Abstract:
A cryogenic air separation process is set forth wherein, in order to provide the refrigeration necessary when at least a portion of the oxygen product is desired as liquid oxygen, LNG-derived refrigeration is used to liquefy a nitrogen stream in the process. A key to the present invention is that, instead of feeding the liquefied nitrogen to the distillation column, the liquefied nitrogen is heat exchanged against the air feed to the distillation column system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the well known process (hereafter “Process”) for the cryogenic separation of an air feed wherein: 
         [0002]    (a) the air feed is compressed, cleaned of impurities that will freeze out at cryogenic temperatures such as water and carbon dioxide, and subsequently fed into an cryogenic air separation unit (hereafter “cryogenic ASU”) comprising a main heat exchanger and a distillation column system which are contained in a large insulated box (generally referred to as the “cold box” in the industry); 
         [0003]    (b) the air feed is cooled in the main heat exchanger by indirectly heat exchanging the air feed against at least a portion of the effluent streams from the distillation column system; 
         [0004]    (c) the cooled air feed is separated in the distillation column system into effluent streams including a stream enriched in nitrogen, a stream enriched in oxygen and, optionally, respective streams enriched in the remaining components of the air feed including argon, krypton and xenon; and 
         [0005]    (d) the distillation column system typically comprises a first column (hereafter “high pressure column” or “HP column”) which separates the air feed into effluent streams including a nitrogen-enriched vapor stream and a crude liquid oxygen stream; and a second column (hereafter, “low pressure column” or “LP column”) which (i) operates at a relatively lower pressure than the HP column, (ii) separates the crude liquid oxygen stream into effluent streams including an oxygen product stream and one or more additional nitrogen-enriched vapor streams and (iii) is thermally linked with the HP column such that at least a portion of the nitrogen-enriched vapor from the HP column is condensed in a reboiler/condenser against boiling oxygen-rich liquid that collects in the bottom (or sump) of the LP column. 
         [0006]    More specifically, the present invention relates to the known embodiment of the Process wherein the refrigeration extracted from liquefied natural gas (hereafter “LNG”) is utilized in order to provide the refrigeration necessary when at least a portion of the oxygen product is desired as liquid oxygen. In particular, the refrigeration is extracted from the LNG by indirectly heat exchanging the LNG in a heat exchanger against one or more nitrogen-enriched vapor streams withdrawn from the distillation column in order to liquefy such nitrogen-enriched stream(s). The skilled practitioner will appreciate the contrast between using LNG to liquefy such nitrogen-enriched stream(s) and the more conventional way of providing the refrigeration necessary to make liquid oxygen product. In particular, the more conventional way consists of turbo expanding a working fluid (typically either nitrogen or air). 
         [0007]    A key to the present invention is what happens to the nitrogen-enriched stream(s) that are liquefied against the boiling LNG. In particular, whereas the prior art introduces such stream(s) into the distillation column system, the present invention introduces such stream(s) into a heat exchanger (preferably the main heat exchanger) to be indirectly heat exchanged against at least a portion of the air feed to the distillation column system in order to liquefy at least a portion of the air feed to the distillation column system. In other words, whereas the prior art provides the LNG-derived refrigeration directly to the distillation column system, the present invention provides such refrigeration to the air feed. As further discussed herein, this has the advantage of both reducing the vapor feed to the high pressure column (thereby by allowing a smaller HP column at a smaller capital cost) and avoiding a safety hazard when, as per the prior art, the liquefied nitrogen is introduced into the distillation column directly after being indirectly heat exchanged against natural gas. In particular, in the event there is a defect in the heat exchanger used for the natural gas/nitrogen heat exchange such that natural gas leaks into the nitrogen, the leaked natural gas will be introduced directly into the distillation and thus have the potential to form very hazardous mixtures with oxygen. 
         [0008]    The above described safety hazard is an important consideration because it leads to some of the unique features found in the below described prior art processes that utilize the refrigeration contained in LNG to aid in liquefaction. 
         [0009]    GB patent application 1,376,678 (hereafter “GB &#39;678”) teaches the very basic concept of how LNG refrigeration may be used to liquefy a nitrogen stream. The LNG is first pumped to the desired delivery pressure then directed to a heat exchanger. The warm nitrogen gas is cooled in said heat exchanger then compressed in several stages. After each stage of compression, the now warmer nitrogen is returned to the heat exchanger and cooled again. After the final stage of compression the nitrogen is cooled then reduced in pressure across a valve and liquid is produced. When the stream is reduced in pressure, some vapor is generated which is recycled to the appropriate stage of compression. 
         [0010]    GB &#39;678 teaches many important fundamental principles. First, the LNG is not sufficiently cold to liquefy a low-pressure nitrogen gas. In fact, if the LNG were to be vaporized at atmospheric pressure, the boiling temperature would be typically above −260° F., and the nitrogen would need to be compressed to at least 15.5 bara in order to condense. If the LNG vaporization pressure is increased, so too will the required nitrogen pressure be increased. Therefore, multiple stages of nitrogen compression are required, and LNG can be used to provide cooling for the compressor intercooler and aftercooler. Second, because the LNG temperature is relatively warm compared to the normal boiling point of nitrogen (which is approximately −320° F.), flash gas is generated when the liquefied nitrogen is reduced in pressure. This flash gas must be recycled and recompressed. 
         [0011]    U.S. Pat. No. 3,886,758 (hereafter “U.S. &#39;758”) discloses a method wherein a nitrogen gas stream is compressed to a pressure of about 15 bara then cooled and condensed by heat exchange against vaporizing LNG. The nitrogen gas stream originates from the top of the lower pressure column of a double-column cycle or from the top of the sole column of a single-column cycle. Some of the condensed liquid nitrogen, which was produced by heat exchange with vaporizing LNG, is returned to the top of the distillation column that produced the gaseous nitrogen. The refrigeration that is supplied by the liquid nitrogen is transformed in the distillation column to produce the oxygen product as a liquid. The portion of condensed liquid nitrogen that is not returned to the distillation column is directed to storage as product liquid nitrogen. 
         [0012]    EP 0,304,355 (hereafter “EP &#39;355”) teaches the use of an inert gas recycle such as nitrogen or argon to act as a medium to transfer refrigeration from the LNG to the air separation plant. In this scheme, the high pressure inert gas stream is liquefied against vaporizing LNG then used to cool medium pressure streams from the air separation unit (ASU). One of the ASU streams, after cooling, is cold compressed, liquefied and returned to the ASU as refrigerant. The motivation here is to maintain the streams in the same heat exchanger as the LNG at a higher pressure than the LNG. This is done to assure that LNG cannot leak into the nitrogen streams, i.e. to ensure that methane cannot be transported into the ASU with the liquefied return nitrogen. The authors also assert that the bulk of the refrigeration needed for the ASU is blown as reflux liquid into a rectifying column. 
         [0013]    U.S. Pat. Nos. 5,137,558, 5,139,547, and 5,141,543 (hereafter “U.S. &#39;558”, “U.S. &#39;547”, and “U.S. &#39;543” respectively) provide a good survey of the prior art up to 1990. These three documents also teach the state-of-the-art at that time. U.S. &#39;558 teaches cold compression to greater than 21 bara such that the nitrogen pressure exceeds the LNG pressure. U.S. &#39;547 deals with the liquefier portion of the process—key features are cold compression to 24 bara and refrigeration recovery from flash gas. U.S. &#39;543 further teaches to use turbo-expansion in addition to LNG for refrigeration to liquefy nitrogen. 
         [0014]    There is little new art in the literature since the early 90&#39;s because the majority of applications for recovery of refrigeration from LNG (LNG receiving terminals) were filled and new terminals were not commonly being built. Recently, there has been resurgence in interest in new LNG receiving terminals and therefore the potential to recover refrigeration from LNG. 
         [0015]    With respect to the ASU operation, a fundamental teaching of U.S. &#39;758 is illustrated in  FIG. 1 . The facility includes an LNG-based nitrogen liquefier ( 2 ) and a cryogenic ASU ( 1 ). In this example, the cryogenic ASU includes a higher pressure column ( 114 ), lower pressure column ( 116 ), and main exchanger ( 110 ). Feed air  100  is compressed in  102  and cleaned of impurities that will freeze out at cryogenic temperatures such as water and carbon dioxide in unit  104  to produce stream  108 . Stream  108  is cooled in  110  against returning gaseous product streams, to produce cooled air feed  112 . Stream  112  is distilled in the double column system to produce liquid oxygen  158 , high pressure nitrogen gas (stream  174 ) and low pressure nitrogen gas (stream  180 ). The nitrogen gases  174  and  180  are warmed in the main exchanger  110  to produce streams  176  and  182 . Streams  176  and  182  are processed in the LNG-based nitrogen liquefier to create liquefied nitrogen product stream  184  and liquid nitrogen refrigerant stream  186 . Liquid nitrogen refrigerant stream  186  is introduced into the distillation columns through valves  136  and  140 . 
         [0016]    The principle laid-out in  FIG. 1  is also taught in JP 2005134036, JP 55-77680 (JP 1978150868), U.S. Pat. No. 4,192,662, U.S. Pat. No. 4,054,433, as well as the above referenced U.S. &#39;758 and EP &#39;355. There are two disadvantages related to processes based on  FIG. 1 . Firstly, should there be a leak of hydrocarbon into ASU refrigerant stream  186 , that hydrocarbon will concentrate in the bottom of the lower pressure column and in liquid oxygen stream  158 . Since build-up of hydrocarbon in oxygen is to be avoided, for reasons of safety, steps must be taken to ensure that such a leak does not occur in the LNG-based nitrogen liquefier. Secondly, since all of the incoming air to the cryogenic ASU (stream  108 ) is introduced to the higher pressure column as vapor, this requires a bigger diameter (and thus higher capital cost) for the higher pressure column. 
         [0017]    It is therefore desired to provide an efficient process that transports refrigeration of the LNG-based nitrogen liquefier to the cryogenic ASU without the disadvantages associated with directly injecting potentially hydrocarbon laden liquid nitrogen to the distillation columns. 
         [0018]    As used herein, “LNG-based nitrogen liquefier” shall be defined as a system that uses the refrigeration contained in LNG to convert gaseous nitrogen into liquid nitrogen. Typical of such systems, the nitrogen will be compressed in stages. If the compression is performed with a cold-inlet temperature, the LNG will be used to cool the compressor discharge by indirect heat exchange. Cooling and or liquefaction of the nitrogen will be accomplished, at least in part, by indirect heat exchange with warming or vaporizing LNG. Examples of LNG-Based Nitrogen Liquefiers can be found in the above referenced GB &#39;678, U.S. &#39;558, U.S. &#39;547, and U.S. &#39;543. 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    The present invention relates to a cryogenic air separation process wherein, in order to provide the refrigeration necessary when at least a portion of the oxygen product is desired as liquid oxygen, LNG-derived refrigeration is used to liquefy a nitrogen stream in the process. A key to the present invention is that, instead of feeding the liquefied nitrogen to the distillation column, the liquefied nitrogen is heat exchanged against the air feed to the distillation column system. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0020]    The present invention as discussed in the Detailed Description is best understood when read in connection with the following drawings: 
           [0021]      FIG. 1  is a schematic diagram that illustrates how the prior art provides LNG-derived refrigeration to the cryogenic ASU. 
           [0022]      FIG. 2  is a schematic diagram of one embodiment of the present invention that illustrates how the present invention provides LNG-derived refrigeration to the cryogenic ASU. 
           [0023]      FIG. 3  is a schematic diagram similar to  FIG. 2  except it includes features and details of the cryogenic ASU omitted from  FIG. 2  for the sake of simplicity. 
           [0024]      FIG. 4  is a schematic diagram that shows one example of how the LNG-based nitrogen liquefier of the present invention could be configured and relates to the worked example. 
           [0025]      FIG. 5  is similar to  FIG. 3  except the cryogenic ASU incorporates a side argon column.  FIG. 5  also relates to the worked example. 
           [0026]      FIG. 6  is a schematic diagram of the prior art that is similar to  FIG. 1  except that, for the purposes of comparing to  FIG. 5  in the worked example, it incorporates FIG.  5 &#39;s version of the cryogenic ASU. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The basic concept of the invention is illustrated in  FIG. 2 . The facility includes an LNG-based Nitrogen Liquefier ( 2 ) and a cryogenic ASU ( 1 ). In this example, the cryogenic ASU includes a higher pressure column ( 114 ), lower pressure column ( 116 ), and main heat exchanger ( 110 ). Feed air  100  is compressed in  102  and cleaned of impurities that will freeze out at cryogenic temperatures such as water and carbon dioxide in unit  104  to produce stream  108 . Stream  108  is split into a first portion  208  and a second portion  230 . Stream  208  is cooled in  110  against returning gaseous product streams, to produce cooled air feed  212 . Stream  230  is first cooled in  110  against returning gaseous product streams then liquefied to produce stream  232 . Liquid air stream  232  is split and is introduced into the distillation columns through valves  236  and  240 . Streams  212  and  232  are distilled in the double column system to produce liquid oxygen  158 , high pressure nitrogen gas (stream  174 ) and low pressure nitrogen gas (stream  180 ). The nitrogen gases  174  and  180  are warmed in the main exchanger  110  to produce streams  176  and  182 . Liquid nitrogen refrigerant stream  186  is directed to the main exchanger where it is vaporized by indirect heat exchange with condensing stream  230  to form vapor nitrogen return stream  288 . Streams  288 ,  176  and  182  are processed in the LNG-based nitrogen liquefier to create liquefied nitrogen product stream  184  and liquid nitrogen refrigerant stream  186 . 
         [0028]    In one key embodiment of the invention, the liquid nitrogen refrigerant stream is vaporized at a pressure less than that of the air stream  108 . This is done to ensure that, should there be a leak of hydrocarbon into the liquid nitrogen refrigerant stream from the LNG-based Nitrogen Liquefier, and should there also be a leak between the liquid nitrogen refrigerant stream and the incoming air (e.g. in the main heat exchanger), the hydrocarbon initially leaked from the LNG-based nitrogen liquefier will not find its way into the distillation columns. In practice, the pressure difference between these two streams can be small, on the order of 0.1 bar. 
         [0029]    In  FIG. 2 , it is preferred that stream  232  be totally condensed. Owing to the differences in latent heat between the air stream  232  and the liquid oxygen stream  158 , the flow of stream  232  will be approximately 1.4 times the flow of liquid oxygen stream  158 . Typically, the flow of oxygen stream  158  is 20 to 21% of incoming air stream  108 , in which case the flow of stream  232  is approximately 28-29% and the flow of stream  212  is 72-71%. In other words, the vapor flow to higher pressure column  114  is approximately 72% of air. In contrast, for the process of  FIG. 1 , vapor flow to higher pressure column  114  is approximately 100% of air. It is apparent then, that this invention has an advantage over the prior art in that the higher pressure column will be of smaller diameter and therefore, of lower cost. 
         [0030]    For the process of  FIG. 2 , the oxygen recovery is maximized if stream  232  is totally condensed. However, it is possible to operate the invention with stream  232  only partially condensed. In this case, the flow of stream  232  will increase because there will still be approximately 28-29% of air as liquid in the stream. In the limit, if the flow of stream  208  were reduced to zero then the flow of stream  232  would be 100% and the liquid fraction of stream  232  would be 28-29%. Operation in this manner has the virtue of making the design of the main exchanger  110  simpler, hence capital cost will be lower, although oxygen recovery will be lower. Therefore the decision between options will depend on the economic trade off of capital and power. 
         [0031]    For the sake of simplicity, many of the features and details of a cryogenic ASU have been omitted from  FIG. 2  which are provided by  FIG. 3 . Atmospheric air  100  is compressed in the main air compressor  102 , purified in adsorbent bed  104  to remove impurities such as carbon dioxide and water, and then divided into two fractions: stream  230  and stream  208 . Stream  208  is cooled in main heat exchanger  110  to become stream  212 , the vapor feed air to the higher pressure column  114 . Stream  230  is cooled to a temperature near that of stream  212 , partially condensed to form stream  232  and then split into streams  334  and  338  which are reduced in pressure across valves  236  and  240  and introduced to the higher pressure column  114  and lower pressure column  116 . The higher pressure column produces a nitrogen-enriched vapor from the top, stream  362 , and an oxygen-enriched stream,  350 , from the bottom. Stream  362  is split into stream  174  and stream  364 . Stream  174  is warmed in the main heat exchanger then passed, as stream  176  to the LNG-based liquefier. Stream  364  is condensed in reboiler-condenser  318  to form stream  366 . A portion of stream  366  is returned to the higher pressure column as reflux (stream  368 ); the remainder, stream  370 , is eventually introduced to the lower pressure column as the top feed to that column through valve  372 . Oxygen-enriched stream  350  is also eventually introduced to the lower pressure column through valve  352 . The lower pressure column produces the oxygen from the bottom, which is withdrawn as liquid stream  158 , and a nitrogen-rich stream,  180 , from the top. 
         [0032]    Nitrogen-rich stream  180  is warmed in main heat exchanger  110  then passed, as stream  182  to the LNG-based liquefier. A waste stream may be removed from the lower pressure column, as stream  390 , warmed in the main exchanger and ultimately discharged as stream  392 . Boil up for the bottom of the lower pressure column is provided by reboiler condenser  318 . Liquid nitrogen refrigerant stream  186  is directed to the main exchanger where it is vaporized by indirect heat exchange with condensing stream  230  to form vapor nitrogen return stream  288 . Streams  288 ,  176  and  182  are processed in the LNG-based nitrogen liquefier to create liquefied nitrogen product stream  184  and liquid nitrogen refrigerant stream  186 . 
         [0033]    In  FIG. 3 , none of the lower pressure column feed streams are cooled prior to their pressure reduction and introduction to the lower pressure column. The action of cooling lower pressure column feeds is commonplace and accomplished by warming a low pressure gas stream, such as stream  180 , in a heat exchanger called a subcooler. Inclusion of a subcooler in the embodiments of the invention usually becomes justified as power cost and/or plant size increases. 
         [0034]    The production of lower pressure nitrogen stream  180  and higher pressure nitrogen stream  174  is optional. For example, if there is no liquid nitrogen product flow (there is no flow in stream  184  from the LNG-based liquefier) then there is no need for either of streams  176  or  182 . In this case, the nitrogen from the cryogenic ASU leaves as waste stream  392 . If the production of liquid nitrogen product stream  184  is modest compared to the production of liquid oxygen product stream  158 , then typically there would be no need for low pressure nitrogen stream  180 , but stream  174  would be used. If the production of liquid nitrogen product stream  184  is large compared to the production of liquid oxygen product stream  158 , then typically there would be no need for high pressure nitrogen stream  174 , but stream  180  would be used. For intermediate production levels of liquid nitrogen, both stream  174  and  180  would be employed. It would be apparent to one of normal skill in the art which combination is best—i.e. it is simply an economic optimization. 
         [0035]    Additionally, the embodiments of the invention could also include the coproduction of gaseous nitrogen product. In such an event, one may elect to use a portion of low pressure stream  182  as nitrogen product. Alternatively, one may elect to use a portion of high pressure stream  176  as nitrogen product. When nitrogen coproduct is withdrawn from the top of the higher pressure column it is also common, though not necessary, to extract the lower pressure column reflux stream,  370 , from a position in the higher pressure column a number of stages below the top of the higher pressure column. In this event, all of reboiler-condenser condensate stream  366  is returned to the higher pressure column. Furthermore, one might elect to recover gaseous nitrogen from the LNG-based liquefier—this might be done if the pressure of the nitrogen exceeds that typical of either streams  176  or  182 . 
         [0036]    Additionally, in  FIGS. 2 to 3  it is shown that the condensed air stream  232  is sent to both columns. It is possible, and often justified, to send all of stream  232  to either the higher pressure column or lower pressure column. Alternatively, all of stream  232  may be sent to the higher pressure column and liquid may be withdrawn from the higher pressure column from the same location as which stream  232  was introduced. As still another alternative, one may eliminate condensed air stream  232  altogether. The associated streams  230 ,  334 ,  338 , and valves  236  and  240 , would also be eliminated. In this event, the single air stream  212  would be partially condensed against the vaporizing nitrogen refrigerant stream  186  and stream  212  would constitute a second feed to the higher pressure column. 
         [0037]    In  FIGS. 2 and 3 , the sole oxygen product from the lower pressure column is stream  158 . Though not shown, one could make gaseous oxygen coproduct as well. This can be accomplished in a number of different ways. For example, oxygen may be withdrawn as a vapor from the bottom the lower pressure column, warmed in the main exchanger, and compressed. Additionally, the vapor oxygen stream may simply be mixed with waste stream  390 . Alternatively, a portion of oxygen stream  158  may be vaporized in the main exchanger and delivered as product. 
         [0038]    In  FIGS. 2 and 3  the condensation of stream  230  and vaporization of stream  186  is shown to take place in the main exchanger. It is within the scope of the present invention to perform this condensation and vaporization by indirect heat exchange in a separate heat exchanger. 
         [0039]    The nature of the LNG-based liquefier is not the focus of the invention, however, an example of an LNG-based Liquefier (unit  2  in  FIGS. 1-3 ) is described in  FIG. 4 . Low pressure nitrogen vapor stream  182  is cooled in liquefier exchanger  404  to make stream  422 , which is subsequently mixed with return vapor stream  464  to form stream  424 . Stream  424  is compressed in LP cold compressor  406  to form stream  426 . Stream  426  is cooled in liquefier exchanger  404  to make stream  428 , which is subsequently mixed with return vapor stream  454  and chilled stream  432  to form stream  434 . 
         [0040]    High pressure nitrogen vapor stream  176  is mixed with vapor nitrogen return stream  288  to form stream  430 , which is subsequently cooled in liquefier exchanger  404  to form stream  432 . Stream  434  is compressed in HP cold compressor  408  to form stream  436 . Stream  436  is cooled in liquefier exchanger  404  to make stream  438 , is compressed in VHP cold compressor  410  to form stream  446 . Stream  446  undergoes cooling and liquefaction in liquefier exchanger  404  to make stream  448 . 
         [0041]    Liquefied stream  448  is further cooled in cooler  412  to form stream  450 . Stream  450  is reduced in pressure across valve  414  and introduced to vessel  416  where the two phase fluid is separated to vapor stream  452  and liquid stream  456 . Liquid stream  456  is split into two streams: stream  460  and stream  186 , which constitutes the liquid nitrogen refrigerant stream that is directed to the cryogenic ASU. Stream  460  is reduced in pressure across valve  418  and introduced to vessel  420  where the two phase fluid is separated to vapor stream  462  and liquid nitrogen product stream  184 . Vapor streams  462  and  452  are warmed in cooler  412  to form streams  464  and  454 , respectively. 
         [0042]    Refrigeration for the LNG-based liquefier is supplied by LNG stream  196 , which is vaporized and or warmed in liquefier exchanger  404  to form stream  198 . 
         [0043]    In the strictest sense, the terms “vaporized” and “condensed” applies to streams that are below their critical pressure. Often, the streams  446  (the highest pressure nitrogen stream) and  196  (the LNG supply) are a pressures greater than critical. It is understood that these streams do not actually condense or vaporize. Rather they undergo a change of state characterized by a high degree heat capacity. One of normal skill in the art will appreciate the similarities between possessing a high degree of heat capacity (at supercritical conditions) and possessing a latent heat (at subcritical conditions). 
         [0044]    There are numerous variation on the liquefier design presented in  FIG. 4 . One distinction of note is as follows. The liquid nitrogen refrigerant stream  186  is shown to be withdrawn from intermediate pressure separator  416 . This is done for reasons of convenience. However, it would be within the spirit of the invention for stream  186  to be withdrawn from lower pressure separator  420 . It would also be possible to send all the liquid produced by the liquefier to storage (not shown) and to withdraw stream  186  from storage. In either of these two cases, it would be desirable to pump stream  186  to a suitable pressure before it is directed to the ASU. 
         [0045]    The following example has been prepared to show possible operating conditions associated with this process. For this example, the invention is depicted by the LNG-based liquefier of  FIG. 4  and the cryogenic ASU of  FIG. 5 . This process is compared to prior art teachings. The prior art teachings would lead to the process depicted by the LNG-based liquefier of  FIG. 4  and the cryogenic ASU of  FIG. 6 . 
         [0046]      FIG. 5  is similar to  FIG. 3  except that an argon column  562  has been added. Referring to  FIG. 5 , a vapor flow is extracted from the lower pressure column as stream  558  and fed to argon column  562 . Argon product is withdrawn from the top of this column as liquid stream  554 . Bottom liquid stream  560  is returned to the lower pressure column. The reflux for the argon column is provided by indirect heat exchange with vaporizing an oxygen-enriched stream, which originates from the higher pressure column as stream  350 . Stream  350  is passed though valve  352  into the reboiler-condenser  564 , and at least partially vaporized to form stream  556 , which is directed to the lower pressure column. Selected results from the rigorous simulation of invention, as indicated by  FIGS. 4 and 5 , is presented in Table 1. In this example, the flow of high pressure nitrogen vapor (stream  176 ) is zero. 
         [0047]    The cryogenic ASU according to the prior art is represented by  FIG. 6 . Referring to  FIG. 6 , liquid nitrogen refrigerant stream  186  is introduced into the higher pressure column through valve  136 . Two alternative cases of the prior art are considered. In the first case, denoted as Prior Art 1 in Table 1, the flow of high pressure nitrogen vapor (stream  176 ) is zero—just as in the example of the invention. In the second case, denoted as Prior Art 2 in Table 1, the flow of high pressure nitrogen vapor (stream  176 ) has been adjusted to yield the same argon production as in the example of the invention. 
         [0048]    The results presented in Table 1 demonstrate that total power of the facility is either less than or equal to that of the prior art. Also the higher pressure column air flow is significantly lower than the prior art, as indicated by stream  212  or  112  in the table. This confirms that the higher pressure column diameter of the invention can be significantly smaller than the prior art. Finally, and most important, the disadvantages associated with directly injecting potentially hydrocarbon laden liquid nitrogen to the distillation columns are mitigated with the invention. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Invention 
                 Prior Art 1 
                 Prior Art 2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Air Flow (108) 
                 Nm3/hr 
                 31,923 
                 30,156 
                 30,124 
               
               
                 Pressure 
                 bara 
                 5.72 
                 5.7 
                 5.71 
               
               
                 Column Air Flow 
                 Nm3/hr 
                 23,974 
                 30,156 
                 30,123 
               
               
                 (212, 112) 
               
               
                 Temperature 
                 C. 
                 −172.4 
                 −173.7 
                 −173.8 
               
               
                 Liquid Air Flow 
                 Nm3/hr 
                 7,949 
                 n/a 
                 n/a 
               
               
                 (232) 
               
               
                 Temperature 
                 C. 
                 −179 
                 n/a 
                 n/a 
               
               
                 Liq. N2 refrigerant 
                 Nm3/hr 
                 8,445 
                 8,536 
                 8,583 
               
               
                 (186) 
               
               
                 Pressure 
                 bara 
                 5.30 
                 5.30 
                 5.30 
               
               
                 Liquid Oxygen Flow 
                 Nm3/hr 
                 5,859 
                 5,847 
                 5,857 
               
               
                 (158) 
               
               
                 Liquid Argon Flow 
                 Nm3/hr 
                 255 
                 277 
                 255 
               
               
                 (554) 
               
               
                 Liquid Nitrogen 
                 Nm3/hr 
                 20,016 
                 20,016 
                 20,016 
               
               
                 Product (184) 
               
               
                 LP N2 Flow (182) 
                 Nm3/hr 
                 20,438 
                 28,974 
                 23,167 
               
               
                 Pressure 
                 bara 
                 1.20 
                 1.20 
                 1.20 
               
               
                 HP N2 Flow (176) 
                 Nm3/hr 
                 0 
                 0 
                 5840 
               
               
                 Pressure 
                 bara 
                 5.23 
                 5.22 
                 5.22 
               
               
                 Vap. N2 refrigerant 
                 Nm3/hr 
                 8,445 
                 n/a 
                 n/a 
               
               
                 (288) 
               
               
                 Pressure 
                 bara 
                 5.16 
                 n/a 
                 n/a 
               
               
                 LNG Supply Flow 
                 Nm3/hr 
                 90,283 
                 90,283 
                 90,283 
               
               
                 (196) 
               
               
                 Pressure 
                 bara 
                 75.9 
                 75.9 
                 75.9 
               
               
                 Temperature 
                 C. 
                 −154 
                 −154 
                 −154 
               
             
          
           
               
                 Power 
               
             
          
           
               
                 Main Air Compressor 
                 kW 
                 2,603 
                 2,458 
                 2,457 
               
               
                 (102) 
               
               
                 LP Compressor(406) 
                 kW 
                 854 
                 1,172 
                 956 
               
               
                 HP Compressor(408) 
                 kW 
                 1,550 
                 1,676 
                 1,650 
               
               
                 VHP Compressor(410) 
                 kW 
                 1,574 
                 1,552 
                 1,520 
               
               
                 Miscellaneous 
                 kW 
                 213 
                 204 
                 204 
               
               
                 Total 
                 kW 
                 6,794 
                 7,062 
                 6,787