Efficient process to produce oxygen

Abstract: The power consumption required by the cryogenic distillation of air in a distillation column system comprising at least one distillation column (198) wherein the boil-up (193; 593 Fig 5; 893 Fig 8) at the bottom of the distillation column (198) producing a oxygen product (172) is provided by condensing a stream (152; 552 Fig 5) whose nitrogen concentration is at least equal to that in the feed air stream (100), is reduced by (a) generating work energy which is at least ten percent of the overall refrigeration demand of the distillation column system by (1) work expanding (139) a first process stream (154 Fig 2; 538 Fig 5; 738 Fig 7; 838 Fig 8) with nitrogen content at least equal to that in the feed air (100) and then condensing at least a portion of the expanded stream (240 Fig 2; 540 Fig 5; 740 Fig 7) by latent heat exchange (194 Fig 2; 394 Fig 3; 594 Fig 5; 794 Fig 7; 894 Fig 8) with (i) a liquid at an intermediate height in the distillation column (198) producing oxygen product and/or (ii) one of the liquid feeds (136) to this distillation column having an oxygen concentration at least equal to the concentration of oxygen in the feed air (100); and/or (2) condensing at least a second process stream (154) with nitrogen content at least equal to that in the feed air (100) by latent heat exchange (194) with at least a portion (136) of a liquid stream which has oxygen concentration at least equal to the concentration of oxygen in the feed air (100) and which is also at a pressure greater than the pressure of the distillation column (198) producing oxygen product, and after vaporization of at least a portion of said liquid stream into a vapor fraction (137) due to latent heat exchange (194), work expanding (139) at least a portion of the resulting vapor stream (137); (b) work expanding (103; 603 Fig 6; 703 Fig 7) a third process stream (104; 604 Fig 6; 704 Fig 7; 904 Fig 9) to produce additional work energy such that the total work generated along with step (a) exceeds the total refrigeration demand of the cryogenic distillation and, if the third process stream (704 Fig 7) is the same as the first process stream (738 Fig 7) in step (a)(1), at least a portion of said third process stream (704) after work expansion (703) is not condensed against either of the two liquid streams described in step (a)(1); and (c) using the work which is generated in excess of the refrigeration need of the distillation column system to cold compress (115; 484 Fig 4; 515 Fig 5; 784 Fig 7) a process stream (114; 482 Fig 4; 551 Fig 5; 782 Fig 7; 851 Fig 8) at a temperature lower than the ambient temperature.

Description: The present invention relates to the efficient production of oxygen by cryogenic air separation. In particular, the present invention relates to cryogenic air separation processes where it is attractive to produce at least a portion of the total oxygen with purity less than 99.5% and, preferably, less than 97%. There are numerous U.S. patents that teach the efficient production of oxygen with purity less than 99.5%. Examples are US-A-4,704,148, US-A-4,936,099, US-A-5 678 427 and EP-A-0556516. US-A-2,753,698 discloses a method for the fractionation of air in which the total air to be separated is prefractionated in the high pressure column of a double rectifier to produce a crude (impure) liquid oxygen (crude LOX) bottoms and a gaseous nitrogen overhead. The so produced crude LOX is expanded to a medium pressure and is completely vaporized by heat exchange with condensing nitrogen. The vaporized crude oxygen is then slightly warmed, expanded against a load of power production and scrubbed in the low pressure column of the double rectifier by the nitrogen condensed within the high pressure column and entered on top of the low pressure column. The bottom of the low pressure column is reboiled with the nitrogen from the high pressure column. This method of providing refrigeration will be referred to hereinafter as CGOX expansion. In this method no other source of refrigeration is used. Thus, the conventional method of air expansion to the low pressure column is replaced by the proposed CGOX expansion. As a matter of fact, it is stated in this patent that the improvement results because additional air is fed to the high pressure column (as no gaseous air is expanded to the low pressure column) and this results in additional nitrogen reflux being produced from the top of the high pressure column. It is stated that the amount of additional nitrogen reflux is equal to the additional amount of nitrogen in the air that is fed to the high pressure column. An improvement in the efficiency of scrubbing with liquid nitrogen in the upper part of the low pressure column is claimed to overcome the deficiency of boil-up in the lower part of the low pressure column. US-A-4,410,343 discloses a process for the production of low purity oxygen which employs a low pressure and a medium pressure column, wherein the bottoms of the low pressure column are reboiled against condensing air and the resultant air is fed into both the medium pressure and low pressure columns. US-A-4,704,148 discloses a process utilizing high and low pressure distillation columns for the separation of air to produce low purity oxygen and a waste nitrogen stream. Feed air from the cold end of the main heat exchangers is used to reboil the low pressure distillation column and to vaporize the low purity oxygen product. The heat duty for the column reboil and oxygen product vaporization is supplied by condensing air fractions. In this process the air feed is split into three substreams. One of the substreams is totally condensed and used to provide reflux to both the low pressure and high pressure distillation columns. A second substream is partially condensed with the vapor portion of the partially condensed substream being fed to the bottom of the high pressure distillation column and the liquid portion providing reflux to the low pressure distillation column. The third substream is expanded to recover refrigeration and then introduced into the low pressure distillation column as column feed. Additionally, the high pressure column condenser is used as an intermediate reboiler in the low pressure column. US-A-4,796,431, Erickson teaches a method of withdrawing a nitrogen stream from the high pressure column, partially expanding this nitrogen to an intermediate pressure and then condensing it by heat exchange against either crude LOX from the bottom of the high pressure column or a liquid from an intermediate height of the low pressure column. This method of refrigeration will be referred to hereinafter as nitrogen expansion followed by condensation (NEC). Generally, NEC provides the total refrigeration need of the cold box. Erickson teaches that only in those applications where NEC alone is unable to provide the refrigeration need that supplemental refrigeration is provided through the expansion of some feed air. However, use of this supplemental refrigeration to reduce energy consumption is not taught. This supplemental refrigeration is taught in the context of a flowsheet incorporating other modifications to reduce the supply air pressure. This reduced the pressure of the nitrogen to the expander and therefore the amount of refrigeration available from NEC. In this patent, Erickson also teaches the use of two NEC. The nitrogen from the high pressure column is split into two streams, and each stream is partially expanded to different pressures and condensed against different liquids. For example, one expanded nitrogen stream is condensed against crude LOX and the other is condensed against an intermediate height liquid from the low pressure column. Erickson claims that the use of a second NEC increases the refrigeration output that can be used to power a cold compressor so as to further increase oxygen delivery pressure. In US-A-4,936,099, Woodward et al use CGOX expansion in conjunction with the production of low purity oxygen. In this case, gaseous oxygen product is produced by vaporizing liquid oxygen from the bottom of the low pressure column by heat exchange against a portion of the feed air. In some air separation plants excess refrigeration is naturally available. This is generally for either of two reasons: (1) an operating equipment constraint leads to excess flow through the expander, and (2) recovery of the product from the distillation system is low and it produces excess waste at an elevated pressure which is then expanded. In such cases, some patents have suggested to use excess refrigeration for compressing a suitable process stream at cryogenic temperatures. This method of compression at cryogenic temperatures will henceforth be referred to as cold compression. An example of the creation of excess refrigeration due to the first reason and then use of cold compression can be found in US-A-4,072,023. In this patent, reversing heat exchangers are used to remove water and carbon dioxide from the feed air. A successful operation of such a reversing heat exchanger requires that a balance stream be used. The balance stream is generally drawn from the distillation column system, then partially warmed in the cold part of the main heat exchanger in indirect heat exchange with the incoming feed air, and then expanded in an expander to provide the needed refrigeration. Unfortunately, the flow rate of this balance stream cannot be reduced below a certain fraction of the feed air flow rate. For large size plants where the refrigeration demand per unit of product flow is not that large, the constraint of having a balance stream flow above a certain fraction of the feed air flow produces excess refrigeration. US-A-4,072,023 teaches to use this excess refrigeration for cold compressing a process stream. Examples of the creation of excess refrigeration due to the second reason and then use of cold compression can be found in US-A-4,966,002 and US-A-5,385,024. In both of these patents, air is fed near the bottom of a single distillation column to produce high pressure nitrogen. Since a single distillation column with no reboiler at the bottom is used, the recovery of nitrogen is low. This produces a large quantity of oxygen-enriched waste stream at an elevated pressure. A portion of this oxygen-enriched waste stream is partially warmed and expanded to provide the needed refrigeration, and the excess refrigeration is used to cold compress another portion of this waste stream. The cold compressed waste stream is recycled to the distillation column. In US-A-5,475,980, cold compression is used to improve the efficiency of cooling in the heat exchanger vaporizing pumped liquid oxygen at a pressure greater than 15 bar (1.5 MPa). For this purpose, an auxiliary stream at an intermediate temperature is taken out from an intermediate location of the heat exchanger. This auxiliary stream is then cold compressed and reintroduced in the heat exchanger and further cooled. At least a portion of the further cooled stream is then expanded in an expander. When the pressure of the auxiliary stream to be cold compressed is much higher than the high pressure column pressure, only a portion of it is expanded to the high pressure column after cold compression and partial cooling. In this case, extra energy is provided at the warm end of the plant to meet the refrigeration and cold compression requirement. However, when the auxiliary stream is withdrawn from the high pressure column then all of it is expanded after cold compression and cooling. This ensures that most of the energy needed for cold compression is recovered from the expander and used for cold compression. As a result, the need for extra vapor flow through the expander to create work energy is minimal and it does not require excess refrigeration as in the earlier cited US-A-4,072,023; US-A-4,966,002 and US-A-5,385,024. In DE-A-28 54 508, a portion of the air feed at the high pressure column is further compressed at the warm level by using work energy from the expander providing refrigeration to the cold box. This further compressed air stream is then partially cooled and expanded in the same expander that drives the compressor. In this scheme, the fraction of the feed air stream which is further compressed and then expanded for refrigeration is the same. As a result, for a given fraction of the feed air, more refrigeration is produced in the cold box. The patent teaches two methods to exploit this excess refrigeration: (i) to produce more liquid products from the cold box and (ii) to reduce flow through the compressor and the expander and thereby increase flow to the high pressure column. It is claimed that an increased flow to the high pressure column would result in a greater product yield from the cold box. The present invention provides a process for the cryogenic distillation of air in a distillation column system that contains a higher pressure ("HP") distillation column and a lower pressure ("LP") distillation column wherein at least a portion of the feed air is fed to the higher pressure distillation column, product oxygen with an oxygen concentration less than 99.5% is produced at the bottom of the lower pressure distillation column and the boil-up at the bottom of the lower pressure distillation column is provided by condensing a stream whose nitrogen concentration is greater than that in the feed air stream. The process of the present invention comprises the steps of: (a) generating work energy which is at least ten percent (10%) of the overall refrigeration demand of the distillation column system by at least one of the following two methods: (1) withdrawing from the higher pressure distillation column a vapor process stream (hereinafter "first vapor process stream") with nitrogen content greater than that in the feed air, work expanding said stream and then condensing at least a portion of the expanded stream by latent heat exchange with at least a portion of a liquid stream that has an oxygen concentration greater than the concentration of oxygen in the feed air and is (i) a liquid at an intermediate height in the lower pressure distillation column and/or (ii) one of the liquid feeds to the lower pressure distillation column having an oxygen concentration greater than the concentration of oxygen in the feed air; and (2) withdrawing from the higher pressure distillation column a process vapor stream (hereinafter "second vapor process stream") with nitrogen content greater than that in the feed air, condensing at least a portion of said second vapor process stream by latent heat exchange with at least a portion of a liquid stream which has oxygen concentration greater than the concentration of oxygen in the feed air and which is also at a pressure greater than the pressure of the lower pressure distillation column, and after vaporization of at least a portion of said liquid stream into a vapor fraction due to latent heat exchange, work expanding at least a portion of the resulting vapor stream; (b) work expanding a "third" process stream to produce additional work energy such that the total work generated along with step (a) exceeds the total refrigeration demand of the distillation column system; and (c) using the work which is generated in excess of the refrigeration demand of the distillation column system to cold compress a process stream at a temperature lower than the ambient temperature. The work expanded third process stream is a portion of feed air that is eventually fed to the lower pressure distillation column or a nitrogen-rich product vapor stream withdrawn from the higher pressure distillation column and is not condensed after work expansion. The present invention teaches more efficient cryogenic processes for the production of low purity oxygen. The low-purity oxygen is defined as a product stream with oxygen concentration less than 99.5% and preferably less than 97%. In the preferred mode, only one of the methods of work expansion from steps (a)(1) and (a)(2) is used; also the second process stream in step (a)(2) will often be the same as the first process stream in step (a)(1). In the most preferred mode, if the work expansion method of step (a)(1) is used then the high pressure nitrogen-rich vapor stream (i.e. the first process vapor stream) is expanded and then condensed by latent heat exchange against a liquid stream at an intermediate height of the LP column or the crude liquid oxygen (crude LOX) stream that originates at the bottom of the HP column and forms the feed to the LP column. In this method, the pressure of the crude LOX stream is dropped to the vicinity of the LP column pressure. The high pressure nitrogen-rich stream can be partially warmed prior to expansion. If the work expansion method of step (a)(2) is used, then the high pressure nitrogen-rich stream (i.e. the second process vapor stream) is condensed by latent heat exchange against at least a portion of the crude LOX stream that is at a pressure higher than the LP column pressure; and the resulting vapor from the at least partial vaporization of the crude LOX is work expanded to the LP column. Prior to the work expansion, the resulting vapor from the at least partial vaporization of the crude LOX could be partially warmed. As an alternative to the crude LOX vaporization, an oxygen-enriched liquid with oxygen content greater than air could be withdrawn from the LP column and pumped to the desired pressure greater than the LP column pressure prior to at least partial vaporization. By work expansion, it is meant that when a process stream is expanded in an expander, it generates work. This work may be dissipated in an oil brake, or used to generate electricity or used to directly compress another process stream. Along with low-purity oxygen, other products can also be produced. This includes high purity oxygen (purity equal to or greater than 99.5%), nitrogen, argon, krypton and xenon. If needed, some liquid products such as liquid nitrogen, liquid oxygen and liquid argon could also be coproduced. The following is a description of embodiments of the invention by way of example only and with reference to the accompanying drawings, in which: Figures 1 through 9 illustrate schematic diagrams of different embodiments of the present invention; Figures 10(a) through 10(c) illustrate schematic diagrams of embodiments of the present invention as configured for use with multiple low pressure distillation columns and Figures 11 and 12 illustrate schematic diagrams of two prior art processes. In Figures 1 through 9, common streams use the same stream reference numbers. Referring to Figure 1, the compressed feed air stream free of heavier components such as water and carbon dioxide is shown as stream 100. The pressure of this compressed air stream is generally greater than 3.5 bar (350 kPa) absolute and less than 24 bar (2.4 MPa) absolute. The preferred pressure range is from 5 bar (0.5 MPa) absolute to 10 bar (1 MPa) absolute. A higher feed air pressure is helpful in reducing the size of the molecular sieve beds used for water and carbon dioxide removal. The feed air stream is divided into two streams 102 and 110. The major fraction of stream 102 is cooled in the main heat exchanger 190 and then fed as stream 106 to the bottom of the higher pressure (HP) column 196. The feed to the high pressure column is distilled into high pressure nitrogen vapor stream 150 at the top and the crude liquid oxygen (crude LOX) stream 130 at the bottom. The crude LOX stream is eventually fed to a lower pressure (LP) column 198 where it is distilled to produce a lower-pressure nitrogen vapor stream 160 at the top and a liquid oxygen product stream 170 at the bottom. Alternatively, oxygen product may be withdrawn from the bottom of the LP column as vapor. The liquid oxygen product stream 170 is pumped by pump 171 to a desired pressure and then vaporized by heat exchange against a suitably pressurized process stream to provide gaseous oxygen product stream 172. The nitrogen vapor stream 160 is warmed in heat-exchanger 192 to provide stream 162 which is further warmed in main heat exchanger 190 to provide a low pressure gaseous nitrogen product (stream 164). The boil-up at the bottom of the LP column is provided by condensing in reboiler/condenser 193 a first portion of the high pressure nitrogen stream from line 150 in line 152 to provide first high pressure liquid nitrogen stream 153. A portion of stream 153 is subcooled in heat exchanger 192 and (stream 158) reduced in pressure to provide reflux to the LP column. The remainder of stream 153 provides reflux to the HP column. According to step (a)(2) of the invention, at least a portion (stream 134) of the crude LOX stream having a concentration of oxygen greater than that in feed air is reduced in pressure across valve 135 to a pressure which is intermediate of the HP and LP column pressures. In Figure 1, prior to pressure reduction, crude LOX is subcooled in subcooler 192 by heat exchange against the returning gaseous nitrogen stream from the LP column. This subcooling is optional. The pressure-reduced crude LOX stream 136 is sent to a reboiler/condenser 194, where it is at least partially boiled by latent heat exchange against the second portion of the high pressure nitrogen stream from line 150 in line 154 (the second process stream of step (a)(2) of the invention) to provide the second high pressure liquid nitrogen stream 156. The first and second high pressure liquid nitrogen streams provide the needed reflux to the HP and LP columns. The vaporized portion of the pressure-reduced crude LOX stream in line 137 (hereinafter referred as crude GOX stream) is partially warmed in the main heat exchanger 190 and then (stream 138) work expanded in expander 139 to the LP column 198 as additional feed (stream 140). Partial warming of crude GOX stream 137 is optional and similarly, after work expansion stream 140 could be further cooled prior to feeding it to the LP column. Non-vaporized pressure-reduced crude LOX from reboiler/condenser 194 (stream 142) is reduced in pressure and fed to the LP column. Similarly, the portion of crude LOX (stream 132) not fed to the reboiler/condenser 194 is reduced in pressure and fed to a higher location of the LP column. According to step (b) of the invention, a portion of the partially cooled air stream is withdrawn as stream 104 (the third process stream) from the main heat exchanger and work expanded in expander 103 and then fed (stream 105) to the LP column. Both expanders 103 and 139 generate more work than is needed for the refrigeration balance of the plant. In a cryogenic air separation plant, all the heat exchangers, distillation columns and the associated valves, pipes and other equipment shown in Figure 1 are enclosed in an insulated box called the cold box. Since the inside of the box is at subambient temperatures, there is a heat leak from the ambient to the cold box. Also, the product streams (such as streams 164 and 172) leaving the cold box are at lower temperatures than the feed air streams. This leads to enthalpy losses due to products leaving the cold box. For a plant to operate, it is essential that both these losses be balanced by extracting an equal amount of energy out from the cold box. Generally, this energy is extracted as work energy. In this invention the work output from both the expanders 103 and 139 exceeds the work that must be extracted to keep the cold box in refrigeration balance. This intentionally generated additional work is then used for cold compression of a process stream within the cold box. This way the additional work does not leave the cold box and the refrigeration balance is maintained. In Figure 1, in order to vaporize the pumped liquid oxygen from pump 171, a portion of the feed air stream 100 in stream 110 is further boosted in an optional booster 113 and cooled against cooling water (not shown in the figure) and then (as stream 112) partially cooled in the main heat exchanger 190. This partially cooled air stream 114 is then cold compressed by cold compressor 115. The energy input in the cold compressor is the additional work energy generated from expanders 103 and 139 (i.e. that not needed for refrigeration). The cold compressed stream 116 is then reintroduced in the main heat exchanger where it cools by heat exchange against the pumped liquid oxygen stream. A portion (stream 120) of the cooled liquid air stream 118 is sent to the HP column and another portion (stream 122) is sent (as stream 124) to the LP column after some subcooling in subcooler 192. Several known modifications can be applied to the example flowsheet in Figure 1. For example, all the crude LOX stream 130 from the HP column may be sent to the LP column and none of it is sent to the reboiler/condenser 194. In lieu of this, a liquid is withdrawn from an intermediate height of the LP column and then pumped to a pressure intermediate of the HP and LP column pressures and sent to the reboiler/condenser 194. The rest of the treatment in reboiler/condenser 194 is analogous to that of stream 134 explained earlier. In another modification, the two high pressure nitrogen streams 152 and 154 condensing in reboiler/condensers 193 and 194, respectively, may not originate from the same point in the HP column. Each one may be obtained at different heights of the HP column and after condensation in their reboilers (193 and 194), each is sent to an appropriate location in the distillation system. As one example, stream 154 could be drawn from a position which is below the top location of the high pressure column, and after condensation in reboiler/condenser 194, a portion of it could be returned to an intermediate location of the HP column and the other portion is sent to the LP column. Figure 2 shows an alternative embodiment where a process stream is work expanded according to step (a)(1). Here subcooled crude LOX stream 134 is let down in pressure across valve 135 to a pressure that is very close to the LP column pressure and then fed to the reboiler/condenser 194. The second portion of the high pressure nitrogen stream in line 254 (now the first process stream of step (a)(1)) is partially warmed (optional) in the main heat exchanger and then (stream 238) work expanded in expander 139 to provide a lower pressure nitrogen stream 240. This stream 240 is then condensed by latent heat exchange in reboiler/condenser 194 to provide stream 242, which after some subcooling is sent to the LP column. The vaporized stream 137 and the liquid stream 142 from the reboiler/condenser 194 are sent to an appropriate location in the LP column. If needed, a portion of the condensed nitrogen stream in line 242 could be pumped to the HP column. Once again, the two nitrogen streams, one condensing in reboiler/condenser 193 and the other condensing in reboiler/condenser 194, could be drawn from different heights of the HP column and could therefore be of different composition. Another variation of Figure 2 using the work expansion according to step (a)(1) is shown in Figure 3. In this scheme, reboiler/condenser 194 is eliminated and all of the crude LOX stream from the bottom of the HP column is sent without any vaporization to the LP column. In place of reboiler/condenser 194, an intermediate reboiler 394 is used at an intermediate height of the LP column. Now the work expanded nitrogen stream 240 from expander 139 is condensed in reboiler/condenser 394 by latent heat exchange against a liquid at the intermediate height of the LP column. The condensed nitrogen stream 342 is treated in a manner which is analogous to that in Figure 2. The other operating features of Figure 3 are also the same as in Figure 2. It is possible to draw several variations of the proposed invention in Figures 1-3. Some of these variations will now be discussed as further examples. The additional work energy extracted from the two expanders can be used to cold compress any suitable process stream. While Figures 1-3 show the cold compression of a portion of the feed air stream which is then condensed against the pumped LOX stream, it is possible to directly cold compress a gaseous oxygen stream. This gaseous oxygen stream may be directly withdrawn from the bottom of the LP column or it could be obtained after the pumped LOX from pump 171 has been vaporized against a suitable process stream. It is also possible to cold compress a stream rich in nitrogen. This nitrogen-rich vapor stream for cold compression can come from any source such as LP column or HP column. Figure 4 shows a variation where this nitrogen-rich vapor stream is withdrawn from the HP column. All the features of Figure 4 are same as Figure 1 except that pumped liquid oxygen from pump 171 is not vaporized by latent heat exchange against a cold compressed air stream but against the cold compressed nitrogen stream from the HP column. While the nitrogen-rich stream for cold compression can be withdrawn from any suitable location of the HP column, in Figure 4 it is shown to be withdrawn from the top of the HP column as stream 480. This stream 480 is then partially warmed (optional) in the main heat exchanger, cold compressed (stream 482) in 484, then (stream 486) condensed by latent heat exchange against the vaporizing liquid oxygen from pump 171. This condensed stream is then sent to the distillation column system. In Figure 4, if needed, nitrogen-rich stream 480 could be first warmed in the main heat exchanger to a temperature close to the ambient temperature and then boosted in pressure by an auxiliary compressor, then partially cooled in the main heat exchanger and then sent to the cold compressor 484. The advantage of cold compressing a nitrogen-rich stream and then condensing it against at least a portion of the liquid oxygen from pump 171 is that it provides significantly more nitrogen reflux to the distillation column system and this improves the recovery and/or purity of nitrogen product. For example, even though not shown in Figure 4, one will be able to coproduce more high pressure nitrogen product from Figure 4 than from the corresponding Figure 1. It should be emphasized that the purpose of cold compression is not limited to raising the pressure of oxygen. It can be used to cold compress any suitable process stream in step (c) of the invention. For example, in Figure 4, either a portion or all of the cold compressed nitrogen stream 486 may not be condensed by further cooling but further warmed in the main heat exchanger to provide a pressurized nitrogen product stream. Another example is shown in Figure 5. The difference between this example and the one in Figure 3 is that all the high pressure nitrogen stream from the top of the HP column 196 is withdrawn in line 554. This stream is then partially warmed in the main heat exchanger (stream 556) and divided into two streams 538 and 551. Stream 538 is further treated in a manner analogous to treatment of stream 238 in Figure 3 by work expansion in expander 139 and (via line 540) condensation in an intermediate reboiler/condenser 594. Stream 551 is cold compressed in compressor 515 according to step (c) of the invention. The cold compressed stream 552 is not condensed against the pumped liquid oxygen from pump 171, but is condensed by latent heat exchange against the liquid in the bottom reboiler/condenser 593 of the LP column. This provides the needed boil-up at the bottom of the LP column. The condensed liquid nitrogen streams in line 542 and 553 are then sent as reflux to the HP and LP columns. If a portion of the lower pressure liquid nitrogen stream 542 is to be sent to the HP column, then a pump 543 would be helpful. In another variation, high pressure nitrogen stream 551 for cold compression may be withdrawn immediately from stream 554. Similarly, the cold compressed nitrogen stream in line 552 may be partially cooled by heat exchange against any suitable process stream prior to condensation in reboiler/condenser 593. These examples clearly illustrate that the present invention can be used to cold compress any suitable process stream. Furthermore, 538 and 551 need not be of the same composition, i.e. each could be drawn from different locations of the HP column. In Figures 1-5, expansion of a portion of the feed air to the LP column is done to meet the requirement of step (b) of the invention. Figure 6 shows an example where a nitrogen-rich stream from the HP column is work expanded. Figure 6 is analogous to Figure 1 except that lines for streams 104 and 105 are eliminated. Instead, a portion of the high pressure nitrogen vapor is withdrawn from the top of the HP column in line 604. This stream is now the third process stream according to step (b) of the invention. The high pressure nitrogen in stream 604 is partially warmed in the main heat exchanger and then work expanded in expander 603. The work expanded stream 605 is then warmed in the main heat exchanger to provide a nitrogen stream in line 606. The pressure of nitrogen stream 606 may be the same or higher than the nitrogen in stream 164. Figures 1-6 show examples where all the first or the second process stream, the third process stream and the cold compressed process stream in steps (a), (b), (c) of the invention do not originate from the same process stream. At least two of these streams have different composition. While such schemes with different process streams can now be easily drawn, Figure 7 shows an example where all the streams for all the three steps of the invention are drawn from the top of the HP column. A portion of the high pressure nitrogen from the top of the HP column is withdrawn in line 754. This stream is then divided into two streams 704 and 780 and both are partially warmed to their respective suitable temperatures in the main heat exchanger. After partial warming of stream 780, it is further divided into two streams 738 and 782. Stream 738 provides the first process stream of step (a)(1) of the invention and is treated in a manner analogous to that of stream 238 in Figure 3 by work expansion in expander 139 and (via line 740) condensation in an intermediate reboiler/condenser 794. Stream 704 provides the third process stream of step (b) of the invention and is treated in a manner analogous to that of stream 604 in Figure 6 by partial warming in the main heat exchanger, work expansion in expander 703 and further warming (via line 705) in the main heat exchanger to provide a nitrogen stream 706. Stream 782 provides the needed process stream for cold compression in compressor 784 in step (c) of the invention and is processed (via lines 786, 787 & 788 and valve 789) in a manner analogous to stream 482 in Figure 4. Note that in Figure 7, the work expanded nitrogen stream 705 from expander 703, is not condensed against any oxygen-rich liquid from or to the LP column in a manner taught for step (a) (1) of the invention. So far, all the example flowsheets show at least two reboiler/condensers. However, it should be emphasized that the present invention does not preclude the possibility of using additional reboiler/condensers in the LP column than those shown in Figures 1-7. If needed, more reboilers/condensers may be used in the bottom section of the LP column to further distribute the generation of vapor in this section. Any suitable process stream may be either totally or partially condensed in these additional reboilers/condensers. For illustration, Figure 8 shows an example where the process in Figure 5 is modified to include another reboiler/condenser in the LP column. While reboilers/condensers 893 and 894 are analogous to reboilers/condensers 593 and 594, reboiler/condenser 895 is the additional reboiler/condenser. The high pressure nitrogen stream 854 (analogous to stream 554) is partially warmed to provide stream 856 (analogous to stream 556) but now divided into three streams. The additional stream in line 857 is condensed in the additional reboiler/condenser 895 against a liquid stream in the LP column and sent (via line 858) for refluxing the high pressure column. Further processing of streams 838 and 851 is the same as for streams 538 and 551 in Figure 5. Figure 8 is just an example of using multiple reboilers/condensers in the LP column. From the known art, it is easy to draw many such examples using the present invention. For illustration, the possibility of condensing a vapor stream withdrawn from an intermediate height of the HP column in a reboiler/condenser located in the LP column may be considered. In such situations, when a stream withdrawn from HP column that contains significant quantities of oxygen is partially condensed, the uncondensed vapor fraction can provide the first process stream of step (a)(1) or the second process stream of step (a)(2). In all those process schemes of the present invention where work is extracted by the method taught in step (a)(1), all of the first process stream after work expansion may not be condensed by latent heat exchange as taught by step (a)(1). A portion of this stream may be recovered as a product stream or used for some other purpose in the process scheme. For example, in the process schemes shown in Figures 2, 3, 5, 7 and 8, at least a portion of the high pressure nitrogen stream from the high pressure column is work expanded in expander 139 according to the step (a)(1) of the invention. A portion of the stream exiting the expander 139 may be further warmed in the main heat exchanger and recovered as a nitrogen product at medium pressure from any one of these process flowsheets. When a portion of the feed air is work expanded, it may be precompressed at near ambient temperatures, prior to feeding it to the main heat exchanger, by using work energy that is extracted from the cold box. For example, Figure 9 shows the process scheme of Figure 1 except that stream 901 is withdrawn from the portion of the feed air in line 102. The withdrawn stream is then boosted in compressor 993, then cooled with cooling water (not shown in the figure) and further cooled in the main heat exchanger to provide stream 904. This stream 904 is further treated in a manner analogous to the treatment of stream 104 in Figure 1 to provide feed stream 905 to the LP column. The work energy needed to drive compressor 993 is derived from the expanders in the cold box. In Figure 9, it is shown that compressor 993 is solely driven by expander 103. An advantage of using such a system is that it provides a potential to extract more excess work from the expanders and therefore, more work energy would be available for cold compression. As an alternative to pressure boosting of a portion of the feed air stream in line 901, it is possible to first warm other process streams which are to be work expanded in the cold box, boost their pressure in a compressor such as 993, partially cool them in appropriate heat exchangers and then feed them to appropriate expanders. There are several methods of transferring extra work energy to the cold compressor. For illustration purpose, some of the alternative methods are listed below: All the work extracted from both the expanders in steps (a) and (b) of the invention may be used external to the cold box and the cold compressor in step (c) of the invention may be driven by an electric motor. For this purpose, either one or both of the expanders may be generator loaded to generate electricity or loaded with a warm compressor to compress a process stream at ambient or above ambient temperatures. All the work extracted from one of the expanders may be recovered external to the cold box and then all the work extracted from the second expander can be used for cold compression. In such a case, the second expander may be directly coupled with the cold compressor through a common shaft to directly transfer the work from the expanded stream to the cold compressed stream. For example in Figure 1, expander 139 may be directly coupled with cold compressor 115 such that it is driven only by expander 139. In such a case, work extracted from expander 103 provides the total refrigeration of the cold box. When suitable, instead of expander 139, expander 103 could be directly coupled to the cold compressor 115 and now expander 139 would provide the needed refrigeration for the plant. It may be possible to directly couple both the expanders to the cold compressor. In such a case, both the expanders will impart at least a portion of the work needed for the cold compression. Also, at least one of the expanders will be loaded external to the cold box to provide the needed refrigeration for the cold box. The cold compressor is directly coupled to an expander and uses up all the work extracted from this expander. The second expander is loaded external to the cold box such that all the work extracted from this expander is rejected outside the cold box. Where work extracted from the second expander exceeds the refrigeration demand of the cold box, the excess work extracted from the second expander can be transferred to the cold compressor through an electric motor assist. It should be apparent to those practicing the art that a single distillation column containing multiple reboilers may be broken into multiple columns, each with one reboiler. The justification for splitting a multi-reboiler column into multiple sections is generally capital cost savings. An example of how this invention may be implemented using multiple low pressure columns is shown in Figure 10. Figure 10(a) is a simplified representation of the process shown in Figure 3, numerous process lines and unit operations have been omitted for clarity. The low pressure column shown in Figure 10(a) contains three distillation sections above the intermediate reboiler and one section below. In Figure 10(b), the section below the intermediate reboiler, and the bottom reboiler, have been relocated to a separate column. Because of elevation differences, it is necessary to add a transfer pump. The advantage of the configuration shown in Figure 10(b) is that the height of the equipment has been reduced. In Figure 10(c), the sections above and including the intermediate reboiler have been relocated to a separate column. The configuration shown in Figure 10(c) results in the lowest equipment height. Reducing the equipment height can be advantageous when the distillation columns are large and the resultant cost savings often offset the capital penalty associated with adding a transfer pump. The method taught in this invention can be used when there are coproducts besides the low-purity oxygen, with oxygen content less than 99.5%. For example, a high purity (99.5% or greater oxygen content) oxygen could be coproduced from the distillation system. One method of accomplishing this task is to withdraw low-purity oxygen from the LP column at a location which is above the bottom and withdraw a high purity oxygen from the bottom of the LP column. If the high purity oxygen stream is withdrawn in the liquid state, it could then be further boosted in pressure by a pump, then vaporized by heat exchange against a suitable process stream. Similarly, a high purity nitrogen product stream at elevated pressure could be coproduced. One method of accomplishing this task would be to take a portion of the condensed liquid nitrogen stream from one of the suitable reboiler/condensers and pump it to the required pressure and then vaporize it by heat exchange with a suitable process stream. The value of the present invention is that it leads to substantial reduction in the energy consumption. This will be demonstrated by comparing it with some known prior art processes, which are listed below. The first prior art process is shown in Figure 11. This is a conventional double column process with an air expander to the LP column. The work energy from the air expander is recovered as electrical energy. The process of Figure 11 differs from the process of Figure 3 in that cold compressor 115, expander 139 and reboiler/condenser 394 and the associated lines are eliminated. The second prior art process is derived on the basis of US-A-4,786,431 (Erickson). For this purpose, compared with the process of Figure 2, cold compressor 115 and air expander 103 are eliminated. Therefore, only one expander 139 is retained to supply the total refrigeration need of the plant. In accordance with Erickson's teaching, the discharge from expander 139 is condensed against a portion of the pressure reduced crude LOX stream 136 in reboiler/condenser 194. The condensed nitrogen stream 242 is sent as reflux to the LP column and streams 137 and 142 from the boiling side of the reboiler/condenser 194 are sent to the LP column. The third prior art process is also derived from US-A-4,796,431 and is shown in Figure 12. In this figure, all the refrigeration is provided by work expansion of the high pressure nitrogen from the top of the HP column. Therefore, any air expander such as expander 103 in Figure 2 is not used. However, the high pressure nitrogen stream 1254 from the HP column is warmed and divided into two streams 1238 and 1255 and each one is work expanded according to the method described in each of Figures 2 and 3. Thus, stream 1238 is work expanded in expander 139 and treated analogous to stream 238 in Figure 2, and stream 1255 is work expanded in expander 1239, condensed in reboiler/condenser 1294 and treated analogous to stream 240 in Figure 3. The excess work extracted from both expanders is used in cold compressor 115 in a manner as shown in Figures 2 and 3. A fourth process for comparison is derived from Figure 1 by retaining everything in Figure 1 except cold compressor 115. Therefore, the work generated from both the expanders 139 and 103 is used to generate electricity. No cold compression of any stream is done within the cold box. Calculations were done to produce 95% oxygen product at 200 psia (1.38 MPa). For all flowsheets, the discharge pressure from the final stage of the main feed air compressor was about 5.3 bar (530 kPa) absolute. The pressure at the top of the LP column was about 1.25 bar (125 kPa) absolute. The net power consumption was computed by calculating the power consumed in the main feed air compressor, the booster air compressor 113 to vaporize pumped liquid oxygen, and taking credit for electrical power generated from any expander. The relative power consumption for several flow schemes are listed below: Case Flow Scheme Relative Power 1 First Prior Art Process (Figure 11) 1.0 2 Second Prior Art Process 1.013 3 Third Prior Art Process (Figure 12) 1.001 4 Fourth Prior Art Process (Figure 1 with no cold compression) 0.986 5 Present Invention, Figure 1 0.946 6 Present Invention, Figure 2 0.957 It is clear from these calculations that the process of the present invention is much superior to any of the prior art processes used for Cases 1 through 3. Also, when Cases 4 and 5 are compared, the huge benefit derived due to cold compression becomes obvious. This is because between these two cases, all the features of the flowsheets are the same except that in Case 4 no cold compression is used, whereas, Case 5 uses cold compression. Another flowsheet according to the present invention in Figure 2 shows substantial improvement, specifically when compared to the prior art process in Case 3 (Figure 12). 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. Rather, various modifications may be made in the details within the scope of the following claims.

Claims: A process for the cryogenic distillation of air in a distillation column system comprising a higher pressure distillation column (196) and a lower pressure distillation column (198) wherein at least a portion (106) of the feed air (100) is fed to the higher pressure distillation column (196), product oxygen (170) with oxygen concentration less than 99.5% is produced at the bottom of the lower pressure distillation column (198) and the boil-up at the bottom of the lower pressure distillation column (198) is provided by condensing (193; 593; 893) a stream (152; 552; 851) whose nitrogen concentration is greater than that in the feed air stream (100), wherein: (a) work energy which is at least ten percent (10%) of the overall refrigeration demand of the distillation column system is generated by: (1) work expanding (139) a vapor process stream (238; 538; 738; 838) (hereinafter "first vapor process stream") withdrawn from the higher pressure distillation column (196) and having nitrogen content greater than that in the feed air and then condensing (194; 394; 594; 794; 894) at least a portion of the expanded first process stream (240; 540; 740) by latent heat exchange with at least a portion of a liquid stream that has an oxygen concentration greater than the concentration of oxygen in the feed air and is (i) a liquid at an intermediate height in the lower pressure distillation column (198) and/or (ii) one of the liquid feeds (136) to the lower pressure distillation column (198); and/or (2) withdrawing from the higher pressure distillation column (196) a vapor process stream (154) (hereinafter "second vapor process stream") with nitrogen content greater than that in the feed air, condensing (194) at least a portion of said second vapor process stream by latent heat exchange with at least a portion of a liquid stream (136) which has oxygen concentration greater than the concentration of oxygen in the feed air and which is also at a pressure greater than the pressure of the lower pressure distillation column (198), and, after vaporization of at least a portion of said liquid stream (136) into a vapor fraction due to latent heat exchange, work expanding (139) at least a portion(138) of the resulting vapor stream (137); (b) additional work energy such that the total work generated along with step (a) exceeds the total refrigeration demand of the cryogenic distillation column system is generated by work expansion (103; 603; 703) of a process stream (hereinafter "third process stream") selected from a portion (104) of feed air that is eventually fed to the lower pressure distillation column (198) and a nitrogen-rich product vapor stream (604; 704; 904) withdrawn from the higher pressure distillation column (196), which third process stream is not condensed after work expansion; and (c) the work which is generated in excess of the refrigeration demand of the distillation column system is used to cold compress (115; 484; 515; 784) a process stream (114; 482; 551; 782; 851) at a temperature lower than the ambient temperature(198)(198). A process according to Claim 1, wherein the process stream of step (a) is said first process stream (254; 538; 738; 838) prior to said condensation (394; 594; 794; 894) and said liquid stream is a liquid at an intermediate height in the lower pressure distillation column (198). A process according to Claim 1, wherein the process stream of step (a) is said first process stream (254) prior to said condensation (194) and said liquid stream is one of the liquid feeds (136) to the lower pressure distillation column (198). A process according to Claim 3, wherein said liquid feed (136) condensing the work expanded first process stream (140) is withdrawn from the higher pressure distillation column (196). A process according to any one of Claims 2 to 4, wherein at least a portion of said condensed expanded first process stream (542) is pumped (543) and sent to the higher pressure distillation column (196). A process according to any one of Claims 2 to 4, wherein all of said condensed expanded first process stream (242; 342) is sent to the lower pressure distillation column (198) as a feed. A process according to Claim 1, wherein the process stream of step (a) is a vapor (137) provided by vaporization of at least a portion of said liquid stream (136) due to said latent heat exchange (194) with at least the second process stream (154), said liquid stream (136) being at a pressure greater than the pressure of the lower pressure distillation column (198). A process according to Claim 7, wherein said vaporized liquid stream (136) is at least a portion of an oxygen enriched liquid (130) which is withdrawn from the higher pressure distillation column (196). A process according to Claim 7 or Claim 8, wherein at least a portion of said condensed (194) second process stream (156) is pumped, if necessary, and sent to the higher pressure distillation column (196). A process according to Claim 7 or Claim 8, wherein at least a portion of said condensed (194) second process stream (156) is sent to the lower pressure distillation column (198) as a feed. A process according to any one of the preceding claims, wherein the third process stream is a portion (104) of the feed air (140) and is eventually fed to the lower pressure distillation column (198). A process according to any one of Claims 1 to 10, wherein the third process stream is a nitrogen rich product vapor stream (604; 704) withdrawn from the higher pressure distillation column (196) and is warmed (190) and discharged from the cold box following expansion. A process according to any one of the preceding claims, wherein the process stream compressed (115) in step (c) is at least a portion (114) of feed air (100). A process according to Claim 13, wherein the oxygen product (170) is withdrawn from the lower pressure distillation column (198) as a liquid and eventually boiled (190) and said cold compressed feed air portion (116) is at least partially condensed by indirect heat exchange (190) with the boiling oxygen. A process according to Claim 14, wherein said feed air portion (114) cold compressed (115) in step (c) is also compressed (113) warm prior to being cooled (190) and subsequently compressed cold (115). A process according to any one of Claims 1 to 14, wherein the process stream compressed in step (c) is a vapor (782; 851) withdrawn from the higher pressure distillation column (196). A process according to Claim 16, wherein the oxygen product (170) is withdrawn from the lower pressure distillation column (198) as a liquid and eventually boiled (190) and at least a portion of said cold compressed higher pressure distillation column vapor (486, 786) is at least partially condensed by indirect heat exchange (190) with the boiling oxygen. A process according to any one of the preceding claims, wherein the expander (139) used for step (a) is direct coupled to the cold compressor (115) used in step (c). A process according to any one of the preceding claims, wherein the oxygen product has a purity less than 97%. Apparatus for the cryogenic distillation of air in a distillation column system by a process as defined in Claim 1 comprising a higher pressure distillation column (196); a lower pressure distillation column (198); means (106) for feeding at least a portion of the feed air (100) to the higher pressure distillation column (196); means for withdrawing product oxygen (170) from the bottom of the lower pressure distillation column (198); heat exchange means (193; 593; 893) providing boil-up at the bottom of the lower pressure distillation column (198) by condensing a stream (152; 552; 851) whose nitrogen concentration is greater than that in the feed air stream; either or both of (1)first work expansion means (139) for expanding a first process vapor stream (254; 538; 738; 838) withdrawn from the higher pressure distillation column (196) and having nitrogen content greater than that in the feed air and first heat exchange means (194; 394; 594; 794; 894) for condensing at least a portion of the expanded stream (240; 540; 740) by latent heat exchange with at least a portion of a liquid stream being (i) a liquid at an intermediate height in the lower pressure distillation column (198) and/or (ii) one of the liquid feeds (136) to this distillation column and having an oxygen concentration greater than the concentration of oxygen in the feed air (100); and (2) second heat exchange means (194) for condensing at least a second process vapor stream (154) withdrawn from the higher pressure distillation column (196) and having nitrogen content greater than that in the feed air by latent heat exchange with at least a portion of a liquid stream (136) which has oxygen concentration greater than the concentration of oxygen in the feed air (100) and which is also at a pressure greater than the pressure of the lower pressure distillation column (198), and second work expansion means (139) for work expanding at least a portion of a vaporized portion (137) of said liquid stream; said first and/or second work expanding means providing at least ten percent (10%) of the overall refrigeration demand of the distillation column system; third work expansion means (103; 603; 703; 103) for work expanding a third processstream, selected from a portion (104) of feed air that is eventually fed to the lower pressure distillation column (198) and a nitrogen-rich product vapor stream (604; 704; 904) withdrawn from the higher pressure distillation column (196), to produce additional work energy such that the total work generated along with the first and/or second work expansion means exceeds the total refrigeration demand of the distillation column system; and cold compression means (115; 484; 515; 784), driven by the work which is generated in excess of the refrigeration demand of the distillation column system, for cold compressing a process stream (114; 482; 551; 782; 851) at a temperature lower than the ambient temperature, said apparatus not having heat exchange means for condensing the work expanded third process stream (105; 605; 705; 905). An apparatus according to Claim 20, comprising said first work expansion means (139) and said first heat exchange means (394; 594; 794; 894), wherein said first heat exchange means (394; 594; 794; 894) condenses the expanded stream (240; 540; 740) against a liquid at an intermediate height in the lower pressure distillation column (198). An apparatus according to Claim 20, comprising said first work expansion means (139) and said first heat exchange means (394; 594; 794; 894), wherein said first heat exchange means (194) condenses the expanded stream (240) against one of the liquid feeds (136) to the lower pressure distillation column (198). An apparatus according to Claim 21 or Claim 22, wherein said liquid feed (136) condensing the work expanded first process stream (140) in the first heat exchange means (194) is withdrawn from the higher pressure distillation column (196). An apparatus according to any one of Claims 21 to 23, comprising pumping means (543) for pumping at least a portion of said condensed expanded first process stream (542) to the higher pressure distillation column (196). An apparatus according to any one of Claims 21 to 23, wherein all of said condensed expanded first process stream (242; 342) is sent to the lower pressure distillation column (198) as a feed. An apparatus according to Claim 20, comprising said second heat exchange means (194) and said second work expansion means (139). An apparatus according to Claim 26, wherein said vaporized liquid stream (136) is at least a portion of an oxygen enriched liquid (130) which is withdrawn from the higher pressure distillation column (196). An apparatus according to Claim 26 or Claim 27, comprising pumping means pumping at least a portion of said condensed (194) second process stream (156) to the higher pressure distillation column (196). An apparatus according to Claim 26 or Claim 27, wherein at least a portion of said condensed (194) second process stream (156) is sent to the lower pressure distillation column (198) as a feed. An apparatus according to any one of Claims 20 to 29, wherein the third process stream is a portion (104) of the feed air (100) and is eventually fed to the lower pressure distillation column (198). An apparatus according to any one of Claims 20 to 29, wherein the third process stream is a nitrogen rich product vapor stream (604; 704) withdrawn from the higher pressure distillation column (196) and is warmed (190) and discharged from the cold box following expansion. An apparatus according to any one of Claims 20 to 31, wherein the process stream compressed in the cold compression means (115) is at least a portion (114) of feed air (100). An apparatus according to Claim 32, wherein the oxygen product (170) is withdrawn from the lower pressure distillation column (198) as a liquid and eventually boiled (190) and said cold compressed feed air portion (116) is at least partially condensed by indirect heat exchange (190) with the boiling oxygen. An apparatus according to Claim 33, comprising warm compression means (113) for compressing said feed air portion (114) prior to being cooled (190) and subsequently compressed in said cold compression means (115). An apparatus according to any one of Claims 20 to 33, wherein the process stream compressed in the cold compression means (115) is a vapor (782; 851) withdrawn from the higher pressure distillation column (196). An apparatus according to Claim 35, wherein the oxygen product (170) is withdrawn from the lower pressure distillation column (198) as a liquid and eventually boiled (190) and at least a portion of said cold compressed higher pressure distillation column vapor (486, 786) is at least partially condensed by indirect heat exchange (190) with the boiling oxygen. An apparatus according to any one of Claims 20 to 36, wherein the first or second expansion means (139) is direct coupled to the cold compressor (115).

Inventors: AGRAWAL RAKESH, HERRON DONN MICHAEL, ZHANG YANPING, AGRAWAL, RAKESH, HERRON, DONN MICHAEL, ZHANG, YANPING
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1999
Main classification: F25J3
IPC codes: F25J3