FIELD OF THE INVENTION 
       [0001]    The present invention relates to a process and apparatus for separating a gaseous mixture within a rectification column by cryogenic rectification to produce a nitrogen product in which an oxygen-enriched liquid composed of a column bottoms is vaporized against condensing a nitrogen containing column overhead to produce reflux for the rectification column. More particularly, the present invention relates to such a process and apparatus in which the oxygen-enriched liquid is successively vaporized and a nitrogen-rich vapor phase produced by the successive vaporization of the oxygen-enriched liquid is condensed and then reintroduced into the column to enhance nitrogen recovery. 
       BACKGROUND OF THE INVENTION 
       [0002]    Nitrogen can be separated from gaseous mixtures that comprise nitrogen and oxygen, for example, air, by cryogenic rectification. Typically, a compressed and purified air stream is cooled to a temperature suitable for its rectification and then rectified within a rectification column to produce a nitrogen-rich vapor as column overhead and an oxygen-enriched column bottoms. Reflux for the column is produced by condensing some of the nitrogen-rich vapor. This condensation is effectuated through indirect heat exchange of a stream of the nitrogen-rich vapor and a stream of the oxygen-enriched column bottoms. Part of the liquid can be taken as a product. 
         [0003]    The resultant vaporization of the oxygen-enriched column bottoms produces a stream that is referred to as a waste stream. The waste stream and a nitrogen product stream are passed through a main heat exchanger in order to cool the incoming compressed and purified air. Refrigeration can be supplied by partially heating the waste stream within the main heat exchanger, passing the waste stream through a turboexpander and then subsequently warming the waste stream within the main heat exchanger. 
         [0004]    In general, the performance of the condenser used to exchange heat between oxygen-enriched liquid and the nitrogen-rich vapor is inefficient. Excess temperature difference exists between the oxygen-enriched liquid stream that is to be vaporized and the overhead nitrogen that is to be condensed. As a consequence, additional compression power is consumed by the process due in large part to the heat transfer within the condenser. Additionally, excess compositional gradients exist near the bottom of the rectification column. In particular, rapid compositional changes occur over few stages. Such steep compositional changes correspond to substantial thermodynamic irreversibility that translates into lost work. In the prior art, substantial efforts have focused primarily on condenser operation and column recovery. 
         [0005]    U.S. Pat. No. 4,867,773 and U.S. Pat. No. 4,872,893 detail similar processes that involve a single column nitrogen rectification process in which at least a portion of the evaporated oxygen-rich bottoms is warmed, compressed and recycled to the column. The recycled stream is fed to a point lower than the feed air. The effect of the modification allows the overhead condenser to operate more efficiently to increase nitrogen recovery. 
         [0006]    U.S. Pat. No. 4,883,519 illustrates another single column nitrogen rectification process wherein the overhead nitrogen is condensed by way of two heat exchangers. In this process the oxygen-rich column bottoms stream is depressurized to a first pressure and partially vaporized. The resulting vapor is recycled to the main air compressor. The remaining oxygen-rich liquid is further depressurized and directed to a second lower pressure heat exchanger where it is substantially vaporized. At least a portion of the resulting vapor is directed to a turbine expander for refrigeration production. 
         [0007]    U.S. Pat. No. 4,927,441 discloses another single column nitrogen process that is similar to that disclosed in U.S. Pat. No. 4,883,519. In this arrangement, the oxygen-rich column bottoms is depressurized to a first pressure and then introduced into a separation vessel which incorporates a small mass-transfer section containing mass-transfer elements such as a packing. The mass-transfer column section serves to further enrich the oxygen content of the stream prior to its depressurization and introduction into a second heat exchanger. The advantage of this arrangement is that the overhead produced in the mass-transfer section has very nearly the composition of air and thus, can be recycled by way of the main air compressor, without associated compositional mixing losses. 
         [0008]    U.S. Pat. No. 5,711,167 discloses a single column nitrogen process in which two overhead condensers are used to generate reflux to the rectification column by successive vaporizations of oxygen-enriched liquid. In a first partial vaporization of the oxygen-enriched liquid conducted in one of the two overhead condensers, the resulting vapor is compressed in a cold compressor and redirected back to the base of the rectification column. The oxygen-rich waste produced from a second partial vaporization of the oxygen-enriched liquid, conducted in the second condenser, is warmed and expanded prior to venting. At least a portion of the shaft work of expansion is directed to the cold compression. 
         [0009]    U.S. Pat. No. 5,899,093 details a process in which the oxygen enriched column bottoms is first partially depressurized and introduced into a dephlegmator-type condenser. The oxygen-rich bottoms, produced in the condenser, is separated into a nitrogen-rich gas which is recycled by way of gas compression within an air compressor. The further oxygen-rich remaining fluid is further depressurized and used to condense an additional portion of the overhead nitrogen from the column. The further oxygen enriched remaining fluid is further depressurized to condense an additional portion of the overhead nitrogen from the column. The nitrogen fraction resulting from the first partial vaporization is recycled to a gas compression and then to the column. 
         [0010]    U.S. Pat. No. 5,934,106 and U.S. Pat. No. 5,868,006 disclose a single column nitrogen generator in which overhead nitrogen is condensed against two streams derived from the column system. A first nitrogen enriched air-like liquid stream is evaporated and recompressed back to the column. A second oxygen-rich column bottoms stream is extracted from the column and also evaporated. The second evaporated fraction is warmed and then expanded. The work of expansion provides the shaft work required for the cold compression of the first recycled stream. 
         [0011]    As will be discussed, the present invention involves a process for recovering nitrogen-rich vapor by the cryogenic rectification of air or other oxygen and nitrogen containing gas within a rectification column in which a nitrogen-rich liquid condensate produced in the course of sequential vaporizations of an oxygen-enriched liquid stream, made-up at least in part from liquid column bottoms, is introduced into the rectification column to increase production of the nitrogen-rich vapor in an energy efficient and cost effective manner. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a method of separating a gaseous mixture comprising nitrogen and oxygen to produce a nitrogen product. In accordance with the method, a purified, pressurized and cooled gaseous stream composed of the gaseous mixture is introduced into a rectification column to produce an overhead nitrogen-rich vapor and an oxygen-enriched liquid bottoms. A first oxygen-enriched liquid stream that is at least in part composed of the oxygen-enriched liquid bottoms is depressurized and then partially vaporized within a first heat exchanger. A vapor phase is disengaged from a liquid phase that is formed by the partial vaporization of the first oxygen-enriched liquid stream. A second oxygen-enriched liquid stream composed at least in part of the liquid phase is depressurized and is then partially vaporized through indirect heat exchange with at least a portion of a vapor phase stream composed of the vapor phase within a second heat exchanger. This substantially condenses the vapor phase stream to form a nitrogen-rich liquid stream. 
         [0013]    A first part of a column overhead nitrogen-rich stream that is composed of the overhead nitrogen-rich vapor is condensed in the first heat exchanger and a second part of the column overhead nitrogen-rich stream is condensed in a third heat exchanger. The condensation of the second part of the column overhead nitrogen-rich stream is conducted through indirect heat exchange with the second oxygen-enriched stream after the partial vaporization thereof, thereby further vaporizing the second oxygen-enriched stream. At least part of the column overhead nitrogen-rich stream after having been condensed is returned to the rectification column as reflux. 
         [0014]    At least part of the nitrogen-rich liquid stream is then introduced into the rectification column, above the purified, pressurized and cooled stream. A product nitrogen stream is produced from part of the nitrogen-rich vapor. 
         [0015]    The first oxygen-enriched liquid stream can be subcooled through indirect heat exchange with the nitrogen product stream and a waste stream composed of a vapor fraction of the second oxygen-enriched liquid stream after having been further vaporized. A compressed and purified stream composed of the gaseous mixture can be cooled by indirect heat exchange with the nitrogen product stream and the waste stream after having subcooled the oxygen-enriched stream and if present, the second part of the nitrogen-rich vapor stream prior to its compression. The cooling of the compressed and purified stream thereby forms at least a portion of the purified, pressurized and cooled stream. 
         [0016]    In a specific embodiment of the present invention, a first part of the vapor phase stream can be substantially condensed within the second heat exchanger to form the nitrogen-rich liquid stream and a second part of the vapor phase stream can be warmed, compressed and cooled and recycled back to the rectification column. The cooling and the recycling can be accomplished by combining the second part of the vapor phase stream, after having been warmed and compressed, with the compressed and purified stream to form a combined compressed and purified stream. In such embodiment, the combined compressed and purified stream is cooled by the nitrogen product stream, the waste stream and the second part of the nitrogen-rich vapor stream. As such, the second part of the vapor phase stream is cooled and recycled back to the rectification column by being combined with the compressed and purified stream. 
         [0017]    Preferably, the waste stream and the nitrogen product stream and if present, the second part of the nitrogen-rich vapor stream prior to its compression, all indirectly exchange heat with the compressed and purified stream within a main heat exchanger. The waste stream can be partially warmed within the main heat exchanger and is then expanded with the performance of work to generate an exhaust stream. The exhaust stream is reintroduced into the main heat exchanger and fully warmed to refrigerate the cryogenic rectification process. It is to be noted that the term “partially warmed” as used herein and in the claims means that the waste stream is warmed to a temperature intermediate the temperatures of the warm and cold ends of the main heat exchanger. The term, “fully warmed” as used herein and in the claims means fully warmed to the warm end temperature of the main heat exchanger. 
         [0018]    The pressure of the nitrogen-rich liquid stream can be adjusted after having been substantially condensed and prior to its being introduced into the rectification column. This adjustment can be effectuated by mechanically pumping the nitrogen-rich liquid stream after having been substantially condensed or by expanding the nitrogen-rich liquid stream. 
         [0019]    In another aspect, the present invention provides an apparatus for separating a gaseous mixture comprising nitrogen and oxygen to produce a nitrogen product. In accordance with this aspect of the present invention a rectification column is connected to the main heat exchanger for rectifying a purified, pressurized and cooled stream composed of the gaseous mixture to produce an overhead nitrogen-rich vapor and an oxygen-enriched liquid bottoms. 
         [0020]    A first valve is provided to depressurize a first oxygen-enriched stream composed at least in part of the oxygen-enriched liquid bottoms. A first heat exchanger is connected to the first valve for partially vaporizing the first oxygen-enriched liquid stream and a phase separator is connected to the first heat exchanger for disengaging a vapor phase from a liquid phase formed by the partial vaporization of the first oxygen-enriched stream. A second valve is connected to the phase separator for depressurizing a second oxygen-enriched liquid stream composed at least in part of the liquid phase and a second heat exchanger is connected to the second valve and to the phase separator for partially vaporizing the second oxygen-enriched liquid stream through indirect heat exchange with at least a portion of a vapor phase stream composed of the vapor phase formed by the partial vaporization of the first oxygen-enriched stream. This substantially condenses the vapor phase stream to produce a nitrogen-rich liquid stream. A third heat exchanger is operated in series with the second heat exchanger for further vaporizing the second oxygen-enriched liquid stream. 
         [0021]    The rectification column is connected to the first heat exchanger and the third heat exchanger for purposes of condensing at least a portion of a column overhead nitrogen-rich stream composed of the overhead nitrogen-rich vapor and returning at least a part of the column overhead nitrogen-rich stream after having been condensed to the rectification column as reflux. 
         [0022]    The second heat exchanger is connected to the rectification column for introducing at least part of the nitrogen-rich liquid stream into the rectification column, above the purified, pressurized and cooled stream. A means is provided for extracting a product nitrogen stream formed from part of the overhead nitrogen-rich vapor. 
         [0023]    In a specific embodiment of the present invention, a main heat exchanger can be provided to cool a compressed and purified stream composed of the gaseous mixture and thereby to form the purified, pressurized and cooled stream. In an alternative embodiment, the second heat exchanger can be connected to the phase separator so that a first part of the vapor phase stream is substantially condensed within the second heat exchanger to form the nitrogen-rich liquid stream. A compressor can be connected in flow communication with the phase separator and to the main heat exchanger and the main heat exchanger can be configured so that a second part of the vapor phase stream is warmed within the main heat exchanger and compressed within the compressor. The main heat exchanger in such alternative is also simultaneously in flow communication with the compressed and purified stream and the compressor such that the second part of the vapor phase stream combines with the compressed and purified stream to form a combined compressed and purified stream. In such embodiment the compressed and purified stream is cooled within the main heat exchanger to form the purified, pressurized and cooled stream. 
         [0024]    A subcooler can be connected to the rectification column so that the first oxygen-enriched liquid stream is subcooled through indirect heat exchange with the nitrogen product stream and a waste stream composed of a vapor fraction of the second oxygen-enriched liquid stream after having been further vaporized. The main heat exchanger is also connected to the subcooler and configured so that the compressed and purified air stream is cooled by indirect heat exchange with a nitrogen product stream and the waste stream after having subcooled the first oxygen-enriched stream. If present, the second part of the vapor phase stream also serves to cool the combined compressed and purified air stream. 
         [0025]    Preferably, the main heat exchanger can also be configured such that the waste stream partially warms within the main heat exchanger and an exhaust stream fully warms within the main heat exchanger to refrigerate the apparatus. An expander is connected to the main heat exchanger so that the waste stream after having been partially warmed is expanded within the expander with the performance of work to generate the exhaust stream. 
         [0026]    A pump can be interposed between the second heat exchanger and the rectification column to pressurize the nitrogen-rich liquid stream after having been substantially condensed and prior to its introduction into the rectification column. Alternatively, a third valve can be interposed between the second heat exchanger and the rectification column in order to reduce the pressure of the nitrogen-rich liquid stream after having been substantially condensed and prior to its introduction into the rectification column. The choice of pump or valve will depend upon the gravitation head generated by the elevation difference between the second condenser and the feed location for the nitrogen-rich liquid. 
         [0027]    As is apparent from the description of the present invention in both aspects of its method and apparatus, the return of the nitrogen-rich liquid to the column has the advantage of increasing the production of the nitrogen-rich vapor. It also decreases compositional variations within the bottom of the column to bring the operating line closer to the vapor liquid equilibrium curve thereby generating greater efficiency. This decreases the lost work which translates into decreased compression requirements. The indirect heat exchange between the oxygen-enriched liquid stream and the nitrogen-rich vapor that is used in refluxing the column can be conducted more efficiently than in the prior art due to the three-stage vaporization process that reduces the log mean temperature differences of the streams subjected to indirect heat exchange. This results in a process that is more efficient than those conducted in the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    While the specification concludes with claims distinctly pointing out the subject matter that Applicant regards as his invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which: 
           [0029]      FIG. 1  is a schematic, process flow diagram of an apparatus that can be used for carrying out a method in accordance with the present invention; and 
           [0030]      FIG. 2  is an alternative embodiment of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    With reference to  FIG. 1 , an apparatus  1  for carrying out a method in accordance with the present invention is illustrated. 
         [0032]    A compressed and purified stream  10  that is composed of nitrogen and oxygen, for instance, air, is cooled in a main heat exchanger  12  to form a purified, pressurized and cooled stream  13  that is then introduced into a rectification column  14  for separation of the oxygen and nitrogen. Within rectification column  14 , the purified, pressurized and cooled purified stream  13  is rectified and separated into an oxygen-enriched liquid bottoms  16  and an overhead nitrogen-rich vapor  18 . A product stream  20  composed of the nitrogen-rich vapor  18  can be fully warmed within the main heat exchanger  12  against cooling compressed and purified stream  10 . 
         [0033]    Rectification column  14  contains mass-transfer contacting elements such as structured packing or trays that are generally disposed within a bottom region  22  of rectification column  14  and the remaining region  24  situated above bottom region  22 . Typically, rectification column  14  will operate in a pressure range of between about 5 bar absolute and about 12 bar absolute. 
         [0034]    Compressed and purified stream  10  can be formed as a result of unit operations being conducted in another process or as known in the art, can also be formed with the use of a compressor and a pre-purification unit utilizing an adsorbent to absorb water, carbon dioxide and potentially dangerous hydrocarbons that could otherwise freeze-out or accumulate within the cryogenic process. As can be appreciated, a compressor and purification unit could be used in connection with the present invention. 
         [0035]    Compressed and purified stream  10  is introduced into rectification column  14  at about its saturation temperature and as such its introduction initiates the formation of an ascending vapor phase that becomes evermore rich in nitrogen to form the overhead nitrogen-rich vapor  18 . A column overhead nitrogen-rich stream  26  that is composed of the overhead nitrogen-rich vapor is withdrawn from rectification column  14  and is divided into a subsidiary column overhead nitrogen-rich stream  28  and nitrogen product stream  20 . It is understood, however, that nitrogen product stream  20  could be separately withdrawn from rectification column  14 . As will be discussed, subsidiary column overhead nitrogen-rich stream  28  is condensed to produce a reflux stream  29  that is introduced into rectification column  14  to initiate the formation of a descending liquid phase which contacts the ascending vapor phase and becomes evermore rich in oxygen as it descends to form oxygen-enriched liquid  16  as the column bottoms. 
         [0036]    A first oxygen-enriched liquid stream  30  composed of the oxygen-enriched liquid bottoms is optionally subcooled within a subcooling heat exchanger  32 . The resulting subcooled liquid has its pressure reduced by a first valve  34 . First oxygen-enriched liquid stream  30  after passage through first valve  34  is then partially vaporized within a first heat exchanger  36  to form a two-phase stream  38 . The liquid and vapor phases within two-phase stream  38  are separated within a phase separator  40  into a liquid phase  42  and a vapor phase  44 . It is to be noted that oxygen-enriched liquid stream  30  after passage through first heat exchanger  36  can typically have a vapor fraction of between about 10% and about 40% and more preferably, about 30%. 
         [0037]    A second oxygen-enriched liquid stream  46  composed of liquid phase  42  is depressurized with the use of a second valve  48  and then partially vaporized within a second heat exchanger  50 . The pressure drop across second valve  48  will typically be in a range of between about 1.0 and 1.5 bar gauge. Also introduced into second heat exchanger  50  is a vapor phase stream  52  composed of vapor phase  44  that is substantially condensed to form a nitrogen-rich liquid stream  54 . The term “substantially condensed” as used herein and in the claims means a stream having a liquid fraction that will typically exceed about 95% by volume. Moreover, it is to be noted that a typical composition for such nitrogen-rich liquid stream is in the range of 75 to 90% nitrogen. 
         [0038]    The second oxygen-enriched liquid stream  46  after having been partially vaporized within second heat exchanger  50  is then introduced into a third heat exchanger  56  which is shown to operate as a natural thermo-siphon to further vaporize the second oxygen-enriched liquid stream  46  and thereby produce a vapor fraction thereof designated by reference number  58  that is discharged as a waste stream  60 . The second oxygen-enriched liquid stream  46  after such partial vaporization within exchanger  50  will have a vapor fraction in a range of between about 40% and about 60%, more preferably, about 45% to about 50%. 
         [0039]    First portion  62  of column overhead nitrogen-rich stream  28  is condensed within first heat exchanger  36  and a second portion  64  is condensed within third heat exchanger  56  to produce a liquid stream  66 . At least part of combined liquid condensate stream  66  is returned as reflux stream  29  to rectification column  14 . An optional liquid product stream  68  can also be taken and directed to suitable storage (not shown). 
         [0040]    It is understood that first heat exchanger  36 , second heat exchanger  50  and third heat exchanger  56  are typically brazed aluminum heat exchangers. Other exchanger types could be employed, for instance shell and tube type. Given the use of brazed aluminum heat exchangers, first heat exchanger  36  and third heat exchanger  56  could be integrated into a single block. Moreover any number of heat transfer flow configurations could be employed. The heat exchangers may be once-through vaporizers as illustrated or they may be configured for recirculated vaporization. An example, all the exchangers may be configured as thermo-siphons of both natural and pump circulation type. It is understood that exchanger  56  contained within vessel  70  attached to rectification column  14  is a natural thermo-siphon in which boiling flow is induced by gravitational liquid head. 
         [0041]    It is also noted that the distribution of two-phase streams into aluminum heat exchangers often require separate liquid and vapor inlet distribution passages. In order to facilitate such distribution, first oxygen-enriched liquid stream  30  entering first heat exchanger  36  and two-phase stream  46  entering second heat exchanger  50  may be subject to phase separation. Additionally, it is possible that compressed and purified stream  10  would be introduced into rectification column  14  as liquid and vapor phase streams that were produced by partial condensation of compressed and purified stream  10 . In yet another variation, first heat exchanger  36  and second heat exchanger  50  could be used for purposes of subcooling other streams, for example stream  68  prior to its being sent to storage. 
         [0042]    Phase separator  40  can be a simple vapor liquid disengagement vessel. Oxygen enrichment of second oxygen-enriched liquid stream  46  may be increased by inclusion of mass-transfer media such as structured packing or trays. A number of streams may be used to impart additional heat to the base of phase separator  40  to facilitate additional oxygen enrichment if warranted. 
         [0043]    Nitrogen-rich liquid stream  54  is introduced into rectification column  14  above the location of compressed and purified stream  10 . This increases the production of nitrogen-rich vapor and also decreases the compositional gradients within the bottom regions of rectification column  14 . As can be appreciated, the nitrogen content of condensed stream  54  will be higher than that of oxygen enriched liquid  16  and thus, it is introduced at a higher level of rectification column  14 . It is to be noted that not all of the nitrogen-rich liquid stream  54  need be introduced into rectification column  14 . A portion of stream  54  may be: combined with stream  10 , vaporized and warmed and/or recycled, or directed to vessel  70  for purposes of heat exchanger control. 
         [0044]    In order to introduce nitrogen-rich liquid stream  54  into rectification column  14 , in most cases, there will have to be a pressure adjustment to condensed stream  54  by way of a device  72 . If for instance, the pressure of liquid stream  54  is too low for entry into rectification column  14 , device  72  can be a pump. If a pump is used, it may be advantageous to employ a vessel to provide additional liquid residence time. Such a vessel would preferably employ a small vapor vent line (which may be connected to the waste stream as necessary). Alternatively, if the static head developed in condensed stream  54  is sufficient due to the placement of components within a cold box, device  72  might simply be a valve. 
         [0045]    As stated above, first oxygen-enriched liquid stream  30  is subcooled within a subcooling unit  32  which can be a brazed aluminum heat exchanger that in fact can be part of heat exchanger  12 . First oxygen-enriched liquid stream  30  is subcooled by partly warming waste stream  60  and nitrogen product stream  20 . After having been partly warmed, waste stream  60  and nitrogen product stream  20  are introduced into main heat exchanger  12  to cool the incoming compressed and purified steam  10 . 
         [0046]    Waste stream  60  upon its discharge from shell  70  typically can have a pressure of between about 2 and about 7 bar absolute. In the illustrated embodiment, the apparatus  1  is refrigerated by partly warming waste stream  60  within a main heat exchanger  12  to form a partly warmed stream  74  that is then expanded within a turboexpander  76  to produce an exhaust stream  78  that is fully warmed within main heat exchanger  12  to a temperature and pressure near ambient, thereby to refrigerate apparatus  1 . The shaft work of expansion may be imparted to a generator or used to compress air, nitrogen or waste stream  60  prior to expansion or dissipated by an oil brake as heat. It is to be noted that a portion of stream  74  could bypass turbine  76  and directed into exhaust stream  78  by use of a valve. Other types of refrigeration are possible with the present invention, including, an external refrigeration source or even air expansion as illustrated in the prior art. 
         [0047]    As illustrated in  FIG. 2 , nitrogen-rich liquid stream  54  is formed by a first part  52   a  of vapor phase stream  52 . A second part  52   b  of vapor phase stream  52  is passed within main heat exchanger  12 ′ and then compressed within a compressor  80 . After having been fully warmed within main heat exchanger  12 ′, second part  52   b  of vapor phase stream  52  can then be combined with compressed and purified stream  10  to form a combined compressed and purified stream  11 . After cooling within main heat exchanger  12 ′, a purified, pressurized and cooled stream  13 ′ is produced for introduction into rectification column  14 . It is to be noted that a similar configuration might involve splitting of partially warmed stream  74  to subject a portion of the flow to compression and return another portion to the base of rectification column  14 . Another alternative would be to recycle second part  52   b  of vapor phase stream  52  back into the distillation column  14  by provision of separate passage provided in the main heat exchanger  12 ′ for such purpose. Moreover, waste stream  60  could be directed to another rectification section or alternately, could be warmed, compressed and then fed to a similar process. 
         [0048]    The present invention is applicable to any number of combinations of rectification columns forming similar functions to that as described. For example, rectification column  14  might employ an auxiliary reboiler for further increase and recovery. In such an arrangement, an additional stream of air and nitrogen would be compressed to a higher pressure and condensed within the reboiler and thereby to provide additional vapor flow. 
         [0049]    Furthermore, rectification column  14  may employ a combination of packing, dumped and structured. Rectification column  14  could be split into multiple sections. As known in the art, a “reflux pump” may be employed to motivate column liquids to and from the column sections. In this regard, device  72  might be a mechanical pump that serves as both a reflux pump and a pressure manipulation device. 
         [0050]    In a further possible embodiment of the present invention, other oxygen-enriched fluid might be extracted from rectification column  14  and added for the make-up of oxygen-enriched liquid stream  30  for purposes of temperature control or to reduce the size of first heat exchanger  36 , second heat exchanger  50  and third heat exchanger  56 . 
         [0051]    It is also possible to utilize certain aspects of the present invention without the direct use of heat exchanger  12  or  12 ′ (or preceding compression and prepurification equipment). For example, a conventional single column nitrogen generator could be utilized to generate the purified, pressurized and cooled stream. Such a stream could be obtained from the vaporized oxygen-enriched bottoms extracted from the condenser in association with the single column nitrogen generator. In this regard, the single column nitrogen generator would preferably operate within a pressure range of between 10 and about 20 bar. Rectification column  14  would operate as described above. Nitrogen product and waste streams would pass through the main heat exchanger in a manner similar to that described with respect to  FIG. 1 . 
         [0052]    While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes and additions can be made without departing from the spirit and the scope of the present invention as set forth in the presently pending claims.
Method and apparatus for distilling nitrogen from a gaseous mixture containing nitrogen and oxygen. Oxygen-enriched bottoms liquid is partially vaporized within a first heat exchanger to condense part of the column overhead to produce reflux. Thereafter, the partially vaporized oxygen-enriched liquid is phase separated. A second oxygen-enriched liquid stream composed of at least part of the liquid phase is used to substantially condense all or part of the vapor stream derived from said phase separation, thereby to form a nitrogen-rich liquid stream. At least part of the nitrogen-rich liquid stream is reintroduced into the column to increase nitrogen recovery. The second oxygen-enriched liquid stream is then used to condense a second part of the reflux for the column.