AIR SEPARATION

Abstract: Pressurised air at a temperature suitable for its separation by cryogenic distillation is passed into a double distillation column 2 comprising a higher pressure column 4 and a lower pressure column 6 and a condenser-reboiler 8. Oxygen-rich liquid is withdrawn from the column 4 through an outlet 12, is sub-cooled in heat exchanger 14 and is separated in column 6 in nitrogen vapour and liquid oxygen. Argon-enriched fluid is withdrawn from the column 6 through outlet 50 and argon is separated from it in a further distillation column 16 having a condenser 18 which is refrigerated by the oxygen-rich liquid. In a typical embodiment of the invention the oxygen-poor liquid is withdrawn though outlet 28 from the column 4 and is expanded through valve 30 into phase separator 32. The vapour phase from the separator 32 is used to provide extra refrigeration to the oxygen-rich liquid by counter current heat exchange in the heat exchanger 14 and is then either taken as product or expanded in a turbine to create refrigeration. Part of the liquid phase from the separator 32 is used as reflux in the column 6 while the remainder is used to provide extra cooling for the condenser 18.

Description: AIR SEPARATION This invention relates to a method or plant for separating air. Air separation is a well known commercial process and its main products, oxygen, nitrogen and argon are widely used in industry. Air separation plants capable of producing more than 100 tonnes per day of products generally employ rectification columns in which the air is separated at cryogenic temperatures. One kind of such plant produces argon and gaseous oxygen products, and, if desired, one or both of a gaseous nitrogen product and liquid nitrogen product. Using such plant, a conventional method of separating oxygen and argon from air as shown e.g. in GB-A-2181 828 or corresponding EP-A-0218467 comprises the steps of : a) passing pressurised air through at least one main heat exchanger so as to reduce its temperature to a level suitable for its separation by cryogenic distillation; b) passing such temperature-reduced air into the higher pressure column of a double rectification column comprising a higher pressure column, a lower pressure column, and a condenser-reboiler which provides reflux for the higher pressure column and reboil for the lower pressure column; c) separating the air in the higher pressure column into an oxygen-rich liquid fraction and an oxygen-poor vapour fraction; d) condensing said oxygen-poor vapour fraction in said condenser-reboiler, and employing some of the condensed vapour as reflux in the higher pressure column and collecting the remainder of the condensed vapour; e) withdrawing oxygen-poor liquid from the higher pressure column, sub-cooling it and employing the sub-cooled liquid as reflux in the lower pressure column; f) withdrawing oxygen-rich liquid from the higher pressure column, sub-cooling it and separating it in the lower pressure column into a nitrogen vapour fraction and a liquid oxygen fraction; g) withdrawing a stream relatively rich in argon from the lower pressure column and separating it in a further rectification column into an argon-enriched fraction and an oxygen fraction, said further column having a condenser associated therwith which is refrigerated by said oxygen-rich liquid upstream of its introduction into the lower pressure column, and withdrawing argon-enriched fluid from the further column; and h) withdrawing nitrogen vapour and oxygen vapour from the lower pressure column and passing the withdrawn oxygen vapour and withdrawn nitrogen vapour through the main heat exchanger countercurrently to the incoming air. Typically, the air is pre-purified by removing constituents of relatively low volatility such as water vapour and carbon dioxide therefrom. Such purification may be accomplished in adsorbers upstream of the main heat exchanger or by forming the main heat exchanger as a reversing heat exchanger. The main requirement for external work in such a method is that of compressing the incoming air. Typically, the air is compressed to about 6 atmospheres. In the developement of air separation plants one of the key objectives has been to reduce the specific power consumption without adversely effecting the purity of the product gases. Our co-pending application GB-A-2 181 828 relates to means for improving the efficiency of air separation by, in effect, adding heat to an intermediate level in the lower pressure column, and to achieve this objective withdraws liquid from an intermediate level of the lower pressure column reboils the liquid in an heat exchanger and returns the thus formed vapour to the lower pressure column. The present invention relates to an alternative approach to improving the efficiency of an air separation method of the above kind. DE-A-2 304 976 relates to a process in which refrigeration for the condenser associated with the argon column is provided solely by oxygen-poor liquid. This reduces the reflux available to the lower pressure column, and adversely affects the argon yield. According to the present invention there is provided a method of separating argon and oxygen products from air, comprising the steps of: (a) passing pressurised air through at least one main heat exchanger so as to reduce its temperature to a level suitable for its separation by cryogenic distillation; (b) passing such temperature-reduced air into the higher pressure column of a double rectification column comprising a higher pressure column, a lower pressure column, and a condenser-reboiler which provides reflux for the higher pressure column and reboil for the lower pressure column; (c) separating the air in the higher pressure column into an oxygen-rich liquid fraction and an oxygen-poor vapour fraction; (d) condensing said oxygen-poor vapour fraction in said condenser-reboiler, and employing some of the condensed vapour as reflux in the higher pressure column and collecting the remainder of the condensed vapour; (e) withdrawing oxygen-poor liquid from the higher pressure column, sub-cooling it and employing the sub-cooled liquid as reflux in the lower pressure column; (f) withdrawing oxygen-rich liquid from the higher pressure column, sub-cooling it and separating it in the lower pressure column into a nitrogen vapour fraction and a liquid oxygen fraction; (g) withdrawing a stream relatively rich in argon from the lower pressure column and separating it in a further rectification column into an argon-enriched fraction and an oxygen fraction, said further column having a condenser associated therewith which is refrigerated by said oxygen-rich liquid upstream of its introduction into the lower pressure column, and withdrawing argon-enriched fluid from the further column; (h) withdrawing nitrogen vapour and oxygen vapour from the lower pressure column and passing the withdrawn oxygen vapour and withdrawn nitrogen vapour through the main heat exchanger countercurrently to the incoming air; (i) vaporising a portion of the oxygen-poor liquid and either withdrawing it as product or expanding it with the performance of external work to perform a refrigeration duty; and (j) transferring refrigeration from the oxygen-poor liquid to the oxygen-rich liquid. The invention also provides a plant for separating argon and oxygen products from air, comprising: (a) at least one main heat exchanger for reducing the temperature of a stream of pressurised air to a level suitable for its separation by cryogenic distillation; (b) a double rectification column comprising a higher pressure column, a lower pressure column and a condenser-reboiler for providing reflux for the higher pressure column and reboil for the lower pressure column; (c) an inlet to the higher pressure column for the reduced temperature air stream, the higher pressure column being able to separate the air into oxygen-poor and oxygen-rich fractions; (d) a first heat exchanger for sub-cooling a stream of oxygen-poor liquid formed by condensation in the condenser-reboiler; (e) an inlet to the lower pressure column for the sub-cooled oxygen-poor liquid stream; (f) a second heat exchanger for sub-cooling a stream of oxygen-rich liquid withdrawn from the bottom of the higher pressure column; (g) an inlet to the lower pressure column for the sub-cooled oxygen-rich liquid stream, the lower pressure column being able to separate the oxygen-rich liquid into oxygen liquid and nitrogen vapour fractions; (h) a further rectification column for separating a stream relatively rich in argon from the lower pressure column into an argon-enriched fraction and an oxygen fraction; (i) a condenser associated with the further rectification column, the condenser having a passage for the sub-cooled stream of oxygen-rich liquid intermediate the second heat exchanger and the inlet to the lower pressure column; (j) an outlet from the further rectification column for argon-enriched fluid; (k) passages through the main heat exchanger for oxygen vapour and nitrogen vapour streams, whereby in use the pressurised air stream is able to be reduced in temperature by countercurrent heat exchange; (l) means for vaporising a portion of the oxygen-poor liquid; (m) an outlet passage for withdrawing as product the vaporised oxygen-poor liquid or an expansion turbine for expanding vaporised oxygen-poor liquid to perform a refrigeration duty; and (n) heat exchange means for transferring refrigeration from the oxygen-poor liquid to the oxygen-rich liquid; The refrigeration duty is conveniently the refrigeration of said at least one main heat exchanger. By taking a portion of the oxygen-poor liquid as product (nitrogen), it is possible to increase the yield of the plant without causing a concomitant increase in the specific power consumption. Moreover, if the nitrogen product is required at elevated pressure, there is a reduction in the amount of compression required since the nitrogen will typically be produced at a pressure intermediate that of the lower and higher pressure columns rather than at that of the lower pressure column. Alternatively, a net reduction in the amount of compression that needs to be performed is made possible by work expansion of the vaporised portion of the oxygen-poor liquid, and preferably, in such embodiments of the invention the vaporised liquid is compressed prior to its expansion. If desired, the expansion machine or turbine may be coupled to the compressor employed to compress the vaporised liquid. Where the said vaporised portion of the oxygen-poor liquid is taken as product, a portion of the compressed air is preferably taken and raised to a higher pressure, cooled in the main heat exchanger and then expanded with the performance of external work to create refrigeration for the main heat exchanger. Such expansion machine or turbine may be coupled to the compressor so as to provide drive for the compressor. Whichever of the above-described means for providing external refrigeration for the main heat exchanger is selected, it may be arranged to provide all the requirements for external refrigeration of that heat exchanger. Alternatively, additional refrigeration means may be provided. The consequence of the transfer of refrigeration is that less oxygen-poor liquid is required to be introduced into the top of the lower pressure column since the oxygen-rich liquid provides more refrigeration to the lower pressure column than in a conventional process, thus allowing a part of the oxygen-poor liquid to be taken as product nitrogen or used to generate refrigeration for the main heat exchanger. Preferably, the refrigeration is transferred from the oxygen-poor to the oxygen-rich liquid by flashing the oxygen-poor liquid into a separator in which the resultant fluid is separated into liquid and vapour phases, withdrawing vapour from the separator and heat exchanging it against the oxygen-rich liquid being sub-cooled. This heat exchange enables the oxygen-rich liquid to be sub-cooled to a lower temperature than in a conventional plant and hence less flash gas is produced on introducing the oxygen-rich liquid into the lower pressure column. It is thus the vapour stream that is taken as product or is expanded to generate refrigeration for the main heat exchanger. The liquid from the separator is typically introduced into the lower pressure column in the normal manner. If desired, the condenser associated with the further column may be located in the phase separator. In embodiments of the invention in which the oxygen-poor liquid is flashed into a phase separator, preferably only some but not all of the oxygen-rich liquid is passed through the condenser associated with the further rectification column, and some other of it is introduced into the lower pressure column without passing through the condenser associated with the further rectification column. An alternative or additional method of transferring refrigeration from the oxygen-poor liquid to the oxygen-rich liquid is to employ some of the sub-cooled oxygen-poor liquid to provide refrigeration for the condenser associated with the further rectification column. Typically, this does not effect a reduction in temperature of the oxygen-rich liquid but means that a greater proportion of the oxygen-rich liquid leaves the condenser associated with the further rectification column in the liquid state. If desired, the portion of the oxygen-poor liquid that is used to provide refrigeration to the condenser associated with the further rectification column may be mixed in an auxiliary liquid-vapour contact column, in which said condenser is located with a stream of liquid oxygen withdrawn from the lower pressure column. A stream of nitrogen or a gas mixture comprising oxygen and nitrogen may be withdrawn from the auxiliary column and taken as product or expanded to provide refrigeration to, for example, the main heat exchanger. If desired, the oxygen-poor liquid that is employed to provide refrigeration for the condenser associated with the further rectification column may be taken from the phase separator into which the oxygen-poor liquid is flashed. The stream withdrawn from the auxiliary column may be mixed with or kept separate from the vapour withdrawn from the separator. In other preferred embodiments of the invention, a stream of oxygen-poor liquid is heat exchanged with a vapour stream that is withdrawn from a level of the lower pressure column intermediate its top and bottom, said vapour stream thereby being at least partially condensed, and the resulting condensate is returned to the lower pressure column. The vapour stream is preferably withdrawn from the same level of the lower pressure column as said stream relatively rich in argon. The oxygen-poor liquid, as typically now vapour, that passes out of said heat exchange relationship with the intermediate condenser may be withdrawn as product or expanded with the performance of external work to generate refrigeration. Heat exchange between the oxygen-poor liquid and said intermediate vapour stream is effective to reduce the requirements for oxygen-poor liquid to be used as reflux at the top of the lower pressure column, thus enabling a part of the oxygen-poor liquid to be taken as product or used to generate refrigeration for the main heat exchanger. The method and plant according to the present invention with now be described by way of example with reference to the accompanying drawings in which : Figure 1 is a schematic circuit diagram of part of a first plant for separating air in accordance with the invention; Figure 2 is a schematic circuit diagram but part of the second plant for performing the method according to the invention; Figure 3 is a schematic circuit diagram but part of the third plant for performing the method according to the invention; Figure 4 is a schematic circuit diagram but part of the fourth plant for performing the method according to the invention; Figure 5 is a schematic circuit diagram of part of a fifth plant for performing the method according to the invention; Figure 6 is a schematic circuit diagram of part of a sixth plant for performing the method according to the invention; Figure 7 is a schematic circuit diagram of part of a seventh plant for performing the method according to the invention; Figure 8 is a schematic circuit diagram of the main heat exchanger refrigeration plant that can be used in conjunction with any one of the plants shown in Figures 1 to 7; and Figure 9 is a circuit diagram illustrating an alternative refrigeration plant to that shown in Figure 8. The accompanying drawings are not to scale. Like parts are identified in the respective Figures of the drawings by the same reference numerals. Referring to Figure 1 of the drawings, an air separation plant includes the double column 2 comprising a higher pressure column 4 and a lower pressure column 6. The columns 4 and 6 are linked by a condenser-reboiler 8 which provides reflux for the column 4 and reboil for the column 6. The higher pressure column 4 typically operates at a pressure in the order of 6 atmospheres absolute. It has an inlet 10 for air that has been purified by removal of water vapour and carbon dioxide therefrom and then cooled in a main heat exchanger (not shown) to a temperature suitable for its subsequent separation in the column 2. In a manner well known in the art, the air admitted to the column 4 is separated into an oxygen-rich fraction that collects at the bottom of the column 4, and a oxygen-poor fraction at the top of the column 4. Typically, the oxygen-poor fraction contains only a small proportion of oxygen and is thus substantially pure nitrogen. Oxygen-rich liquid is withdrawn from the bottom of the column 4 through an outlet 12 and is sub-cooled in a heat exchanger 14. The resulting sub-cooled liquid is divided into two parts. A first part is employed to provide refrigeraton for a condenser 18 associated with a side or argon column 16 that is employed to produce a crude argon product from a fluid stream withdrawn from the lower pressure column 6. Oxygen-rich fluid leaving the condenser 18 is then flashed through an expansion valve 20 into the lower pressure column 6 through an inlet 22. The other part of sub-cooled oxygen-rich liquid is flashed through valve 24 into the column 6 via an inlet 26. Oxygen-poor liquid is withdrawn from the top of the column 4 through an outlet 28 and is flashed through an expansion valve 30 into a phase separator 32. The separator 32 effective to separate under gravity the residual liquid from the flash gas. The oxygen-poor liquid from the separator 32 is sub-cooled by passage through heat exchanger 40 and the resulting sub-cooled oxygen-poor liquid is then flashed through valve 34 into the top of the column 6 via an inlet 36. The oxygen-rich liquid entering the column 6 is separated into oxygen and nitrogen fractions. Liquid oxygen is reboiled in the condenser-reboiler 8 and liquid nitrogen reflux is provided by the oxygen-poor liquid entering the top of the column 6 through the inlet 36. Typically, the column 6 operates at a pressure in the order of one and half atmospheres absolute. A gaseous oxygen product is withdrawn from the column 6 through an outlet 44 and a gaseous nitrogen product is withdrawn from the top of the column 6 through an outlet 46. In addition, an impure nitrogen stream is withdrawn from the column 6 through the outlet 48. The streams withdrawn from the column 6 through the outlets 46 and 48 are respectively passed through the heat exchangers 40 and 14, in that sequence, countercurrently to the liquid being sub-cooled. Heat exchange with these streams effects the sub-cooling of the oxygen-rich liquid and the oxygen-poor liquid. As is well known in the art, a local maximum argon concentration tends to occur at an intermediate level of the low pressure column 6. An outlet 50 is located at such intermediate level and a fluid stream comprising oxygen and argon is withdrawn through the outlet 50 and is passed into the bottom of the rectification column 16, in which it is separated into oxygen and argon. Liquid oxygen is returned through inlet 52 to the column 6 and a crude product liquid argon stream is withdrawn through outlet 54 from the top of column 16, if desired be further purified. Referring to the separator 32, a stream of vapour is withdrawn from the top thereof and is passed through the heat exchanger 14 countercurrently to the oxygen-rich liquid. This vapour stream is therefore effective to produce additional sub-cooling for the oxygen-rich liquid. It thus provides a means for transferring refrigeration from the oxygen-poor liquid to the oxygen-rich liquid. Moreover, the expansion valve 30 and separator 32 are preferably arranged so that the pressure at which the vapour is provided is between the operating pressures of the columns 4 and 6 and may for example be about 2.8 atmospheres. The pressure available in this vapour stream may be utilised by withdrawing the stream 42 as an additional nitrogen product, with additional compression of the product if desired, or by employing it as a working fluid in a refrigeration cycle used to provide refrigeration for the main heat exchanger or to perform some other heat exchange duty. For example, if the plant shown in Figure 1 is associated with the nitrogen liquefier (not shown) the stream 42 may be used as working fluid in a refrigeration cycle employed in such a liquefier. The vapour stream withdrawn from the separator 32 enables the oxygen-rich liquid to be sub-cooled to a lower temperature than conventional in the heat exchanger 14. Accordingly, appreciably less flash gas than normal is produced by passage of the liquid through the expansion valves 20 and 24, and such reduction of the amount of flash gas that is produced is believed to be beneficial to the column 6 and in effect enables a proportion of the oxygen-poor liquid to be diverted from its normal duties of providing reflux to the top of the column 6 and to be taken either as product or for the purposes of acting as working fluid in a refrigerant cycle. Generally, we prefer the amount of refrigeration provided to the column 6 by the oxygen-rich liquid and the oxygen-poor liquid to be substantially the same as in a conventional process. Accordingly, it will be appreciated that a greater proportion of this refrigeration will be provided by the introduction of the oxygen-rich liquid into the column than is normal. Since the oxygen-rich liquid is introduced into the column at higher temperatures than the oxygen-poor liquid, more refrigeration is provided at a higher temperature thereby enabling a more efficient separation to take place in the column 6. From such thermodynamic considerations, it will be appreciated that the increased efficiency in the operation of the column 6 enables additional product to be withdrawn from the oxygen-poor liquid. The plant shown in Figure 2 has many similarities to that shown in Figure 1 and the parts with a similar function to corresponding parts in Figure 1 will not be described again below. One main difference between the plant shown in Figure 1 and Figure 2 is that there is no flash separation of the oxygen-poor liquid withdrawn through the outlet 28 of the column 4. However, as in the plant shown in Figure 1, not all of the liquid withdrawn through the outlet 28 flows through the expansion valve 34. Some of the withdrawn liquid is taken from the stream flowing to the column 6 at a location upstream the heat exchangers 14 and 40. This part of the sub-cooled liquid is flashed through valve 63 and the resulting fluid is introduced through an inlet 62 into a bottom region of an auxiliary liquid-vapour contact column 60 whose function is to mix the fluid with a liquid oxygen stream withdrawn through outlet 64 from the bottom of the lower pressure column 6. (A process using such an auxiliary liquid vapour contact column 60 is the subject of our application EP-A-0 269 342.) This liquid oxygen is introduced into the column 60 at the top thereof through inlet 66. The mixing of the two streams in the column 60 is effective to provide additional refrigeration for the condenser 18 thereby allowing for an enhanced rate of argon condensation in the condenser 18 and hence an enhanced rate of production of liquid argon. Typically, the nitrogen entering the column 60 through the inlet 62 enters a volume of liquid nitrogen in which the condenser 18 is immersed or partially immersed. The condenser 18 thus functions as a reboiler for the column 60. A stream of gas is withdrawn from the column 60 through an outlet 68. This stream typically comprises a mixture of the nitrogen introduced therein through the inlet 62 and the oxygen introduced into the column 60 through the inlet 66. The stream withdrawn from the column 60 through the outlet 68 passes through the heat exchangers 40 and 14 in sequence counter-currently to the liquid being sub-cooled in such heat exchangers. The column 60 preferably operates at a pressure intermediate that of the higher pressure column 4 and the lower pressure column 6. For example, the stream withdrawn from the outlet 68 may have a pressure of 2.8 atmospheres and may be utilised, if relatively pure, as product nitrogen, or may be used as a working fluid in a refrigeration cycle. Similarily to the plant shown in Figure 1 the stream of oxygen-poor liquid introduced into the column 60 throught the inlet 62 is effective to provide refrigeration to the sub-cooled oxygen-rich liquid. In this instance, the transfer of refrigeration takes place in the condenser 18 of the argon side column 16. Unlike the plant shown in Figure 1, however, this transfer of refrigeration does not provide any substantial degree of additional sub-cooling, Rather, a greater proportion of the oxygen-rich liquid exiting the condenser 18 is in the liquid state and thus the proportion of the oxygen-rich fluid leaving the condenser 18 in the liquid state is greater. This has the effect of rendering the operation of the column 6 more efficient and thereby allows removal of the stream 68 from the column 60, which stream is in effect withdrawn from the oxygen-poor liquid, without loss of efficiency. Referring now to Figure 3, a further alternative plant is illustrated. In this plant there is also transfer of refrigeration from the oxygen-poor liquid to the oxygen-rich liquid in the condenser 18. However, in this example, no auxiliary column 60 is employed. Instead, the portion of the oxygen-rich liquid that is withdrawn from the sub-cooled liquid at a region intermediate the heat exchangers 38 and 40 is passed through the condenser 18 without undergoing any mixing with liquid oxygen or other oxygen stream taken from the column. After passage through the condenser 18, the oxygen-poor liquid is returned through the heat exchangers 40 and 14 in sequence, flowing counter-currently to the liquid being sub-cooled. The liquid nitrogen entering the condenser 18 is preferably expanded through an expansion valve 72 upstream of the condenser 18 such that it enters the condenser 18 at a pressure that is intermediate the average pressures of the columns 4 and 6, for example, a pressure of about 2.8 atmospheres absolute. The resulting nitrogen fluid leaving the condenser 18 after its passage through the heat exchangers 40 and 14 may be taken as a stream 70 of nitrogen product or alternatively may be used as a working fluid in a refrigeration cycle. The advantages to be obtained from the plant shown in Figure 3 are analogous to those to be obtained from the use of the plant shown in Figure 2, save that there is an additional advantage since it is not necessary to divert liquid oxygen from the lower pressure column 6 through the outlet 64 to the auxiliary column 60. The plant shown in Figure 4 of the accompanying drawings is particularly preferred since it enables refrigeration to be effectively transferred from the oxygen-poor liquid to the oxygen-rich liquid at two locations namely in the sub-cooling heat exchanger 14 and in the condenser 18. The plant shown in Figure 4 thus combines the phase separator 32 and associated expansion valve 30 of Figure 1 with the column 60 of Figure 2. Thus, in the plant shown in Figure 4 it is the liquid separated in the separator 32 which is subsequently divided intermediate the heat exchangers 14 and 40 to provide one portion of sub-cooled liquid that is introduced into the column 16 through the valve 34 and inlet 36 as reflux and another portion of sub-cooled liquid that is introduced through the inlet 62 into the column 60. The advantages provided are therefore the same as the advantages provided by the plants shown in Figures 1 and 2. Typically, though not necessarily, the phase separator 32 and the auxiliary column 60 operate at the same pressure as one another. The vapour stream from the phase separator 32 may be kept separate from the stream withdrawn from the column 60 through the outlet 68 or may be mixed with such stream. The steam of sub-cooled liquid that passes through the valve 24 may typically have a temperature of 89K upstream of the valve 24 such that the fluid entering the column 6 through the inlet 26 contains 5.7% by volume of flash gas. Referring now to Figure 5, just as in effect the plant shown in Figure 4 combines the phase separator 32 (with its associated expansion valve 30) with the auxilliary liquid-vapour contact column 60 of Figure 2 so the plant in Figure 5 combines the phase-separator 32 (and its associated expansion valve 30) with the oxygen-poor fluid passing through the condenser 18 that is illustrated in Figure 3 of the accompanying drawings. As in the plant shown in Figure 4, it is the sub-cooled liquid taken from the separator 32 that is divided into two parts intermediate the heat exchangers 14 and 40. Typically, the oxygen-poor stream exiting the condenser 18 is at substantially the same pressure as that of the vapour stream exiting the phase separator 32, though, if desired, these two fluids may be at different pressures from one another. Streams 42 and 70 may be kept separate from one another or may be mixed. Referring now to Figure 6, there is illustrated a plant in which there is transfer of refrigeration from the oxygen-poor liquid to a vapour stream withdrawn from an intermediate level of the low pressure column 6, this vapour stream being thereby condensed and the resulting liquid returned to the low pressure column. It is possible to take vaporised oxygen-poor liquid resulting from the heat exchange with the stream withdrawn from the column 6 and employ the vaporised liquid either as product or as working fluid in a refrigerant cycle to refrigerate the heat exchanger 6. Accordingly, a portion of the oxygen-poor liquid collecting at the top of the column 4 is withdrawn through an outlet 100, and is passed through a throttling or expansion valve 102 so as to reduce the pressure to which it is subjected to 4.7 atmospheres absolute. The resulting fluid is introduced into a heat exchanger 104 and is vaporised therein by heat exchange with a stream of vapour withdrawn from the column 6 through an outlet 106 which is at the same level as the outlet 50. The stream of vapour is condensed in the heat exchanger 106, and the condensate is returned to the low pressure column 6 through an inlet 108. The vaporised oxygen-poor liquid leaving the heat exchanger 104 is passed back through the heat exchanger 14 countercurrently to the oxygen-rich liquid withdrawn from the column through the outlet 12 and thus provides refrigeration to this stream. After leaving the heat exchanger 14, the oxygen-poor liquid flows out of the illustrated plant as stream 110 which may be used to provide refrigeration for the main heat exchanger 6 or may be taken as product. The oxygen-poor liquid withdrawn through the outlet 28 may all be employed as reflux in the low pressure column 6, as shown in Figure 6, or may be passed through throttling valve 30, and into phase separator 32, with only the resulting liquid being used as such reflux, while the resulting vapour flows through the heat exchanger 14 countercurrently to the oxygen-rich liquid and may be taken as stream 42 and used as product or as working fluid in a refrigeration cycle. Referring now to Figure 8 of the accompanying drawings, there is illustrated a main heat exchange unit 80 for use in association with any one of the plants shown in Figures 1 to 7. There are shown a number of passages through the heat exchanger 80. There is a passage 82 for a low pressure gaseous oxygen stream which is the one withdrawn from the outlet 44 of any one of the low pressure columns shown in Figures 1 to 7, a passage 84 for a low pressure product nitrogen gas stream which is taken from the product nitrogen stream exiting the heat exchanger 14 in any one of the plants shown in Figures 1 to 5, and a passage 86 for a low pressure waste nitrogen stream which is taken from the waste nitrogen stream exiting the heat exchanger 14 in any one of the plants shown in Figures 1 to 7. In addition, there is a passage 88 for air from which low volatility impurities such as water and carbon dioxide have been removed. The air typically enters the heat exchanger unit 80 at a pressure in the order of 6 atmospheres absolute. There is also a passage 90 for a product nitrogen stream at a pressure greater than the average operating pressure of the low pressure column 6 but lower than that of the high pressure column 4. The gaseous streams flow through the passages 82, 84, 86 and 90 countercurrently to the incoming air flowing through the passage 88. If the plant shown in Figure 8 is to be used in conjunction with that shown in Figure 1, it is the stream 42 shown in Figure 1 that passes through the passage 90. If the plant shown in Figure 8 is used in conjunction with the plant shown in Figure 2, it is the stream withdrawn from the outlet 68 of the column 60 that passes through the passage 90. If the plant shown in Figure 8 is to be used in conjunction with the plant shown in Figure 3, it is the stream 70 that flows through the passage 90. If the plant shown in Figure 8 is to be used in conjunction with the plant shown in Figure 4, the stream flowing through the passage 90 may be a mixture of the stream 42 with the gas withdrawn from the outlet 68 of the column 60. If the plant shown in Figure 8 is to be used in conjunction with the plant shown in Figure 5 the stream flowing through the passage 90 may be a mixture of the streams 42 and 70. If the plant shown in Figure 8 is to be used in conjunction with the plant shown in Figure 6, the stream flowing through the passage 90 may be the stream 110, and if the plant shown in Figure 8 is to be used in conjunction with the plant shown in Figure 7, the stream flowing through the passage 90 may be a mixture of the streams 42 and 110. In order to provide refrigeration for the heat exchanger 80, a portion of the pressurised air is withdrawn from upstream of the warm end of the heat exchanger 80 and is further compressed in a booster-compressor 92. This air then flows through the heat exchanger 90 co-currently with the rest of the air, is withdrawn from the heat exchanger 90 at an intermediate location thereof, is expanded with the performance of external work in an expansion turbine 94, which is desirably coupled to the compressor 92, and is then united with the waste nitrogen flowing through the passage 86 immediately upstream of its entry into the heat exchanger 80. The outlet pressure of the booster-compressor 92 and the outlet pressure of the turbine 94 may be selected such that the refrigeration provided by the turbine 94 is effective to meet all the requirements for refrigeration of the heat exchanger 80. Alternatively, for example, part of the waste nitrogen stream may be employed in a similar refrigeration circuit including a booster-compressor and a turbine to provide the rest of the refrigeration requirements of the heat exchanger 80. Figure 9 shows a heat exchanger similar to the one illustrated in Figure 8. In this example, however, it is the gas passing through the passage 90 that is first compressed and then expanded to provide refrigeration for the heat exchanger 80, and all the incoming air 88 is passed through the heat exchanger 80. Accordingly, the nitrogen stream leaving the passage 90 at the cold end of the heat exchanger 80 is compressed in a booster-compressor 96 and is then returned through the heat exchanger 80 co-currently with the air flowing through the passage 88 and is then withdrawn from the heat exchanger 80 at an intermediate region thereof and is expanded in an expansion turbine 98 for the performance of external work. Similarly, to the arrangement shown in Figure 8, the cold gas exiting the expansion turbine 98 is united with the waste nitrogen stream immediately upstream of the warm end of the heat exchanger 80. The expansion turbine 98 is preferably coupled to the booster-compressor 96.

Claims: A method of separating argon and oxygen products from air, comprising the steps of: (a) passing pressurised air through at least one main heat exchanger so as to reduce its temperature to a level suitable for its separation by cryogenic distillation; (b) passing such temperature-reduced air into the higher pressure column of a double rectification column comprising a higher pressure column, a lower pressure column, and a condenser-reboiler which provides reflux for the higher pressure column and reboil for the lower pressure column; (c) separating the air in the higher pressure column into an oxygen-rich liquid fraction and an oxygen-poor vapour fraction; (d) condensing said oxygen-poor vapour fraction in said condenser-reboiler, and employing some of the condensed vapour as reflux in the higher pressure column and collecting the remainder of the condensed vapour; (e) withdrawing oxygen-poor liquid from the higher pressure column, sub-cooling it and employing the sub-cooled liquid as reflux in the lower pressure column; (f) withdrawing oxygen-rich liquid from the higher pressure column, sub-cooling it and separating it in the lower pressure column into a nitrogen vapour fraction and a liquid oxygen fraction; (g) withdrawing a stream relatively rich in argon from the lower pressure column and separating it in a further rectification column into an argon-enriched fraction and an oxygen fraction, said further column having a condenser associated therewith which is refrigerated by said oxygen-rich liquid upstream of its introduction into the lower pressure column, and withdrawing argon-enriched fluid from the further column; (h) withdrawing nitrogen vapour and oxygen vapour from the lower pressure column and passing the withdrawn oxygen vapour and withdrawn nitrogen vapour through the main heat exchanger countercurrently to the incoming air; (i) vaporising a portion of the oxygen-poor liquid and either withdrawing it as product or expanding it with the performance of external work to perform a refrigeration duty; and (j) transferring refrigeration from the oxygen-poor liquid to the oxygen-rich liquid. A method as claimed in claim 1, wherein the refrigeration duty is the refrigeration of said at least one main heat exchanger. A method as claimed in claim 1 or claim 2, in which said portion of the oxygen-poor liquid is compressed at a stage intermediate its vaporisation and its expansion. A method as claimed in claim 3, in which the said expansion of the oxygen-poor liquid meets all the requirements for refrigeration of said main heat exchanger. A method as claimed in claim 1, in which some air is diverted from the incoming air, is further compressed, is cooled in the main heat exchanger, is expanded with the performance of external work, and is then employed to refrigerate the main heat exchanger. A method as claimed in claim 5, in which the expansion of the air meets the requirements of the main heat exchanger for refrigeration. A method as claimed in any one of the preceding claims, in which the transfer of refrigeration is effected by flashing the oxygen-poor liquid into a phase separator in which the resultant fluid is separated into liquid and vapour phases, withdrawing vapour from the separator and heat exchanging it against the oxygen-rich liquid being sub-cooled, the vapour subsequently being taken as product or expanded with the performance of external work, and the separated liquid forming the oxygen-poor liquid that is introduced into the lower pressure column. A method as claimed in claim 7, in which only some of the oxygen-rich liquid is passed through the condenser associated with the further rectification column, and some other of the oxygen-rich liquid is introduced into the further rectification column without passing through the condenser associated with the further rectification column. A method as claimed in claim 7 or claim 8, in which in order to transfer refrigeration from the oxygen-poor liquid to the oxygen-rich liquid, some of the sub-cooled oxygen-poor liquid is used to provide refrigeration for the condenser associated with the further rectification column. A method as claimed in any one of claims 7 to 9, in which the oxygen-poor liquid used to provide refrigeration for the condenser associated with the further rectification column is taken from said phase separator. A method as claimed in any one of claims 1 to 7, in which a stream of oxygen-poor liquid is heat exchanged with a vapour stream that is withdrawn from a level of the lower pressure column intermediate its top and bottom, said vapour stream thereby being at least partially condensed, and the resulting condensate is returned to the lower pressure column, and said oxygen-poor liquid being vaporised and then withdrawn as product or expanded with the performance of external work to perform a refrigeration duty. A method as claimed in claim 11, in which the said vapour stream is withdrawn from the same level of the lower pressure column as said stream relatively rich in argon. A plant for separating argon and oxygen products from air, comprising: (a) at least one main heat exchanger (30) for reducing the temperature of a stream of pressurised air to a level suitable for its separation by cryogenic distillation; (b) a double rectification column comprising a higher pressure column (4), a lower pressure column (6), and a condenser-reboiler (8) for providing reflux for the higher pressure column (4) and reboil for the lower pressure column (6); (c) an inlet (2) to the higher pressure column (4) for the reduced temperature air stream, the higher pressure column being able to separate the air into oxygen-poor and oxygen-rich fractions; (d) a first heat exchanger (40) for sub-cooling a stream of oxygen-poor liquid formed by condensation in the condenser-reboiler (8); (e) an inlet (36) to the lower pressure column (6) for the sub-cooled oxygen-poor liquid stream; (f) a second heat exchanger (14) for sub-cooling a stream of oxygen-rich liquid withdrawn from the bottom of the higher pressure column (4); (g) an inlet (26) to the lower pressure column (6) for the sub-cooled oxygen-rich liquid stream, the lower pressure column (6) being able to separate the oxygen-rich liquid into oxygen liquid and nitrogen vapour fractions; (h) a further rectification column (16) for separating a stream relatively rich in argon from the lower pressure column (6) into an argon-enriched fraction and an oxygen fraction; (i) a condenser (18) associated with the further rectification column (16), the condenser (18) having a passage for the sub-cooled stream of oxygen-rich liquid intermediate the second heat exchanger (14) and the inlet (26) to the lower pressure column (6); (j) an outlet (54) from the further rectification column (16) for argon-enriched fluid; (k) passages through the main heat exchanger (80) for oxygen vapour and nitrogen vapour streams, whereby in use the pressurised air stream is able to be reduced in temperature by countercurrent heat exchange; (l) means (30, 60, 18, 104) for vaporising a portion of the oxygen-poor liquid; (m) an outlet passage (42, 70, 110) for withdrawing as product the vaporised oxygen-poor liquid or an expansion turbine (98) for expanding vaporised oxygen-poor liquid to perform a refrigeration duty; and (n) heat exchange means (14, 18) for transferring refrigeration from the oxygen-poor liquid to the oxygen-rich liquid;

Inventors: RATHBONE, THOMAS
Applicants: THE BOC GROUP PLC
Application year: 1987
Main classification: F25J3
IPC codes: F25J3