Process for the cryogenic distillation of air

Abstract

Pressurized gaseous nitrogen (“GAN”) is produced in a process in which liquid nitrogen (“LIN”) is produced in a cryogenic air separation unit (“ASU”). The pressure of at least a portion of the LIN is increased to produce pressurized LIN. A fluid having an oxygen concentration at least equal to that of air is separated in an auxiliary cryogenic distillation column to produce nitrogen-rich overhead vapor and oxygen-enriched bottoms liquid. Heat and optionally mass is transferred between at least a portion of the nitrogen-rich overhead vapor and at least a portion of the pressurized LIN to produce nitrogen-rich liquid and pressurized GAN. At least a portion of said nitrogen-rich liquid is used as reflux to the ASU after suitable pressure adjustment. Such a process allows the production of high purity GAN without the loss of oxygen recovery typically observed in conventional pumped LIN cycles.

1 . A process for the production of pressurized gaseous nitrogen (“GAN”) comprising; 
 producing liquid nitrogen (“LIN”) in a cryogenic air separation unit (“ASU”);    increasing the pressure of at least a portion of said LIN to produce pressurized LIN;    separating a fluid having an oxygen concentration at least equal to that of air in an auxiliary cryogenic distillation column to produce nitrogen-rich overhead vapor and oxygen-enriched bottoms liquid;    transferring heat and optionally mass between at least a portion of said nitrogen-rich overhead vapor and at least a portion of said pressurized LIN to produce nitrogen-rich liquid and pressurized GAN; and    feeding at least a portion of said nitrogen-rich liquid as reflux to the ASU after suitable pressure adjustment.    
   
   
       2 . The process as claimed in  claim 1  wherein at least a portion of said oxygen-enriched liquid is fed to the ASU after suitable pressure adjustment.  
   
   
       3 . The process as claimed in  claim 1  wherein said fluid is air.  
   
   
       4 . The process as claimed in  claim 1  wherein said fluid is oxygen-rich fluid from the ASU.  
   
   
       5 . The process as claimed in  claim 1  wherein a portion of said nitrogen-rich liquid is used to reflux said auxiliary column.  
   
   
       6 . The process as claimed in  claim 1  wherein said portion of LIN is pumped to increase the pressure thereof.  
   
   
       7 . The process as claimed in  claim 1  wherein heat is exchanged indirectly between at least a portion of said nitrogen-rich overhead vapor and at least a portion of said pressurized LIN thereby condensing said nitrogen-rich overhead vapor to produce nitrogen-rich liquid and vaporizing said pressurized LIN to produce pressurized GAN.  
   
   
       8 . The process as claimed in  claim 7  wherein said heat is exchanged indirectly using a reboiler/condenser.  
   
   
       9 . The process as claimed in  claim 1  wherein heat and mass are transferred directly between at least a portion of said nitrogen-rich overhead vapor and at least a portion of said pressurized LIN to produce nitrogen-rich liquid and pressurized GAN.  
   
   
       10 . The process as claimed in  claim 9  wherein the auxiliary column has at least a main distillation zone and vapor/liquid contact promoting means provided above the main distillation zone, said process comprising contacting directly nitrogen-rich overhead vapor with pressurized LIN in said contact promoting means to produce said nitrogen-rich liquid and pressurized GAN.  
   
   
       11 . The process as claimed in  claim 10  wherein said contact promoting means is a further distillation zone.  
   
   
       12 . The process as claimed in  claim 1  wherein said fluid is gaseous.  
   
   
       13 . The process as claimed in  claim 1  wherein said fluid is liquid.  
   
   
       14 . The process as claimed in  claim 13  wherein at least a portion of said liquid is vaporized by indirect heat exchange against a process stream using a reboiler/condenser provided in the sump of said auxiliary column.  
   
   
       15 . The process as claimed in  claim 1  wherein the ASU comprises a higher pressure (“HP”) distillation column and a lower pressure (“LP”) distillation column, said HP column being thermally integrated with said LP column via an ASU reboiler/condenser, said process further comprising: 
 separating air in said HP column into HP nitrogen-rich overhead vapor and oxygen-rich bottoms liquid;    separating at least a portion of said oxygen-rich bottoms liquid in said LP column into LP nitrogen-rich overhead vapor and liquid oxygen (“LOX”);    cooling and at least partially condensing at least a portion of said HP nitrogen-rich overhead vapor in said ASU reboiler/condenser by indirect heat exchange against LOX to produce said LIN; and    refluxing said HP column with a portion of said LIN.    
   
   
       16 . The process as claimed in  claim 15  further comprising using a portion of said oxygen-rich bottoms liquid as at least a portion of said fluid after suitable pressure adjustment.  
   
   
       17 . The process as claimed in  claim 15  wherein the operating pressure of said auxiliary column is higher than the operating pressure of said LP column.  
   
   
       18 . The process as claimed in  claim 15  wherein the operating pressure of said auxiliary column is higher than the operating pressure of said HP column.  
   
   
       19 . The process as claimed in  claim 1  wherein the operating pressure of the auxiliary column is from about 0.10 MPa (1.0 bara) to about 3.0 MPa (30 bara).  
   
   
       20 . The process as claimed in  claim 1  wherein the pressurized GAN is produced at a pressure from about 0.15 MPa (1.5 bara) to about 2.5 MPa (25 bara).  
   
   
       21 . The process as claimed in  claim 1  wherein the purity of said pressurized GAN is from about 99.9 mol % nitrogen to about 99.9999 mol % nitrogen.  
   
   
       22 . Apparatus for the production of pressurized GAN comprising: 
 a cryogenic ASU comprising at least one distillation column, for producing LIN;    pressurizing means for pressurizing LIN;    conduit means for feeding LIN from the top of the column of the ASU producing said LIN to the pressurizing means;    an auxiliary cryogenic distillation column comprising a main distillation zone for separating a fluid having an oxygen concentration at least equal to that of air into nitrogen-rich overhead vapor and oxygen-enriched bottoms liquid;    transfer enabling means for enabling heat and optionally mass transfer between at least a portion of said nitrogen-rich overhead vapor and at least a portion of said pressurized LIN to produce nitrogen-rich liquid and pressurized GAN;    conduit means for feeding pressurized LIN from the pressurizing means to the transfer enabling means;    pressure reducing means for reducing the pressure of nitrogen-rich liquid to produce reduced pressure nitrogen-rich liquid;    conduit means for feeding nitrogen-rich liquid from the auxiliary column to the pressure reducing means; and    conduit means for feeding reduced pressure nitrogen rich liquid from said pressure reducing means to the ASU as reflux.    
   
   
       23 . Apparatus as claimed in  claim 22  wherein the transfer enabling means is condensing means for condensing at least a portion of nitrogen-rich overhead vapor by indirect heat exchange against pressurized LIN to produce nitrogen-rich liquid and pressurized GAN.  
   
   
       24 . Apparatus as claimed in  claim 22  wherein said transfer enabling means is vapor/liquid contact promoting means for promoting direct contact between nitrogen-rich overhead vapor and pressurized LIN, said contact promoting means being located within the auxiliary distillation column above the main distillation zone.  
   
   
       25 . Apparatus as claimed in  claim 24  wherein the contact promoting means is a further distillation zone in the auxiliary column.  
   
   
       26 . Apparatus as claimed in  claim 22  further comprising: 
 pressure reducing means for reducing the pressure of oxygen-enriched liquid to produce reduced pressure oxygen-enriched liquid;    conduit means for feeding oxygen-enriched liquid from the auxiliary column to the pressure reducing means; and    conduit means for feeding reduced pressure oxygen-enriched liquid from the pressure reducing means to the ASU.