Method for regulating a device for separating air by cryogenic distillation

Abstract

In a method for regulating a device for separating air by cryogenic distillation, during the change from a low-flow mode to a high-flow mode, two flow rates of reflux sent to the low-pressure column of a double column are increased more rapidly than the flow rate of gaseous air sent to the medium-pressure column of the double column in order to ensure the stability of the oxygen content in the argon bulge of the low-pressure column during the mode change.

1 . A method for regulating a device for separating air by cryogenic distillation in a device comprising a first column operating at a first pressure and a second column operating at a second pressure lower than the first pressure, the bottom of the second column being heated by a gas from the first column, and a third column, wherein
 sending cooled, purified gaseous air to the first column;   sending a first, oxygen-enriched, liquid from the bottom of the first column to the second column;   sending a second liquid that is liquefied air or that is withdrawn from an intermediate level of the first column to the second column;   sending a third nitrogen-enriched liquid and optionally a fourth nitrogen-enriched liquid from an upper part of the first column to the second column;   withdrawing an oxygen-enriched fluid from the second column;   sending an argon-enriched gas from the second column to the third column for separation therein;   withdrawing a fluid enriched in argon compared to the argon-enriched gas that feeds the third column at the top of the third column; and   operating the device in two modes including a first mode in which the gaseous air sent to the first column has a first flow rate and a second mode in which the gaseous air sent to the first column has a second flow rate that is greater than the first flow rate,   wherein:
 i) upon a condition in which the flow rate of gaseous air sent to the first column increases by at least n % per minute when changing from the first mode to the second mode, the flow rate of the first liquid arriving in the second column is increased by at least % per minute, where x is non-zero, and the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where x′ is non-zero, during at least part of a period during which the increase in the flow rate of air is at least n % per minute, and/or 
 ii) upon a condition in which the flow rate of gaseous air sent to the first column reduces by at least n % per minute when changing from the second mode to the first mode, the flow rate of the first liquid arriving in the second column is reduced by at least % per minute, where y is non-zero, and the flow rate of the second liquid arriving in the second column is reduced by at least % per minute, where y′ is non-zero, during at least part of a period during which the reduction in the flow rate of air is at least n % per minute, 
   wherein n is equal to or greater than 1.   
     
     
         2 . The method according to  claim 1 , wherein the first, second, third and optionally fourth liquids are sent to the second column without passing through a storage capacity. 
     
     
         3 . The method according to  claim 1 , wherein
 i) upon a condition in which the flow rate of gaseous air sent to the first column increases by at least n % per minute when changing from the first mode to the second mode, the flow rate of the third liquid arriving in the second column sent from the first column to the second column is reduced by at least % per minute, where w is non-zero, and the flow rate of the fourth liquid arriving in the second column from the first column is optionally reduced by at least % per minute, where w′ is non-zero, during at least part of the period during which the increase in the flow rate of air is at least n % per minute, and/or   ii) upon a condition in which the flow rate of gaseous air sent to the first column reduces by at least n % per minute when changing from the second mode to the first mode, the flow rate of the third liquid sent to the second column from the first column is increased by at least % per minute, where z is non-zero, and the flow rate of the fourth liquid sent to the second column from the first column is optionally increased by at least % per minute, where z′ is non-zero, during at least part of the period during which the reduction in the flow rate of air is at least n % per minute.   
     
     
         4 . The method according to  claim 3 , wherein for feature i) the flow rate of the third and/or fourth liquid sent to the second column is reduced, by increasing the flow rate of a part of the third and/or fourth liquid produced as a final product, compared to this flow rate during the second mode, or part of the third and/or fourth liquid is discharged as a final product whereas none is produced during the second mode. 
     
     
         5 . The method according to  claim 3 , wherein for feature i), the flow rate of the third and/or fourth liquid withdrawn from the first column is reduced by at least % per minute and at least % per minute respectively. 
     
     
         6 . The method according to  claim 1 , wherein the flow rate of air sent to the first column increases or reduces during a time t between the first and second modes, at least equal to quarter of an hour, and
 i) the flow rate of the first liquid arriving in the second column is reduced by at least % per minute, where x is non-zero, and/or the flow rate of the second liquid arriving in the second column is reduced by at least % per minute, where x′ is non-zero, during at least the first quarter of the time t and at most three quarters of the time t, and/or   ii) the flow rate of the first liquid arriving in the second column is increased by at least % per minute, where y is non-zero, and/or the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where y′ is non-zero, during at least the first quarter of the time t and at most three quarters of the time t.   
     
     
         7 . The method according to  claim 1 , wherein while, preferably only while, the reduction in the flow rate of gaseous air sent to the first column is greater than n %/minute, and
 a. at least part of the argon-enriched fluid is sent to a denitrogenation column with an increased flow rate compared to the flow rate during the second mode and/or   b. at least part of the argon-enriched fluid is sent in liquid or gaseous form to the top of the second column or mixed with a nitrogen-enriched gas withdrawn from the second column and/or   c. at least part of the argon-enriched fluid is stored in liquid form in a storage capacity attached to the third column and/or   d. at least part of the argon-enriched fluid is vented in gaseous form and/or   e. at least part of the argon-enriched fluid is drained in liquid form.   
     
     
         8 . The method according to  claim 7 , wherein the argon-enriched fluid is sent after, preferably only after, the flow rates of the first and second liquids have been reduced by at least % per minute and at least % per minute respectively while the reduction in the flow rate of gaseous air sent to the first column is at least n % per minute,
 wherein n is equal to or greater than 1.   
     
     
         9 . The method according to  claim 1 , wherein the argon-enriched fluid contains at least 97 mol % argon. 
     
     
         10 . The method according to  claim 1 , wherein when changing from the second mode to the first mode, the second flow rate of gaseous air entering the first column being equal to m % of the first flow rate of air entering the first column, m being greater than 100, the flow rate of the first liquid has a value of M % of V during the second mode and has a value of V during the period between the second and first modes, M preferably being greater than m. 
     
     
         11 . The method according to  claim 1 , comprising the steps of:
 detecting the flow rate of gaseous air sent to the first column; and   regulating the flow rate of the first and/or second liquid as a function of the flow rate of air detected upon a condition in which the flow rate of gaseous air sent to the first column detected increases by at least n % per minute when changing from the first mode to the second mode,
 i) the flow rate of the first liquid arriving in the second column is modified by at least % per minute, where x is non-zero, and/or 
 ii) the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where x′ is non-zero, during at least part of a period during which the increase in the flow rate of air is at least n % per minute. 
   
     
     
         12 . The method according to  claim 1 , comprising the steps of
 detecting the flow rate of gaseous air sent to the first column, and   regulating the flow rate of the first and/or second liquid as a function of the flow rate of air detected upon a condition in which the flow rate of gaseous air sent to the first column detected reduces by at least n % per minute when changing from the second mode to the first mode,
 i) the flow rate of the first liquid arriving in the second column is reduced by at least % per minute, where y is non-zero, and/or 
 ii) the flow rate of the second liquid arriving in the second column is reduced by at least % per minute, where y′ is non-zero, during at least part of a period during which the reduction in the flow rate of air is at least n % per minute. 
   
     
     
         13 . The method according to  claim 1 , wherein upon a condition in which the flow rate of gaseous air sent to the first column increases by at least n % per minute when changing from the first mode to the second mode, the flow rate of the first liquid arriving in the second column is increased by at least % per minute, where x is non-zero, and the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where x′ is non-zero, during at least part of a period during which the increase in the flow rate of air is at least n % per minute.