Patent Publication Number: US-2011067444-A1

Title: Processes and Device for Low Temperature Separation of Air

Description:
This U.S. patent application claims priority of German patent document DE 10 2009 042410.5 filed on Sep. 21, 2009 and German patent document DE 10 2009 0484156.6 filed on Oct. 7, 2009, the entireties of which are incorporated herein by reference. 
     FIELD OF INVENTION 
     The invention is directed to a method and device for the low temperature separation of air with a distillation-column system. 
     BACKGROUND OF INVENTION 
     A process in which a product flow of liquid at pressure is vaporized against a heat carrier and is finally obtained as a gaseous compressed product is also called an internal compression process. The process is particularly widespread for obtaining compressed oxygen, but can also be used to obtain compressed nitrogen or compressed argon. For the case of a supercritical pressure in a main heat exchanger, no phase transformation occurs in a real sense; the product stream is then “pseudo-vaporized”. 
     Compared to the (pseudo-)vaporized product stream, a heat carrier under high pressure is liquefied in the main heat exchanger (or pseudo-liquefied if it is under supercritical pressure), namely a fractional stream of air, which here is called a “throttling stream”. 
     It is customary to bring a throttling stream and a turbine stream together in a recompressor or in the main air compressor at a higher pressure, as is required for the distillation. This pressure must be sufficiently high for the vaporization or pseudo-vaporization of the product stream made liquid at pressure and can be, for example, 20 or 60 bars. The turbine stream is then, of course, also expanded at this pressure (“second pressure”) at roughly the operating pressure of the high-pressure column. Alternatively, the throttling stream is further compressed at a still higher pressure (“third pressure”). 
     The turbine stream serves initially for refrigeration. But in systems with internal compression, it has a second function. The turbine stream helps the throttling stream to evaporate (or to pseudo-evaporate) an internally-compressed stream (nitrogen, oxygen, and/or argon). The larger the turbine stream, and the more this stream is cooled down in the main heat exchanger (the greater the temperature difference between inlet and outlet), the more heat is made available for the (pseudo-) evaporation of the internally-compressed product stream, and the smaller the throttling stream. The average temperature difference in the heat exchanger is thereby smaller, the temperature profile more favorable, and the system more efficient. This means it is always advantageous to cool the turbine stream down in the heat exchanger as much as possible. This generally leads to the stream at the turbine outlet not being gaseous, but in fact partially liquefied. 
     Lowering the temperature at the turbine inlet is however not unconditionally possible but, with the machines generally used, a maximum liquid fraction of roughly 6% to a maximum of 10% (design criterion) is provided. Higher liquid fractions can lead to turbine damage. The inlet temperature in an air turbine is limited by this restriction, for example, with 60 bars at the inlet and roughly 85% efficiency at about 169 K. For an inlet pressure of 20 bars, the smallest possible turbine inlet temperature is roughly 125 K. A goal is to set the turbine inlet temperature lower without violating the turbine design criterion, resulting in a more efficient process. 
     SUMMARY OF INVENTION 
     The present invention is based on the problem of achieving an energy-efficient process and a corresponding device, with a comparatively low equipment cost. 
     This problem is resolved by the present invention. The turbine stream is no longer taken off in the operation of cooldown from an intermediate position of the main heat exchanger, but passes further through the main heat exchanger, so that the turbine stream, at subcritical pressure up to roughly the dewpoint temperature, is either cooled down more or, at supercritical pressure, is pseudo-liquefied. Finally, the stream is expanded in the main heat exchanger at an intermediate pressure optimized with respect to expansion to produce work and to the temperature profile. The stream is preferably expanded with a throttle valve, and is heated up again in the main heat exchanger at the intermediate temperature, which corresponds to the inlet temperature of expansion to produce work and is as low as possible, so that the turbine design criterion is not violated. This intermediate temperature lies, for example, below 169 K for a 60-bar turbine stream or below 125 K for a 20-bar turbine stream. 
     The cooldown and (pseudo-)liquefaction of the turbine stream in the main heat exchanger can then, if its pressure is equal to that of the throttling stream, occur along with the throttling stream or separately from it. The intermediate pressure, at which the turbine stream is expanded before its expansion to produce work, is equal to or higher than √{square root over (P throttling stream ·P high-pressure column ))}. That is, for a 60-bar throttling stream, the intermediate pressure would lie at 18 bars or higher or for a 20-bar throttling stream at 10.5 bars (under the assumption that the pressure in the high-pressure column amounts to 5.5 bars). Expansion at the intermediate pressure is preferably carried out in a throttle valve. Expansion to produce work is performed in an expansion machine, which preferably is constructed as a turbine. 
     In a first variant of the present invention, a recompressor operates on external power; both the throttling stream and the turbine stream are under the second pressure during cooldown in the main heat exchanger. Using a recompressor without intermediate offtake, the equipment cost can be kept low. 
     “Operates on external power” means that the corresponding compressor is not operated by power self-produced in the air-separation process but, for example, by an electric motor, a steam turbine, or a gas turbine. 
     In a second variant of the present invention, the recompressor is driven by an expansion machine, which is operated with a process stream of the procedure, in particular by an expansion machine which is operated with the turbine stream, in which the air compressor represents the only machine operated on external power for air compression. 
     The “only machine” is understood here to be a single-stage or multistage compressor, whose stages are all connected to the same drive, in which all the stages are put into the same housing or are connected to the same gear system. In this second variant, the “first pressure” is above the highest pressure of the distillation-column system; in particular, it is clearly above the operating pressure of the high-pressure column. This pressure difference amounts to, for example, at least 4 bars and is preferably between 6 and 16 bars. In this variant, the total air in the air compressor (except for possible smaller fractions such as, for example, instrument air) is preferably completely divided up into the throttling stream and the turbine stream. 
     The process stream, which is used to drive the recompressor, can instead of the turbine stream be formed, for instance, by a third air stream, which is expanded at the operating pressure of the low-pressure column (Lachmann turbine) or by compressed nitrogen from the distillation-column system, particularly from a high-pressure or low-pressure column. The compressed nitrogen can, at the inlet into the corresponding expansion machine, be almost at ambient temperature, or it is heated in front of an inlet into the expansion machine at a temperature above ambient (“hot gas expander”). 
     In both variants of the present invention, the throttling stream can be under a higher pressure than the turbine stream. That is, the turbine stream during cooldown in the main heat exchanger is under the second pressure and the throttling stream during cooldown in the main heat exchanger is under a third pressure, which is identical to the second pressure or is higher than the second pressure. 
     For the further secondary compression from the second at pressure, a second recompressor is installed in the second variant, which is driven by an expansion machine that is operated with a process stream of the procedure. Preferably, the recompressor that runs at the second pressure is driven by the expansion machine that is operated with the turbine stream, and the process stream that is used for driving the second recompressor is formed by a third air stream, which is expanded at the operating pressure of the low-pressure column (Lachmann turbine) or by compressed nitrogen from the distillation-column system, particularly from a high-pressure or low-pressure column. Alternatively, the two drives can be switched. 
     In a modification of the first variant, the present invention comprises at least two stages instead of the recompressor and can also be driven with external power. The further compression at the second pressure then occurs in at least a first stage of the recompressor; the throttling stream is further compressed downstream of the branching off of the turbine stream at least in the last stage of the recompressor at a third pressure, which is higher than the second pressure. The steps according to the invention of cooldown, expansion, and heat-up of the turbine stream create so much additional flexibility that the process can attain a high efficiency, even if the construction-dependent intermediate offtake pressures of the recompressors are in themselves unfavorable. 
     Preferably, the intermediate pressure is 1.5 to 5 bars below the second pressure, that is, the turbine stream in front of the inlet into the expansion machine is expanded by this pressure difference. This relatively low throttling at the low temperature causes practically no power loss and still allows the desired reduction in the inlet temperature of the expansion machine. 
     Preferably, the distillation-column system comprises a high-pressure column and a low-pressure column, which is located above a main condenser relative to heat exchange. The main condenser is constructed as a condenser-evaporator. The turbine stream is expanded in the expansion machine preferably at roughly the operating pressure of the high-pressure column and is fed at least in part into the high-pressure column. 
     A liquid oxygen stream, a liquid nitrogen stream, and/or a liquid argon stream can be used as the liquid product stream from the distillation-column system. If more than one product is internally compressed, many independent and of course appropriate types of equipment for increasing pressure (as a rule, pumps or pairs of pumps) and independent paths must be provided through the main heat exchanger. 
     It is favorable if a second stream of air is formed out of another portion of the purified main air stream and the second stream of air is cooled down in the main heat exchanger under the first pressure and is conducted to the distillation-column system. This second stream of air is also called a direct air stream. Preferably, the main air stream, aside from a small portion of the instrument air used, if necessary, is divided up into precisely the three parts cited here, namely the direct air stream, the turbine stream, and the throttling stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention as well as further details of the invention are further clarified in the following with the aid of the embodiments schematically represented in the drawings. 
         FIG. 1  shows an embodiment of the first variant of the invention; 
         FIG. 2  shows a first embodiment of the second variant of the invention with a single turbine; 
         FIG. 3  shows a further embodiment of the second variant of the invention with two turbines; 
         FIG. 4  shows a further embodiment of the second variant of the invention with two turbines; 
         FIG. 5  shows a heat-exchange diagram (temperature relative to specific enthalpy) for a process according to prior art without throttling of the turbine stream; and 
         FIG. 6  shows a heat-exchange diagram for the process of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the embodiment of  FIG. 1 , the distillation-column system  50  comprises, in the part which serves for the nitrogen-oxygen separation, a high-pressure column  14 , a low-pressure column  15 , and a main condenser  16  constructed as a condenser-evaporator, at which the two columns stand relative to heat exchange. 
     Atmospheric air is sucked in as main air stream  1  to an air compressor  2 , and is brought there to a first pressure that corresponds roughly to the operating pressure of the high-pressure column  14 , is cooled down in a primary cooling  3  to roughly ambient temperature, and is conducted to an adsorptive air purification  4 . A first portion of the purified main air stream  5  is further compressed as a “first air stream”  6  in a recompressor  7  at a second pressure of at least 50 bars, for example roughly 60 bars. The high-pressure air  8  is conducted to the hot end of a main heat exchanger  9  and is cooled down and pseudo-liquefied in the main heat exchanger. The pseudo-liquefied air is drawn off through piping  10  from the cold end of the main heat exchanger and is then split up into a throttling stream  11  and a turbine stream  17 . Conversely, the throttled and turbine streams after the joint recompression  7  are also cooled down and pseudo-liquefied together in the main heat exchanger. Alternatively, the turbine stream  17  could be taken off somewhat above the cold end of the main heat exchanger  9  (see  FIG. 2 .) 
     The throttling stream (“JT Air”)  11  is expanded in a throttle valve  12  to roughly the operating pressure of the high-pressure column and is conducted through piping  13  in a liquid state, at least in part, into the high-pressure column  14 . Instead of the throttle valve  12 , a fluid turbine can be installed. One portion  43  of the throttling stream can be immediately drawn out again from the high-pressure column and after cooldown  31  can be fed through piping  44  to the low-pressure column  15  at an intermediate position. 
     The turbine stream  17 , which is pseudo-liquefied along with throttling stream, is expanded in a throttle valve  18  at an intermediate pressure between the operating pressure of the high-pressure column and the second pressure and is then conducted again to the cold end of the main heat exchanger  9 . In the main heat exchanger, it is again heated up to an intermediate temperature that is between 140 and 150 K. At this intermediate temperature, the turbine stream is drawn through piping  70  out of the main heat exchanger  9  and conducted to a turbine  19 , which in the example is slowed down by a generator  20 . In the turbine  19 , the air is expanded to produce work at roughly the operating pressure of the high-pressure column. The expanded turbine stream  21  is conducted into a separator (phase separator)  22  in order to separate out liquid fractions, if necessary. Such liquid fractions  23  are fed in through piping  24  to a suitable location in the low-pressure column  15 . The gaseous fraction  25  is conducted through piping  26  as gaseous feed air into the high-pressure column  14 . 
     The remainder of the purified main air stream  5  is passed, without pressure-altering steps, through the main heat exchanger  9  as a direct air stream (“second air stream”)  27 ,  28  and flows further through piping  26  into the high-pressure column  14 . 
     In a first version of the embodiment (system without argon yield), raw liquid oxygen  29  flows from the sump of the high-pressure column  14  through piping  30 , undercooling counterflow  31 , and further through piping  32  to an intermediate position on the low-pressure column. The gaseous nitrogen head  33  of the high-pressure column  14  is condensed at least for the portion  34  in the liquefaction space of the main condenser  16 . Another portion can be passed over piping  35  through the main heat exchanger  9  and can finally be drawn off through piping  36  as a gaseous intermediate-pressure product (PGAN). 
     The condensed nitrogen  37  is delivered from the main condenser  16  to a first portion  38  as a return flow at the high-pressure column  14 . A second portion  39  is cooled down in the undercooling counterflow  31  and passed through piping  40  to the low-pressure column  15  as return flow. 
     Likewise, a nitrogen-enriched stream  41 ,  42  can be conducted from an intermediate position on the high-pressure column  14  through the undercooling counterflow  31  to an intermediate position on the low-pressure column  15 . 
     From the sump of the low-pressure column, a low-pressure gaseous oxygen product  45  (GOX) can be directly taken off, heated in the main heat exchanger  9 , and be drawn off through piping  46  as a low-pressure product. 
     The oxygen desired as a gaseous compressed product is drawn off as a liquid (LOX) out of the low-pressure column or out of the evaporation space of the main condenser  16  and passes as a “first liquid product stream  47  of internal compression (IC-LOX). Here it is brought in a liquid state by an oxygen pump  48  to the desired increased pressure (first increased pressure) and conducted through piping  49  to the cold end of the main heat exchanger  9 . In the main heat exchanger  9 , the liquid oxygen stream  49  is vaporized or pseudo-vaporized under the increased pressure and heated up to roughly ambient temperature. It finally leaves the system through piping  51  as a first gaseous compressed product (HP-GOX). 
     If desired, a further gaseous oxygen product  53 ,  54  (MP-GOX) can be obtained under an intermediate pressure that is between the operating pressure of the low-pressure column  15  and the increased pressure downstream of the pump  48 , in which this fraction branches off downstream of the pump  48 , is appropriately throttled down  52 , and is finally vaporized and heated up separately in the main heat exchanger  9 . 
     Alternatively, or in addition to the internally compressed oxygen stream or streams, nitrogen can be passed on for internal compression. What is more, a third portion  55  of the condensed nitrogen  37  is brought as a second “liquid product stream” out of the main condenser  16  (HP-LIN) into a nitrogen pump  56  at a second increased pressure that corresponds to the desired product pressure and which must not be the same as the first increased pressure. The high-pressure nitrogen is conducted through piping  57 ,  58  to the cold end of the main heat exchanger  9 . In the main heat exchanger  9 , the liquid or supercritical nitrogen stream  58  is vaporized or pseudo-vaporized under the increased pressure and is heated up to roughly ambient temperature. It finally leaves the system through piping  59  as a second gaseous compressed product (HP-GAN). 
     If desired, a further gaseous nitrogen product  61 ,  62  (MP-GAN) can be obtained under an intermediate pressure that is between the operating pressure of the high-pressure column  16  and the increased pressure downstream of the pump  56 , in which this portion branches off downstream of the pump  56 , accordingly throttled down  60 , and finally vaporized and heated up separately in the main heat exchanger  9 . 
     As further return flows, unpurified nitrogen  63 ,  64 ,  65  and unpurified nitrogen  66 ,  67 ,  68  are drawn gaseous out of the low-pressure column  15  into the undercooling counterflow  31  and are further heated up in the main heat exchanger  9  and drawn off as low-pressure products (GAN, UN2). Finally, a portion of the products are also obtained as liquid, for example liquid nitrogen (LIN)  69  or a portion of the liquid oxygen (LOX)  47  from the sump of the low-pressure column  15 . 
     The process of the embodiment of the first version can, for example, also be operated with only one liquid product stream and one gaseous compressed product (for instance, either oxygen or nitrogen), or alternatively with any combination of the streams depicted  49 ,  53 ,  58 , and  61  made liquid at pressure. 
     In an embodiment of the second version, the distillation-column system of the embodiment additionally exhibits an argon fraction  100  for the equipment for nitrogen-oxygen separation, which serves to yield pure liquid argon (LAR)  105 . The argon fraction comprises one or more raw-argon columns for argon-oxygen separation and a pure-argon column for argon-nitrogen separation, which is operated in the known manner. The lower end of the raw-argon column communicates through the piping  101  and  102  with an intermediate area of the low-pressure column  15 . The raw liquid oxygen  29  is conducted out of the high-pressure column  11 , in this case through the pipes  129  (systems with argon), into the argon fraction and is partially vaporized, particularly at least in part in the top condenser of the raw-argon column(s) (not depicted). The at least partially vaporized raw oxygen is fed in through piping  103  into the low-pressure column  15 , which remains liquid through piping  132 . Likewise, a gaseous residue stream (waste)  104  is drawn off from the argon fraction  100 . 
     Alternatively, or in addition to the internally compressed products described for the first version, the pure liquid argon  105  can be passed on for internal compression, in which it is brought as a third “liquid product stream” into an argon pump  106  at a third increased pressure that corresponds to the desired product pressure and which must not be the same as the first and/or second increased pressure. The high-pressure argon is conducted through piping  107  to the cold end of the main heat exchanger  9 . In the main heat exchanger  9 , the argon stream  107  is vaporized or pseudo-vaporized under the increased pressure and is heated up to roughly ambient temperature. It finally leaves the system through piping  108  as a third gaseous compressed product (HP-GAR). 
     The main heat exchanger can be executed as integral or split. The drawings show only the function of the exchanger: hot streams are cooled to cold. 
       FIG. 2  corresponds in large part to  FIG. 1 . Hence the same references are used as for the procedural steps and apparatus parts already described above, and the air compressor, the air purification, and the distillation-column system are not depicted in  FIG. 2 . 
     A difference from  FIG. 1  is the higher outlet pressure of the air compressor (“first pressure”), which in  FIG. 2  clearly lies above the operating pressure of the high-pressure column and amounts to 17 bars in the actual example. On this basis, the direct air stream ( 27  in  FIG. 1 ) is also missing. Rather, the total air  8  is divided downstream of the recompressor  7  under roughly 22 bars (“second pressure”) at  203  into the turbine stream  17  and the throttling stream  11 . (In  FIG. 2 , the cooldown of the turbine and throttling streams could also be executed together, whereby the fractionation within the main heat exchanger  9  could be executed just in front of its cold end). The temperature of the turbine stream  17  in the example, before throttling  18 , is 1 K to 50 K above the temperature of the cold end and is also above the temperature at which the throttling stream  11  leaves the main heat exchanger. Alternatively, the turbine stream could also, as shown in  FIG. 1 , be passed on as far as the cold end of the main heat exchanger  9 . 
     In addition, the aftercooler  202  of the recompressor is represented in  FIG. 2 , which may also used in the process according to  FIG. 1  but is not shown. The reference number  201  indicates the optional cooldown of the main air stream  5  downstream of the recompressor  7  in the main heat exchanger  9 . 
       FIG. 3  is distinguished from  FIG. 1  by a second expansion machine  319 , a recompressor  304 , and an aftercooler  305 . The branch-off of turbine stream and throttling stream takes place here in heat at  303 , whereby the throttling stream in the second recompressor  304  with the second pressure (here, for example, 22 bars) is further compressed to a third pressure (here, for instance 25 bars). The aftercooler  302  behind the first recompressor  7  can be omitted if the pre-cooling  201  of the main air stream is used. 
     The process stream  270 , with which the second expansion machine is operated, can be formed by one of the following streams:
         A partial stream of the turbine stream  70  (in this case, the expanded stream  325  is mixed with the stream  25  from the first expansion machine (both expansion machines in parallel).   A further air stream, which under the inlet pressure of the first recompressor  7  or under the outlet pressure of the first  7  or second  304  recompressors is taken off at an intermediate temperature from the main heat exchanger and is fed downstream of the expansion machine  319  into the low-pressure column or into the high-pressure column ( 15  and  14  in  FIG. 1 ) (Lachmann or a second Claude turbine).   A compressed nitrogen stream ( 35 ,  64 ,  67  in  FIG. 1  or a partial stream respectively thereof) out of the high-pressure column or the low-pressure column.       

     Alternatively, the coupling between the turbines  19 ,  319  and the recompressors  7 ,  304  is also the converse of that depicted in  FIG. 3 . 
       FIG. 4  is thereby distinguished from  FIG. 3 , in that the second recompressor  403 , which further compresses only the throttling stream, is constructed as a cold compressor. 
     In  FIGS. 5  (for prior art) and  6  (for  FIG. 2 ), the effect of throttling according to the invention downstream of the expansion machine is read off on the H-T diagram of the main heat exchanger. In  FIG. 6 , the turbine inlet temperature (stream  70  in FIG.  2 ; Tin) is clearly lower than in  FIG. 5 . The curves for the cooled-down streams (above) and the heated-up streams (below) lie considerably closer to one another. The exchange losses are accordingly smaller.