Air separation unit and method for producing nitrogen and argon using distillation column system with 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 the production of high purity nitrogen for electronic applications, and include a nitrogen recirculation loop required to obtain higher purity nitrogen products. In addition to the medium pressure kettle column, the air separation unit of the present invention and associated methods employ a direct flow argon condenser and a direct flow kettle column reboiler, a direct flow kettle column condenser, preferably disposed within the low pressure column.

Description

Technical Field

[0001] The systems and methods of the present invention relate to separating air by cryogenic distillation, and more particularly, to an air separation unit and method customized for producing a high purity nitrogen product stream and an argon product stream from a distillation column system, the air separation unit and method utilizing a high pressure column, a low pressure column, an intermediate pressure kettle column, and an argon column apparatus. 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.

[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.

[0014] The above-mentioned prior art systems are specifically directed to large oxygen production air separation units, and there is no configuration with a medium pressure column proposed for air separation units customized for electronic applications for producing high purity nitrogen and argon and compressed product oxygen and / or high purity oxygen air separation units. In addition, there are various air separation units that provide an air separation cycle based on three distillation columns, which focuses on the production of high purity nitrogen, argon and oxygen products. The distillation columns in these three-distillation column-based air separation units are usually operated at a higher pressure than conventional three-column air separation units, and have significant nitrogen recycle to achieve high purity nitrogen, and also produce argon at a good recovery rate. However, the power required for nitrogen recycle in these air separation units is relatively large. Moreover, the capital cost of these air separation units increases because the nitrogen recycle requires an increase in the size of the main heat exchanger, main condenser-reboiler, high pressure column and low pressure column.

[0015] Further improvements to these air separation units with nitrogen recycle that produce high purity nitrogen and argon are needed to potentially reduce power costs while improving argon recovery and ensuring adequate availability of the higher purity nitrogen product stream. Summary of the invention

[0016] The present invention can be broadly characterized as an air separation unit comprising: (i) a main heat exchanger configured to cool a purified compressed feed air stream via indirect heat exchange with one or more product streams, a recycle stream, and a waste stream to produce a liquid air stream; (ii) a high pressure column configured to receive the liquid air stream, a nitrogen recycle stream, and a first reflux stream, and produce a nitrogen-rich overhead stream, a kettle liquid, and a dirty shelf nitrogen stream; (iii) a low pressure column configured to receive the dirty shelf nitrogen stream from the high pressure column as a second reflux stream, and produce a nitrogen overhead stream, a waste nitrogen stream, an oxygen liquid bottoms stream, and an argon-oxygen side stream; and (iv) a main condenser-reboiler disposed in the low pressure column and configured to cool at least one of the nitrogen-rich overhead streams from the high pressure column to a temperature of 100°C. (v) an intermediate-pressure kettle column configured to receive kettle liquid from the high-pressure column at an intermediate position of the kettle distillation column and produce an oxygen-rich bottoms product and a nitrogen-rich overhead product; (vi) a direct-flow kettle column reboiler; (vii) a direct-flow kettle column condenser; (viii) an argon column unit including one or more argon columns and an argon condenser, the argon column being configured to receive a second portion of the argon-oxygen side stream from the low-pressure column and produce an argon-rich overhead product and an oxygen-rich liquid stream returned to an intermediate position of the low-pressure column; and (ix) a nitrogen recycle loop configured to recycle all or a portion of the nitrogen overhead product from the low-pressure column to the high-pressure column. A portion of the nitrogen-rich overhead from the higher pressure column is cooled in the main heat exchanger and provided as a higher purity nitrogen product stream, and a portion of the oxygen liquid bottoms is provided as an oxygen product stream.

[0017] The once-through kettle column reboiler is configured to boil a portion of the descending liquid in the 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 returning to an intermediate position of the low-pressure column. The once-through kettle column condenser is configured to condense all or a portion of the nitrogen-rich tower top distillate of the kettle column by contacting it with a portion of the oxygen-rich tower bottom distillate of the kettle column.

[0018] In a preferred embodiment, the nitrogen recycle loop further comprises heating the nitrogen overhead from the low pressure column in a nitrogen superheater and a main heat exchanger to produce a heated recycle stream. The heated nitrogen overhead stream or heated recycle stream is then further compressed in a recycle compressor to produce a compressed recycle stream. The compressed recycle stream is then recooled in a main heat exchanger and introduced into the high pressure column.

[0019] In other embodiments, the direct flow kettle column condenser is configured to condense a first portion of the nitrogen-rich overhead distillate of the intermediate-pressure kettle column by contacting it with a first portion of the oxygen-rich bottom distillate of the intermediate-pressure kettle column, while a second portion of the nitrogen-rich overhead distillate of the intermediate-pressure kettle column is used as a medium-pressure nitrogen product stream having a lower purity than the high-purity nitrogen product stream from the high-pressure column. In addition, a first portion of the condensate from the direct flow kettle column condenser is directed to the intermediate-pressure kettle column as a reflux, and a second portion of the condensate from the direct flow kettle column condenser is directed to the low-pressure column as a third reflux stream.

[0020] All of the above embodiments may include an exhaust gas turbine configured to expand a heated waste stream to produce a waste exhaust stream. The waste exhaust stream is heated in a main heat exchanger to provide supplemental refrigeration for the air separation cycle. Preferably, the heated waste stream is a mixture of heated waste nitrogen and waste oxygen streams, the waste nitrogen being heated in a nitrogen superheater.

[0021] In addition to the high purity nitrogen product stream taken from the high pressure column and the gaseous oxygen product stream produced by heating a portion of the liquid oxygen taken from the low pressure column, embodiments of the air separation unit of the present invention may also produce a liquid nitrogen product stream as a portion of the liquid nitrogen withdrawn from the main condenser-reboiler. Additionally, another portion of the oxygen liquid column bottoms may be used as a liquid oxygen product stream.

[0022] 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 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

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

[0024] 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;

[0025] Figure 2A 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;

[0026] 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; and

[0027] Figure 4 A schematic diagram of a process flow diagram for one embodiment of an air separation unit and associated air separation process configured to produce a high purity nitrogen product stream is shown. DETAILED DESCRIPTION

[0028] The air separation unit and method disclosed in the present invention use Figure 3 The four-tower apparatus shown separates air by cryogenic distillation, includes a high pressure column, a low pressure column, an intermediate pressure column and an argon column apparatus, and is specifically adapted to produce 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] The once-through kettle column condenser 75 is configured to condense all or a portion of the nitrogen-rich kettle overhead 76A of the kettle column 70 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.

[0045] 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 about 2 bara to 3 bara.

[0046] 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 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.

[0047] 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 .

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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 in this configuration is much larger than the prior art.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] In some respects, Figure 4 The process flow diagram depicted in Figure 3 The process flow diagram of the intermediate pressure reactor is somewhat similar to that of the intermediate pressure reactor, and for the sake of brevity, many descriptions of the detailed arrangement of the intermediate pressure reactor will not be repeated. Instead, the following discussion will focus on Figure 4 The key features of the air separation unit 10 for producing high purity nitrogen and argon are shown, and Figure 3 The process flow diagram of the air separation unit shown is compared Figure 4 The differences in the process flow diagram depicted in .

[0057] As noted above, there is a need to improve these air separation units with nitrogen recycle that produce high purity nitrogen and argon to potentially reduce power costs while improving argon recovery and ensuring adequate availability of a higher purity nitrogen product stream. One such improvement is the use of an intermediate pressure kettle column 70 to further fractionate kettle liquid 48 from the high pressure column 40, which improves the available nitrogen reflux 78B to the low pressure column 50. With the additional nitrogen reflux 78B from the intermediate pressure kettle column 70, the nitrogen recycle rate can be reduced, thereby saving power costs. Although the intermediate pressure kettle column 70 presents additional capital costs compared to the prior art three-column air separation unit with a nitrogen recycle loop, the additional capital costs can be partially offset by reducing the size of other equipment, including the main heat exchanger 20, the main condenser-reboiler 60, the high pressure column 40 and the low pressure column 50, and the cost of many other equipment will be saved due to the reduced nitrogen recycle rate.

[0058] Now turn to Figure 4 , which shows an embodiment of an air separation unit 10 for producing high purity nitrogen and argon with nitrogen recycle. Figure 3 The key differences between the illustrated embodiments are the nitrogen recycle loop, generation of ultra high purity nitrogen from the high pressure column, oxygen generation, and supplemental refrigeration using a waste gas expander configured to expand the waste nitrogen and waste oxygen streams.

[0059] like Figure 4 As shown, the nitrogen recycle loop takes nitrogen overhead 52 from the low pressure column 50 and heats the product stream in the nitrogen superheater 21 and the main heat exchanger 20. The heated nitrogen stream is then compressed in the nitrogen recycle compressor 350 and post-cooled in the aftercooler. The compressed recycle stream 352 is further cooled in the main heat exchanger 20 before being directed to the high pressure column 40. By continuously recycling the nitrogen overhead 52 from the top of the low pressure column 50 to the high pressure column 40, the purity of the nitrogen overhead 42 from the high pressure column 40 is increased to a certain value, which enables a portion of the nitrogen overhead 42 from the high pressure column 40 to be used as an ultra-high purity nitrogen vapor stream 44, which is heated in the main heat exchanger 20 to produce an ultra-high purity nitrogen product stream 144.

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

[0061] and Figure 3 Similar to the air separation unit in Figure 4 The mass transfer driving force in the portion of the low-pressure column of the air separation unit 10 depicted in the drawing is unnecessarily high, and this feature can be exploited by using a vapor stream 58 originating from an intermediate position of the low-pressure column 50 to produce vapor in the bottom of the intermediate-pressure kettle column 70 via a kettle column reboiler 71, and by condensing a nitrogen overhead distillate 76A from the intermediate-pressure kettle column 70 in a kettle column condenser 75 using liquid bottoms 72, 73 from the intermediate-pressure kettle column 70 to produce liquid nitrogen streams 78A, 78B.

[0062] In addition, with reference Figure 3 The implementation scheme discussed is similar to Figure 4 The kettle tower reboiler 71 and the kettle tower condenser 75 in the embodiment are direct flow devices. Since the boiling streams in the kettle tower reboiler 71 and the kettle tower condenser 75 have mixed purity, the direct flow design results in much larger ΔTs than the device using a pool boiler. The best way to benefit from this is to drive the kettle tower reboiler 71 and the kettle tower condenser 75 to the maximum level limited by ΔT or other size standards. This maximizes the benefits achieved by the medium-pressure kettle tower 70. The kettle tower reboiler 71 and the kettle tower condenser 75 are generally preferred direct flow devices that flow upward. Direct flow devices that flow downward can provide slightly more benefits, but the cost of constructing them is generally quite high. In the kettle tower reboiler 71 and the kettle tower condenser 75, the vapor fraction on the boiling side is very low, so the kettle tower reboiler 71 and the kettle tower condenser 75 can be safely operated because these devices will not approach unsafe conditions, such as boiling to dryness.

[0063] Although not shown, a portion of the nitrogen overhead 76A from the intermediate pressure kettle column 70 can be used as a high pressure nitrogen vapor product stream. Removing a portion of the nitrogen overhead from the intermediate pressure kettle column as a high pressure nitrogen vapor product stream enhances the recovery benefits of the intermediate pressure kettle column and can further reduce the nitrogen recycle stream. In other embodiments, the distillation column system of the air separation unit may also include a fifth distillation column, an ultra-high purity oxygen column (not shown), where it is desired to produce an ultra-high purity (UHP) oxygen product stream.

[0064] 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, nitrogen and argon from a purified compressed feed air stream, the air separation unit comprising: a main heat exchanger configured to cool the purified compressed feed air stream via indirect heat exchange with one or more product streams, recycle streams, and waste streams to produce a liquid air stream; a high pressure column configured to receive the liquid air stream, the nitrogen recycle stream, and the first reflux stream and produce a nitrogen-rich overhead, a bottoms liquid, and a dirty shelf nitrogen stream; a low pressure column configured to receive the dirty shelf nitrogen stream from the high pressure column as a second reflux stream and to produce a nitrogen overhead stream, a waste nitrogen stream, an oxygen liquid bottoms stream, and an argon-oxygen 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 bottoms in the low pressure column to produce the first reflux stream and the oxygen waste stream; a medium-pressure kettle tower configured to receive the kettle liquid from the high-pressure tower at an intermediate position of the kettle-type distillation tower and produce a bottom distillate rich in oxygen and a top distillate rich in nitrogen; a once-through kettle column reboiler configured to boil a portion of the oxygen-rich bottoms 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 intermediate-pressure kettle column and an argon-oxygen liquid stream that is returned to an intermediate position of the low-pressure column; a once-through kettle tower condenser configured to condense all or a portion of the nitrogen-rich overhead product of the intermediate-pressure kettle tower by contacting it with a portion of the oxygen-rich bottom product of the intermediate-pressure kettle tower; and an argon column apparatus comprising one or more argon columns and an 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 liquid stream that are returned to the intermediate location of the low pressure column; a nitrogen recycle loop configured to recycle all or a portion of the nitrogen overhead from the low pressure column to the high pressure column; and wherein a portion of the nitrogen-rich overhead from the higher pressure column is heated in the main heat exchanger and provided as a higher purity nitrogen product stream, and a portion of the oxygen liquid bottoms is provided as an oxygen product stream.

2. The air separation unit of claim 1, wherein the nitrogen recycle loop further comprises: a nitrogen superheater configured to heat the nitrogen overhead from the low pressure column, wherein the hot stream is further heated in the main heat exchanger to produce a heated nitrogen overhead stream; a recycle compressor configured to compress the heated nitrogen overhead stream to produce a compressed recycle stream; and wherein the compressed recycle stream is cooled in the main heat exchanger and then directed to the high pressure column.

3. The air separation unit according to claim 1, wherein the once-through kettle column condenser is configured to condense a first portion of the nitrogen-rich overhead product of the intermediate-pressure kettle column by contacting it with a first portion of the oxygen-rich bottom product of the intermediate-pressure kettle column, and wherein a second portion of the nitrogen-rich overhead product of the intermediate-pressure kettle column is used as a low-pressure nitrogen product stream.

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

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; and A pre-purification unit is configured to remove contaminants and water vapor from the compressed feed air stream to produce the purified compressed feed air stream.

6. The air separation unit of claim 2, wherein: Waste nitrogen from the low pressure column is heated in the superheater and then mixed with the oxygen waste stream to form a mixed waste stream; The mixed waste stream is heated in the main heat exchanger to produce a heated waste stream.

7. The air separation unit according to claim 6, further comprising: an exhaust gas turbine configured to expand the heated waste stream to produce an exhaust gas stream; and Wherein the waste exhaust gas stream is heated in the main heat exchanger to provide supplemental refrigeration.

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

9. The air separation unit of claim 1 wherein another portion of the oxygen liquid column bottoms is provided as a liquid oxygen product stream.

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