Abstract:
An apparatus for producing liquid nitrogen is provided. The apparatus includes a heat exchanger, a pair of turbine-boosters, a warm compressor, an air separation unit having a single column, a top condenser and a bottom reboiler, a liquid/gas separator, and an optional subcooler. The apparatus is configured to produce merchant or non-merchant grade liquid nitrogen using the pair of turbine-boosters to provide refrigeration and energy for the process.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus for separating air into its components. More specifically, embodiments of the present invention are related to an apparatus for producing merchant or non-merchant grade liquid nitrogen using a pair of turbo-boosters to provide refrigeration and energy for the process. 
       SUMMARY OF THE INVENTION 
       [0002]    An apparatus is provided for producing nitrogen through the cryogenic separation of air. In one embodiment, the apparatus can include a heat exchanger configured to receive a main air feed comprising filtered purified and compressed air at a pressure of at least 5 bar; an air separation unit in fluid communication with a cool side of the heat exchanger, the air separation unit configured to receive cooled air from the heat exchanger and produce gaseous nitrogen and waste gaseous oxygen, wherein the air separation unit comprises a single column having a bottom reboiler and a top condenser; a recycle compressor in fluid communication with a warm side of the heat exchanger such that the recycle compressor is configured to receive a nitrogen recycle from the heat exchanger, wherein at least a portion of the nitrogen recycle is made up of gaseous nitrogen from the air separation unit; a first turbine-booster having a first booster and a first turbine, the first booster in fluid communication with the recycle compressor such that the first booster is configured to receive a compressed nitrogen recycle from the recycle compressor; a second turbine-booster having a second booster and a second turbine, the second booster in fluid communication with the first booster such that the second booster is configured to receive a boosted nitrogen from the first booster, wherein an outlet of the second booster is in fluid communication with the heat exchanger such that the boosted nitrogen from the second booster is cooled within the heat exchanger, wherein the second turbine is in fluid communication with the heat exchanger such that the second turbine is configured to receive a cooled fluid under pressure from the heat exchanger and then expand the cooled fluid to provide refrigeration for the apparatus; and a liquid/gas separator in fluid communication with the heat exchanger, the liquid/gas separator configured to receive an expanded fluid comprising nitrogen from the second booster turbine and separate the expanded fluid into a nitrogen-enriched gas and a nitrogen-enriched liquid, the liquid/gas separator in fluid communication with the second booster and the heat exchanger such that the liquid/gas separator is configured to receive a portion of the boosted nitrogen after fully cooling in the heat exchanger, the liquid/gas separator is also configured to send the nitrogen-enriched gas to the cool side of the heat exchanger. 
         [0003]    According to other optional aspects of the invention:
       the top condenser is in fluid communication with the single column, wherein the bottom reboiler is in fluid communication with the single column, wherein the top condenser is configured to provide condensing duty for the single column, wherein the bottom reboiler configured to provide reboiling duty for the single column;   the apparatus includes a subcooler in fluid communication with the single column such that the subcooler is configured to receive a fluid from the single column that is operable to provide subcooling for the subcooler;   the fluid received from the single column is an oxygen-rich liquid from the bottom of the single column;   the fluid received from the single column is an oxygen-rich liquid from a middle section of the single column;   the subcooler is in fluid communication with the liquid/gas separator, the subcooler being configured to subcool the nitrogen-enriched liquid from the liquid/gas separator;   the second turbine is mechanically coupled to the second booster;   the first turbine is mechanically coupled to the first booster; and   the recycle compressor is in fluid communication with the heat exchanger, the bottom reboiler and the single column, such that the recycle compressor is configured to have a fraction of partially compressed nitrogen recycle withdrawn from an internal stage of recycle compressor, cooled in the heat exchanger, used by the bottom reboiler as a boiler heating fluid; and then introduced flashed into a top portion of the single column.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
           [0013]      FIG. 1  provides an embodiment of the present invention. 
           [0014]      FIG. 2  provides another embodiment of the present invention. 
           [0015]      FIG. 3  provides an additional embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims. 
         [0017]      FIG. 1  provides a basic embodiment of the present invention. Main air feed  2 , which has already been purified and compressed to a pressure of about 5 to about 6 bar, is introduced to heat exchanger  10  and cooled down to a temperature near its dew point or lower to form fully cooled air feed  12 . Fully cooled air feed  12  is then introduced to air separation unit  19 , in order to separate the various components of air. Waste gaseous oxygen  22  is recovered from air separation unit  19  and is passed through the cold side of heat exchanger  10  in order to provide cooling to heat exchanger  10 . After exiting heat exchanger  10 , waste gaseous oxygen  22  can be vented to the atmosphere, used to regenerate the air adsorbers (not shown) or sent to a system of columns (not shown) if recovery of the oxygen is desired. 
         [0018]    Gaseous nitrogen  28  is also withdrawn from air separation unit  19  and passed through the cold side of heat exchanger  10  to provide additional cooling. However, instead of venting to the atmosphere, gaseous nitrogen  28  is recycled in the process. Nitrogen recycle  38  exits air separation unit  19  and is introduced to recycle compressor  40  and compressed to form compressed nitrogen recycle  46 . Compressed nitrogen recycle  46  is then cooled in second aftercooler  43  before being boosted in first booster  50  and cooled in third aftercooler  51  to form boosted nitrogen  52 . Boosted nitrogen  52  is then introduced to second booster  53  in order to further compress boosted nitrogen  52  before being cooled in fourth aftercooler  55  to form fully boosted nitrogen  56 . In one embodiment fully boosted nitrogen  56  can be at ambient temperature and a pressure of about 45 to about 65 bar prior to entering heat exchanger  10 . 
         [0019]    Fully boosted nitrogen  56  is then introduced to heat exchanger  10  for cooling. In one embodiment, one portion of fully boosted nitrogen  56  is fully cooled in heat exchanger  10  to form liquefied nitrogen  58 , which is subsequently introduced to liquid/gas separator  60  by flashing via valve  59 . In another embodiment, another portion of fully boosted nitrogen  56  is only partially cooled in heat exchanger  10  to form partially cooled boosted nitrogen  78 . In one embodiment, partially cooled boosted nitrogen  78  is at or above its super critical pressure. Partially cooled boosted nitrogen  78  is then introduced into second turbine  80  in order to expand partially cooled boosted nitrogen  78  to form second expanded nitrogen  82 . In one embodiment, second expanded nitrogen  82  can have a temperature that is near or below its dew point and a pressure of about 5 to about 6 bar. In one embodiment, second expanded nitrogen  82  is a two phase fluid consisting of gas and liquid phases. In a preferred embodiment, second expanded nitrogen  82  is introduced to liquid/gas separator  60  in order to separate any gaseous nitrogen from liquid nitrogen. Recovered liquid nitrogen  62  is withdrawn from liquid/gas separator  60  and collected as product. In one embodiment, gaseous nitrogen  68  is withdrawn from a top portion of liquid/gas separator  60  and combined with gaseous nitrogen  28  before introduction to the cold side of heat exchanger  10  and subsequently recycled. 
         [0020]    In one embodiment, fraction of compressed nitrogen recycle  48  is withdrawn from compressed nitrogen recycle  46  and fed to the warm end of heat exchanger  10 , where fraction of compressed nitrogen recycle  48  is partially cooled before being expanded in first turbine  70  to form first expanded nitrogen  72 . In one embodiment, first expanded nitrogen  72  is reintroduced to heat exchanger  10 , preferably at an intermediate point, and combined with gaseous nitrogen  28  and subsequently recycled. In one embodiment, first turbine  70  is connected by a common shaft with first booster  50  and helps to provide the energy needed for first booster  50  to compress compressed nitrogen recycle  46 . Likewise, second turbine  80  is connected by a common shaft with second booster  53  and helps to provide the energy needed for second booster  53  to compress boosted nitrogen  52 . In one embodiment, first turbine  70  and second turbine  80  provide substantially all of the refrigeration needs for the process. 
         [0021]    First turbine  70  and second turbine  80  produce refrigeration by work expansion. Their respective boosters, first booster  50  and second booster  53 , utilize the produced work to further compress their respective nitrogen streams. 
         [0022]      FIG. 2  provides an alternate embodiment of the invention, which includes two recycle compressors (recycle compressor  40  and second recycle compressor  45 ), recycle aftercooler  41 , single column  20  and subcooler  30 . In one embodiment, single column  20  operates at about 5 bar. In one embodiment, nitrogen recycle  38  is partially compressed in recycle compressor  40  and cooled in recycle aftercooler  41  to form partially compressed nitrogen recycle  42 . In one embodiment, partially compressed nitrogen recycle  42  has a pressure of about 8 bar. Partially compressed nitrogen recycle  42  is then further compressed in second recycle compressor  45  and cooled in second aftercooler  43  to form compressed nitrogen recycle  46 , which preferably has a pressure of about 25 to about 27 bar. Fraction of partially compressed nitrogen recycle  44  is withdrawn from partially compressed nitrogen recycle  42  and fed to the warm end of heat exchanger  10 . Whereas fraction of compressed nitrogen recycle  48  is only partially cooled in heat exchanger  10 , fraction of partially compressed nitrogen recycle  44  is fully cooled in heat exchanger  10 . In one embodiment, the split between compressed nitrogen recycle  48  and compressed nitrogen recycle  46  is about 40/60. In another embodiment, the split can be determined by balancing the needs for obtaining the desired temperature approach at the warm end of heat exchanger  10  and maintaining appropriate turbine side versus booster side flow rates based on each device&#39;s efficiencies. 
         [0023]    After exiting the cold end of heat exchanger  10 , fraction of partially compressed nitrogen recycle  44  is used to provide heat to bottom boiler  21  before being introduced via valve  93  near a top portion of single column  20 . Those of ordinary skill in the art will recognize that even though recycle compressor  40  and second recycle compressor  45  are pictured as two different compressors, it is possible to use one compressor and remove fraction of partially compressed nitrogen recycle  44  from an inner stage of that single compressor. 
         [0024]    As with all distillation columns, liquids tend to collect near the bottom, while gases rise to the top. In this embodiment, oxygen-rich liquid  24  is withdrawn from the bottom of single column  20  and introduced to subcooler  30  via valve  29 . In one embodiment, oxygen-rich condensing fluid  26  is withdrawn from oxygen-rich liquid  24  and introduced via valve  35  near top condenser  23 . In one embodiment, top condenser is a bath type condenser. 
         [0025]    Gaseous nitrogen near the top of single column  20  travels up tube  27 , with a portion being withdrawn as gaseous nitrogen  28  and the rest condensing within top condenser  23  before being reintroduced to single column  20 . Oxygen-rich condensing fluid  26  introduced near top condenser  23  provides the needed cooling to condense the nitrogen. Waste gaseous oxygen  22  is withdrawn and used to provide refrigeration to heat exchanger  10 . In one embodiment, safety purge  83  can be withdrawn as a safety precaution. 
         [0026]    Recovered liquid nitrogen  62  is then introduced to subcooler  30  in order to further cool recovered liquid nitrogen  62  to produce liquid nitrogen product  64 . Oxygen-rich liquid  24  is used to provide the necessary cooling. Any gas forming within subcooler  30  is withdrawn as oxygen-rich waste gas  34  and may be combined with waste gaseous oxygen  22  before entering the warm end of heat exchanger  10 . In one embodiment not shown, oxygen-rich waste gas  34  may be warmed in heat exchanger  10  separately from waste gaseous oxygen  22  in order to allow for deeper subcooling of liquid nitrogen product  64 . Oxygen purge  32  can be withdrawn from the bottom of subcooler  30  as necessary. Recycled liquid nitrogen  66  can be withdrawn from recovered liquid nitrogen  62  and introduced to the top of single column  20  as reflux via valve  36 . In an optional embodiment (shown as dotted line  66   a ), recycled liquid nitrogen  66  can originate from liquefied nitrogen  58  via line  66   a.    
         [0027]      FIG. 3  provides an additional embodiment of the invention.  FIG. 3  is similar to  FIG. 2 , except that  FIG. 3  provides for increased safety by reducing the risk of hydrocarbons concentrating in subcooler  30 . In this embodiment, oxygen-rich fluid  25  is withdrawn from single column  20  at a point above the bottom portion of single column  20  and introduced to subcooler  30  instead of using the oxygen-rich liquid from the bottom of single column  20 . Essentially, instead of sending a portion of the bottoms liquid to subcooler  30 , all of the withdrawn bottoms liquid is sent to top condenser  23  as oxygen-rich condensing fluid  26  via valve  35 . Recycled liquid nitrogen  67  can be withdrawn from liquefied nitrogen  58  and introduced to the top of single column  20  as reflux via valve  36 . 
         [0028]    In certain embodiment, the feed gas to the single column is air, as opposed to a feed gas having a concentration having higher nitrogen content. The single column has both a bottom reboiler and a top condenser, and in certain embodiments, the reboiler is driven by gaseous nitrogen withdrawn from the recycle compressor, preferably at a first stage discharge of the recycle compressor. In another embodiment, the single column can be partly refluxed with liquid nitrogen split-off from a Joule-Thompson stream (e.g., high pressure nitrogen stream exiting the cool end of the heat exchanger such as stream  58 ). In another embodiment, column bottoms may be split for both product subcooling and for driving the top condenser, or all of column bottoms can used for driving the top condenser with product subcooling being done via an oxygen-rich liquid column sidedraw stream. 
         [0029]    While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device. 
         [0030]    The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0031]    Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
         [0032]    Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.