Abstract:
The invention relates to a method and a device used for the low-temperature separation of air in a distillation column system, comprising at least one high-pressure column ( 11 ) and a low-pressure column ( 12 ). The method has a high pre-liquifaction of 30% or more. Feed air is introduced into the distillation column system. The distillation column system further has a pre-column ( 10 ), the operating pressure of which is higher than the operating pressure of the high-pressure column ( 11 ). A first partial stream ( 1 ) of the feed air is introduced into the pre-column ( 10 ). The pre-column ( 10 ) has a head condenser ( 14 ), which is configured as a condenser-evaporator having a condensation chamber and an evaporation chamber. A gaseous fraction ( 30, 31 ) from the upper region of the pre-column ( 10 ) is introduced into the condensation chamber of the head condenser ( 14 ). Fluid ( 6 ) formed in the condensation chamber is at least partially applied to the pre-column ( 10 ) as runback ( 7 ). A second partial stream ( 2   a;    2   b ) of the feed air is introduced into the evaporation chamber of the head condenser ( 14 ).

Description:
This U.S. patent application is a national stage application of PCT/EP2009/000431 filed on 23 Jan. 2009 and claims priority of both European patent document 08009400.6 filed on 19 Jun. 2008 and German patent document 10 2008 006 431.9 filed on 28 Jan. 2008, the entireties of which are incorporated herein by reference. 
     FIELD OF INVENTION 
     The invention relates to a method for low-temperature separation of air. 
     BACKGROUND OF THE INVENTION 
     Methods and devices for low-temperature separation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik [Low-temperature technology], 2nd edition 1985, Chapter 4 (pages 281 to 337). 
     The distillation column system of the invention comprises a two-column system (for example a classical Linde double-column system) for separating nitrogen/oxygen having a high-pressure column and a low-pressure column which are in a heat-exchange relationship with one another. The heat-exchange relationship between high-pressure column and low-pressure column is generally effected by a main condenser in which overhead gas of the high-pressure column is liquefied against vaporizing bottom liquid of the low-pressure column. In addition to the columns for separating nitrogen/oxygen, the distillation column system can comprise other devices, for example for producing other air components, in particular noble gases, for example an argon production stage which comprises at least one crude argon column, or a krypton-xenon production stage. The distillation column system, in addition to the distillation columns, also comprises the heat exchangers directly assigned thereto, which heat exchangers are generally constructed as condenser-evaporators. 
     The majority of modern air separation plants are constructed on the basis of what is termed a double column. This system of two coupled columns having differing working pressures enables not only the production of gaseous oxygen-, argon- and nitrogen-containing products, but also liquid fractions. These liquids can be taken off from the air separation plant as liquid end products or are internally compressed (brought to the higher pressure in a pump and warmed), so they are then available as gaseous pressurized products. 
     If such liquid fractions are taken off from the double-column system, a corresponding amount of air must be preliquefied before being fed into the double column, that is to say some of the air is passed into the double-column system in the gaseous state (feed air to the high-pressure column and, e.g., air from the Lachmann turbine, which is fed directly into the low-pressure column) and some of the air is fed into the double-column system in the liquid state (throttling stream and liquid air from Claude turbine, where present). If many products are taken off in the liquid state, the proportion of preliquefied air increases correspondingly. 
     Since only the lower sections of both columns are charged with liquid air, the preliquefied air shares only few of the rectification processes in the double column (compared with gaseous air). Therefore, the preliquefaction has an adverse effect on the rectification processes in the double column. With increasing air preliquefaction, the oxygen yield decreases (and also the argon yield, if the system produces argon). The efficiency and economic efficiency of the air separation plant decrease. 
     In order to intensify the rectification (in particular in the upper sections of both columns), resort is made to measures such as what is termed a “feed compressor” (which compresses some of the product from the upper part of the low-pressure column to the pressure of the high-pressure column and this is fed into the high-pressure column) and/or attempts are made to use what is termed a nitrogen cycle for generating cold (the air in this case is not liquefied upstream of the double column but within the pressure column by liquid nitrogen). These measures, however, mean a higher energy consumption and make the overall plant more expensive as a result of a higher number of heat exchangers and/or machinery. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to increase the oxygen yield (and argon yield, if argon is produced) of an air separation plant even in the case of a high preliquefaction (for example greater than 30 mol %, in particular greater than 40 mol %, of the total feed air) without using additional machinery and heat exchangers. 
     This object is achieved by the features of patent claim  1 . In this case an additional third column (“precolumn”) is connected upstream of the conventional double column. At least some of the gaseous air (the “first substream”) is firstly passed into this third column and (similarly to in the high-pressure column of the double column) separated into liquid nitrogen fractions and crude oxygen. This upstream column is cooled by preliquefied air (the “second substream”) by means of a top condenser (generally placed above the column). This liquid is vaporized in this process and fed in the gaseous state into the distillation column system, preferably into the high-pressure column. 
     The third column is operated at a pressure which is higher than the pressure of the high-pressure column of the double column in order that the air which vaporizes in the top condenser can be introduced into the high-pressure column. 
     Preferably, the pressure ratio between precolumn and high-pressure column (in each case measured at the top) is at least 1.4 and is in particular between 1.4 and 1.8, preferably between 1.5 and 1.7. 
     Liquid nitrogen from the precolumn (or from the condensation compartment of the top condenser thereof) is then fed into the high-pressure column, liquid crude oxygen from the lower region of the precolumn into the high-pressure column and/or into the low-pressure column, or alternatively or additionally into the argon part, where present. 
     By means of these connections the following advantages are achieved:
         The preliquefied air is vaporized in the top condenser of the precolumn and passed in the gaseous state into the double column. In this manner the adverse effect of the preliquefaction is markedly reduced.   The rectification in the double column can be improved by feeding in one or more wash-LIN fraction(s) from the precolumn or top condenser thereof.   The oxygen yield increases markedly and so customary yields can be achieved even in the case of preliquefaction of greater than 50%. The same applies to the argon yield if the plant additionally generates argon.   The dimensions of columns, especially the high-pressure column and the precolumn, are relatively small.   From the precolumn, pressurized nitrogen (VHPGAN—very high pressure gaseous nitrogen) can be obtained at a pressure which is higher than the pressure of the high-pressure column of the double column.   For generation of cold, the air can be expanded in a turbine not only to the pressure of the low-pressure column (Lachmann turbine) or pressure of the high-pressure column (HPC Claude turbine), but also to the pressure of the precolumn or top condenser thereof (PC Claude turbine).       

     According to a fundamental concept of the invention, as far as possible all process streams available at high pressure which are suitable for cooling the precolumn are used for cooling thereof (this does not exclude, however, that in individual cases some of these process streams are introduced into the distillation column system at another point). In particular, preferably the entire preliquefied air, in any case more than 80 mol %, or more than 90 mol % of the preliquefied air, is introduced into the vaporization compartment of the top condenser of the precolumn. 
     The invention additionally relates to a device for low-temperature air separation according to patent claim  12 . 
     The following variants are possible in the scope of the invention and can if appropriate also be combined with one another:
     1. Precolumn by the side of the double column (high-pressure column and low-pressure column one above the other).   2. All three columns side by side.   3. Three columns with PC Claude turbine, the gaseous air expanded into the precolumn and liquid air expanded into the top condenser of the precolumn.   4. Use in methods having compression of all of the air to markedly above precolumn pressure; in this case, regularly a part is liquefied in the context of what is termed internal compression or (at supercritical pressure) pseudoliquefied and subsequently throttle-expanded; the remainder is work-producingly expanded in one or more turbines, in particular to the pressure of the precolumn or top condenser thereof.   5. Three columns having an HPC Claude turbine which expands air into the high-pressure column.   6. Three columns having a Lachmann turbine which expands air into the low-pressure column.   7. Three columns in combination with two turbines (PC Claude turbine with HPC Claude turbine, PC Claude turbine with Lachmann turbine, HPC Claude turbine with Lachmann turbine).   8. Three columns with three turbines (PC Claude turbine, HPC Claude turbine and Lachmann turbine).   9. With or without argon production.   10. The heat exchangers can be split or integrated.   

     The invention and also further details of the invention will be explained in more detail hereinafter with reference to exemplary embodiments shown in the drawings. In this case, in the drawings: 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a first exemplary embodiment of the method according to the invention, 
         FIG. 2  shows a second exemplary embodiment with a depiction of the main heat exchanger and a PC Claude turbine as single expansion machine, 
         FIG. 3  shows a modification of  FIG. 2  in which the entire gaseous feed air (first substream) originates from the PC Claude turbine, 
         FIG. 4  shows a fourth exemplary embodiment with an HPC Claude turbine as sole expansion machine, 
         FIG. 5  shows a fifth exemplary embodiment with a Lachmann turbine as sole expansion machine and 
         FIG. 6  shows a fifth exemplary embodiment for production of impure oxygen with compression of the entire air to markedly above precolumn pressure. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In  FIG. 1 , the compression, purification and cooling of the feed air is not shown. The distillation column system comprises here a precolumn  10 , a high-pressure column  11  and a low-pressure column  12 , and also the condenser-evaporator linked thereto, the main condenser  13  and the top condenser  14  of the precolumn. Optionally, the distillation column system can additionally comprise an argon part  15  which contains, in particular, at least one crude argon column and top condenser thereof; in addition, the argon part can comprise a pure argon column for argon/nitrogen separation. 
     The separation columns for nitrogen/oxygen separation in the example have the following operating pressures (in each case at the top): 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 precolumn 10 
                 7.5 to 12 bar, 
               
               
                   
                 high-pressure column 11 
                 5.0 to 6.5 bar, 
               
               
                   
                 low-pressure column 12 
                 1.3 to 1.6 bar. 
               
               
                   
                   
               
             
          
         
       
     
     A first substream  1  of the feed air comes in the gaseous state from the cold end of the main heat exchanger (which is not shown) or from a turbine. It is at a pressure which is just above the operating pressure of the precolumn  13  and is introduced immediately above the bottom. 
     The precolumn  10  comprises a top condenser  14 , into the evaporation compartment of which a second substream of air in the liquid state is introduced. This “second substream” is formed in the example by two subdivided streams  2   a ,  2   b . Subdivided stream  2   a  originates from the outlet of a PC Claude turbine, subdivided stream  2   b  originates from the cold end of the main heat exchanger (which is not shown) and was condensed or (at supercritical pressure) pseudocondensed against a taken off from the distillation column system in the liquid state and subsequently brought to pressure in the liquid state. During the introduction into the evaporation compartment of the top condenser  14 , the second substream  2   a ,  2   b  consists essentially (85 to 95 mol %) of liquid. The liquid fraction thereof comprises 30 to 50 mol % of the total feed air. The remaining feed air is introduced into the distillation column system in the gaseous state. The gaseous introduction proceeds—except for possible gaseous fractions in the streams  2   a  and  2   b  and the turbine stream  3 —completely via the first substream  1  into the interior of the precolumn  10 . 
     In the example, furthermore, an additional liquid stream  4  is passed into the vaporization compartment of the top condenser  14 . This originates from an intermediate point of the precolumn  10  which is arranged about 8 to 16 theoretical or practical plates above the bottom. 
     The entire bottom liquid  5  of the precolumn is introduced here into the high-pressure column  11 , more precisely immediately at the bottom thereof. Alternatively, or additionally, the bottom liquid  5  of the precolumn or a part thereof—after cooling in the subcooling countercurrent heat exchanger  37 , can be fed into the low-pressure column  12  and/or the argon part  15  (which is not shown in the drawing). The liquid  6  generated in the condensation compartment of top condenser  14  from a part  31  of the top nitrogen  30  of the precolumn  10  is fed into the precolumn  10  as a first part as top reflux  7  and as a second part  8  to the top of the high-pressure column  11 . Furthermore, a nitrogen-enriched impure fraction  9  can be passed from the precolumn into the high-pressure column; this impure fraction  9  is taken off at an intermediate point of the precolumn  10  which is arranged about 8 to 16 theoretical or practical plates below the top and passed to the high-pressure column  11  at an intermediate point. 
     The vaporized fraction  16  formed in the evaporation compartment of the top condenser is passed via line  17  to the bottom of the high-pressure column, together with a third substream  18  of the feed air which originates from the outlet of an HPC Claude turbine. The purge liquid  32  from the top condenser  14  of the precolumn  10  is fed to the high-pressure column  10  at an intermediate point in the lower region. 
     In the example, furthermore, a further liquid stream  4  is passed into the evaporation compartment of the top condenser  14 . This further liquid stream originates from an intermediate point of the precolumn  10  which is arranged about 8 to 16 theoretical or practical plates above the bottom. 
     Otherwise, the double column  11 / 12 / 13  and the optional argon part  15  function in the generally known manner. 
     From the high-pressure column  11 , liquid crude oxygen  33  at the bottom, a liquid air fraction  34  at the intermediate point at which the purge liquid  32  is introduced, impure nitrogen  35  from an intermediate point situated further above and liquid pure oxygen from the condensation compartment of the main condenser  13  are cooled in a subcooling countercurrent heat exchanger  37  in indirect heat exchange with backflows and introduced into the low-pressure column  12  via the lines  38 ,  39 ,  40  or  41  at the suitable points. Furthermore, gaseous air  42  from a Lachmann turbine and/or liquid air  43  from an HPC Claude turbine can be fed into the low-pressure column  12 . 
     If the plant does not have an argon part, the following products can be withdrawn:
         gaseous nitrogen (GAN)  44 ,  45  from the top of the low-pressure column  12     liquid nitrogen (LIN)  46  from the top of the low-pressure column  12     gaseous impure nitrogen (UN2)  47 ,  48  from an intermediate point in the upper region of the low-pressure column  12     gaseous oxygen (GOX)  49  directly above the bottom of the low-pressure column  12     liquid oxygen (LOX)  50  from the bottom of the low-pressure column  12     gaseous pressurized nitrogen (HPGAN)  51  from the top of the high-pressure column  11     liquid pressurized nitrogen (HP-LIN)  52  from the condensation compartment of the main condenser  13  or from the high-pressure column  11     gaseous nitrogen of particularly high pressure (VHPGAN)  53  from the top of the precolumn  10         

     The plant can, but need not, generate all of these products simultaneously. 
     The gaseous product streams are warmed in a main heat exchanger which is not shown in indirect heat exchange with feed air. The main heat exchanger can consist of a block or of two or more parallel and/or serially connected blocks. The liquid oxygen can be produced as a liquid product; alternatively, or additionally, at least a part of the oxygen withdrawn in the liquid state from the low-pressure column is brought to pressure in the liquid state and subsequently vaporized or (at supercritical pressure) pseudo-vaporized in the main heat exchanger and warmed and subsequently withdrawn as gaseous pressurized product (what is termed internal compression). 
     In a variant of the exemplary embodiment of  FIG. 1 , the system comprises an argon part  15  for producing liquid pure argon (LAR)  54 . The argon part contains one or more crude argon columns for argon/oxygen separation and a pure argon column for argon/nitrogen separation which are operated in the generally known manner. The lower end of the crude argon column communicates via lines  61  and  62  with an intermediate region of the low-pressure column  12 . The liquid crude oxygen from the high-pressure column  11  is passed in this case via the line  33 A into the argon part and, in particular at least in part of the top condenser of the crude argon column(s), at least in part vaporized (which is not shown). The at least in part gaseous crude oxygen is fed via line  38 A into the low-pressure column  12 . From the argon part  15 , in addition, a gaseous residual stream (waste)  55  is withdrawn. 
     From the exemplary embodiment of  FIG. 1 , the following variants deviating from the drawing can be derived:
         The line  4  can be omitted or remain out of operation. The top condenser  14  is then cooled exclusively by liquefied air  2   a ,  2   b.      The bottom liquid  5  of the precolumn  10  can be introduced in part or completely after subcooling in  37  into the low-pressure column  12  instead of into the high-pressure column  11 . If argon is produced, a part or the entire subcooled liquid can be used before introduction thereof into the low-pressure column for cooling the top condenser of the crude argon column.       

       FIG. 2  shows a drawing with a depiction of the main heat exchanger  260  and a PC Claude turbine  261  as sole expansion machine. The turbine can be braked either by means of an oil brake  262  or by means of a generator or by means of a recompressor which compresses either the turbine stream or throttle stream  2   b  (upstream of the [pseudo]liquefaction thereof in the main heat exchanger  260 ). The turbine-expanded and at least in part liquefied air  263  is introduced into a phase separation unit  264 . The liquid fraction  264  is introduced into the evaporation compartment of the top condenser  14  of the precolumn  10 . The gaseous fraction  270  is combined with the gaseous air from the main heat exchanger  260  and fed into the precolumn  10  via line  1 . 
     In  FIG. 2 , the production of gaseous pressurized oxygen  293 ,  294  by internal compression is also shown. Here, at least a part (IC-LOX) of the liquid oxygen  50  is fed from the bottom of the low-pressure column  12  via line  290  to an oxygen pump  291 , there brought to an elevated pressure and at least a first part vaporized or pseudo-vaporized at this elevated pressure in the main heat exchanger  260  and withdrawn as high-pressure product  294 . Another part can be reduced in pressure ( 292 ) and at this reduced pressure vaporized or pseudo-vaporized in the main heat exchanger  260  and finally be withdrawn as medium-pressure product  293 . 
     Additionally or alternatively, one or two nitrogen products  296 ,  297  can be produced at very high pressure in a similar manner by internal compression by bringing the liquid high-pressure nitrogen  52  in a nitrogen pump  295  to a correspondingly high pressure and, at this pressure (and if appropriate in part at a somewhat lower intermediate pressure), (pseudo-)vaporizing and warming it in the main heat exchanger  260 . 
     The exemplary embodiment of  FIG. 3  differs from  FIG. 2  in that the total gaseous feed air (the “first substream”)  301  originates from the PC Claude turbine  361 . 
       FIG. 4  shows a fourth exemplary embodiment having an HPC Claude turbine  465  as sole expansion machine. The turbine can be braked either by means of an oil brake  466  or by means of a generator or by means of a recompressor which compresses either the turbine stream or throttle stream (upstream of the [pseudo]liquefaction thereof in the main heat exchanger  260 ). The turbine-expanded and at least in part liquefied air  467  is introduced into a phase separation unit  468 . The liquid fraction  469  is passed via line  471  into the low-pressure column  12 . The gaseous fraction  470  is combined with the gaseous air  16  from the top condenser of the precolumn  10  and fed into the high-pressure column  11  via line  417 . 
     In the exemplary embodiment of  FIG. 5 , a Lachmann turbine is the sole expansion machine. The turbine can be braked either by means of an oil brake  562  or by means of a generator or by means of a recompressor which compresses the turbine stream (upstream of its [pseudo]liquefaction in the main heat exchanger  260 ). The turbine-expanded gaseous air  563  is fed into the low-pressure column  12 . 
     In  FIG. 6 , a variant of the method according to the invention is shown which is suitable, in particular, for producing impure oxygen. Here the total air is compressed to significantly above precolumn pressure. Otherwise this variant substantially corresponds to that of  FIG. 3 ; an argon production stage, however, is generally not expedient here. 
     The feed air is here brought in a main air compressor  601  to a pressure of, for example, 5.5 to 24 bar, fed at this pressure to a precooler  602  and further to prepurification  603  which is constructed, for example, as a molecular sieve adsorber station. The total purified feed air is subsequently further compressed in a recompressor  604  to a pressure of, for example, up to 40 bar. The resultant high-pressure air  605  is divided into a first branch stream  606  and a second branch stream  607 . 
     The first branch stream  606  is brought in a further recompressor  661  which is driven by the PC Claude turbine  361  to a still higher pressure and serves as throttle stream  2   b . The second branch stream  607  is introduced into the main heat exchanger  260  at the exit pressure of the recompressor  604  and expanded in the PC Claude turbine  361 . 
     All of the processes and plants shown are to be understood as exemplary. The drawings are intended primarily to illustrate the functional relationships. Although high-pressure column and low-pressure column are shown one above the other and with an integrated main condenser, in the context of the invention, however, any other known arrangement of columns and condensers is also possible. 
     The columns can be equipped with sieve trays, structured packings or non-structured packings or else contain combinations of said types of mass-transfer elements. 
     The main condenser is constructed as falling film or bath evaporator. In the case of a bath evaporator, it can be constructed as a single storey or multistorey (cascade condenser). The top condenser of the precolumn is preferably constructed as a bath condenser. 
     Some streams or column sections can be absent in the actual connection. In terms of the process this means that the amount of the corresponding stream is equal to zero or the number of theoretical plates in the relevant section is equal to zero. With respect to the device this generally means that the corresponding line or the corresponding column section is absent. 
     The main heat exchanger can in each case be constructed in an integrated or split manner, the drawings show only the unit function of the exchanger—warm streams are cooled by cold streams. 
     In all exemplary embodiments of the invention no pump is used to transport a liquid from one column to another column.