AIR SEPARATION

Abstract: Air from which carbon dioxide and water have been removed is subjected to fractional distillation in a first column 4 to form oxygen-nitrogen vapor and argon-enriched oxygen; argon is separated from the latter in a second column 10 operating at substantially lower pressure than the first; a portion of the nitrogen vapor is liquefied in a cycle including heat exchange with incoming air and compression to a pressure above that of the first column 4, and a portion of the liquid nitrogen is taken as product and a second portion is returned to the first column 4 as reflux. Liquid oxygen and nitrogen vapor from the first column are mixed in a liquid-vapor contact column 14, impure liquid nitrogen is withdrawn therefrom and used as reflux in the first column 4. A remixed oxygen-nitrogen stream is withdrawn from the column 14 and expanded to create refrigeration for the system.

Description: This invention relates to a method and apparatus or plant for separating air according to the first part of claims 1 and 12, resp. Air is traditionally separated by fractional distillation at cryogenic temperatures. Either a single or double distillation column may be employed in plant for separating air by fractional distillation. Typically, but not necessarily, both nitrogen and oxygen products are produced. In addition, by taking an argon-enriched vapour stream from the single or double distillation column and subjecting it to distillation in a further column, an argon product may also be produced. When, for example, both oxygen and nitrogen are required as products in the gaseous state, the incoming air is typically compressed to a pressure in the range of 6-7 atmospheres absolute in a plant using a double distillation column. Since there is a large demand for both liquid nitrogen and liquid oxygen, there is a need for plants that produce both these products in the liquid states. Liquid oxygen that is collected at the bottom of the distillation column may be taken therefrom as product. The demand for liquid nitrogen may be met taking a gaseous nitrogen product and liquefying at least some of it. Modern nitrogen liquefiers typically include compressors that raise the pressure of the nitrogen to 40 atmospheres or more. Alternatively, the liquefaction plant may be integrated with the air separation plant such that the liquid nitrogen product can be taken in substantial quantities directly from the single or double distillation column. FR-A-2 305 700 shows a heat pump cycle for a single distillation column which enables some liquid nitrogen to be withdrawn as product. US-A-4 705 548 relates to a double distillation column cycle which uses an intermediate reboiler in the lower pressure column to condense nitrogen as part of a nitrogen liquefaction cycle. In plants that produce essentially gaseous oxygen and nitrogen products, there have been proposed methods of providing increased argon production which involve the use of an additional liquid-vapour contact column to remix gaseous nitrogen and liquid oxygen and provide additional liquid nitrogen reflux to the column from which the argon-enriched vapour stream is taken for further separation. Such methods are described in EP 136 926 and international (PCT) patent application W087/00609. In addition, GB-A-2 174 916 and EP 259 070 relate to such use of an additional liquid-vapour contact column to remix gaseous nitrogen and liquid oxygen when argon is required as the sole or primary product of air separation. The air separation cycle illustrated in Figure 2 of EP-A-259 070 forms liquid nitrogen for the purpose of providing reflux to the columns used therein. the invention relates to air separation methods, apparatus and plant which employ remixing of liquid oxygen and nitrogen vapour and also produce a liquid nitrogen product employing a nitrogen liquefaction cycle sharing heat exchangers with the air separation cycle and using a turbine and a reboiler of one of the distillation columns. According to the present invention there is provided a method of separating air into oxygen, nitrogen and argon, comprising the steps of extracting carbon dioxide and water vapour from the air and cooling the air to a cryogenic temperature suitable for its separation by fractional distillation; subjecting the air to fractional distillation in a first distillation column operating at a first pressure and withdrawing oxygen, nitrogen vapour and argon-enriched oxygen from the first distillation column; subjecting the argon-enriched oxygen to further separation in a second distillation column operating at a second pressure substantially lower than the first pressure; withdrawing argon from the second distillation column; liquefying at least some of the nitrogen vapour by a cycle which warms the nitrogen vapour, which employs compression of warmed nitrogen vapour to a third pressure substantially in excess of the first pressure and which reduces the temperature of the compressed nitrogen upstream of its liquefaction; introducing a part of the liquid nitrogen into said first distillation column as reflux, wherein liquid oxygen and impure nitrogen are withdrawn from the first distillation column and remixed in a liquid-vapour contact column, impure liquid nitrogen is withdrawn from the said liquid-vapour contact-column and used as reflux in the said first distillation column, and a remixed oxygen-nitrogen stream is withdrawn from the said liquid-vapour contact column and is subjected to expansion with the performance of external work so as to create refrigeration, characterised in that in the nitrogen liquefaction cycle the warming of the nitrogen vapour is performed in countercurrent heat exchange with the air being cooled; at least some of the cooled nitrogen is subjected to expansion with the performance of external work; the thus expanded nitrogen is passed through a reboiler associated with the first distillation column to provide reboil for the distillation in that column, the nitrogen leaving the reboiler is subjected to further cooling and temperature reduction by heat exchange, and part of the resulting liquid nitrogen is taken as product. The invention also provides apparatus or plant for separating air into oxygen, nitrogen and argon, comprising means for extracting carbon dioxide and water vapour from the air; heat exchange means for cooling air to a cryogenic temperature suitable for its separation by fractional distillation; a first distillation column for subjecting the air to fractional distillation, said column having outlets for the withdrawal of liquid oxygen, nitrogen vapour and argon-enriched oxygen; a second distillation column communicating with said outlet for argon-enriched oxygen operable to separate argon therefrom; an outlet for argon from said second distillation column; and means for liquefying at least some of the nitrogen vapour by performing a cycle which includes heat exchange means for warming the nitrogen vapour, at least compressor for raising the pressure of the warmed nitrogen to a pressure substantially in excess of the pressure at which the first distillation column operates, heat exchange means for cooling and reducing the temperature of a compressed nitrogen and an inlet for returning a part of the liquid nitrogen as reflux to said first distillation column wherein the apparatus or plant additionally includes a liquid-vapour contact column having an inlet for liquid oxygen in communication with the outlet for liquid oxygen from the first distillation column and an inlet for impure nitrogen vapour in communication with an outlet for such vapour from the first distillation column, and also having an outlet for impure liquid nitrogen, in communication with an inlet for such nitrogen to the first distillation column, and another outlet for a remixed oxygen-nitrogen mixture communicating with at least one machine for expanding said mixture with the performance of external work so as to create refrigeration characterised in that the nitrogen liquefaction cycle utilises at least part of the heat exchange means for cooling the air as the heat exchange means for warming the nitrogen vapour and as the heat exchange means for cooling the compressed nitrogen and utilises an expansion turbine for expanding at least a part of the cooled, compressed, nitrogen with the performance of external work; and in that the outlet of the expansion turbine communicates with a reboiler associated with the first distillation column; the reboiler has an outlet for nitrogen communicating with a part of said heat exchange means for cooling the compressed nitrogen, and the apparatus or plant has an outlet for product liquid nitrogen. Typically, the liquid-vapour contact column (also referred to herein as the mixing column) is operated at essentially the same pressure as the first distillation column. This pressure is preferably at least 3 atmospheres and more preferably at least 5 atmospheres. Remixing of the oxygen and nitrogen can be conducted with greater thermodynamic efficiency at pressures of at least 3 atmospheres than it can at substantially lower pressures. Moreover by operating the liquid-vapour contact column at such pressures, work expansion of the oxygen-nitrogen mixture withdrawn can provide a significant amount of refrigeration which helps to meet the overall requirements for refrigeration of the process and thus makes possible choice of said third pressure lower than that which would be required for a fully comparable method not employing a mixing column. Preferably, the remixed stream of oxygen and nitrogen is withdrawn from an intermediate level of the liquid-vapour contact column. It is further preferred that another remixed oxygen-nitrogen stream containing a greater proportion of oxygen than does the first stream is withdrawn from the mixing column, is condensed in heat exchange with a stream of boiling liquid from one of the distillation columns, and is returned to the mixing column, the boiled liquid being returned to one of the distillation columns. The withdrawal of the second remixed stream of oxygen and nitrogen offers an improvement in the thermodynamic efficiency with which the liquid-vapour contact column operates. Preferably, a third part of said liquid nitrogen is used to condense nitrogen vapour from the liquid-vapour contact column, resulting condensate being returned to the column, said third part being thereby vaporised and at least part of the vapour being recycle and for liquefaction. In addition to employing a stream of nitrogen to reboil the first distillation column, a stream of air can be used for this purpose. In one preferred example, a stream of air preferably at its dew point is used to provide some of the reboil for the first distillation column and resulting liquid air is then introduced into the first distillation column. The nitrogen withdrawn from the first 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 a temperature in the range 150 to 170K). 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 the 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 associated with the first distillation column. Preferably, liquid nitrogen leaving the reboiler is sub-cooled by heat exchange 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 may be used. Preferably, liquid nitrogen from a first flash separation step is employed to condense nitrogen vapour from the liquid-vapour contact column, resulting condensate being returned to this column as reflux. The first distillation column may comprise one or more vessels. Typically, it comprises an upper vessel and a lower vessel. The two vessels interchange in pure nitrogen, vapour passing from the lower vessel to the upper vessel and liquid passing in the opposite direction. Reflux for the upper vessel is provided from the nitrogen liquefaction cycle, whereas a reflux for the lower vessel is provided by the mixing column. The air to be separated is fed to the lower vessel, and the liquid oxygen and impure nitrogen feeds for the mixing column are withdrawn from a lower vessel as is the argon-enriched oxygen stream. The second distillation column is preferably reboiled. Reboil is preferably provided by the first distillation column, or alternatively an additional distillation column to which a part of the air feed is sent may be used for this purpose. Reboiling of the second distillation column makes it possible to withdraw a relatively pure oxygen product, typically in the liquid state, from the second distillation column. When liquid oxygen is required as the product, this is advantageous, as the second distillation column operates at a pressure nearer to atmospheric than the first distillation column. Typically, the second distillation column operates at a pressure between one and two atmospheres absolute while the first distillation column and the mixing column operate at pressures between 5.5 and 6.5 atmospheres absolute. Method, apparatus and plant according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram illustrating one arrangement of distillation columns with a mixing column for use in performing the inventions; Figure 2 is a schematic diagram illustrating an alternative arrangement of columns to the one shown in Figure 1; Figure 3 illustrates a detail of a further modified arrangement of columns for use in performing the invention; Figure 4 illustrates a plant for performing method according to the invention which utilises the arrangement of columns shown in Figure 2; and Figure 5 shows an alternative plant to the one illustrated in Figure 4; Referring to Figure 1 of the drawings, there is shown an arrangement of distillation columns suitable for performing the method according to the invention. For ease of illustration, all heat exchangers, compressors, expansion turbines and valves are omitted from Figure 1 save for one heat exchanger. The illustrated apparatus includes a first distillation column 2 comprising a lower vessel 4 and an upper vessel 6. The first distillation column 2 is employed to separate air into oxygen, nitrogen and argon-enriched oxygen. The argon-enriched oxygen is separated in a second distillation column 10. The second distillation column 10 is reboiled by a third distillation column 12 which receives a portion of the air feed. In addition, there is a liquid-vapour contact column 14 in which liquid oxygen and nitrogen vapour from the lower vessel 4 are remixed such that nitrogen reflux can be provided to the lower vessel 4. Air typically at its dew point is fed at an elevated pressure, typically 6 atmospheres absolute, through an inlet 16 into the lower vessel 4. The air is subjected to fractional distillation in the lower vessel 4, reboil being provided at the bottom of the vessel 4 by means of a reboiler 18 and impure liquid nitrogen reflux being provided to the top of the vessel 4 from the upper vessel 6 via conduit 20 and from the bottom of the mixing column 14 via conduit 22. Accordingly, there is established in the column 4 a downward flow of liquid which becomes progressively richer in oxygen and an upward flow of vapour that becomes progressively richer in nitrogen. Commercially pure liquid oxygen is withdrawn from the lower vessel 4 of the distillation column 2 through an outlet 24 at its bottom an impure nitrogen vapour stream is withdrawn through an outlet 26 at the top of the vessel 4 and an argon-enriched liquid stream is withdrawn through an outlet 29 which is at a level intermediate those of the air feed inlet 16 and the liquid oxygen outlet 24. The position of the outlet 29 for the argon-enriched oxygen stream is at a level where the concentration of argon in the vapour phase in the vessel 4 is at or near a maximum such that the stream withdrawn through the outlet 29 typically contains in the order of about 8 to 10% by volume of argon. The stream withdrawn from the top of the vessel 4 is typically an impure nitrogen vapour stream containing the order of 1 to 2% by volume of oxygen (for example, 1.4%). A part of this stream is then passed to the upper vessel 6 in which there is an upward flow of nitrogen vapour that becomes progressively richer in nitrogen and a downward flow of liquid which becomes progressively richer in oxygen. Impure liquid nitrogen is withdrawn from the bottom of the upper vessel 6 and typically contains in the order of 2% by volume of oxygen. This liquid nitrogen is employed as a part of reflux in the lower vessel 4. Reflux for the upper vessel 6 is provided by a stream of liquid nitrogen entering near the top of the vessel 6 through a conduit 28. Liquid nitrogen is supplied from a nitrogen liquefaction cycle (not shown) which uses heat exchangers (not shown) to cool the air to be liquefied. A substantially pure gaseous nitrogen stream is withdrawn from the vessel 6 through an outlet 30. The other part of the stream of impure nitrogen withdrawn from the lower vessel 4 through the outlet 26 is passed to the bottom of the liquid-vapour contact or mixing column 14. This impure nitrogen is remixed in the column 14 with a part of the liquid oxygen withdrawn from the bottom of the vessel 4 through the outlet 24, such liquid oxygen being introduced into the column 14 through its top. There is thus established in the column 14 an upward flow of vapour which undergoes mass exchange with a downward flow of liquid, such that the liquid as it descends becomes progressively richer in nitrogen and the vapour as it ascends becomes progressively richer in oxygen. In order to enhance the downward flow of liquid, the column 14 is provided with a condenser 32 at its top such that oxygen vapour is condensed at the top of the column 14. For ease of illustration, the means of providing cooling fluid to the condenser 32 is not shown in Figure 1. Typically, the liquid nitrogen which is returned via the conduit 22 from the column 14 to the vessel 4 as reflux contains from 3 to 6% by volume of oxygen, (for example 4.9% by volume). A mixed oxygen-nitrogen stream is withdrawn from an intermediate level thereof through a conduit 34. Typically, this mixed oxygen-nitrogen stream contains about 21% by volume of oxygen, that is about the same level as in air. If desired, however, the mixed stream may contain a lower or higher concentration of oxygen. The stream withdrawn through the conduit 34 is subjected to expansion with the performance of external work by means not shown in Figure 1. Withdrawal of this stream maintains a mass balance in the column 14 and its withdrawal from an intermediate position helps to improve the efficiency with which the column 14 operates. The efficient operation of the column 14 is also facilitated by the choice of an operating pressure in excess of 3 atmospheres absolute. The column 14 and the upper vessel 6 are typically operated at substantially the same pressure as the lower vessel 4, that is in the order of 6 atmospheres although, because of liquid head effects, the average pressure in the column 14 and in the vessel 6 will be a little less than in the vessel 4 (say, in the order of 5.8 atmospheres absolute). With the aim of enhancing the efficiency with which the column 14 operates, a stream of a mixed vapour comprising oxygen and nitrogen is withdrawn from the column 14 through a conduit 36 at a level above that of the conduit 34 but below that of the lowest level in the column 14 at which mass exchange takes place between liquid and vapour. Typically, the vapour stream withdrawn through the conduit 36 contains about 45-50% by volume of oxygen (typically, about 47% by volume). This vapour is condensed in a condenser 38 and the resulting liquid is returned to the column 14 through a conduit 40 at a level such that the composition of the returning liquid is approximately the same as the composition of the liquid to which it is returned. Refrigeration for the condenser 40 is provided by withdrawing through conduit 42 a stream of oxygen-enriched liquid from the lower vessel 4 of distillation column 2 and passing it to the condenser 38 such that it undergoes heat exchange with the stream withdrawn from the column 14 through the conduit 36, condenses upstream, and is itself reboiled. The resulting vapour is then returned to the vessel 4 by way of an inlet 44 which is located at a level such that the composition of the returning vapour matches that of the liquid to which it is returned. In order to produce an argon product, the argon-enriched oxygen liquid stream withdrawn from the vessel 4 through the outlet 29 is passed to a sub-cooler (not shown) and thereby reduced in temperature, passed through a throttling valve (not shown) and then introduced through an inlet 46 into the column 10 where it is separated by distillation into oxygen and argon products, the latter typically containing in the order of 2% by volume of oxygen impurity. Accordingly, in the column 10, a generally downwardly flowing stream of liquid is contacted by an upwardly flowing stream of vapour and mass exchange takes place between them such that the liquid becomes progressively richer in oxygen as it descends the column 10 and the vapour becomes progressively richer in argon as it ascends the column 10. The column 10 is provided with a condenser 48 at its top to provide liquid argon reflux for the column and a reboiler at 50 at the bottom to provide an ascending stream of vapour. The reboiler 50 also functions as a condenser for the column 12 to provide reflux for this column. A liquid argon product is withdrawn from the column 10 through an outlet 52 at its top and a liquid oxygen product is similarly withdrawn through an outlet 54 at its bottom. The column 10 is typically operated at a pressure a little above atmospheric, for example 1.3 atmospheres absolute. In order to provide a suitable temperature difference across the condenser- reboiler 50 the column 12 is operated at 6 atmospheres absolute (that is at substantially the same pressure as the lower vessel 4). Since it is generally desirable to take liquid oxygen product from the apparatus shown in Figure 1 at a relatively low pressure, some of the liquid oxygen withdrawn from the bottom of the lower vessel 4 through the outlet 24 is transferred to the bottom of the column 10 via an expansion valve 56. Accordingly, the rate at which liquid oxygen is withdrawn from the column 10 through the outlet 54 is enhanced. As aforementioned, the column 12 provides reboil for the column 10. A minor proportion of the incoming air is introduced into the column 12 through an inlet 58, the air having been pre-cooled to its dew point. The air is separated in the column 12 into an oxygen-rich fraction collecting at the bottom of the column and a nitrogen fraction collecting at the top of the column 12. The oxygen-rich liquid is withdrawn from the column 12 through an outlet 60 and introduced into the column 4 through an inlet 62. Since there is withdrawal of liquid from the bottom of column 12, there is no need to provide any reboil for it. A nitrogen vapour-liquid mixture is withdrawn from the top of the column 12 and is united with the liquid nitrogen being introduced into the upper vessel 6 of the distillation column 2 through the inlet 28. Each of the columns 2, 10, 12 and 14 typically includes a multiplicity of liquid-vapour contact trays (not shown) to effect mass transfer between the liquid and vapour phases. Alternatively, structured packings can be used for this purpose. Typically, if trays are used, they are of the sieve tray type. The lower vessel 4 may typically be operated with 89 theoretical trays, the upper vessel 6 with 16.5 theoretical trays, the second distillation column 10 with 73 theoretical trays, the additional distillation column 12 with 42 theoretical trays, and the mixing column 14 with 35 theoretical trays. (The number of actual trays employed will be the number of theoretical trays multiplied by the inverse of the tray efficiency.) Referring now to Figure 2, the apparatus shown therein comprises a similar arrangement of columns to the one shown in Figure 1 save that the first distillation column is employed to reboil the second distillation column with the result that apparatus shown in Figure 2 employs one less distillation column than the apparatus shown in Figure 1. The apparatus shown in Figure 2 includes a first distillation column 72, comprising a lower vessel 74 and an upper vessel 76, a second distillation column 78, and a liquid-vapour contact or mixing column 79. Air typically at a pressure of 6 atmospheres absolute and at its dew point is passed into the lower vessel 74 of the first distillation column 72 and is in this column separated by fractional distillation into oxygen, argon-enriched oxygen and impure nitrogen fractions. The vessel 74 is provided with two sources of impure liquid nitrogen reflux. The first source is the upper vessel 76 which provides liquid nitrogen to the vessel 74 via conduit 82. The second source is the mixing column 79 which provides liquid nitrogen via a conduit 84 placing the bottom of the mixing column 79 in communication with the top of the vessel 74. The lower vessel 74 is also provided near its bottom a reboiler 86 which is adapted to provide an upward flow of vapour to come into mass transfer relationship with a downward flow of liquid such that the liquid as it descends becomes richer in oxygen and the vapour as it ascends becomes richer in nitrogen. For ease of illustration, the source of the fluid providing the reboil is not shown in Figure 2. Liquid oxygen is withdrawn from the bottom of the vessel 74 through an outlet 88. A part of this liquid oxygen may be taken as product directly or first introduced into the column 76 and taken therefrom as product, whereas the remainder of the liquid oxygen leaving the vessel 74 through the outlet 88 is passed to the top of the mixing column 78. Impure nitrogen vapour is withdrawn from the top of the vessel 74 in two streams. A first stream of such impure nitrogen is withdrawn from the top of the vessel 74 through a conduit 90 and enters the bottom of the upper vessel 76. The vapour ascends the vessel 76 and comes into mass transfer relationship with a descending stream of liquid. The descending stream of liquid is created by condensing a portion of the vapour at the top of the vessel 76. A condenser 92 is provided for this purpose. The condenser 92 also serves to provide reboil for the second distillation column 78. Further liquid nitrogen reflux is provided by introducing into the upper vessel 76 liquid nitrogen through an inlet 94. The liquid nitrogen is produced in a liquefier cycle (not shown in Figure 2) which uses heat exchangers (not shown) employed to cool the incoming air. A second stream of impure nitrogen vapour is withdrawn from the lower vessel 74 of distillation column 72 and is passed along conduit 98 into the bottom of the liquid-vapour contact column 79. The nitrogen vapour is mixed with liquid oxygen supplied through an inlet 100 to the top of the column 79 from the outlet 88 at the bottom of the vessel 74 of distillation column 72. The operation of the mixing column 79 is generally similar to that of the mixing column 14 shown in Figure 1. In particular, the mixing column 79 is provided with a condenser 102 to provide additional liquid reflux for the column. The column is operated at substantially the same pressure as the lower vessel 74 of the distillation column 72 though typically there will be a slight difference in pressure to allow for the flow of liquid under gravity from the column 79 back into the vessel 74. Such liquid flow takes place via the conduit 84 as aforesaid. As the liquid descends the column 79 so it contacts ascending vapour and there is mass exchange there-between with the result that the descending liquid becomes progressively richer in nitrogen and the ascending vapour becomes progressively richer in oxygen. The vapour at the top of the column is condensed by condenser 102. Typically, the liquid nitrogen returned to the vessel 72 to act as reflux therein is impure, typically containing up to 6 mole per cent of oxygen. A mixed oxygen-nitrogen stream is withdrawn from the mixing column 79 at intermediate level thereof and passes along a conduit 106 for expansion with the performance of external work in for example an expansion turbine (not shown in Figure 2). The stream withdrawn through the conduit 106 typically contains a proportion of oxygen the same as or similar to the proportion of oxygen in air. There is also withdrawn from the mixing column 79 a mixed stream of oxygen and nitrogen containing a greater proportion of oxygen than does the stream withdrawn through the conduit 106. The stream of higher oxygen concentration is withdrawn through an outlet 108, is passed to the heat exchanger 110 in which it is condensed, and the resulting liquid is returned to the column 79 through an inlet 112 located at a level such that the composition of the returning liquid is approximately the same as that of the liquid to which it is returned. Condensation of the vapour in the heat exchanger (or condenser) 110 is effected by heat exchange with a stream of liquid withdrawn from the lower vessel 74 of distillation column 72 through an outlet 116. This liquid is reboiled in the heat exchanger 110 and is returned as a vapour to the vessel 74 through an inlet 118 located such that the composition of the returning vapour is approximately the same as that of the vapour into which it is returned. In addition to the withdrawal of substantially pure liquid oxygen through the outlet 88 and impure nitrogen through the conduits 90 and 98, a stream of argon-enriched oxygen in the liquid phase is also withdrawn from the vessel 74. This stream is taken at a level below that of the air inlet 80 where there is a relatively high concentration of argon in the vapour phase, for example, 8-10% by volume. The argon-enriched oxygen stream passes from the vessel 74 into a conduit 120 is then sub-cooled in a heat exchanger (not shown in Figure 2), is passed through an expansion valve 122 and is then introduced into the column 78 which typically operates at a pressure a little above atmospheric (say 1.3 atmospheres absolute). The argon-enriched oxygen is separated in the column 78 into oxygen and argon fractions. The argon fraction typically contains up to 2% by volume of oxygen. The column 78 is provided as aforesaid with a reboiler 92 to reboil liquid at the bottom and a condenser 124 at its top to condense vapour at the top and provide liquid reflux for the column 78. Accordingly, liquid descends the column 78 and comes into mass transfer relationship with an ascending vapour. Liquid becomes progressively richer in the less volatile constituent (oxygen) as it descends the column while the vapour similarly becomes richer in the more volatile component (argon) as it ascends the column 78. Liquid oxygen product is withdrawn from the column 78 through an outlet 126 at its bottom and liquid argon product is withdrawn from the top of the column 78 through an outlet 128. As with the apparatus shown in Figure 1, typically all the columns have liquid-vapour contact trays to facilitate mass transfer between the liquid and vapour phases or are provided with suitable packings for this purpose. Typically the vessel 74 may have at least 93 theoretical trays as follows: 35 theoretical trays between the top of the vessel 174 and the level of the air inlet 80; 5.4 theoretical trays between the level of the air inlet and the level of the inlet 118; 17 theoretical trays between the level of the inlet 118 and the level of the inlet to the conduit 120, and 35.6 theoretical trays below the level of the inlet to the conduit 120. The vessel 76 may have 18.2 theoretical trays; the column 78 may have 81.3 theoretical trays, 45.9 above the level of the air feed, and 35.4 below the level of the air feed, and the column 79 may have 35 theoretical trays, 17 below the level of the inlet to the conduit 106; 8 between the inlet to the conduit 106 and the level of the outlet 108, and 10 above the level of the outlet 108. Referring now to Figure 3, there is shown a modification to the upper vessel 76 of the distillation column 72. Typically, the liquid nitrogen produced in the upper vessel 76 of the distillation column 72 contains up to 200 vpm of impurities (excluding argon). If nitrogen of a higher purity is required, the liquid nitrogen reflux is introduced into the column 72 through the inlet 94 at a level several trays (eg 5) below that at which a pure gaseous nitrogen product is withdrawn through a conduit 99. Accordingly, most of the impurities (excluding argon) are transferred from the ascending vapour phase to the descending liquid phase between the level of the inlet 94 and that of the outlet 99 such that the resultant gaseous nitrogen typically contains in the order of 1 vpm only of impurities (excluding argon). In the apparatus shown in Figure 1 and Figure 2, the first distillation column is shown as being formed of two discrete vessels. If desired, this column may comprise just a single vessel. There will now be described two examples of processes and plant employing apparatus such as that shown in Figures 1 and 2. Referring first to Figure 4, there is shown a plant employing a column system analogous to that shown in Figure 2, though as illustrated the first distillation column comprises just a single vessel. Referring to Figure 4,130,000 sm³/hr of air flow into a compressor 200 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.) Water vapour, carbon dioxide and the like are removed from the air in a purifier 201 which typically comprises a plurality of beds of an adsorbent that selective adsorbs water vapour and carbon dioxide from the air. The construction and operation of such purifiers are well known in the art. 44967sm³/hr of the purified compressed air is further compressed in a compressor 202 to a pressure of 30 atmospheres absolute while the remainder by-passes the compressor 202. The two air streams then flow through a first heat exchanger 204 in which they are cooled from a temperature of 298K to a temperature of 235K. The larger air stream is then further cooled in a heat exchanger 206 to a temperature of 159K. The minor (30 atmosphere) air stream by-passes the heat exchanger 206 and is instead expanded in an expansion turbine 208, with the performance of external work, to a pressure of 6.1 atmospheres absolute. The resulting expanded air leaves the turbine 208 at a temperature of 159K and is reunited with the other air stream downstream of the heat exchanger 206 upstream of a further heat exchanger 210. The combined air stream is then cooled in the 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 at a pressure of 6 atmospheres absolute through an inlet 218 into a first or main distillation column 216 in which it is separated into a nitrogen fraction at the top of the column 216 and an oxygen fraction at the bottom of the column 216. The distillation column 216 is provided at its bottom with a reboiler 220, and at its top with an inlet 222 for substantially pure liquid nitrogen reflux. In addition, there is a condenser-reboiler 224 which condenses vapour at the top of the column 216 and provides reboil at the bottom of a second distillation column 226. Nitrogen that passes through the 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 111755 sm³/hr and a temperature of 96K and is united with a further 13772 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 end to its warm end and is thereby raised in temperature to 98K. It then flows through the heat exchangers 212, 210, 206 and 204 countercurrently to the incoming air flow and leaves the heat exchanger 204 at a temperature of 298K. The nitrogen stream leaving the heat exchanger 204 is mixed with a stream of nitrogen passing from a compressor 233 at a rate of 20,081 sm³/hr and a pressure of about 5 1/2 atmospheres absolute and the mixture is then further compressed in a compressor 234 to a pressure of 11 atmospheres. The resulting mixed stream at a pressure of 11 atmospheres is mixed at a temperature of 298K with a yet further stream of nitrogen and flowing at a rate of 33988 sm³/hr. This mixed stream is then compressed to a pressure of 59 atmospheres in a compressor 236. The resulting compressed stream flowing at a rate of 179596 sm³/hr then passes through the heat exchangers 204 and 206 co-currently with the incoming air, being thereby cooled to a temperature of 159K. The stream is then divided into two parts. The major part comprises a flow of 134213 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 238 and leaves the outlet of 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 the column 216, the nitrogen itself being at least partially 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 Joule-Thomson or throttling valve 240 at a rate of 118424 sm³/hr and is thereby reduced in pressure to 11 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 11 atmosphere stream of nitrogen intermediate the compressors 233 and 236. Most of the resulting liquid collected in the phase separator 242 is used to make one contribution to a two-phase stream which is passed to a further phase separator 230. Accordingly, a first flow of 71052 sm³/hr of this liquid is flashed through a throttling or Joule-Thomson valve 244 and the resultant 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 3384 sm³/hr from the bottom of the phase separator 242 (at a temperature of 105K), sub-cooling the stream to a temperature of 98K by passage through the heat exchanger 214, and then flashing it 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 15789 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 or Joule-Thomson valve 250 and is then united with the liquid-vapour mixture passing to the phase separator 230. A fourth and final 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 45383 sm³/hr and a pressure of 59 atmospheres absolute and continues its passage through the heat exchangers, 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 or Joule-Thomson valve 252 to reduce its pressure to 5.8 atmospheres. The resulting liquid-vapour is as aforesaid mixed with the rest of the liquid-vapour mixture passing to the phase separator 230. The liquid nitrogen collecting in the phase separator 230 is used for three purposes. A part of it is returned to the inlet 222 of the first distillation column 216 to serve as reflux. A second part of it is eventually collected as product. A third part of it used to provide condensation of vapour at the top of the second distillation column 226 in which a liquid argon product is formed. For the time being, only the first flow of liquid nitrogen back to the distillation column 216 will be described. This flow is at a rate of 61555 sm³/hr. In order to provide additional reflux for the column 216 a stream of impure nitrogen vapour is withdrawn from the column 216 at a rate of 97287 sm³/hr through an outlet 254. This nitrogen vapour typically contains in the order of 2% by volume of oxygen. The stream is introduced into the bottom of a mixing column 256. In the column 256 an ascending stream of vapour is intimately contacted with a descending stream of liquid. Accordingly, a liquid oxygen stream is withdrawn from the bottom of the column 216 through an outlet 258 and the major portion of such liquid oxygen is passed at a rate of 14772 sm³/hr into the top of the mixing column 256. Thus, there is a downward flow of liquid through the column 256 that becomes progressively richer in nitrogen and an upward flow of vapour that becomes progressively richer in oxygen. Impure liquid nitrogen is withdrawn from the bottom of the column 256 at a rate of 47060 sm³/hr through an outlet 260 and is returned to the distillation column 216 to act as further reflux for the column. Additional reflux for the column 256 is created by operation of a condenser 262 therein. The condenser is cooled by a yet further portion of liquid nitrogen taken from the phase separator 242. Thus, liquid nitrogen at a rate of 18703 sm³/hr flows into the condenser 262 at a pressure of 11 atmospheres and a temperature of 105K and is thus effective to condense oxygen vapour at the top of the column 256. The nitrogen is itself vaporised and is used to form part of that flow of nitrogen at 11 atmospheres that is mixed with the nitrogen passing from the compressor 234 to the compressor 236. The remainder of such flow is made up of 15285 sm³/hr of nitrogen gas withdrawn from the phase separator 242. The flow of nitrogen gas from the phase separator 242 is mixed with the stream of vaporised nitrogen passing out of the condenser 262 and flows back through the heat exchangers 212, 210, 206 and 204 countercurrently to the incoming air before being mixed with the nitrogen intermediate the compressors 234 and 236. In addition to the stream of impure nitrogen withdrawn from the bottom of the mixing column 256 through the outlet 260, a stream of nitrogen- oxygen vapour mixture is withdrawn from an intermediate region of the column 256 through an outlet 264 at a flow rate of 65,000 standard cubic metres per hour. The mixed stream typically contains 21% by volume of oxygen. It flows through the heat exchangers 212, 210 and 206 in sequence counter-currently to the flow of incoming air and is thus warmed to a temperature of 230K. The mixed nitrogen-oxygen stream is then expanded with the performance of external work in an expansion turbine 266. The stream of oxygen and nitrogen leaves the expansion turbine 266 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. In addition to producing nitrogen and oxygen fractions, the distillation column 216 also provides an argon-enriched and oxygen feed to the second distillation column 226. Accordingly, argon-enriched liquid oxygen typically containing in the order of 8% by volume of argon is withdrawn through outlet 268 at a rate of 13050 sm³/hr from a level in the column 216 below that of the air inlet 218 and is sub-cooled by passage through the heat exchangers 212 and 214. The sub-cooled argon-oxygen liquid mixture is flashed through a throttling or Joule-Thomson valve 270 and is introduced into the column 226 through an inlet 272 at a pressure of 1.3 atmospheres absolute. Reboil for the column 226 is provided by the condenser-reboiler 224 and reflux is provided by operation of a condenser 276 in the top of the column 226. Cooling for the condenser 276 is provided by taking a stream of liquid nitrogen from the phase separator 230 at a rate of 70281 sm³/hr and sub-cooling it in a heat exchanger 278 thereby reducing its temperature from 96K to 90K. The resulting sub-cooled nitrogen is then flashed through a throttling or Joule-Thomson valve 280 and the resulting liquid-vapour mixture is passed to a phase separator 282 operating at a pressure of 3 atmospheres absolute. A first stream of liquid is withdrawn from the phase separator 282 at a rate of 41389 sm³/hr and is passed through the condenser 276 thus condensing vapour and hence providing reflux in the column 226 while being vaporised itself. The resulting vapour is the mixed with vapour withdrawn from the top of the phase separator 282, and the mixture is returned through the heat exchanger 278 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. Gaseous nitrogen product at a pressure of 2.8 atmospheres is taken from this stream of nitrogen at a flow rate of 26213 sm³/hr whereas the remainder of the flow (20081 sm³/hr) is that compressed in the compressor 233. A second stream of liquid nitrogen is withdrawn from the phase separator 282 at a flow rate of 23989 sm³/hr and is sub-cooled in a heat exchanger 284, its temperature thereby being reduced from 90K to 88K. The sub-cooled liquid nitrogen is then flashed through a throttling or Joule-Thomson valve 286 and the resulting two-phase mixture is collected in a phase separator 288. Saturated liquid nitrogen product at a pressure of 1.3 atmospheres absolute is withdrawn from the phase separator 288 at a rate of 21699 sm³/hr through an outlet 290. Nitrogen vapour is withdrawn from the top of the phase separator 288 at a rate of 2287 sm³/hr and is progressively warmed to 298K by passage through heat exchangers 284, 278, 214, 212, 210, 206 and 204 in sequence. This gaseous nitrogen is also collected as product. By providing reboil and reflux in the column 226, it is possible to separate liquid argon and liquid oxygen streams therein. Thus, 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 292 positioned at or near the top of the column 226. Liquid oxygen product is withdrawn at a flow rate of 13592 sm³/hr and a pressure of 1.4 atmospheres absolute from the bottom of the column 226 through an outlet 294. This liquid oxygen product comprises that formed by fractionation in the column 226, supplemented with a stream of liquid oxygen withdrawn from the column 216 through the outlet 258 and flashed through a throttling or Joule-Thomson valve 296 at a rate of 1720 sm³/hr, the resultant 2-phase oxygen mixture entering the bottom of the column 226. It will be appreciated that net refrigeration for the heat exchanger 206 is provided by the expansion of the mixed oxygen-nitrogen stream from the mixing column 256 in the expansion turbine 266. Accordingly, the stream withdrawn through the outlet 264 of the mixing column 256 makes an important contribution to the overall refrigeration requirements of the process. It will further be appreciated that the expansion turbines 208 and 238 contribute refrigeration to the plant shown in Figure 5. Typically, the refrigeration requirements of the heat exchanger 204 operating between the highest temperature limits are met by a mechanical refrigeration machine 298 using Freon (registered trademark) as a working fluid. If desired, the heat exchangers 204, 206, 210 and 212 may be made as one heat exchange block. Typically, the compressors 233, 234 and 236 may comprise separate stages of a single multi-stage rotary compressor. Similarly, the compressors 200 and 202 may also comprise separate stages of another plural or 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 266 may each drive a booster compressor used in the compression of the incoming air or nitrogen. Many modifications to the plant shown in Figure 4 are possible without departing from the invention. In particular, it is possible to substitute a column arrangement analogous to the one shown in Figure 1 for that of Figure 4. It is desirable to provide the mixing column 256 with a condenser for condensing a stream of vapour withdrawn at a level intermediate that of the outlet 264 and the top of the column where the liquid oxygen from the distillation column 216 is introduced so as to increase the operating efficiency of the mixing column 256. The condensed oxygen-nitrogen mixture is then returned to the mixing column 256. The operation of such a condenser may be as described with reference to Figure 2. A third modification that may be made to the plant shown in Figure 4 is illustrated in Figure 5. Like parts occurring in Figures 4 and 5 are indicated by the same reference numerals. Referring to Figure 5, not all of the reboil requirements of the 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 the 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 absolute and a temperature of 113.6K. The resultant expanded air then passes through the reboiler 220 and is condensed by passage therethrough. The condensed 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 throttling or Joule-Thomson valve 304 and a 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 around 8% of the total air introduced into the distillation column 216. By introducing some of the air into the distillation column 216 as liquid the overall cycle and column efficiencies are improved. In all other respects, the plant shown in Figure 5 and its operation is similar to the plant shown in Figure 4 and its operation. It is to be appreciated that the plants shown in Figures 4 and 5 are capable of flexible operation in that the relative rates of production argon, oxygen and nitrogen products may be varied. Thus, the greater are the flow rates of oxygen and nitrogen to the remixing column, the greater is the rate of argon production but the lower the rates of oxygen and nitrogen production.

Claims: 1. A method of separating air into oxygen, nitrogen and argon, comprising the steps of extracting carbon dioxide and water vapour from the air and cooling the air to a cryogenic temperature suitable for its separation by fractional distillation; subjecting the air to fractional distillation in a first distillation column (2; 72; 216) operating at a first pressure and withdrawing oxygen, nitrogen vapour and argon-enriched oxygen from the first distillation column; subjecting the argon-enriched oxygen to further separation in a second distillation column (10; 78; 226) operating at a second pressure substantially lower than the first pressure; withdrawing argon from the second distillation column (10; 78; 226); liquefying at least some of the nitrogen vapour by a cycle which warms the nitrogen vapour, which employs compression of warmed nitrogen vapour to a third pressure substantially in excess of the first pressure and which reduces the temperature of the compressed nitrogen upstream of its liquefaction; introducing a part of the liquid nitrogen into said first distillation column (2; 72; 216) as reflux, wherein liquid oxygen and impure nitrogen are withdrawn from the first distillation column and remixed in a liquid-vapour contact column (14; 79; 256), impure liquid nitrogen is withdrawn from the said liquid-vapour contact column (14; 79; 256) and used as reflux in the said first distillation column (2; 72; 216), and a remixed oxygen-nitrogen stream is withdrawn from the said liquid-vapour contact column (14; 79; 256) and is subjected to expansion with the performance of external work so as to create refrigeration, characterised in that in the nitrogen liquefaction cycle the warming of the nitrogen vapour is performed in countercurrent heat exchange with the air being cooled; at least some of the cooled nitrogen is subjected to expansion with the performance of external work; the thus expanded nitrogen is passed through a reboiler (18; 86; 220) associated with the first distillation column (2; 72; 216) to provide reboil for the distillation in that column (2; 72; 216), the nitrogen leaving the reboiler (18; 86; 220) is subjected to further cooling and temperature reduction by heat exchange, and part of the resulting liquid nitrogen is taken as product. 2. A method as claimed in Claim 1, in which a third part of said liquid nitrogen is used to condense nitrogen vapour from the liquid-vapour contact column (14; 79; 256), resulting condensate being returned to the liquid-vapour contact column (14; 79; 256), said third part of said liquid nitrogen being thereby vaporised and at least part of the vapour being recycled for liquefaction. 3. A method as claimed in Claim 1 or Claim 2, in which the liquid-vapour contact column (14; 79; 256) operates at a pressure of at least 5 atmospheres. 4. A method as claimed in any one of the preceding claims, in which the remixed stream of oxygen and nitrogen is withdrawn from an intermediate level of the liquid-vapour contact column (14;79; 256). 5. A method as claimed in Claim 4, in which another remixed oxygen-nitrogen stream (containing a greater proportion of oxygen than does the other such stream) is withdrawn from the liquid-vapour contact column (14), is condensed by heat exchange with a stream of boiling liquid from one of the distillation columns (4) and is returned to the liquid-vapour contact column (14), the boiled liquid being returned to one of the distillation columns (4). 6. A method as claimed in any one of the preceding claims, in which reboil for the first distillation column (2; 72; 216) is provided by a stream of nitrogen at least part of which is liquefied downstream of where is provides reboil for the first distillation column (2; 72; 216). 7. A method as claimed in any one of the preceding claims, in which a stream of air is used to provide some of the reboil for the first distillation column (2; 72; 216) and resulting liquid air is then introduced into the first distillation column. 8. A method as claimed in any one of the preceding claims, in which the nitrogen withdrawn from the first distillation column (2; 72; 216) is compressed to a pressure in excess of its critical pressure, and is taken for expansion with the performance of external work at a pressure in the range 50 to 75 atmospheres; at the completion of work expansion the nitrogen is a saturated vapour at a pressure in the range 12 to 20 atmospheres; liquid nitrogen leaving the reboiler (18; 86; 220) is subjected to a plurality of flash separation steps to provide liquid nitrogen and a plurality of flash gas streams; and liquid nitrogen from a first flash separation step is employed to condense nitrogen vapour from the liquid vapour contact column (14; 79; 256), resulting condensate being returned to the liquid-vapour contact column (14; 79; 256) as reflux. 9. A method as claimed in any one of the preceding claims, in which the second distillation column (10; 78; 226) is reboiled. 10. A method as claimed in Claim 9, in which reboil for the second distillation column (10; 78; 226) is provided by nitrogen vapour from the first distillation column (2; 72; 216). 11. A method as claimed in Claim 10, in which additional reboil is provided by nitrogen vapour from an additional distillation column (12) to which a part of the air feed is sent. 12. Apparatus or plant for separating air into oxygen, nitrogen and argon, comprising means (201) for extracting carbon dioxide and water vapour from the air; heat exchange means (204; 206; 210; 212) for cooling air to a cryogenic temperature suitable for its separation by fractional distillation; a first distillation column (2; 72; 216) for subjecting the air to fractional distillation, said column having outlets (24; 30; 29; 88; 96; 120; 258; 228; 268) for the withdrawal of liquid oxygen, nitrogen vapour and argon-enriched oxygen; a second distillation column (10; 78; 226) communicating with said outlet (29; 120; 268) for argon-enriched oxygen operable to separate argon therefrom; an outlet (52; 128; 292) for argon from said second distillation column (10; 78; 226); and means for liquefying at least some of the nitrogen vapour by performing a cycle which includes heat exchange means (204; 206; 210; 212; 214) for warming the nitrogen vapour, at least one compressor (234; 236) for raising the pressure of the warmed nitrogen to a pressure substantially in excess of the pressure at which the first distillation column (2; 72; 216) operates, heat exchange means (204; 206; 210; 212; 214) for cooling and reducing the temperature of the compressed nitrogen and an inlet (28; 94; 222) for returning a part of the liquid nitrogen as reflux to said first distillation column (2; 72; 216), wherein the apparatus or plant additionally includes a liquid-vapour contact column (14; 79; 256) for remixing liquid oxygen and impure nitrogen vapour, said liquid-vapour contact column (14; 79; 256) having an inlet (100) for liquid oxygen in communication with the outlet (24; 88; 258) for liquid oxygen from the first distillation column (2; 72; 216) and an inlet for impure nitrogen vapour in communication with an outlet (26; 98; 254) for such vapour from the first distillation column (2; 72; 216), and also having an outlet (22; 84; 260) for impure liquid nitrogen, in communication with an inlet for such nitrogen to the first distillation column (2; 72; 216), and another outlet (34; 264) for a remixed oxygen-nitrogen mixture communicating with at least one machine (266) for expanding said mixture with the performance of external work so as to create refrigeration characterised in that the nitrogen liquefaction cycle utilises at least part of the heat exchange means (204; 206; 210; 212) for cooling the air as the heat exchange means (204; 206; 210; 212; 214) for warming the nitrogen vapour and as the heat exchange means (204; 206; 210; 212; 214) for cooling the compressed nitrogen, and utilises an expansion turbine (238) for expanding at least a part of the cooled, compressed, nitrogen with the performance of external work; and in that the outlet of the expansion turbine (238) communicates with a reboiler (18; 86; 220) associated with the first distillation column (2; 72; 216); the reboiler (18; 86; 220) has an outlet for nitrogen communicating with a part (212; 214) of said heat exchange means (204; 206; 210; 212; 214) for cooling the compressed nitrogen, and the apparatus or plant has an outlet (290) for product liquid nitrogen.

Inventors: LAVIN, JOHN TERENCE, LAYLAND, DAVID JOHN
Applicants: THE BOC GROUP PLC
Application year: 1989
Main classification: F25J3
IPC codes: F25J3