Patent Application
20240035742
The present system and method relates to separating air by cryogenic distillation, and more particularly, to an air separation unit and method tailored for producing high purity nitrogen and argon product streams from a distillation column system that employs a higher pressure column, a lower pressure column, an intermediate pressure kettle column, and an argon column arrangement.
The conventional air separation cycle employing a higher pressure column, a lower pressure and an argon column is the standard choice for an air separation unit when the oxygen product is needed at 99.5% purity or higher, which is often referred to as ‘normal purity oxygen’ together with an argon product. However, for normal purity oxygen and argon production, a three column arrangement exhibits a significant operational deficiency, as illustrated in the McCabe-Thiele diagram of
McCabe-Thiele diagrams are instructive because they illustrate the magnitude of the mass transfer driving forces in the distillation columns of an air separation unit. A McCabe-Thiele diagram for the lower pressure column is key for analysis of a cryogenic oxygen based air separation process. The space in a McCabe-Thiele diagram between the equilibrium line and the operating line is indicative of the driving force for that portion of the column. A process that has large distillation driving forces will tend to have short distillation columns (i.e. not many stages of separation), but it will also be high in power consumption. Ideally, a very efficient cryogenic distillation process will have close, and fairly constant, approaches between the operating line and the equilibrium line. A McCabe-Thiele diagram normally plots the light key component in the liquid on the x-axis and the light key component in the vapor on the y-axis. In
The McCabe-Thiele diagram illustrated in
A feature of most conventional three column arrangements producing normal purity oxygen is the tight approach between the equilibrium line and the operating line in the bottom section of the lower pressure column depicted in the McCabe-Thiele diagram of
Note, that in
Another key observation with respect to the conventional three column arrangement is that the production of high quality nitrogen reflux by the higher pressure column is limited by the equilibrium between the feed air and the kettle liquid. That is, even if the higher pressure column contains a very high number of stages, the amount of reflux generated for supply to the lower pressure column will be limited. The equilibrium between the feed air and kettle liquid necessarily means that a large amount of nitrogen escapes in the kettle liquid and cannot be converted into nitrogen reflux in conventional three column arrangements.
To address these problems, the use of an intermediate pressure column, or kettle column has been suggested. In the intermediate pressure column, the kettle liquid from the higher pressure column is further fractionated to produce additional nitrogen reflux. Examples of the intermediate pressure column are disclosed in U.S. Pat. Nos. 5,675,977; 5,657,644; 5,862,680; and 6,536,232.
U.S. Pat. No. 5,675,977 discloses the use of an intermediate pressure column for the production of low purity oxygen, where the intermediate pressure column is driven with nitrogen vapor from the higher pressure column. By diverting a portion of nitrogen vapor from the higher pressure column to the intermediate pressure column re-boiler, the loss of nitrogen boil-up in the lower pressure column is perfectly tolerable for low purity oxygen production. However, this configuration is not suitable for normal purity oxygen production, where all the available nitrogen gas from the higher pressure column must be used in the main condenser-reboiler to produce sufficient nitrogen vapor boil-up in the lower pressure column. Also note that U.S. Pat. No. 5,675,977 does not disclose the production of any argon product and therefore the disclosed cycle has limited utility due to the lack of argon production.
U.S. Pat. No. 5,657,644 discloses an air separation unit and cycle that employs a higher pressure column, a lower pressure column, an intermediate pressure column, and an argon column that is configured to produce a liquid or crude argon product, a pumped oxygen product and a low pressure nitrogen product taken from the overhead of the lower pressure column. In the four column arrangement disclosed in U.S. Pat. No. 5,657,644, a stream of kettle liquid from the higher pressure column is introduced into a lower section of the intermediate pressure column which produces an oxygen-enriched liquid bottoms and a nitrogen enriched overhead. The intermediate pressure column also includes a bottom reboiler heated by an argon-oxygen containing stream from the lower pressure column and an overhead condenser that condenses a portion of the nitrogen enriched overhead against a portion of the oxygen-enriched liquid bottoms. Another portion of the oxygen-enriched liquid bottoms drives the argon condenser disposed above the argon column while the remaining portion of the oxygen-enriched liquid bottoms is returned to the lower pressure column.
While the air separation cycle disclosed in U.S. Pat. No. 5,657,644 is economically advantageous compared to conventional three column air separation units in that there is a reduced total power consumption as well as an increased argon recovery and oxygen recovery, there is a continuing need to find further improvements to further reduce the total power consumption and provide additional product flexibility, that includes a crude argon product or a refined argon product.
Another example is disclosed in U.S. Pat. No. 6,536,232 which also discloses an air separation unit that employs a higher pressure column, a lower pressure column, an intermediate pressure column, but without an argon column. The intermediate pressure column includes a bottom reboiler heated by an argon-oxygen containing stream from the lower pressure column and an overhead condenser that condenses a portion of the nitrogen enriched overhead against a portion of the oxygen-enriched liquid bottoms. Another portion of the oxygen-enriched liquid bottoms is returned to the lower pressure column.
The air separation cycle disclosed in U.S. Pat. No. 6,536,232 differs from the air separation cycle disclosed in U.S. Pat. No. 5,657,644 mainly due to the absence of the argon column and no production of argon. As indicated above, such improved air separation cycle has limited utility because there is no argon production as well as limited nitrogen production.
The above-described prior art systems are specifically targeted for large oxygen producing air separation units and there has been no configurations with intermediate pressure columns proposed for air separation units that are tailored for electronics applications that produce high purity nitrogen and argon as well as compressed product oxygen and/or high purity oxygen air separation units. There also exists various air separation units that provide a three distillation column based air separation cycle that focus on production of high purity nitrogen, argon, and oxygen products. The distillation columns in these three distillation column based air separation units are generally operated at higher pressures than conventional three column air separation units and with significant nitrogen recycle to achieve the high purity nitrogen and also produce argon at satisfactory recovery rates. However, the power required for the nitrogen recycle in these air separation units is relatively large. Also, the capital costs of these air separation units are increased as the nitrogen recycling requires increasing the size of the main heat exchanger, the main condenser-reboiler, the higher pressure column, and the lower pressure column.
What is needed is further enhancements to these high purity nitrogen and argon producing air separation units with nitrogen recycle to potentially reduce power costs while concurrently improving argon recovery and ensuring sufficient availability of higher purity nitrogen product streams.
The present invention may be broadly characterized as an air separation unit comprising: (i) a main heat exchanger configure to cool the purified, compressed feed air stream via indirect heat exchange against one or more product streams, a recycle stream and a waste stream to yield a liquid air stream; (ii) a higher pressure column configured to receive the liquid air steam, a nitrogen recycle stream, and a first reflux stream and yield a nitrogen-rich overhead, a kettle liquid, and a dirty shelf nitrogen stream; (iii) a lower pressure column configured to receive the dirty shelf nitrogen stream from the higher pressure column as a second reflux stream, and yield a nitrogen overhead stream, a nitrogen waste stream, an oxygen liquid bottoms, and an argon-oxygen side stream; (iv) a main condenser-reboiler disposed in the lower pressure column and configured for thermally coupling the higher pressure column and the lower pressure column by liquefying at least a portion of the nitrogen-rich overhead from the higher pressure column against oxygen liquid bottoms in the lower pressure column to yield the first reflux stream and an oxygen waste stream; (v) an intermediate pressure kettle column configured to receive the kettle liquid from the higher pressure column at an intermediate location of the kettle rectification column and yield an oxygen-rich bottoms and a nitrogen rich overhead; (vi) a once-through kettle column reboiler; (vii) a once through kettle column condenser; (viii) an argon column arrangement comprising one or more argon columns and an argon condenser, the argon column is configured to receive a second part of the argon-oxygen side stream from the lower pressure column and yield an argon-rich overhead and an oxygen-rich liquid stream that is returned to the intermediate location of the lower pressure column; and (ix) a nitrogen recycle circuit configured to recycle all or a portion of the nitrogen overhead from the lower pressure column to the higher pressure column. A portion of the nitrogen-rich overhead from the higher pressure column is cooled in the main heat exchanger and taken as a higher purity nitrogen product stream and a portion of the oxygen liquid bottoms is taken as an oxygen product stream.
The once-through kettle column reboiler is configured to boil a portion of the descending liquid in the kettle column against a first part of the argon-oxygen side stream to yield an ascending vapor stream in the kettle column and an argon-oxygen liquid stream that is returned to an intermediate location of the lower pressure column. The once-through kettle column condenser is configured to condense all or a portion of the nitrogen rich overhead of the kettle column against a portion of the oxygen-rich bottoms of the kettle column.
In the preferred embodiments, the nitrogen recycle circuit further includes warming the nitrogen overhead from the lower pressure column in a nitrogen superheater and the main heat exchanger to yield a warmed recycle stream. The warmed nitrogen overhead stream or warmed recycle stream is then further compressed in a recycle compressor to yield a compressed recycle stream. The compressed recycle stream is then re-cooled in the main heat exchanger and introduced into the higher pressure column.
In other embodiments, the once through kettle column condenser is configured to condense a first portion of the nitrogen rich overhead of the intermediate pressure kettle column against a first portion of the oxygen-rich bottoms of the intermediate pressure kettle column while a second portion of the nitrogen rich overhead of the intermediate pressure kettle column is taken as a intermediate pressure nitrogen product stream having a lower purity than the higher purity nitrogen product stream from the higher pressure column. Also, a first portion of the condensate from the once through kettle column condenser is directed as reflux to the intermediate pressure kettle column and a second portion of the condensate from the once through kettle column condenser is directed to the lower pressure column as a third reflux stream.
All of the above-described embodiments may include a waste turbine configured to expand a warmed waste stream to produce a waste exhaust stream. The waste exhaust stream is warmed in the main heat exchanger to provide supplemental refrigeration for the air separation cycle. The warmed waste stream is preferably a mixture of warmed waste nitrogen, warmed in the nitrogen superheater and the waste oxygen stream.
In addition to the higher purity nitrogen product stream taken from the higher pressure column and the gaseous oxygen product stream produced by warming a portion of the liquid oxygen taken from the lower pressure column, embodiments of the present air separation units may also produce a liquid nitrogen product stream taken as a portion of the liquid nitrogen exiting the main condenser-reboiler. Also, another portion of the oxygen liquid bottoms may be taken as a liquid oxygen product stream.
In other embodiments of the present air separation unit and method designed to also produce an argon product, the argon column arrangement further comprises a first argon column configured to receive the second part of the argon-oxygen side stream from the lower pressure column and yield the argon-rich overhead and the oxygen-rich bottoms that is directed back to the lower pressure column and a high ratio column configured to receive a portion of a crude argon stream from the once-through argon condenser and rectify the portion of the crude argon stream to yield an argon-rich liquid and an overhead vapor. The argon column arrangement further includes a high ratio column reboiler disposed at the bottom of the high ratio column and a high ratio column condenser. In these embodiments, a portion of the argon-rich liquid at the bottom of the high ratio column is taken as liquid argon product.
It is believed that the claimed invention will be better understood when taken in connection with the accompanying drawings in which:
The disclosed air separation unit and method for separating air by cryogenic distillation using a four column arrangement shown in
By generating additional nitrogen reflux and/or supplemental elevated pressure (i.e. intermediate pressure) nitrogen product from the intermediate pressure column, or kettle column, the present air separation unit system and method enables higher oxygen recovery, higher argon recovery with improved efficiency and reduce power consumption relative to conventional three column arrangements and many of the prior art intermediate column arrangements. On the McCabe-Thiele diagram of
A key principle or characteristic of the present four column arrangement and associated air separation cycle is that: (1) the mass transfer driving force in the top section of the lower pressure column is increased by taking advantage of the excess driving force in the section between the kettle liquid feed and argon column draw; and (2) the mass transfer driving force of the bottom section of the lower pressure column is not reduced. In addition, the use of present four column arrangement and cycle reduces or avoids the need to draw product nitrogen from the top of the lower pressure column due to its ability to produce supplemental nitrogen product and/or nitrogen reflux from the intermediate pressure column.
Turning now to
The warm-end arrangement is configured for conditioning a feed air stream for separation into its constituent components, namely argon, oxygen, and nitrogen, The warm-end arrangement receives a feed air stream, compresses the feed air stream in a series of main air compression stages and purifies the compressed air stream in a pre-purification unit to produce a compressed and purified air stream 12.
A first main portion of the compressed and purified air stream 12 is directed to the main heat exchanger 20 where it is cooled to temperatures suitable for rectification in the distillation column system 30 and exits the main heat exchanger as a cooled, compressed and purified stream 22. A second portion of the compressed and purified air stream 14 is further compressed in a first booster compressor 13 and cooled in aftercooler. A part of the further compressed second portion of the compressed and purified air stream is still further compressed in a second booster compressor 15 and cooled in aftercooler to yield a booster air stream 16 that is also directed to the main heat exchanger 20. The booster air stream 16 is cooled in main heat exchanger 20 to yield a liquid air stream 26 that is directed to the higher pressure column 40 of the distillation column system 30.
The remaining part of the further compressed first portion of the compressed and purified air stream is diverted as stream 18 that is further compressed in another booster compressor 17 to yield a turbine air stream 25 that is then partially cooled in main heat exchanger 20. The partially cooled stream is the expanded in turbine 27 yielding an exhaust stream 28 that is also directed to the higher pressure column 40 of the distillation column system 30. Note the exhaust stream 28 may be combined with the cooled, compressed and purified air stream 22.
The first portion of the compressed and purified air stream 12 as well as the turbine air stream 18 and booster air stream 16 are cooled in the main heat exchanger 20 via indirect heat exchange with a plurality of streams from the distillation column system 30 including: a clean shelf nitrogen stream 44; a pumped liquid oxygen stream 53; a pumped high pressure gaseous nitrogen stream 68; a waste nitrogen stream 59; and a low pressure gaseous nitrogen stream 52. The warmed streams exit the main heat exchanger 20 as: a product grade gaseous nitrogen stream 144; a product grade gaseous oxygen stream 153; a product grade high pressure gaseous nitrogen stream 168; a warmed waste nitrogen stream 159; and a product grade low pressure gaseous nitrogen stream 152.
The illustrated distillation column system 30 comprises: a higher pressure column 40, an intermediate pressure column or kettle column 70; a lower pressure column 50; an integrated argon condenser 65; an argon column 80; and a high ratio column 90.
The higher pressure column 40 configured to receive one or more streams of compressed, purified air including the liquid air stream 26, the cooled, compressed and purified air stream 22, as well as the exhaust stream 28 together with a reflux stream and yields a nitrogen-rich overhead 42, a clean shelf vapor stream 44, a dirty shelf nitrogen stream 46, a kettle liquid 48, and a synthetic liquid air stream 45 taken from an intermediate location of the higher pressure column 40.
The lower pressure column 50 is configured to receive the synthetic liquid air stream 45, an oxygen-rich bottoms 83 and or more reflux streams or other streams to yield a low pressure product grade nitrogen overhead 52, an oxygen liquid 51 at the bottom of the column to be taken as a liquid oxygen stream 53, and an argon-oxygen containing side stream 56 taken from an intermediate location of the lower pressure column 50. A portion of the liquid oxygen stream 53 may be taken as a liquid oxygen product 154 while the majority of the liquid oxygen stream 53 is pumped via pump 55 and vaporized in the main heat exchanger to produce the gaseous oxygen product 153. The one or more reflux streams introduced into the lower pressure column 50 preferably include stream 78B from the kettle column condenser 75 and the dirty shelf nitrogen stream 46 from the higher pressure column which streams may be combined to yield a mixed shelf reflux stream 47. The purity of the dirty shelf nitrogen streams 78B, 46 from the higher pressure column 40 and from the kettle column 70 is optimized for feed to the lower pressure column 50 at or near a location where waste nitrogen is withdrawn from lower pressure column.
The lower pressure column 50 also houses a main condenser-reboiler configured for thermally coupling higher pressure column 40 and lower pressure column 50 by liquefying at least a portion of the nitrogen-rich overhead 42 from the higher pressure column 40 against the oxygen liquid 51 at the bottom of the lower pressure column 50 to yield a nitrogen reflux stream 62 directed to the higher pressure column 40 and another nitrogen stream 61, a portion of which is directed as reflux stream 63 to the top of the lower pressure column 50. Another portion of nitrogen stream 61 is preferably taken as high pressure product grade nitrogen stream 68 that is pumped via pump 67 and directed to the main heat exchanger 20 while the remaining portion of nitrogen stream 61 is preferably taken as a liquid nitrogen product stream 164.
A once-through argon condenser is also disposed within the lower pressure column 50 at a location above the intermediate location of the lower pressure column. The argon condenser is configured to condense an argon-rich overhead taken from the argon column against all or a portion of the descending liquid in the lower pressure column 50 including feed streams 79, 97, 98, and optionally a diverted portion 66 of the liquid air stream 45 to produce a crude argon stream 69, a portion of which is a reflux stream 81 for the argon column 80.
In some cases, it may be preferred to use the diverted portion of the subcooled synthetic liquid air 66 to further increase the temperature driving force of the argon condenser 65. Doing this introduces this liquid air to a non-ideal location within the lower pressure column, resulting in a small penalty in argon recovery. Unlike the prior art systems and methods, using a small, diverted portion of the subcooled synthetic liquid air 66 to drive the argon condenser 65 in the present air separation unit and method can enable a further increase in the driving force of the kettle column reboiler 71 and kettle column condenser 75. When this is desirable, it results in a further increase in oxygen recovery and a further reduction in power consumption.
An intermediate pressure column or kettle column 70 is configured to receive the kettle liquid 48 from the higher pressure column 40 and yield an oxygen-rich kettle bottoms 72 and a nitrogen rich kettle overhead 76A. The kettle liquid 48 is preferably subcooled in the nitrogen superheater 21 and routed through the high ratio column reboiler 95. Preferably, the subcooled kettle liquid 48 is then introduced into the kettle column 70, preferably at an intermediate location of the kettle column several stages above the bottom section. Operatively associated with the kettle column 70 is a once-through kettle column reboiler 71 and a once-through kettle column condenser 75. The once-through kettle column reboiler 71 is configured to boil a portion of the descending liquid in the kettle column 70 against a first part 58 of the argon-oxygen side stream 56 to yield an ascending vapor stream in the kettle column 70 and an argon-oxygen liquid stream 77 that is returned at or near the intermediate location of the lower pressure column 50. For that reason, the kettle column 70 is spatially disposed preferably above the intermediate location of lower pressure column 50 so that the return liquid from the kettle column reboiler and the transferred kettle can be fed to the lower pressure column 50 by gravity. Also, unlike some of the prior art disclosures related to intermediate pressure columns, none of the synthetic liquid air or liquid air feed is directed to the kettle column.
The once-through kettle column condenser 75 is configured to condense all or a portion of the nitrogen rich kettle overhead 76A of the kettle column 70 against a first major portion 73 of the oxygen-rich kettle bottoms 72 of the kettle column 70 to yield a nitrogen reflux stream 78A for the kettle column 70, a shelf nitrogen liquid stream 78B and a boil-off vapor stream or transferred kettle stream 79 that is returned to the lower pressure column 50. The first major portion 73 of the oxygen-rich kettle bottoms 72 is let down in pressure and then fed to the Kettle Column condenser 75. The remaining or second minor portion 74 of the oxygen-rich kettle bottoms 72 of the kettle column 70 is also let down in pressure and preferably directed to a high ratio column condenser 96.
In the disclosed embodiment of
As indicated above, the kettle column of
The argon column 80 is configured to receive a second part 57 of the argon-oxygen side stream 56 from the lower pressure column 50 and yield an argon-rich overhead 82 that is directed to the once through argon condenser 65 and an oxygen-rich bottoms 83 that is returned at or near the intermediate location of the lower pressure column 70.
The high ratio column 90 is configured to receive a portion 84 of the crude argon stream 69 from the once-through argon condenser 65 and rectify the portion 84 of the crude argon stream 69 to yield an argon-rich liquid 94 and an overhead vapor 92. A portion of the argon-rich liquid 94 at the bottom of the high ratio column 90 is taken as liquid argon product 194. Associated with the high ratio column 90 is a high ratio reboiler 95 and a high ratio column condenser 96. The high ratio column reboiler 95 is disposed at the bottom of the high ratio column 90 and configured for reboiling another portion of the argon-rich liquid at the bottom of the high ratio column 90 against a stream of the kettle liquid 48 to produce an ascending vapor stream in the high ratio column 90. The high ratio column condenser 96 is configured to condense the overhead vapor 92 from the high ratio column 90 and return all or a portion of the condensate as a high ratio column reflux stream 97. All or a portion of the high ratio column condenser boil-off vapor 98 as well as a portion of the excess condensing media 99 is be returned to the lower pressure column 50. Together the argon column 80, the high ratio column 90, the once-through argon condenser 65, the high ratio column reboiler 95, and the high ratio column condenser 96 make up an argon column arrangement.
It is essential that the argon condenser 65 is once-through on the boiling side and this feature provides a large advantage over the prior art disclosures of four column arrangements, due in part, to the impure boiling stream. A once-through up-flow configuration for the argon condenser 65 is preferred due to its lower cost and simplicity, although a once-through downflow configuration for the argon condenser 65 would also provide an advantage.
A key difference between the present air separation unit and associated methods and those disclosed in the prior art references related to four column arrangements with an intermediate pressure column, is the argon condenser 65 is an integrated unit disposed within the lower pressure column and therefore not directly coupled to the kettle column. By locating the Argon condenser within the lower pressure column, the boiling flow through the argon condenser is much greater, and by locating the argon condenser at the optimal location in the lower pressure column, the composition of the boiling stream (i.e. descending liquid) can be higher in nitrogen content. The optimal location for the argon condenser within the lower pressure column is such that the ΔT of the argon condenser does not limit the ability to drive the kettle column before the ΔTs of the kettle column reboiler and kettle column condenser, while not penalizing the separation within the lower pressure column, for example, by locating the argon condenser too high within the column.
The benefit of using the once-through kettle column reboiler and kettle column condenser, as well as the integrated once-through argon condenser is large. This configuration naturally increases the temperature differences of each device due to the lower purity boiling streams compared to a pool boiling (i.e. thermosyphon) configurations for such devices. But rather than designing and operating the air separation cycle with large temperature differences, which would make the kettle column reboiler, kettle column condenser, and argon condenser smaller, and save some capital cost, it is far better to use these larger temperature driving forces to dramatically increase the capacity of the intermediate pressure column or kettle column. Increasing the capacity of the kettle column results in much more liquid reflux production (or product nitrogen generation) from the kettle column, and a much larger advantage for its use. The greater production of nitrogen from the kettle column means that the oxygen-rich kettle bottoms 72 is richer in oxygen. This results in reduced temperature differences in the kettle column reboiler, kettle column condenser, and argon condenser. Ultimately, size and minimum temperature difference design constraints of these devices limit the capacity of the kettle column, but at a much greater magnitude in this configuration than for the prior art.
The transferred kettle stream 79 exiting the kettle column condenser 75 is fed to the lower pressure column 50. The transferred kettle stream 79 is likely a two phase stream and it may be transferred to the lower pressure column as a two phase stream or it may be separated in a phase separator (not shown) before transferring to the lower pressure column 50. If a phase separator is used, then the kettle column condenser 75 may be contained within the phase separator. In the illustrated embodiment, the transferred kettle stream 79 is preferably fed to the lower pressure column 50 just above the location of the integrated once-through argon condenser 65. Within the lower pressure column 50 the liquid portion of the transferred kettle stream 79 is combined with the downflowing liquid in the lower pressure column 50.
Similar to the argon condenser, it is essential that both the kettle column reboiler and kettle column condenser are once-through on the boiling side, A once-through up-flow configuration of the kettle column reboiler as well as the kettle column condenser is preferred. A once-through downflow configuration for the kettle column reboiler and kettle column condenser would provide some additional advantage but will be more costly and would probably not be justified in most cases. Due to the large feed liquid flow to the boiling side of the once-through kettle column reboiler, the vapor fraction of the exiting fluid is low. This means that the once-through up-flow or once through downflow configurations can be used safely. With the once through up-flow or downflow kettle column reboiler, the vapor fraction of its outlet is minimized which enables the kettle column reboiler to operate at high duty with an appropriate ΔT so that its size is reasonable, and within safe operating criteria with its walls being sufficiently wetted. For the kettle column condenser and argon condenser similar points can be made. They handle the feed of large liquid flow rates. As a result, the vapor fraction exiting is low. This enables safe operation and maximizes the ΔT at a given heat duty.
Since the boiling fluid within the kettle column reboiler is very impure, a once-through kettle column reboiler provides a large benefit. If a pool boiler (i.e., thermosyphon reboiler) were used instead, the boiling flow would be significantly higher in oxygen concentration which would decrease its ΔT. The resulting penalty would be a greatly reduced ability to drive the kettle column, with much less production of nitrogen. The kettle column reboiler is preferably driven by the same vapor source from the lower pressure column that feeds the argon column. Also, the liquid returned from the kettle column condenser is fed to the same general location in the lower pressure column as the vapor source.
Unlike some of the prior art disclosures related to four column arrangements with an intermediate pressure column which splits the oxygen-rich kettle bottoms between the kettle column condenser and the argon condenser severely limiting the kettle column capacity, none of the oxygen-rich kettle bottoms 72 in the present air separation unit and method are directed to the argon condenser 65. Rather, the oxygen-rich kettle bottoms 72 are supplied mainly to the kettle column condenser except for a very minor takeoff that is directed to the high ratio column condenser, if it is used.
In some respects, the process flow diagrams depicted in
As indicated above, there is a need for enhancements to high purity nitrogen and argon producing air separation units with nitrogen recycle in order to potentially reduce power costs while concurrently improving argon recovery and ensuring sufficient availability of higher purity nitrogen product streams. One such improvement is the use of an intermediate pressure kettle column 70 to further fractionate the kettle liquid 48 from the higher pressure column 40 which improves the available nitrogen reflux 78B to the lower pressure column 50. With the additional nitrogen reflux 78B from the intermediate pressure kettle column 70, the rate of nitrogen recycle can be reduced, saving power costs. While the intermediate pressure kettle column 70 represents an additional capital cost compared to the prior art three column air separation units with a nitrogen recycle circuit, this is partially off-set by the capital cost reduction in the reduction in other equipment including the size of the main heat exchanger 20, the main condenser-reboiler 60, the higher pressure column 40, and the lower pressure column 50 there will be cost savings in much of the other equipment due to the reduced rate of nitrogen recycle.
Turning now to
As shown in
Similar to the air separation unit in
Also, similar to the embodiment discussed with reference to
Although not shown, a portion of the nitrogen overhead 76A from the intermediate pressure kettle column 70 may be taken as an elevated pressure nitrogen vapor product stream. Taking a portion of the nitrogen overhead from the intermediate pressure kettle column as an elevated pressure nitrogen vapor product stream enhances the recovery benefit of the intermediate pressure kettle column and enables possible further reduction of the nitrogen recycle flow. In other embodiments, the distillation column system of the air separation unit may further include a fifth distillation column, namely an ultra-high purity oxygen column (not shown) where production of an ultra-high purity (UHP) oxygen product stream is required.
While the present invention has been described with reference to a preferred embodiment or embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present invention as set forth in the appended claims.
