Nitrogen generation method and apparatus

Abstract: Compressed and purified air is separated in a distillation column 12 into a nitrogen-rich vapour and an oxygen-rich liquid. Nitrogen-rich vapour is condensed in a condenser 18 in indirect heat exchange with a stream of the oxygen-rich liquid. A part of the condensate is employed s reflux in the column 12 and another part taken as liquid nitrogen product. The condenser 10 is also cooled by a supplemental liquid nitrogen stream. Both the oxygen-rich stream and the supplemental liquid nitrogen stream are vaporised. One part of the oxygen rich vapour is compressed in a compressor 38 which drives a turboexpander 42 coupled thereto through a heat dissipative brake 46. The other part of the oxygen-rich vapour is expanded in the turboexpander 42. The vaporised supplemental nitrogen stream is rewarmed and reliquefied in a nitrogen liquefier.

Description: The present invention relates to a nitrogen generation method and apparatus in which air is separated in a distillation column into nitrogen-rich vapour and oxygen-rich liquid fractions comprising the features of the preamble of claim 1 and claim 6, respectively. Such a method and apparatus are known, for example, from EP-A-0 624 767. There are numerous known processes and apparatus in which air is distilled in a distillation column to produce a nitrogen-rich vapour which is taken as a product. In one type of air separation process and apparatus employing a single distillation column, air, after having been filtered, compressed and purified, is cooled in a main heat exchanger to a temperature suitable for its rectification. Thereafter, the air is introduced into the single column and separated into nitrogen-rich vapour and oxygen-rich liquid fractions. In order to reflux the column, a head condenser is employed in which oxygen-rich liquid is used to condense nitrogen-rich vapour. The vaporised oxygen-rich liquid is then recompressed and re-introduced into the column in order to increase nitrogen production. This compression can take place at a temperature of either the warm or cold ends of the main heat exchanger. Part of the vaporised rich liquid can be partially heated and then expanded with a performance of work. It would seem inviting to apply all this work of expansion to recompression of the vaporised rich liquid. However, for the case where compression occurs at the temperature of the cold end of the main heat exchanger, heat of compression is produced. If this heat of compression is dissipated within the main heat exchanger no net refrigeration is made. Thus, a great proportion of the work of expansion must be rejected from the plant by way of an energy dissipative brake. Typically, such plants as have been described above, make their entire product as a gas. In order to convert the product into a liquid, the product gas must be liquefied in a separate liquefier. Such liquefaction is not accomplished without increased energy costs. At the same time, if high purity nitrogen is desired, the equipment involved in the liquefaction can act to contaminate the high purity nitrogen produced by the nitrogen generator. Thus, provision must be made for downstream cleaning of the liquid nitrogen if such liquid nitrogen is to be utilised in a high purity application. As will be discussed, the present invention provides a nitrogen generation method and apparatus in which more of the work of expansion can be applied to the compression to enhance liquid nitrogen production in an energy efficient manner. Additionally, such liquid nitrogen production is accomplished without the use of a downstream liquefier of the nitrogen product. The present invention provides a method of producing nitrogen. The method comprises cooling compressed, purified feed air to a temperature suitable for its rectification. The compressed, purified feed air is then introduced into a distillation column to produce a nitrogen rich column overhead of high purity ("high purity" as used herein and in the claims meaning less than 100 ppb of oxygen) and an oxygen-rich liquid as column bottoms. At least part of a nitrogen-rich stream, composed of the nitrogen-rich column overhead is condensed and part of the resulting condensate is introduced back into the distillation column as reflux. A nitrogen product stream is formed from a remaining part of the resulting condensate. A recycle stream is compressed and then cooled to the temperature suitable for the rectification of the feed air. The recycle stream is introduced into the distillation column to increase recovery of the nitrogen product. A refrigerant stream is expanded with the performance of (external) work to form a primary refrigerant stream. Heat is indirectly exchanged between the primary refrigerant stream and the compressed and purified air. A part of the work of expansion is applied to the compression of the recycle stream. A supplemental refrigerant stream is vaporised and then reliquefied. The supplemental refrigerant stream is at least partly vaporised by indirect heat exchange between the part of the nitrogen-rich stream, thereby to help effect the condensation of the part of the nitrogen-rich stream. Prior to the reliquefaction of the supplemental refrigerant stream, heat is indirectly exchanged between said supplemental refrigerant stream and the compressed and purified air to increase the portion of the work able to be supplied to the compression, over that obtainable had the supplemental refrigeration not been added. This increases the compression and further increases recovery of the nitrogen product. In another aspect, the present invention provides a nitrogen generator. A main heat exchange means is configured for cooling compressed, purified feed air to a temperature suitable for its rectification. A distillation column is connected to the main heat exchange means to rectify the compressed and purified feed-air and thereby to produce a nitrogen rich overhead of high purity and an oxygen-rich liquid column bottoms. A head condenser is connected to the distillation column for condensing at least part of a nitrogen-rich stream composed of the nitrogen-rich tower overhead and for re-introducing part of the resultant condensate back into the distillation column as reflux so that a remaining part of the resulting condensate can be removed as a product stream. A compressor is provided for compressing a recycle stream. A main heat exchange means is interposed between the compressor and the distillation column so that the recycle stream cools to the temperature at which the air is rectified and is introduced into the distillation column to increase recovery of the nitrogen product. A turboexpander is provided for expanding a refrigerant stream with the performance of work to form a primary refrigerant stream. The turboexpander communicates the main heating exchange means so that the primary refrigerant stream indirectly exchanges heat with the compressed and purified air. A means is provided for coupling the turboexpander to the compressor so that a portion of the work is applied to the compression of the recycle stream. A supplemental refrigerant circuit is provided for circulating a supplemental refrigerant stream vaporised during the circulation. The supplemental refrigerant circuit includes the head condenser and the main heat exchange means. The head condenser is configured such that the supplementary refrigerant stream is at least partly vaporised through indirect heat exchange with the at least part of the nitrogen-rich stream. The main heat exchange means is also configured to indirectly exchange heat between the supplemental refrigerant stream and the compressed and purified air to increase the amount of work able to be supplied to the compression, over that obtainable had the supplemental refrigeration not been added. This increases compression and further increases recovery of the nitrogen product. The supplemental refrigerant circuit also includes a liquefier interposed between the main heat exchange means and the head condenser to re-liquefy the supplemental refrigerant stream after having been vaporised. The addition of the supplemental refrigerant stream allows more of the work of expansion to go to the compression of the vaporised rich liquid oxygen stream to be re-introduced back into the distillation column. Thus, for a given supply rate of air, more nitrogen will be produced and more nitrogen can be removed from the head condenser as a liquid. As will be discussed, the supplemental refrigerant stream can be a nitrogen stream which adds its supplemental refrigeration to the plant in the main heat exchanger. However, since such stream leaves the main heat exchanger without a high pressure drop, the amount of energy required for reliquefaction is not as great as if a vaporised nitrogen stream were to be separately liquefied in a non-integrated liquefier. Hence, more liquid nitrogen can be produced at an energy savings over the prior art. Additionally, since the nitrogen can be produced at high purity within a nitrogen generator of the present invention, and the liquefier is integrated through indirect heat exchange, there is no contamination to the product that might otherwise occur had the liquefier been integrated to liquefy the nitrogen product, downstream of the nitrogen generator. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic flow diagram of a nitrogen generator in accordance with the present invention; and Figure 2 is a schematic view of a nitrogen liquefier to be integrated into the nitrogen generator illustrated in Figure 1. With reference to Figure 1, a nitrogen generator 1 in accordance with the present invention is illustrated. Air after being filtered to remove dust particles is compressed and then purified to remove carbon dioxide and water. Thereafter, the air is cooled as air stream 10 to a temperature suitable for its rectification within a main heat exchanger 11. Air stream 10 is introduced at pressure into a distillation column 12 which is configured to produce an oxygen rich liquid as column bottoms and a high purity nitrogen-rich vapour as column overhead and which operates at a superatmospheric pressure typically in the range of 5 to 10 bar so as to enable a high pressure nitrogen product to be taken from the top of the distillation or rectification column 12. The oxygen-rich liquid preferably has a relatively high nitrogen content, e.g. in the range of 30 to 70% by volume, preferably 40 to 60% by volume, and may alternatively be referred to as a waste nitrogen stream. A nitrogen-rich stream 14 is produced from the nitrogen-rich vapour. A part 16 of the nitrogen-rich stream 14 is condensed within a head condenser 18 to produce a condensed stream 20. A part 22 of the condensed stream is re-introduced back into distillation column 12. Another part, which in the illustrated embodiment is a remaining part of the condensed stream 20, is extracted as a liquid product stream 23 which preferably after having been subcooled within a subcooling unit 24 is expanded by passage through an expansion valve 26 prior to being sent to storage, a product gaseous nitrogen product stream may, as shown, be taken from the stream of nitrogen-rich stream 14 is a possible modification of the illustrated embodiment. An oxygen rich liquid stream 28 is subcooled with a subcooling unit 30 and is then expanded through an expansion valve 32 to a sufficiently low temperature to effect the condensation of the part 16 of the aforesaid nitrogen-rich stream 14. The oxygen-rich liquid stream 28, after expansion, is introduced into head condenser 18 to produce a vaporised oxygen-rich liquid stream 34. A part 36 of the vaporised oxygen-rich liquid stream is re-compressed within a recycle compressor 38 and then cooled in Section 11B of main heat exchanger 11 to the temperature of distillation column 12. The now compressed, vaporised oxygen-rich liquid stream is re-introduced into distillation column 12. A remaining part 40 of vaporised oxygen-rich liquid stream 34 is warmed to an intermediate temperature, above the temperature at which the rectification of the air takes place. This occurs within Section 11 B of main heat exchanger 11. The remaining part 40 of oxygen-rich liquid stream forms a refrigerant stream which is expanded within a turboexpander 42 to produce a primary refrigerant stream 44. Turboexpander 42 is coupled to compressor 38. Part of the work of expansion is dissipated by an energy dissipative brake 46 which if desired may take the form of an electrical generator and a remaining part of the energy of expansion is used to power compressor 38. Primary refrigerant stream 44 warms within subcooling unit 30 and then is fully warmed within main heat exchanger 11 where it is discharged from the plant as waste. It is to be noted that embodiments of the present invention are possible in which a stream of liquid is extracted at a column location above the bottom of the column and then, after vaporisation during use in the distillation process, is recompressed, cooled and reintroduced into the column. Additionally, the present invention is not limited to nitrogen generation plants in which a refrigerant stream is formed from vaporised column bottoms liquid although such generators are preferred. A supplemental refrigerant stream 48 is supplied from a nitrogen liquefying unit (labelled "NLU") that will be discussed hereinafter. A part 50 of supplementary refrigerant stream 48 is vaporised within head condenser 18 and then is further warmed within subcooling unit 30. Thereafter, it is introduced into main heat exchanger 11 where it is fully rewarmed and then returned back to the nitrogen liquefying unit. An embodiment of the present invention is possible in which the supplementary refrigerant stream is partly vaporised within head condenser 18 and then goes on to be fully vaporised within main heat exchanger 11. Supplemental refrigeration is thus supplied to nitrogen generator 1. A remaining part 51 of the incoming supplementary refrigerant stream is expanded by passage through a valve 52 and then is phase separated within phase separator 54 to produce a liquid stream 56. Liquid stream 56 acts to subcool liquid product stream 23. A vapour stream 58 composed of the vapour phase of the separated supplemental refrigerant is combined with stream 56 and returned to the nitrogen liquefying unit as a stream 59. With reference to Figure 2, a nitrogen liquefying unit 2 for use in association with a nitrogen generator according to the present invention is illustrated. Part 50 of supplementary refrigerant stream 48 is combined with a recycle stream 60 and stream 59 after having been warmed in a manner that will be discussed hereinafter. The resultant combined stream is then recompressed within a compression unit 62 to form a compressed stream 64. The heat of compression is removed from compressed stream 64 by an after-cooler 66. Compressed stream 64 is then introduced into a first booster compressor 68 and the heat of compression is removed by a first after-cooler 70. Compressed stream 64 is then introduced into a second booster compressor 72 and the heat of compression is then removed from compressed stream 64 by a second after-cooler 74. Thereafter, the major part of compressed stream 64 is cooled within a heat exchanger 76 and valve expanded to liquefaction by valve 77 to produce supplementary refrigerant stream 48. After compressed stream 64 has partly cooled within heat exchanger 76, a subsidiary stream 78 is separated from compressed stream 64. Subsidiary stream 78 is expanded within a first turboexpander 80 linked to second booster compressor 72 to produce an expanded stream 82. After formation of subsidiary stream 78, compressed stream 64 is further cooled and a subsidiary stream 84 is then separated therefrom. Subsidiary stream 84 is expanded within a second turboexpander 86 operating at a lower temperature than that of first turboexpander 80. Second turboexpander 86 is linked to first compressor booster 68. The resultant expanded stream 88 is then partly rewarmed within heat exchanger 76 and combined with expanded stream 82 to form recycle stream 60. Recycle stream 60 is fully rewarmed within main heat exchanger 76 prior to its combination with the part 50 of supplemental refrigerant stream 48 that . enters liquefying unit 2. Stream 59 also fully warms within heat exchanger unit 76 and is then compressed in a compressor 90 to enable it to also combine with part 50 of supplemental refrigerant stream 48. An optimised production of one or both of liquid nitrogen and gaseous nitrogen products is made possible by the nitrogen generator illustrated in the accompanying drawings.

Claims: A method of producing nitrogen, said method comprising: cooling compressed, purified feed air to a temperature suitable for its rectification; introducing said compressed, purified feed air into a distillation column to produce by rectification a nitrogen rich column overhead of high purity and oxygen-rich liquid as column bottoms; condensing at least part of a nitrogen-rich stream composed of said nitrogen-rich column overhead and introducing part of the resulting condensate into said distillation column as reflux; forming a nitrogen product stream from a remaining part of the resulting condensate; compressing a recycle stream, cooling said recycle stream to said temperature and introducing said recycle stream into said distillation column; expanding a refrigerant stream with the performance of work to form a primary refrigerant stream and indirectly exchanging heat between said primary refrigerant stream and said compressed and purified air and said recycle stream; applying an amount of said work to said compression of said recycle stream; characterised by vaporising and then reliquefying a supplemental refrigerant stream; said supplemental refrigerant stream being at least partly vaporised by indirectly exchanging heat with said at least part of said nitrogen-rich stream, thereby to help effect said condensation of said part of said nitrogen-rich stream; and prior to said reliquefaction of said supplemental refrigerant stream, indirectly exchanging heat between said supplemental refrigerant stream and said compressed and purified air and said recycle stream. A method according to claim 1, wherein: a stream of said oxygen-rich liquid is withdrawn from said distillation column, valve expanded, and passed in indirect heat exchange with said nitrogen-rich stream to help condense said at least part of said nitrogen-rich stream and thereby to form a vaporised oxygen-rich stream; said recycle stream is formed from part of said vaporised oxygen-rich stream; and said refrigerant stream is formed from a remaining part of said vaporised oxygen-rich liquid stream. A method according to claim 2, wherein said supplemental refrigerant stream is completely vaporised by said indirect heat exchange with said nitrogen-rich tower overhead. A method according to claim 3, wherein said supplemental refrigerant stream is liquefied by compressing said supplemental refrigerant stream and expanding said supplemental refrigerant stream at two temperature levels. A method according to claim 2, wherein: said nitrogen product comprises part of said condensate and is divided into two product streams; one of said product streams is vaporised through indirect heat exchange with said compressed and purified air; the other of said product streams is subcooled through indirect heat exchange with a subsidiary stream composed of part of said supplemental refrigerant stream; and said subsidiary stream is combined with a remaining part of said supplemental refrigerant stream prior to liquefaction. A nitrogen generator comprising: main heat exchange means configured for cooling compressed, purified feed air to a temperature suitable for its rectification; a distillation column communicating with said main heat exchange means to rectify said compressed and purified feed air and thereby to produce a nitrogen rich column overhead of high purity and oxygen-rich liquid as column bottoms; a head condenser connected to said distillation column for condensing at least part of a nitrogen-rich stream composed of said nitrogen rich column overhead and for reintroducing part of the resultant condensate back into said distillation column as reflux so that a remaining part of the resultant condensate can be removed as a product stream; a compressor for compressing a recycle stream; said main heat exchange means being in a position intermediate said compressor and said distillation column so that said recycle stream cools to said temperature and is introduced into said distillation column to increase recovery of said nitrogen product; a turboexpander for expanding a refrigerant stream with performance of work to form a primary refrigerant stream; said turboexpander being in communication with said main heat exchange means so that said primary refrigerant stream indirectly exchanges heat with said compressed and purified air; and means for coupling said turboexpander to said compressor so that an amount of said work is applied to said compression of said recycle stream; characterised in that the nitrogen generator comprises a supplemental refrigerant circuit for circulating a supplemental refrigerant stream vaporised during the circulation, said supplemental refrigerant circuit including, said head condenser, said head condenser being configured such that said supplementary refrigerant stream is at least party vaporised through indirect heat exchange with said at least part of the nitrogen-rich stream, said main heat exchange means, said main heat exchange means also being configured indirectly to exchange heat between the supplemental refrigerant stream and said compressed and purified air; and a liquefier in a position between said main heat exchange means and said head condenser to re-liquefy the vaporised supplemental refrigerant stream. A nitrogen generator according to claim 6, wherein said head condenser is also configured so as indirectly to exchange heat with a stream of said oxygen-rich liquid; additionally including an expansion valve intermediate said head condenser and said distillation column for valve expanding said stream of said oxygen-rich liquid, thereby to form a vaporised oxygen rich stream; and wherein said compressor and turboexpander communicate with said head condenser so that said recycle stream comprises part of said vaporised oxygen-rich liquid stream and said refrigerant stream comprises a remaining part of said vaporised oxygen-rich liquid stream. A nitrogen generator according to claim 6 or claim 7, wherein supplemental refrigerant stream liquefier comprises a nitrogen liquefier having two turboexpanders operating at two different temperature levels. A nitrogen generator according to any one of the preceding claims, in which the recycle compressor is coupled to the turboexpander through an energy dissipative brake.

Inventors: NAUMOVITZ JOSEPH P, NAUMOVITZ, JOSEPH P.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1996
Main classification: F25J3
IPC codes: F25J3