more purified compressed air streams and a first reflux stream, and produce a nitrogen-rich tower top distillate, a bottom liquid; (vi) a low-pressure column, the low-pressure column being configured to receive a split liquid air stream and a second reflux stream, and produce a low-pressure product-grade nitrogen tower top distillate, an oxygen liquid at the bottom of the tower, and an argon-oxygen-containing side stream; (vii) a main condenser-reboiler disposed in the low-pressure column and configured to heat the tower top distillate by passing at least a portion of the nitrogen-rich tower top distillate from the high-pressure column through a condenser connected to the bottom of the low-pressure column. (viii) an intermediate-pressure kettle tower apparatus, the intermediate-pressure kettle tower apparatus comprising a kettle rectification tower, the kettle rectification tower being configured to receive kettle liquid from the high-pressure tower and produce an oxygen-rich bottom distillate and a nitrogen-rich top distillate; and (ix) an argon tower apparatus, the argon tower apparatus comprising one or more argon towers and a direct-flow argon condenser, the argon tower being configured to receive a second portion of the argon-oxygen side stream from the low-pressure tower and produce an argon-rich top distillate and an oxygen-rich bottom distillate which are returned to an intermediate position of the low-pressure tower.

[0016] The medium-pressure kettle column device preferably includes a direct-flow kettle column reboiler configured to boil a portion of the descending liquid in the kettle rectification column by contacting it with the first portion of the argon-oxygen side stream to produce an ascending vapor stream in the kettle rectification column and an argon-oxygen liquid stream returned to an intermediate position of the low-pressure column. The medium-pressure kettle column device also includes a direct-flow kettle column condenser configured to condense all or a portion of the nitrogen-rich tower top distillate of the kettle rectification column by contacting it with a portion of the oxygen-rich tower bottom distillate of the kettle rectification column.

[0017] The invention also features an air separation process comprising the steps of: (a) rectifying one or more purified compressed feed air streams with a first reflux stream in a higher pressure column to produce a nitrogen-rich overhead product and bottoms liquid; (b) condensing all or a portion of the nitrogen-rich overhead product by contact with oxygen liquid at the bottom of a lower pressure column in a main condenser-reboiler to produce a liquid nitrogen stream and an ascending vapor stream, a first portion of the liquid nitrogen stream serving as a first reflux stream and another portion of the liquid nitrogen stream serving as a first reflux stream. (c) rectifying the split liquid air stream and the rising vapor stream with the second reflux stream in the low-pressure column to produce a low-pressure product-grade nitrogen tower top distillate, an oxygen liquid at the bottom of the low-pressure column, and an argon-oxygen-containing side stream; (d) rectifying the kettle liquid with the third reflux stream in the medium-pressure kettle distillation column to produce an oxygen-rich tower bottom distillate and another nitrogen-rich tower top distillate; (e) in a direct flow kettle column reboiler disposed in the medium-pressure kettle distillation column, making the oxygen-rich tower bottom distillate (f) condensing all or a portion of the nitrogen-rich overhead product from the intermediate-pressure kettle distillation column by contacting with a portion of the oxygen-rich bottom product in a once-through kettle column condenser to produce a liquid nitrogen stream and an evaporated vapor stream that is returned to the low-pressure column, a portion of the liquid nitrogen stream being used as a third reflux stream and another portion being used as a shelf nitrogen stream that is returned to the low-pressure column; (g) ) rectifying a second portion of the argon-oxygen containing side stream with an argon reflux stream in the argon column to produce an argon-rich overhead product and an oxygen-rich bottoms product, the oxygen-rich bottoms product being returned to an intermediate position in the low pressure column; (g) condensing the argon-rich overhead product by contact with descending liquid in the low pressure column in a once-through argon condenser disposed in the low pressure column to produce an argon reflux stream and a crude argon stream; and (h) producing one or more standard purity oxygen product streams from the oxygen liquid at the bottom of the low pressure column.

[0018] In some embodiments of the above systems and methods, the split liquid air stream is a synthetic liquid air stream withdrawn from an intermediate position of the higher pressure column. The kettle liquid is preferably subcooled and then introduced at an intermediate position of the kettle distillation column.

[0019] In other embodiments of the systems and methods of the present invention designed to also produce an argon product, the argon column apparatus further comprises: a first argon column configured to receive a second portion of the argon-oxygen side stream from the low-pressure column and produce an argon-rich overhead distillate and an oxygen-rich bottom distillate directed back to the low-pressure column; and a high ratio column configured to receive a portion of the crude argon gas stream from the direct-flow argon condenser and rectify a portion of the crude argon gas stream to produce an argon-rich liquid and an overhead vapor. The argon column apparatus further comprises a high ratio column reboiler and a high ratio column condenser disposed at the bottom of the high ratio column. In these embodiments, a portion of the argon-rich liquid at the bottom of the high ratio column is used as a liquid argon product. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] It is believed that the claimed invention will be better understood when taken in conjunction with the accompanying drawings, in which:

[0021] Figure 1 A McCabe-Thiele diagram depicting a conventional three-column apparatus and cycle known in the art wherein the flow rate of the high pressure nitrogen product is approximately the same as the flow rate of the oxygen product;

[0022] Figure 2 A McCabe-Thiele diagram depicting one embodiment of the systems and methods of the present invention, comprising a four-column apparatus including a high pressure column, a low pressure column, an intermediate pressure column, and an argon column;

[0023] Figure 3 a schematic diagram showing a process flow diagram of an embodiment of the systems and methods of the present invention; and

[0024] Figure 4 A schematic diagram of a process flow diagram illustrating an alternative embodiment of the systems and methods of the present invention. DETAILED DESCRIPTION

[0025] The system and method of the present invention for separating air by cryogenic distillation using a four-column apparatus including a high pressure column, a low pressure column, an intermediate pressure column and an argon column is particularly suitable for producing standard purity oxygen, argon and one or more nitrogen products, wherein the nitrogen production rate of the high pressure nitrogen product and / or the liquid nitrogen product exceeds 50% of the total standard purity oxygen production rate.

[0026] By generating additional nitrogen reflux and / or supplemental high pressure nitrogen product from the intermediate pressure column or kettle column, the system and method of the present invention can achieve higher oxygen recovery, higher argon recovery and reduced power consumption with improved efficiency relative to conventional three-column arrangements and many prior art intermediate column arrangements. Figure 1The tight clamping in the upper portion of the low pressure column is relieved by utilizing the excess distillation driving force in the selected portion of the low pressure column on the McCabe-Thiele diagram of FIG. In addition, certain benefits may be realized, including efficiency benefits in applications where the high pressure nitrogen product and / or liquid nitrogen product production rate is less than 50% of the total oxygen production rate.

[0027] Figure 2 A McCabe-Thiele diagram of an embodiment of a four-column apparatus and cycle of the present invention is shown. As described above, the space between the equilibrium line 102 and the operating line 103 represents the driving force for that portion of the column. Note that the driving force for mass transfer is reduced in the portion of the low pressure column between the kettle liquid feed and the argon column draw. The reduction in the driving force for mass transfer in the portion of the low pressure column between the kettle liquid feed and the argon column draw is achieved by driving the reboiler of the intermediate pressure column or kettle column with vapor from the base of that portion of the low pressure column, preferably from the same source as the vapor fed to the argon column from the low pressure column.

[0028] The key principles or features of the four-column apparatus and associated air separation cycle of the present invention are: (1) the mass transfer driving force in the top portion of the low pressure column is increased by utilizing the excess driving force in the portion between the kettle liquid feed and the argon column draw; and (2) the mass transfer driving force in the bottom of the low pressure column is not reduced. In addition, the use of the four-column apparatus and cycle of the present invention reduces or avoids the need to draw product nitrogen from the top of the low pressure column because the four-column apparatus and cycle of the present invention can generate supplemental nitrogen product and / or nitrogen reflux from the intermediate pressure column.

[0029] Now turn to Figure 3 , an air separation unit 10 including a hot end device and a cold end device is shown, which 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 one main heat exchanger 20 and a nitrogen superheater 21 or subcooler.

[0030] The hot end device is configured to condition a feed air stream for separation into its component parts, namely argon, oxygen and nitrogen, the hot end device receiving the feed air stream, compressing the feed air stream in a series of main air compression stages, and purifying the compressed air stream in a pre-purification unit to produce a compressed and purified air stream 12.

[0031] A first major portion of the compressed and purified air stream 12 is directed to the main heat exchanger 20 where it is cooled to a temperature suitable for rectification in the distillation column system 30 and leaves 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 the first booster compressor 13 and cooled in an aftercooler. A portion of the further compressed second portion of the compressed and purified air stream is further compressed in the second booster compressor 15 and cooled in an aftercooler to produce a boosted pressure air stream 16, which is also directed to the main heat exchanger 20. The boosted pressure air stream 16 is cooled in the main heat exchanger 20 to produce a liquid air stream 26, which is directed to the high pressure column 40 of the distillation column system 30.

[0032] The remainder of the further compressed first portion of the compressed and purified air stream is branched off as stream 18, which is further compressed in another booster compressor 17 to produce a turbine air stream 25, which is then partially cooled in the main heat exchanger 20. The partially cooled stream is expanded in the turbine 27 to produce an off-gas stream 28, which is also directed to the high-pressure column 40 of the distillation column system 30. It is noted that the off-gas stream 28 can be combined with the cooled, compressed and purified air stream 22.

[0033] The first portion of compressed and purified air stream 12, as well as turbine air stream 18 and boosted pressure air stream 16, are cooled in main heat exchanger 20 via indirect heat exchange with a plurality of streams from distillation column system 30, including: clean shelf nitrogen stream 44; pumped liquid oxygen stream 53; pumped high pressure gaseous nitrogen stream 68; waste nitrogen stream 59; and low pressure gaseous nitrogen stream 52. The hot streams exit main heat exchanger 20 as: product-level gaseous nitrogen stream 144; product-level gaseous oxygen stream 153; product-level high pressure gaseous nitrogen stream 168; hot waste nitrogen stream 159; and product-level low pressure gaseous nitrogen stream 152.

[0034] The illustrated distillation column system 30 includes: a high pressure column 40; an intermediate pressure column or kettle rectification column 70; a low pressure column 50; an integrated argon condenser 65; an argon column 80; and a high ratio column 90.

[0035] The high-pressure column 40 is configured to receive one or more purified compressed air streams, including the liquid air stream 26, the cooled, compressed and purified air stream 22, and the waste gas stream 28 and the reflux stream, and produce a nitrogen-rich overhead distillate 42, a clean shelf vapor stream 44, a dirty shelf nitrogen stream 46, kettle liquid 48, and a synthetic liquid air stream 45 withdrawn from an intermediate position of the high-pressure column 40.

[0036] The low pressure column 50 is configured to receive the synthetic liquid air stream 45, the oxygen-rich bottoms 83, and / or more reflux streams or other streams to produce a low pressure product grade nitrogen overhead 52, oxygen liquid 51 as a liquid oxygen stream 53 at the bottom of the column, and an argon-oxygen containing side stream 56 withdrawn from an intermediate position of the low pressure column 50. A portion of the liquid oxygen stream 53 may be used as a liquid oxygen product 154, while a majority of the liquid oxygen stream 53 is pumped via pump 55 and vaporized in a main heat exchanger to produce a gaseous oxygen product 153. The one or more reflux streams introduced into the low pressure column 50 preferably include stream 78B from the kettle condenser 75 and the dirty shelf nitrogen stream 46 from the high pressure column 40, which streams may be combined to produce a mixed shelf reflux stream 47. The dirty shelf nitrogen streams 78B, 46 from the higher pressure column 40 and from the kettle rectification column 70 are optimized for purity in order to be fed to the lower pressure column 50 at or near the point where the waste nitrogen is taken from the lower pressure column.

[0037] The low-pressure column 50 also houses a main condenser-reboiler configured to thermally couple the high-pressure column 40 and the low-pressure column 50 by liquefying at least a portion of the nitrogen-rich overhead 42 from the high-pressure column 40 by contacting with the oxygen liquid 51 at the bottom of the low-pressure column 50 to produce a nitrogen reflux stream 62 directed to the high-pressure column 40 and another nitrogen stream 61, a portion of which is directed to the top of the low-pressure column 50 as a reflux stream 63. Another portion of the nitrogen stream 61 is preferably used as a high-pressure product-grade nitrogen stream 68, which is pumped via a pump 67 and directed to the main heat exchanger 20, while the remaining portion of the nitrogen stream 61 is preferably used as a liquid nitrogen product stream 164.

[0038] A direct-flow argon condenser is also disposed within the low-pressure column 50 at a position above the midpoint of the low-pressure column. The argon condenser is configured to condense the argon-rich overhead product taken from the argon column by contact with all or a portion of the descending liquid in the low-pressure column 50 (including feed streams 79, 97, 98 and optionally a split 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 an argon column 80.

[0039] In some cases, it may be preferred to use a split portion of the subcooled synthetic liquid air 66 to further increase the temperature driving force of the argon condenser 65. In doing so, the liquid air is introduced into a non-ideal location within the low pressure column, resulting in a slight decrease in argon recovery. Unlike the systems and methods of the prior art, the use of a small split portion of the subcooled synthetic liquid air 66 to drive the argon condenser 65 in the systems and methods of the present invention can further increase the driving force of the kettle column reboiler 71 and the kettle column condenser 75. When it is desired to do so, this results in a further increase in oxygen recovery and a further reduction in power consumption.

[0040] The intermediate pressure column or kettle distillation column 70 is configured to receive kettle liquid 48 from the high pressure column 40 and produce an oxygen-rich kettle bottoms product 72 and a nitrogen-rich kettle top product 76A. The kettle liquid 48 is preferably subcooled in the nitrogen superheater 21 and directed through the high ratio column reboiler 95. Preferably, the subcooled kettle liquid 48 is then introduced into the kettle distillation column 70, preferably at an intermediate position of the kettle distillation column, a few stages above the bottom portion. A straight-through kettle column reboiler 71 and a straight-through kettle column condenser 75 are operably associated with the kettle distillation column 70. The straight-through kettle column reboiler 71 is configured to boil a portion of the descending liquid in the kettle distillation column 70 by contacting with the first portion 58 of the argon-oxygen side stream 56 to produce an ascending vapor stream in the kettle distillation column 70 and an argon-oxygen liquid stream 77 that is returned at or near an intermediate position of the low pressure column 50. Therefore, the kettle distillation column 70 is preferably spatially arranged above the middle position of the low pressure column 50 so that the return liquid from the kettle column reboiler and the transferred kettle can be gravity fed to the low pressure column 50. Moreover, unlike some prior art disclosures involving medium pressure columns, no synthetic liquid air or liquid air feed is directed to the kettle distillation column.

[0041] The once-through kettle condenser 75 is configured to condense all or a portion of the nitrogen-rich kettle overhead 76A of the kettle rectification column 70 by contacting it with the first major portion 73 of the oxygen-rich kettle bottoms 72 of the kettle rectification column 70 to produce a nitrogen reflux stream 78A for the kettle rectification column 70, a nitrogen liquid shelf stream 78B, and an evaporated vapor stream or a transported kettle stream 79 returned to the low pressure column 50. The first major portion 73 of the oxygen-rich kettle bottoms 72 is reduced in pressure and then fed to the kettle condenser 75. The remaining portion or second minor portion 74 of the oxygen-rich kettle bottoms 72 of the kettle rectification column 70 is also reduced in pressure and is preferably directed to the high ratio column condenser 96.

[0042] exist Figure 3 In the disclosed embodiment, the kettle distillation column 70 preferably has between 15 and 30 stages of separation, and structured packing or trays may be used, although structured packing is preferred. When only between 15 and 30 stages of separation are used, the shelf nitrogen stream 78B withdrawn from the kettle distillation column 70 has a lower purity and is referred to as a dirty shelf nitrogen stream. In this embodiment, the pressure of the medium pressure column or kettle distillation column 70 is set by the temperature difference of the kettle column condenser 75 and the kettle column reboiler 71, typically in the range of about 2 bara to 3 bara.

[0043] As mentioned above, Figure 3The dirty shelf nitrogen stream 78B increases the dirty shelf reflux stream 46 produced by the high pressure column 40 and forms a mixed reflux stream 47 for the low pressure column 50. Compared to other configurations using a higher purity nitrogen stream or a clean shelf nitrogen stream from a kettle distillation column with more than 30 separation stages, Figure 3 The dirty rack liquid configuration maximizes the power savings of the air separation unit 10 but sacrifices some argon recovery.

[0044] Argon column 80 is configured to receive second portion 57 of argon-oxygen side stream 56 from low pressure column 50 and produce an argon-rich overhead 82 that is directed to once-through argon condenser 65 and an oxygen-rich bottoms 83 that is returned at or near an intermediate location in low pressure column 70 .

[0045] The high ratio column 90 is configured to receive a portion 84 of the crude argon gas stream 69 from the once-through argon condenser 65 and rectify the portion 84 of the crude argon gas stream 69 to produce 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 used as a liquid argon product 194. A high ratio reboiler 95 and a high ratio column condenser 96 are associated with the high ratio column 90. The high ratio column reboiler 95 is disposed at the bottom of the high ratio column 90 and is configured to reboil another portion of the argon-rich liquid at the bottom of the high ratio column 90 by contacting with a stream of 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. The high ratio column condenser evaporates all or a portion of the vapor 98 and a portion of the excess condensing medium 99 back to the low pressure column 50. Argon column 80, high ratio column 90, once-through argon condenser 65, high ratio column reboiler 95 and high ratio column condenser 96 together constitute an argon column apparatus.

[0046] Importantly, 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. The upward-flowing once-through configuration for the argon condenser 65 is preferred due to its lower cost and simplicity, although the downward-flowing once-through configuration for the argon condenser 65 will also provide advantages.

[0047] The key difference between the system and method of the present invention and those disclosed in the prior art references involving four column arrangements with an intermediate pressure column is that the argon condenser 65 in the system and method of the present invention is an integrated unit disposed within the low pressure column and is therefore not directly coupled to the kettle column. By locating the argon condenser within the low pressure column, the boil-up flow through the argon condenser is much greater, and by locating the argon condenser at an optimal location in the low pressure column, the composition of the boil-up flow (i.e., the descending liquid) can have a higher nitrogen content. The optimal location of the argon condenser within the low pressure column is such that the ΔT of the argon condenser does not limit the ability to drive the kettle column ahead of the ΔT of the kettle column reboiler and kettle column condenser, while not penalizing separation within the low pressure column, for example, by locating the argon condenser too high within the column.

[0048] The benefit of using a direct-flow kettle tower reboiler and kettle tower condenser and an integrated direct-flow argon condenser is great. Compared with the pool boiling (i.e., thermosyphon) configuration of this device, due to the boiling flow of lower purity, this configuration naturally increases the temperature difference of each device. However, compared with designing and operating an air separation cycle with a large temperature difference (which will make kettle tower reboilers, kettle tower condensers, and argon condensers smaller, and save some capital costs), it is better to use these larger temperature driving forces to significantly increase the capacity of the medium pressure tower or kettle distillation tower. Increasing the capacity of the kettle distillation tower causes more liquid reflux from the kettle distillation tower to be generated (or product nitrogen generation), and causes its use to have greater advantages. The kettle distillation tower produces more nitrogen, indicating that the kettle tower bottom distillate 72 rich in oxygen is rich in oxygen. This causes the temperature difference in the kettle tower reboiler, kettle tower condenser, and argon condenser to be reduced. Ultimately, the size of these devices and the minimum temperature difference design constraints limit the capacity of the kettle distillation tower, but the size in this configuration is much larger than the prior art.

[0049] The transmitted kettle stream 79 leaving the kettle rectification column condenser 75 is fed to the low pressure column 50. The transmitted kettle stream 79 may be a two-phase stream and may be transmitted to the low pressure column as a two-phase stream, or may be separated in a phase separator (not shown) before being transmitted to the low pressure column 50. If a phase separator is used, the kettle column condenser 75 may be contained within the phase separator. In the illustrated embodiment, the transmitted kettle stream 79 is preferably fed to the low pressure column 50 just above the location of the integrated direct flow argon condenser 65. Within the low pressure column 50, the liquid portion of the transmitted kettle stream 79 is combined with the downwardly flowing liquid within the low pressure column 50.

[0050] Similar to the argon condenser, it is important that both the kettle tower reboiler and the kettle tower condenser are direct current on the boiling side, and the upward flow direct current configuration of the kettle tower reboiler and the kettle tower condenser is preferred. The downward flow direct current configuration of the kettle tower reboiler and the kettle tower condenser will provide some additional advantages, but will be more expensive and may not be justified in most cases. Due to the large amount of feed liquid flowing to the boiling side of the direct current kettle tower reboiler, the vapor fraction of the discharged fluid is low. This shows that the upward flow direct current or downward flow direct current configuration can be used safely. The vapor fraction at the outlet of the upward flow or downward flow direct current kettle tower reboiler is minimized, which enables the kettle tower reboiler to operate at high load with an appropriate ΔT so that its size is reasonable and within the safe operating standards while fully wetting its walls. Similar points can be made for the kettle tower condenser and the argon condenser. The kettle tower condenser and the argon condenser handle feeds with large liquid flow rates. As a result, the discharged vapor fraction is low. This enables safe operation and maximizes ΔT at a given heat load.

[0051] Since the boiling fluid in the kettle column reboiler is very impure, the direct current kettle column reboiler provides a big benefit. If a pool boiler (i.e., a thermosyphon reboiler) is used instead, the oxygen concentration of the boiling stream will be significantly higher, which will reduce its ΔT. The resulting loss is that the ability to drive the kettle distillation column is greatly reduced, and much less nitrogen is produced. The kettle column reboiler is preferably driven by the same vapor source from the low pressure column that feeds the argon column. In addition, the liquid returned from the kettle column condenser is fed to the same general location in the low pressure column as the vapor source.

[0052] Unlike some prior art disclosures involving four column arrangements with an intermediate pressure column, where the intermediate pressure column splits the oxygen-rich kettle bottoms between a kettle column condenser and an argon condenser, severely limiting kettle distillation column capacity, in the system and method of the present invention no oxygen-rich kettle bottoms 72 is directed to the argon condenser 65. Instead, the oxygen-rich kettle bottoms 72 is primarily supplied to the kettle column condenser, except for a very small take-off that is directed to the high ratio column condenser (if used).

[0053] Figure 4 The process flow diagram depicted in Figure 3 The process flow diagram of is very similar, and for the sake of brevity, many descriptions of the detailed arrangement will not be repeated. Instead, the following discussion will focus on Figure 4 The process flow diagram depicted in Figure 3 The differences when compared to the process flow diagram depicted in .

[0054] exist Figure 4In the schematic diagram shown, the medium pressure column or kettle distillation column 70 comprises 30 or more separation stages, more preferably 30 to 50 separation stages. All the shelf liquid nitrogen streams 63 and 78C transmitted from the high pressure column 40 and the kettle distillation column 70 to the low pressure column 50 are relatively high purity liquid nitrogen streams 63 and 78C, which are usually referred to as "clean shelf" liquid nitrogen, and Figure 3 In the embodiment of the present invention, all of the shelf liquid nitrogen streams 46, 78B transferred from the higher pressure column 40 and the kettle distillation column 70, respectively, to the lower pressure column 50 are lower purity nitrogen, commonly referred to as "dirty shelf" liquid nitrogen.

[0055] Use a clean rack of liquid nitrogen Figure 4 The embodiment provides a better performance than using dirty rack liquid nitrogen Figure 3 The embodiment has a higher argon recovery rate, but is also higher than Figure 3 The dirty rack liquid nitrogen implementation is less power efficient. Figure 4 The illustrated embodiment is typically used in markets where the value of the argon product is sufficiently high relative to the cost of electricity.

[0056] While the invention has been described with reference to one or more preferred embodiments, it should be understood that various additions, modifications and omissions may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

1. A distillation column system for producing oxygen, nitrogen and argon from a purified compressed feed air source, the distillation column system comprising: a high pressure column configured to receive one or more purified compressed air streams and a first reflux stream and produce a nitrogen-rich overhead, a bottoms liquid; a low pressure column configured to receive the split liquid air stream and the second reflux stream and to produce a low pressure 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 low-pressure column and configured to thermally couple the high-pressure column and the low-pressure column by liquefying at least a portion of the nitrogen-rich overhead from the high-pressure column by contacting with the oxygen liquid at the bottom of the low-pressure column to produce the first reflux stream and the second reflux stream; A medium-pressure kettle tower device, the medium-pressure kettle tower device comprising a kettle distillation tower, the kettle distillation tower being configured to receive the kettle liquid from the high-pressure tower and produce a bottom distillate rich in oxygen and a top distillate rich in nitrogen; The intermediate-pressure kettle tower apparatus further includes a once-through kettle tower reboiler, the once-through kettle tower reboiler being configured to boil a portion of the descending liquid in the kettle rectification tower by contacting it with a first portion of the argon-oxygen side stream to produce an ascending vapor stream in the kettle rectification tower and an argon-oxygen liquid stream that is returned to an intermediate position of the low-pressure tower; The medium-pressure kettle tower device further comprises a direct-flow kettle tower condenser, which is configured to condense all or a portion of the nitrogen-rich tower top distillate of the kettle distillation tower by contacting with a portion of the oxygen-rich tower bottom distillate of the kettle distillation tower; and an argon column apparatus comprising one or more argon columns and a once-through argon condenser, the argon column being configured to receive a second portion of the argon-oxygen side stream from the low pressure column and to produce an argon-rich overhead and an oxygen-rich bottoms that are returned to the intermediate location of the low pressure column, The argon condenser is disposed in the low-pressure column at a position above the middle position of the low-pressure column, and the argon-rich overhead product is condensed by contact with all or part of the descending liquid in the low-pressure column.

2. The distillation column system according to claim 1, wherein: The split liquid air stream is a resultant liquid air stream withdrawn from an intermediate position of the higher pressure column.

3. The distillation column system according to claim 1, wherein: The kettle-type distillation column is configured to receive the kettle liquid at an intermediate position of the kettle-type distillation column.

4. The distillation column system according to claim 1, wherein: The argon tower device also includes: a first argon column configured to receive the second portion of the argon-oxygen side stream from the low pressure column and to produce the argon-rich overhead and the oxygen-rich bottoms that are directed back to the low pressure column; the once-through argon condenser being configured to receive the argon-rich overhead from the first argon column and to condense the argon-rich overhead to produce a crude argon gas stream; and a high ratio column configured to receive a portion of the crude argon gas stream from the once-through argon condenser and rectify the portion of the crude argon gas stream to produce an argon-rich liquid and an overhead vapor; Part of the argon-rich liquid at the bottom of the high ratio tower is used as liquid argon product.

5. The distillation column system according to claim 4, wherein: The argon tower device also includes: a high ratio column reboiler disposed at the bottom of the high ratio column and configured to reboil another portion of the argon-rich liquid at the bottom of the high ratio column to produce an ascending vapor flow in the high ratio column; A high ratio column condenser is 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.

6. The distillation column system according to claim 5, wherein: A first portion of the oxygen-rich bottoms from the kettle column is directed to the once-through kettle column condenser, and a second portion of the oxygen-rich bottoms from the kettle column is directed to the high ratio column condenser.

7. The distillation column system according to claim 6, wherein: The ratio of the first portion of the oxygen-enriched pot bottoms to the second portion of the oxygen-enriched pot bottoms from the pot column is between about 25:1 and 100:

1.

8. The distillation column system according to claim 5, wherein: The kettle liquid is subcooled in a heat exchanger and in the high ratio column reboiler.

9. The distillation column system according to claim 8, wherein: The medium pressure kettle column produces one or more medium pressure nitrogen product streams.

10. The distillation column system according to claim 1, wherein: The kettle-type distillation tower is spatially arranged above the middle position of the low-pressure tower.

11. An air separation process for producing normal pure oxygen, the air separation process comprising the steps of: rectifying one or more purified compressed feed air streams with a first reflux stream in a higher pressure column to produce a nitrogen-enriched overhead and bottoms liquid; condensing all or a portion of the nitrogen-rich overhead product by contact with the oxygen liquid at the bottom of the low-pressure column in a main condenser-reboiler to produce a liquid nitrogen stream and an ascending vapor stream, a first portion of the liquid nitrogen stream serving as a first reflux stream and another portion of the liquid nitrogen stream serving as a second reflux stream; rectifying the split liquid air stream and the ascending vapor stream with the second reflux stream in a lower pressure column to produce a low pressure nitrogen column overhead, the oxygen liquid at the bottom of the lower pressure column, and an argon-oxygen containing side stream; rectifying the kettle liquid with a third reflux stream in a medium-pressure kettle-type distillation column to produce a bottom distillate enriched in oxygen and another top distillate enriched in nitrogen; reboiling the oxygen-rich column bottoms by contacting them with a first portion of the argon-oxygen-containing side stream in a once-through column reboiler disposed in the medium-pressure kettle distillation column to produce an ascending vapor stream in the medium-pressure kettle distillation column; Condensing all or a portion of the nitrogen-rich overhead product from the intermediate-pressure kettle rectification column by contacting with a portion of the oxygen-rich bottom product in a once-through kettle column condenser to produce a liquid nitrogen stream and an evaporated vapor stream returned to the low-pressure column, a portion of the liquid nitrogen stream serving as the third reflux stream and another portion of the liquid nitrogen stream serving as a shelf nitrogen stream returned to the low-pressure column; rectifying a second portion of the argon-oxygen containing side stream with an argon reflux stream in an argon column to produce an argon-rich overhead and an oxygen-rich bottoms, the oxygen-rich bottoms being returned to an intermediate position in the lower pressure column; Condensing the argon-rich overhead product in a once-through argon condenser disposed in the low-pressure column by contacting it with a descending liquid in the low-pressure column to produce the argon reflux stream and a crude argon stream; as well as One or more standard purity oxygen product streams are produced from the oxygen liquid in the bottom of the lower pressure column.

12. The method according to claim 11, wherein: The split liquid air stream is a resultant liquid air stream withdrawn from an intermediate position of the higher pressure column.

13. The method according to claim 11, further comprising the steps of: subcooling the kettle liquid; and The subcooled kettle liquid is directed to a middle position of the kettle-type distillation tower.

14. The method according to claim 13, further comprising the steps of: rectifying the crude argon stream in a high ratio column to produce an argon-rich liquid and an overhead vapor; as well as A liquid argon product stream is produced from the argon rich liquid at the bottom of the high ratio column.

15. The method according to claim 14, further comprising the steps of: dividing the oxygen-rich bottoms of the medium-pressure kettle distillation tower into a first part and a second part; directing the first portion of the oxygen-rich bottoms of the intermediate-pressure kettle distillation column to the kettle column condenser; as well as The second portion is directed to a high ratio column condenser associated with the high ratio column.

16. An air separation unit, comprising: a main air compression device configured to receive a feed air stream and to compress the feed air stream in a series of main air compression stages to produce a compressed feed air stream; a pre-purification unit configured to remove contaminants and water vapor from the compressed feed air stream to produce a purified compressed feed air stream; wherein the purified compressed feed air stream is divided into one or more purified compressed air streams; one or more heat exchangers configured to cool the one or more purified compressed air streams via indirect heat exchange with an oxygen product stream and one or more nitrogen streams; and The distillation tower system further comprises: a high pressure column configured to receive one or more purified compressed air streams and a first reflux stream and produce a nitrogen-rich overhead, a bottoms liquid; a low pressure column configured to receive the split liquid air stream and the second reflux stream and to produce a low pressure 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 low-pressure column and configured to thermally couple the high-pressure column and the low-pressure column by liquefying at least a portion of the nitrogen-rich overhead from the high-pressure column by contacting with the oxygen liquid at the bottom of the low-pressure column to produce the first reflux stream and the second reflux stream; A medium-pressure kettle tower device, the medium-pressure kettle tower device comprising a kettle distillation tower, the kettle distillation tower being configured to receive the kettle liquid from the high-pressure tower and produce a bottom distillate rich in oxygen and a top distillate rich in nitrogen; The intermediate-pressure kettle tower device further comprises a once-through kettle tower reboiler, wherein the once-through kettle tower reboiler is configured to boil a portion of the descending liquid in the kettle rectification tower by contacting with the first portion of the argon-oxygen side stream to produce an ascending vapor flow in the kettle rectification tower and an argon-oxygen liquid flow returned to an intermediate position of the low-pressure tower; Wherein, the medium-pressure kettle tower device further comprises a direct-flow kettle tower condenser, and the direct-flow kettle tower condenser is configured to condense all or part of the nitrogen-rich tower top distillate of the kettle distillation tower by contacting with a part of the oxygen-rich tower bottom distillate of the kettle distillation tower; and an argon column apparatus comprising one or more argon columns and a once-through argon condenser, the argon column being configured to receive a second portion of the argon-oxygen side stream from the low pressure column and to produce an argon-rich overhead and an oxygen-rich bottoms that are returned to the intermediate location of the low pressure column, The argon condenser is disposed in the low-pressure column at a position above the middle position of the low-pressure column, and the argon-rich overhead product is condensed by contact with all or part of the descending liquid in the low-pressure column.

17. The air separation unit of claim 16, wherein: At least one of the one or more purified compressed air streams exiting the one or more heat exchangers is a liquid air stream.

18. The air separation unit of claim 16, wherein: The one or more heat exchangers include a main heat exchanger configured for cooling the one or more purified compressed air streams via indirect heat exchange with a stream selected from the group consisting of: a high pressure gaseous nitrogen stream from the high pressure column, a waste nitrogen stream from the low pressure column; a low pressure product grade nitrogen overhead stream from the low pressure column; a pumped liquid oxygen stream from the low pressure column; and a pumped high pressure liquid nitrogen stream.

19. The air separation unit of claim 16, wherein: The one or more heat exchangers also include a nitrogen superheater configured to cool one or more streams selected from the group consisting of: the kettle stream, the first reflux stream, the second reflux stream, the split liquid air stream, and a portion of the liquid from the kettle column condenser condensed by contact with the waste nitrogen stream from the low pressure column, and optionally a low pressure product grade nitrogen column overhead stream from the low pressure column.

20. The air separation unit of claim 16, wherein: The split liquid air stream is a resultant liquid air stream withdrawn from an intermediate position of the higher pressure column.

21. The air separation unit of claim 16, wherein: The kettle-type distillation column is configured to receive the kettle liquid at an intermediate position of the kettle-type distillation column.

22. The air separation unit of claim 16, wherein: The argon tower device also includes: a first argon column configured to receive the second portion of the argon-oxygen side stream from the low pressure column and to produce the argon-rich overhead and the oxygen-rich bottoms that are directed back to the low pressure column; the once-through argon condenser being configured to receive the argon-rich overhead from the first argon column and to condense the argon-rich overhead to produce a crude argon gas stream; and a high ratio column configured to receive a portion of the crude argon gas stream from the once-through argon condenser and rectify the portion of the crude argon gas stream to produce an argon-rich liquid and an overhead vapor; Part of the argon-rich liquid at the bottom of the high ratio tower is used as liquid argon product.

23. An air separation unit according to claim 22, wherein: The argon tower device also includes: a high ratio column reboiler disposed at the bottom of the high ratio column and configured to reboil another portion of the argon-rich liquid at the bottom of the high ratio column to produce an ascending vapor flow in the high ratio column; A high ratio column condenser is 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.

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