Patent Publication Number: US-2016245585-A1

Title: System and method for integrated air separation and liquefaction

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system and method for producing one or more liquid product streams by cryogenic rectification of air or other feed stream, and more particularly, a system and method for converting upwards of 20% of the incoming air or other feed stream into one or more liquid product streams. 
     BACKGROUND OF THE INVENTION 
     Oxygen and nitrogen are produced commercially in large quantities by the cryogenic distillation of air in air separation units (ASU) where incoming feed air is compressed; purified of contaminants; and then cooled in a main heat exchanger to a temperature suitable for its rectification. The compressed, purified and cooled air stream is then cryogenically distilled typically in a thermally integrated dual pressure column system in which air is separated into at least oxygen and nitrogen-rich product streams. As well known in the art, additional columns may be employed to produce argon or additionally rare gases. 
     Oxygen that is separated from the incoming air feed can be taken as a liquid product that can be produced in the low pressure column as an oxygen-rich liquid column bottoms. Liquid product can additionally be taken from part of the nitrogen-rich liquid used in refluxing the columns. As known in the art, the oxygen liquid product can be pumped and then in part taken as a pressurized liquid product, and also heated in the main heat exchanger to produce an oxygen product as a vapor or as a supercritical fluid depending on the degree to which the oxygen is pressurized by the pumping. The liquid nitrogen can similarly be pumped and taken as either as pressurized liquid product, a high pressure vapor or a supercritical fluid. 
     In order to operate the distillation column system, refrigeration must be supplied to offset ambient heat leakage, warm end heat exchange losses and to allow the production of liquid products. Supplemental refrigeration is typically supplied by expanding part of the air, a waste or product stream from the low pressure column within a turbo-expander to generate a cold exhaust stream. The cold exhaust stream is then introduced into the distillation column or the main heat exchanger. External refrigeration can also be imparted by refrigerant streams introduced into the main heat exchanger. Refrigeration can also be generated through closed loop, external refrigeration cycles. 
     Many times it is desirable to produce significant fractions of the incoming feed air, preferably between about 10% and 15% of the feed air, as a liquid product. Liquid oxygen, liquid nitrogen and/or liquid argon may be produced directly from the distillation column system in what is known as an air liquefaction process. Such liquid products are typically distributed and sold as merchant products and may significantly enhance the profitability of an air separation unit. However, the air liquefaction process requires additional refrigeration to be supplied to the air separation unit which drives up the capital costs and power consumption of the air separation unit. In order to produce significant liquid fractions of more than about 10% of the feed air, it is often necessary to use two or more gas turbo-expanders to effectively deliver the additional supplemental refrigeration to the air separation unit over a range of temperatures. The optimal distribution of refrigeration with respect to temperature reduces overall cycle power consumption. 
     There are a number of critical process design aspects to integrated air distillation and liquefaction processes. For example, since a cost effective means is required to generate the necessary compression power for the turbo-expansion, the shaft work of turbo-expansion must be used in a manner that extracts the maximum value to the air separation unit. Compression power and capital cost problems associated with the distillation column based air liquefaction processes are particularly challenging in small to medium sized air separation plants. 
     The use of warm recycle compression-expansion circuits in such small to medium sized plants to improve the air liquefaction process has been tried but is somewhat expensive and introduces unwanted process complexities to the overall air separation process. See, for example, U.S. Pat. Nos. 4,883,518; 5,287,704; 5,400,600; 5,454,226; 5,758,515; 5,806,341; 6,257,020; and 8,397,535. 
     Accordingly, there is a need for the development of small and medium air separation plants and processes with improved liquid production capabilities. Specifically, there is a need to lower the operating and capital costs associated with warm recycle compression systems used in small to medium sized plants while maintaining high liquefaction efficiency. In addition, there is also the need to further push the liquid production capacity of small and medium sized air separation plants over 15% of the incoming feed air. 
     SUMMARY OF THE INVENTION 
     The present invention may be characterized as a method of separating a feed mixture comprised of air components in a cryogenic separation plant having a main heat exchanger and a distillation column system, the method comprising the steps of: (i) producing a pressurized and purified feed stream; (ii) directing a first portion of the compressed and purified feed stream to the main heat exchanger to cool the first portion of the compressed and purified feed stream; (iii) liquefying the cooled first portion of the compressed and purified feed stream to produce a liquid feed stream suitable for rectification in the distillation column system; (iv) diverting a second portion of the compressed and purified feed stream to a turboexpander section of the cryogenic separation plant; (v) partially cooling at least a portion of the second compressed and purified feed stream in the main heat exchanger; (vi) expanding the partially cooled second portion of the compressed and purified feed stream in the turboexpander to produce work and a gaseous exhaust stream at a temperature suitable for the rectification in the distillation column system; (vii) conducting a cryogenic distillation process to separate the first and second portions in the distillation column system thereby producing at least one liquid product stream; (viii) compressing a working fluid in a supplemental refrigeration circuit using the work produced by the turbo-expansion of the partially cooled second portion of the compressed and purified feed stream; (ix) cooling and expanding the compressed working fluid in a turboexpander disposed within the supplemental refrigeration circuit; (x) directing the expanded working fluid to the main heat exchanger and warming the expanded working fluid in the main heat exchanger to impart a portion of the refrigeration required by the cryogenic separation plant; and (xi) recirculating at least a portion of the warmed working fluid to a compressor section within the supplemental refrigeration circuit after having passed through the main heat exchanger or otherwise exchanged heat with at least one cooling stream directed to the air separation plant/unit. 
     The present invention may also be characterized as a supplemental refrigeration system for producing liquefied products in a cryogenic separation plant comprising: (a) an intake conduit configured to receive a working fluid; (b) a first compressor coupled to the intake conduit and configured to compress the working fluid, the first compressor mechanically coupled to a first turboexpander of the cryogenic separation plant and using the work produced by the first turboexpander of the cryogenic separation plant; (c) a second compressor coupled to the first compressor and configured to further compress the compressed working fluid; (d) a second turboexpander operatively coupled to the second compressor configured to expand the further compressed working fluid to generate an expanded working fluid; (e) a heat exchanger configured to receive the expanded working fluid from the second turboexpander and warm the expanded working fluid to impart a portion of the refrigeration required by the cryogenic separation plant; and (f) a recirculating conduit configured to return the warmed expanded working fluid from the heat exchanger to the first compressor section. 
     The compressed and purified feed stream is preferably produced by first compressing an air feed stream in a multistage main air compression system of the cryogenic separation plant to produce a compressed feed air stream and then purifying the compressed feed air stream in a pre-purification unit of the cryogenic separation plant. In some embodiments of the present system and method, the compressed and purified feed stream may be at a pressure of greater than the critical pressure of the feed stream. 
     The first portion of the compressed and purified feed air stream directed to the main heat exchanger is preferably less than about 35 percent by volume of the total compressed and purified feed air stream whereas the second portion of the compressed and purified feed stream diverted to the turbo-expander section of the cryogenic separation plant is greater than about 65 percent by volume of the compressed and purified feed stream. 
     In some embodiments of the present system and method, the working fluid is compressed in a first booster compressor coupled to the lower column turbo- expander or other turbo-expander of the cryogenic separation plant and then further compressed in a second booster compressor coupled to a turbo-expander disposed within the supplemental refrigeration circuit. 
     Cooling of the compressed working fluid is preferably accomplished in one or more stages of cooling prior to expanding the compressed working fluid in the turbo-expander wherein one such stage may comprise partially cooling the compressed working fluid in the main heat exchanger. The compressed working fluid may also be cooled using a supplemental cooling circuit wherein such cooling occurs via indirect heat exchange with the expanded working fluid or other refrigerating fluid or a vapor compression refrigerant, such as R134a. 
     In some embodiments, the cooled, expanded working fluid imparts a portion of the refrigeration required by the cryogenic separation plant via the main heat exchanger. Alternatively, the expanded working fluid may be warmed in an auxiliary heat exchanger via indirect heat exchange with a boosted compressed stream that is directed to the cryogenic separation plant thereby imparting a portion of the refrigeration required by the cryogenic separation plant. 
     One aspect that differentiates the present systems and methods from the prior art systems and methods is that the shaft work resulting from the turbo-expansion of the second air stream provides the necessary power for compression of the warmed expanded stream. Similarly, in some embodiments, the warmed expanded stream is further compressed in a secondary compressor preferably imparted by way of the shaft work generated through the turbo-expansion of a third stream, namely the recycled and recompressed working fluid stream. In the present system and method, the working fluid is comprised of air, nitrogen or a mixture of air constituents having an oxygen content not greater than air. The working fluid preferably comprises a portion of the compressed feed air stream. 
     Advantageously, the recycle compression necessary to power warm level refrigeration is generated without an externally powered compressor. The elimination of a separate drive means, power supply and lubrication system offers a significant capital cost savings. In addition, the main driver for product liquefaction is primarily shifted toward the main air compressor. In particular, the discharge pressure of the main feed air compressor system is substantially increased over many conventional main feed air compressor systems. By increasing the overall compression ratio on the main feed air compressor system one can achieve enhance liquefaction capability with a minimum in incremental capital expense. 
     Another aspect that differentiates the present invention from the prior art systems is the use or introduction of a warm gas turbo-expansion as supplemental refrigeration. In general, it has been found that the introduction of a warm gas turbo-expansion will enable the production of an additional 5% to 10% of the incoming feed air as liquefied product. This incremental liquid production can be typically gained at low incremental power consumption which provides the main economic advantage of the present system and method. By recycling the streams only from the warm turbine, the irreversibility associated with prior art cold recycle systems is eliminated. In particular, the incremental liquefaction from about 10%-15% of the incoming feed air to greater than 15% of the incoming feed air is achieved using warm turbo-expansion at roughly half the unit power typically associated with cold turbo-expansion that is used or contemplated in various prior art systems. 
     Yet another advantage of the present system and method over many prior art liquefaction processes is that no separate liquefaction heat exchanger is required to produce the greater than about 15% of the air as liquid and more preferably greater than about 18% of the air as liquid. In addition, the main heat exchanger in the present system operates at high efficiency. In general, the incremental liquefaction unit power consumption will be on the order of about 10 kw-hr/kcf or less. This represents a unit liquefaction power reduction of greater that about 35% when compared to prior art systems that employs the liquefaction process in a segregated liquefier. 
     In addition to lower power consumption, by producing a significant liquid fraction, i.e., over 15% of the feed air produced as liquid, the present system and method enables the near complete liquefaction of the oxygen contained in the incoming feed air. In such situations, the pump and piping associated with prior art pumped liquid oxygen product stream can be eliminated with a further appreciable capital cost savings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the present invention concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which: 
         FIG. 1  is a schematic process flow diagram of a cryogenic rectification plant in accordance with the present invention for carrying out a method of the present invention; and 
         FIG. 2  is a schematic process flow diagram of an alternate embodiment of a cryogenic rectification plant in accordance with the present invention for carrying out a method of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a double column, cryogenic air separation plant  10  is illustrated that is integrated with a closed loop supplemental refrigeration circuit or warm recycle expansion circuit  20 , discussed hereinafter, to increase production of liquid products such as liquid oxygen or liquid nitrogen. The operation of this thermally linked, double column distillation system is well known to the art of air separation. It is also understood by those skilled in the art, that if argon were a necessary or desired product, an argon column (or columns) could be incorporated into the distillation system. 
     Feed air stream  1  is first compressed in a multi-stage air compression system  100  with intercooling and condensate removal (not shown for simplicity) to a substantially elevated pressure in the range of about 40 to about 60 bar. The compressed air stream  2  is then directed to a pre-purification unit  110 . The pre-purification process undertaken by the pre-purification unit  110  may comprise several unit operations, including but not limited to, direct contact water cooling, refrigeration based chilling, direct contact with chilled water, phase separation and/or absorption. In adsorption-based processes, the pre-purification unit typically contains beds of alumina and/or molecular sieve operating in accordance with a temperature and/or pressure swing adsorption cycle in which moisture and other higher boiling impurities are adsorbed. Also, as known in the art, such higher boiling impurities are typically, carbon dioxide, water vapor and hydrocarbons. While one bed is operating, another bed is regenerated. The compressed, pre-purified air stream  3  is a clean dry air stream that is subsequently split into two or more portions. 
     A first portion of the compressed, pre-purified air stream  3 , preferably about 65 to 80 volume % is taken as clean dry air stream  32  which is directed to a main heat exchanger  200  where it is partially cooled to a temperature in the range of about 160° K to 220° K and subsequently expanded in turbo-expander  122 . The turbine exhaust stream  37  exiting the turbo-expander  122  is then introduced to the base of the moderate to high pressure column  300  as a primary gaseous air feed. The shaft work of turbo-expander  122  is imparted to a recycle compression stage  505  which will be described in greater detail below. 
     A second portion of the compressed, pre-purified air stream  3 , preferably the remaining 20 to 35 volume % is taken as stream  21 . Stream  21  is further cooled in the main heat exchanger  200  and exits the main heat exchanger  200  as a dense phase liquid and sub-cooled stream  22 . 
     Part of liquid sub-cooled stream  22  is depressurized by way of expansion valve  400  and is directed as incoming stream  23  into an intermediary location in the moderate to high pressure column  300 . A second part of liquid sub-cooled stream  22  is directed through expansion valve  420  and introduced as incoming stream  24  into an intermediary location of low pressure column  310 . It should be noted that dense phase expanders (e.g. liquid turbines) can be employed to supplement plant refrigeration in lieu of simple Joule-Thompson (J-T) expansion valves. Such dense phase expanders (not shown) may be employed upon stream  23  and stream  24  prior to or in lieu of expansion valves  400 / 420 . Alternatively, the entire stream  22  may be expanded in a liquid turbine. 
     Columns  300  and  310  represent distillation columns in which vapor and liquid are counter-currently contacted in order to affect a gas/liquid mass-transfer based separation of the respective feed streams. As is well known to the art, columns  300  and  310  will preferably employ packing (e.g. structured packing or dumped packing) or trays or combinations thereof. The moderate to high pressure column  300  and the low pressure column  310  are linked in a heat transfer relationship wherein the overhead of  300  exchanges latent heat with the bottoms liquid of column  310 . 
     In the illustrated embodiment, air streams  37  and  23  are directed to the moderate to high pressure column  300  which separates the respective streams into a nitrogen rich vapor column overhead and an oxygen rich bottoms stream  40 . The nitrogen-rich vapor column overhead, extracted from the top of the moderate to high pressure column  300  as a stream  50 , is condensed within a condenser-reboiler  220  located in the base of low pressure column  310  against boiling an oxygen-rich liquid column bottoms of column  310  where the latent heat of condensation is thereby imparted to the oxygen rich bottoms fluid of column  310 . The boiling of oxygen-rich liquid column bottoms initiates the formation of an ascending vapor phase within low pressure column  310 . The condensation produces a liquid nitrogen stream  51  that is divided into streams  56 ,  55  that reflux the higher pressure column  300  and the lower pressure column  310 , respectively to initiate the formation of descending liquid phases in such columns. An oxygen enriched liquid  40  is also withdrawn from column  300  and is then directed through pressure reduction valve  430  prior to entry into low pressure column  310  as stream  41 . 
     Low pressure column  310  operates at a pressure in the range of about 1.1 to 1.5 bar. Nitrogen rich liquid stream  52  is first subcooled in exchanger  210  and exits as stream  53  which may be split into a product liquid stream  54  and the reflux liquid stream  55 . The reflux nitrogen stream  55  is expanded through valve  435 . It should be noted that oxygen rich bottoms stream  40  may alternatively be subcooled prior to expansion valve  430 . Subcooling of the oxygen rich bottoms stream  40  may be accomplished by way of an additional subcooler or an extension of subcooler  210 . It should be noted that subcooler  210  may also be integrated into main heat exchanger  200 . 
     Within the low pressure column  310 , incoming streams  55 ,  24  and  41  are further separated into nitrogen-rich overhead streams  60  and  70  and an oxygen rich bottoms liquid  80 . Nitrogen-rich streams  60 ,  70  are warmed to about ambient temperatures by indirect heat exchange within subcooler  210  and/or main heat exchanger  200 . The resulting warmed nitrogen-rich streams subsequently emerge as warmed, lower pressure nitrogen streams  62  and  72 . It should be noted that warmed, low pressure nitrogen stream  62  may be taken as a co-product nitrogen stream and compressed as necessary. The warmed, low pressure nitrogen stream  72  often finds use as a purge/sweep fluid for purposes of regenerating adsorbent systems which may form part of pre-purification unit  110 . 
     An oxygen rich liquid stream  80  is extracted from the base of lower pressure column  310 . This oxygen rich liquid stream  80  is then compressed by a combination of the gravitational head and by mechanical pump  440  to form a pressurized liquid oxygen stream  81 . The pressurized liquid oxygen stream  81  may then be split into an oxygen product liquid stream  84  and directed to storage (not shown) as well as liquid oxygen stream  82 . Liquid oxygen stream  82  is shown as directed to passages within the main heat exchanger  200  where it is vaporized and warmed to near ambient temperatures and exits as a gaseous oxygen product stream  86  and may be directed to a pipeline or utilized directly. 
     As discussed above, air separation plant  10  is capable of producing liquid products, namely, a nitrogen-rich liquid stream and a liquid oxygen product stream. In order to increase the production of such products, additional refrigeration is supplied by a refrigeration system that is illustrated as a closed loop supplemental refrigeration circuit or warm recycle expansion circuit  20  that may use air as the refrigerant. In this regard, part of the compressed and purified air stream may be used to charge the closed loop supplemental refrigeration circuit  20  by way of conduit and valving (not shown). After having been charged. 
       FIG. 1  also depicts a warm recycle expansion circuit  20  configured to provide supplemental refrigeration. In the supplemental refrigeration circuit or warm recycle expansion circuit  20 , a refrigerant stream  93  enters warm turbo-expander  520  at a temperature in the range of between about 298° K to 220° K. In the illustrated embodiments, stream  93  is cooled within main heat exchanger  200  prior to expansion in turbo-expander  520 . As illustrated, warm turbo-expander  520  serves to generate the supplemental refrigeration through the expansion of refrigerant stream  93  to a pressure in the range of between about 5 bar and about 15 bar. The expanded and cooled turbo-expander exhaust stream  94  is directed to an intermediary location of main heat exchanger  200  where it is subsequently warmed to ambient temperature and exits the main heat exchanger  200  as warmed refrigerant stream  95 . Warmed refrigerant stream  95  is then directed to recycle compressors  505 ,  510  which may be cooled as necessary with intercoolers  506  and  511 , respectively. In the preferred embodiment, compressor  505  is coupled to turbo-expander  122  whereas compressor  510  is couple to turbo-expander  520 . Coupled in this regard indicates that the shaft work of expansion, indicated by a dotted lines  600 ,  602  is imparted directly to the coupled compressor wheels, preferably through a common shaft. 
     The pressurized recycle stream  92  exiting compressor  510  will typically exit at a pressure in the range of between about 40 bar and 60 bar. This pressurized recycle stream  92  may be further cooled within intercooler  511  by cooling water, ambient air and/or sub-ambient utility (e.g. chilled water) prior to cooling in main heat exchanger  200  and prior to expansion in turbo-expander  520 . 
     In the embodiment of  FIG. 1 , compressors  505  and  510  will exhibit some small leakage and a means must be employed to replenish the working fluid. Such replenishment may be accomplished by supplying a make-up stream through a small line and associated valves. For example, a stream of compressed, purified air may be diverted from stream  3  and directed into the supplemental refrigeration circuit or warm recycle expansion circuit  20  at a suitable location. Depending upon the pressure of operation of turbine  520 , a stream of cold gas from the inlet or exhaust of turbine  122  may be used as a make-up stream. Alternatively, nitrogen can be used as a working fluid and the makeup stream may be extracted from the lower column  300  or from a product nitrogen compressor associated with stream  62 . 
       FIG. 2  depicts an alternate embodiment of the present system and method for producing liquefied products in a cryogenic air separation plant. This alternate embodiment is arranged to further increase the overall energy efficiency of the air separation process. In many regards, the embodiments of  FIG. 1  and  FIG. 2  are very similar and thus, only the differences between the embodiments of  FIG. 1  and  FIG. 2  will discussed in detail in the paragraphs that follow. 
     With reference to  FIG. 2 , a portion of the compressed, purified air (e.g. about 7 percent of the incoming air feed) is introduced as a supplemental stream  96  through flow control valve  512  and into the warm recycle expansion circuit  20 . The supplemental stream  96  is combined with the discharge of compressor  505  where it is further compressed, cooled and then expanded to an elevated pressure. Alternatively, the supplemental stream  96  may be introduced into the discharge of compressor  510  where it is then cooled and then expanded. The operation of the supplemental refrigeration circuit or warm recycle expansion circuit  20  in  FIG. 2  is basically the same as discussed above with reference to  FIG. 1 . However, in the embodiment of  FIG. 2 , a stream  98  comparable in flow to that of supplemental stream  96  (e.g. about 7% of the incoming air feed) is diverted or extracted from the turbo-expander exhaust stream  94 . The pressure of diverted stream  98  will typically be in the range of between about 6 bar and about 15 bar. The diverted stream  98  is further cooled and liquefied by way of main heat exchanger  200 . The resulting liquefied air stream  97  is then depressurized through valve  513  and is introduced into an intermediary location of the low pressure column  310 . It should be noted that the resulting liquid stream  97  may also be introduced into the higher pressure column  300  or combined with stream  23  and/or stream  24  prior to column entry. In general, between about 5% to about 10% of the air can be fed to the column system by way of diverted stream  98 . In the embodiment illustrated in  FIG. 2 , the primary liquefaction air flow is reduced to about 18 percent of the total feed air which, in turn, reduces the total power consumption of the illustrated air separation process by about 1.5 percent compared to the embodiment of  FIG. 1 . 
     Although the above-described supplemental refrigeration circuit or warm recycle expansion circuit  20  preferably employs air as a working fluid or refrigerant, other working fluids such as nitrogen, argon or another refrigerant may be employed. In applications where nitrogen or air are the working fluids in the warm recycle expansion circuit, the recycle expansion circuit need not operate in a closed circuit but may also be operated in an open or partially open circuit. In its broadest terms, the working fluid can be any fluid mixture with oxygen content no more than air and preferably a dew point below the exhaust temperature of the warm turbine. Alternative fluid components may include fluorocarbons and inorganics such as carbon dioxide. 
     In order to enhance the refrigerating effect of the warm recycle expansion circuit  20 , stream  92  may be cooled within intercooler  511  by way of chilled water or by way of a separate refrigeration system (not shown) using a commercially available low temperature refrigerant like ammonia or R134a. A particularly advantageous arrangement would involve the use of nitrogen as a working fluid for the warm expansion within the recycle expansion circuit  20  and chilled water or a low temperature refrigerant in intercoolers  506  and  511 . 
     Although not shown, it may also be advantageous to introduce additional compression stages into the supplemental refrigeration circuit or warm recycle expansion circuit  20 . In particular, a booster compression stage may be configured upstream or downstream of the inlet and/or the outlet of compressors  505  and  510 . Also, a variable high speed motor may be operatively coupled to compressors  505  and  510  and configured to increase the compression ratio/power of compressor stages within the warm recycle expansion circuit  20 . Alternatively, a high speed generator may be configured so that shaft work of expander  122  may partly be recouped as electrical energy. When using the high speed generator arrangement, it may be advantageous to periodically shut down or turn down the warm turbo-expander  520  and warm recycle expansion circuit  20  from operation diverting the flow within the supplemental refrigeration circuit or warm recycle expansion circuit  20  using a recirculation or bypass circuit (not shown). 
     It is to be noted that although the air separation plant  10  is illustrated as having a high pressure column and a low pressure column connected in a heat transfer relationship by provision of condenser-reboiler  220 , other types of air separation plants are possible. For example, the present system and method can be used in low purity oxygen plants where the high pressure column and low pressure column are not connected in a latent heat transfer relationship as shown in  FIGS. 1 and 2 . Rather, the lowermost reboil of the low pressure column in a low purity oxygen plant is typically provided by the condensation or partial condensation of a compressed air stream that is afterwards fed into the high pressure column. 
     It should be noted that any number of alternative air distillation processes and configurations may be employed with the present system and method. In particular, argon may be recovered from the base double column system. Additional columns may be employed for purposes of reducing power consumption, for the recovery of rare gases, the further purification of argon or the production of ultra-high purity products. 
     Also, the production of a high pressure, pumped oxygen stream as shown in  FIGS. 1 and 2  is optional as it is entirely possible to extract a cold gas from the low pressure column  310  and warm it directly in the main heat exchanger  200  while venting the oxygen or compressing a portion of the produced oxygen for subsequent use. Alternatively, gaseous oxygen can be extracted from column  310  and directed into the waste nitrogen stream  71  that is subsequently warmed to ambient temperatures. 
     From the foregoing, it should be appreciated that the present invention thus provides a system and method for high efficiency separation of an incoming feed air stream in a cryogenic separation plant having a warm gas turbo-expansion cycle to supply the supplemental refrigeration required to produce liquid products in excess of about 15% of the incoming feed air. While the invention herein disclosed has been described by means of specific embodiments and processes associated therewith, numerous modifications and variations can be made thereto by those skilled in the art without departing from the scope of the invention as set forth in the appended claims or sacrificing all of its features and advantages.