Air separation

Abstract: Cooled and purified air is introduced into a higher pressure rectifier 6 in at least partly vaporous state and is separated therein into oxygen-enriched liquid air and nitrogen. One part of the nitrogen so separated is condensed in a reboiler-condenser 12 and another part in a reboiler-condenser 16. Some of the condensate is used as reflux in the rectifier 6, and the rest of the condensate as reflux in a low pressure rectifier 4. Oxygen-enriched liquid air is taken from the bottom of the higher pressure rectifier 6 and is separated into oxygen and nitrogen in the lower pressure rectifier 4. A liquid argon-enriched oxygen stream is withdrawn from the lower pressure rectifier 4 through an outlet 42 and is separated into argon and oxygen fractions in a further rectifier 18. The further rectifier 18 is reboiled by the reboiler-condenser 16.

Description: This invention relates to a method and apparatus for separating air. The most important method commercially of separating air is by rectification. The most frequently used air separation cycles include the steps of compressing a stream of air. purifying the resulting stream of compressed air by removing water vapour and carbon dioxide, and pre-cooling the stream of compressed air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure and a lower pressure rectification column i.e. one of the two columns operates at higher pressure than the other. Most if not all of the air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and liquid nitrogen vapour. The nitrogen vapour is condensed. A part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column, is sub-cooled, and is introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid is separated into substantially pure oxygen and nitrogen products in the lower pressure column. These products are withdrawn in the vapour state from the lower pressure column and form the returning streams against which the incoming air stream is heat exchanged. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it, and passing it into the top of the lower pressure column through a throttling or pressure reduction valve. Conventionally, liquid oxygen at the bottom of the lower pressure column is used to meet the condensation duty at the top of the higher pressure column. Accordingly, nitrogen vapour from the top of higher pressure column is heat exchanged with liquid oxygen in the bottom of the lower pressure column. Sufficient liquid oxygen is able to be evaporated thereby to meet the requirements of the lower pressure column for reboil and to enable a good yield of pure gaseous oxygen product to be achieved. Sometimes additional columns are added to assist in the separation of oxygen from the air. Examples of such processes are given in US-A- 4 533 375. The pre-characterising parts of claims 1 and 12 are based on US-A- 4 533 375. An alternative to the conventional process is to use a part of the feed air to provide the necessary heat to reboil liquid in a first reboiler-condenser at the bottom of the low pressure column. This alternative removes the link between the top of the higher pressure column and the bottom of the lower pressure column. Accordingly, the operating pressure ratio between the two columns can be reduced, thus reducing the energy requirements of the air separation process. Nitrogen separated in the higher pressure column is condensed in a second reboiler-condenser by heat exchange with liquid withdrawn from an intermediate mass-exchange region of the lower pressure rectification column. This alternative kind of process is sometimes referred to as a "dual reboiler" process. Examples of such processes are given in EP-A- 0 450 768. In addition Figures 1 and 2 of JP-A-60-42583 disclose such processes in which, in addition, an argon product is separated. The bottom of the argon rectification column is reboiled by vapour from the top of the higher pressure rectification column. A pure oxygen-enriched liquid stream is fed from the bottom of the lower pressure rectification column to the bottom of the argon rectification column. A further stream of unspecified state and composition is fed from the lower pressure rectification column to the argon rectification column. One disadvantage of dual reboiler processes is a difficulty in obtaining an argon product by rectification of an argon-enriched oxygen stream withdrawn from the lower pressure rectification column. In order to produce such an argon product effectively, it is desirable to operate the bottom section of the lower pressure rectification column at a relatively high reboil rate so as to achieve conditions therein close to minimum reflux. To achieve such a high reboil rate, air would need to be condensed in the first reboiler-condenser at a relatively high rate with an attendant high rate of condensation of the air. Introduction of such liquid air into the higher pressure column reduces the rate of formation of liquid nitrogen reflux available to the lower pressure column. As a result, attempts to achieve an adequate argon recovery by increasing the reboil rate beyond a certain limit would become self-defeating. It is an aim of the present invention to provide a method and apparatus that ameliorate this problem. According to the present invention the e is provided a method of separating argon as set out in claim 1 below. The invention also provides apparatus for separating air as set out in claim 5. By the term "rectifier" as used herein is meant a fractionation or rectification column in which, in use, ascending vapour phase is contacted with a descending liquid phase, or a plurality of such columns operating at generally the same pressure as one another. References herein to "reboiling" a rectifier mean that a liquid feed or liquid taken out of mass exchange relationship with ascending vapour in a rectifier is boiled at least in part so as to create an upward flow of vapour through the rectifier. The boiling is typically performed by indirect heat exchange with condensing vapour in a condenser-reboiler. The condenser-reboiler may be located within or outside the rectifier. If the liquid is taken from an intermediate mass exchange region of a rectifier, the reboiling may be said to be performed in an "intermediate" reboiler. The reboiling of the further rectifier in which the argon product is separated has the consequence of reducing the amount of air that needs to be condensed in reboiling the lower pressure rectifier (in comparison with similar processes in which the feed to the further rectifier is taken from the lower pressure rectifier in vapour state and therefore no reboiling of the further rectifier takes place). Accordingly, a greater proportion of the oxygen product of the method and apparatus according to the invention may be of a relatively high purity (i.e. above 99% by volume of oxygen) and a greater yield or recovery of argon can be achieved. Preferably, the further column employs random or structured packing to effect liquid-vapour contact therein. A low pressure drop packing (e.g. that sold under the trade mark MELLAPAK) is preferably employed. By a low pressure drop packing is meant one that has a pressure drop of less than 2 millibars per theoretical stage. By reducing the pressure of the feed to the further rectifier and by employing a low pressure drop packing in the further rectifier, it is possible to widen the temperature difference between the bottom and the top of the further rectifier, thereby making possible an enhancement of argon recovery. Preferably, a liquid stream is withdrawn from the bottom of the further rectifier as an oxygen product. The purity of this oxygen product depends on the amount of separation of oxygen from the argon that takes place in the further rectifier below the level at which the argon-enriched liquid feed is introduced. Operation of the intermediate pressure rectifier enhances the rate at which liquid nitrogen reflux may be supplied to the higher and lower pressure rectifiers and thereby makes possible a further enhancement in the proportion of the argon in the incoming air that can be recovered and further increase in the proportion of the oxygen product that can be produced at a purity greater than 99% by volume. Another stream of liquid air further enriched in oxygen is preferably taken from the bottom of the intermediate pressure rectifier column, is reduced in pressure, and is employed to condense nitrogen-enriched vapour separated in the intermediate pressure rectifier. The condensation is preferably performed in a condenser-reboiler with resulting reboiled further-enriched liquid being introduced into the lower pressure rectifier as feed. Preferably a part of the condensed nitrogen-enriched vapour is employed as reflux in the intermediate pressure rectifier and another part of the condensed nitrogen-enriched vapour is preferably nitrogen of essentially the same purity as that separated in the higher pressure rectifier. If desired, a yet further part of the condensed nitrogen-enriched vapour may be taken as a nitrogen product. In some examples, nitrogen separated in the higher pressure rectifier is also used to reboil the intermediate pressure rectifier, this nitrogen also being condensed. Accordingly in such examples there are several different sources of liquid nitrogen reflux and as a result well in excess of 40% of the argon in the air fed to the method can be recovered as product and well in excess of 30% of the oxygen product can be produced at a purity of 99.5%. Typically, however, it is not possible in such examples to produce all the oxygen product at a purity of 99.5%: it is necessary to take some of the oxygen product at a lower purity. Air is condensed as a result of the reboiling of the lower pressure rectifier. A part or all of the air stream used to reboil the lower pressure rectifier may be so condensed. If all of the air stream is so condensed, there is a separate feed of vaporous air to the higher pressure rectifier. If the air stream is only partly condensed, it may form the flow to the higher pressure rectifier of compressed and cooled feed air. Alternatively, the liquid and vapour phases may be disengaged from one another with the vapour phase sent to the higher pressure rectifier and the liquid phase sent to one or more of the lower pressure rectifier, the higher pressure rectifier, and, if employed, the intermediate pressure rectifier. Similarly, if all the air stream used to reboil the lower pressure rectifier is condensed, it may be distributed to one or more of the aforesaid rectifiers. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a simplified schematic, flow diagram illustrating an arrangement of rectifiers. The arrangement shown in Figure 1 is not within the scope of any of the claims. Figure 2 is a schematic flow diagram of a first air separation plant for performing the method according to the invention. The drawings are not to scale. Referring to Figure 1 of the drawings, a first stream of compressed vaporous air which has been purified by removal of its components of low volatility, particularly water vapour and carbon dioxide, and cooled to approximately its saturation temperature is partially condensed by passage through the condensing passages (not shown) of a condenser-reboiler 2. The reboiling passages (not shown) of the condenser-reboiler 2 are arranged to provide reboil for a lower pressure rectifier 4 as will be described below. The partially condensed stream of air flows from the condenser-reboiler 2 into the bottom of a higher pressure rectifier 6 through an inlet 8. The higher pressure rectifier 6 is fed with a second stream of compressed and purified liquid air through an inlet 10. The higher pressure rectifier 6 contains liquid-vapour contact devices (not shown) whereby a descending liquid phase is brought into intimate contact with an ascending vapour phase such that mass transfer between the two phases takes place. The descending liquid phase becomes progressively richer in oxygen and the ascending vapour phase progressively richer in nitrogen. The liquid-vapour contact means may comprise an arrangement of liquid-vapour contact trays or may comprise structured or random packing. Liquid collects at the bottom of the higher pressure rectifier 6. The inlets Band 10 are located such that the liquid so collected is approximately in equilibrium with incoming vaporous air. Accordingly, since oxygen is less volatile than the other main components (nitrogen and argon) of the air, the liquid collecting at the bottom of the rectifier 6 is enriched in oxygen and typically contains in the order of from 30 to 35% by volume of oxygen. A sufficient number of trays or a sufficient height of packing is included in the higher pressure rectifier 6 for the vapour to be produced at its top to be essentially pure nitrogen. The nitrogen is condensed so as to provide a downward flow of reflux for the higher pressure rectifier 6 and also to provide such reflux for the lower pressure rectifier 4. Condensation of the nitrogen is effected primarily by indirect heat exchange of a stream of it in the condensing passages (not shown) of another condenser-reboiler 12 with boiling liquid in the liquid passages (not shown) thereof. The condenser-reboiler 12 is associated with an intermediate region of the lower pressure rectifier 4 and provides intermediate reboil for this rectifier 4. Thus liquid is withdrawn from an intermediate mass exchange region of the lower pressure rectifier 4 and is reboiled in the boiling passages (not shown) of the condenser-reboiler 12. A part of the condensed nitrogen is returned to the higher pressure rectifier 6 as reflux. Another part is sub-cooled, is passed through a throttling valve 14 and is introduced into the top of the lower pressure rectifier 4 as reflux. Another stream of nitrogen vapour separated in the higher pressure rectifier 6 is reduced in pressure by passage through a throttling valve 15 and is condensed by indirect heat exchange in the condensing passages (not shown) of another condenser-reboiler 16 which is associated with the bottom of a further rectifier 18 in which argon and impure oxygen products are separated. The resulting nitrogen condensate is returned by a pump 20 to the higher pressure rectifier 6 as liquid nitrogen reflux. A stream of oxygen-enriched liquid is withdrawn from the bottom of the higher pressure rectifier 6 through an outlet 22, is sub-cooled, and is divided into two subsidiary streams. One of the subsidiary streams is reduced in pressure by passage through a throttling valve 24 to a pressure a little above the operating pressure of the lower pressure rectifier 4. The pressure-reduced stream of oxygen-enriched liquid air is employed in a condenser 26 to condense argon separated in the further rectifier 18. The pressure-reduced stream of oxygen-enriched liquid air is thus vaporised and the resulting vapour stream is introduced as feed into the lower pressure rectifier 4 through an inlet 28 at an intermediate level thereof. The other subsidiary stream of sub-cooled, oxygen-enriched liquid air flows through a throttling valve 30 and is thereby reduced in pressure. Downstream of the throttling valve 30, the other subsidiary stream of sub-cooled, oxygen-enriched liquid air flows into an intermediate region of the lower pressure rectifier 4 through an inlet 32 at a level above that of the inlet 28. The lower pressure rectifier 4 also receives a feed stream of liquid air through an inlet 34 located above the inlet 32 and a feed stream of vaporous air through an inlet 36 located at the same level as the inlet 32. The various air streams fed to the lower pressure rectifier 4 are separated therein into oxygen and nitrogen products. In order to effect the separation, liquid-vapour contact devices (not shown), for example distillation trays or random or structured packing, are provided in the rectifier 4 to effect intimate contact between ascending vapour and descending liquid therein, thereby enabling mass transfer to take place between the two phases. The downward flow of liquid is created by the introduction of the liquid nitrogen into the top of the rectifier 4 and by the introduction of the liquid streams into the rectifier 4 through the inlets 32 and 34. The upward flow of vapour is created by operation of the condenser-reboilers 2 and 12 and by the introduction of vapour streams into the lower pressure rectifier 4 through the inlets 28 and 36. An essentially pure vaporous nitrogen product is withdrawn from the low pressure rectifier through an outlet 38. An oxygen product (typically 99.5% pure) is withdrawn in liquid state from the bottom of the rectifier 4 through an outlet 40. Although air contains only about 0.93% by volume of argon, a peak argon concentration typically in the order of 8% is created at an intermediate region of the lower pressure rectifier 4 below the condenser-reboiler 12. The lower pressure rectifier is thus able to act as a source of argon-enriched oxygen for separation in the further rectifier 18. An argon-enriched liquid oxygen stream typically containing about 5 mole per cent of argon is withdrawn from the lower pressure rectifier 4 through an outlet 42, is reduced in pressure by passage through a throttling valve 44 and is introduced into the further rectifier 18 through an inlet 46. The further rectifier 18 contains a low pressure drop structured or random packing in order to effect intimate liquid-vapour contact and hence mass transfer between a descending liquid phase and an ascending vapour phase. Packing is located in the further rectifier 18 both above and below the level of the inlet 46. The downward flow of liquid through the further rectifier 18 is created by operation of the condenser 26, and is augmented in the bottom region of the rectifier 18 by the liquid feed introduced through the inlet 46. The upward flow of vapour through the further rectifier 18 is created by operation of the condenser-reboiler 16 to reboil liquid at the bottom of the rectifier 18. A liquid argon product is withdrawn from the condenser 26 through an outlet 48. The purity of the argon product depends on the height of packing in the further rectifier 18 above the level of the inlet 46. If a sufficient height of packing to provide in the order of 180 theoretical plates is employed above the level of the inlet 46, an essentially oxygen-free argon product is produced. Alternatively, however, a substantially smaller height of packing, providing substantially fewer theoretical plates, may be used above the level of the inlet 46. An argon product containing, say, from 0.2 to 2% by volume of oxygen impurity may thereby be produced. Such an argon product may be purified by catalytic reaction with hydrogen, adsorptive removal of water vapour and yet further rectification to remove nitrogen and hydrogen impurities. An impure oxygen product is withdrawn in liquid state through an outlet 50 from the bottom of the further rectifier 18. The oxygen products may be produced at elevated pressure by raising the pressure of the products in pumps (not shown) and vaporising the respective pressurised oxygen streams. Various heat exchangers (not shown) may be employed to effect the cooling and sub-cooling of streams flowing to and from the columns. One or more feed air streams or one or more product nitrogen streams may be expanded with the performance of external work in order to create refrigeration for the method and thereby to maintain a heat balance. The further rectifier 18 is preferably operated at a pressure in the range of 1 bar to 1.1 bar at its top and the lower pressure rectifier 4 is preferably operated with a pressure in the range of 1.2 to 1.5 bar at its top. Since the bottom of the lower pressure rectifier 4 is not thermally linked by a condenser-reboiler to the top of the higher pressure rectifier 6 (which is the arrangement in a conventional double rectification column for the separation of air) the higher pressure rectifier 6 may be operated at a lower pressure (at its top) than in a conventional double rectification column. Indeed, the higher pressure rectifier 6 is preferably operated at a pressure in the range of 3.75 to 4.5 bar. The arrangement of rectifiers 4, 6 and 18 shown in Figure 1 make possible the production of an argon product by virtue of the fact that the operation of the condenser-reboiler 16 enhances the rate at which liquid nitrogen reflux is produced while at the same time reducing the reboil duty on the condenser-reboiler 2 and thus reducing the proportion of the incoming air that needs to be condensed in the condenser-reboiler 2. Nonetheless, the yield of argon that can be achieved and the proportion of the oxygen product that can be produced are still limited by a pinch appearing in the lower pressure rectifier 4 at the inlet 28. This pinch point effectively limits the proportion of the higher pressure rectifier's condensation duty that can be used to provide reboil for the further rectifier 18, and hence limits the argon recovery to approximately 40% of that contained in the feed air. In Figure 2 of the accompanying drawings there is shown an air separation plant with an improved arrangement of columns in accordance with the invention which is able to enhance the rate at which liquid nitrogen reflux is produced and thus increase the argon yield and the proportion of the total oxygen product that can be produced at relatively high purity. Referring to Figure 2 of the drawings, a feed air stream is compressed in a compressor 52 and the resulting compressed feed air stream is passed through a purification unit 54 effective to remove water vapour and carbon dioxide therefrom. The unit 54 employs beds (not shown) of adsorbent to effect this removal of water vapour and carbon dioxide. The beds are operated out of sequence with one another such that while one or more beds are purifying the feed air stream, the remainder are being regenerated, for example by being purged with a stream of hot nitrogen. Such a purification unit and its operation are well known in the art and need not be described further. The purified feed air stream is divided into three subsidiary air streams. A first subsidiary air stream flows through a main heat exchanger 56 from its warm end 58 to its cold end 60 and is thereby cooled from about ambient temperature to just above its saturation temperature (or other temperature suitable for its separation by rectification). The thus cooled air stream flows through a condenser-reboiler 62 and is partially condensed therein. The resulting partially condensed air stream is introduced into a higher pressure fractionation column 64 through an inlet 66. An alternative arrangement (which is not shown) is to divide the first subsidiary air stream downstream of the cold end 60 of the main heat exchanger 56 and introduce one part directly into the higher pressure fractionation column 64 and to condense entirely the other part in the condenser-reboiler 62 upstream of its introduction into the column 64. In addition to the feed through the inlet 66, the higher pressure fractionation column 64 is also fed with a liquid air stream. To this end, a second subsidiary stream of purified air is further compressed in a compressor 68 and cooled to its saturation temperature by passage through the main heat exchanger 56 from its warm end 58 to its cold end 60. The thus cooled second subsidiary air stream is divided into three parts. One part flows through a throttling valve 70 and is introduced into the higher pressure fractionation column 64 through an inlet 72. The use to which the other parts of the cooled second subsidiary air stream is put will be described below. The higher pressure fractionation column 64 contains liquid-vapour contact devices (not shown) whereby a descending liquid phase is brought into intimate contact with an ascending vapour phase such that mass transfer between the two phases takes place. The descending liquid phase becomes progressively richer in oxygen and the ascending vapour phase progressively richer in nitrogen. The liquid-vapour contact devices may comprise an arrangement of liquid-vapour contact trays or may comprise structured or random packing Liquid collects at the bottom of the higher pressure fractionation column 64. The inlets 66 and 72 are located such that the liquid so collected is approximately in equilibrium with incoming vaporous air. Accordingly, since oxygen is less volatile than the other main components (nitrogen and argon) of the air, the liquid collecting at the bottom of the column 64 is enriched in oxygen and typically contains in the order of from 30 to 35% by volume of oxygen. A sufficient number of trays or a sufficient height of packing is included in the higher pressure fractionation column 64 for the vapour produced at the top of the column 64 to be essentially pure nitrogen. The nitrogen is condensed so as to provide a downward flow of liquid nitrogen reflux for the column 64 and also to provide such reflux for a lower pressure rectification column 74 with which boiling passages (not shown) of the first condenser-reboiler 62 are associated. Condensation of the nitrogen is effected in three further condenser-reboilers 76, 78 and 80. The boiling passages (not shown) of the condenser-reboiler 76, 78 and 80 are respectively associated with an intermediate mass transfer region of the lower pressure rectification column 74, the bottom of an intermediate pressure rectification column 82, and the bottom of a further rectification column 84 for producing argon and oxygen products. That part of the nitrogen condensed in the condenser-reboiler 76 which is not required as reflux in the higher pressure rectification column 64 is sub-cooled in a heat exchanger 86, is passed through a throttling valve 88, is introduced through an inlet 90 into the top of the lower pressure rectification column 74, and provides liquid nitrogen reflux for that column. A stream of oxygen-enriched liquid is withdrawn from the bottom of the higher pressure fractionation column 64 through an outlet 92, is sub-cooled in the heat exchanger 86, is reduced in pressure by passage through a throttling valve 94, and is introduced into the bottom of the intermediate pressure rectification column 82. The intermediate pressure rectification column 82 is also fed with one of the two parts of the cooled second subsidiary air stream that are not sent to the higher pressure fractionation column 64. This part is reduced in pressure by passage through a throttling valve 96 upstream of its introduction in liquid state into the intermediate pressure rectification column 82 through an inlet 98. The intermediate rectification column 82 separates the air into firstly liquid air further enriched in oxygen and secondly nitrogen. The column 82 is provided with liquid-vapour contact devices (not shown) such as trays or structured packing to enable an ascending vapour phase to come into intimate contact with a descending liquid phase, thereby enabling mass transfer to take place between the two phases. The upward flow of vapour is created by boiling the liquid that collects at the bottom of the intermediate rectification column 82. This boiling is carried out in the boiling passages (not shown) of the condenser-reboiler 78, by indirect heat exchange with condensing nitrogen. A sufficient number of trays or a sufficient height of packing is included in the intermediate pressure column 82 to ensure that essentially pure nitrogen is produced at its top. A stream of this nitrogen vapour is withdrawn from the top of the intermediate pressure rectification column 82 and is condensed in a condenser 100. One part of the condensate is used as liquid nitrogen reflux in the intermediate pressure rectification column 82. Another part is pressurised by a pump 102 and is passed through the main heat exchanger 56 from its cold end 60 to its warm end 58. The pressurised nitrogen stream is thus vaporised and emerges from the warm end 58 of the main heat exchanger 56 as a high pressure nitrogen product at approximately ambient temperature. A third part of the nitrogen condensed in the condenser 100 is reduced in pressure by passage through a throttling valve 104, and is introduced into the top of the lower pressure rectification column 74 as reflux through an inlet 106. It will be appreciated, therefore, that operation of the intermediate pressure rectification column 82 enhances the rate at which nitrogen separated in the higher pressure fractionation column 64 can be condensed, and enhances the rate at which liquid nitrogen reflux can be provided to the columns 64 and 74. A stream of liquid air further enriched in oxygen (typically containing about 40% by volume of oxygen) is withdrawn through an outlet 108 from the bottom of the intermediate pressure rectification column 82. The stream is divided into two parts. One part flows through a throttling valve 110 in order to reduce its pressure to a little above that at which the lower pressure rectification column 74 operates. The pressure reduced stream of further enriched liquid air flows through the condenser 100 in indirect heat exchange relationship with condensing nitrogen. Cooling is thus provided for the condenser 100 and the further-enriched liquid air is reboiled by the heat exchange. The resulting vaporised further enriched air stream is introduced through an inlet 112 into the lower pressure rectification column 74 at an intermediate liquid vapour contact region thereof. The other part of the further-enriched liquid air stream that is withdrawn from the bottom of the intermediate pressure rectification column 82 is divided again into two streams. One of these streams is reduced in pressure by passage through a throttling valve 114 and is introduced into the lower pressure rectification column 74 through an inlet 116 at a level above that of the inlet 112. The other stream of further enriched liquid air flows through a throttling valve 118 in order to reduce its pressure. The pressure-reduced further-enriched liquid air stream flows from the valve 118 through a condenser 120 which is associated with the head of the further rectification column 84. (The column 84 is located by the side of and fed from the lower pressure rectification column 74.) The stream of further-enriched liquid air flowing through the condenser 120 is reboiled and the resulting vapour is introduced into the lower pressure rectification column 74 through an inlet 122 at the same level as the inlet 112. Further air feed streams for the lower pressure rectification column 74 are provided. First, the third part of the cooled second subsidiary air stream is taken from downstream of the cold end 60 of the main heat exchanger 56, is sub-cooled by passage through the heat exchanger 86, is passed through a throttling valve 124, and is introduced into the lower pressure rectification column 74 as a liquid stream through an inlet 126 at a level above that of the inlet 116 but below that of the inlets 90 and 106. Second, the third subsidiary purified air stream is employed as a feed to the lower pressure rectification column 74. This stream is further compressed in a compressor 128, cooled to a temperature of about 150K by passage through the main heat exchanger 56 from its warm end 58 to an intermediate region thereof, is withdrawn from the intermediate region of the main heat exchanger 56, is expanded to a pressure a little above that of the lower pressure rectification column 74 in an expansion turbine 130, and is introduced into the column 74 through an inlet 132 at the same level as the inlet 116. Expansion of the third subsidiary air stream in the turbine 130 takes place with the performance of external work which may, for example, be the driving of the compressor 128. Accordingly, if desired, the rotor (not shown) of the turbine 130 may be mounted on the same drive shaft as the rotor (not shown) of the compressor 128. Operation of the turbine 130 generates the necessary refrigeration for the air separation process. The amount of refrigeration required depends on the proportion of the incoming air that is separated into liquid product. In the plant shown in the drawing, only argon is produced in liquid state. Accordingly, only one turbine is required. The various air streams fed to the lower pressure rectification column 74 are separated therein into oxygen and nitrogen products. In order to effect the separation, liquid-vapour contact devices (not shown), for example distillation trays or random or structured packing, are provided in the column 74 to effect intimate contact between ascending vapour and descending liquid therein, thereby enabling mass transfer to take place between the two phases. The downward flow of liquid is created by the introduction of liquid nitrogen reflux into the column 74 through the inlets 106 and 90. Indirect heat exchange of liquid at the bottom of the column 74 with condensing air in the condenser-reboiler 62 provides an upward flow of vapour in the column 74. This upward flow is augmented by operation of the condenser-reboiler 76 which reboils liquid withdrawn from mass exchange relationship with vapour at an intermediate level of the column 74, typically below that of the inlets 112 and 122. An essentially pure nitrogen product is withdrawn from the top of the lower pressure rectification column 74 through an outlet 133, is warmed by passage through the heat exchanger 86 countercurrently to the streams being sub-cooled therein, and is further warmed by passage through the main heat exchanger 56 from its cold end 60 to its warm end 58. A pure nitrogen product at a relatively low pressure is thus able to be produced at approximately ambient temperature. A relatively pure oxygen product (typically containing 99.5% oxygen) is withdrawn in liquid state through an outlet 134 at the bottom of the column 74 and is pressurised by a pump 136 to a desired elevated supply pressure. The resulting pressurised liquid oxygen stream is vaporised by passage through the heat exchanger 56 from its cold end 60 to its warm end 58. Although the incoming air contains only about 0.93% by volume of argon, a higher peak argon concentration is created at an intermediate region of the lower pressure rectification column 74. The column 74 is thus able to act as a source of argon-enriched oxygen for separation in the further rectification column 84. An argon-enriched oxygen stream in liquid state is taken from an intermediate liquid-vapour contact region of the low pressure rectification column 74 where the argon concentration is about 7% by volume (and only traces of nitrogen are present). The liquid argon-enriched oxygen stream is withdrawn from the column 74 through an outlet 138, is reduced in pressure by passage through a throttling valve 140 and is introduced into an intermediate region of the further rectification column 84 through an inlet 142. The further rectification column 84 contains a low pressure drop packing (preferably structured packing) (not shown) to enable ascending vapour to come into intimate contact with descending liquid. Packing is provided in the column both below and above the level of the inlet 142. The descending flow of liquid above the level of the inlet 142 is created by condensation in the condenser 120 of vapour taken from the head of the further rectification column 84. A part only of the condensate provides the reflux for the further column 84; the remainder of the condensate is taken as argon product through an outlet 144. The upward flow of vapour through the rectification column 84 is created by reboiling of liquid collecting at the bottom of the column 84. The reboiling is performed in the condenser-reboiler 80 by indirect heat exchange with nitrogen separated in the higher pressure fractionation column 64. A stream of such nitrogen is supplied via a throttling valve 146 to the condensing passage of the condenser-reboiler 80, is condensed therein and is returned as reflux to the higher pressure rectification column 64 by a pump 148. An impure oxygen product typically containing 98.5% by volume of oxygen is withdrawn from the bottom of the further rectification column 84 through an outlet 150 by a pump 152 which raises the oxygen to a supply pressure. The resulting impure oxygen product is vaporised by passage through the main heat exchanger 56 from its cold end 60 to its warm end 58. The pressure at which the second subsidiary purified air stream is passed through the main heat exchanger 56 is selected so as to maintain a close match between the temperature-enthalpy profile of this stream and that of the vaporising liquid oxygen streams. In a typical example of the operation of the plant shown in Figure 2 of the drawings, the higher pressure fractionation column 64 operates at a pressure in the range of 3.75 to 4.5 bar at its top; the intermediate pressure rectification column 82 at a pressure in the range of 2.4 to 2.8 bar at its top; the lower pressure rectification column 74 at a pressure of about 1.3 bar at its top; and the argon rectification column 84 at a pressure of about 1.05 bar at its top. The impure and pure oxygen products are typically produced in this example at a pressure of 8 bar and the pressurised nitrogen product at a pressure of 10 bar. Further, in this example, the compressor 68 has an outlet pressure of 24 bar and the compressor 128 an outlet pressure of 7 bar. By virtue of the operation of the intermediate pressure rectification column 82, it is possible in this example to recover up to 50% of the argon in the incoming air as an argon product and to produce up to 35% of the oxygen product at a purity of 99.5%. Although the argon recovery of the plant shown in Figure 2 is not limited by conditions in the top section of the lower pressure rectification column 74, a limitation would nonetheless appear at a maximum argon condensation duty in the condenser 120. If further enriched liquid is vaporised at too high a rate in the condenser 120, a pinch in the lower pressure rectification column occurs at the point where the vapour is introduced into it.

Claims: A method of separating argon from air comprising the steps of introducing a flow of compressed and cooled feed air in at least partly vapour state into a higher pressure rectifier and separating the flow into oxygen-enriched liquid air and nitrogen; condensing nitrogen so separated and employing one part of the condensate as reflux in the higher pressure rectifier and another part of it as reflux in a lower pressure rectifier; separating in the lower pressure rectifier a stream of oxygen-enriched liquid air derived indirectly from the higher pressure rectifier withdrawing a stream of argon-enriched liquid oxygen from the lower pressure rectifier and separating it by rectification in a further rectifier to produce an argon product, wherein the lower pressure rectifier is reboiled with a vapour stream of feed air and at least part of the said nitrogen is condensed by being employed to reboil the further rectifier, the lower pressure rectifier's reboiled at an intermediate level in addition to its being reboiled by the said stream of feed air, the reboiling at the intermediate level being performed by nitrogen vapour separated in the higher pressure rectifier, the nitrogen vapour thereby being condensed, the argon-enriched liquid oxygen stream is reduced in pressure upstream of its introduction to the further rectifier and liquid-vapour contact devices are employed below as well as above the level at which the argon-enriched liquid feed is introduced into the further rectifier, whereby separation takes place within the further rectifier both above and below said level, and a stream of oxygen-enriched liquid air is introduced into an intermediate pressure rectifier in which nitrogen-enriched vapour is separated therefrom, and a liquid air stream further enriched in oxygen is withdrawn from the intermediate pressure rectifier and fed to the lower pressure rectifier. A method as claimed in claim 1, wherein a part of the stream of liquid air further enriched in oxygen which is fed to the lower pressure rectifier is employed to condense argon separated in the further rectifier, and a part of the resulting argon condensate is returned to the further rectifier as reflux, and another part is taken as product; another stream of liquid air further enriched in oxygen is withdrawn from the bottom of the intermediate pressure rectifier, is reduced in pressure, and is employed to condense nitrogen-enriched vapour separated in the intermediate pressure rectifier by indirect heat exchange therewith; the other stream of liquid air is reboiled by its heat exchange with the nitrogen-enriched vapour, and the resulting reboiled stream of further-enriched air is introduced into the lower pressure rectifier. A method as claimed in claim 2 in which nitrogen separated in the higher pressure rectifier is employed to reboil the intermediate pressure rectifier the said nitrogen thereby being condensed. A method as claimed in any one of the preceding claims in which a relatively impure oxygen product is withdrawn from the bottom of the further rectifier and a relatively pure oxygen product is withdrawn from the bottom of the lower pressure rectifier. Apparatus for separating air comprising a higher pressure rectifier (6;64) for separating compressed and cooled feed air into oxygen-enriched liquid air and nitrogen; a plurality of condensers (12, 16; 76, 76, 80) for condensing nitrogen so separated so as to enable in use part of the condensed nitrogen to be employed in the higher pressure rectifier (6; 64) as reflux and another part of it in a lower pressure rectifier (4; 74) also as reflux; means for taking oxygen-enriched air from the higher pressure rectifier (6; 64) and for introducing it via a further separating means (82) into the lower pressure rectifier (4; 74) for separation therein; and a further rectifier (18; 84) for producing an argon product having an inlet (46; 142) for an argon-enriched liquid oxygen stream communicating with an outlet (42; 142) from the lower pressure rectifier (4; 74), wherein the apparatus additionally includes a reboiler (2; 62) associated with the lower pressure rectifier (4; 74) having condensing passages in communication with a source of compressed and cooled feed air in vapour state; one (16; 80) of the said plurality of condensers (12, 16; 76, 78, 80) acts as a reboiler for the further rectifier (18; 84), another of the said plurality of condensers (12, 16; 76, 78, 80) is arranged to provide reboil at an intermediate level of the lower pressure rectifier the inlet (46; 142) for the argon-enriched liquid oxygen stream communicates with the outlet (42; 138) from the lower pressure rectifier (4; 74) via a throttling valve (44; 140); and there are liquid-vapour contact devices in the lower pressure rectifier (4; 74) both above and below the level of said inlet for the argonenriched liquid oxygen stream, and said further separating means (82) comprises an intermediate pressure rectifier (82), said intermediate pressure rectifier (82) having an outlet (108) for liquid air further enriched in oxygen communicating with the lower pressure rectifier (74). Apparatus as claimed in claim 5, in which there is an outlet (50; 150) for oxygen product at the bottom of the further rectifier (18; 84).

Inventors: RATHBONE THOMAS, RATHBONE, THOMAS
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1995
Main classification: F25J3
IPC codes: F25J3