Liquid nitrogen production

Abstract

An improved process for liquid nitrogen production by cryogenic air separation using a distillation column system to enhance the product recovery.

1 . A system for liquid nitrogen production by cryogenic air separation comprising:
 a) a first column operating to distill a gaseous air feed stream and a liquid air feed stream into a liquid nitrogen stream and an oxygen-enriched liquid stream;   b) second and third columns, at least one operating to distill the oxygen-enriched liquid stream into a liquid nitrogen stream and an oxygen-enriched liquid stream with enhanced content of oxygen;   c) three condenser-reboilers associated with the respective first, second and third columns and operating to effect indirect heat exchange between the nitrogen vapor at the top of the column and the oxygen-enriched liquid at the bottom of the higher column or columns;   d) the upper condenser-reboiler operating to effect indirect heat exchange between the nitrogen vapor at the top of the third or second column and the oxygen-enriched liquid in an upper reboiler;   e) means for passing a first portion of the pressurized air feed through a cold expander and feeding a part of the air exhaust from said expander as a gaseous air stream into the bottom of the first column;   f) means for passing a second portion of the pressurized air feed through a main heat exchanger for cooling and liquefying, passing the second portion of air through a throttling valve and feeding as a liquid air stream into the middle of the first column;   h) means for passing the oxygen-enriched liquid stream from the first column to the second column, from the second column to the third column or to the upper reboiler, and from the third column to the upper reboiler;   i) means for removing the liquid nitrogen in three streams as liquid nitrogen products;   j) wherein the three columns have a serial arrangement whereby oxygen enriched liquid is introduced successively from the high pressure first column to the intermediate pressure second column, and then from the intermediate pressure column to the low pressure third column.   
     
     
         2 . The system of  claim 1  further comprising in combination:
 a) means for subcooling the liquid nitrogen product by evaporating a part of liquid nitrogen stream at reduced pressure; 
 b) means for subcooling the liquid nitrogen product by passing the liquid nitrogen streams removed from the distillation columns through throttling valves and into a liquid separator; 
 c) one throttling valve passing the air exhaust from the cold expander to the first column; 
 d) an additional expander at the stream passing of the air exhaust from the cold expander to the first column; 
 e) one or more trays above the upper reboiler to provide two separate streams comprising a regeneration gas with decreased oxygen content, and a waste gas; 
 f) two upper reboilers operated in series to provide two separate streams; a regeneration gas with decreased oxygen content, and a waste gas. 
 
     
     
         3 . A method of separating liquid nitrogen products from air comprising:
 a) providing first and second distillation columns;   b) passing a portion of pressurized air feed through an expander;   c) passing part of the exhaust from said expander to the first distillation column;   d) distilling air in the first distillation column to produce 99 to 99.99% pure nitrogen gas,   and condensing said produced nitrogen gas in a first reboiler to produce a condensed nitrogen product;   d) passing bottoms from the first distillation column, after cooling, to the second distillation column to join with descending reflux in that column;   e) wherein vapor ascending in the second distillation column is progressively rectified until pure N 2  is produced, removed and condensed in a second reboiler, as product nitrogen.   
     
     
         4 . The method of  claim 3  wherein the expander is a turbo expander that produces said air exhaust. 
     
     
         5 . The method of  claim 3  wherein said turbo expander drives at least one compressor which pressurizes said air feed. 
     
     
         6 . The method of  claim 3  further comprising: removing H 2 O and CO 2  from the air feed between stages of compression. 
     
     
         7 . The method of  claim 6  wherein said part of the air exhaust from the expander is fed as vapor to the first distillation column. 
     
     
         8 . The method of  claim 6  further comprising: providing and operating a primary heat exchanger configured to cool the air flow to the turbo expander and to operate the turbo expander. 
     
     
         9 . The method of  claim 3  wherein another part of the pressurized air feed is passed through a valve operating to form cooled liquid air. 
     
     
         10 . The method of  claim 3  wherein vapor exiting a second reboiler associated with the second distillation column is passed through a secondary heat exchanger and then to a primary heat exchanger for recovery of refrigeration. 
     
     
         11 . A system for separating liquid nitrogen produced from air comprising:
 a) first and second distillation columns;   b) means for passing a portion of pressurized air feed through an expander and passing part of air exhaust from said expander to the first distillation column, and for passing a liquefied portion of the air feed to the first distillation column;   c) the first distillation column configured to distill air in the first column to produce 99 to 99.99% pure nitrogen gas, and to condense said produced nitrogen gas in a first reboiler to produce a condensed nitrogen product;   d) means for passing an oxygen enriched liquid stream from the first distillation column, after cooling, to the second distillation column to join with descending reflux in the second distillation column, and for passing an oxygen enriched liquid stream from the second distillation column to a third distillation column, liquid nitrogen being withdrawn in separate streams from the three distillation columns;   e) wherein the three distillation columns have a serial arrangement whereby oxygen enriched liquid is introduced successively from the first distillation column at a high pressure to the second distillation column at an intermediate pressure, and then from the second distillation column to the third distillation column at a low pressure.   
     
     
         12 . The system of  claim 11  wherein the expander is a turbo expander producing said air exhaust, said expander driving at least one compressor raising the pressure of the air feed. 
     
     
         13 . The system of  claim 12  including means feeding part of the air exhaust from the expander in vapor state to the first distillation column. 
     
     
         14 . The system of  claim 11  including means for flowing part of the pressurized air feed through a valve operating to form cooled liquid air. 
     
     
         15 . The system of  claim 11  including means for flowing vapor exiting a first reboiler associated with the first distillation column to a secondary heat exchanger and then to a primary heat exchanger for recovery of refrigeration. 
     
     
         16 . The combination of  claim 11  wherein the first distillation column is a high pressure column and the second distillation column is a low pressure column.