Method and device for producing compressed nitrogen

Abstract

A method and a device that serve for producing compressed nitrogen by low-temperature decomposition of air in a distillation column system which has a high-pressure column, a low-pressure column, a high-pressure column head condenser and a low-pressure column head condenser. A main air compressor constitutes the only gas compressor powered by external energy. An internally compressed nitrogen stream is formed by a part-stream of the liquid nitrogen stream from the high-pressure column head condenser and/or a part-stream of the liquid nitrogen stream from the low-pressure column head condenser; the internally compressed nitrogen stream is brought in the liquid state to a product pressure which is between 15 and 100 bars; then the internally compressed nitrogen stream is heated in the main heat exchanger and then extracted as a gaseous compressed nitrogen product below the product pressure.

1 . A method of producing compressed nitrogen by low-temperature fractionation of air in a distillation column system having a high-pressure column and a low-pressure column and a high-pressure column top condenser and a low-pressure column top condenser, both in the form of condenser-evaporators, wherein
 the low-pressure column has the lowest operating pressure of all the separating columns in the distillation column system,   feed air is compressed in a main air compressor to an overall air pressure, forming a high-pressure overall air stream,   the main air compressor is the only external energy-driven gas compressor which is used in the process,   a first substream of the high-pressure overall air stream is decompressed to perform work and introduced into the distillation column system,   a second substream of the high-pressure overall air stream is cooled down in a main heat exchanger and introduced at least partly in liquid form into the distillation column system,   gaseous top nitrogen from the high-pressure column is at least partly liquefied in the high-pressure column top condenser, forming a first liquid nitrogen stream,   a first substream of the first liquid nitrogen stream is applied as reflux liquid to the high-pressure column,   gaseous top nitrogen from the low-pressure column is at least partly liquefied in the low-pressure column top condenser, forming a second liquid nitrogen stream,   a first substream of the second liquid nitrogen stream is applied as reflux liquid to the low-pressure column,   an internal compression nitrogen stream which is formed by a second substream of the first liquid nitrogen stream and/or a second substream of the second liquid nitrogen stream, in the liquid state, is brought to a product pressure higher than the operating pressure of the high-pressure column,   the internal compression nitrogen stream which has been brought to the product pressure is warmed in the main heat exchanger and then drawn off as gaseous compressed nitrogen product stream under the product pressure,   at least one oxygen-enriched product gas stream is drawn off from the distillation column system and warmed in the main heat exchanger and   all oxygen-enriched product gas streams are drawn off from the distillation column system in the gaseous state,   
       characterized in that
 the exit pressure of the first substream of the high-pressure overall air stream in the work-performing decompression is equal to or higher than the operating pressure of the high-pressure column, 
 the feed air is compressed in the main air compressor to an overall air pressure at least 5 bar above the operating pressure of the high-pressure column, and 
 the internal compression nitrogen stream in the liquid state is brought to a product pressure between 15 and 100 bar. 
 
     
     
         2 . The method as claimed in  claim 1 , characterized in that at least a portion of the second portion of the feed air is introduced as cooling fluid into the evaporation space of the high-pressure column top condenser, with formation of the entirety of the cooling fluid for the high-pressure column top condenser by feed air. 
     
     
         3 . The method as claimed in  claim 1 , characterized in that the main air compressor has a single drive unit which is especially formed by a gas turbine unit, a steam turbine, a gas engine or a diesel engine. 
     
     
         4 . The method as claimed in  claim 1 , characterized in that at least a portion of the second substream of the high-pressure overall air stream, after it has been cooled down in the main heat exchanger, is decompressed to perform work in a fluid turbine. 
     
     
         5 . The method as claimed in  claim 1 , characterized in that vapor generated in the evaporation space of the high-pressure column top condenser is introduced into the bottom region of the low-pressure column, this vapor forming the entirety of the gas ascending in the lower section of the low-pressure column. 
     
     
         6 . The method as claimed in  claim 1 , characterized in that a third substream of the first liquid nitrogen stream is applied as reflux to the low-pressure column and the internal compression nitrogen stream is formed by a second substream of the second liquid nitrogen stream. 
     
     
         7 . The method as claimed in  claim 1 , characterized in that the internal compression nitrogen stream is formed by a second substream of the first liquid nitrogen stream and a second substream of the second liquid nitrogen stream, these two substreams being brought separately to the product pressure. 
     
     
         8 . The method as claimed in  claim 1 , characterized in that the first substream of the high-pressure overall air stream, downstream of the work-performing decompression thereof, is at least partly introduced into the high-pressure column. 
     
     
         9 . The method as claimed in  claim 1 , characterized in that the bottoms liquid of the high-pressure column is evaporated in a high-pressure column reboiler which takes the form of a condenser-evaporator, with a third portion of the high-pressure overall air stream which is formed at least partly by at least a portion of the first substream which has been decompressed to perform work being introduced into the liquefaction space of the high-pressure column reboiler. 
     
     
         10 . The method as claimed in  claim 1 , characterized in that the second substream of the high-pressure overall air stream is recompressed in a turbine-driven recompressor to a second air pressure higher than the overall air pressure. 
     
     
         11 . The method as claimed in  claim 1 , characterized in that at least a portion of the second substream of the high-pressure overall air stream is decompressed in a decompression turbine and hence liquefied and then introduced into the distillation column system. 
     
     
         12 . The method as claimed in  claim 1 , characterized in that
 a circulation stream is drawn off from the evaporation space of the high-pressure column top condenser and compressed in a circulation compressor which takes the form of a cold compressor, and   the compressed circulation stream is cooled in the main heat exchanger and   fed to the high-pressure column.   
     
     
         13 . The method as claimed in  claim 1 , characterized in that
 a circulation stream is drawn off from the upper region of the low-pressure column and compressed in a circulation compressor which takes the form of a cold compressor, and   the compressed circulation stream is cooled in the main heat exchanger and   fed to the high-pressure column top condenser.   
     
     
         14 . An apparatus for producing compressed nitrogen by low-temperature fractionation of air comprising
 a distillation column system having a high-pressure column and a low-pressure column, and a high-pressure column top condenser and a low-pressure column top condenser, both in the form of condenser-evaporators, wherein   the low-pressure column in the operation of the plant has the lowest operating pressure of all the separating columns in the distillation column system, and comprising   a main air compressor for production of a high-pressure overall air stream by compressing feed air to an overall air pressure, wherein   the main air compressor is the only external energy-driven gas compressor in the apparatus,   means of work-performing decompression a first substream of the high-pressure overall air stream,   means of introducing the first substream decompressed to perform work into the distillation column system,   a main heat exchanger for cooling a second substream of the high-pressure overall air stream.   means of introducing the cold second substream at least partly in the liquid state into the distillation column system,   means of introducing gaseous top nitrogen from the high-pressure column into the liquefaction space of the high-pressure column top condenser,   means of withdrawing a first liquid nitrogen stream from the liquefaction space of the high-pressure column top condenser,   means of introducing a first substream of the first liquid nitrogen stream as reflux liquid into the high-pressure column,   means of introducing top nitrogen from the low-pressure column into the liquefaction space of the low-pressure column top condenser,   means of withdrawing a second liquid nitrogen stream from the liquefaction space of the low-pressure column top condenser,   means of introducing a first substream of the second liquid nitrogen stream as reflux liquid into the low-pressure column,   means of forming an internal compression nitrogen stream by a second substream of the first liquid nitrogen stream and/or a second substream of the second liquid nitrogen stream,   means of increasing the pressure of the internal compression nitrogen stream in the liquid state to a product pressure higher than the operating pressure of the high-pressure column,   means of warming the internal compression nitrogen stream brought to the product pressure in the main heat exchanger,   means of drawing off the warmed internal compression nitrogen stream as gaseous compressed nitrogen product stream below the product pressure,   means of drawing off at least one oxygen-enriched product gas stream from the distillation column system,   means of warming the oxygen-enriched product gas stream in the main heat exchanger, and comprising   means of drawing off all oxygen-enriched product gas streams in the gaseous state from the distillation column system,   
       characterized in that
 the exit of the means for work-performing decompression is flow-connected to the high-pressure column, 
 the main air compressor is designed for the compression of the feed air to an overall air pressure at least 5 bar above the operating pressure of the high-pressure column, and 
 the means of increasing the pressure of the internal compression nitrogen stream in the liquid state are designed for increasing the pressure to a product pressure between 20 and 100 bar. 
 
     
     
         15 . The apparatus as claimed in  claim 14 , characterized in that the apparatus does not have any means of drawing off a liquid oxygen-enriched product stream from the distillation column system and more particularly does not have any pump for bringing a liquid oxygen-enriched stream to an elevated pressure.