AIR SEPARATION

Abstract: Air is fractionated in a double distillation column 4 comprising lower pressure and higher pressure columns 8 and 6 respectively. Condenser-reboiler 42 provides reboiled oxygen to the bottom of the column 8. Liquid nitrogen reflux is introduced into the top of the column 8 through an inlet 50. Oxygen, nitrogen and argon-enriched streams are withdrawn from the column 8 through outlets 52, 54 and 58 respectively. The argon-enriched stream is separated in a further distillation column 40 and a product argon stream is withdrawn through outlet 62. A liquid stream is withdrawn through outlet 70 from an intermediate level of the column 8 is at least partially reboiled in heat exchanger 72, and is returned to the column 8 through inlet 74 at a level where the composition of the vapour corresponds approximately to that of the reboiled liquid. Heating for the heat exchanger 72 is provided by a vapour stream withdrawn through outlet 76 from the column 40, which vapour stream condenses and is returned to the column 40 through inlet 75. The aforesaid reboiling of the stream from the intermediate level of the column 8 makes possible more efficient air separation.

Description: This invention relates to air separation. In "An Approach to Minimum Power Consumption in Low Temperature Gas Separation", Trans Instn Chem Engrs, Vol 36, 1958, G.G. Haselden identifies the irreversibility of the distillation columns as a key source of inefficiency in the operation of cryogenic air separation processes. It is pointed out in this paper that because of the change of slope of the reboil requirement curve in the lower part of an air separation column occuring at a vapour composition of about 50% oxygen, it is possible to make a simple approach towards ideal column operation by adding about half the reboil heat at a single level in the column a little below the feed, say at a temperature of 88K, the remaining half being added at the terminal temperature of 92.7K. It is further observed that any practical attempt to approach ideal non-adiabatic column operation by the use of distributed heating and cooling sources operating over extended zones of the column will be most effective for moderate product purities. A cycle is proposed utilising the column operating principles identified in the aforesaid paper. Even with the use of an auxiliary column, forty percent of the oxygen is produced at medium purity. In US patent specification 4 025 398, (G.G. Haselden) it is proposed that two distilling systems be arranged to interchange heat with each other in order to achieve a close approach to the kind of thermodynamic ideal discussed hereinabove. One distilling system comprises a first column having a rectifying section in which there are varying amounts of reflux, and a second column having a stripping section in which there are varying amounts of reboil. Thermal linkage between the two columns is provided by taking vapour from the variable reflux column, partially condensing it in the stripping column, and returning the resulting liquid-vapour mixture to the variable reflux column. The partial condensation takes place in passages formed in distillation trays of the stripping column. Heat is thus extracted from the stripping column and is transferred to the variable reflux column. In the drawings accompanying the aforesaid US patent specification, four trays are shown provided with such heat exchange passages and hence there are four associated liquid outlets from the variable reflux column and four associated inlets to the variable reflux column for the liquid-vapour mixture that is formed by partial evaporation of the liquid in the heat exchange passages. The streams of vapour are taken from the variable reflux column just below the level of chosen trays and the liquid-vapour mixture is returned to the column just above the respective trays. Although the proposals in US patent specification 4 025 398 represent an advance in the art, difficulties arise in fabricating a distillation system in accordance therewith to operate at cryogenic temperatures. First, it is not easy to provide a piece of apparatus that can function adequately as both a distillation tray and as a heat exchanger to enable the partial condensation of the vapour from the variable reflux rectifier to be effected. Moreover, in a practical distillation system operating at cryogenic temperatures a large number of trays are typically required. In order to approach the thermodynamic ideal set out in US patent specification 4 025 398 with such a system, it becomes necessary to provide a mutiplicity of passages extending from the variable reflux rectifier to a large number of heat exchangers in the stripping column and a further multiplicity of passages for returning the resulting liquid-vapour mixture to the variable reflux column. The use to produce oxygen of the process described is US patent specification 4 025 398 is discussed in "Energy Conservation and Medium Purity Oxygen", J.R. Flower, 1. Chem E Symposum Series No 79, pp F5-F14. The process is summarised in this paper as involving the taking of a number of vapour sidestreams from a first column and condensing them in heat transfer baffle elements immersed in the two phase mixtures on selected distillation stages of the second column. The condenser products would pass back to stages in the first column where the compositions matched. From analysis of this cycle, it was found that the advantages of distribution of heat flux decreased sharply as the product (oxygen) purity changed from 95 to 99% and that the critical part of the design involved the matches at the base of the second column for liquid (oxygen) compositions greater than 85%. It is therefore concluded that the cycle is primarily of use in producing medium purity oxygen. It is further reported in the aforesaid paper by Flower that in the absence of suitable heat transfer baffles, more recent work has employed a series of reboiler - condensers situated between the first and second columns, each fed by a separate vapour sidestream and a separate liquid sidestream. The condenser products and evaporator products are returned to the first and second columns. It is also reported by Flower that the advantages of using such existing heat exchange equipment are offset by a requirement for higher air feed pressures partly as a result of liquid hydrostatic effects. It can therefore be seen that these existing proposals for distributing the necessary heat and refrigeration over a distillation column generally require a multiplicity of links between a pair of columns, and in the example of the production of oxygen are not effective to produce high purity oxygen. In general, the industrial demand for high purity oxygen is far greater than that for so-called medium purity oxygen. Moreover, when medium purity oxygen is produced, it is generally not possible to obtain in the distillation system a sufficient local concentration of argon to justify the inclusion of an additional column to produce pure argon. Our analysis of the distillation of air shows that disproportionately more work needs to be in producing a given percentage change in a composition containing less than 80% nitrogen than in one containing more than 80% nitrogen. Accordingly, in air separation there is a greater need for reboiling of compositions intermediate air and pure oxygen than there is for condensation of compositions intermediate air and pure nitrogen. This appreciation of the relative merits of 'intermediate' reboil and 'intermediate' condensation is not shown in the prior art. Indeed, we have noted two prior proposals, US-A-2 812 645, and DE-A-2 202 206, which disclose an intermediate condensation step but not an intermediate reboiling step. Although WO 86/06462 discloses the use of an 'intermediate' reboil step using an argon-enriched stream from an argon column as the heat exchange medium, but returns the reboiled liquid to the same mass-transfer stage as that from which the liquid is taken thereby introducing inefficiency into the distillation process. It is an aim of the present invention to provide a method and apparatus for separating air in which reboil is effectively provided at more than one level in a distillation column employed to separate the air, while making possible the production of an argon product and a relatively pure oxygen product. According to the present invention there is provided a method of separating air, comprising fractionating air in a first distillation column, providing reboil at a bottom region and reflux at a top region of the first distillation column, withdrawing a product oxygen stream from a bottom region of the column, withdrawing a nitrogen stream from a top region of the column, withdrawing a stream enriched in argon from an intermediate level in the column, and separating it in a second distillation column to form a product argon stream, wherein at least one liquid stream having a composition intermediate the extremes of composition that occur in the first column is taken from the first column and is at least partially boiled externally to the first distillation column by heat exchange with fluid taken from the second distillation column, resulting boiled liquid is returned to said first distillation column, and said fluid is returned to the second distillation column, wherein the said resulting boiled liquid is returned to said first distillation at a different level from the one from which the said liquid stream is taken, said different level being where the composition of the vapour closely matches that of said resulting boiled liquid. The invention also provides apparatus for separating air, comprising a first distillation column, means for introducing air into the column, means for providing reboil at a bottom region of the column, means for providing reflux to a top region of the column, a first outlet from a bottom region of the column for the withdrawal of an oxygen product stream, a second outlet from top region of the column for the withdrawal of nitrogen, and a third outlet from an intermediate level of the column for the withdrawal of a stream enriched in argon, said third outlet communicating with a second distillation column for separating an argon product from said stream relatively rich in argon, wherein the first column has a fourth outlet for the withdrawal from the first column of at least one liquid stream of composition intermediate the extremes of composition that occur in the first column, and there is provided heat exchange means having a first pass communicating at one of its ends with said fourth outlet and at its other end with an inlet to said first column, and a second pass communicating at one of its ends with an outlet from said second column and at its other end with an inlet to said second column, whereby in operation, the liquid stream of intermediate composition is able to be at least partially boiled by heat exchange with fluid from the second column, with resulting vapour being returned to the first distillation column and the fluid from the second column being returned thereto, wherein the inlet to said first column with which said first pass communicates is at a different level from that of said fourth outlet, said level being selected such that in operation the composition of the vapour at that level matches closely that of said resulting vapour. The fluid taken from the second column is preferably a vapour, which is desirably at least partially condensed by heat exchange with the liquid stream of intermediate composition. The heat exchange means thus functions as a reboiler condenser. The first column is preferably the lower pressure column of a double column system. By providing a thermal link through the heat exchange means between the argon column and the lower pressure column, it becomes possible to improve the efficiency of the lower pressure column without detriment to the purity of the oxygen and argon products. In a double column system, the improvement in efficiency can be utilised to enhance the yield of oxygen. Thus air can be introduced directly into the lower pressure column as well as the higher pressure column. In order to keep down the irreversible work associated with the operation of the first column, it is preferred that the composition of the boiled stream matches more closely the composition of the vapour to which it is returned than the vapour in mass-exchange relationship with the liquid from which it is taken. In general, it is desirable for this reason that the composition of the boiled steam matches closely the composition of the vapour into which it is introduced on being returned to the first column. Preferably, the liquid stream of intermediate composition includes 40 to 60% by volume of oxygen. Although it is possible to take more than one liquid stream of intermediate composition for reboiling from the first column, this expedient is not preferred. Indeed, the method and apparatus according to the invention render it unnecessary to use the large number of additional inlets and outlets to the column of the kind shown in the drawings accompanying US patent specification 4 025 398. It is not essential to the method according to the present invention that the heat exchange between the liquid stream of intermediate composition and the heat exchange fluid be effective to boil all the steam. When there is incomplete phase change, the resulting liquid-vapour bi-phase may be separated into liquid and vapour, and a stream of the boiled fluid returned to the first column in accordance with the invention. The remaining liquid is desirably passed into liquid of a similar composition in a column forming part of the apparatus according to the invention (and typically the first column). Instead of returning the residual liquid to a liquid-vapour contact column, it may be subjected to further heat exchange in order to complete the phase change. The vapour produced as a result of such further heat exchange has a different composition from that of the vapour produced as a result of the first heat exchange. Accordingly, it is typically returned to a liquid-vapour contact column (typically the first column) forming part of the apparatus according to the invention and introduced into vapour therein having a similar composition to said vapour produced as a result of further heat exchange. Typically, the or each liquid stream of intermediate composition taken from a chosen level for reboiling comprises from 20 to 50% by volumne of the liquid flow at that level. Figure 1 is a schematic diagram illustrating the mass exchange that takes place on two adjacent trays of a distillation column for separating a binary mixture of nitrogen and oxygen. Figure 2 is a McCabe-Thiele diagram representing operation of a distillation shown in Figure 1 to separate a binary mixture of oxygen and nitrogen; Figure 3 is a graph representing the irreversibilities (excluding pressure drop) entailed in operating a distillation column along the operating line AB in Figure 2; Figure 4 is another McCabe-Thiele diagram representing operation of a distillation column to separate a binary mixture of oxygen and nitrogen, but with additional heat being supplied to one tray of the column below the feed level; Figure 5 is a graph representing the irreversibilities (excluding pressure drop) entailed in operating a distillation column along the operating line A′ B′ C′ D′ of Figure 4; Figure 6 is a further McCabe-Thiele diagram representing operation of a distillation column to separate a binary mixture of oxygen and nitrogen with a liquid stream of intermediate composition being withdrawn from the column, reboiled and returned into a lower level of the column. Figure 7 is a graph representing the irreversibilities (excluding pressure drop) entailed in operating the column along the operating line A″ B″ C″ D″ of Figure 6; Figure 8 is a schematic drawing illustrating a first air separation plant in accordance with the invention utilising the principle of reboiling a liquid stream of intermediate composition. In Figure 1 of the drawings, there are shown two communicating trays (n-i) and n of a distillation column. On these two trays mass exchange takes place between liquid and vapour. Figure 1 shows vapour V₁ and liquid L₁ passing out of mass exchange relationship with one another from tray n. Liquid L₁ flows through the downcomer onto tray (n-i) where it comes into contact with vapour ascending from the tray below tray (n-i). As a result, a liquid L₂ leaves the tray (n-i) and a vapour V₂ ascends to tray n. In the context of this specification we refer to the vapour V₁ as "corresponding" with the liquid L₁. For a theoretical tray, V₁ is in equilibrium with L₁ and V₂ is in equilibrium with L₂. At minimum reflux the compositon of L₁ approaches that of L₂ and the composition of V₂ approaches that of V₁. The part of the equilibrium line from the bottom of the column to a feed point for an oxygen-nitrogen system is represented in the McCabe-Thiele diagram in Figure 2. In practice, such minimum reflux conditions are not achievable throughout the column. Irreversible work is thus entailed in mixing liquid with vapour on each tray. Referring again to Figure 2, the operating line AB thus follows a different path from the equilibrium line. Both lines do however pass through the origin as no nitrogen is removed with the oxygen at the bottom of the column. It is a general principle that as the operating line approaches the equilibrium line, the column approaches reversibility at that point since there are only minute changes in composition between communicating trays, and hence losses arising from mixing streams of different composition are minimised. It can be seen from Figure 2 that between the feed point B and the bottom of the column (point A, where pure oxygen is produced) the operating line diverges considerably from the equilibrium line. Considerable irreversible work of mixing is thus entailed. The total amount of irreversible work (ignoring pressure drop) done in operating the column shown in Figure 1 is represented by the cross-hatched area in Figure 3. The area of the graph below the cross-hatched area represents the reversible work of separating oxygen from nitrogen. The abscissa in Figure 3 can be plotted in terms of the liquid phase, or the vapour phase, or both. Suppose some external heat is provided on a tray n within three trays below the level of the feed and where the difference between the proportion of the nitrogen in the vapour phase V₁ and the proportion of the nitrogen in the liquid phase L₁ (see Figure 1) is relatively large compared with other regions of the column below the feed. As shown in Figure 4, by supplying an appropriate amount of heat, the operating line can be "lifted" at the level of tray n back to near the equilibrium line. Part A′B′ of the line passes through the origin as a pure oxygen product is obtained at the bottom of the column. Since providing extra heat at the level of the tray n does not change the mass flux on that part of the column, the slope of the other part C′D′ is such that if it were extended downwards it would also pass through the origin. The result therefore of providing heat at the level of tray n is that the irreversible work of mixing in that part of the column below tray n is reduced while that above tray n remains unaltered. This fact is illustrated in Figure 5 in which the cross- hatched area should be compared with the corresponding area in Figure 3. A further reduction in the irreversible work can be achieved by withdrawing a stream of liquid of intermediate composition from a tray n, reboiling it externally of the column, and returning the reboiled stream to the column at a level (tray m) where the composition of the vapour is substantially the same as that of the reboiled stream. The effect of such reboil is shown in Figure 6. The line A″B″C″D″ is origin and is the operating line for the part of the column below the tray m. Since different mass balance conditions prevail below the tray m from those that prevail above it, the length B" C" does not pass through the origin. Thus, the invention makes it possible to achieve a closer approach to absolute reversibility with the liquid for reboil being taken from the downcomer serving tray n and the resulting vapour being returned to the vapour space above tray m than is achieved when no such intermediate reboil is carried out even through in the latter case external heat may be applied to the tray n. The reduction in the amount of irreversible work of mixing that needs to be done is illustrated in Figure 7 of the accompanying drawings which is to be compared with Figures 3 and 5. In particular, it can be seen that the irreversible work of mixing associated with the operation of the part of the column below the tray n is substantially reduced in comparison with operation of the column in accordance with Figures 4 and 5. Generally, there will be a number of different positions available for the return of the reboiled vapour stream such that the composition of the vapour matches more closely that of the vapour leaving the liquid vapour on the tray m that it does the vapour in the vapour space above the tray n. It is not critical to the invention which one of these possible return positions is selected. (Indeed, it is possible for the vapour stream formed by intermediate reboil to be divided, with one part of it being returned to the column at one such position and the remainder being returned at one or more other such positions.) Each of these "matching" positions results in there being a relatively close proximity between the point B″ in Figure 6, and the equilibrium line, and therefore, if selected for the returning reboiled liquid, makes it possible to keep down the amount of irreversible work of mixing that needs to be done. For a binary mixture, the position for such return is desirably selected so as to minimise the irreversible work that is done in the column. Irreversible work of mixing is not the sole source of such irreversible or lost work; there are also losses arising out of pressure drop in the column. In general, the greater the number of trays in the column, the greater the pressure drop. Accordingly, so far as distillation of ternary mixtures of nitrogen, oxygen and argon is concerned although in some instances it may be desirable to select the position of return of the reboiled liquid of intermediate composition so as to minimise the irreversible work, in other instances, it may be desirable to select a different return position so as to reduce the number of trays in the column needed to give a produce or products of desired purity. In the above description of the operating lines the presence of argon has been ignored. Since argon constitutes less than 1% by volume of air, its presence in the oxygen-nitrogen mixture does to some extent affect the amount of lost work that can be saved in accordance with the invention and the composition of the stream selected for intermediate reboil. When argon is to be obtained as a product by taking a side draw of a mixture relatively rich in argon and subjecting the mixture to further distillation in another column, the selection of the level in the column at which the reboiled liquid stream is returned is also influenced by the desirability of maximising the yield of argon. Indeed, in some instances, this criterion may take priority over the other criteria affecting the selection of the return position. Thus, in order to increase argon yield, it may be desirable to select a return position where the irreversibility of the column is greater than could be achieved with a different return position. For mixtures comprising three or more components, the closeness of matching may be assessed by calculating the work involved in mixing the respective fluids, the less the calculated work, the closer the match. A plant for producing oxygen, argon and nitrogen that utilises the principle of intermediate reboil and is in accordance with the invention is shown in Figure 8 of the accompanying drawings. Referring to Figure 8, an air stream at a pressure of about 6.5 atmospheres (absolute) is passed at a temperature of about 300K into the warm end of a reversing heat exchanger 2 and leaves the cold end of the reversing heat exchanger 2 at a temperature of about 103K. The air then passes into the higher pressure column 6 of a double column system, indicated generally by the reference number 4, through an inlet 10 below the level of the lowest tray in the column. A stream of air is immediately withdrawn from the column 6 through an outlet 12. One portion of this stream is returned to the cold end of the reversing heat exchanger 2. This portion of the air stream flows through the heat exchanger 2 countercurrently to the incomming air stream. The portion is then withdrawn from an intermediate location of the heat exchanger at a temperature of about 157 K and is divided into two streams. One of the streams is expanded in expansion turbine 14 to a pressure of about 1.21 atmospheres. The expanded air leaves the turbine 14 at a temperature of about 107 K and is mixed with an impure or waste nitrogen stream from the low pressure column 8 of a double column system 4. The resulting mixture is then introduced into a heat exchanger 16, which it leaves at a temperature of about 101K and then flows back through the reversing heat exchanger from the cold end to the warm end thereof, and is then vented to the atmosphere. If desired, instead of taking an air stream out of the column 6 through the outlet 10 and then returning it partially through the heat exchanger 2 prior to expanding it in the turbine 14, the air for the turbine 14 may be taken directly from the incoming air flow at an intermediate region of the heat exchanger 2. The second stream of air that is formed by dividing the air leaving the heat exchanger 2 at an intermediate temperature is expanded to a pressure of about 1.42 atmospheres in expansion turbine 18. This air leaves the expansion turbine 18 in a superheated state at a temperature of 111 K and is introduced into the lower pressure column 8 through an inlet 20. Referring to the stream of air that is withdrawn through the outlet 12 from the higher pressure column 6, the second portion of this air is reboiled and returned to the column 6 through inlet 22. One part of this portion of the air is condensed in a heat exchanger 24, and the other part is condensed in the heat exchanger 16, the other part flowing through the heat exchanger 16 countercurrently to the mixture of air and waste nitrogen. In the higher pressure column 6, the air is separated at a pressure of about 6 atmospheres into an oxygen-rich liquid and a nitrogen liquid fraction. The oxygen-rich liquid is used as the main feed for the lower pressure column 8 which is employed to separate the liquid to produce a substantially pure oxygen product, a substantially pure nitrogen product, an argon-enriched air stream which is separated in a further column 40 operating at substantially the same pressure as the lower pressure column 8 to form a substantially pure argon product. The oxygen-rich liquid is withdrawn from the bottom of the column 6 through an outlet 26. It is then sub-cooled in a heat exchanger 28 which it enters as temperature of about 102 K. One part of the sub-cooled liquid is passed through a throttling valve 30 and is then introduced into the low pressure column 8 through an inlet 32. The other part of the sub-cooled liquid is passed through a throttling valve 34, and then as a liquid-vapour biphase enters a condenser 36 associated with the argon column 40. The stream of liquid-vapour mixture entering the condenser 36 provides cooling for the condenser, and after leaving the condenser 36 enters the column 8 as vapour through an inlet 38 positioned below the level of the inlet 32. Nitrogen rising to the top of the column 6 enters a condenser -reboiler 42 that provides a thermal link between the columns 6 and 8 of the double column system 4. The nitrogen vapour is condensed against a flow of liquid oxygen from the bottom of the column 8 and part of the resulting condensed nitrogen is employed as reflux for the column 6. The remainder of the condensed nitrogen is withdrawn from the column 6 through an outlet 44 at a temperature of about 97 K and sub-cooling it to a temperature of about 81 K by heat exchange in a heat exchanger 46. Sub-cooled liquid nitrogen is then passed through a throttling valve 48 and is introduced into the top of the column 8 through an inlet 50. The liquid nitrogen introduced into the top of the column 8 through the inlet 50 serves as reflux for the column 8. The liquid becomes progressively richer in oxygen as it descends the column 8, and the ascending vapour stream becomes progressively richer in nitrogen. Reboil for the column 8 is provided as aforesaid by the condenser - reboiler 42. A portion of the reboiled oxygen is withdrawn from the bottom of the column 8 at a temperature of about 95 K through an outlet 52 and is warmed to a temperature of about 101 K by flow through the heat exchanger 24 countercurrently to the air flow through that heat exchanger. This product oxygen stream is thereby warmed to a temperature of about 102 K and is then passed through the heat exchanger 2 countercurrently to the incoming flow of air the oxygen product stream, which is typically 99.8% pure, leaves the warm end of the heat exchanger 2 at a temperature of about 297 K. A gaseous nitrogen product stream is taken from the top of the lower pressure column 8 through an outlet 54 at a temperature of about 79 K and a pressure of about 1.25 atmospheres. The nitrogen product stream is first warmed in heat exchanger 46, flowing countercurrently to the nitrogen stream taken from the condenser reboiler 42. It leaves the heat exchanger 46 and is then warmed by passage through the heat exchanger 28 countercurrently to the oxygen-enriched liquid stream taken from the column 6 via the outlet 26. The product nitrogen stream is further warmed to about 101 K by passage through the heat exchanger 16 cocurrently with the mixture of expanded air and waste nitrogen. The product nitrogen stream then enters the reversing heat exchanger 2 and flows therethrough countercurrently to the incoming air flow, leaving the heat exchanger 2 at a temperature of about 290 K. In order to provide a waste nitrogen stream which may be used to cleanse the reversing heat exchanger 2 well known in the art by subliming solid, frozen deposits of water and carbon dioxide, impure nitrogen typically containing about 50 volumes per million by volume of oxygen is withdrawn from the column 8 at a level a few trays below the uppermost tray in that column but above the level of the inlet 32. The waste nitrogen stream is withdrawn at a temperature of about 79K through an outlet 56 and is then passed through the heat exchangers 46 and 28 cocurrently with the product nitrogen stream. It is then united with the expanded air stream from the turbine 14 and passed through the heat exchangers 16 and 2 as hereinbefore described. Sufficient reflux is provided in the column 8 to ensure that there is a local maximum of argon in the vapour phase at a level of the column intermediate its top and bottom. At the level of the local maximum of argon, a stream of vapour is withdrawn through an outlet 58 and passed to the column 40 entering it at level below the bottom tray thereof through an inlet 60. In the column 40, the argon-enriched stream is fractionated to provide argon product at the top of the column. Argon vapour reaching the top of the column is condensed in condenser 36 and a part of the resulting liquid argon is withdrawn through outlet 62 as liquid product, another part being used as the reflux for the column 40. Oxygen-rich liquid collects at the bottom of column 40 and is withdrawn therefrom through an outlet 64 and returned to the column 8 through an inlet 66 at a level below that of the outlet 58. In accordance with the invention, the efficiency with which the column 8 operates is enhanced by a withdrawal of a liquid stream containing about 50% by volume of oxygen from the column 8 through an outlet 70 at a level below that of the inlet 38 and above that of the outlet 58. The liquid stream is totally reboiled in a heat exchanger 72 and is returned to the column 8 through inlet 74 at a level below that of the outlet 70 but above that of the outlet 58 where the vapour matches closely with the composition of the reboiled liquid. The heating for the heat exchanger 72 is provided by passing a stream of oxygen-rich vapour (containing more than 65% by volume of oxygen) from the argon side column 40 through the heat exchanger 72 countercurrently to the stream that is reboiled therein. The stream withdrawn from the argon column 40 through the outlet 75 is typically condensed in the heat exchanger 72, and the resulting liquid is returned to the column through an inlet 76. Reboiling of the stream taken from the outlet 70 of the column 8 renders the operation of the column 8 more thermodynamically efficient for the reasons discussed herein with reference to Figures 1 to 7. It is therefore possible to enhance the production of the plant illustrated in Figure 8 by introduction of the expanded air stream into a low pressure column through the inlet 20. Typically, about 5 to 6% of the net air flow to the columns is expanded in the turbine 18 and a similar quantity of air is expanded in the turbine 14. Instead of employing a reversing heat exchanger to remove carbon dioxide and water vapour from the incoming air, the plant may be provided with preliminary beds of molecular sieve of a kind that preferentially adsorbs carbon dioxide and water vapour from the incoming air. The construction and operation of apparatus employing beds of molecular sieve to remove water vapour and carbon dioxide from the incoming air are well known in the air separation art and need not be further described herein.

Claims: A method of separating air, comprising fractionating air in a first distillation column, providing reboil at a bottom region and reflux at a top region of the first distillation column, withdrawing a product oxygen stream from a bottom region of the column, withdrawing a nitrogen stream from a top region of the column, withdrawing a stream enriched in argon from an intermediate level in the column, and separating it in a second distillation column to form a product argon stream, wherein at least one liquid stream having a composition intermediate the extremes of composition that occur in the first column is taken from the first column, is at least partially boiled externally to the first distillation column by heat exchange with fluid taken from the second distillation column, resulting boiled liquid is returned to said first distillation column, and said fluid is returned to the second distillation column, characterised in that the said resulting boiled liquid is returned to said first distillation at a different level from the one from which the said liquid stream is taken, said different level being where the composition of the vapour closely matches that of said resulting boiled liquid. A method as claimed in Claim 1, in which said fluid is a vapour, which is at least partially condensed by heat exchange with the liquid stream of intermediate composition. A method as claimed in Claim 1 or Claim 2, in which the first column is the lower pressure column of a double column system comprising the lower pressure column and a higher pressure column. A method as claimed in Claim 3, in which air is introduced into said higher and said lower pressure columns. A method as claimed in Claim 4, in which the air that is introduced into the lower pressure column is expanded from the operating pressure of the higher pressure column to a pressure suitable for its introduction into the lower pressure column. A method as claimed in any one of the preceeding claims, in which a product nitrogen stream and a waste nitrogen stream are also produced. A method as claimed in any one of the preceeding claims, in which the liquid stream of intermediate composition includes from 40 to 60% by volume of oxygen. A method as claimed in any one of the preceeding claims, in which only part of the liquid stream of intermediate composition is reboiled. A method as claimed in Claim 8, in which the residual liquid is returned to the first column. A method as claimed in Claim 8, in which the residual liquid is subjected to a further stage of heat exchange and is thereby reboiled, the resulting vapour being returned to the first distillation column. A method as claimed in any one of the preceeding claims, in which the liquid stream of intermediate composition comprises from 20 to 50% by volume of the liquid flow at the level in the first column from which it is taken. Apparatus for separating air, comprising a first distillation column, means for introducing air into the column, means for providing reboil at a bottom region of the column, means for providing reflux to a top region of the column, a first outlet from a bottom region of the column for the withdrawal of an oxygen product stream, a second outlet from the top region of the column for the withdrawal of nitrogen, and a third outlet from an intermediate level of the column for the withdrawal of a stream enriched in argon, said third outlet communicating for a second distillation column for separating an argon product from said stream relatively rich in argon, wherein the first column has a fourth outlet for the withdrawal from the first column of at least one liquid stream of composition intermediate the extremes of composition that occur in the first column, and there is provided heat exchange means having a first pass communicating one of its ends with said fourth outlet and at its other end with an inlet to said first column, and a second pass communicating one of its ends with an outlet from said second column and at its other end with an inlet to said second column, whereby in operation, the liquid stream of intermediate composition is able to be at least partially boiled by heat exchange with fluid from the second column, with resulting vapour being returned to the first distillation column and the fluid from the second column being returned thereto, characterised in that the inlet to said first column with which said first pass communicates is at a different level from that of said fourth outlet, said level being selected such that in operation the composition of the vapour at that level matches closely that of said resulting vapour. Apparatus as claimed in Claim 12, in which said heat exchange means also functions as a condenser for condensing said fluid. Apparatus as claimed in Claim 12 or Claim 13, in which said first column is the lower pressure column of a double column system comprising higher and lower pressure columns, and the means for introducing air into the column includes a conduit placing the lower pressure column in communication with oxygen-enriched liquid air collecting at the bottom of the higher pressure column. Apparatus as claimed in Claim 14, in which said means for introducing air into the first column additionally includes an expansion turbine having its inlet in communication with a source of air at substantially the pressure of the higher pressure column and its outlet in communication with an inlet to the lower pressure column. Apparatus as claimed in Claim 14 or Claim 15, in which said second outlet is for the withdrawal of a product nitrogen stream, there being a further outlet from the lower pressure column for the withdrawal of a waste or impure nitrogen stream.

Inventors: GLADMAN, DAVID CHARLES FLETCHER, OAKEY, JOHN DOUGLAS
Applicants: THE BOC GROUP PLC
Application year: 1987
Main classification: F25J3
IPC codes: F25J3