Air separation unit and method for cryogenic separation of air using distillation column system comprising medium pressure kettle column

Abstract

An air separation unit and associated method for separating air by cryogenic distillation using a distillation column system including a high pressure column, a low pressure column, a medium pressure kettle column, and an argon column arrangement is provided. The air separation units and methods disclosed herein are particularly useful for producing oxygen products as well as several nitrogen products, wherein the nitrogen overhead is a portion of the medium pressure kettle column as a medium pressure nitrogen product. The air separation unit and associated methods of the present invention employ a once-through kettle column reboiler, a once-through kettle column condenser, and a 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 relate to separating air by cryogenic distillation, and more particularly, to an air separation unit and method using a high pressure column, a low pressure column, an intermediate pressure kettle column and an argon column. Background Art

[0002] A conventional air separation cycle employing a high pressure column, a low pressure column, and an argon column is the standard choice for air separation units when an oxygen product of 99.5% or greater purity is desired, as well as an argon product, which is often referred to as "standard purity oxygen." However, for the production of standard purity oxygen and argon, a three-column apparatus exhibits significant operating disadvantages, as shown in the McCabe-Thiele diagram of FIG1 .

[0003] McCabe-Thiele diagrams are helpful because they show the magnitude of the mass transfer driving force in the distillation columns of the air separation unit. The McCabe-Thiele diagram of the low-pressure column is the key to analyzing the cryogenic oxygen-based air separation process. The space between the equilibrium line and the operating line in the McCabe-Thiele diagram represents the driving force of this part of the column. Methods with large distillation driving forces tend to have short distillation columns (i.e., without many separation stages), but the power consumption is also high. Ideally, a very efficient cryogenic distillation process will 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, 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 portion of the low-pressure column, while nitrogen is the light key in the rest of the low-pressure column. By plotting the sum of the argon and nitrogen fractions in Figure 1, the entire low-pressure column can be clearly characterized.

[0004] The McCabe-Thiele diagram shown in Figure 1 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] A characteristic of most conventional three-tower plants producing standard purity oxygen is the close proximity between the equilibrium line and the operating line in the bottom portion of the low pressure column depicted in the McCabe-Thiele diagram of Figure 1. 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 sufficient 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 in Figure 1 that 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 illustrates 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 and an intermediate pressure nitrogen vapor product stream.

[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 improved air separation cycle has limited utility because no argon is produced and nitrogen production is limited, and there is no medium-pressure nitrogen vapor product stream. Summary of the invention

[0014] The present invention can be broadly characterized as 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; (iii) one or more heat exchangers configured to cool the one or more purified compressed air streams via indirect heat exchange with one or more waste and product streams; and (iv) a distillation column system having a high pressure column, a low pressure column, an intermediate pressure kettle column producing an intermediate pressure nitrogen vapor product stream, and an argon column device having an argon condenser disposed in the low pressure column, the argon condenser being configured to condense an argon-rich overhead distillate by contact with a portion of a descending liquid in the low pressure column to produce a crude argon stream.

[0015] More specifically, within the distillation tower system, the high-pressure tower is configured to receive one or more purified compressed air streams and a first reflux stream, and produce a nitrogen-rich tower top distillate and a kettle liquid. The low-pressure tower is configured to receive a split liquid air stream and a second reflux stream, and produce a low-pressure nitrogen tower top distillate, an oxygen liquid at the bottom of the tower, and an argon-oxygen-containing side stream. The main condenser-reboiler is disposed in the low-pressure tower and is configured to thermally couple the high-pressure tower and the low-pressure tower by liquefying at least a portion of the nitrogen-rich tower top distillate from the high-pressure tower by contacting with the oxygen liquid at the bottom of the low-pressure tower to produce a high-pressure nitrogen product stream, the first reflux stream, and the second reflux stream. The argon tower device includes one or more argon towers and a direct-flow argon condenser.

[0016] The intermediate-pressure kettle column is configured to receive kettle liquid from the high-pressure column and produce a bottom distillate rich in oxygen and a top distillate rich in nitrogen, a portion of which is used as an intermediate-pressure nitrogen product stream. The once-through kettle column reboiler is configured to boil a portion of the descending liquid in the intermediate-pressure kettle column by contacting it with a first portion of the argon-oxygen side stream to produce an ascending vapor stream in the kettle column and an argon-oxygen liquid stream returned to an intermediate position of the low-pressure column, and the once-through kettle column condenser is configured to contact all or a portion of the nitrogen-rich top distillate of the kettle column with a portion of the oxygen-rich bottom distillate of the kettle column for condensation.

[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 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 column to produce an oxygen-rich tower bottom distillate and another nitrogen-rich tower top distillate; in a direct-flow kettle column reboiler disposed in the medium-pressure kettle column, the oxygen-rich tower bottom distillate is passed through the first part of the reboiler with the argon-oxygen-containing side stream to produce a low-pressure nitrogen tower top distillate, an oxygen liquid at the bottom of the low-pressure column and an argon-oxygen-containing side stream; (e) condensing all or a portion of the nitrogen-rich overhead product from the intermediate-pressure kettle 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; (f) rectifying a second portion of the argon-oxygen-containing side stream in an argon column with the argon reflux stream (g) condensing the argon-rich overhead product in a direct argon condenser disposed in the low-pressure column by contacting it with a descending liquid in the low-pressure column to produce an argon reflux stream and a crude argon stream; and (h) producing one or more oxygen product streams from the oxygen liquid at the bottom of the low-pressure column, and producing one or more nitrogen products including an intermediate-pressure nitrogen vapor product stream from the intermediate-pressure kettle column.

[0018] In some embodiments of the above-described air separation unit and associated air separation process, the kettle column has 15 to 30 separation theoretical plates, and a portion of the condensed nitrogen-rich overhead product discharged from the once-through kettle column condenser is directed to another intermediate location of the low-pressure column as a dirty rack reflux stream. In this embodiment, the high-pressure column produces a dirty rack nitrogen stream that is taken from another intermediate location of the high-pressure column, and the dirty rack nitrogen stream is mixed with the dirty rack reflux stream and directed to another intermediate location of the low-pressure column.

[0019] In other embodiments of the above-described air separation unit and associated air separation process, the kettle column has more than 30 separation theoretical plates, and a portion of the condensed nitrogen-rich overhead product discharged from the once-through kettle column condenser is directed to the top of the low-pressure column as a third reflux stream. In such other embodiments, a portion of the nitrogen-rich overhead product from the kettle column is used as a medium-pressure nitrogen vapor product.

[0020] In all of the above embodiments, the split liquid air stream may be a resultant liquid air stream taken from an intermediate position of the higher pressure column, or may be a portion of the liquid air stream withdrawn from the main heat exchanger. The kettle liquid from the higher pressure column is subcooled and then introduced at an intermediate position of the kettle column.

[0021] In other embodiments of the air separation unit and method 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 stream from the direct-flow argon condenser and rectify a portion of the crude argon 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

[0022] It is believed that the claimed invention will be better understood when taken in conjunction with the accompanying drawings, in which:

[0023] FIG1 depicts a McCabe-Thiele diagram of 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;

[0024] Figure 2 A McCabe-Thiele diagram depicting one embodiment of the air separation unit and process 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;

[0025] Figure 3 A schematic diagram showing a process flow diagram of an air separation unit having a distillation column system including an intermediate pressure kettle column;

[0026] Figure 4 A schematic diagram showing an alternative embodiment of a process flow diagram for an air separation unit and associated air separation process wherein a portion of the nitrogen overhead medium pressure kettle column is used as a medium pressure nitrogen product stream;

[0027] Figure 5 a schematic diagram showing a process flow diagram of another alternative embodiment of an air separation unit and associated air separation process of the present invention; and

[0028] Figure 6 A schematic diagram showing a process flow diagram of another embodiment of an air separation unit and associated air separation process of the present invention. DETAILED DESCRIPTION

[0029] The air separation unit and process of the present invention for separating air by cryogenic distillation using a four-column arrangement comprising 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.

[0030] By generating additional nitrogen reflux and / or supplemental high pressure (i.e., medium pressure) nitrogen product from the medium pressure column or kettle column, the air separation unit 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 plants and many prior art intermediate tower plants. On the McCabe-Thiele diagram of FIG. 1 for a conventional three-column plant, the tight clamping in the upper portion of the low pressure column is relieved by utilizing excess distillation driving force in selected portions of the low pressure column. In addition, certain benefits can also be achieved, including efficiency benefits in applications when the high pressure nitrogen product and / or liquid nitrogen product production rate is less than 50% of the total oxygen production rate.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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: gaseous nitrogen stream 144; gaseous oxygen stream 153; high pressure gaseous nitrogen stream 168; hot waste nitrogen stream 159; and low pressure gaseous nitrogen stream 152.

[0038] The illustrated distillation column system 30 includes: a high pressure column 40; an intermediate pressure column or kettle column 70; a low pressure column 50; an integrated argon condenser 65; an argon column 80; and a high ratio column 90.

[0039] 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.

[0040] 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 nitrogen overhead 52, oxygen liquid 51 at the bottom as a liquid oxygen stream 53, and an argon-oxygen containing side stream 56 withdrawn from an intermediate location 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 column condenser 75 and a 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 purity of the dirty shelf nitrogen streams 78B, 46 from the high pressure column 40 and from the kettle column 70 are optimized so as to be fed to the low pressure column 50 at or near the location where the waste nitrogen is withdrawn from the low pressure column.

[0041] 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 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.

[0042] 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.

[0043] 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 prior art systems and methods, the use of a small split portion of the subcooled synthetic liquid air 66 to drive the argon condenser 65 in the air separation unit and method of the present invention can further increase the driving force of the kettle column reboiler 71 and 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.

[0044] The intermediate pressure column or kettle 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 column 70, preferably at an intermediate position of the kettle column, several 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 column 70. The straight-through kettle column reboiler 71 is configured to boil a portion of the descending liquid in the kettle 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 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 column 70 is preferably spatially disposed above the middle of the low pressure column 50 so that return liquid from the kettle column reboiler and the transferred kettle can be gravity fed to the low pressure column 50. Also, unlike some prior art disclosures involving intermediate pressure columns, no synthetic liquid air or liquid air feed is directed to the kettle column.

[0045] 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 by contacting it with the first major portion 73 of the oxygen-rich kettle bottoms 72 of the kettle column 70 to produce a nitrogen reflux stream 78A for the kettle 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 column 70 is also reduced in pressure and is preferably directed to the high ratio column condenser 96.

[0046] exist Figure 3 In the disclosed embodiment, the kettle 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 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 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 2 bara to 3 bara.

[0047] As mentioned above, Figure 3 The 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 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.

[0048] 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 .

[0049] 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 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.

[0050] 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.

[0051] The key difference between the air separation unit and associated methods of the present invention and those systems and methods disclosed in prior art references involving four column arrangements with intermediate pressure columns is that the argon condenser 65 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.

[0052] 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. But compared with designing and operating an air separation cycle with a large temperature difference (which will make the kettle tower reboiler, kettle tower condenser, and argon condenser 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 tower. Increasing the capacity of the kettle tower leads to more liquid reflux generation (or product nitrogen generation) from the kettle tower, and leads to greater advantages in its use. The kettle tower produces more nitrogen, indicating that the kettle tower bottom distillate 72 rich in oxygen is rich in oxygen. This leads to a reduction in the temperature difference in the kettle tower reboiler, kettle tower condenser, and argon condenser. Ultimately, the size of these devices and the minimum temperature difference design constraints limit the capacity of the kettle tower, but the size is much larger than the prior art.

[0053] The transmitted kettle stream 79 leaving the kettle 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 once-through 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.

[0054] 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.

[0055] Since the boiling fluid in the kettle column reboiler is very impure, a once-through 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 a greatly reduced ability to drive the kettle column, producing much less nitrogen. 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.

[0056] 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 column capacity, in the air separation unit and process of the present invention no oxygen-rich kettle bottoms 72 is directed to the argon condenser 65. Instead, oxygen-rich kettle bottoms 72 is primarily supplied to the kettle condenser, with the exception of a very small take-off directed to the high ratio column condenser, if used.

[0057] Figures 4 to 6 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 , Figure 5 and Figure 6 The process flow diagram depicted in Figure 3 The differences when compared to the process flow diagram depicted in .

[0058] exist Figure 4 In the embodiment depicted in FIG. 1 , the intermediate pressure column or kettle column 70 comprises 30 or more separation stages, more preferably 30 to 50 separation stages. In this embodiment, the intermediate pressure column or kettle column 70 does not produce a higher purity liquid nitrogen stream, commonly referred to as "clean rack" liquid nitrogen, for use as reflux to the low pressure column. Instead, all of the "clean rack" liquid nitrogen 63 used as reflux to the low pressure column 50 is withdrawn from the main condenser-reboiler 60, while in FIG. Figure 3 In an embodiment of the present invention, the shelf liquid nitrogen stream is transferred from both the high pressure column (via the main condenser-reboiler) and the kettle column via the kettle column condenser.

[0059] In this embodiment, the medium-pressure kettle column 70 produces a medium-pressure nitrogen vapor product 176. The medium-pressure nitrogen vapor 76D from the medium-pressure kettle column 70 is preferably heated in the nitrogen superheater 21 before it passes through the main heat exchanger 20.

[0060] exist Figure 4In the embodiment of the present invention, since the high pressure column 40 generates a considerable shelf liquid reflux, the main condenser-reboiler 60 has a higher duty, while the kettle column condenser 75 has a lower duty. The higher main condenser-reboiler duty and lower kettle column condenser duty translate into more boiling vapor in the bottom portion of the low pressure column 50 and less boiling vapor from the kettle column condenser 75, which is directed to the low pressure column 50 as part of the transmitted kettle stream 79, resulting in oxygen recovery and argon recovery in this embodiment. Figure 3 There is a significant improvement compared to the implementation scheme.

[0061] On the other hand, withdrawing the medium-pressure nitrogen vapor product 176 from the medium-pressure kettle column 70 and reducing the pumped high-pressure nitrogen products 68, 168 taken from the condensate 61 of the main condenser-reboiler 60 means that the compression power for the overall increased product nitrogen stream will be higher unless the supply of medium-pressure nitrogen vapor 76D from the kettle column 70 at medium pressure is acceptable and the medium-pressure nitrogen vapor product stream 176 does not need to be further compressed.

[0062] As a result of this tradeoff, if the desired total product nitrogen rate (i.e., liquid nitrogen and compressed gaseous nitrogen) is greater than about 100% of the desired oxygen rate, then Figure 4 The air separation unit 10 configuration depicted in Figure 3 If the argon product is high value, then Figure 4 The air separation unit 10 configuration depicted in FIG. 1 is also preferred. Regardless of whether the high pressure column 40 produces only a "clean" rack liquid transfer 63 (e.g. Figure 4 This configuration can be used whether the argon product is a low purity or "dirty" rack nitrogen or a portion of the rack nitrogen delivery is a low purity or "dirty" rack nitrogen. However, in situations where the argon product has a high value, using only "clean" rack nitrogen is the preferred configuration.

[0063] Figure 5 The embodiment depicted in Figure 4 , wherein the medium-pressure column or kettle column 70 also preferably comprises 30 to 50 separation stages and produces medium-pressure nitrogen vapor 76D which is heated in the nitrogen superheater 21 before it passes through the main heat exchanger 20. However, Figure 5 The embodiment depicted in is most suitable for applications where the demand for high pressure nitrogen is less than what the intermediate pressure kettle column 70 can produce. Therefore, to maximize the benefit of the intermediate pressure kettle column 70, additional nitrogen is produced from the kettle column 70 as shelf liquid nitrogen 78B, which is directed to the low pressure column 50, and no dirty shelf liquid nitrogen is brought from the high pressure column to the low pressure column.

[0064] In some cases, some high pressure gaseous nitrogen is produced by pumping liquid nitrogen 53 from the main condenser-reboiler 60 via pump 55 and heating the pumped liquid nitrogen in the main heat exchanger 20 to produce a high pressure gaseous nitrogen product stream 153. Alternatively, a portion of the shelf liquid nitrogen from the kettle column condenser can be diverted as a liquid nitrogen source (instead of or in addition to the liquid nitrogen from the main condenser-reboiler) to be vaporized in the main heat exchanger to produce a high pressure gaseous nitrogen product stream. In another case, the shelf liquid nitrogen from the high pressure column (via the main condenser-reboiler) to the pump is preferably subcooled in a nitrogen superheater, otherwise, when the shelf liquid nitrogen is combined with the liquid nitrogen from the intermediate pressure kettle column, it will flash upstream of the pump.

[0065] Figure 6 The implementation scheme depicted in Figure 5 The embodiment is the same as that of , wherein the medium-pressure column or kettle column 70 also preferably comprises 30 to 50 separation stages and produces medium-pressure nitrogen vapor 76D, which is heated in the nitrogen superheater 21 before passing through the main heat exchanger 20. In addition, Figure 6 The embodiment shown in FIG. 1 shows that high pressure shelf nitrogen vapor 44 is withdrawn from the high pressure column and heated in the main heat exchanger to produce a high pressure shelf nitrogen vapor product stream 144 . Figure 6 The embodiment described in Figure 5 The implementation scheme is useful in different scenarios and can achieve greater power reduction in some situations.

[0066] Specifically, in the preferred cycle selection, all the rack liquid nitrogen delivered from the medium pressure kettle column (see Figure 3 ) is switched to produce medium-pressure nitrogen vapor product from the medium-pressure kettle column (see Figure 4 ), it may be beneficial to produce both a medium pressure nitrogen vapor product and a shelf liquid nitrogen stream for use as reflux in the low pressure column. Figure 6 The embodiment depicted in satisfies this goal. In the product rate range where the total nitrogen product rate is about 100% of the total oxygen product rate, Figure 3 The configuration shown allows very high oxygen recovery, while Figure 4 The oxygen recovery of the configuration is relatively low. Figure 6 The configuration provides additional efficiency benefits.

[0067] At very high oxygen recovery, Figure 4 shown Figure 4Configuration has limited benefit to maximizing the capacity of the intermediate pressure kettle column and the duties of the kettle column reboiler and kettle column condenser. When the intermediate pressure kettle column capacity is maximized, the corresponding increase in oxygen recovery in this case is small, while the intermediate pressure nitrogen vapor production from the intermediate pressure kettle column continues to increase linearly. The increase in oxygen recovery saves power, but the increase in intermediate pressure nitrogen vapor production and the corresponding reduction in high pressure tower nitrogen vapor production maintain the total high pressure nitrogen product supplied to the customer, resulting in a power loss. This power loss is due to the lower pressure of the intermediate pressure nitrogen vapor production compared to the high pressure tower vapor production.

[0068] Figure 6 The configuration or embodiment depicted in provides an improvement in this case. By supplying some of the nitrogen from the intermediate pressure kettle column as liquid reflux to the low pressure column, the design and operation of the air separation unit can be placed at a more ideal position on the oxygen recovery curve. As a result, Figure 4 Compared to the configuration shown, oxygen recovery is reduced to a lesser extent, but the rate of high pressure nitrogen vapor product from the high pressure column is increased to a greater extent, and the high pressure nitrogen vapor product from the intermediate pressure kettle column is reduced accordingly. Therefore, power consumption is reduced in this case.

[0069] Alternatively, "dirty" rack liquid may be produced from the high pressure column and the intermediate pressure kettle column instead of "clean" rack liquid. Alternatively, a combination of "dirty" rack liquid and "clean" rack liquid may be produced from the high pressure column and the intermediate pressure kettle column. The transfer of "dirty" rack liquid nitrogen to the low pressure column will provide greater power savings, but lower argon recovery. In addition, if product pure nitrogen is produced from the low pressure column overhead, a corresponding amount of clean rack nitrogen liquid is required to achieve the product purity specification of the low pressure column overhead nitrogen.

[0070] 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. An air separation unit for producing oxygen and nitrogen from a purified compressed feed air source, the air separation unit 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 and 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, oxygen liquid at the bottom, 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 it with the oxygen liquid at the bottom of the low-pressure column to produce a high-pressure nitrogen product stream, the first reflux stream, and the second reflux stream; a medium-pressure kettle tower device, the medium-pressure kettle tower device comprising a kettle tower, the kettle 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, a portion of the top distillate being used as a medium-pressure nitrogen product stream; The intermediate pressure kettle tower apparatus further comprises 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 tower by contacting it with a first portion of the argon-oxygen side stream to produce an ascending vapor stream in the kettle 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, wherein the direct-flow kettle tower condenser is configured to condense all or a portion of the nitrogen-rich tower top distillate of the kettle tower by contacting with a portion of the oxygen-rich tower bottom distillate of the kettle 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; wherein the argon condenser is disposed within the low pressure column at a position above the intermediate position of the low pressure column, and the argon-rich overhead product is condensed by contact with a portion of the descending liquid in the low pressure column to produce a crude argon stream.

2. The air separation unit of claim 1 , wherein the kettle column further comprises more than 30 separation theoretical plates, and a portion of the condensed nitrogen-rich overhead product discharged from the once-through kettle column condenser is directed to the top of the low-pressure column as a third reflux stream.

3. The air separation unit of claim 2, wherein a portion of the nitrogen-rich overhead product from the kettle column is provided as a medium-pressure nitrogen vapor product.

4. The air separation unit of claim 1, wherein the kettle column further comprises more than 30 separation theoretical plates, and a portion of the nitrogen-rich overhead product from the kettle column is used as a medium-pressure nitrogen vapor product.

5. The air separation unit according to claim 1, further 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; as well as 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.

6. The air separation unit according to claim 5, wherein: At least one of the one or more purified compressed air streams exiting the one or more heat exchangers is a feed liquid air stream.

7. The air separation unit according to claim 6, wherein: The one or more heat exchangers include a main heat exchanger configured to cool 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 nitrogen overhead stream from the low pressure column; a pumped liquid oxygen stream from the low pressure column; and a pumped high pressure stream of liquid nitrogen.

8. The air separation unit according to claim 5, 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: a 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 a low pressure nitrogen column overhead stream from the low pressure column.

9. The air separation unit 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.

10. The air separation unit according to claim 1, wherein: The kettle column is configured to receive the kettle liquid at an intermediate position of the kettle column.

11. The air separation unit 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 stream; and a 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 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.

12. The air separation unit according to claim 11, 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.

13. 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 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 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 column to produce a bottom distillate enriched in oxygen and another top distillate enriched in nitrogen; reboiling the oxygen-rich column bottoms by contacting with a first portion of the argon-oxygen containing side stream in a once-through column reboiler disposed in the medium-pressure column to produce an ascending vapor stream in the medium-pressure column; condensing all or a portion of the nitrogen-rich overhead product from the intermediate-pressure kettle column by contacting it 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 oxygen product streams are produced from the oxygen liquid at the bottom of the low pressure column, and one or more nitrogen products including a medium pressure nitrogen vapor product stream are produced from the medium pressure kettle column.

14. The method according to claim 13, wherein: The split liquid air stream is a resultant liquid air stream withdrawn from an intermediate position of the higher pressure column.

15. The method according to claim 13, further comprising the steps of: subcooling the kettle liquid; and The subcooled kettle liquid is directed to an intermediate position of the kettle column.

16. The method according to claim 15, 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.

17. The method according to claim 16, further comprising the steps of: dividing the oxygen-rich bottoms of the medium-pressure kettle column into a first portion and a second portion; directing the first portion of the oxygen-rich bottoms of the intermediate-pressure kettle 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.

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