Method and apparatus for air separation by cryogenic distillation

Abstract

The invention relates to a method for air separation by cryogenic distillation in a column system that includes at least a first column operating at a first pressure, and a second column operating at a second pressure that is lower than the first pressure, the head of the first column being thermally connected to the tank of the second column via a vaporizer-condenser, a first portion of a nitrogen-enriched gas is drawn from the head of the first column, compressed in a compressor having an inlet temperature no higher than −150° C. and condensed in the vaporizer-condenser, an oxygen-rich fluid is drawn from the lower portion of the second column and heated in the exchange line, a nitrogen-rich gas is drawn from the upper portion of the second column and heated in the exchange line, and a second portion of the nitrogen-enriched gas is expanded in a turbine without being compressed.

1 - 13 . (canceled) 
     
     
         14 . A process for separating air by cryogenic distillation in a system of columns comprising at least a first column operating at a first pressure and a second column operating at a second pressure below the first pressure, the top of the first column being thermally coupled to the bottom of the second column by a vaporizer-condenser wherein:
 i) cooling purified air in an exchange line and sending the purified air to the first column under conditions effective for separation;   ii) withdrawing a nitrogen-enriched gas from the top of the first column and divided into two portions, a first portion is compressed in a compressor having an inlet temperature of at most −150° C. and is condensed in the vaporizer-condenser;   iii) reheating a second portion of the nitrogen-enriched gas in the exchange line and expanded in an expansion turbine;   iv) sending an oxygen-enriched liquid or a liquid derived from this liquid from the bottom of the first column to the second column in order to be separated therein, without having been reheated against a nitrogen-enriched gas from the first column;   v) sending a nitrogen-enriched liquid from the top of the first column to the top of the second column;   vi) withdrawing an oxygen-rich fluid from the bottom of the second column and is reheated in the exchange line; and   vii) withdrawing a nitrogen-rich gas from the upper part of the second column and is reheated in the exchange line;   wherein the vaporizer-condenser being the only means of vaporizing liquid in the lower part of the second column,   wherein the second portion of the nitrogen-enriched gas is reheated in the exchange line and is expanded in the turbine, without having been compressed.   
     
     
         15 . The process as claimed in  claim 14 , wherein the oxygen-rich fluid contains at least 97% oxygen, or even at least 99% oxygen. 
     
     
         16 . The process as claimed in  claim 14 , wherein the nitrogen-enriched liquid sent from the first column to the second column is less rich in nitrogen than the nitrogen-enriched gas sent to the vaporizer-condenser. 
     
     
         17 . The process as claimed in  claim 14 , wherein all the gas condensed in the vaporizer-condenser is sent back as reflux to the first column. 
     
     
         18 . The process as claimed in  claim 14 , wherein the oxygen-rich fluid from step v) is an oxygen-rich gas. 
     
     
         19 . The process as claimed in  claim 18 , wherein the oxygen-rich gas is compressed after reheating in the exchange line to its production pressure. 
     
     
         20 . The process as claimed in  claim 14 , wherein the argon-enriched gas is withdrawn from the second column and sent to a third column in order to produce a flow richer in argon than the argon-enriched gas and a nitrogen-rich gas is withdrawn from the first column as product. 
     
     
         21 . The process as claimed in  claim 14 , wherein the compressor is driven by a motor and/or by a turbine that expands air or a nitrogen-enriched flow originating from the system of columns. 
     
     
         22 . The process as claimed in  claim 14 , according to first and second operating modes wherein, during the first operating mode, the compressor is shut down and at least one liquid is withdrawn from the system of columns as final product and during the second operating mode, the compressor operates and no liquid is withdrawn from the system of columns as final product or at least three times less liquid is withdrawn from the system of columns as during the first operating mode. 
     
     
         23 . An apparatus for separating air by cryogenic distillation that has a system of columns comprising at least a first column operating at a first pressure and a second column operating at a second pressure below the first pressure, the top of the first column being thermally coupled to the bottom of the second column by a vaporizer-condenser, an exchange line, means for sending purified air to be cooled into the exchange line and for sending the cooled air to the first column in order to be separated therein, means for withdrawing a nitrogen-enriched gas from the top of the first column, a compressor for compressing a first portion of the nitrogen-enriched gas having an inlet temperature of at most −150° C., means for sending a second portion of the nitrogen-enriched gas to the exchange line, a turbine for expanding the second portion of the nitrogen-enriched gas that is connected to the exchange line, means for sending the compressed gas to be condensed into the vaporizer-condenser, means for sending an oxygen-enriched liquid or a liquid derived from this liquid from the bottom of the first column to the second column in order to be separated therein, means for sending a nitrogen-enriched liquid from the top of the first column to the top of the second column, means for withdrawing an oxygen-rich fluid from the bottom of the second column, means for sending the oxygen-rich fluid to be reheated into the exchange line, means for withdrawing a nitrogen-rich gas from the upper part of the second column and for sending the gas withdrawn to be reheated into the exchange line, any means for reheating the oxygen-enriched liquid sent to the second column against a nitrogen-enriched gas from the first column and the vaporizer-condenser being the only means of vaporizing liquid in the lower part of the second column. 
     
     
         24 . The apparatus as claimed in  claim 23 , comprising an oxygen compressor for compressing the oxygen-rich gas, after reheating in the exchange line, to its production pressure. 
     
     
         25 . The apparatus as claimed in  claim 23 , comprising a third column , means for sending an argon-enriched gas from the second column to the third column, means for withdrawing a flow richer in argon than the argon-enriched gas from the third column and means for withdrawing a nitrogen-rich gas from the first column as product. 
     
     
         26 . The apparatus as claimed in  claim 23 , wherein the compressor is driven by a motor and/or by a turbine that expands air and/or the turbine that expands the second portion of the nitrogen-enriched flow.