Method and apparatus for separating air by cryogenic distillation

Abstract

In a method for separating air by cryogenic distillation, cooled air purified to remove water is sent to a first column operating at a first pressure, where it is separated into a nitrogen-enriched gas as an oxygen-enriched liquid; a gas enriched in argon relative to the air is withdrawn from the second column; at least a portion of the oxygen-enriched liquid is vaporized by heat exchange with the argon-enriched gas; and the vaporized, oxygen-enriched liquid is sent to an intermediate level of the second column.

1 . A method for separating air by cryogenic distillation, the method comprising the steps of:
 (a) sending a cooled air that has previous been purified to remove water to a first column operating at a first pressure, where the cooled air is separated into a nitrogen-enriched gas and an oxygen-enriched liquid;   (b) withdrawing a liquid enriched in nitrogen relative to the air from the first column and sending said liquid enriched in nitrogen to the top of a second column that is connected thermally to the first column and operates at a second pressure, wherein the second pressure is lower than the first pressure;   (c) withdrawing a liquid enriched in oxygen relative to the air from the first column;   (d) withdrawing a gas enriched in argon relative to the air from the second column;   (e) at least partially vaporizing at least a portion of the oxygen-enriched liquid by heat exchange with the argon-enriched gas to form a vaporized oxygen-enriched fluid, and then sending the vaporized oxygen-enriched fluid to an intermediate level of the second column;   (f) returning at least one condensed portion of the argon-enriched gas to a third column, wherein the third column is also fed with an argon-enriched gas flow originating from the second column, wherein an argon-enriched top gas is withdrawn at a top portion of the third column, and an argon-depleted liquid is returned from the third column to the second column;   (g) sending a portion of the oxygen-enriched liquid to an overhead condenser of the third column;   (h) vaporizing the oxygen-enriched liquid sent to the overhead condenser in the overhead condenser and then sending the resulting oxygen-enriched vapor to the second column,   wherein the portion of the oxygen-enriched liquid that is sent to the overhead condenser of the third column has not been reheated against the argon-enriched gas flow.   
     
     
         2 . A method according to  claim 1 , wherein the vaporized oxygen-enriched fluid is at a pressure at least 1 bar greater than the pressure of the second column, and is expanded in a turbine and then sent to an intermediate level of the second column. 
     
     
         3 . A method according to  claim 1 , wherein at least one condensed portion of the argon-enriched gas is returned to the second column. 
     
     
         4 . A method according to  claim 1 , wherein the vapour produced is sent to the second column, by being mixed with the flow expanded in the turbine. 
     
     
         5 . A method according to  claim 1 , wherein the third column is disposed inside the second column and the at least one portion of the oxygen-enriched liquid is vaporized by heat exchange with the argon-enriched gas inside the second column. 
     
     
         6 . A method according to  claim 1 , wherein the argon-enriched gas sent to the exchanger in which the heat exchange takes place has a condensation temperature greater than the vaporization temperature of the oxygen-enriched liquid in the exchanger. 
     
     
         7 . An apparatus for separating air by cryogenic distillation, the apparatus comprising:
 a first column operating at a first pressure;   a second column connected thermally to the first column and operating at a second pressure, which is lower than the first pressure;   a heat exchanger;   means for sending cooled air that has been purified to remove water, to the first column, operating at a first pressure, where the cooled air is separated into a nitrogen-enriched gas and an oxygen-enriched liquid;   means for withdrawing a liquid enriched in nitrogen relative to the air from the first column;   means for sending the nitrogen-enriched liquid to the top of the second column;   means for withdrawing a liquid enriched in oxygen relative to the air from the first column;   means for withdrawing a gas enriched in argon relative to the air from the second column;   means for sending a portion of the oxygen-enriched liquid to the heat exchanger for at least partial vaporization thereof by heat exchange with the argon-enriched gas;   means for sending the oxygen-enriched liquid vaporized in the heat exchanger to an intermediate level of the second column;   a third column;   means for sending at least one condensed portion of the argon-enriched gas in the heat exchanger to the third column; and   means for sending an argon-enriched gas flow originating from the second column to the third column;   means for withdrawing an argon-enriched flow at the top of the third column;   means for sending an argon-depleted liquid from the third column to the second column;   means for sending a portion of the oxygen-enriched liquid to an overhead condenser of the third column; and   means for sending the vapour produced by vaporizing the oxygen-enriched liquid in the overhead condenser to the second column;   wherein the means for sending the portion of the oxygen-enriched liquid to the overhead condenser are connected directly to the first column without passing via the heat exchanger.   
     
     
         8 . The apparatus according to  claim 7 , further comprising a turbine connected to an intermediate level of the second column fed with the vaporized oxygen-enriched fluid. 
     
     
         9 . The apparatus according to  claim 7 , further comprising means for returning at least one condensed portion of the argon-enriched gas to the second column. 
     
     
         10 . The apparatus according to  claim 7 , further comprising means for sending the vapour produced to the second column, by being mixed with the flow expanded in the turbine. 
     
     
         11 . The apparatus according to  claim 7 , wherein the third column is disposed inside the second column, and the apparatus further comprises means for vaporizing the at least one portion of the oxygen-enriched liquid by heat exchange with the argon-enriched gas inside the second column. 
     
     
         12 . The apparatus according to  claim 7 , wherein the third column contains fewer than 50 theoretical stages. 
     
     
         13 . The apparatus according to  claim 7 , wherein the third column contains fewer than 10 theoretical stages.