Nitrogen Generator
Numerous processes for the nitrogen generator from air are known in the art
. Where the primary product is nitrogen, single column processes for the separation of air at cryogenic conditions utilizing an oxygen-enriched stream for expansion and refrigeration for the process is well known.
One such basic process and apparatus for the generation of nitrogen using waste oxygen-enriched stream expansion is described in U.S. Pat. No. 4,222,756. In this basic process, compressed dried feed air with impurities removed is cooled to near the dew point and fed to the lower part of a single distillation column wherein it is separated into a nitrogen-enriched stream at the top of the column and an oxygen-enriched stream at the bottom of the column. The oxygen-enriched liquid is withdrawn from the bottom of the column and following expansion is delivered to a reboiler/condenser wherein heat transfer by indirect heat exchange from a portion of the nitrogen-rich vapor from the top of the column. Following the indirect heat exchange, the gaseous oxygen-enriched stream is withdrawn from the reboiler/condenser and expanded to provide refrigeration for the process. The nitrogen-enriched vapors condensed in the reboiler/condenser are returned to the single distillation column as reflux. Typically, the gaseous oxygen-enriched stream comprises between about 35% and 38% oxygen.
U.S. Pat. No. 4,848,996 discloses modifications to the basic nitrogen generation process described above with reference to U.S. Pat. No. 4,222,756. In this process, distillation stages are added in a fractionation section above the reboiler/condenser for the purpose of stripping oxygen from the gaseous oxygen-enriched stream. With a relatively lower oxygen content, the gaseous stream removed from the reboiler/condenser above the second fractionation section is described to be of a composition similar to air, and the "synthetic air" is recycled for compression and mixing with the main feed air stream to the bottom of the main distillation column. To provide refrigeration for the process, a gaseous stream above the reboiler/condenser, and below the fractionation section of the upper column, is removed from the upper fractionation column and expanded in an expansion device. The oxygen-enriched vapor stream withdrawn for expansion has an oxygen content of between about 40% and 45%.
In U.S. Pat. No. 4,927,441, further modifications to the process described in U.S. Pat. No. 4,848,996 are disclosed. The process described in U.S. Pat. No. 4,927,441 retains the use of a distillation section in the upper column which also comprises the reboiler/condenser for the stripping of oxygen from the oxygen-rich stream from the bottom of the main column and to produce a "synthetic air". However, following the separation by distillation of the oxygen-enriched stream from the main column into a second oxygen-enriched liquid and synthetic air at the top of the second column, a liquid stream comprising the second oxygen-enriched liquid is withdrawn and expanded into a second reboiler condenser where it is vaporized and thereafter flowed to an expansion device to provide refrigeration for the process. As with the process described in U.S. Pat. No. 4,848,996, the synthetic air stream in this process is also produced to enable mixing directly with main feed air to the main distillation column. The process of U.S. Pat. No. 4,927,441 describes returning the synthetic air either to an interstage of the main air compressor or to a separate recycle compressor and thereafter combining the compressed synthetic air with the feed air stream prior to cooling. Among other factors, the shortcomings of the processes described above is the complexities of the air convection train which includes a side feed stream. Additionally, the purification and cooling sections are required to be larger in size, resulting in a greater capital cost.
Another modification of the basic single column nitrogen generator is disclosed in U.S. Pat. No. 4,966,002. In this process, oxygen-enriched liquid is withdrawn from the bottom of the single distillation column and expanded into the reboiler/condenser to provide for condensation of nitrogen-enriched vapors which in turn are returned as reflux to the top of the distillation column. From the reboiler/condenser a gaseous stream is withdrawn and thereafter split into two streams. The first such divided stream is expanded in an expansion device to provide process refrigeration. The second such divided stream, which along with the first divided stream has an oxygen content of between about 45% and 50%, is warmed to ambient temperature, and thereafter, compressed, cooled, and recycled to the distillation column, or alternatively, to a cold compressor and returned to the column. The ratio of the divided waste nitrogen streams may vary in the process of U.S. Pat. No. 4,966,002, however, the composition of both such divided streams is the same. The waste stream and the recycled stream have the same concentration of nitrogen, which results in loss of potential nitrogen recovery in the overall process.
by: ericfu
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