AIR SEPARATION

Abstract: Air is compressed in compressor 200, is reduced in temperature to its dew point by passage through successive heat exchangers 204, 206, 210 and 212 (water and carbon dioxide being removed therefrom in heat exchangers 204 and 206). The air is introduced into a first distillation column 216 through an inlet 218. It is separated into oxygen and nitrogen therein. Nitrogen vapour is withdrawn from the top of the column 216 through outlet 228, is warmed, is compressed in compressors 234 and 236, is cooled, is expanded in expansion turbine 238 and is then passed through a reboiler 220 to provide reboil for the column 216. A resulting liquid stream is passed through a throttling valve 244 and liquid nitrogen is collected in phase separator 230. Some of this liquid nitrogen is returned to the column 216 as reflux, while another part is used to provide product. Argon-­ enriched oxygen vapour is withdrawn from the column 216 through an outlet 260, and is separated to form argon product in a further distillation column 226 which is reboiled by vapour from the top of the column 216.

Description: This invention relates to a process and plant for air separation. It is well known to separate air by cryogenic distillation into oxygen and nitrogen products. If a proportion of the nitrogen product is required in liquid state, a gaseous nitrogen product may be taken from the distillation means and liquefied. The liquefier may be independent of the air separation plant or may be integrated into the air separation plant. A liquid oxygen product may also be produced. The present invention relates to a process and plant of the aforementioned integrated kind. An example of a known integrated air separation-nitrogen liquefaction process and plant is disclosed in GB-A-1 258 568. This patent specification discloses using a single distillation column to separate incoming air into oxygen and nitrogen. Reboil for the bottom of the distillation column is provided by a high pressure nitrogen stream which, after condensation in the reboiler, is sub-cooled and used partly to provide reflux for the distillation column and also to provide liquid nitrogen product. Refrigeration for the plant is provided by taking portions of the high pressure nitrogen upstream of the reboiler and expanding each such portion in a turbine. It is found that this arrangement is relatively inefficient thermodynamically and there is scope for its improvement. It is an aim of the first aspect of the present invention to provide process and plant utilising an improved cycle for effecting reboil of a distillation column employed to separate the air into oxygen and nitrogen, providing reflux for the distillation, and providing refrigeration for the liquefaction of the nitrogen. According to the present invention there is provided a method of separating air, comprising removing carbon dioxide and water vapour from compressed air, reducing the temperature of the compressed air in heat exchange means to a temperature suitable for its separation into oxygen and nitrogen by cryogenic distillation, separating the air into nitrogen and oxygen in at least one distillation column, taking nitrogen vapour from said distillation column, warming the nitrogen countercurrently to the air in said heat exchange means, compressing some of the warmed nitrogen, cooling and reducing the temperature of such compressed nitrogen in said heat exchange means, taking at least some of the cooled nitrogen and subjecting it to expansion with the performance of external work, passing nitrogen through a reboiler associated with said at least one distillation column to provide reboil for the distillation, subjecting nitrogen leaving the reboiler to further cooling and temperature reduction and employing a part of the resulting liquid nitrogen as reflux in the distillation and taking another part of the resulting liquid nitrogen as product, characterised in that the nitrogen passing through the reboiler is the nitrogen that has been subjected to said expansion with the performance of external work, and the nitrogen leaving the reboiler is subjected to said further cooling and temperature reduction in said heat exchange means. The invention also provides a plant for separating air comprising at least one compressor for compressing the air, means for removing carbon dioxide and water vapour from the compressed air, heat exchange means for reducing the temperature of the air to a value suitable for its separation into oxygen and nitrogen by cryogenic distillation, at least one distillation column for separating air into oxygen and nitrogen, an outlet for nitrogen vapour from said at least one distillation column communicating with the inlet of at least one nitrogen compressor via said heat exchange means, at least one expansion turbine having an inlet for communicating with said nitrogen compressor, a reboiler associated with the said at least one distillation column, the outlet of said reboiler communicating with means for providing liquid nitrogen reflux for said at least one distillation column and also with an outlet for product liquid nitrogen, characterised in that said inlet to said expansion turbine communicates with said nitrogen compressor via the heat exchanger means, the said expansion turbine has an outlet communicating with an inlet to said reboiler, and the outlet of said reboiler communicates via said heat exchange means with said means for providing liquid nitrogen reflux. Preferably, a gaseous nitrogen product is also taken from said at least one distillation column. It is also preferred to take an oxygen product from said at least one distillation column, typically in liquid state. The column for which the reflux is provided is preferably the same column as that with which the reboiler is associated. The nitrogen withdrawn from the distillation column is typically compressed in a multi-stage compressor to a pressure in excess of its critical pressure. The compressed nitrogen is preferably taken for expansion with the performance of external work at a pressure in the range 50 to 75 atmospheres and at a temperature preferably in the range 150 to 170 K. It is not essential to take all the compressed nitrogen for expansion with the performance of external work. If desired, some of the compressed nitrogen may be liquefied without passing through work-expansion means and the reboiler associated with the distillation column. At the completion of work expansion the nitrogen preferably has a pressure in the range 12 to 20 atmospheres absolute and is preferably a saturated vapour. Liquefaction of the nitrogen is then preferably effected in the reboiler. The work expanion is typically conducted in a single turbine which if desired may be employed to drive a compressor employed in the compression of the nitrogen or the air. Preferably, the liquid nitrogen leaving the reboiler is sub-cooled in the heat exchange means and then subjected to a plurality of flash separation steps, to provide liquid nitrogen and a plurality of flash gas streams. The flash gas streams are desirably returned through the heat exchange means countercurrently to the incoming air and therefore provide refrigeration for the heat exchange means. If desired, at least three flash separation steps or alternatively just two such steps may be used. Additional refrigeration for the heat exchange means may be obtained by withdrawing a waste nitrogen vapour stream from the said distillation column, increasing its temperature in said heat exchange means, subjecting it to expansion with the performance of external work, typically in an expansion turbine, and returning the gas through the heat exchange means. The waste nitrogen may then be vented to the atmosphere. Net refrigeration for the heat exchange means between ambient temperature and the temperature of the compressed nitrogen at the start of its work expansion may be provided by any conventional means. Typically, a further expansion turbine employing nitrogen as the working fluid may be used to provide net refrigeration in the lower part of this temperature range, and a Freon (fluorocarbon refrigerant) refrigeration cycle used to provide net refrigeration for the rest of this temperature range. Alternatively a mixed refrigerant cycle may be used to provide refrigeration over the whole of this temperature range. Typically, at least one stream of argon-enriched fluid is withdrawn from the said distillation column and subjected to separation in a further distillation column to provide an argon product and preferably further oxygen product. The argon-enriched stream may be withdrawn as vapour or liquid. Alternatively, both liquid and vapour streams may be withdrawn. Preferably, the method according to the invention additionally includes the steps of taking a stream of compressed air, reducing the temperature of the stream by heat exchange, taking at least some of the stream and subjecting it to expansion with the performance of external work, employing the expanded stream (typically at its dew point) to further cooling and temperature reduction by heat exchange whereby to form a sub-cooled liquid air stream, and passing the liquid air stream through a throttling valve into the distillation column. Preferably, from 5 to 10% of the total air introduced into distillation column or columns for separation. The method and plant according to the present invention will now be described by way of example with reference to the accompanying drawings which: Figure 1 is a schematic flow diagram of a first plant according to the invention for separating air, Figure 2 is a schematic flow diagram of a second plant according to the invention for separating air, and Figure 3 is a schematic flow diagram of a third plant according to the invention for separating air. Referring to Figure 1 of the accompanying drawings, 122 854 sm³/hr of air flow into a compressor 2 and are compressed to a pressure of 6.2 atmospheres absolute. (As used herein, 1 sm³/hr = 1 m³/hr at 15°C and 1 atmosphere absolute). The resulting compressed air is cooled in a water after cooler 4 and is then passed through a purification unit 6 typically comprising molecular sieve adsorbers effective to remove water vapour and carbon dioxide from the air. The compressed air then enters heat exchange means 8 comprising heat exchangers 10, 12 and 14. If desired, the heat exchangers 10, 12 and 14 may be fabricated as a single heat exchange block. The air enters the heat exchanger 10 at approximately ambient temperature and leaves it at a temperature in the order of 113 K, at which temperature it enters the heat exchanger 12. The air leaves the heat exchanger 12 at its dew point and is then divided into two parts. The major portion of the air flows at a rate of 100,000 sm³/hr into a single distillation column 18 through an inlet 20. The column 18 operates at a pressure of about 6 atmospheres absolute and is adapted to separate the air into oxygen and nitrogen fractions. The distillation column 18 is provided with a reboiler 22 at its bottom to form oxygen vapour and an inlet 24 at its top for liquid nitrogen reflux. The reboiler 22 boils liquid oxygen collecting at the bottom of the column 18 and causes vapour to ascend the column, while the inlet 24 for liquid nitrogen is able to provide a downward flow of liquid nitrogen reflux. Nitrogen vapour is withdrawn from the column 18 from an outlet 26 and passed through the heat exchangers 14, 12 and 10 in sequence. A minor proportion (13851 sm³/hr) is withdrawn as product while the major proportion (178 310 sm³/hr) enters a multi-stage compressor 36 which raises the pressure of the nitrogen from 5.6 atmospheres absolute typically to 59 atmospheres absolute. The compressed nitrogen is then cooled in a water cooler 38 and is passed into the heat exchanger 10 and flows therethrough co-currently with the incoming air. 148 758 sm³/hr of compressed nitrogen is withdrawn from the heat exchanger 10 at a temperature of 159 K and is passed into an expansion turbine 40 in which it is expanded to a pressure of 17.7 atmospheres (to give a reboiler delta T of 1.3 K) with the performance of external work. The nitrogen leaves the expansion turbine 40 as saturated vapour at a temperature of 113.6 K. It then passes through the reboiler 32 and thus provides the necessary heating to effect reboiling of liquid oxygen in the bottom of the column 18 while being itself condensed so that it leaves outlet 34 of the reboiler 22 as a saturated liquid. This liquid is then divided into two parts. A major stream of liquid is taken therefrom at a rate of 115 630 sm³/hr and is flashed through throttling valve 44 into a phase separator 46 operating at a pressure of 8.2 atmospheres. The flash gas from the separator 46 passes through the heat exchangers 12 and 10 countercurrently to the incoming air at a flow rate of 23 499 sm³/hr and is then returned to a suitable stage of the compressor 36. Liquid flows out of the phase separator 46 at a rate of 92 031 sm⁻³/hr and a major part of it passes through the heat exchanger 14 from its warm end to its cold end at a flow rate of 70 734 sm³/hr. It then flashes through a throttling valve 48. The remainder of the liquid flashes through a further throttling valve 49. The remainder of the liquid nitrogen leaving the boiler 22 enters the warm end of the heat exchanger 12 at a rate of 33 228 sm³/hr and leaves this heat exchanger at a temperature of about 101 K. It then flows through the heat exchanger 14 from its warm end to its cold end leaving the cold end at a temperature of about 98 K. The liquid then flashes through a throttling valve 50 and the resulting 2-phase mixture is mixed with those issuing from the throttling valves 48 and 49. The fluid issuing from the valves 48 and 50 is further combined with that part of the compressed nitrogen stream that does not flow through the expansion turbine 40. Such part of the compressed nitrogen stream exits the cold end of the heat exchanger at a temperature of 113 K and then flows through the heat exchangers 12 and 14 leaving the cold end of the latter at a temperature of about 98.5 K (and at a flow rate of 53 051 sm³/hr). This fluid is then flashed through throttling valve 52 and is united with the fluid mixtures leaving the throttling valves 48, 49 and 50. The resulting fluid flows at a rate of 178 310 sm³/hr into a phase separator 56 where it is separated into liquid and gas at a pressure of 5.8 atmospheres. A first stream of liquid is taken from the separator 56 at a rate of 107 004 sm³/hr and forms the predominant part of the reflux stream introduced into the column 18 through the inlet 24. In addition, gas is withdrawn from the separator 56 at a rate of 6122 sm³/hr and is combined with the nitrogen stream leaving the top of the distillation column 18 through the outlet 26. In can thus be seen that there is a nitrogen circuit extending from the outlet 26 of the distillation column 18 through the compressor 36, the expansion turbine 40, the reboiler 22 and returning to the heat exchanger via the phase separator 56. The circuit is able provide most of the reflux, and all of the reboil for the distillation column 18 as well as providing a considerable amount of the refrigeration required for the heat exchangers 10, 12 and 14. Moreover, these results may be achieved at a relatively high thermodynamic efficiency in comparison with the comparable parts of the plant descibed in the drawing accompanying UK patent specification 1 258 568. A liquid nitrogen product is obtained from the separator 56 by taking a second stream liquid nitrogen at a flow rate of 65 184 sm³/hr therefrom and passing it through a sub-cooling heat exchanger 57, flashing it through throttling valve 58 into a phase separator 60 operating at a pressure of 2.7 atmospheres absolute. Flash gas is withdrawn from the phase separator 60 at a rate of 5381 sm³/hr and passed through the heat exchanger 57 countercurrently to the second stream of liquid nitrogen withdrawn from the phase separator 56. A liquid nitrogen product stream is withdrawn from the phase separator 60 at a flow rate 25 748 sm³/hr. Further liquid nitrogen is withdrawn from the phase separator 60 and is utilised in a manner to be described below. In addition to providing nitrogen product and heat exchange fluid, the distillation column 18 also provides liquid oxygen product which is withdrawn from the bottom of the column through an outlet 42 at a rate of 18 470 sm³/hr. In addition, the column 18 is used to provide a stream of oxygen relatively rich in argon. This stream is taken from the outlet 28 at a level a little below that at which the argon concentration in the column 18 is a maximum. It is separated in a further distillation column 62 operating at a pressure of about 1.3 atmospheres. The column 62 is provided with a condenser 64 at its top and a condenser-reboiler 66 at its bottom. The condenser reboiler 66 provides reflux for a second distillation column 68 having an inlet 70 for a minor portion (22 854 sm³/hr) of the compressed air withdrawn from the cold end of the heat exchanger 12. The column 68 operates at a similar pressure to the column 18 and provides for the column 18 a stream of oxygen-rich liquid which is withdrawn from the column 68 through the outlet 72 and enters the distillation column 18 through the inlet 30. This stream of oxygen-rich liquid helps to render the operation of the column 18 more efficient by reducing its overall demand for liquid nitrogen reflux through the inlet 24. The column 68 more importantly provides the necessary heat for reboiling liquid oxygen separated in the column 62. Column 68 also provides a stream of oxygen-poor liquid at a rate of 9 996 sm³/hr which is withdrawn from an outlet 74 at an upper region thereof and is united with the first stream of liquid nitrogen withdrawn from the phase separator 56 to provide the liquid nitrogen reflux that is introduced into the column 18 through the inlet 24. The feed for the column 62 is provided by withdrawing the argon enriched oxygen from the column 18 through the outlet 28 at a flow rate of 8350 sm³/hr and introducing the stream into the heat exchanger 10 at its cold end, and withdrawing it from an intermediate region of the heat exchanger 10 at a temperature of about 137 K passing it to an expansion turbine 76 in which it is expanded with the performance of external work to the operating pressure of the column 62. The expanded fluid is then introduced into the column 62 through an inlet 78. Reflux for the column 62 is provided by withdrawing a second stream of liquid nitrogen from the phase separator 60 at a flow rate of 33 562 sm³/hr and passing it through the condenser 64. The resultant vaporised nitrogen leaving the condenser 64 is united with the flash gas separator 60 upstream of the cold end of the heat exchanger 57. The combined gases after leaving the warm end of the heat exchanger 57 flow through the heat exchangers 14, 12 and 10, in sequence, and thus a product nitrogen stream may be formed a flow rate of 38 444 sm³/hr and a pressure of about 2.5 atmospheres. By employing both a condenser 64 and a reboiler 66 the operation of the argon column 62 may be made relatively efficient in comparison with that described in the aforementioned UK patent specification. Accordingly, a relatively high number of trays, for example in the order of 100, may be employed in the column 62. A crude liquid argon product typically containing in the order of 2% by volume of oxygen is withdrawn from the top of the column 62 through an outlet 80 at a rate of 1058 sm³/hr and a further liquid oxygen product stream is withdrawn from the bottom of the column 62 through an outlet 82 at a rate of 7292 sm³/hr. The requirements for refrigeration of the above described process are not met wholly by the operation of the expansion turbine and associated circuits. Further refrigeration is provided by withdrawing a stream of waste nitrogen from a few trays below the top of the column 18 through the outlet 54 at a flow rate of 17000 sm³/hr and passing the stream through the heat exchangers 14 and 12 in sequence and then introducing into the heat exchanger 10. The stream of waste nitrogen is then withdrawn from the heat exchanger 10 at a temperature of 140 K and expanded to about atmospheric pressure in a further expansion turbine 84. The resulting expanded waste nitrogen stream is then introduced at a temperature of 96 K into the cold end of the heat exchanger 14 and flows through the heat exchanger 14, the heat exchanger 12 and the heat exchanger 10 in sequence and is then vented to the atmosphere at about ambient temperature or preferably used regenerate molecular sieve adsorbers employed to extract carbon dioxide and water vapour from the incoming air. Refrigeration for the warm end of the heat exchanger 10 is provided by refrigeration unit or means 86. Such unit may comprise a mixed refrigerant cascade cycle or a combination of Freon refrigeration unit and a "warm" nitrogen expansion turbine cycle which turbine may typically have an inlet temperature in the order of 200 K and an outlet temperature of about 160 K. Many changes and modifications to the plant shown in Figure 1 are possible without departing from the scope of the inventions described herein. For example, reduction in pressure of the second liquid nitrogen stream withdrawn from the phase separator 56 may be accomplished in at least two successive flash separation stages rather than the single stage (comprising valve 58 and phase separator 60) as shown in Figure 1. In addition, the liquid oxygen product withdrawn from the column 18 through the outlet 42 may be sub-cooled and subjected to a plurality of flash separation stages in order to provide a liquid oxygen product at nearer atmospheric pressure and gaseous oxygen product which can be returned through the heat exchangers 14, 12 and 10 in sequence, thereby providing additional refrigeration for such heat exchangers. Furthermore, it is not essential to use molecular sieve adsorbers or other means 6 to remove the carbon dioxide and water vapour from the incoming air. Instead, the heat exchanger 10 may be built as a reversing heat exchanger. In this instance, however, the waste nitrogen stream withdrawn from the column 18 will typically be used as the stream for the regenerating the heat exchanger 10 and consequently its flow rate will need to be substantially greater than described above. In addition, additional boost compressors (not shown) may be employed to provide further compression of the nitrogen leaving the compressor 36 or the air leaving the compressor 2. For example, three such booster-compressors may be employed, one driven by the turbine 40, another by the turbine 76, and a third by the turbine 84. A further boost-compressor may be associated with any turbine employed in the refrigeration means 86. Another improvement that can be made to the plant shown in Figure 1, is to withdrawn argon-enriched liquid from the distillation column 18 (typically from below the outlet 18) and pass it through an expansion valve into the column 62 (typically at a level below the inlet 78) to enhance the proportion of liquid oxygen produced by the column 62. It is alternatively or additionally possible to pass a liquid oxygen stream from the column 18 into the column 62. There is considerable flexibility in the relative rates at which oxygen and nitrogen products can be produced by the plant shown in the drawing. In the above described example all the oxygen product from the column 18 is produced as liquid. If desired the flow of nitrogen through the reboiler 22 can be increased to produce gaseous oxygen product (that can be taken from the column 18 at a level below the lowermost tray (not shown) in the column 18). Referring to Figure 2 of the accompanying drawings, 130 000 sm³/hr of air flow into a compressor 200 and are compressed to a pressure of 6.2 atmospheres absolute. The air stream then flows through a first heat exchanger 204 in which it is cooled from a temperature of 298K to a temperature of 235K. The air stream is then further cooled in a heat exchanger 206 to a temperature of 159K, and in a heat exchanger 210 to a temperature of 113.6K. The air is then further cooled in a heat exchanger 212 to a temperature of 101K (its dew point) and is introduced into a first or main distillation column 216 at a pressure of 6 atmospheres absolute through an inlet 218. The distillation column 216 is provided at its top with an inlet 222 for substantially pure liquid nitrogen reflux and at its bottom with a reboiler 220. In addition, there is a condenser-reboiler 224 which condenses vapour at the top of the column 216 (to provide additional reflux for the column) and provides reboil at the bottom of a second distillation column 206. Nitrogen that passes through a reboiler 220 and into the inlet 222 of the column 216 is provided in a nitrogen refrigeration and liquefaction cycle that starts and ends in the column 216. Thus, substantially pure nitrogen vapour is withdrawn from the top of the column 216 through an outlet 228 at a rate of approximately 206,747 sm³/hr and a temperature of 96K and is mixed with approximately a further 9,407 sm³/hr of nitrogen taken from a phase separator 230 (whose place in the cycle will be described below). The combined nitrogen stream then flows through a heat exchanger 214 from its cold and to its warm end and is thereby raised in temperature to 98K. It then flows through the heat exchangers 212,210, and 206 countercurrently to the incoming air flow and leaves the heat exchanger 206 at a temperature of about 230K. The stream is then divided into minor and major parts. The major part of this nitrogen stream (156 249 sm³/hr) is then expanded in expansion turbine 208 with the performance of external work. The expanded nitrogen stream leaves the turbine 208 at a temperature of 155K and a pressure of 1.1 atmospheres absolute. The expanded nitrogen stream is then warmed to about 298K by passage through the heat exchanger 206 and then the heat exchanger 204. The expanded nitrogen stream is then divided. A first subsidiary stream flowing at a rate of 51,575 sm³/hr is taken as product, and the remainder forms a second subsidiary stream flowing at a rate of 104 674 sm³/hr which is compressed in a compressor 231. The nitrogen stream leaves the compressor 231 at a pressure of about 2.8 atmospheres absolute and is mixed with a further stream of nitrogen (whose formation will be described below). The mixed stream is compressed in a further compressor 232. The nitrogen stream leaves the compressor 232 at a rate of 151137 sm³/hr and a pressure of about 5 1/2 atmospheres absolute. It is then mixed with the minor part of the nitrogen stream (51249 sm³/hr) from the heat exchanger 206 and the resulting mixed stream is compressed in a compressor 234 to a pressure of 8 atmospheres. The resulting mixed stream at a pressure of 8 atmospheres is mixed at a temperature of 298K with a yet further stream of nitrogen flowing at a rate of 26089 sm³/hr and is compressed in compressor 236. The resulting compressed stream flowing at a rate of 237131 sm³/hr then passes through the heat exchangers 204 and 206 co-currently with the incoming air, thereby being cooled to a temperature of 159K. The stream is then divided into two parts. The major part comprises a flow of 174640 sm³/hr which is passed to the inlet of an expansion turbine 238. The nitrogen stream is expanded with the performance of external work in the turbine at a pressure of 17.6 atmospheres and a temperature of 113.6K. This fluid stream then passes through the reboiler 220 of the first distillation column 216 and thus provides reboil at the bottom of column 216, the nitrogen itself being at least partially, and normally fully condensed. The resulting nitrogen leaves the reboiler 220 and is then divided into a major stream and a minor stream. The major stream is flashed through a throttling valve 240 at a rate of 130610 sm³/hr and is thereby reduced in pressure to 8 atmospheres. The resulting two-phase mixture is then separated in a phase separator 242. A vapour stream is withdrawn from the separator 242, is warmed to 298K by passage through the heat exchangers 212, 210, 206 and 204 in sequence and is used as the nitrogen which is mixed with the 8 atmosphere stream of nitrogen between the compressors 234 and 236. The liquid collected in the phase separator 242 is used to form a further two-phase stream which is passed to a further phase separator 230. Accordingly, a first stream of this liquid is flashed through a throttling valve 244 at a rate of 86434 sm³/hr and the resulting liquid-vapour mixture passes to the phase separator 230. Upstream of the phase separator 230, this liquid-vapour mixture is mixed with a further stream of liquid-vapour mixture which is formed by taking another stream of liquid nitrogen at a rate of 18087 sm³/hr from the bottom of the phase separator 242 (at a temperature of 101K), sub-cooling the stream to a temperature of 98K by passage through the heat exchanger 214, and then flashing through a throttling valve 246, thereby reducing its pressure to 5.8 atmospheres absolute. Another contribution to the liquid-vapour mixture passing to the phase separator 230 is formed from the minor stream of liquid from the reboiler 220 which by-passes the valve 240 and flows at a rate of 44030 sm³/hr (being at a pressure of 17.6 atmospheres absolute) through the heat exchanger 212, being thereby cooled to a temperature of 101K. The resulting liquid is then further cooled by passage through heat exchanger 214 to a temperature of 98K. This cooled nitrogen is then flashed through a throttling valve 250 and is then united with the liquid vapour mixture passing to the phase separator 230. A fourth contribution to the liquid vapour mixture passing to the phase separator 230 is formed by the minor part of the nitrogen stream from the heat exchanger 206 that by-passes the expansion turbine 238. This part of the nitrogen stream flows at a rate of 62491 sm³/hr and a pressure of 59 atmospheres absolute and continued its passage through the heat exhangers, flowing from the warm end to the cold end of heat exchangers 210, 212 and 214 in sequence. The nitrogen leaves the warm end of the heat exchanger 214 at a temperature of 98K and is then passed through a throttling valve 252 to reduce its pressure to 5.8 atmospheres. The resulting liquid-vapour mixture is as aforesaid mixed with the rest of the liquid-vapour mixture passing to the phase separator 230. A first stream of liquid nitrogen is withdrawn from the phase separator 230 at a rate of 201635 sm³/hr and is introduced into the top of the distillation column 216 through inlet 222 to serve as reflux. As will be described more fully below, a second stream of liquid nitrogen withdrawn from the phase separator 230 is used to form nitrogen product, and to provide condensation of vapour at the top of the second distillation column 226 in which a liquid argon product is formed. A stream of impure nitrogen, typically containing about 0.2% of oxygen is withdrawn from the first distillation column 216 at a rate of 19500 sm³/hr through an outlet 254. This stream flows through the heat exchangers 212, 210 and 206 in sequence countercurrently to the flow of incoming air and is thus cooled to a temperature of 230K. The stream is then expanded with the performance of external work in an expansion turbine 256. The stream leaves the expansion turbine 256 at a pressure of 1.1 atmospheres absolute and a temperature of 155K. It is then warmed to 298K by passage through the heat exchangers 206 and 204 in sequence. The resultant waste stream is vented to the atmosphere. Liquid oxygen is also withdrawn from the first distillation column 216 at a rate of 15388 sm³/hr through an outlet 258 at the bottom thereof. The liquid oxygen is then preferably passed through a throttling valve (not shown) in the column 226, and liquid oxygen product is taken from the column 226 as described below. In addition to providing nitrogen and oxygen fractions, the distillation column 216 also provides an argon-enriched oxygen-vapour feed to the second distillation column 226. Accordingly, argon-enriched oxygen vapour typically containing in the order of 9% by volume of argon is withdrawn through an outlet 260 at a rate of 13050 sm³/hr from a level in the column 216 below that of the air inlet 218 and is passed to the warm end of the heat exchanger 212 and is then liquefied by passage through the heat exchanger 212. The resulting liquid argon-oxygen mixture at a temperature of 101K is then sub-cooled by passage through the heat exchanger 214. The sub-cooled argon-oxygen liquid mixture is passed through a throttling valve 262 and is introduced into the second column 226 through an inlet 264 at a pressure of 1.3 atmospheres absolute. Reboil for the second distillation column 226 is provided by the condenser-reboiler and reflux is provided by operation of a condenser 266 in the top of the column 226. Cooling for the condenser 266 is provided by taking a stream of liquid nitrogen from the phase separator 230 at a rate of 76950 sm³/hr and sub- cooling it in a heat exchanger 268, thereby reducing its temperature from 96 to 90K. The resulting sub-cooled nitrogen is then flashed through a throttling valve 270 and the resulting liquid-vapour mixture is passed to a phase separator 272 operating at a pressure of 3 atmospheres absolute. A first stream of liquid is withdrawn from the phase separator 272 at a rate of 41389 sm³/hr and is passed through the condenser 266 thus condensing vapour and hence providing reflux in the column 226 while being vaporised itself. The resulting vapour is mixed with vapour withdrawn from the top of the phase separator 272, and thus-formed mixture is returned through the heat exchanger 268 countercurrently to the flow therethrough of liquid nitrogen from the phase separator 230. The nitrogen vapour is thus warmed to 94K. It is subsequently warmed to 298K by passage through the heat exchangers 214, 212, 210, 206 and 204 in sequence and forms the gas stream that is mixed with the one leaving the compressor 231. A second stream of liquid nitrogen is withdrawn from the phase separator 272 at a flow rate of 30486 sm³/hr and is sub-cooled in a heat exchanger 274, its temperature thereby being reduced from 90 to 88K. The sub-cooled liquid nitrogen is then flashed through a throttling valve 276 and the resulting two-phase mixture is collected in a phase separator 278. Saturated liquid nitrogen product at a pressure of 1.3 atmospheres absolute is withdrawn from the phase separator 278 at a rate of 27579 sm³/hr through an outlet 280. Nitrogen vapour is withdrawn from the top of the phase separator 278 at a rate of 2907 sm³/hr and is progressively warmed to 298K by passage through heat exchangers 274, 268, 214, 212, 210 206 and 204 in sequence. This gaseous nitrogen is also collected as product. By providing reboil and reflux in the second distillation column, it is possible to separate a liquid oxygen product as well as a liquid argon product therein. A stream of liquid argon typically containing up to 2% by volume of oxygen impurity is withdrawn from the distillation column 226 at a flow rate of 1178 sm³/hr and a pressure of 1.2 atmospheres absolute through an outlet 281 positioned at or near the top of the column 226. Liquid oxygen product is withdrawn from the bottom of the column 226 through an outlet 282 at a flow rate of 27260 sm³/hr and a pressure of 1.4 atmospheres absolute. This liquid oxygen product comprises that formed by fractionation in the column 226 supplemented by the liquid oxygen withdrawn through the outlet 258 from the first distillation column 216 which is, if desired, sub-cooled, passed through a throttling valve (not shown) and introduced into the bottom of the column 226. It will be appreciated that refrigeration for the heat exchanger 206 is provided by the expansion of the nitrogen stream in turbine 208 and the impure nitrogen stream in the expansion turbine 266, while net refrigeration for the heat exchanger 204 operating between 235 and 300K is met by a mechanical refrigeration machine 284 using Freon (registered trademark) as a working fluid. If desired the heat exchangers 204, 206, and 210 may be made as one heat exchange block. It is additionally or alternatively desirable to form the heat exchangers 204 and 206 as a reversing heat exchanger such that the waste nitrogen stream from the turbine 256 can be used to sublime deposits of ice and solid carbon dioxide left on the heat exchange passages in such heat exchangers by the passage of air therethrough. The operation of reversing heat exchangers is well known in the art and will not be described further herein. Typically, the compressors 231, 232, 234 and 236 may comprise separate stages of a single multi-stage rotary compressor. Each such compressor will have its own water cooler associated therewith to remove the heat of compression. In addition, the expansion turbines 208, 238 and 256 may each drive a booster compressor (not shown) used in the compression of the incoming air or nitrogen. Many modifications to the plant shown in Figure 2 are possible without departing from the invention. For example there may be several liquid-vapour contact trays in the column 216 disposed between the level of the outlet 228 and the condenser-reboiler 224, with there being an additional outlet for nitrogen provided above the uppermost of these trays. This enables a particularly pure nitrogen stream, typically containing less than 1vpm of oxygen, to be withdrawn from the column 216. In another modification the column 216 may be provided as two separate vessels, typically arranged one above the other, with the lower vessel passing vapour from its top to the bottom of the upper vessel and receiving liquid at its top from the bottom of the upper vessel. The upper vessel may be used as a nitrogen impurification vessel, the waste nitrogen stream being wit drawn through the outlet 254 from the lower vessel. A further modification to the plant shown in Figure 2 is illustrated in Figure 3. Like parts occurring in Figures 2 and 3 are indicated by the same reference numerals. Referring to Figure 3, not all of the reboil requirements of the first distillation column 216 are met by the nitrogen flowing through the reboiler 220. Instead, there is an additional reboil cycle in which the working fluid is air. Accordingly, air is compressed in a compressor 300 to a pressure of 47 atmospheres absolute. After removal of its heat of compression by a water cooler (not shown) the compressed air is cooled to a temperature of 159K by passage through heat exchangers 204 and 206 in sequence. This air stream then passes out of the heat exchanger 206 and is expanded in an expansion turbine 302 to a pressure of 15.6 atmospheres and a temperature of 113.6K. The resultant expanded air then passes through the reboiler 220 and is condensed by passage therethrough. The condenser air then enters the warm end of the heat exchanger 212 at a temperature of 113.6K and flows through the heat exchangers 212 and 214 in sequence leaving the cold end of the heat exchanger 214 at a temperature of 98K. The resulting sub-cooled liquid air is then flashed through a throttling valve 304 and the resultant liquid-vapour mixture enters the column 216 at a pressure of 5.9 atmospheres absolute through an inlet 308 located a few trays above that of the inlet 218. Typically, the air flow through the turbine 302 is about 7% of the total gas flow through the reboiler 220, and about 8% of the total air introduced into the first distillation column 216. By introducing some of the air into the first distillation column 216 as liquid the overall column and cycle efficiencies are improved.

Claims: A method of separating air, comprising removing carbon dioxide and water vapour from compressed air, reducing the temperature of the compressed air in heat exchange means to a temperature suitable for its separation into oxygen and nitrogen by cryogenic distillation, separating the air into nitrogen and oxygen in at least one distillation column, taking nitrogen vapour from said distillation column, warming the nitrogen countercurrently to the air in said heat exchange means, compressing some of the warmed nitrogen, cooling and reducing the temperature of such compressed nitrogen in said heat exchange means, taking at least some of the cooled nitrogen and subjecting it to expansion with the performance of external work, passing nitrogen through a reboiler associated with said at least one distillation column to provide reboil for the distillation, subjecting nitrogen leaving the reboiler to further cooling and temperature reduction and employing a part of the resulting liquid nitrogen as reflux in the distillation and taking another part of the resulting liquid nitrogen as product, characterised in that the nitrogen passing through the reboiler is the nitrogen that has been subjected to said expansion with the performance of external work, and the nitrogen leaving the reboiler is subjected to said further cooling and temperature reduction in said heat exchange means. A method as claimed in claim 1, in which the column for which the reflux is provided is the same column as that with which the reboiler is associated. A method as claimed in claim 1 or claim 2, in which the compressed nitrogen is taken for expansion at a pressure in the range 50 to 75 atmospheres and at a temperature in the range 150 to 170 K. A method as claimed in any one of the preceding claims, in which some of the compressed nitrogen is liquefied without being subjected to expansion with the performance of external work and without being passed through the reboiler associated with the said distillation column. A method as claimed in any one of the preceding claims, in which at the completion of work expansion, the nitrogen has a pressure in the range 12 to 20 atmospheres absolute. A method as claimed in claim 5 in which the nitrogen at the end of work expansion is in saturated vapour state. A method as claimed in any one of the preceding claims, in which nitrogen leaves the reboiler in liquid state and at least some of it is sub-cooled in the heat exchange means and is then subjected to a plurality of flash separation steps to form liquid nitrogen and a plurality of flash gas streams. A method as claimed in any one of the preceding claims, in which a gaseous nitrogen product and an oxygen product are also produced, and additionally including the step of withdrawing a stream of argon-enriched fluid from the said distillation column and subjecting it to separation in a further distillation column to provide an argon product. A method as claimed in any preceding claim, additionally including the steps of taking a stream of compressed air, reducing the temperature of the stream in the heat exchange means, taking at least some of the stream and subjecting it to expansion with the performance of external work, employing the expanded stream to provide additional reboil in a reboiler the said at least one distillation column, subjecting the stream leaving the reboiler to further cooling and temperature reduction in the heat exchange means whereby to form a sub-cooled liquid air stream, and passing the liquid air stream through a throttling valve into the distillation column. A plant for separating air comprising at least one compressor (2;200) for compressing air, means (6;204,206) for removing carbon dioxide and water vapour from the compressed air, heat exchange means (10,12,14; 204,206,210,212,214) for reducing the temperature of the air to a value suitable for its separation into oxygen and nitrogen by cryogenic distillation, at least one distillation column (18;216) for separating air into oxygen and nitrogen, an outlet (26;228) for nitrogen vapour from said at least one distillation column (18;216) communicating with the inlet of at least one nitrogen compressor (36,234) via said heat exchange means (10, 12, 14; 204, 206, 210, 212, 214), at least one expansion turbine (40;238) having an inlet for communicating with said nitrogen compressor (36;224), a reboiler (22,220) associated with the said at least one distillation column (18,216), the outlet of said reboiler (22,220) communicating with means (48, 49, 50, 244; 246; 250) for providing liquid nitrogen reflux for said at least one distillation column (18; 216) and also with an outlet (LN; 280) for product liquid nitrogen, characterised in that said inlet to said expansion turbine (40;238) communicates with said nitrogen compressor (36;224) via the heat exchange means (10,12,14; 204,206,210,212,214); the said expansion turbine (40;238) has an outlet communicating with an inlet to said reboiler (22;220) and the outlet of said reboiler communicates via said heat exchange means (10, 12, 14; 204, 206, 210, 212, 214) with said means (48, 49, 50; 244, 246, 250) for providing liquid nitrogen reflux. A plant as claimed in claim 10, in which the column (18; 216) for which the reflux is provided is the same column (18;216) as that with which the reboiler (22;220) is associated.

Inventors: LAVIN, JOHN TERENCE, LAYLAND, DAVID JOHN
Applicants: THE BOC GROUP PLC
Application year: 1988
Main classification: F25J3
IPC codes: F25J3