A three column cryogenic cycle for the production of impure oxygen and pure nitrogen

Abstract: Impure oxygen (120) and/or substantially pure nitrogen (116) are produced by cryogenic separation of air using a double column system (60,62) and a third column (24) operating at a pressure between the higher pressure column (60) and the lower pressure column (62) of the double column system. A portion (38,48,52) of the feed air (10) is separated in the double column system, and another portion (50) of the feed air (10) is distilled in the medium pressure column (24). Crude liquid oxygen (100,110) from the higher pressure column (60) and/or the medium pressure column (24) is reduced in pressure (107) and vaporized by heat exchange (106) against nitrogen overhead of the medium pressure column (24). The vaporized crude liquid oxygen (108) is subsequently introduced as a vapor feed to the lower pressure column (62) thereby reducing irreversibilities of separation in said column (62).

Description: The present invention pertains to the production of substantially pure nitrogen and impure oxygen in a cryogenic air separation system. Substantially pure nitrogen (namely nitrogen purity of at least 99.9 mole %) and impure oxygen (namely oxygen purity lower than 98 mole %) are increasingly used in industry. For example, nitrogen and impure oxygen are used in petrochemical plants, gas turbines for power generation, glass production, and in the pulp and paper industry. In certain circumstances, only impure oxygen is required as a product from a cryogenic distillation plant and nitrogen is discarded as waste. In other cases, such as with nitrogen generators, impure oxygen constitutes a waste stream and nitrogen is the desired product. Generally, in a cryogenic distillation plant, production of impure oxygen can be combined with production of pure nitrogen. Numerous processes for the production of impure oxygen and/or nitrogen are known. For example, US-A-3,210,951 discloses a dual reboiler process in which a portion of the feed air is condensed in a reboiler/condenser providing reboil for the bottom section of the low pressure column. Overhead vapor from the high pressure column is condensed in a second reboiler/condenser vaporizing an intermediate liquid stream, which is then delivered to the low pressure column. In comparison with a classic double column, single reboiler cycle, this dual reboiler arrangement reduces the irreversibility of the distillation process in the low pressure column and consequently decreases the feed air pressure, thereby saving power. US-A-4,702,757 discloses a dual reboiler process in which a portion of the feed air is only partially condensed, reducing the feed air pressure even more. US-A-4,453,957 describes a cryogenic rectification process for the production of nitrogen at relatively high purity and at relatively high pressure in a classic double column arrangement with an additional reboiler/condenser at the top of the low pressure column. An impure oxygen waste stream is vaporized at the top reboiler/condenser to provide necessary reflux for the low pressure column. US-A-4,617,036 discloses another cryogenic rectification process to recover nitrogen in large quantities and at relatively high pressure. In this system, an additional side reboiler/condenser is used to condense high pressure nitrogen gas against waste oxygen at reduced pressure. In US-A-5,069,699, a three column nitrogen generator is described. Specifically, the system includes a classic two column, dual reboiler/condenser distillation system and an additional, discrete third column having a pressure higher than the pressure of the high pressure column of the two column system. The bottom reboiler/condenser in the low pressure column is used to condense nitrogen, and crude oxygen is fed to the low pressure column as a liquid. EP-A-0,694,745 relates primarily to a method of separating argon from air by separating a stream of compressed and cooled feed air into nitrogen and oxygen enriched streams then further rectifying the oxygen enriched stream to lead, following further rectification of the resulting argon-enriched liquid, to an argon product. The invention also provides apparatus for separating compressed and cooled air into an oxygen-enriched product and nitrogen. The system disclosed in Figure 2 for use in the above processes comprises a higher pressure, a lower pressure and an intermediate pressure column and condensers to enable the use of part of the nitrogen-enriched liquid condensate for reflux. The preambles of present Claims 1 and 18 are based on said Figure. A conventional double column, dual reboiler cycle which has been used to produce these gases is shown in Fig. 1. The inclusion of a second reboiler/condenser in the low pressure column serves to reduce the specific power of the double column cycle. The cycle shown in Fig. 1 is considered to be one of the most efficient cycles for the production of impure oxygen. Nonetheless, analysis of composition profiles in the low pressure column for this system demonstrate a significant region of process irreversibility. This region is graphically represented by the area between the operating line "O" and the equilibrium line "E" shown in Fig. 2. In a strongly competitive market, there is a demand to reduce this irreversibility and the power required by this cycle even further. The present invention is directed to a method for cryogenically distilling air using a system having a higher pressure column, a lower pressure column, and a medium pressure column to produce at least one of nitrogen and impure oxygen. Preferably, the cycle includes a classic dual column system, along with a discrete medium pressure column having a pressure between the pressures of the higher pressure column and the lower pressure column. The present invention reduces irreversibilities of separation in the lower pressure column by delivering crude oxygen as a vapor to the lower pressure column. In addition, a portion of the feed air is introduced directly to the medium pressure column, introducing the second feed air stream into the medium pressure column for rectification into a medium pressure, oxygen-enriched liquid and a medium pressure nitrogen overhead; introducing the first feed air stream into the higher pressure column for rectification into a higher pressure, oxygen-enriched liquid and a higher pressure nitrogen overhead; at least partially condensing the higher pressure nitrogen overhead against a liquid from the lower pressure column to form higher pressure nitrogen condensate and returning at least a portion of the higher pressure nitrogen condensate to the higher pressure column as reflux; at least partially condensing the medium pressure nitrogen overhead against a first reduced-pressure, oxygen-enriched liquid, resulting in an oxygen-enriched vapor stream and a medium pressure nitrogen condensate, and returning at least a portion of the medium pressure nitrogen condensate to the medium pressure column as reflux; introducing the oxygen-enriched vapor stream into the lower pressure column as feed; withdrawing an oxygen-enriched product stream from a position near the bottom of the lower pressure column; and withdrawing a nitrogen-enriched product stream from a position near the top of said lower pressure column, characterised in that said first reduced-pressure, oxygen-enriched liquid is formed by reducing the pressure of at least a portion of at least one of the medium pressure, oxygen-enriched liquid and the higher pressure, oxygen-enriched liquid; and a remaining portion of at least one of the higher pressure nitrogen condensate and the medium pressure nitrogen condensate is introduced into the lower pressure column as reflux. Preferably, both a portion of the higher pressure nitrogen condensate and a portion of the medium pressure nitrogen condensate are introduced into the lower pressure column as reflux. In one presently preferred embodiment, the at least partial condensation of the higher pressure nitrogen overhead includes introducing at least a portion of the overhead into an intermediate reboiler/condenser of the lower pressure column; a third compressed and cooled feed air stream is condensed in a bottom reboiler/condenser of the lower pressure column to form liquefied feed air; and at least a portion of said liquefied feed air is fed to at least one of the higher pressure column, the medium pressure column, and the lower pressure column. Usually, a first portion of the liquefied feed air is introduced into the higher pressure column; a second portion of the liquefied feed air is introduced to the medium pressure column; and a third portion of the liquefied feed air is introduced into the lower pressure column. The oxygen-enriched product stream can be withdrawn as a liquid and pressurized to form a pressurized oxygen-enriched product stream; the pressurized stream vaporized against a condensing high pressure feed air stream at a pressure higher than the first pressure; and the condensed stream reduced in pressure and at least a portion thereof fed to at least one of the higher pressure column, the medium pressure column, and the lower pressure column. Usually, a first portion of the liquefied feed air is introduced into the higher pressure column; a second portion of the liquefied feed air is introduced to the medium pressure column; and a third portion of the liquefied feed air is introduced into the lower pressure column. In another presently preferred embodiment, the at least partial condensation of the higher pressure nitrogen overhead includes introducing at least a portion of the overhead into a bottom reboiler/condenser of the lower pressure column and the oxygen-enriched product stream is withdrawn as a liquid and introduced into a top reboiler/condenser of the lower pressure column to provide additional reflux to the column and to vaporize the oxygen-enriched product. In a further presently preferred embodiment, the at least partial condensation of the higher pressure nitrogen overhead includes introducing a first portion of the overhead into a bottom reboiler/condenser of the lower pressure column and introducing a second portion of the overhead into a side reboiler/condenser of the lower pressure column; and the oxygen-enriched product stream is withdrawn as a liquid, reduced in pressure and vaporized in the side reboiler/condenser A compressed and cooled further feed air stream at a pressure less than the second pressure often will be introduced into the lower pressure column. Conveniently, the reflux to the lower pressure column is subcooled by heat exchange against the nitrogen-enriched product stream The at least partial condensation of the medium pressure nitrogen overhead can include introducing the first reduced-pressure oxygen-enriched liquid into a top reboiler/condenser of the medium pressure column to form the oxygen-enriched vapor stream and to condense the medium pressure nitrogen overhead. The pressure of the higher pressure, oxygen-enriched liquid can be reduced to form an intermediate reduced-pressure oxygen-enriched liquid and combined with the medium pressure, oxygen-enriched liquid to form a combined oxygen-enriched liquid and the pressure of at least a portion of the combined oxygen-enriched liquid reduced to form the first reduced-pressure oxygen-enriched liquid. The pressure of a second portion of the combined oxygen-enriched liquid can be reduced to form a further reduced-pressure oxygen-enriched liquid which is introduced into the lower pressure column or the pressure of all of the combined oxygen-enriched liquid reduced to form the first reduced-pressure oxygen-enriched liquid. Alternatively, the higher pressure, oxygen-enriched liquid can be reduced in pressure and introduced into the medium pressure column. Usually the first, second and any further feed air streams will be provided from a single main air feed. Suitably, the main air feed is first compressed to the second pressure to provide the second feed air stream and a portion of the compressed feed air is further compressed to provide the first feed air stream or the main air feed is first compressed to the first pressure to provide the first feed air stream and a portion of the compressed feed air expanded to provide the second feed air stream. The oxygen-enriched vapor can be formed by at least partially condensing the medium pressure nitrogen overhead against the first reduced-pressure, oxygen-enriched liquid separated into a first portion having a first oxygen concentration and a second portion having a higher second oxygen concentration; said first portion introduced into a first location of the lower pressure column; and said second portion introduced into a lower second location of the lower pressure column. The present invention also provides an apparatus for producing substantially pure nitrogen and impure oxygen by a method as defined in Claim 1; said apparatus comprising: a higher pressure column; a lower pressure column; a medium pressure column; means for providing a first compressed and cooled feed air stream at a first pressure; means for providing a second compressed and cooled feed air stream at a second pressure less than the first pressure; means for introducing said second feed air stream into the medium pressure column for rectification into a medium pressure, oxygen-enriched liquid and a medium pressure nitrogen overhead; means for introducing said first feed air stream into the higher pressure column for rectification into a higher pressure, oxygen-enriched liquid and a higher pressure nitrogen overhead; means for at least partially condensing the higher pressure nitrogen overhead against a liquid from the lower pressure column to form higher pressure nitrogen condensate; means for returning at least a portion of the higher pressure nitrogen condensate to the higher pressure column as reflux; means for at least partially condensing the medium pressure nitrogen overhead against a first reduced-pressure, oxygen-enriched liquid, resulting in an oxygen-enriched vapor stream and a medium pressure nitrogen condensate; means for returning at least a portion of the medium pressure nitrogen condensate to the medium pressure column as reflux; means for introducing said oxygen-enriched vapor stream into said lower pressure column as feed; means for withdrawing an oxygen-enriched product stream from a position near the bottom of said lower pressure column; and means for withdrawing a nitrogen-enriched product stream from a position near the top of said lower pressure column, characterised in that the apparatus further comprises means for reducing the pressure of at least a portion of at least one of the medium pressure, oxygen-enriched liquid and the higher pressure, oxygen-enriched liquid to form the first reduced-pressure, oxygen-enriched liquid; and means for introducing a remaining portion of at least one of the higher pressure nitrogen condensate and said medium pressure nitrogen condensate into the lower pressure column as reflux. It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. The invention is best understood from the following detailed description when read in connection with the accompanying drawings, in which: Fig. 1 is a schematic diagram of a conventional double-column, dual reboiler cycle; Fig. 2 is a McCabe-Thiele diagram showing the equilibrium curve and operating curve of a system corresponding to Fig. 1; Fig. 3 is a schematic diagram of a first embodiment of the present invention; Fig. 4 is a McCabe-Thiele diagram showing the equilibrium curve and operating curve of a system corresponding to Fig. 3; Fig. 5 is a schematic diagram of a second embodiment of the present invention; Fig. 6 is a schematic diagram of a third embodiment of the present invention; Fig. 7 is a schematic diagram of a fourth embodiment of the present invention; Fig. 8 is a schematic diagram of a fifth embodiment of the present invention; Fig. 9 is a schematic diagram of a sixth embodiment of the present invention; and Fig. 10 is a schematic diagram of a seventh embodiment of the present invention. Reference numerals identify the same elements in all of the figures. In general, the present invention calls for feed air to be introduced to, for example, at least one compressor, at least one heat exchanger, and at least one expander to provide (a) a medium pressure feed air stream and (b) a higher pressure feed air stream. In the preferred embodiment of the present invention shown in Fig. 3, which is a three-column, dual reboiler, impure oxygen cycle, a feed air stream in line 10 is compressed in compressor 12, cooled in heat exchanger 14, cleaned of water and carbon dioxide, preferably in molecular sieve adsorption unit 16, and divided into two streams: the medium pressure feed air stream in line 18 and stream in line 30. Medium pressure feed air stream in line 18 is cooled in a main heat exchanger 20 to a cryogenic temperature and introduced as feed in line 22 to the medium pressure column 24. There, the medium pressure feed air stream (along with another feed discussed below) is rectified into a medium pressure, oxygen-enriched liquid (withdrawn as a bottom product via line 110) and a medium pressure nitrogen overhead stream (withdrawn as an overhead vapor in line 105). Compressed feed air stream in line 30 is further compressed in compressor 32, cooled in heat exchanger 34 against an external cooling fluid, and split into two streams in lines 36 and 70. Stream in line 36 is cooled in main heat exchanger 20 close to its dew point and divided into two streams: a first fraction of the higher pressure feed air stream in line 38 and a second fraction of the higher pressure feed air stream in line 40. The first fraction of the higher pressure feed air stream in line 38 is introduced as a feed into the higher pressure column 60 for rectification (along with another feed discussed below) into a higher pressure, oxygen-enriched liquid (withdrawn as a bottom product via line 100) and a higher pressure nitrogen overhead stream 80. The second fraction of the higher pressure feed air stream in line 40 is condensed in a bottom reboiler/condenser 42, located in the bottom of the lower pressure column 62, thereby forming liquefied feed air in line 46 and providing a part of the reboil necessary for the separation in the lower pressure column 62. Liquefied feed air in line 46 may be divided into three streams: a first portion in line 48, a second portion in line 50, and a third portion in line 52, which form liquefied air feeds to higher pressure column 60, medium pressure column 24 and lower pressure column 62, respectively. Alternatively, liquefied feed air in line 46 may be directed to only one of higher pressure column 60, medium pressure column 24 or, preferably, lower pressure column 62, or any combination of any two of them. The operating pressures of the three columns can vary over wide ranges, such as 18-180 psia (125-1256 kPa) for lower pressure column 62, 35-250 psia (250-1750 kPa) for medium pressure column 24, and 55-350 psia (375-2400 kPa) for higher pressure column 60. The portion of the further compressed feed air stream in line 70 is compressed, then cooled and expanded and introduced as a lower pressure feed air stream to lower pressure column 62. Specifically, the stream in line 70 is compressed in compander compressor 72, cooled in heat exchanger 74 against an external cooling fluid, cooled in main heat exchanger 20, and expanded in turbo-expander 76. Then, the stream is introduced via line 78 to lower pressure column 62 as a lower pressure feed air stream. As mentioned above, the first fraction of the higher pressure feed air stream in line 38 and the first portion of the liquefied air feed in line 48 are introduced to higher pressure column 60, where they are rectified into the higher pressure, oxygen-enriched liquid withdrawn in line 100 and a higher pressure nitrogen overhead stream withdrawn in line 80. The higher pressure nitrogen overhead stream in line 80 is condensed against a liquid from lower pressure column 62 to form higher pressure nitrogen condensate in line 84, a portion of which is returned to higher pressure column 60 in line 86 as reflux. Specifically, the higher pressure nitrogen overhead stream is condensed in an intermediate reboiler/condenser 82 located in lower pressure column 62 above bottom reboiler/condenser 42. As an alternative to using the intermediate reboiler/condenser 82 in lower pressure column 62, a separate device, disposed near and connected to lower pressure column 62 by appropriate vapor and liquid lines, may be utilized. The remaining portion of the higher pressure nitrogen condensate is withdrawn via line 88, subcooled in a heat exchanger 90, reduced in pressure across an isenthalpic Joule-Thompson valve 89 and flashed in a separator 92. The resulting low pressure nitrogen reflux is introduced via line 94 close to the top of lower pressure column 62. As mentioned above, medium pressure feed air stream in line 22 and second portion of liquefied feed air in line 50 are introduced to medium pressure column 24, where they are rectified into a medium pressure, oxygen-enriched liquid (withdrawn via line 110 as a bottom product) and a medium pressure nitrogen overhead stream, which is condensed in a top reboiler/condenser 106 via line 105. A portion of the medium pressure nitrogen condensate provides reflux for medium pressure column 24, and the remaining portion in line 112 is subcooled in heat exchanger 90 and reduced in pressure across an isenthalpic Joule-Thompson valve 91. The stream is then flashed in separator 92 to provide additional reflux to lower pressure column 62 via line 94. In all of the embodiments of the present invention, at least a portion of at least one of the medium pressure, oxygen-enriched liquid and the higher pressure, oxygen-enriched liquid is reduced in pressure to form a first reduced-pressure, oxygen-enriched liquid, and the first reduced-pressure, oxygen-enriched liquid is used as the cooling medium to condense the medium pressure nitrogen overhead stream in the top reboiler/condenser 106 of medium pressure column 24. In the embodiment shown in Fig. 3, higher pressure, oxygen-enriched liquid in line 100 is first subcooled in heat exchanger 103, reduced in pressure across an isenthalpic Joule-Thompson valve 101 to form a second reduced-pressure oxygen-enriched liquid, then combined with medium pressure, oxygen-enriched liquid from line 110 coming from the bottom of medium pressure column 24 to form a combined oxygen-enriched liquid, and either split into two streams in lines 102 and 104 or directed entirely to line 104. Stream in line 104 is reduced in pressure across an isenthalpic Joule-Thompson valve 107 and then vaporized in top reboiler/condenser 106, serving as the first reduced-pressure, oxygen-enriched liquid. The refrigeration provided by stream in line 104 provides the necessary reflux for medium pressure column 24. The resulting vapor stream in line 108 is introduced to lower pressure column 62, as an oxygen-enriched vapor stream. Stream in line 102 is optional, and for some operating conditions not necessary (i.e., the flow in line 102 may be zero). When there is flow in line 102, the stream in line 102 is reduced in pressure across an isenthalpic Joule-Thompson valve 109 and introduced into lower pressure column 62. Introducing the oxygen-enriched stream in line 108 as a vapor, not as a liquid, to lower pressure column 62 greatly reduces the irreversibility in the lower pressure column 62. The corresponding McCabe-Thiele diagram for a system of Fig. 3 is shown in Fig. 4. When comparing this diagram to Fig. 2, it can be seen that the graphical representation of process irreversibilities, namely the area between the operating line "O" and the equilibrium line "E", is reduced in Fig. 4. In all of the embodiments of the present invention, two product streams are withdrawn: (1) an oxygen-enriched product from a position near the bottom of the lower pressure column; and a nitrogen-enriched product from a position near the top of the lower pressure column. Either product may be withdrawn as a liquid or a gas depending on the particular needs, although nitrogen is preferably withdrawn as a gas. In the embodiment shown in Fig. 3, gaseous nitrogen product in line 116 is withdrawn from the top of lower pressure column 62 in line 114, combined with any flash gases from separator 92, and warmed up in: (1) heat exchanger 90 against higher pressure nitrogen condensate in line 88 and medium pressure nitrogen condensate in line 112, (2) heat exchanger 103 against higher pressure, oxygen-enriched liquid in line 100, and (3) main heat exchanger 20 against medium pressure feed air stream in line 22 and higher pressure feed air stream in line 36 and the stream from compander compressor 72 and heat exchanger 74. Also in the embodiment shown in Fig. 3, oxygen product 120 is recovered as a vapor from the bottom of lower pressure column 62 in line 118 and is warmed up in main heat exchanger 20 against medium pressure feed air stream in line 22 and higher pressure feed air stream in line 36 and the stream from compander compressor 72 and heat exchanger 74. Turning to the other embodiments of the present invention shown in Figs. 5-10, in which the same reference numerals refer to the same elements as discussed above in connection with Fig. 3, the embodiments shown in Fig. 5 and in Fig. 6 are directed to using the medium pressure column with a nitrogen generator. Such nitrogen plants also produce impure oxygen as a waste. A significant irreversibility region in the stripping section of the lower pressure column exists when crude oxygen is supplied to the low pressure column as a liquid feed. The irreversibilities are greatly reduced by introduction of the third, medium pressure column, which allows crude oxygen to be supplied to the low pressure column in the form of vapor instead of liquid, as discussed above in connection with Fig. 3. The embodiment shown in Fig. 5 differs from that of Fig. 3 in that there is no intermediate reboiler/condenser but instead there is a top reboiler/condenser 130 of lower pressure column 62. Also, in the embodiment shown in Fig. 5, all of the further compressed feed air stream in line 36 is directed via line 38 to higher pressure column 60. In this embodiment, the step of condensing higher pressure nitrogen overhead stream in line 80 against a liquid from lower pressure column 62 includes introducing higher pressure nitrogen overhead stream in line 80 to a bottom reboiler/condenser 42 of lower pressure column 62. In this embodiment, the oxygen-enriched stream is withdrawn as a liquid via line 132 from a position near the bottom of lower pressure column 62 and introduced to top reboiler/condenser 130 of lower pressure column 62 to provide additional reflux to lower pressure column 62 and to vaporize the oxygen-enriched stream, which could be classified as a product for some uses, but is typically a waste stream in this embodiment. This oxygen-enriched stream is warmed in heat exchangers 90 and 103, as well as in main heat exchanger 20. The embodiment shown in Fig. 6 differs from that of Fig. 3 in that there is no intermediate reboiler/condenser but instead there is a side reboiler/condenser 134 of lower pressure column 62. Also, as in the embodiment shown in Fig. 5, all of the further compressed feed air stream in line 36 is directed via line 38 to higher pressure column 60. In the embodiment shown in Fig. 6, the step of condensing higher pressure nitrogen overhead stream includes the steps of introducing a first portion of higher pressure nitrogen overhead stream to bottom reboiler/condenser 42 of lower pressure column 62 and introducing a second portion of higher pressure nitrogen overhead stream to side reboiler/condenser 134 of lower pressure column 62. Side reboiler/condenser 134 can be contained within the column of lower pressure column 62 or situated next to it. Furthermore, the step of withdrawing an oxygen-enriched product from a position near the bottom of lower pressure column 62 includes first withdrawing an oxygen-enriched product as a liquid from a position near the bottom of lower pressure column 62 via line 136. This stream is reduced in pressure across an isenthalpic Joule-Thompson valve 137 to form a reduced-pressure, oxygen-enriched product which is delivered to side reboiler 134 and used to condense the second portion of the higher pressure nitrogen overhead stream. Another embodiment of the present invention is shown in Fig. 7. This cycle differs from the cycle presented in Fig. 3 in the manner in which the higher pressure, oxygen-enriched liquid in line 100 is used. Specifically, the higher pressure, oxygen-enriched liquid stream in line 100 is reduced in pressure across valve 101 and introduced to the bottom of medium pressure column 24 where it is flashed, thus providing extra reboil for medium pressure column 24 and additional nitrogen reflux to the lower pressure column. The medium pressure, oxygen-enriched liquid in line 110 is cooled in heat exchanger 103, reduced in pressure in an isenthalpic Joule-Thompson valve 107 in line 104, then introduced to top reboiler/condenser 106 of medium pressure column 24. A portion of the medium pressure, oxygen-enriched liquid may be delivered to lower pressure column 62 via line 102. The embodiment shown in Fig. 8 differs from the embodiment of Fig. 3 in that the entire feed air stream is compressed to a higher pressure to form the higher pressure feed air stream in line 30, then a portion of higher pressure feed air stream in line 70 is expanded in an expander 76 to form medium pressure feed air stream in line 22, as opposed to being delivered to lower pressure column 62. The embodiment shown in Fig. 9 differs from the embodiment of Fig. 3 in that a small section of stages or packing 150 is added above top reboiler/condenser 106 of medium pressure column 24. With the inclusion of additional stages or packing 150, the reduced-pressure, oxygen-enriched liquid is partially separated as it is being vaporized. Specifically, it is separated into two portions: (1) a first portion having a first concentration which is withdrawn in line 152; and (2) a second portion having a second concentration, less pure in oxygen than the first concentration, which is withdrawn in line 108. Streams in line 152 and 108 are introduced to lower pressure column 62 at different locations. Specifically, stream in line 108 is introduced above the point at which stream in line 152 is introduced to lower pressure column 62. This embodiment further reduces the irreversibilities of separation in the lower pressure column resulting in additional power savings. The embodiment shown in Fig. 10 differs from the cycle of Fig. 3 by the manner in which oxygen product is withdrawn. Specifically, the embodiment shown in Fig. 10 is desirable if oxygen product is needed at a high pressure without the need to include an expensive oxygen compressor in the system. In this embodiment, oxygen-enriched product is withdrawn as a liquid from the bottom of lower pressure column 62 via line 300. This stream may be pumped via pump 310 to the desired higher pressure. Alternatively, pump 310 may not be needed if a lower oxygen pressure is desired; specifically, several pounds (kPas) of oxygen product pressure can be obtained due to the static head gain caused by the height difference between the point at which liquid oxygen is withdrawn from the lower pressure column 62 and the point where it is boiled. Pressurized oxygen-enriched product in line 320 is then introduced to a heat exchanger 250, where it is vaporized and heated, exiting as stream in line 330. Stream in line 330 is further warmed in main heat exchanger 20. The medium directed to heat exchanger 250, which is used to heat the pressurized oxygen-enriched product from line 320, is a highest pressure feed air stream in line 240. Stream in line 240 is obtained by removing a portion of stream in line 70 via line 200, boosting this portion to a sufficient pressure in auxiliary compressor 210, and cooling the stream in heat exchanger 220 to form stream in line 230 which is cooled further in main heat exchanger 20. Stream in line 240 is condensed in heat exchanger 250 to form liquefied feed air 260 which is joined with liquid air stream 48 to form liquefied air stream 49, which is subsequently delivered to higher pressure column 60. Optionally, liquid air stream 260 could be introduced also to streams in lines 46, 50, or 52. Finally, separate heat exchanger 250 may not be necessary as oxygen could be boiled in main heat exchanger 20 under certain conditions. EXAMPLES In order to demonstrate the efficacy of the present invention, the following example was developed. In Table 1 below, the stream parameters are listed for the embodiment shown in Fig. 3. In Table 2, the mole fractions of the various streams are provided. The basis of the simulations was to produce gaseous oxygen at 95% purity at atmospheric pressure from 100 lbmol/h (45 kgmol/h) of air at atmospheric conditions. In the simulations, the number of theoretical trays in higher pressure column 60 was 25, the number of theoretical trays in medium pressure column 24 was 20, and the number of theoretical trays in lower pressure column 62 was 35. Stream Temperature Pressure Flow Rate in Line Number (°F) (K) (psi) (kPa) (Ibmol/ hour) gmole/s 10 80.0 299.8 14.7 101 100.0 12.60 18 90.0 305.4 47.0 324 29.6 3.73 22 -292.6 92.8 46.0 317 29.6 3.73 30 90.0 305.4 47.0 324 70.4 8.87 36 90.0 305.4 61.2 422 60.4 7.61 38 -287.5 95.6 58.7 405 21.7 2.73 40 -287.5 95.6 58.7 405 38.7 4.88 46 -291.9 93.2 57.7 398 38.7 4.88 48 -291.9 93.2 57.7 398 2.2 0.28 50 -291.9 93.2 57.7 398 3.0 0.38 52 -291.9 93.2 57.7 398 33.6 4.23 70 90.0 305.4 61.2 422 10.0 1.26 78 -255.2 113.6 18.0 124 10.0 1.26 88 -295.3 91.3 57.9 399 12.0 1.51 94 -317.5 79.0 17.5 121 28.0 3.53 100 -287.3 95.8 59.1 408 11.8 1.49 102 -300.0 88.7 58.6 404 0.1 0.01 104 -300.0 88.7 58.6 404 11.7 1.47 108 -302.1 87.5 20.0 138 27.6 3.48 110 -292.3 93.0 47.0 324 15.9 2.00 112 -300.1 88.7 46.0 317 16.7 2.10 114 -317.9 78.8 17.0 117 77.6 9.78 116 83.6 301.8 14.9 103 78.2 9.85 118 -293.9 92.1 18.4 127 21.7 2.73 120 83.6 301.8 17.4 120 21.7 2.73 Stream In Line Number Mole Fraction Nitrogen Argon Oxygen 10 0.7812 0.0093 0.2095 18 0.7812 0.0093 0.2095 22 0.7812 0.0093 0.2095 30 0.7812 0.0093 0.2095 36 0.7812 0.0093 0.2095 38 0.7812 0.0093 0.2095 40 0.7812 0.0093 0.2095 46 0.7812 0.0093 0.2095 48 0.7812 0.0093 0.2095 50 0.7812 0.0093 0.2095 52 0.7812 0.0093 0.2095 70 0.7812 0.0093 0.2095 78 0.7812 0.0093 0.2095 88 0.9867 0.0042 0.0090 94 0.9867 0.0042 0.0090 100 0.5717 0.0145 0.4138 102 0.5717 0.0145 0.4138 104 0.5717 0.0145 0.4138 108 0.5679 0.0148 0.4172 110 0.5652 0.0150 0.4197 112 0.9871 0.0039 0.0090 114 0.9933 0.0030 0.0036 116 0.9933 0.0030 0.0036 118 0.0180 0.0320 0.9500 120 0.0180 0.0320 0.9500 In another example, selected flow rates and pressures in the three-column dual reboiler cycle (shown in Fig. 3) and in the conventional dual reboiler cycle (shown in Fig. 1), both producing 95% oxygen, were compared. This comparison is shown in Table 3 below. Using the cycle shown in Fig. 3 results in a power savings. Specifically, because a significant portion of the feed is separated in the medium pressure column in the cycle of Fig. 3, a smaller amount of the feed needs to be compressed to the high pressure column pressure. In this example, the power of the three-column cycle (of Fig. 3) is 4% lower than the power of the conventional dual reboiler cycle (of Fig. 1). Stream or Apparatus Number Unit Present Invention Fig. 3 Dual Reboiler Cycle Fig. 1 Feed 10 mole/s 100 100 Oxygen Product 120 mole/s 21.7 21.7 Nitrogen Product 116 mole/s 78.2 78.2 Compressor Flow 10 mole/s 100 100 Compressor Discharge Pressure 12 kPa 331.3 442.7 Compressor Flow 30 mole/s 70.4 -- Compressor Discharge Pressure 32 kPa 435.6 -- Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown.

Claims: A method of producing substantially pure nitrogen (116) and impure oxygen (120) by cryogenic distillation in a system having a higher pressure column (60), a lower pressure column (62), and a medium pressure column (24), said method comprising the steps of: providing a first compressed and cooled feed air stream (38) at a first pressure and a second compressed and cooled feed air stream (22) at a second pressure less than said first pressure; introducing said second feed air stream (22) into said medium pressure column (24) for rectification into a medium pressure, oxygen-enriched liquid (110) and a medium pressure nitrogen overhead (105); introducing said first feed air stream (38) into said higher pressure column (60) for rectification into a higher pressure, oxygen-enriched liquid (100) and a higher pressure nitrogen overhead (80); at least partially condensing (82 in Figs. 3 & 7-10; 42 in Figs. 5 & 6) said higher pressure nitrogen overhead (80) against a liquid from said lower pressure column (62) to form higher pressure nitrogen condensate (84) and returning at least a portion (86) of said higher pressure nitrogen condensate to said higher pressure column (60) as reflux; at least partially condensing (106) said medium pressure nitrogen overhead (105) against a first reduced-pressure, oxygen-enriched liquid, resulting in an oxygen-enriched vapor stream (108) and a medium pressure nitrogen condensate, and returning at least a portion of said medium pressure nitrogen condensate to said medium pressure column (24) as reflux; introducing said oxygen-enriched vapor stream (108) into said lower pressure column (62) as feed; withdrawing an oxygen-enriched product stream (118 in Figs 3 & 7-9, 132 in Fig 5, 136 in Fig 6; 300 in Fig 10) from a position near the bottom of said lower pressure column (62); and withdrawing a nitrogen-enriched product stream (114) from a position near the top of said lower pressure column (62) characterised in that said first reduced-pressure, oxygen-enriched liquid is formed by reducing (107) the pressure of at least a portion (104) of at least one of said medium pressure, oxygen-enriched liquid (110) and said higher pressure, oxygen-enriched liquid (100); and a remaining portion (88,112) of at least one of said higher pressure nitrogen condensate (84) and said medium pressure nitrogen condensate is introduced into said lower pressure column (62) as reflux. A method as claimed in Claim 1, wherein both a portion (88) of said higher pressure nitrogen condensate (84) and a portion (112) of said medium pressure nitrogen condensate are introduced into said lower pressure column (62) as reflux. A method as claimed in Claim 1 or Claim 2, wherein said at least partial condensation of said higher pressure nitrogen overhead (80) includes introducing at least a portion of said overhead (80) into an intermediate reboiler/condenser (82 in Figs 3 & 7-10) of said lower pressure column (62); a third compressed and cooled feed air stream (40) is condensed in a bottom reboiler/condenser (42 in Figs 3 & 7-10) of said lower pressure column (62) to form liquefied feed air (46); and at least a portion (48,50,52 in Figs 3 & 5-9) of said liquefied feed air (46) is fed to at least one of said higher pressure column (60), said medium pressure column (24), and said lower pressure column (62). A method as claimed in Claim 1 or Claim 2, wherein said at least partial condensation of said higher pressure nitrogen overhead (80) includes introducing at least a portion of said overhead (80) into a bottom reboiler/condenser (42 in Fig 5) of said lower pressure column (62) and said oxygen-enriched product stream (132) is withdrawn as a liquid and introduced into a top reboiler/condenser (130) of said lower pressure column (62) to provide additional reflux to said column (62) and to vaporize said oxygen-enriched product (132). A method as claimed in Claim 1 or Claim 2, wherein said at least partial condensation of said higher pressure nitrogen overhead (80) includes introducing a first portion of said overhead (80) into a bottom reboiler/condenser (24 in Fig 6) of said lower pressure column (62) and introducing a second portion of said overhead (80) into a side reboiler/condenser (134) of said lower pressure column (62); and said oxygen-enriched product stream (136) is withdrawn as a liquid, reduced (137) in pressure and vaporized in said side reboiler/condenser (134). A method as claimed in any one of Claims 1 to 3, wherein said oxygen-enriched product stream (300) is withdrawn as a liquid and pressurized (310) to form a pressurized oxygen-enriched product stream (320); said pressurized stream (320) is vaporized against a condensing high pressure feed air stream (240) at a pressure higher than said first pressure; and the condensed stream (260) is reduced in pressure and at least a portion thereof (48,50,52 in Fig 10) is fed to at least one of said higher pressure column (60), said medium pressure column (24), and said lower pressure column (62). A method as claimed in Claim 3 or Claim 6, wherein a first portion (48,49) of said liquefied feed air (42,260) is introduced into said higher pressure column (60); a second portion (50) of said liquefied feed air (42,260) is introduced to said medium pressure column (24); and a third portion (52) of said liquefied feed air (42,260) is introduced into said lower pressure column (62). A method as claimed in any one of the preceding claims, wherein a compressed and cooled further feed air stream (78) at a pressure less than said second pressure is introduced into said lower pressure column (62). A method as claimed in any one of the preceding claims, wherein said reflux (88,112) to the lower pressure column (62) is subcooled by heat exchange against said nitrogen-enriched product stream (114). A method as claimed in any one of the preceding claims wherein said at least partial condensation of said medium pressure nitrogen overhead (105) includes introducing said first reduced-pressure oxygen-enriched liquid into a top reboiler/condenser (106) of said medium pressure column (24) to form said oxygen-enriched vapor stream (108) and to condense said medium pressure nitrogen overhead (105). A method as claimed in any one of the preceding claims, wherein the pressure of said higher pressure, oxygen-enriched liquid (100) is reduced (101) to form an intermediate reduced-pressure oxygen-enriched liquid; combining said intermediate reduced-pressure oxygen-enriched liquid with said medium pressure, oxygen-enriched liquid (110) to form a combined oxygen-enriched liquid (102 & 104); and reducing (107) the pressure of at least a portion (104) of said combined oxygen-enriched liquid (102 & 104) to form said first reduced-pressure oxygen-enriched liquid. A method as claimed in Claim 11, wherein the pressure of a second portion (102) of said combined oxygen-enriched liquid (102 & 104) is reduced (109) to form a further reduced-pressure oxygen-enriched liquid which is introduced into said lower pressure column (62). A method as claimed in Claim 11, wherein all of said combined oxygen-enriched liquid (102 & 104) is reduced in pressure to form said first reduced-pressure oxygen-enriched liquid. A method as claimed in any one of Claims 1 to 10, wherein said higher pressure, oxygen-enriched liquid (100) is reduced (101) in pressure and introduced into said medium pressure column (24). A method as claimed in any one of the preceding claims, wherein feed air (10) is first compressed (12) to said first pressure to provide said first feed air stream (38) and a portion (70) of said compressed feed air is expanded (76) to provide said second feed air stream (22). A method as claimed in any one of Claims 1 to 14, wherein feed air (10) is first compressed (12) to said second pressure to provide said second feed air stream (22) and a portion (30) of said compressed feed air is further compressed (32) to provide said first feed air stream (36). A method as claimed in any one of the preceding claims, wherein oxygen-enriched vapor formed by at least partially condensing (106) said medium pressure nitrogen overhead (105) against said first reduced-pressure, oxygen-enriched liquid is separated (150) into a first portion (108) having a first oxygen concentration and a second portion (152) having a higher second oxygen concentration; said first portion (108) is introduced into a first location of said lower pressure column (62); and said second portion (152) is introduced into a lower second location of said lower pressure column (62). An apparatus for producing substantially pure nitrogen (116) and impure oxygen (120) by a method as defined in Claim 1, said apparatus comprising: a higher pressure column (60); a lower pressure column (62); a medium pressure column (24); means (10-16, 30-36 & 20 in Figs 3, 5-7, 9 &10 & 10-16, 30, 36, & 20 in Fig 8) for providing a first compressed and cooled feed air stream at a first pressure; means (10-16, 18 & 20 in Figs 3, 5-7, 9 &10 & 10-16, 30, 70-74, 20 & 76 in Fig 8) for providing a second compressed and cooled feed air stream at a second pressure less than said first pressure; means (22) for introducing said second feed air stream into said medium pressure column (24) for rectification into a medium pressure, oxygen-enriched liquid and a medium pressure nitrogen overhead; means (38) for introducing said first feed air stream into said higher pressure column (60) for rectification into a higher pressure, oxygen-enriched liquid and a higher pressure nitrogen overhead; means (82 in Figs. 3 & 7-10; 42 in Figs. 5 & 6) for at least partially condensing said higher pressure nitrogen overhead against a liquid from said lower pressure column (62) to form higher pressure nitrogen condensate; means (86) for returning at least a portion of said higher pressure nitrogen condensate to said higher pressure column (60) as reflux; means (106) for at least partially condensing said medium pressure nitrogen overhead against a first reduced-pressure, oxygen-enriched liquid, resulting in an oxygen-enriched vapor stream and a medium pressure nitrogen condensate; means for returning at least a portion of said medium pressure nitrogen condensate to said medium pressure column (24) as reflux; means (108) for introducing said oxygen-enriched vapor stream into said lower pressure column (62) as feed; means (118 in Figs 3 & 7-9, 132 in Fig 5, 136 in Fig 6; 300 in Fig 10) for withdrawing an oxygen-enriched product stream from a position near the bottom of said lower pressure column (62); and means (114) for withdrawing a nitrogen-enriched product stream from a position near the top of said lower pressure column (62) characterised in that the apparatus further comprises means (107) for reducing the pressure of at least a portion of at least one of said medium pressure, oxygen-enriched liquid and said higher pressure, oxygen-enriched liquid to form said first reduced-pressure, oxygen-enriched liquid; and means (88, 112 & 90-94) for introducing a remaining portion of at least one of said higher pressure nitrogen condensate and said medium pressure nitrogen condensate into said lower pressure column (62) as reflux.

Inventors: AGRAWAL RAKESH, FIDKOWSKI ZBIGNIEW TADEUSZ, AGRAWAL, RAKESH, FIDKOWSKI, ZBIGNIEW TADEUSZ
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1997
Main classification: F25J3
IPC codes: F25J3