Process for producing liquid nitrogen

Abstract

A process for producing a liquid nitrogen product through the cryogenic separation of air in a nitrogen production plant is provided. The nitrogen production plant can include a main air compressor; a heat exchanger; an air separation unit having a single column, a top condenser, and a bottom reboiler; a recycle compressor; at least one turbine-booster having a booster and a turbine; a liquid/gas separator; and a subcooler. The reboiler can be driven by gaseous nitrogen withdrawn from the recycle compressor, preferably at a first stage discharge of the recycle compressor. Additionally, the single column can be partly refluxed with liquid nitrogen split-off from a Joule-Thompson stream.

I claim: 
     
         1 . A process for producing nitrogen through the cryogenic separation of air in a nitrogen production plant, the nitrogen production plant comprising a main air compressor, a heat exchanger, an air separation unit, a recycle compressor, a first turbo-booster having a first booster and a first turbine, a second turbo-booster having a second booster and a second turbine, and a liquid/gas separator, the process comprising the steps of:
 (a) obtaining a main air feed comprising purified and compressed air at a pressure of at least 5 bar;   (b) fully cooling the main air feed in the heat exchanger to form a fully cooled air feed;   (c) withdrawing the fully cooled air feed from the heat exchanger and introducing the fully cooled air feed to the air separation unit under conditions effective for rectification of the cooled air feed into gaseous nitrogen and waste oxygen, wherein the air separation unit comprises a single column;   (d) warming the waste oxygen in the heat exchanger;   (e) warming the gaseous nitrogen in the heat exchanger;   (f) compressing the gaseous nitrogen in the recycle compressor to form a compressed nitrogen recycle;   (g) further compressing the compressed nitrogen recycle in the first booster to form a boosted nitrogen;   (h) further compressing the boosted nitrogen in the second booster to form a fully boosted nitrogen;   (i) cooling the fully boosted nitrogen in the heat exchanger to form liquefied nitrogen;   (j) introducing the liquefied nitrogen to the liquid/gas separator under conditions effective to separate gaseous nitrogen from the liquefied nitrogen, such that gaseous nitrogen is withdrawn from the liquid/gas separator and combined with the gaseous nitrogen from the air separation unit before warming in the heat exchanger;   (k) withdrawing recovered liquid nitrogen from the liquid/gas separator as product;   (l) withdrawing a fraction of compressed nitrogen recycle from the compressed nitrogen recycle and partially cooling the fraction of compressed nitrogen recycle before expanding the fraction of compressed nitrogen recycle in the first turbine to form a first expanded nitrogen;   (m) introducing the first expanded nitrogen into the heat exchanger and combining with the gaseous nitrogen from the air separation unit prior to compression in the recycle compressor;   (n) withdrawing a partially cooled boosted nitrogen from the fully boosted nitrogen from an intermediate point of the heat exchanger;   (o) expanding the partially cooled boosted nitrogen using a second turbine to form second expanded nitrogen; and   (p) introducing the second expanded nitrogen to the liquid/gas separator.   
     
     
         2 . The process as claimed in  claim 1 , wherein the main air feed has a composition substantially the same as atmospheric air. 
     
     
         3 . The process as claimed in  claim 1 , wherein the air separation unit further comprises a top condenser and a bottom reboiler, the top condenser being in fluid communication with the single column, the bottom reboiler being in fluid communication with the single column, the top condenser configured to provide condensing duty for the single column, the bottom reboiler configured to provide reboiling duty for the single column. 
     
     
         4 . The process as claimed in  claim 3 , further comprising withdrawing an oxygen-rich liquid from the bottom of the single column and introducing the oxygen-rich liquid to a subcooler, such that the oxygen-rich liquid provides subcooling for the subcooler. 
     
     
         5 . The process as claimed in  claim 3 , further comprising withdrawing an oxygen-rich liquid from the bottom of the single column and introducing the oxygen-rich liquid to the top condenser, such that the oxygen-rich liquid provides cooling for the top condenser. 
     
     
         6 . The process as claimed in  claim 3 , further comprising withdrawing an oxygen-rich fluid from a middle section of the single column and introducing the oxygen-rich fluid to a subcooler, such that the oxygen-rich fluid provides subcooling for the subcooler. 
     
     
         7 . The process as claimed in  claim 6 , wherein the oxygen-rich fluid from the middle section contains less hydrocarbons than an oxygen-rich liquid withdrawn from the bottom of the single column. 
     
     
         8 . The process as claimed in  claim 1 , wherein the second turbine is mechanically coupled to the second booster. 
     
     
         9 . The process as claimed in  claim 1 , wherein the first turbine is mechanically coupled to the first booster. 
     
     
         10 . The process as claimed in  claim 1 , further comprising the steps of withdrawing a fraction of partially compressed nitrogen recycle from an internal stage of recycle compressor; introducing the partially compressed nitrogen recycle to the heat exchanger for cooling before using the partially compressed nitrogen recycle as the heating fluid for the bottom reboiler; and then flashing the partially compressed nitrogen recycle into a top portion of the single column. 
     
     
         11 . The process as claimed in  claim 1 , further comprising subcooling the recovered liquid nitrogen from the liquid/gas separator in a subcooler. 
     
     
         12 . The process as claimed in  claim 1 , wherein the liquefied nitrogen is introduced to the liquid/gas separator by flashing. 
     
     
         13 . A process for producing a liquid nitrogen product through the cryogenic separation of air in a nitrogen production plant, the nitrogen production plant comprising a main air compressor, a heat exchanger, an air separation unit having a single column, a top condenser, and a bottom reboiler, a recycle compressor, at least one turbo-booster having a booster and a turbine, a liquid/gas separator, and a subcooler, the process comprising the steps of:
 (a) obtaining a main air feed comprising purified and compressed air at a pressure of at least 5 bar;   (b) fully cooling the main air feed in the heat exchanger to form a fully cooled air feed;   (c) withdrawing the fully cooled air feed from the heat exchanger and introducing the fully cooled air feed to the air separation unit under conditions effective for rectification of the cooled air feed into gaseous nitrogen and waste oxygen, wherein the air separation unit comprises a single column;   (d) warming the gaseous nitrogen in the heat exchanger;   (e) compressing the gaseous nitrogen in the recycle compressor to form a compressed nitrogen recycle;   (f) withdrawing a fraction of partially compressed nitrogen recycle from an internal stage of the recycle compressor, cooling said fraction of partially compressed nitrogen recycle in the heat exchanger, condensing said fraction of partially compressed nitrogen recycle in the bottom reboiler, and then flashing said fraction of partially compressed nitrogen recycle into a top portion of the single column as reflux;   (g) compressing and expanding the compressed nitrogen recycle using the at least one turbine-booster to provide the refrigeration for the process and produce an expanded nitrogen stream;   (h) introducing the expanded nitrogen stream to the liquid/gas separator;   (i) subcooling the nitrogen-enriched liquid from the liquid/gas separator in the subcooler to form the liquid nitrogen product; and   (j) withdrawing an oxygen-enriched fluid from the single column to provide subcooling for the subcooler.   
     
     
         14 . A process for producing a liquid nitrogen product through the cryogenic separation of air in a nitrogen production plant, the nitrogen production plant comprising a main air compressor; a heat exchanger; an air separation unit having a single column, a top condenser, and a bottom reboiler; a recycle compressor; at least one turbine-booster having a booster and a turbine; a liquid/gas separator; and a subcooler, the process comprising the steps of:
 introducing a main air feed to the single column under conditions effective for separating air to produce oxygen and nitrogen;   using gaseous nitrogen withdrawn from the recycle compressor as a heating fluid for the bottom reboiler; and   refluxing the single column using a pressurized nitrogen stream cooled in the heat exchanger.   
     
     
         15 . The process as claimed in  claim 14 , further comprising the step of withdrawing column bottoms from the single column and using a first portion of the column bottoms to provide subcooling for a nitrogen product, and using a second portion of the column bottoms for driving the top condenser. 
     
     
         16 . The process as claimed in  claim 14 , further comprising the steps of withdrawing column bottoms from the single column and using the column bottoms to drive the top condenser, and withdrawing an oxygen-rich liquid column side draw stream from the single column and using the oxygen-rich liquid column side draw stream to provide subcooling for a nitrogen product.