Process for the production of oxygen and nitrogen

Abstract: Oxygen and nitrogen are produced by cryogenic separation in which at least a portion (112) of feed air is introduced to a first (130) of at least three distillation columns (130, 164, 166). An oxygen-lean stream (132) from or near the top of the first column (130) is at least partially condensed in a reboiler-condenser (141; 135) of the second or third column (164; 166) to provide reflux for the first column. Oxygen-enriched liquid (168) from the bottom of the first column (130) is fed to the second or third column (164; 166). Nitrogen enriched liquid (154), which can be a portion (150) of the condensed oxygen-lean stream from the first column (130), is fed to the second column(164). Oxygen-enriched liquid bottoms (160) from the second column (164) is fed to the third column (166) and nitrogen-rich vapour overheads (194; 182) are withdrawn from the second and third columns (164; 166). A liquid oxygen-rich stream (173) from the third column (130) is elevated in pressure (173) and warmed (110), at least in part, by indirect heat exchange with a pressurized stream (116) having a nitrogen content greater than or equal to that in the feed air. The pressurized stream is cooled without being subjected to distillation and is fed to any one or combination of the three columns (130, 164, 166).

Description: The present invention relates generally to the production of oxygen and nitrogen from a cryogenic air separation plant, and more particularly to the production of pressurized oxygen using pumped-LOX (liquid oxygen) and the production of at least a portion of nitrogen as pressurized nitrogen. The most well known cryogenic process for the production of both oxygen and nitrogen is the double-column cycle. This process uses a distillation column system comprising a higher pressure column, a lower pressure column and a reboiler-condenser which thermally links the two columns. Early versions of the double-column cycle produced both nitrogen and oxygen as vapours from the lower pressure column. Recently, it has become commonplace to withdraw the oxygen product from the distillation column system as a liquid ("LOX"), raise the pressure of the liquid oxygen by using either static head or a pump, and warm it in a main heat exchanger by cooling some suitably pressurized stream. This method of oxygen delivery is referred to as pumped-LOX. When large quantities of pressurized nitrogen are also required it is typical to elevate the pressure of the lower pressure column to recover nitrogen at some pressure greater than atmospheric. Processes of this type are often called elevated pressure, or EP, cycles. Numerous examples of elevated pressure, double column, pumped-LOX cycles exist in the prior art. An example of one such prior art cycle is shown in Figure 9. A commercial application for such a process is the production of low purity oxygen (less than 98 mole % oxygen) and nitrogen for Coal Gasification Combined Cycle ("CGCC") power and chemical plants. Since an objective of such applications is to produce power, it is essential that the air separation process be energy efficient. The need for high efficiency has given rise to many modifications to the conventional elevated pressure, double-column, pumped-LOX cycle. One solution for improving the efficiency of the double-column cycle is to utilize a third distillation column as in US-A-5,682,764 (Agrawal, et al .). This patent teaches the use of a third column which operates at a pressure intermediate that of the higher and lower pressure columns. This third column receives a vapour air feed which is at a lower pressure than the main air feed to the higher pressure column. This intermediate pressure column has a condenser but no reboiler, and produces liquid nitrogen reflux for the lower pressure column. Power consumption is reduced by only having to compress a fraction of the feed air to the pressure of the higher pressure column. Another patent which teaches the use of a third column to improve efficiency is US-A-5,678,426 (Agrawal, et al .). This patent also teaches the use of a third column which operates at a pressure intermediate that of the higher and lower pressure columns. This third column receives oxygen-enriched liquid from the bottom of the higher pressure column as a feed. This intermediate pressure column has both a reboiler and a condenser, and produces a nitrogen-rich stream from its top and a further-oxygen-enriched liquid from its bottom. Another patent which teaches the use of a third column to improve efficiency is disclosed in US-A-4,254,629 (Olszewski). Olszewski teaches the use of a third intermediate pressure column which functions much like that of US-A-5,682,764. Olszewski also discloses a four-column version which has a pair of double columns in parallel. As taught by Olszewski, both lower pressure columns operate at essentially the same pressure. One higher pressure column operates at a lower pressure than the other. This is achieved by maintaining the composition in the bottom of one lower pressure column more oxygen-lean than the other - - the higher pressure column which is thermally linked to the lower pressure column having the more oxygen-depleted composition can thereby operate at lower pressure. Olszewski also teaches to pass oxygen-depleted vapour to the other lower pressure column. None of the three patents discussed above teaches modes of operation using pumped-LOX. US-A-4,433,989 (Erickson) also teaches the use of a third column to improve efficiency. Erickson teaches the use of a third intermediate pressure column in conjunction with a double-column process which includes: 1) passing all the air to the higher pressure column; 2) passing essentially all the oxygen-enriched liquid from the higher pressure column into the intermediate pressure column; 3) distilling in the intermediate pressure column to produce a nitrogen-rich vapour and a further oxygen enriched liquid; 4) passing the further oxygen-enriched liquid to the lower pressures column; 5) refluxing both intermediate pressure column and lower pressure column with nitrogen-enriched liquid from the higher pressure column; and 6) providing boilup to both the intermediate pressure column and the lower pressure column by indirect heat exchange with condensing vapour from the higher pressure column Erickson also suggests an operating method using pumped-LOX in which pressurized air is passed to the bottom of a fourth distillation column which produces a nitrogen-rich liquid from its top and an oxygen-enriched liquid from its bottom - - much like a typical higher pressure column would. The condenser for this fourth column is operated by vaporizing the oxygen product at elevated pressure. US-A-5,341,646 (Agrawal et al) discloses the cryogenic separation of air to produce an oxygen product and a nitrogen product using a triple column system comprising high, medium and low pressure columns. Characterizing features of the process include producing an oxygen product with a purity of less than 98% oxygen; producing no argon product; producing from the medium and/or high pressure columns a gaseous nitrogen product which represents greater than 35% of the feed air; recovering a major portion of the oxygen product from the low pressure column; condensing at least a portion of the high pressure nitrogen overhead from the high pressure column by heat exchange against a liquid stream in the medium pressure column and utilizing at least a portion of the condensed portion to provide reflux to the high pressure column. In one exemplified embodiment, a first portion of compressed air is fed to the high-pressure column; oxygen-enriched bottoms liquid from the high-pressure column is fed to the medium-pressure column; overhead vapour from the high-pressure column is condensed in intermediate and bottom reboiler-condensers in the medium-pressure column to provide reflux to the high- and medium-pressure columns; oxygen-enriched bottoms liquid from the medium-pressure column is fed to the low-pressure column; part of the overhead vapour from the medium-pressure column is condensed in a reboiler-condenser in the low-pressure column to provide further reflux to the medium-pressure column and the remainder provides the nitrogen product; overhead vapour from the low-pressure column is eventually discharged as a waste nitrogen stream; bottoms liquid from the low-pressure columnis pumped and warmed against inter alia a further portion of the air feed to provide the oxygen product and a cooled air feed; portions of the cooled air feed are fed to the medium- and low- pressure columns; and a liquid stream from an intermediate location of the medium-pressure column provides reflux to the low-pressure column. In an unexemplified variation, boilup at the bottom of the low-pressure column is provided by condensing another suitable process stream such as a portion of the feed air stream. WO-A-8404957 (published 20th December 1984) discloses a cryogenic process for separating air to produce oxygen and nitrogen in which there is latent heat exchange at intermediate locations between lower pressure and medium pressure columns of a distillation column system to ensure high reboil flow through the argon stripping section of the lower pressure column and provide midsection reboil in the medium pressure column. In the exemplified embodiment of Figure 3, the distillation column system has a first distillation column ("HP column"), a second distillation column ("MP column "), a third distillation column ("LP column ") and an argon side-arm column. The HP column is at pressure higher than the MP column, which is at a pressure higher than the LP column. Compressed air is fed to the HP column to provide oxygen-enriched bottoms liquid and oxygen-lean overhead vapour. Respective portions of the bottoms liquid are fed to the MP and LP columns. One portion of the overhead is condensed in a reboiler/condenser in the bottom of the LP column and the condensate retumed to the top of the HP column to provide reflux therein. Another portion of the overhead is condensed in a reboiler/condenser at a lower intermediate location of the MP column and respective portions of the condensate fed to the tops of the HP, MP and LP columns to provide reflux therein. A further portion of the overhead is withdrawn as high pressure nitrogen product. Bottom reboil to the MP column is provided by oxygen-lean vapour from an intermediate location of the HP column. Further intermediate reboil to the MP column is provided by a reboiler/condenser at an upper intermediate location thereof and fed with a vapour withdrawn from the LP column at an intermediate location above the argon stripping section thereof. The resultant condensate is retumed to the LP column at said intermediate location. Oxygen-enriched liquid is withdrawn from the bottom of the MP column and is fed to the LP column. Respective nitrogen-rich vapours are withdrawn from the tops of the MP and LP columns and removed from the distillation column system without condensation. Liquid oxygen is withdrawn from the bottom of the LP column, elevated in pressure and vaporized by indirect heat exchange with a pressurized argon recycle stream subsequently providing reflux to the argon side-arm column. WO 8504000 (published 12th September 1985) discloses a cryogenic process for separating air to produce oxygen and nitrogen in which additional nitrogen reflux is provided by condensing an elevated pressure portion of the feed air against vaporising liquid oxygen to provide liquid nitrogen-enriched and liquid oxygen-enriched feeds for a distillation column system. In the exemplified embodiment of Figure 1, the distillation column system has a first distillation column ("HP column"), a second distillation column ("MP column "), and a third distillation column ("LP column "). The HP column is at pressure higher than the MP column, which is at a pressure higher than the LP column. A first portion of compressed air is fed to the HP column and a second portion of compressed air is further compressed and fed to a rectifier in which reflux is provided by indirect heat exchange with boiling pressurized liquid oxygen from the LP column. The rectifier provides liquid nitrogen-enriched and liquid oxygen-enriched intermediate feeds to the HP column after exchange of sensible heat with the pressurized liquid oxygen prior to reflux duty. Oxygen-enriched liquid is withdrawn from the bottom of the HP column and fed to the MP column. Oxygen-lean vapour is withdrawn from the top of the HP column, condensed in a reboiler-condenser in the bottom of the LP column and retumed to the top of the HP column to provide reflux thereto. Boilup for MP column is provided by a bottom reboiler/condenser fed with an oxygen-lean vapour withdrawn from the top or an intermediate location of the HP column, Respective portions of condensate from the MP reboiler/condenser are fed to tops of the MP and LP columns to provide reflux thereto. Oxygen-enriched liquid is withdrawn from the bottom of the MP column and is fed to the LP column. Respective nitrogen-rich vapours are withdrawn from the tops of the MP and LP columns and removed from the distillation column system without condensation. Liquid oxygen is withdrawn from the bottom of the LP column and elevated in pressure to provide the pressurized liquid oxygen required for reflux duty in the rectifier. US-A-5,675,977 (published 14th October 1997), upon which the respective preambles of the independent claims are based, discloses a cryogenic process for separating air to produce oxygen and nitrogen in which additional nitrogen reflux is provided to a distillation column having thermally integrated higher and lower pressure columns by processing bottoms liquid from the higher pressure column in a column reboiled by vapour withdrawn from an intermediate location of that column. In the exemplified embodiment, the distillation column system has a first distillation column ("HP column"), a second distillation column ("MP column "), and a third distillation column ("LP column "). The HP column is at pressure higher than the MP column, which is at a pressure higher than the LP column. Feed air portions are fed to all three columns. An oxygen-enriched liquid is withdrawn from the bottom of the HP column and fed to the MP column. An oxygen-lean vapour is withdrawn from the top of the HP column and divided into two portions, one of which is fed to a reboiler/condenser to provide reboil to the bottom of the LP column and the other provides a high pressure gaseous nitrogen product. Respective portions of the condensate from the reboiler-condenser are fed to the tops of the HP and LP columns to provide reflux therein. Boilup for the MP column is provided by a bottom reboiler-condenser fed with an oxygen-lean vapour from an intermediate location of the HP column and the resultant condensate is retumed to the same or a higher location in the HP column. An oxygen-lean vapour is withdrawn from the top of the MP column and fed to an intermediate reboiler/condenser in the LP column. Respective portions of the condensate from that reboiler/condenser are fed to the tops of the MP and LP columns to provide reflux thereto. An oxygen-enriched liquid is withdrawn from the bottom of the MP column and is fed to the LP column. A low pressure gaseous nitrogen product is withdrawn from the top of the LP column and removed from the distillation column system without condensation. Liquid oxygen is withdrawn from the bottom of the LP column and is elevated in pressure before being warmed by indirect heat exchange with feed air for the distillation column system. Research Disclosure 42544: EP-A-1030148; and US-A-4533375 also disclose triple column systems for the cryogenic separation of air. It is desired to have an efficient process for separating air to produce oxygen and nitrogen, wherein the oxygen is produced as a pressurized product and at least a portion of the nitrogen is produced as a pressurized product. It also is desired to have an efficient mode of utilizing pumped-LOX in a multi-column cycle comprising three or more distillation columns. The present invention is a process for separating air to produce oxygen and nitrogen using a distillation column system having at least three distillation columns. The invention also includes a cryogenic air separation unit using the process. The invention provides a process for separating air to produce oxygen and nitrogen using a distillation column system having at least three distillation columns, including a first distillation column, a second distillation column, and a third distillation column. The first distillation column is at a first pressure, the second distillation column is at a second pressure lower than the first pressure, and the third distillation column is at a third pressure lower than the second pressure. At least a first portion of a stream of compressed air having a first nitrogen content is fed to the first distillation column. A first oxygen-enriched stream is withdrawn from the bottom of the first distillation column and at least a portion thereof is fed to the second distillation column and/or the third distillation column. A first oxygen-lean vapour stream is withdrawn from or near the top of the first distillation column, at least a first portion thereof is fed to a first reboiler-condenser in the bottom of the second distillation column or of the third distillation column, and at least partially condensed, thereby forming a first nitrogen-enriched liquid. Boilup for the bottom of the other of the second and third distillation columns is provided at least in part by indirect heat exchange with an oxygen-lean vapour stream from the first distillation column or a fourth distillation column of the distillation column system. At least a first portion of the first nitrogen-enriched liquid is fed to the top of the first distillation column. A second nitrogen-enriched liquid and/or a second portion of the first nitrogen-enriched liquid is fed to the top of the second distillation column. A second oxygen-enriched liquid stream is withdrawn from the bottom of the second distillation column and is fed to the third distillation column. A first nitrogen-rich vapour stream is withdrawn from the top of the second distillation column and a second nitrogen-rich vapour stream is withdrawn from the top of the third distillation column. A liquid oxygen stream is withdrawn from the bottom of the third distillation column and is elevated in pressure before being warmed at least in part by indirect heat exchange with a pressurized stream having a nitrogen content at least equal to the first nitrogen-content and selected from feed air and oxygen-lean vapour withdrawn from the first distillation column, said pressurized stream being cooled without being subjected to distillation. At least a portion of the cooled pressurized stream is fed to any one or combination of the first, second, and third distillation columns. At least one nitrogen-enriched liquid process stream provides feed to the top of the third column. Said second nitrogen-enriched liquid stream and said at least one nitrogen-enriched liquid process stream are oxygen-lean liquid(s) derived from the first or, if present, fourth distillation columns and both said first and second nitrogen-rich vapour streams are withdrawn from the distillation column system without condensation. Usually, said nitrogen-enriched feed to the third column is provided by one or more of the first nitrogen-enriched liquid, a nitrogen-enriched liquid withdrawn from an intermediate location of the first column, condensed overhead from a fourth column of the distillation system, or the cooled pressurized stream when it is a nitrogen-enriched stream. In one embodiment, the pressurized stream is the first portion of the stream of compressed air. In another embodiment, the pressurized stream is another portion of the stream of compressed air, which can be a further compressed portion. In a further embodiment, the pressurized stream is a compressed portion of an oxygen-lean vapour stream withdrawn from the distillation column system. Boilup for the second distillation column can be provided at least in part by indirect heat exchange with the first portion of the oxygen-lean vapour stream and boilup for the third distillation column can be provided at least in part by indirect heat exchange with another portion of the first oxygen-lean vapour stream. In an embodiment of the invention, a second portion of the first oxygen-lean vapour stream from the first distillation column is fed to the bottom of a fourth distillation column; a third nitrogen-enriched liquid stream is withdrawn from the bottom of the fourth distillation column and at least a portion thereof fed to the second distillation column and/or the third distillation column; a second oxygen-lean vapour stream is withdrawn from or near the top of the fourth distillation column and at least a first portion thereof is fed to a second reboiler-condenser of the second distillation column or of the third distillation column and at least partially condensed, thereby forming a fourth nitrogen-enriched liquid; at least a portion of the fourth nitrogen-enriched liquid is fed to the top of the fourth distillation column; and a high purity nitrogen stream withdrawn from the second oxygen-lean vapour stream or the fourth nitrogen-enriched liquid. Boilup for the second distillation column can be provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapour stream, and boilup for the third distillation column is provided at least in part by indirect heat exchange with the first portion of the second oxygen-lean vapour stream. In another embodiment of the present invention, another portion of the stream of compressed air is fed to the bottom of the fourth distillation column; a third oxygen-enriched liquid stream is withdrawn from the bottom of the fourth distillation column and at least a portion thereof is fed to the second distillation column and/or the third distillation column, a second oxygen-lean vapour stream is withdrawn from or near the top of the fourth distillation column and at least a portion thereof is fed to a second reboiler-condenser of the second distillation column or of the third distillation column, and at least partially condensed, thereby forming the second nitrogen-enriched liquid; and at least a portion of the second nitrogen-enriched liquid is fed to the top of the fourth distillation column. The fourth distillation column can be at a pressure greater than the pressure of the first distillation column or at a pressure less than the pressure of the first distillation column. Boilup for the third distillation column can be provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapour stream, and boilup for the second distillation column can be provided at least in part by indirect heat exchange with the second oxygen-lean vapour stream. In a further embodiment of the present invention, a vapour stream is withdrawn from the first distillation column at an intermediate location, fed to a second reboiler-condenser of the second distillation column or of the third distillation column, and at least partially condensed, thereby forming an intermediate reflux stream, which is fed to the first distillation column at or near the intermediate location; the second nitrogen-enriched liquid is withdrawn from the first distillation column at or near the intermediate location and at least a portion thereof is fed to the top of the second distillation column or the third distillation column. Boilup for the second distillation column can be provided at least in part by indirect heat exchange with the vapour stream withdrawn at the intermediate location, and boilup for the third distillation column can be provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapour stream. Alternatively, boilup for the third distillation column can be provided at least in part by indirect heat exchange with the vapour stream withdrawn at the intermediate location, and boilup for the second distillation column can be provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapour stream. The present invention also provides a cryogenic air separation unit using a process of the invention as discussed above. In particular, in one aspect, the present invention provides an apparatus for cryogenic air separation by a process of the invention, said apparatus comprising: a distillation column system having at least three distillation columns, including a first distillation column, a second distillation column, and a third distillation column, said second and third columns not having overhead condensers; means for feeding at least a first portion of a stream of compressed air having a first nitrogen content to the first distillation column; means for withdrawing a first oxygen-enriched liquid stream from the bottom of the first distillation column and feeding at least a portion thereof to the second distillation column and/or the third distillation column ; means for withdrawing a first oxygen-lean vapour stream from or near the top of the first distillation column, feeding at least a first portion thereof to a first reboiler-condenser in the bottom of the second distillation column or of the third distillation column, and at least partially condensing said first portion, thereby forming a first nitrogen-enriched liquid; means providing boilup for the other of the second and third distillation columns by indirect heat exchange with an oxygen-lean vapour stream from the first distillation column or a fourth distillation column of the distitlation column system; means for feeding at least a first portion of the first nitrogen-enriched liquid to the top of the first distillation column ; means for feeding a second nitrogen-enriched liquid and/or a second portion of the first nitrogen-enriched liquid to the top of the second distillation column ; means for withdrawing a second oxygen-enriched liquid stream from the bottom of the second distillation column and feeding said second oxygen-enriched liquid stream to the third distillation column; means for withdrawing a first nitrogen-rich vapour stream from the top of the second distillation column; means for withdrawing a second nitrogen-rich vapour stream from the top of the third distillation column; means providing boilup for the bottom of the other of the second and third distillation columns by indirect heat exchange with an oxygen-lean vapour stream from the first distillation column or a fourth distillation column of the distillation column system; means for withdrawing a liquid oxygen stream from the bottom of the third distillation column; means for elevating the pressure of said liquid oxygen stream; means for warming the elevated pressure liquid oxygen stream at least in part by indirect heat exchange with a pressurized stream having a nitrogen content at least equal to the first nitrogen content and selected from feed air and oxygen-lean vapour withdrawn from the first distillation column, said pressurized stream being cooled without being subjected to distillation; means for feeding at least one nitrogen-enriched process stream to the top of the third distillation column; and means for feeding at least a portion of the cooled pressurized stream to any one or combination of the first, second, and third distillation columns and wherein said second nitrogen-enriched liquid stream and said at least one nitrogen-enriched liquid process stream are oxygen-lean liquid(s) derived from the first or, if present, fourth distillation columns and neither means for withdrawing said first and second nitrogen-rich vapour streams condenses said stream.. The present invention is a process for the production of oxygen and nitrogen using a distillation column system. The process is applicable when the oxygen product is withdrawn from the distillation column system as a liquid, pumped to an elevated pressure, and warmed at least in part by cooling a suitably pressurized stream. In the preferred mode of operation, nitrogen product is produced at a pressure greater than 20 psia (140 kPa) and the purity of the oxygen product is less than 98 mole% (low purity oxygen). In the most preferred mode of operation, the nitrogen product is produced at a pressure greater than 30 psia (200 kPa) and the ratio of nitrogen production to oxygen production is greater than 1.5 mole/mole. The term "oxygen-rich" is understood to represent the oxygen product and corresponds to an oxygen content less than 99.9 mole%, preferably greater than 85 mole% and preferably less than 98 mole%. It also is understood that the term "nitrogen-rich" represents nitrogen product and corresponds to a nitrogen content greater than 95 mole%, preferably greater than 98 mole%. The term "oxygen-enriched" is understood to mean having an oxygen concentration greater than that of air. The term "nitrogen-enriched" is understood to mean having a nitrogen concentration greater than that of air. The concentration of a "nitrogen-enriched" stream is typically similar to that of a "nitrogen-rich" stream. The term "oxygen-lean" means having an oxygen concentration less than that of air. An "oxygen-lean" stream could have a composition similar to a "nitrogen-enriched" stream, but it could contain much less oxygen than a nitrogen-enriched or nitrogen-rich stream (e.g., it could be a nitrogen product with an oxygen level of only a few parts per million (ppm)). The following is a description by way of example only and with reference to the accompanying drawings of presently preferred embodiment of the invention. In the drawings: Figure 1 is a schematic diagram of a first embodiment of the present invention; Figure 2 is a schematic diagram of a second embodiment of the present invention; Figure 3 is a schematic diagram of a third embodiment of the present invention; Figure 4 is a schematic diagram of a fourth embodiment of the present invention; Figure 5 is a schematic diagram of a fifth embodiment of the present invention; Figure 6 is a schematic diagram of a sixth embodiment of the present invention; Figure 7 is a schematic diagram of a seventh embodiment of the present invention; Figure 8 is a schematic diagram of an eighth embodiment of the present invention; and Figure 9 is a schematic diagram of a conventional elevated pressure, double-column, pumped-LOX process. One embodiment of the invention is shown in Figure 1. This embodiment comprises a first distillation column 130, a second distillation column 164, and a third distillation column 166. The oxygen product is removed from the distillation column system as an oxygen-rich liquid stream 172. Two nitrogen-rich streams are produced from the distillation column system as a first nitrogen-rich vapour stream 194, a vapour from the top of the second distillation column 164, and a second nitrogen-rich vapour stream 182, a vapour from the top of the third distillation column 166. Air stream 100 is compressed in a main air compressor 102 and purified in unit 104 to remove impurities such as carbon dioxide and water thereby forming a compressed and purified air feed 106 for the process. The pressure of the compressed air is generally between 75 psia (500 kPa) and 250 psia (1,700 kPa) and preferably between 100 psia (700 kPa) and 200 psia (1,400 kPa). Stream 106 is split into two portions, stream 108 and stream 114. Stream 108 is cooled in a main heat exchanger 110 to form cooled air stream 112, which subsequently is introduced to the bottom of the first distillation column 130. Stream 114, which is typically 25% to 30% of the incoming air, is further compressed in a booster compressor 115 to form a pressurized stream 116. Stream 116 is cooled in the main heat exchanger 110 to form stream 118. Stream 118 is reduced in pressure across valve 121 to form stream 122, which constitutes a feed to the third distillation tolumh 166. The first distillation column 130 produces an oxygen-lean fraction from the top, vapour stream 132, and a first oxygen-enriched liquid stream 168 from the bottom. Stream 132 is split into two portions, stream 134 and stream 140. Stream 134 is condensed in reboiler-condenser 135, located in the bottom of the third distillation column 166, to form stream 136; stream 140 is condensed in reboiler-condenser 141, located in the bottom of the second distillation column 164, to form stream 142. In this embodiment, stream 136 and stream 142 are combined to form stream 144. A portion of stream 144 is returned to the first distillation column 130 as reflux stream 145. The other portion of stream 144 constitutes nitrogen-enriched liquid stream 150, which is split into stream 152 and stream 156. Stream 152 is reduced in pressure across valve 153 to form stream 154, which constitutes a feed to the top of the second distillation column 164. Stream 156 is reduced in pressure across valve 157 to form stream 158, which constitutes a feed to the top of the third distillation column 166. First oxygen-enriched liquid stream 168, which has an oxygen content of approximately 35 to 40 mole%, is reduced in pressure across valve 169 to form stream 170, which constitutes a feed to the second distillation column 164. The second distillation column 164 produces a first nitrogen-rich vapour stream 194 from the top and a second oxygen-enriched liquid stream 160 from the bottom. Upward vapour flow for distillation is provided by reboiler-condenser 141. First nitrogen-rich vapour stream 194 is warmed in the main heat exchanger 110 to form stream 196. Second oxygen-enriched liquid stream 160 has an oxygen content of approximately 50 to 80 mole% and more preferably 55 to 70 mole%. Stream 160 is reduced in pressure across valve 161 to form stream 162, which constitutes a feed to the third distillation column 166. The third distillation column 166 produces second nitrogen-rich vapour stream 182 from the top and liquid oxygen-rich stream 172 from the bottom. Upward vapour flow for distillation is provided by reboiler-condenser 135. Second nitrogen-rich vapour stream 182 is warmed to intermediate temperature in the main heat exchanger 110. A portion of partially warmed stream 182 is removed at an intermediate temperature as stream 184; the remainder is completely warmed to form stream 192. Stream 184 is reduced in pressure across turbo-expander 185 to form stream 186 and thereby produce refrigeration for the process. Stream 186 is then fully warmed in the main heat exchanger to form stream 188. Liquid oxygen-rich stream 172 is elevated in pressure through pump 173 to form stream 174. Stream 174 is warmed in the main heat exchanger 110 to form stream 176. At least a portion of the energy needed to warm stream 174 is provided, through indirect heat exchange, by cooling pressurized stream 116. The warming of oxygen-rich stream 174 may include vaporization, and cooling of pressurized stream 116 may include condensation. Pressurized stream 116 is cooled without being subjected to distillation. A tabulation of representative temperatures, pressures and flows for selected streams in Figure 1 is provided in Table 1 below. Streams such as, for example, streams 118, 150, 160, 168, 182, and 184 can be subjected to optional processing. For example, streams 118, 150, 160, and 168 may be further cooled before being reduced in pressure, and streams 182 and 194 may be warmed before being introduced to the main heat exchanger 110. Such cooling and warming often is performed in a subcooler (not shown), procedures commonly known in the field of cryogenics. For clarity, the optional use of single or multiple subcoolers is not described. A noteworthy feature of the embodiment shown in Figure 1 is that all of the first oxygen-enriched liquid stream 168 is introduced to the second distillation column 164, and all of the cooled pressurized stream 118 is introduced to the third distillation column 166. Alternatively, all of the first oxygen-enriched liquid stream 168 could be introduced to the third distillation column 166, and all of the cooled pressurized stream 118 could be introduced to the second distillation column 164. It has been discovered that efficient operation requires that at least a portion of one of streams 118 or 168 be introduced to the second distillation column and that at least a portion of one of streams 118 or 168 be introduced to the third distillation column. Figure 2 illustrates another embodiment of the invention. This second embodiment shares many similarities with the embodiment of Figure 1. Streams in Figure 2 which are common with those of Figure 1 are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in Figure 2. As shown in Figure 2, a cooled pressurized stream 118 is divided into stream 220 and stream 222. Stream 222 is reduced in pressure across valve 223 to form stream 224, which constitutes a feed to the second distillation column 164. Stream 220 is reduced in pressure across valve 121 to form stream 122, which constitutes a feed to the third distillation column 166. This embodiment produces some improvement in efficiency by increasing the production of the first nitrogen-rich vapour stream 194 at the expense of decreasing the production of the second nitrogen-rich vapour stream 182. In the more typical cases, when the pressure of the second distillation column is greater than the pressure of the third distillation column, nitrogen product compression power may be reduced. As an alternative, all of the cooled pressurized stream 118 may be introduced to the second distillation column 164 and first oxygen-enriched liquid stream 168 may be split into two fractions, with one fraction forming a feed to the second distillation column 164 and the other fraction forming a feed to the third distillation column 166. As a further alternative, both stream 118 and stream 168 may be split and be introduced to both the second distillation column and the third distillation column. Figure 3 shows an embodiment of the invention which illustrates an alternative processing step for the cooled pressurized stream 118. This embodiment shares many similarities with the embodiment of Figure 1. Streams in Figure 3 which are common with those of Figure 1 are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in Figure 3. As shown in Figure 3, cooled pressurized stream 118 is reduced in pressure across valve 121 to form stream 122. In this embodiment, stream 122 is first introduced as a feed to the first distillation column 130. Liquid stream 318 is withdrawn from an intermediate location of the first distillation column and is reduced in pressure across valve 321 to from stream 322, which constitutes a feed to the second distillation column 164. In this embodiment, first oxygen-enriched liquid stream 168 is withdrawn from the bottom of the first distillation column 130 and is reduced in pressure across valve 169 to form stream 170, which constitutes a feed to the third distillation column 166. As an alternative, stream 322 may be a feed to the third distillation column and stream 170 may be a feed to the second distillation column. As a further alternative, either or both of streams 168 and 318 may be split between both the second and third distillation columns. Introducing the cooled pressurized stream 118 into the first distillation column 130 and then removing a quantity of liquid from an intermediate location, such as stream 318, is a common technique in cryogenic air separation. This is done for simplicity of design as well as for improving efficiency, since some vapour may be present in stream 122 as it enters the distillation column system. Persons skilled in the art will recognize that the flow of stream 318 need not be the same as the flow of stream 122; in fact, the flow of stream 318 is often approximately 50-75% of the flow of stream 122. Persons skilled in the art also will recognize that stream 318 need not be removed from first column 130 from the same location as stream 122 is introduced. As an alternative, stream 122 may be split into fractions outside the first distillation column 130. In such an event, different fractions may be directed to any or all of the first, second or third distillation columns. Figure 4 illustrates how an additional nitrogen product may be recovered. This embodiment shares many similarities with the embodiment of Figure 1. Streams in Figure 4 which are common with those of Figure 1 are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in Figure 4. As shown in Figure 4, reboiler-condenser 135 and reboiler-condenser 141 condense different oxygen-lean vapours. Vapour stream 132 exits the top of the first distillation column 130 and is split into stream 440 and stream 134. Stream 134 is condensed in reboiler-condenser 135 to form stream 136, which is returned to the first distillation column as top reflux. Stream 440 is warmed in the main heat exchanger 110 to form nitrogen product stream 442. Vapour stream 140 is removed from an intermediate location of the first distillation column 130, condensed in reboiler-condenser 141 to form stream 142, and returned to the first distillation column as intermediate reflux. Nitrogen-enriched liquid stream 150 is removed from the first distillation column at a location at or near the location that intermediate reflux stream 142 enters the first distillation column. This embodiment in Figure 4 is useful when it is desired to produce a high purity nitrogen product from the distillation column system. In this embodiment, such a high purity nitrogen product is represented by stream 440. Typical purity requirement for such a stream may be as low as 1 parts per million (ppm), which is usually much more stringent than the purity requirement for the major nitrogen products such as streams 182 and 194. In such cases, it is advantageous to withdraw the nitrogen-enriched liquid stream 150 from a location near, but not at, the top of the first distillation column 130. This embodiment also shows that high purity nitrogen stream 440 leaves the first distillation column as a vapour. Alternatively, stream 440 may be removed as a liquid, for example as a portion of stream 136, then pumped to delivery pressure before being warmed in the main heat exchanger 110. A modification of the embodiment illustrated in Figure 4 would be to exchange the reboiler-condenser duties. For example, stream 134 could be condensed in reboiler-condenser 141 and stream 140 could be condensed in reboiler-condenser 135. Figure 5 illustrates an embodiment which uses an alternative pressurized stream. This embodiment shares many similarities with the embodiment of Figure 1. Streams in Figure 5 which are common with those of Figure 1 are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in Figure 5. As shown in Figure 5, oxygen-lean vapour stream 132 from the first distillation column 130 is split into recycle stream 540 in addition to streams 134 and 140. Recycle stream 540 is warmed to near ambient temperature to form stream 542, compressed in booster compressor 115 to form stream 116, then cooled in the main heat exchanger 110 to form cooled pressurized stream 118. Stream 118 is reduced in pressure across valve 121 to form stream 122, which in this case is a second feed to the top of the third distillation column 166. The embodiment of Figure 5 may be attractive to employ when booster compressor 115 can be incorporated into other compression services. This is often the case since nitrogen-rich product streams 192 and 196 are typically compressed before being delivered to an end user. Since the composition of stream 542 is nominally the same as streams 192 and 196, compression of stream 542 may be performed in the same compressor. There are numerous modifications and alternatives to the embodiment shown in Figure 5, including but not limited to: 1) recycle stream 540 may originate from a location below the top of the first distillation column 130; 2) recycle stream 540 may originate from at, or below, the top of either the second distillation column 164 or the third distillation column 166; 3) the recycle stream may be derived from any of streams 188, 192 or 196; and 4) cooled pressurized stream 118 may be introduced to any or all of the first, second, or third distillation columns. As another alternative, one may combine elements of the embodiment of Figure 1 with the embodiment of Figure 5. In this case, two pressurized streams might be cooled to warm the oxygen-rich stream: one derived from further compression of feed air, and one derived from a recycle from the process such as described in Figure 5. Figure 6 is another embodiment of the invention, which shows the use of a fourth distillation column 646. This embodiment shares many similarities with the embodiment of Figure 1. Streams in Figure 6 which are common with those of Figure 1 are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in Figure 6. As shown in Figure 6, oxygen-lean vapour stream 638 from first distillation column 130 is split into streams 640 and 644. Stream 640 is condensed in reboiler-condenser 141 to form stream 642, which is returned to the first distillation column as top reflux. Stream 644 is introduced to the bottom of the fourth distillation column 646. Fourth distillation column 646 produces a further oxygen-lean fraction from the top, stream 132, and the nitrogen-enriched liquid stream 150 from the bottom. Stream 132 is split into two portions, stream 134 and stream 440. Stream 440 is warmed in the main heat exchanger 110 to form stream 442. Stream 134 is condensed in reboiler-condenser 135 to form stream 136. In this embodiment, the entirety of stream 136 is returned to the fourth distillation column as reflux. Stream 150 is split into stream 152 and stream 156. Stream 152 is reduced in pressure across valve 153 to form stream 154, which constitutes a feed to the top of the second distillation column 164. Stream 156 is reduced in pressure across valve 157 to form stream 158, which constitutes a feed to the top of the third distillation column 166. This embodiment is useful when it is desired to produce a high purity nitrogen product from the distillation column system. In this embodiment, such a high purity nitrogen product is represented by stream 440. Typical purity requirement for such a stream may be as low as 1 ppm, which is usually much more stringent than the purity requirement for the major nitrogen products such as streams 182 and 194. In such cases, it is advantageous to withdraw the nitrogen-enriched reflux stream 150 from the bottom of the fourth distillation column 646. This embodiment also shows that high purity nitrogen stream 440 is extracted from the distillation system as a vapour. Alternatively, stream 440 may be removed as a liquid, for example as a portion of stream 136, then pumped to delivery pressure before being warmed in the main heat exchanger 110. A modification of the embodiment illustrated in Figure 6 would be to exchange the reboiler-condenser duties. For example, stream 134 could be condensed in reboiler-condenser 141 and stream 640 could be condensed in reboiler-condenser 135. Figure 7 is another embodiment of the invention which shows an alternative use of a fourth distillation column 720. This embodiment shares many similarities with the embodiment of Figure 1. Streams in Figure 7 which are common with those of Figure 1 are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in Figure 7. As shown in Figure 7, a third portion of feed air is withdrawn from booster compressor 115 as side stream 716. Stream 716 is cooled in the main heat exchanger 110 to form stream 718, which is the feed to the bottom of the fourth distillation column 720. First distillation column 130 produces a first oxygen-lean fraction from the top, vapour stream 132, and a first oxygen-enriched liquid stream 168 from the bottom. Stream 132 is condensed in reboiler-condenser 135 to form stream 136. In this embodiment, a portion of stream 136 is returned to the first distillation column 130 as reflux stream 145. The other portion of stream 136 constitutes a first nitrogen-enriched liquid stream 750. Fourth distillation column 720 produces a second oxygen-lean fraction from the top, stream 140, and a fourth oxygen-enriched liquid stream 722 from the bottom. Stream 140 is condensed in reboiler-condenser 141 to form stream 142. In this embodiment, a portion of stream 142 is returned to the fourth distillation column 720 as reflux stream 752. The other portion of stream 142 constitutes a second nitrogen-enriched liquid stream 754. In this embodiment, streams 750 and 754 are combined to form a third nitrogen-enriched liquid stream 150, and streams 168 and 722 are combined to form stream 170. This embodiment is useful for adjusting the relative pressures of the nitrogen-rich streams produced from the second and third distillation columns. There are numerous modifications and alternatives of the embodiment shown in Figure 7. For example, as illustrated, the pressure of the fourth distillation column 720 is greater than the pressure of the first distillation column 130. As an alternative, the pressure of the fourth distillation column 720 may be less than the pressure of first distillation column 130. In such a case: 1) air feed 716 could be at a lower pressure than air feed 108; or 2) stream 718 could be derived by turbo-expanding a portion of air feed 108, thereby providing refrigeration for the process and eliminating turbo-expander 185. Another modification of the embodiment illustrated in Figure 7 would be to exchange the reboiler-condenser duties. For example, stream 132 could be condensed in reboiler-condenser 141 and stream 140 could be condensed in reboiler-condenser 135. Persons skilled in the art will recognize that the two air feed streams 108 and 716 may be derived from different sources. For example, each of these two streams may be compressed and purified in separate unit operations. Such an operation would be appropriate when the oxygen production rate is so large as to make using two smaller compressors and/or purifiers economical. Furthermore, separate main heat exchangers could be used. Taken to the extreme, pairs of columns could be operated as separate processes. For example, referring to Figure 7, the first distillation column 130 and the third distillation column 166 may be built as one plant, complete with a dedicated compressor, purifier, and main heat exchanger; the fourth distillation column 720 and the second distillation column 164 may be built as another plant, complete with a dedicated compressor, purifier, and main heat exchanger. In this alternative, the second oxygen-enriched stream 160 would be transferred from one plant to the other. Numerous additional alternatives can be derived and will be known to persons skilled in the art. Figure 8 is another embodiment of the invention which illustrates that first oxygen-enriched liquid stream 168 may be preprocessed outside either the second distillation column 164 or the third distillation column 166. This embodiment shares many similarities with the embodiment of Figure 1. Streams in Figure 8 which are common with those of Figure 1 are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in Figure 8. As shown in Figure 8, the first oxygen-enriched stream 168 is reduced in pressure across valve 169 to form stream 170. Stream 170 is introduced to a vessel 841 which encloses reboiler-condenser 141. Stream 170 is at least partially vaporized by the reboiler-condenser 141 to produce vapour stream 842 and liquid stream 840. Vapour stream 842 is introduced to the bottom of the second distillation column 164. The bottom liquid from the second distillation column, stream 844, is combined with liquid stream 840 to form second oxygen-enriched stream 160 The mode of operation suggested by Figure 8 is essentially equivalent to operating the process of Figure 1 with the bottom section removed from the second distillation column 164 of Figure 1. It is therefore within the scope of the present invention to equate vaporizing a liquid feed outside a column and transferring the vapour to the column with transferring the liquid to the column and vaporizing within the column. Persons familiar with distillation will understand that it is also possible to pass streams 844 and 840 separately to the third distillation column 166. It also will be understood that a fraction of stream 170 may be split, prior to being introduced to vessel 841, and sent directly to either the second distillation column 164 or the third distillation column 166. Finally, the use of vessel 841 is illustrative and it is known in the field of heat transfer that stream 170 may be sent directly to reboiler-condenser 141. In Figures 1 to 8 the mode of refrigeration supply is through expansion of stream 184 in turbo-expander 185. Other alternatives exist and are known in the field of cryogenic air separation, including but are not limited to: 1) turbo-expansion of a portion of the nitrogen-rich vapour from the second distillation column; 2) turbo-expansion of a portion of pressurized stream 116 to either of the first, second or third distillation columns; 3) turbo-expansion of a portion of incoming air stream 108 to either of the second or third distillation columns; and 4) turbo-expansion of a vapour stream taken from either of the first, second, or third distillation columns, said vapour stream being withdrawn from any location in said columns. As illustrated in Figure 1, pressurized stream 118 is shown as being reduced in pressure across a valve 121. It will be known to persons familiar with cryogenics that valve 121 may be replaced with a work producing device, such as a dense fluid expander. In Figures 1 to 8 only one oxygen product is produced. It will be known to persons skilled in the art that multiple oxygen products may be produced. These oxygen products may differ in their pressure and/or purity. Examples of ways to make multiple purity oxygen products include, but are not limited to: 1) withdraw the lower purity oxygen product from a location above the bottom of the third distillation column and withdraw the higher purity oxygen product from the bottom of the third distillation column; and 2) withdraw the lower purity oxygen product from the bottom of the second distillation column and withdraw the higher purity oxygen product from the bottom of the third distillation column. In Figures 3 and 6 it is shown that an additional nitrogen-rich product is made from the first distillation column 130. Persons skilled in the art will recognize that an additional nitrogen-rich product may be made from the first distillation column in any of the embodiments of the present invention. Persons skilled in the art also will recognize that none of the nitrogen-rich products need be the same composition. For example, it is found that in some cases it is advantageous to produce stream 196 and 192 at different purities, so that when combined, they meet the specification of the process. Conversely, all the nitrogen products may be of similar purity and compressed in a common product compressor. In Figures 1 to 8 the main heat exchanger 110 is shown as a single heat exchanger. Persons skilled in the art will recognize that such a depiction is not limiting to the invention. Typically, large plants require multiple heat exchangers in parallel. Furthermore, one may elect to pass different streams to different parallel heat exchangers. One common example, with reference to Figure 1, would be to pass oxygen-rich stream 174, pressurized stream 116, and a portion of either stream 192 or stream 196 to a first parallel heat exchanger and to pass the remaining streams to a second parallel heat exchanger. Persons skilled in the art will recognize also that one need not recover both streams 192 and 196 as products. For example, referring to the embodiment of Figure 1, if the quantity of nitrogen desired is not large, one may elect to operate the third distillation column 166 at a reduced pressure and pass all of partially warmed stream 182 to turbo-expander 185. The resultant flow of stream 192 would thereby become zero. In this case, the only nitrogen product produced by the process would be stream 196, along with any optionally produced nitrogen-rich product from the first distillation column 130. In another example, the third distillation column may be operated at near atmospheric pressure and the second nitrogen-rich vapour stream 182 may constitute a waste byproduct rather than a nitrogen product. In such a case, an alternative means of provided refrigeration, such as those previously discussed, would be applied. In the application of the embodiment of Figures 1 to 5 it is possible to spatially locate the three columns in a number of different ways. For example, if minimization of plot size is key, the three columns may be stacked on top of one another. In such a case, six combinations are possible. One configuration of note would be to install the second distillation column 164 on top of the third distillation column 166 and to install the third distillation column on top of the first distillation column 130. This particular configuration is advantageous because stream 160, the second oxygen-enriched stream from the second distillation column, may easily flow downward to the third distillation column. Alternatively, if minimization of equipment height is key, all three columns may be located along side one another. In such a case, such as in Figure 1, a pump would be needed to transfer liquid reflux stream 145 to the top of the first distillation column 130. In some circumstances it may be advantageous to locate one of the reboiler-condensers on top of the first distillation column. In such an event a pump would be needed to transfer liquid from the bottom of one or both of the second distillation column 164 and/or third distillation column 166. An intermediate configuration strategy could install one of the columns on top of the other and have the remaining column located along side. There are six possible combinations of this type. One configuration of note would be to install the third distillation column 166 on top of the first distillation column 130 and to install the second distillation column 164 along side the first distillation column. In principle, any liquid made in reboiler-condenser 141 of the second distillation column would need to be pumped if it was necessary to return liquid to the top of the first distillation column. In the practice of this invention, it is possible to operate in such a manner that the reflux needed for the first distillation column is provided entirely by reboiler-condenser 135 of the third distillation column and it would not be necessary to pump reflux from reboiler condenser 141. Analogously, a configuration may call for installing the second distillation column on top of the first distillation column and installing the third distillation column along side the first distillation column. This configuration is most appropriate when reboiler-condenser 141 of the second distillation column provides all the necessary reflux to the top of the first distillation column. For the case where the second distillation column 164 and the third distillation column 166 are stacked on one another with the first distillation column 130 installed along side, the preferred configuration would install the second distillation column on top of the third distillation column. This configuration has two advantages: 1) stream 160 may be freely transferred to the third distillation column; and 2) reboiler-condenser 141 may supply all the reflux to the first distillation column and, if elevated properly, said reflux could be transferred without a pump. As with the case where all columns are located along side one another, in some circumstances it may be advantageous to locate one of the reboiler-condensers on top of the first distillation column. In such an event a pump may or may not be needed to transfer liquid from the bottom of one of the second or third distillation columns. In the application of the embodiments of Figures 6 and 7 it is possible to spatially locate the four columns in even more different ways. Although the number of combinations is relatively large, the combinations are easily enumerated. In one possible arrangement, all four columns are installed along side one another. For the case where three columns are stacked on top of one another and one column is installed along the side, there are 24 possible combinations: six configurations with the first distillation column 130 installed along the side, six configurations with the second distillation column 164 installed along the side, and so on. For the case where two of the columns are stacked on one another and the other two columns are stacked on one another, and the stacked pairs are installed along side of one another, there are twelve possible combinations. For example, as implied by Figure 6, the third distillation column 166 may be stacked on top of the fourth distillation column 646 and the second distillation column 164 may be stacked on top of the first distillation column 130. For the case where all four distillation columns are stacked on top of one another, there are 24 possible combinations. For example, referring to Figure 6, the second distillation column 164 may be on top of the third distillation column 166 which may be on top of the fourth distillation column 646 which may be on top of the first distillation column 130. Persons skilled in the art will recognize that a reboiler-condenser associated with a column pair may be physically installed: 1) in the bottom of the column receiving the boilup; 2) in the column receiving the reflux; or 3) external to either column. Thus, the spatial location of a reboiler-condenser is also a variable for construction. For example, referring to Figure 8, reboiler-condenser 141 is shown to be external to the second distillation column 164. In this case, one may elect to place vessel 841, and its contained reboiler-condenser 141, near or below the second distillation column 164, on near or above the first distillation column 130, or even near or above the third distillation column 166. In the application of the embodiments illustrated in Figures 1 to 8, and those alternatives discussed in the text, the selection of the proper spatial location is a cost optimization exercise. Factors which play a role in selecting the optimal configuration include but are not limited to: 1) individual column diameters and column heights; 2) shipping and installation limitations on maximum height; 3) allowable plot space; 4) avoiding the use of liquid pumps; 5) whether the equipment enclosures are shop-fabricated or field-erected; and 6) the existence of other major equipment items, such as main heat exchanger 110. Although, the number of possible options can be large, they are finite and can be readily identified . Therefore, persons skilled in the art may easily evaluate the cost of each configuration and select the optimal arrangement. EXAMPLE In order to demonstrate the efficacy of the present invention and to compare the present invention to more conventional processes, the following example is presented. The basis for comparison follows. The prior art process is a standard elevated pressure, double-column, pumped-LOX cycle as illustrated in Figure 9. As shown in Figure 9, air stream 100 is compressed in a main air compressor 102 and purified in unit 104 to remove impurities such as carbon dioxide and water, thereby forming a compressed and purified air feed stream 106 for the process. Stream 106 is split into two portions, stream 108 and stream 114. Stream 108 is cooled in a main heat exchanger 110 to form cooled air stream 112, which is subsequently introduced to a higher pressure column 130. Stream 114 is further compressed in a booster compressor 115 to form pressurized stream 116. Stream 116 is cooled in the main heat exchanger 110 to form stream 118. Stream 118 is reduced in pressure across valve 121 to form stream 122, which constitutes a feed to a lower pressure column 166. The higher pressure column 130 produces an oxygen-lean fraction from the top, stream 132, and a first oxygen-enriched liquid stream 168 from the bottom. Stream 132 is condensed in reboiler-condenser 135 to form stream 136. A portion of stream 136 is returned to the higher pressure column 130 as reflux stream 145. The other portion of stream 136 constitutes a nitrogen-enriched liquid stream 150. Stream 150 is reduced in pressure across valve 157 to form stream 158, which constitutes a feed to the top of the lower pressure column 166. First oxygen-enriched liquid stream 168 is reduced in pressure across valve 169 to form stream 170, which constitutes a feed to the lower pressure column 166. The lower pressure column 166 produces a nitrogen-rich vapour stream 182 from the top and a liquid oxygen-rich stream 172 from the bottom. Upward vapour flow for distillation is provided by reboiler-condenser 135. Nitrogen-rich vapour stream 182 is warmed to an intermediate temperature in the main heat exchanger 110. A portion of partially warmed stream 182 is removed at an intermediate temperature as stream 184; the remainder of stream 182 is completely warmed to form stream 192. Stream 184 is reduced in pressure across a turbo-expander 185 to form stream 186 and thereby produce refrigeration for the process. Stream 186 is then fully warmed in the main heat exchanger to form stream 188. Liquid oxygen-rich stream 172 is elevated in pressure through pump 173 to form stream 174. Stream 174 is warmed in the main heat exchanger 110 to form stream 176. A portion of the energy needed to warm stream 174 is provided, through indirect heat exchange by cooling pressurized stream 116. The embodiment of the present invention chosen for comparison with the prior art process corresponds to Figure 1. The production basis is: 1) Oxygen=4,210 lb mole/h (1,910 kg/mole/h) at > 95 mole% and 400 psia (2,750 kPa); 2) Nitrogen=12,960 lb mole/h (5,880 kg/mole/h) at > 99 mole% and 150 psia (1,400 kPa). Computer simulations of the two processes were developed. Selected results are presented in Table 1. A summary of the power consumed by the two processes is presented in Table 2. The results show that the present invention saves almost 1,000 kW or nearly 6% of the main air compressor power. POWER SUMMARY - kW Prior Art Figure 9 Present Invention Figure 1 Main Air Compressor 17,855 18,285 Booster Compressor 5,195 5,196 Nitrogen Compressor 8,238 6,817 Total 31,288 30,298 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 or described. Rather, various modifications may be made in the details within the scope of the claims.

Claims: A process for separating air to produce oxygen and nitrogen, said process using a distillation column system having at least three distillation columns, including a first distillation column (130) at a first pressure, a second distillation column (164) at a second pressure lower than the first pressure, and a third distillation column (166) at a third pressure lower than the second pressure, comprising the steps of: feeding at least a first portion (108, 112) of a stream of compressed air (106) having a first nitrogen content to the first distillation column (130); withdrawing a first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) and feeding at least a portion (170; 840, 842) thereof to the second distillation column (164) and/or the third distillation column (166); withdrawing a first oxygen-lean vapour stream (132; 638) from or near the top of the first distillation column (130), feeding at least a first portion (134; 140; 640) thereof to a first reboiler-condenser (141; 135) providing boilup to the bottom of one of the second and third distillation columns (164, 166), and at least partially condensing said first portion, thereby forming a first nitrogen-enriched liquid (144; 136; 642); feeding at least a first portion (145; 136; 642) of the first nitrogen-enriched liquid (144) to the top of the first distillation column (130); feeding a second nitrogen-enriched liquid and/or a second portion (152) of the first nitrogen-enriched liquid (144) to the top of the second distillation column (164); withdrawing a second oxygen-enriched liquid stream (160; 844) from the bottom of the second distillation column (164) and feeding said second oxygen-enriched liquid stream (160; 844) to the third distillation column (166); withdrawing a first nitrogen-rich vapour stream (194) from the top of the second distillation column (164); withdrawing a second nitrogen-rich vapour stream (182) from the top of the third distillation column (166); providing boilup for the bottom of the other of the second and third distillation columns (164, 166) at least in part by indirect heat exchange (135, 141) with an oxygen-lean vapour stream (132; 140) from the first distillation column (130) or from a fourth distillation column (646; 720) of the distillation column system; withdrawing a liquid oxygen stream (172) from the bottom of the third distillation column (166) and elevating (173) said liquid oxygen stream in pressure before being warmed at least in part by indirect heat exchange (110) with a pressurized stream (116) having a nitrogen content at least equal to the first nitrogen content and selected from feed air and oxygen-lean vapour withdrawn from the first distillation column (130), said pressurized stream (116) being cooled without being subjected to distillation; feeding at least a portion (122) of the cooled pressurized stream (118) to any one or combination of the first, second, and third distillation columns (130, 164, 166); and feeding at least one nitrogen-enriched liquid process stream (158; 122) to the top of the third distillation column (166), characterized in that said second nitrogen-enriched liquid stream (152) and said at least one nitrogen-enriched liquid process stream (158; 122) are oxygen-lean liquid(s) derived from the first or, if present, fourth distillation columns (130; 646; 720) and both said first and second nitrogen-rich vapour streams (194, 182) are withdrawn from the distillation column system without condensation. A process as claimed in Claim 1, wherein the pressurized stream (116) is the first portion (108, 112) of the stream of compressed air (106). A process as claimed in Claim 1, wherein the pressurized stream (116) is another portion (114) of the stream of compressed air (106). A process as claimed in Claim 3, wherein said another portion (114) is further compressed (115). A process as claimed in Claim 1, wherein the pressurized stream (116) is a compressed (115) portion (540, 542) of an oxygen-lean vapour stream (132) withdrawn from the first distillation column. A process as claimed in Claim 5, wherein a nitrogen-enriched liquid process stream feed (122) to the third column (166) is provided by the cooled pressurized stream (118). A process as claimed in any one of the preceding claims, wherein a nitrogen-enriched liquid process stream feed (158) to the third column (166) is provided by the first nitrogen-enriched liquid (144). A process as claimed in any one of the preceding claims, wherein a nitrogen-enriched liquid process stream (158) feed to the third column (166) is provided by a nitrogen-enriched liquid (150) withdrawn from an intermediate location of the first column (130). A process as claimed in any one of the preceding claims, wherein a nitrogen-enriched liquid process stream feed (159) to the third column (130) is provided by condensed overhead (142, 754) from a fourth column (720) of the distillation system. A process as claimed in any one of the preceding claims, wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange (141) with the first portion (140) of the first oxygen-lean vapour stream (132), and wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange (135) with another portion (134) of the first oxygen-lean vapour stream (132). A process as claimed in any one of the preceding claims, wherein: a second portion (644) of the first oxygen-lean vapour stream (638) from the first distillation column (130) is fed to the bottom of a fourth distillation column (646); a third nitrogen-enriched liquid stream (150) is withdrawn from the bottom of the fourth distillation column (644) and at least a portion (152; 156) thereof fed to the second distillation column (164) and/or the third distillation column (166); a second oxygen-lean vapour stream (132) is withdrawn from or near the top of the fourth distillation column (646) and at least a first portion (134) of the second oxygen-lean vapour stream (132) is fed to a second reboiler-condenser(135) of the second distillation column (164) or of the third distillation column (166) and at least partially condensed, thereby forming a fourth nitrogen-enriched liquid (136); at least a portion (136) of the fourth nitrogen-enriched liquid (136) is fed to the top of the fourth distillation column (646); and a high purity nitrogen stream (440) is withdrawn from the second oxygen-lean vapour stream (132) or the fourth nitrogen-enriched liquid (136). A process as claimed in any one of Claims 1 to 10, wherein: another portion (716, 718) of the stream of compressed air (106) is fed to the bottom of a fourth distillation column (720); a third oxygen-enriched liquid stream (722) is withdrawn from the bottom of the fourth distillation column (720), and at least a portion (722) thereof is fed to the second distillation column (164) and/or the third distillation column (166); a second oxygen-lean vapour stream (140) is withdrawn from or near the top of the fourth distillation column (720), and at least a portion (140) thereof is fed to a second reboiler-condenser (141) of the second distillation column (164) or of the third distillation column (166), and at least partially condensed, thereby forming the second nitrogen-enriched liquid (142); and at least a first portion (752) of the second nitrogen-enriched liquid (142) is fed to the top of the fourth distillation column (720). A process as claimed in Claim 12, wherein the fourth distillation column (720) is at a fourth pressure greater than the pressure of the first distillation column (130). A process as claimed in Claim 12, wherein the fourth distillation column (720) is at a fourth pressure less than the pressure of the first distillation column (130). A process as claimed in any one of Claims 12 to 14, wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange (135) with the first portion (132) of the first oxygen-lean vapour stream (132), and wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange (141) with the second oxygen-lean vapour stream (140). A process as claimed in any one of the preceding claims, wherein: a vapour stream (140) is withdrawn from the first distillation column (130) at an intermediate location, fed to a second reboiler-condenser (141) of the second distillation column (164) or of the third distillation column (166), and at least partially condensed, thereby forming an intermediate reflux stream (142); the intermediate reflux stream (142) is fed to the first distillation column (130) at or near the intermediate location; and the second nitrogen-enriched liquid (150) is withdrawn from the first distillation column (130) at or near the intermediate location and at least a portion thereof (152; 156) is fed to the top of the second distillation column (164) or the third distillation column (166). A process as claimed in Claim 16, wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange (141) with the vapour stream (140) withdrawn at the intermediate location, and wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange (135) with the first portion (134) of the first oxygen-lean vapour stream (132). A process as claimed in Claim 16, wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange with the vapour stream (140) withdrawn at the intermediate location, and wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange with the first portion (134) of the first oxygen-lean vapour stream (132). A process of Claim 1, wherein: the first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) is fed to at least one of the second and third distillation columns (164; 166); respective portions (140, 134) of the first oxygen-lean vapor stream (132) from or near the top of the first distillation column (130) are fed to the first reboiler-condenser (141) of the second distillation column (164) and the first reboiler-condenser (135) of the third distillation column (166) and at least partially condensed therein thereby providing boilup to the second and third distillation columns and forming the first nitrogen-enriched liquid (144); respective portions (154, 158) of the first nitrogen-enriched liquid (144) are fed to the tops of the second and third distillation columns (164, 166); and the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream (174) is a compressed air stream (116) that is fed (122) to at least one of the second or third distillation column (164;166) so that each of the second or third distillation columns receives at least a portion of one of the first oxygen-enriched liquid stream (168) and the compressed air stream (116). A process of Claim 1, wherein: at least a portion of the first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) is fed to the second distillation column (164); respective portions (140, 134) of the first oxygen-lean vapor stream (132) from or near the top of the first distillation column (130) are fed to the first reboiler-condenser (141) of the second distillation column (164) and the first reboiler-condenser (135) of the third distillation column (166) and at least partially condensed therein thereby providing boilup to the second and third distillation columns and forming the first nitrogen-enriched liquid (144); respective portions (154, 158) of the first nitrogen-enriched liquid (144) are fed to the tops of the second and third distillation columns (164; 166); and the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream (174) is a compressed air stream (116) that has at least a portion (222) fed to the second distillation column (164) such that the third distillation column (166) receives at least a portion of the first oxygen-enriched liquid stream (168) or of the compressed air stream (116). A process of Claim 1, wherein: respective portions (140; 134) of the first oxygen-lean vapor stream (132) from or near the top of the first distillation column (130) are fed to the first reboiler-condenser (141) of the second distillation column (164) and the first reboiler-condenser (135) of the third distillation column (166) and at least partially condensed therein thereby providing boilup to the second and third distillation columns and forming the first nitrogen-enriched liquid (144); respective portions (154, 158) of the first nitrogen-enriched liquid (144) are fed to the tops of the second and third distillation columns (164, 166); the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream is a compressed air stream (116) that is fed (122) to the first distillation column (130); and a liquid stream (318) from an intermediate location of the first column (130) is fed to at least one of the second and third distillation columns (164; 166) such that each of the second and third distillation columns receives at least a portion of said intermediate liquid stream (318) or at least a portion of the first oxygen-enriched liquid stream (168). A process of Claim 1, wherein: the first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) is fed to the second distillation column (164); at least a portion (134) of the first oxygen-lean vapor stream (132) from or near the top of the first distillation column (130) is fed to the first reboiler-condenser (141; 135) of one of the second and third distillation columns (164; 166) and at least partially condensed therein thereby providing boilup to said distillation column and a nitrogen-enriched liquid that is fed to the top of the first distillation column (130); an oxygen-lean vapor stream (140) from an intermediate location of the first distillation column (130) is fed to the first reboiler-condenser (141; 135) of the other of the second and third distillation columns (164; 166) and at least partially condensed therein thereby providing boilup to said distillation column and forming a nitrogen-enriched liquid stream (142) that is returned to the first distillation column (130) as an intermediate reflux stream; withdrawing a first nitrogen-enriched liquid (150) from the first distillation column (130) at a location at or near the feed of the intermediate reflux stream (142) and feeding respective portions (154, 158) of the first nitrogen-enriched liquid (150) to the tops of the second and third distillation columns (164, 166); and the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream (174) is a compressed air stream (116) that is fed (122) to the third distillation column (166). A process of Claim 1, wherein: the first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) is fed to the second distillation column (164); respective portions (140, 134) of the first oxygen-lean vapor stream (132) from or near the top of the first distillation column (130) are fed to the first reboiler-condenser (141) of the second distillation column (164) and the first reboiler condenser (135) of the third distillation column (166) and at least partially condensed therein thereby providing boilup to the second and third distillation columns and forming the first nitrogen-enriched liquid (144); respective portions (154, 158) of the first nitrogen-enriched liquid (144) are fed to the tops of the second and third distillation columns (164, 166); and the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream (174) is a compressed (115) nitrogen-enriched recycle stream (540, 542, 116) obtained from one of the first, second and third distillation columns (130; 164; 166) and is fed (122) to any or all of said distillation columns. A process of Claim 1, wherein: the first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) is fed to the second distillation column (164); a portion of the first oxygen-lean vapor stream (638) from or near the top of the first distillation column (130) is fed to the first reboiler-condenser (141; 035) of one of the second and third distillation columns (164; 166) and at least partially condensed therein thereby providing boilup to said column and forming a nitrogen-enriched liquid (642) that is fed to the top of the first distillation column (130); the fourth distillation column (646) is present and is fed with another portion (644) of the first oxygen-lean vapor stream (638) from or near the top of the first distillation column (130); a nitrogen-enriched liquid (150) is withdrawn from the bottom of the fourth distillation column (646) and respective portions (154, 158) thereof fed to the second and third distillation columns (164, 166), a second oxygen-lean vapor stream (132) is withdrawn from or near the top of the fourth distillation column (646) and fed to the first reboiler-condenser (141; 135) of the other of the second and third distillation columns (164; 166) and at least partially condensed therein thereby providing boilup to said column and forming a second nitrogen-enriched liquid (136) that is fed to the top of the fourth distillation column (646); and the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream (174) is a compressed air stream (116) that is fed (122) to the third distillation column (166). A process of Claim 1, wherein: the first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) is fed to the second distillation column (164); the first oxygen-lean vapor stream (132) from or near the top of the first distillation column (130) is fed to the first reboiler-condenser (141; 135) of one of the second and third distillation columns (164; 166) and at least partially condensed therein thereby providing reboil to said column and forming the first nitrogen-enriched liquid (750); respective portions (154, 158) of the first nitrogen-enriched liquid (750) are fed to the tops of the second and third distillation columns (164, 166); the fourth distillation column (720) is present and is fed with another portion (716, 718) of the compressed air (106); a third oxygen-enriched liquid (722) is withdrawn from the bottom of the fourth distillation column (720) and fed to the second distillation column (164), a second oxygen-lean vapor stream (140) is withdrawn from or near the top of the fourth distillation column (720) and fed to the first reboiler-condenser (141; 135) of the other of the second and third distillation columns (164; 166) and at least partially condensed therein thereby providing reboil to said column and forming a second nitrogen-enriched liquid (142); feeding respective portions (752, 154 (via 754), 158 (via 754)) of the second nitrogen-enriched liquid (142) to the tops of the first, second and third distillation columns (130, 164, 166); and the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream (174) is a compressed air stream (116) that is fed (122) to the third distillation column (166). A process of Claim 1, wherein: respective portions (140, 134) of the first oxygen-lean vapor stream (132) from or near the top of the first distillation column (130) are fed to the first reboiter-condenser (141) of the second distillation column (164) and the first reboiler-condenser (135) of the third distillation column (166) and at least partially condensed therein thereby providing boilup to the second and third distillation columns and forming the first nitrogen-enriched liquid (144); respective portions (154, 158) of the first nitrogen-enriched liquid (144) are fed to the tops of the second and third distillation columns (164, 166); the first oxygen-enriched liquid stream (168) from the bottom of the first distillation column (130) is partially vaporized by the reboiler-condenser (141) of the second column (164) to provide a vapor fraction (842) that is fed to the second distillation column (164) and a liquid fraction (840) that is fed to the third column (166); and the pressurized stream having a nitrogen content at least equal to the first nitrogen content warming the pressurized liquid oxygen stream (174) is a compressed air stream (116) that is fed (122) to the third distillation column (166). An apparatus for cryogenic air separation by a process as defined in Claim 1, said apparatus comprising: a distillation column system having at least three distiltation columns, including a first distillation column (130), a second distillation column (164), and a third distillation column (166); means (108, 112) for feeding at least a first portion of a stream of compressed air (106) having a first nitrogen content to the first distillation column (130); means (168, 169, 170) for withdrawing a first oxygen-enriched liquid stream from the bottom of the first distillation column (130) and feeding at least a portion thereof to the second distillation column (164) and/or the third distillation column (166); means (132, 134; 132, 140) for withdrawing a first oxygen-lean vapour stream from or near the top of the first distillation column (130), feeding at least a first portion thereof to a first reboiler-condenser (141; 135) providing boilup to the bottom of one of the second distillation column and third distillation columns (164, 166), and at least partially condensing said first portion, thereby forming a first nitrogen-enriched liquid (142; 136); means (145) for feeding at least a first portion of the first nitrogen-enriched liquid (142; 136) to the top of the first distillation column (130); means (150, 152, 153, 154) for feeding a second nitrogen-enriched liquid and/or a second portion of the first nitrogen-enriched liquid (142; 136) to the top of the second distillation column (164); means (160, 161, 162) for withdrawing a second oxygen-enriched liquid stream from the bottom of the second distillation column (164) and feeding said second oxygen-enriched liquid stream to the third distillation column (166); means (194) for withdrawing a first nitrogen-rich vapour stream from the top of the second distillation column (164); means (182) for withdrawing a second nitrogen-rich vapour stream from the top of the third distillation column (166); means (135,141) providing boilup for the bottom of the other of the second and third distillation columns (164, 166) by indirect heat exchange with an oxygen-lean vapour stream (132; 140) from the first distillation column (130) or a fourth distillation column (646; 720) of the distillation column system; means (172) for withdrawing a liquid oxygen stream from the bottom of the third distillation column; means (173) for elevating the pressure of said liquid oxygen stream; means (110) for warming the elevated pressure liquid oxygen stream at least in part by indirect heat exchange with a pressurized stream (116) having a nitrogen content at least equal to the first nitrogen content and selected from feed air and oxygen-lean vapour withdrawn from the first distillation column, said pressurized stream being cooled without being subjected to distillation; means (158; 122) for feeding at least one nitrogen-enriched process stream to the top of the third distillation column (166); and means (118, 121, 122) for feeding at least a portion of the cooled pressurized stream to any one or combination of the first, second, and third distillation columns (130, 164, 166), characterized in that said second nitrogen-enriched liquid stream (152) and said at least one nitrogen-enriched liquid process stream (158; 122) are oxygen-lean liquid(s) derived from the first or, if present, fourth distillation columns (130; 646; 720) and in that neither means (194; 182) for withdrawing said first and second nitrogen-rich vapour streams (194, 182) condenses said stream. An apparatus as claimed in Claim 27, wherein the pressurized stream (116) is provided by another portion (114) of the stream of compressed air (106) and the apparatus includes means (115) to further compress said another portion (114). An apparatus as claimed in Claim 27, wherein the pressurized stream (116) is provided by a portion (540, 542) of an oxygen-lean vapour stream (132) withdrawn from the first distillation column and the apparatus includes means (115) to further compress said portion (540, 542). An apparatus as claimed in Claim 29, wherein a nitrogen-enriched liquid process stream feed (122) to the third column (166) is provided by the cooled pressurized stream (118). An apparatus as claimed in any one of Claims 27 to 30, wherein a nitrogen-enriched liquid process stream feed (158) to the third column (166) is provided by the first nitrogen-enriched liquid (144). An apparatus as claimed in any one of Claims 28 to 31, wherein a nitrogen-enriched liquid process stream (158) feed to the third column (166) is provided by a nitrogen-enriched liquid (150) withdrawn from an intermediate location of the first column (130). An apparatus as claimed in any one of Claims 27 to 32, wherein the apparatus includes a fourth column (720) and a nitrogen-enriched liquid process stream feed (159) to the third column (130) is provided by condensed overhead (142, 754) from said fourth column (720). An apparatus as claimed in any one of Claims 28 to 33, wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange (141) with the first portion (140) of the first oxygen-lean vapour stream (132) , and wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange (135) with another portion (134) of the first oxygen-lean vapour stream (132). An apparatus as claimed in any one of Claims 28 to 34, wherein: the apparatus includes a fourth column (646); a second portion (644) of the first oxygen-lean vapour stream (638) from the first distillation column (130) is fed to the bottom of said fourth distillation column (646); a third nitrogen-enriched liquid stream (150) is withdrawn from the bottom of the fourth distillation column (644) and at least a portion (152; 156) thereof fed to the second distillation column (164) and/or the third distillation column (166); a second oxygen-lean vapour stream (132) is withdrawn from or near the top of the fourth distillation column (646) and at least a first portion (134) of the second oxygen-lean vapour stream (132) is fed to a second reboiler-condenser (135) of the second distillation column (164) or of the third distillation column (166) and at least partially condensed, thereby forming a fourth nitrogen-enriched liquid (136); at least a portion (136) of the fourth nitrogen-enriched liquid (136) is fed to the top of the fourth distillation column (646); and a high purity nitrogen stream (440) is withdrawn from the second oxygen-lean vapour stream (132) or the fourth nitrogen-enriched liquid (136). An apparatus as claimed in any one of Claims 28 to 32, wherein: the apparatus includes a fourth column (720); another portion (716, 718) of the stream of compressed air (106) is fed to the bottom of said fourth distillation column (720); a third oxygen-enriched liquid stream (722) is withdrawn from the bottom of the fourth distillation column (720), and at least a portion (722) thereof is fed to the second distillation column (164) and/or the third distillation column (166); a second oxygen-lean vapour stream (140) is withdrawn from or near the top of the fourth distillation column (720), and at least a portion (140) thereof is fed to a second reboiler-condenser (141) of the second distillation column (164) or of the third distillation column (166), and at least partially condensed, thereby forming the second nitrogen-enriched liquid (142); and at least a first portion (752) of the second nitrogen-enriched liquid (142) is fed to the top of the fourth distillation column (720). An apparatus as claimed in Claim 36, wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange (135) with the first portion (132) of the first oxygen-lean vapour stream (132), and wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange (141) with the second oxygen-lean vapour stream (140). An apparatus as claimed in any one of claims 27 to 37, wherein: a vapour stream (140) is withdrawn from the first distillation column (130) at an intermediate location, fed to a second reboiler-condenser (141) of the second distillation column (164) or of the third distillation column (166), and at least partially condensed, thereby forming an intermediate reflux stream (142); the intermediate reflux stream (142) is fed to the first distillation column (130) at or near the intermediate location; and the second nitrogen-enriched liquid (150) is withdrawn from the first distillation column (130) at or near the intermediate location and at least a portion thereof (152; 156) is fed to the top of the second distillation column (164) or the third distillation column (166). An apparatus as claimed in Claim 38, wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange (141) with the vapour stream (140) withdrawn at the intermediate location, and wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange (135) with the first portion (134) of the first oxygen-lean vapour stream (132). An apparatus as claimed in Claim 38, wherein boilup for the third distillation column (166) is provided at least in part by indirect heat exchange with the vapour stream (140) withdrawn at the intermediate location, and wherein boilup for the second distillation column (164) is provided at least in part by indirect heat exchange with the first portion (134) of the first oxygen-lean vapour stream (132).

Inventors: AGRAWAL RAKESH, HERRON DONN MICHAEL, HIGGINBOTHAM PAUL, AGRAWAL, RAKESH, HERRON, DONN MICHAEL, HIGGINBOTHAM, PAUL
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 2001
Main classification: F25J3
IPC codes: F25J3