Abstract:
A method and apparatus of separating air to produce an oxygen product. In accordance with the method and apparatus the air is rectified within a double column arrangement. The lower pressure column has lower and intermediate reboilers. Nitrogen from the higher pressure column is compressed and sent to the lower reboiler and oxygen tower overhead from the higher pressure column is fed to the intermediate reboiler. The resultant liquid is used to reflux both columns. The advantages in the arrangement set forth above is that the higher pressure column may be made to operate at a lower pressure to conserve energy.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for separating air to produce an oxygen product. More particularly, the present invention relates to such a method and apparatus in which air is separated in double column arrangement having higher and lower pressure columns. Even more particularly, the present invention relates to such a method and apparatus in which the lower pressure column is reboiled with compressed nitrogen vapor from the higher pressure column and the vapor rate is increased at an intermediate location thereof by generation of vaporized liquid. 
     Air is commonly separated in a double column arrangement having higher and lower pressure columns. Prior to separation, air is filtered and compressed. After removing the heat of compression, the air is purified by removing impurities such as carbon dioxide, moisture and heavy hydrocarbons. The resultant compressed and purified air stream is then cooled in a main heat exchanger to a temperature suitable for its rectification and introduced into double column arrangement. Liquid oxygen is produced as a column bottoms of the lower pressure column. An oxygen product is extracted as a liquid stream that may be pumped to pressurize the liquid. The liquid is then vaporized in the main heat exchanger against cooling the incoming air. 
     In order to reboil the lower pressure column, a condenser reboiler can be provided to condense incoming air against boiling the liquid oxygen. The air may be partially or fully condensed and is introduced into the higher pressure column. Examples of this can be found in U.S. Pat. No. 5,626,036 and WO 885893. In both of these patents the air is partially condensed to reboil the lower pressure column. Such partial condensation is advantageous in that the majority of the air may be compressed in the main compressor to a pressure below 4 bar absolute. This minimum compression will produce a minimum amount of boiling in the lower pressure column so that a liquid product may be withdrawn. Additionally, in both of these patents, an increase in the vapor rate is effected at an intermediate location of the lower pressure column by means of an intermediate reboiler in which nitrogen vapor constitutes the coolant. The condensate of such intermediate reboiler is returned to both the higher and lower pressure columns as reflux. 
     As will be discussed, the present invention produces greater efficiency than such prior art patents. 
     SUMMARY OF THE INVENTION 
     The present invention provides air separation method separating air to produce an oxygen product. In accordance with the method, compressed and purified air is cooled to a temperature suitable for its rectification. The cooled and compressed air is then introduced into a double rectification column system having a higher pressure column and a lower pressure column. The compressed and purified air is then rectified in the double rectification system so that a nitrogen-rich tower overhead and an oxygen-rich liquid column bottoms are produced within the higher pressure column. An oxygen liquid column bottoms is produced within the lower pressure column. The lower pressure column is reboiled by cold compressing a first nitrogen stream composed of the nitrogen-rich tower overhead and introducing the first nitrogen stream into a reboiler associated with a bottom region of the lower pressure column, thereby to form a nitrogen liquid stream. The lower pressure column is reboiled at an intermediate location thereof with a second nitrogen rich stream composed of the nitrogen-rich tower overhead, thereby to form an additional nitrogen liquid stream. The lower and higher pressure columns are refluxed with liquid nitrogen contained within the nitrogen liquid stream and the additional nitrogen rich liquid stream. A product stream composed of the oxygen liquid column bottoms is extracted from the lower pressure column and is fully warmed through indirect heat exchange with the compressed and purified air, thereby to form the oxygen product. 
     In another aspect, the present invention provides an apparatus for separating air to produce an oxygen product. In accordance with this aspect of the present invention, a main heat exchanger is provided for cooling compressed and purified air to temperature suitable for its rectification. A double rectification column system is also provided. The double rectification system has a higher and lower pressure column configured to rectify the air to produce a nitrogen-rich tower overhead and an oxygen-rich liquid column bottoms. An oxygen liquid column bottoms is produced within a lower pressure column. The main heat exchanger is connected to the double rectification column system so that the compressed and purified air is introduced therein. A lower reboiler is located within a bottom region of the lower pressure column. A cold compressor is interposed between the lower reboiler and the higher pressure column to compress a first nitrogen stream composed of the nitrogen-rich tower overhead and to introduce the first nitrogen stream into the lower reboiler to form a nitrogen liquid stream. An intermediate reboiler is associated with an intermediate region of the lower pressure column and connected to the higher pressure column so that a rich liquid stream, composed of the oxygen-rich column bottoms, indirectly exchangers heat with a second nitrogen rich stream composed of the nitrogen rich tower overhead, thereby to form an additional nitrogen liquid steam and a partially vaporized rich liquid stream. The lower and intermediate reboilers and the higher and lower pressure columns are all associated with one another so that the liquid nitrogen contained within the nitrogen liquid stream and the additional nitrogen liquid stream reflux the higher and lower pressure columns and the vaporized rich liquid stream is introduced into an intermediate location of the lower pressure column. The lower pressure column is connected to the main heat exchanger so that product stream composed of the oxygen liquid column bottoms as fully warmed through a direct heat exchange with the cooled and compressed air, thereby to form the oxygen product. 
     In a conventional double column arrangement, in which nitrogen is used to reboil the lower pressure column, the lower pressure column pressure and the higher pressure column pressure are tied to one another because the nitrogen must be at a sufficient pressure to vaporize oxygen against its own condensation, In the present invention, since cold compression is provided, that is, compression at the rectification temperature of the air, the higher pressure column may be made to operate at a lower pressure than otherwise would be required. Therefore, the main air compressor may be made to operate at a lower pressure and thus utilize less energy. At the same time, since vaporized rich liquid is being introduced into an intermediate location of the lower pressure column, boil up is increased within the lower pressure column to approximate a more ideal case. It has been calculated by the inventors therein that the present invention allows overall power requirements of an air reboiled plant to be reduced by about 2.5%. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims distinctly pointing out the subject mater that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying sole FIGURE which is a schematic representation of an apparatus used in carrying out a method in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to the FIG., an apparatus  1  in accordance with the present invention is illustrated. Air after having been cooled in main heat exchanger  10  to a temperature suitable for its rectification is rectified within a double column rectification system having a higher pressure column  12  and a lower pressure column  14 . Although not illustrated, higher and lower pressure columns  12  and  14  are filled with mass transfer elements which can be trays, or packing such as structured packing or random packing. 
     In the higher pressure column  12 , the air is distilled to form a nitrogen-rich tower overhead and an oxygen-rich column bottoms. The air is further refined in lower pressure column  14  to produce a liquid oxygen column bottoms within a bottom region  16  thereof. A product stream  82  (to be discussed hereinafter) composed of the liquid oxygen column bottoms is extracted and then totally warmed with main heat exchanger  10 . 
     It is to be noted that as used herein and in the claims, the term “fully warmed” means warmed to a temperature at which the compressed and purified air enters in heat exchanger  10 . The term “fully cooled” means cooled to a temperature which the cryogenic rectification is conducted which is normally at the temperature of the cold end of main heat exchanger  10 . The terms “partly cooled” or “partly warmed” mean warmed to a temperature between that of fully warmed and fully cooled. 
     More specifically, the air after having filtered in filter  18  is compressed in a compressor  20  having stages  22  and  24 . The compressed air is then purified within the prepurification unit  26  which may be beds of alumina operating out of phase to remove moisture and carbon dioxide. The resultant compressed and purified air is divided into the first and second subsidiary streams  28  and  30 . First subsidiary stream  28  is further compressed in a compressor  32  having stages  34  and  36  to form a further compressed stream  38 . Second subsidiary  30  after having been partially cooled is divided into two parts. A first of the two parts  40  is expanded within a turboexpander  42  with performance of work to form a refrigerant stream  44 . After refrigerant stream  44  is fully cooled, it is then introduced lower pressure column  14 . The second of the two parts, designated by reference numeral  46 , is fully cooled and then introduced higher pressure column  12 . Further compressed stream  38  is valve expanded within a valve  48  and introduced into higher pressure column  12 . Depending upon the exact cycle, further compressed stream  38  may be sufficiently cooled in main heat exchanger  10  so as to form liquid air. 
     Lower pressure column  14  is provided with a lower reboiler  50  located within bottom region  16  of lower pressure column  14 . A cold compressor  52  is interposed between lower reboiler  50  and higher pressure column  16  to compress a first nitrogen stream  54  composed of the nitrogen-rich tower overhead. The liquid oxygen column bottoms vaporizes and thereby condenses within lower reboiler  50  to form a nitrogen liquid stream  56  which is then valve expanded to operational pressure of higher pressure column  12  by an expansion valve  58 . An intermediate reboiler  60  is associated with intermediate location of lower pressure column  14  to provide reboil in such section. Intermediate reboiler  60  is connected to higher pressure column  12  to condense a second nitrogen rich stream  62  composed of nitrogen-rich tower overhead. Second nitrogen rich stream  62  condenses therein to form an additional nitrogen liquid steam  64 . Nitrogen liquid steam  56  and additional nitrogen liquid stream  64  are used to provide liquid nitrogen to reflux higher and lower pressure columns  12  and  14 . As illustrated, this is effectuated by introducing a reflux stream  66  into higher pressure column  12  and another reflux stream  68  into lower pressure column  14  in order to effectuate the foregoing introduction. Reflux stream  68  is valve expanded in an expansion valve  70  to the operational pressure of lower pressure column  14 . 
     A crude liquid stream  72 , composed of the oxygen rich liquid column bottoms of higher pressure column  12 , is valve expanded within expansion valve  74  to the operational pressure of lower pressure column  14 . The crude liquid stream  72  is passed into intermediate reboiler  60  and partially vaporized against the condensation of nitrogen. The resulting vapor stream is introduced into lower pressure column  14  to further refine the air. 
     It should be noted that intermediate reboiler  60  is illustrated as lying outside of lower pressure column  14 . As would be known to those skilled in the art, an intermediate reboiler having the same function as intermediate reboiler  60  could be positioned within lower pressure column  14  at the same level of introduction of crude liquid stream  72  after its partial vaporization. A further point is that a reboiler having the function of lower reboiler  50  could similarly be positioned outside of lower pressure column  14 . Such reboiler would have to be provided with passes to boil liquid oxygen. In any event, the term “intermediate location” is meant to designate a location between the top and bottom of lower pressure column  14 . Its exact location simply be a matter of design with a view towards optimization of the performance of lower pressure column  14  by bringing the operating line of the distillation being conducted closer to the vapor-liquid equilibrium line as would be graphically illustrated in a McCabe-Theile Diagram. In the illustrated embodiment, intermediate location was selected to be a level of the column in which the liquid concentration is equal to that of the oxygen-enriched liquid columns bottoms of higher pressure column  12 . 
     Further compressed air stream  38 , after having been liquefied, is valve expanded within expansion valve  48 . This produces two phase flow mixture of liquid and vapor. The liquid component of this mixture preferably extracted as a liquid air stream  78  that is expanded in an expansion valve  79  to the operational pressure of lower pressure column  14 . Thereafter, liquid air stream  78  is introduced into lower pressure column  14  for further refinement. Thus, higher pressure column  12  is acting as a phase separator which, although less preferably, similarly could be provided by an external pot. 
     The waste nitrogen stream  76  is then fully warmed within main heat exchanger  10  and is discharged as waste nitrogen, labeled “WN”. As illustrated, liquid nitrogen contained within reflux stream  68 , crude liquid stream  72 , and liquid air stream  78  are subcooled within a subcooling unit  80  which is preferably provided to subcool the foregoing streams before their introduction into lower pressure column  14 . Subcooling is produced through indirect heat exchange with waste nitrogen  76 . 
     Product stream  82  is extracted from bottom region  16  of lower pressure column  14  and then is vaporized within main heat exchanger  10  to produce the oxygen product as a vapor. As would be known to those skilled in the art, product stream  82  could be pressurized by being pumped before being vaporized. It is intended by the inventors herein that such pumping not be excluded from the coverage of the claims appended hereto. 
     In the illustrated embodiment, higher pressure column  12  designed to operate with air compressor  20  producing a compressed and purified air stream at a pressure approximately 3.4 bar (a). Cold compressor  52  designed to boost pressure to 5.2 bar (a). The pressure of lower pressure column  14  is 1.3 bar (a) and the flow to reboilers  50  and  60  is in the ratio of approximately 0.45. 
     While the present invention has been described with reference to preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention.