Method for separating air by cryogenic distillation

Abstract

In a method for separating air by cryogenic distillation in a system of columns comprising a first column operating at a first pressure and a second column operating at a second pressure which is lower than the first column, the temperature T1 at which an airflow leaves, after cooling, the heat exchanger by rising towards the cold end of said heat exchanger and enters the first column is at least 1° C., preferably at least 2° C., higher than the dew point of the airflow.

1 - 14 . (canceled) 
     
     
         15 . A method for separating air by cryogenic distillation in a column system comprising a first column operating at a first pressure and a second column operating at a second pressure lower than the first pressure, the top of the first column being thermally coupled to the bottom of the second column, in which:
 i) cooling an air flow that has been purified of water and carbon dioxide in a heat exchanger and then sending said air flow to the first column in gaseous form;   ii) withdrawing an oxygen-enriched liquid from the bottom of the first column, subcooling the oxygen-enriched liquid in a subcooler, and then sending said oxygen-enriched liquid to the second column;   iii) withdrawing a nitrogen-enriched liquid from the upper part of the first column, subcooling the nitrogen-enriched liquid in a subcooler, and then sending said nitrogen-enriched liquid to the second column;   iv) withdrawing a nitrogen-rich gas and an oxygen-rich fluid from the second column and then heating in the heat exchanger;   wherein the heat exchanger is positioned below the first column, which in turn is positioned below the second column,   wherein the air flow cooling in the heat exchanger traversing the heat exchanger from bottom to top,   wherein the temperature T 1  at which the air flow leaves the heat exchanger and enters the first column is at least 1° C. higher than the dew point of the air flow.   
     
     
         16 . The method as claimed in  claim 15 , wherein the temperature T 1  at which the air flow leaves the heat exchanger and enters the first column is at least 2° C. higher than the dew point of the air flow. 
     
     
         17 . The method as claimed in  claim 15 , wherein another air flow that has been purified of water and carbon dioxide is cooled in the heat exchanger, leaves the heat exchanger at a temperature T 2 , is expanded in a turbine, is returned to the heat exchanger at a temperature T 3  and is cooled in the exchanger to a temperature T 4  before being sent to the second column in gaseous form, the temperature T 2  being higher than T 1 . 
     
     
         18 . The method as claimed in  claim 17 , wherein T 4  is at least 1° C. higher than the dew point of the expanded flow. 
     
     
         19 . The method as claimed in  claim 18 , wherein T 4  is at least 2° C. higher than the dew point of the expanded flow. 
     
     
         20 . The method as claimed in  claim 15 , wherein another air flow purified of water and carbon dioxide substantially at the second pressure is cooled in the heat exchanger to a temperature at least 1° C. higher than its dew point and sent to the second column without having been expanded. 
     
     
         21 . The method as claimed in  claim 20 , wherein another air flow purified of water and carbon dioxide substantially at the second pressure is cooled in the heat exchanger to a temperature at least 2° C. higher than its dew point and sent to the second column without having been expanded. 
     
     
         22 . The method as claimed in  claim 15 , wherein the oxygen-enriched liquid is cooled in the subcooler to a temperature lower than the temperature at which the nitrogen-enriched liquid enters the subcooler, the two subcooled liquids each being expanded in a respective valve before being sent to the second column. 
     
     
         23 . The method as claimed in  claim 15 , wherein the first column and potentially the second column are fed with air solely by gaseous air flows. 
     
     
         24 . The method as claimed in  claim 15 , wherein the first column and the second column only produce gas flows as final products. 
     
     
         25 . The method as claimed in  claim 15 , wherein the heat exchanger and the subcooler are made up of a single body of aluminum plates brazed together. 
     
     
         26 . The method as claimed in  claim 15 , wherein the pressure drop of the air flow purified of water and carbon dioxide intended for the first column by cooling in the heat exchanger does not exceed 120 mbar. 
     
     
         27 . The method as claimed in  claim 26 , wherein the pressure drop of the air flow purified of water and carbon dioxide intended for the first column by cooling in the heat exchanger does not exceed 100 mbar. 
     
     
         28 . The method as claimed in  claim 15 , wherein a liquid flow is withdrawn at an intermediate level of the first column, is subcooled in the subcooler to an intermediate temperature between the temperature at the outlet of the subcooler of the oxygen-enriched liquid and the temperature at the outlet of the subcooler of the nitrogen-enriched liquid, and sent to the second column.