With reference to FIG. 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 into a distillation column 12 which is configured to produce an oxygen rich liquid as column bottoms and a high purity nitrogen-rich vapor as tower overhead.
A nitrogen rich stream 14 is produced from the nitrogen-rich vapor. 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 valve expanded by a expansion valve 26 prior to being sent to storage. As would occur to those skilled in the art, a product stream composed of another part 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 vaporized oxygen-rich liquid stream 34.
A part 36 of the vaporized 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, vaporized oxygen-rich liquid stream is re-introduced into distillation column 12. A remaining part 40 of vaporized 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 11B 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 or possibly 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 vaporization 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 vaporized column bottoms liquid.
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 vaporized 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 warmed and then returned back to the nitrogen liquefying unit. An embodiment of the present invention is possible in which the supplementary refrigerant stream partly vaporizes within head condenser 18 and then goes on to fully vaporize 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 valve expanded within 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 vapor stream 58 composed of the vapor 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 FIG. 2, a nitrogen liquefying unit 2 in accordance with 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 warmed within heat exchanger 76 and combined with expanded stream 82 to form recycle stream 60. Recycle stream 60 is fully warmed 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.
As will be understood by those skilled in the art, although the present invention has been described with reference to a preferred embodiment, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention.