Method for restarting an air separation unit

Abstract

A method and apparatus for restarting an air separation unit comprising a system of columns including at least a first distillation column and a heat exchanger positioned below the first column with its main axis vertical and with its cold end adjacent to the bottom of the first column, in which, during the restart, air is sent to the exchanger and at least one portion of the cooled air is sent in gaseous form to the atmosphere.

1 - 13 . (canceled) 
     
     
         14 . A method for restarting an air separation unit comprising a system of columns comprising at least a first distillation column and a heat exchanger by indirect exchange of heat for cooling the air purified of water and of carbon dioxide and intended to be separated in the first column by sending it to one end of the heat exchanger called the hot end and extracting it at the other end of the heat exchanger called the cold end, the exchanger also serving to heat at least two gases coming from the system of columns, the heat exchanger being sited below the first column with its main axis vertical and its cold end adjacent to the bottom of the first column, and
 i. during normal operation, at least part of the air cooled in the heat exchanger and intended to be distilled is sent to the system of columns, for example to the first column, and at least two gases from the system of columns are sent to the heat exchanger;   ii. while the unit is shut down, air cooled in the heat exchanger is no longer sent to the system of columns, and fluids are no longer sent from the system of columns to the heat exchanger; and   iii. during the restart, air is sent to the hot end of the heat exchanger and from the cold end of the heat exchanger in gaseous form to the system of columns, where it is separated, and at least one gas extracted from the system of columns is sent to the heat exchanger, wherein during the restart:
 a) the at least one part of the gaseous air coming from the cold end is divided into two, a first portion thereof is sent to the system of columns, and a second portion thereof is vented to the atmosphere, and/or 
 b) at least part of a gas extracted from the system of columns is vented to the atmosphere. 
   
     
     
         15 . The method as claimed in  claim 14 , wherein, during the restart, a flow of cooled air corresponding to at least 5%, preferably to at least 10% of the overall flow-rate of gases cooled in the heat exchanger during normal operation is vented to the atmosphere at the cold end of the exchanger. 
     
     
         16 . The method as claimed in  claim 14 , wherein, during the restart, a flow of gas coming from the system of columns and having a flow-rate corresponding to at least 5%, preferentially to at least 10% of the overall flow-rate of gas to be heated in the heat exchanger during normal operation is vented to the atmosphere at the cold end of the exchanger. 
     
     
         17 . The method as claimed in  claim 14 , wherein the gas extracted from the system of columns is enriched with nitrogen or with oxygen or with argon. 
     
     
         18 . The method as claimed in  claim 14 , wherein the heat exchanger is a brazed plate and fin exchanger. 
     
     
         19 . The method as claimed in  claim 14 , wherein:
 in normal operation, a first flow of air at a first pressure and a second flow of air at a second pressure lower than the first pressure are cooled in the heat exchanger and are sent respectively to the first column of the system of columns and to a second column of the system of columns operating at a lower pressure than the first column, and   during the restart, the flow-rate of the first flow is smaller compared to that sent during normal operation, or even zero and the second flow is divided in two, a portion being sent to the second column and the other portion being vented to the atmosphere.   
     
     
         20 . The method as claimed in  claim 14 , wherein the system of columns comprises a single column which is the first column, and in normal operation all the air that is to be separated is sent to the first column, in a shutdown, no flow of air is sent to the first column and during the restart, the air coming from the cold end is divided in two, and a first portion thereof is sent to the first column and a second portion thereof is vented into the air. 
     
     
         21 . The method as claimed in  claim 14 , wherein, during normal operation and/or during a restart or during a shutdown, part of the air or of a fluid extracted from the system of columns is vented to the atmosphere if, and preferably only if, the pressure in a column of the system of columns exceeds a threshold. 
     
     
         22 . The method as claimed in  claim 14 , wherein, during the restart, the first portion is sent to the second column. 
     
     
         23 . The method as claimed in  claim 14 , wherein, during the restart, the at least one part of the gaseous air coming from the cold end is divided into two, a first portion thereof is sent in gaseous form to the system of columns and a second portion thereof is sent in gaseous form to the atmosphere. 
     
     
         24 . A cryogenic-distillation air separation unit comprising a system of columns comprising at least a first distillation column, a heat exchanger using indirect exchange of heat to cool the air intended to be separated in the first column, means for sending air to one end of the heat exchanger called the hot end, means for extracting the cooled air from the other end of the heat exchanger called the cold end, these means being connected to the system of columns, means for sending at least two fluids coming from the system of columns to be heated in the exchanger, the heat exchanger being sited below the first column with its main axis vertical and its cold end adjacent to the bottom of the first column, characterized in that it comprises means for sending to the atmosphere, in gaseous form:
 a. air cooled in the heat exchanger, the venting means for venting to the atmosphere being connected to the means for extracting the cooled air from the other end of the heat exchanger; and/or   b. at least a portion of one of the at least two fluids produced by the system of columns upstream of the heat exchanger.   
     
     
         25 . The unit as claimed in  claim 24  further comprising means for preventing transfer of fluid between the heat exchanger and the air processing means and between the heat exchanger and the means for venting at least one product from the system to the atmosphere in the event of a distillation shutdown. 
     
     
         26 . The unit as claimed in  claim 24 , wherein the means for venting to the atmosphere air cooled in the heat exchanger are connected to the first column or to the second column.