Abstract:
A system is set forth to increase the capacity of an LNG-based liquefier in a cryogenic air separation unit wherein, in a low production mode, the nitrogen that is fed to the LNG-based liquefier consists only of at least a portion of the high pressure nitrogen from the distillation column system while in a high production mode, a supplemental compressor is used to boost the pressure of at least a portion of the low pressure nitrogen from the distillation column system to create additional (or replacement) feed to the LNG-based liquefier. A key to the present invention is the supplemental compressor and the associated heat exchange equipment is separate and distinct from the LNG-based liquefier. This allows its purchase to be delayed until a capacity increase is actually needed and thus avoid building an oversized liquefier based on a speculative increase in liquid product demand.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention concerns 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 “ASU”) comprising a main heat exchanger and a distillation column system;  
         [0003]     (b) the air feed is cooled (and optionally at least a portion condensed) 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 and 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 comprises a higher pressure column and a lower pressure column;  
         [0006]     (e) the higher pressure column separates the air feed into effluent streams including a high pressure nitrogen stream withdrawn from the top of the higher pressure column, and a crude liquid oxygen stream withdrawn from the bottom of the higher pressure column and fed to the lower pressure column for further processing;  
         [0007]     (f) the lower pressure column separates the crude liquid oxygen stream into effluent streams including an oxygen product stream withdrawn from the bottom of the lower pressure column, and a low pressure nitrogen stream withdrawn from the top of the lower pressure (and often a waste nitrogen stream which is withdrawn from an upper location of the lower pressure column); and  
         [0008]     (g) the higher pressure column and lower pressure column are thermally linked such that at least a portion of the high pressure nitrogen is condensed in a reboiler/condenser against boiling oxygen-rich liquid that collects in the bottom (or sump) of the lower pressure column and used as reflux for the distillation column system.  
         [0009]     More specifically, the present invention concerns the known embodiment of the above-described Process wherein, in order to provide the refrigeration necessary when at least a portion of the product is desired as liquid, refrigeration is extracted from liquefied natural gas (hereafter “LNG”) by feeding nitrogen from the distillation column system to an insulated liquefier unit (hereafter “LNG-based liquefier”) where it is liquefied. If at least a portion of the liquid product desired is liquid oxygen, at least a portion of the liquefied nitrogen is returned to the distillation column system (or optionally the main heat exchanger). Otherwise, the liquefied nitrogen is withdrawn as product.  
         [0010]     Typical of LNG-based liquefiers, the nitrogen is compressed in stages and cooled between stages by indirect heat exchange against LNG. If the compression is performed with a cold-inlet temperature, the LNG will also be used to cool the feed to the compressor as well as the discharge by indirect heat exchange. Examples of LNG-Based liquefiers can be found in GB patent application 1,376,678 and U.S. Pat. No. 5,137,558, 5,139,547 and 5,141,543, all further discussed below.  
         [0011]     The skilled practitioner will appreciate the contrast between an LNG-based liquefier and the more conventional liquefier where the refrigeration necessary to make liquid product is derived from turbo-expanding either nitrogen or air feed.  
         [0012]     An LNG-based liquefier is typically oversized to accommodate a projected increase in demand of liquid products after the initial years of operation. This is particularly true for liquid nitrogen since the demand for liquid nitrogen out of any particularly ASU often grows faster than the demand for liquid oxygen above the base load of liquid oxygen for which the plant is designed. A problem with this oversizing approach however is the incremental capital cost incurred does not begin to pay off until the projected demand increase is actually realized (if at all). Furthermore, capital costs are particularly sensitive for LNG-based liquefiers since, as opposed to conventional liquefiers which are typically located near the customers of the liquid products, LNG-based liquefiers must be located near an LNG receiving terminal and thus incur a product transportation cost penalty.  
         [0013]     To address this problem, the present invention is a system to increase the capacity of the LNG-based liquefier comprising a supplemental compressor that is separate and distinct from the auxiliary compressor(s) contained in the LNG-based liquefier. This allows the supplemental compressor and the associated heat exchange equipment to be purchased and installed when the projected demand increase is actually realized, if at all. In this fashion, the incremental capital that would have otherwise been spent on oversizing the LNG-based liquefier from the start does not get spent until it is actually needed. Another benefit of the present invention is that the capacity increase is primarily directly toward the ability to produce liquid nitrogen which, as noted above, will often have a demand that grows faster than the demand for the liquid oxygen from the plant.  
         [0014]     The skilled practitioner will appreciate that, as an alternative to the present invention, the capacity of an LNG-based liquefier can be increased by adding a dense fluid expander. However, only modest capacity increases can be achieved in this manner.  
         [0015]     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.  
         [0016]     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.  
         [0017]     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.  
         [0018]     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.  
         [0019]     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. In all three of these documents, the nitrogen feed to the liquefier is made up of lower pressure and higher pressure nitrogen streams from the ASU. The lower pressure nitrogen stream originates from the lower pressure column; the higher pressure nitrogen stream originates from the higher pressure column. No direction is given as to the ratio of the lower pressure to higher pressure nitrogen streams.  
         [0020]     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.  
       BRIEF SUMMARY OF THE INVENTION  
       [0021]     The present invention relates to a cryogenic air separation unit which utilizes an LNG-based liquefier to provide the refrigeration necessary when at least a portion of the product is desired as liquid. The present invention is a system to increase the capacity of the LNG-based liquefier wherein, in a low production mode, the nitrogen that is fed to the LNG-based liquefier consists only of at least a portion of the high pressure nitrogen from the distillation column system while in a high production mode, a supplemental compressor is used to boost the pressure of at least a portion of the low pressure nitrogen from the distillation column system to create additional (or replacement) feed to the LNG-based liquefier. A key to the present invention is the supplemental compressor is separate and distinct from the LNG-based liquefier. This allows its purchase to be delayed until a capacity increase is actually needed and thus avoid building an oversized liquefier based on a speculative increase in liquid product demand. 
     
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0022]      FIG. 1   a  is a schematic diagram showing one embodiment of the prior art to which the system of the present invention pertains.  
         [0023]      FIG. 1   b  is a schematic diagram showing the basic concept of the present invention in relation to  FIG. 1   a.    
         [0024]      FIG. 2  is a schematic diagram identical to  FIG. 1   b  in terms of showing the basic concept of the present invention, but differs slightly with respect to the configuration between the LNG-based liquefier ( 2 ) and the ASU ( 1 ).  
         [0025]      FIG. 3   a  is a schematic diagram showing the detail for one example of an LNG-based liquefier for the flowsheet of  FIG. 2 .  
         [0026]      FIG. 3   b  is a schematic diagram showing one embodiment of the present invention, particularly as it relates to the integration between the supplemental processing unit and the LNG-based liquefier of  FIG. 3   a.    
         [0027]      FIG. 3   c  is a schematic diagram of a second embodiment of the present invention, particularly as it relates to the integration between the supplemental processing unit and the LNG-based liquefier of  FIG. 3   a.    
         [0028]      FIG. 4 &#39;s schematic diagram of the flowsheet that served as the basis for the worked example and includes a more detailed air separation unit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     The present invention is best understood when read in connection with the drawings.  
         [0030]      FIG. 1   a  is a schematic diagram showing one embodiment of the prior art to which the system of the present invention pertains. Referring now to  FIG. 1   a,  the facility includes an LNG-based 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 dried in  104  to produce stream  108 . Stream  108  is cooled in main exchanger  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 main exchanger  110  to produce streams  176  and  182 . Stream  182  is ultimately rejected to the atmosphere. Stream  176  is processed in the LNG-based liquefier ( 2 ) to create liquefied nitrogen product stream  188  and liquid nitrogen refrigerant stream  186 . Liquid nitrogen refrigerant stream  186  is introduced into the distillation columns through valves  136  and  140 . Refrigeration for LNG-based liquefier is provided from LNG stream  194 , which is vaporized and heated to produce stream  198 . In  FIG. 1   a,  the only nitrogen feed to the LNG-based liquefier is stream  176 , which originates from the higher pressure column  114 .  
         [0031]      FIG. 1   b  is a schematic diagram showing the basic concept of the present invention in relation to  FIG. 1   a.  Referring now to  FIG. 1   b,  feed air  100  is compressed in  102  and dried in  104  to produce stream  108 . Stream  108  is cooled in main exchanger  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 main exchanger  110  to produce streams  176  and  182 . Stream  182  is transformed utilizing a supplemental compressor and the associated heat exchange equipment (referred to hereunder as the “supplemental processing unit” which is depicted as unit  3  in  FIG. 1   a ) to become stream  184 , then mixed with stream  176 , to form a feed to the LNG-based liquefier ( 2 ). Liquefied nitrogen product stream  188  and liquid nitrogen refrigerant stream  186  are produced within the LNG-based liquefier. Liquid nitrogen refrigerant stream  186  is introduced into the distillation columns through valves  136  and  140 . In contrast to  FIG. 1   a,  the source of the nitrogen feed to the LNG-based liquefier leaves the ASU as two streams,  182  and  176 .  
         [0032]     As noted above, the term supplemental processing unit as used hereunder means the present invention&#39;s supplemental compressor and the associated heat exchange equipment. It should be noted however that the term does not necessarily mean the supplemental compressor and the associated heat exchange equipment are contained in a single physical unit. The exact nature of the supplemental processing unit ( 3 ) is described in detail with reference to the embodiments of the invention depicted in  FIGS. 3   b  and  3   c.    
         [0033]     Operation of  FIG. 1   b  where, similar as shown in  FIG. 1   a,  stream  182  is vented and not fed the supplemental processing unit ( 3 ), is preferred when the ratio of liquid nitrogen product to liquid oxygen product (stream  188 /stream  158 ) is relatively low and hereafter is referred to as “low production mode”. When operating in this mode, it is appropriate to extract all of the nitrogen to be liquefied from the higher pressure column. Operation as shown in  FIG. 1   b,  hereafter referred to as “high production mode” is preferred when the ratio of liquid nitrogen product to liquid oxygen product (stream  188 /stream  158 ) is relatively high. In such a case, so much nitrogen needs to be liquefied that it is appropriate to extract the nitrogen to be liquefied from both the higher pressure column and lower pressure column.  
         [0034]     In  FIG. 1   b,  the supplemental processing unit ( 3 ) is inserted to transform the state of stream  184  relative to stream  182  so that it may be mixed with stream  176  prior to introduction to the LNG-based liquefier. By doing so, the design and operation of the LNG-based liquefier may be similar in both high and low production modes. In fact, the design of the LNG-based liquefier can be exactly the same and the equipment simply operated at “turn-down” in the low production mode.  
         [0035]      FIG. 2  is a schematic diagram identical to  FIG. 1   b  in terms of showing the basic concept of the present invention, but differs slightly with respect to the configuration between the LNG-based liquefier ( 2 ) and the ASU ( 1 ). In particular, whereas liquefied nitrogen stream  186  is fed to the distillation column system in  FIG. 1   b,  stream  186  is fed to the main heat exchanger in  FIG. 2 . Referring now to  FIG. 2 , feed air  100  is compressed in  102  and dried in  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 . In low production mode, stream  182  is vented and streams  288  and  176  are processed in the LNG-based liquefier to create liquefied nitrogen product stream  188  and liquid nitrogen refrigerant stream  186 . In high production mode, stream  182  is transformed in the supplemental processing unit ( 3 ) to become stream  184 , then mixed with stream  176 . The mixed stream, plus stream  288 , are processed in the LNG-based liquefier to create liquefied nitrogen product stream  188  and liquid nitrogen refrigerant stream  186 .  
         [0036]     The exact nature of the LNG-based liquefier is not the focus of the present invention, however, how the liquefier integrates with the supplemental processing unit ( 3 ) is important to understand so an example of an LNG-based liquefier (unit  2  in  FIG. 2 ) is described in  FIG. 3   a .  FIG. 3   b  and  3   c  will give examples of the same LNG-based liquefier with inclusion of different embodiments of the supplemental processing unit ( 3 ).  
         [0037]     Referring to  FIG. 3   a , high pressure nitrogen vapor stream  176  is mixed with vapor nitrogen return stream  288  to form stream  330 , which is subsequently cooled in liquefier exchanger  304  to form stream  332 . Stream  334  is compressed in a first auxiliary compressor (HP cold compressor  308 ) to form stream  336 . Stream  336  is cooled in liquefier exchanger  304  to make stream  338 , then is compressed in a second auxiliary compressor (VHP cold compressor  310 ) to form stream  346 . Stream  346  undergoes cooling and liquefaction in liquefier exchanger  304  to make stream  348 .  
         [0038]     Liquefied stream  348  is further cooled in cooler  312  to form stream  350 . Stream  350  is reduced in pressure across valve  314  and introduced to vessel  316  where the two phase fluid is separated to vapor stream  352  and liquid stream  356 . Liquid stream  356  is split into two streams: stream  360  and stream  186 , which constitutes the liquid nitrogen refrigerant stream that is directed to the cryogenic ASU. Stream  360  is reduced in pressure across valve  318  and introduced to vessel  320  where the two phase fluid is separated to vapor stream  362  and liquid nitrogen product stream  188 . Vapor streams  362  and  352  are warmed in cooler  312  to form streams  364  and  354 , respectively. Stream  364  is further warmed in exchanger  304  to form gaseous nitrogen vent stream  366  from the LNG-based liquefier.  
         [0039]     Refrigeration for the LNG-based liquefier is supplied by LNG stream  194 , which is vaporized and or warmed in liquefier exchanger  304  to form stream  198 .  
         [0040]     In the strictest sense, the terms “vaporized” and “condensed” applies to streams that are below their critical pressure. Often, the streams  346  (the highest pressure nitrogen stream) and  194  (the LNG supply) are at 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).  
         [0041]     Referring now to  FIG. 3   b,  in high production mode of operation, lower pressure nitrogen stream  182  is an additional source of nitrogen that ultimately needs to be liquefied. Per the present invention, the supplemental processing unit ( 3 ) has been added to transform low pressure nitrogen stream  182  into a higher pressure nitrogen stream  184 . Stream  182  is combined with warm, low pressure gaseous nitrogen vent stream  366  to form stream  370 . Stream  370  is cooled in pre-cooling heat exchanger  322  to produce cooled nitrogen stream  372 . Stream  372  is mixed with cold, low pressure gaseous nitrogen vent stream  386  from the LNG-based liquefier to form stream  374 . Stream  374  is compressed cold in the supplemental compressor (LP compressor  306 ) to form stream  184 , then mixed with high pressure liquefier feed streams  288  and  176  to form stream  330 . The refrigeration for cooling stream  370  is provided by LNG stream  394 , which is vaporized and/or warmed in precooling heat exchanger  322  to form stream  396 .  
         [0042]     Operation of LNG-based liquefier ( 2 ) in  FIG. 3   b  is very similar to that described in  FIG. 3   a  with some exceptions. As in  FIG. 3   a , stream  330  is cooled in liquefier exchanger  304  to form stream  332 . Stream  334  is compressed in HP cold compressor  308  to form stream  336 . Stream  336  is cooled in liquefier exchanger  304  to make stream  338 , is compressed in VHP cold compressor  310  to form stream  346 . Stream  346  undergoes cooling and liquefaction in liquefier exchanger  304  to make stream  348 .  
         [0043]     As in  FIG. 3   a , liquefied stream  348  is further cooled in cooler  312  to form stream  350 . Stream  350  is reduced in pressure across valve  314  and introduced to vessel  316  where the two phase fluid is separated to vapor stream  352  and liquid stream  356 . Liquid stream  356  is split into two streams: stream  360  and stream  186 , which constitutes the liquid nitrogen refrigerant stream that is directed to the cryogenic ASU. Stream  360  is reduced in pressure across valve  318  and introduced to vessel  320  where the two phase fluid is separated to vapor stream  362  and liquid nitrogen product stream  188 . Vapor streams  362  and  352  are warmed in cooler  312  to form streams  364  and  354 , respectively.  
         [0044]      FIG. 3   b  is different from  FIG. 3   a in that stream  364 , which is a low pressure nitrogen stream, need not be warmed and vented because the supplemental compressor (LP cold compressor  306 ) exists. There are two possible ways to combine stream  364  with stream  182 . In the more thermodynamically preferred case, valve  380  is closed and valve  382  is open. In this event stream  364  flows through valve  382  to become gaseous nitrogen vent stream  386  from the LNG-based liquefier, which is then blended with cold nitrogen feed stream  372 . In the less thermodynamically preferred case, valve  380  is open and valve  382  is closed. In this event stream  364  flows through valve  380  to become stream  384 , is warmed in heat exchanger  304  to become gaseous nitrogen vent stream  366  from the LNG-based liquefier, then blended with warm nitrogen feed stream  182 . The more thermodynamically preferred option (valve  380  closed) would be employed if the cold valves  380  and  382  were incorporated into the liquefier at the design point; the less thermodynamically preferred option (valve  382  closed) would be employed if the inclusion of the supplemental processing unit ( 3 ) was executed as a retrofit. In the latter event, valves  380  and  382  might not exist and line  382  would not be present.    
         [0045]     Finally in  FIG. 3   b,  and as in  FIG. 3   a , refrigeration for the LNG-based liquefier is supplied by LNG stream  194 , which is vaporized and or warmed in liquefier exchanger  304  to form stream  198 .  
         [0046]     As indicated above, the refrigeration to cool the lower pressure nitrogen in precooling heat exchanger  322  is by vaporizing and/or warming LNG stream  394 . As an alternative, it is possible to extract a cold nitrogen stream from the cold or intermediate location of the liquefier heat exchanger  304 , warm that stream in exchanger  322 , then re-cool that stream in exchanger  304 . This might be done to eliminate the need to pipe LNG to precooling heat exchanger  322  as shown by stream  394  in  FIG. 3   b.  Any suitable stream may be used as the source of the cold nitrogen gas, such as streams  332 ,  338 , or  348 .  
         [0047]     Referring now to  FIG. 3   c,  a simpler supplemental processing unit might be employed. Once again, in high production mode of operation lower pressure nitrogen stream  182  is an additional source of nitrogen that ultimately needs to be liquefied. Per the present invention, the supplemental processing unit ( 3 ) has been added to transform low pressure nitrogen stream  182  into a higher pressure nitrogen stream  184 . Stream  182  is combined with warm, low pressure nitrogen gaseous nitrogen vent stream  366  from the LNG-based liquefier to form stream  370 . Stream  370  is compressed in the supplemental compressor (warm LP compressor  324 ), then cooled in aftercooler heat exchanger  326  (typically using cooling water or glycol as the cooling medium) to form stream  184 . Stream  184  is subsequently mixed with high pressure liquefier feed streams  288  and  176  to form stream  330 . The operation of the LNG-Based liquefier is similar to that described in  FIG. 3   a , except stream  366  is not vented.  
         [0048]     As noted previously, the supplemental processing unit as depicted as unit ( 3 ) in  FIGS. 3   b  and  3   c  does not necessarily refer to single physical unit. For example, the supplemental compressor can be contained in a housing with other compressors while the supplemental heat exchanger can be contained in a housing with other heat exchangers. It should also be noted that while the supplemental compressor and heat exchanger operate at above ambient temperature in  FIG. 3   c &#39;s embodiment of the present invention, this equipment operates at below ambient temperatures in  FIG. 3   b &#39;s embodiment and therefore must be insulated.  
       EXAMPLE  
       [0049]     A worked example has been prepared to demonstrate possible operating conditions associated with the present invention and clarify what is different and common between operating modes. Three cases will be given: Case 1 corresponds to low production mode operation without the supplemental processing unit ( 3 ) while Cases 2 and 3 correspond to high production mode operation with the supplemental processing unit ( 3 ) in place. For this example, Case 1 is depicted by the LNG-based liquefier ( 2 ) of  FIG. 3   a ; Cases 2 and 3 are depicted by the LNG-based liquefier ( 2 ) and the supplemental processing unit ( 3 ) of  FIG. 3   b . For Cases  2  and  3 , referring to  FIG. 3   b,  valve  380  is closed and valve  382  is open. The cryogenic ASU in shown in greater detail in  FIG. 4  and described below.  
         [0050]     Referring to  FIG. 4 , 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, 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  then at least partially condensed to form stream  232 , then eventually 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  462 , and an oxygen-enriched stream,  450 , from the bottom. Stream  462  is split into stream  174  and stream  464 . Stream  174  is warmed in the main heat exchanger then passed, as stream  176  to the LNG-based liquefier ( 2 ). Stream  464  is condensed in reboiler-condenser  418  to form stream  466 . A portion of stream  466  is returned to the higher pressure column as reflux (stream  468 ); the remainder, stream  470 , is eventually introduced to the lower pressure column as the top feed to that column through valve  472 . Oxygen-enriched stream  450  is passed to the argon column&#39;s reboiler-condenser  484  through valve  452 , and at least partially vaporized to form stream  456 , which is directed to the lower pressure column.  
         [0051]     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. Nitrogen-rich stream  180  is warmed in main heat exchanger  110  to form stream  182 . A waste stream may be removed from the lower pressure column, as stream  490 , warmed in the main exchanger and ultimately discharged as stream  492 . Boilup for the bottom of the lower pressure column is provided by reboiler-condenser  418 . A vapor flow is extracted from the lower pressure column as stream  478  and fed to argon column  482 . Argon product is withdrawn from the top of this column as liquid stream  486 . Bottom liquid stream  480  is returned to the lower pressure column. The reflux for the argon column is provided by indirect heat exchange with the vaporizing oxygen-enriched stream, which originates from the higher pressure column as stream  450 .  
         [0052]     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 .  
         [0053]     In low production mode of operation (Case 1) stream  182  is vented to atmosphere from the ASU (as stream  486 ), stream  366  is vented to atmosphere from the LNG-Based liquefier, and the flow of streams  184  and  386  are zero. In high production mode (Cases 2 and 3) streams  182  (as stream  488 ) and  386  are passed to the supplemental processing unit, and the flow of stream  366  is zero. For these particular Case 2 and 3 examples, the flow of stream  176  (originating from the higher pressure column) is also zero. That is, in Cases 2 and 3, the entire portion of the high pressure nitrogen  462  from the high pressure column is condensed in reboiler/condenser [ 418 ] and used as reflux for the distillation column system such that, as between the boosted pressure nitrogen and the high pressure nitrogen, only the boosted pressure nitrogen is fed to the LNG-based liquefier in high production mode. Although this is not mandatory, it is a typical scenario in high production mode. The distinction between Case 2 and 3 is the liquid nitrogen production in Case 3 is higher.  
         [0054]     Cases 1-3 are intended to illustrate how liquid production can be increased. Several balance points can be gleaned from the Table as indicated by Notes 1-5 therein which are explained below:  
         [0000]     Note 1: The liquid oxygen production increases by 33% in going from Case 1 to Case 2; liquid oxygen production is the same in Case 2 and 3.  
         [0000]     Note 2: The liquid nitrogen production increases 60% in going from Case 1 to Case 2; liquid nitrogen production increases 140% in going from Case 1 to Case 3.  
         [0000]     Note 3: The high pressure nitrogen flow is sufficient to meet the liquid nitrogen production requirement in Case 1, but is zero in Cases 2 and 3.  
         [0055]     Note 4: Even though the liquid oxygen production is significantly less in Case 1, the air flow to the ASU is roughly the same for all three cases. This is an important feature. When one elects to produce nitrogen from the ASU as high pressure nitrogen then the oxygen recovery declines. As a result, the use of the present invention allows one to use the same air compressor and same Cryogenic ASU for all three cases.  
         [0056]     Note 5: Case 1 operates with no LP Compressor (the supplemental processing unit ( 3 ) is not needed)  
                                                             TABLE 1                                   Case 1   Case 2   Case 3   Notes                                    Liquid Oxygen Flow (158)   Nm3/hr   4,399   5,848   5,859   1       Liquid Nitrogen Product Flow (188)   Nm3/hr   8340   13344   20016   2       Liquid Argon Flow (486)   Nm3/hr   121   255   255       LP N2 Flow exit ASU (182)   Nm3/hr   7,469   18,956   20,438       Pressure   bara   1.2   1.2   1.2       LP N2 to vent (486)   Nm3/hr   7,469   5,400   104       LP N2 to Unit 3 (488)   Nm3/hr   0   13556   20334   5       HP N2 Flow exit ASU (176)   Nm3/hr   9,184   0   0   3       Pressure   bara   5.2   n/a   n/a       Vap. N2 refrigerant exit ASU (288)   Nm3/hr   6,298   8,354   8,445       Pressure   bara   5.2   5.2   5.2       LP N2 from Unit 2 to Vent (366)   Nm3/hr   1562   0   0       LP N2 to Unit 3 (386)   Nm3/hr   n/a   2499   3666       Pressure   bara   n/a   1.1   1.1       Temperature   C.   n/a   −179.6   −179.6       N2 from Unit 3 (184)   Nm3/hr   n/a   16055   24000       Pressure   bara   n/a   5.0   5.0       Temperature   C.   n/a   −49.7   −49.5       Air Flow (108)   Nm3/hr   29,831   30,598   31,923   4       Pressure   bara   5.7   5.8   5.7       Liq. N2 refrigerant from Unit 2 (186)   Nm3/hr   6,298   8,354   8,445       Pressure   bara   5.3   5.3   5.3       LNG Supply Flow to Unit 2 (194)   Nm3/hr   45142   64190   82291       LNG Supply Flow to Unit 3 (394)   Nm3/hr   0   5329   7994       Pressure   bara   76.53   75.84   75.84       Temperature   C.   −153.9   −153.9   −153.9                  
 
         [0057]     In the description of  FIG. 4 , gaseous nitrogen stream  174  from the high pressure column that is warmed in the main heat exchanger and fed as stream  176  to the liquefier could alternatively be condensed in reboiler-condenser [ 418 ]. In this scenario, after being condensed in reboiler-condenser [ 418 ], the liquid nitrogen stream  174  would be vaporized and warmed in the main heat exchanger.  
         [0058]     Finally, as can be appreciated by one skilled in the art, even though the supplemental compressor of the present invention is separate and distinct from the auxiliary compressor(s) for the LNG-based liquefier, a common machine could drive both in high production mode. In this scenario, the machine installed for driving the auxiliary compressor(s) when the plant is built could contain a vacant pinion for eventually adding the supplemental compressor. Alternately, the auxiliary compressor(s) and the supplemental compressor are driven by separate machines in high production mode.