Integrated Process for Urea and Melamine Production

Abstract

In an integrated process for urea and melamine production, urea is produced in a urea plant ( 10 ) comprising a high pressure urea synthesis section ( 11 ) from which an aqueous solution comprising urea, ammonium carbamate and ammonia is obtained and a urea recovery section ( 21 ) operating at low pressure, and melamine is produced in a melamine plant ( 40 ) wherein off-gases resulting as by-products of the melamine synthesis are discharged from said plant at a medium pressure and recycled to the high-pressure urea synthesis section ( 11 ).

1 . An integrated process for urea and melamine production, wherein urea is produced in a urea plant comprising a high-pressure urea synthesis section from which a aqueous solution comprising urea, ammonium carbamate and ammonia is obtained, and a low-pressure urea recovery section, and melamine is produced in a melamine plant wherein off-gases resulting as by-products of the melamine synthesis are discharged therefrom at a medium pressure and recycled to said high pressure urea synthesis section, comprising the steps of:
 feeding at least a part of said aqueous solution comprising urea, ammonium carbamate and ammonia coming from said urea synthesis section to a medium-pressure treatment section of the urea plant for recovering ammonium carbamate and ammonia contained in it;   subjecting said part of aqueous solution comprising urea, ammonium carbamate and ammonia to dissociation in said medium-pressure treatment section, obtaining a urea aqueous solution and a vapour phase comprising ammonia, carbon dioxide and water;   feeding said urea aqueous solution obtained from dissociation in said treatment section to a decomposer of a urea recovery section operating at a predetermined low pressure,   subjecting said urea an solution to decomposition in said decomposer of said urea recovery section, obtaining a concentrated urea solution and a second vapour phase comprising ammonia, carbon dioxide and water;   subjecting said second vapour phase to condensation in a condenser of said urea recovery section in fluid communication with said decomposer, obtaining a recycle ammonium carbamate aqueous solution;   feeding said off-gases coming from said melamine plant and said recycle ammonium carbamate solution to a condensation section of said medium-pressure treatment section of the urea plant;   condensing said offs-gases with said recycle carbamate aqueous solution in said condensation unit of the medium-pressure treatment section, obtaining a concentrated carbamate aqueous solution; and   recycling said carbamate aqueous solution to said high pressure urea synthesis section.   
   
   
       2 . The process according to  claim 1 , wherein said medium-pressure treatment section of the urea plant operates to pressure substantially equal or lower than that of said off-gases. 
   
   
       3 . The process according to  claim 1 , wherein said recycle carbamate solution coming from the urea recovery section is fed directly to said medium-pressure treatment section for condensing said off-gases. 
   
   
       4 . The process according to  claim 1 , wherein said recycle carbamate solution coming from the urea recovery section is compressed to a pressure substantially corresponding to the operating pressure of said medium-pressure treatment section for condensing said off-gases, before feeding it into said condensation unit. 
   
   
       5 . The process according to  claim 1 , wherein said concentrated carbamate aqueous solution coming from said condensation unit of the medium-pressure treatment section is compressed to a pressure substantially corresponding to the operating pressure of said urea synthesis section, before feeding it to said urea synthesis section. 
   
   
       6 . The process according to  claim 1 , wherein said condensation unit of the medium-pressure treatment section comprises a single condenser and in that the process further comprises the steps of:
 feeding said vapour phase comprising ammonia, carbon dioxide and water, said off-gases and said recycle ammonium carbamate solution in said single condenser of the medium-pressure treatment section;   condensing said vapour phase comprising ammonia, carbon dioxide and water as well as said off-gases with said recycle ammonium carbamate solution in said single condenser of said medium-pressure treatment section, obtaining a concentrated ammonium carbamate aqueous solution; and   recycling said concentrated ammonium carbamate aqueous solution to said high pressure urea synthesis section,   
   
   
       7 . The process according to  claim 1 , wherein said condensation unit of the medium-pressure treatment section comprises a first condenser and a second condenser in fluid communication to each other and in that the process further comprises the steps of:
 feeding said off-gases coming from said melamine plant and said recycle ammonium carbamate solution to said first condenser of the medium-pressure treatment section of the urea plant;   condensing said off-gases with said recycle carbamate aqueous solution in said first condenser of the medium-pressure treatment section, obtaining a first concentrated ammonium carbamate aqueous solution;   feeding said first concentrated ammonium carbamate aqueous solution in said second condenser of the medium-pressure treatment section;   feeding said vapour phase comprising ammonia, carbon dioxide and water obtained from dissociation of said part of the aqueous solution comprising urea, ammonium carbamate and ammonia, in said second condenser of the medium-pressure treatment section; and   condensing said vapour phase comprising ammonia, carbon dioxide and water with said first concentrated ammonium carbamate aqueous solution in said second condenser of the medium-pressure treatment section, obtaining a second concentrated ammonium carbamate aqueous solution; and   recycling said second concentrated ammonium carbamate aqueous solution to said high pressure urea synthesis section.   
   
   
       8 . The process according to  claim 1 , wherein it comprises the further steps of:
 feeding carbon dioxide to said condenser of said urea recovery section; and   subjecting said carbon dioxide and said second vapour phase to condensation in said condenser of said urea recovery section, obtaining a recycle ammonium culminate aqueous solution.   
   
   
       9 . The process according to  claim 8 , characterized by feeding a carbon dioxide amount from 1 to 10 wt. % of the totality of feed carbon dioxide to said condenser of said urea recovery section. 
   
   
       10 . The process according to  claim 1 , wherein said part of aqueous solution comprising urea, ammonium carbamate and ammonia fed to said treatment section operating at medium pressure is comprised between 10 and 50 wt. % of said aqueous solution comprising urea, ammonium carbamate and ammonia obtained in said synthesis section. 
   
   
       11 . The process according to  claim 1 , wherein said medium pressure of the treatment section is comprised between 10 and 70 bar. 
   
   
       12 . An integrated plant for urea and melamine production, wherein urea is produced in a urea plant comprising a high-pressure urea synthesis section and a low-pressure urea recovery section comprising a decomposer and a condenser, said sections being in fluid communication to each other, and melamine is produced in a melamine plant wherein off-gases resulting as by-products of the melamine synthesis are discharged from said plant at a medium pressure and recycled to said high-pressure urea synthesis section, the plant comprising:
 a medium-pressure treatment section of the urea plant of a part of the urea solution produced in said synthesis section, comprising a dissociator and a condensation unit;   connecting means between said melamine synthesis section and said condensation unit of the medium-pressure treatment section for feeding said off-gases coming from said melamine synthesis section to said condensation unit of the medium-pressure treatment section;   connecting means between said condenser of the urea recovery section and said condensation unit of the medium-pressure treatment section for feeding a recycle ammonium carbamate coming from said condenser of the urea recovery section to said condensation unit of the medium-pressure treatment section; and   connecting means between said dissociator of the medium-pressure treatment section and said decomposer of the low-pressure urea recovery section for feeding a urea aqueous solution obtained from dissociation in said treatment section to said decomposer of the urea recovery section.   
   
   
       13 . The integrated plant according to  claim 12 , wherein it further comprises connecting means between said dissociator and said condensation unit of the medium-pressure treatment section for feeding a vapour phase comprising ammonia, carbon dioxide and water from said dissociator to said condenser of the medium-pressure treatment section. 
   
   
       14 . The integrated plant according to  claim 12 , comprising a first compressing section located, in fluid communication, between the condenser of the urea recovery section and the condensation unit of the medium-pressure treatment section for compressing a recycle carbamate solution coming from said condenser of the urea recovery section to a pressure corresponding to the operative pressure of said condensation unit of the medium-pressure treatment section. 
   
   
       15 . The integrated plant according to  claim 12 , wherein it further comprises a second compressing section located, in fluid communication, between said condensation unit of the medium-pressure treatment section and said high-pressure urea synthesis section for compressing a concentrated carbamate solution coming from said condensation unit of the medium-pressure treatment section to a pressure corresponding to the operative pressure of said high-pressure urea synthesis section. 
   
   
       16 . The integrated plant according to  claim 12 , wherein it further comprises connecting means for feeding feed carbon dioxide to said condenser of the low-pressure area recovery section. 
   
   
       17 . The integrated plant according to  claim 12 , characterized in that said condensation unit of the medium-pressure treatment section comprises a single condenser. 
   
   
       18 . The integrated plant according to  claim 17 , wherein said single condenser of the medium-pressure treatment section comprises a conventional tube bundle, in fluid communication, on the tube side, with the concentrated urea solution exiting the decomposer of the low-pressure urea recovery section and, on the shell side, with the vapour phase comprising ammonia, carbon dioxide and water coming from the dissociator of the medium-pressure treatment section, with off-gases coming from the melamine synthesis section as well as with the recycle carbamate aqueous solution coming from the condenser of the low-pressure urea recovery section. 
   
   
       19 . The integrated plant according to  claim 12 , wherein said condensation unit of the medium-pressure treatment section comprises a first condenser and a second condenser and in that it further comprises:
 connecting means between said melamine synthesis section and said first condenser of the medium-pressure treatment section for feeding said off-gases coming from said melamine synthesis section to said first condenser of the medium-pressure treatment section;   connecting means between said condenser of the urea recovery section and said first condenser of the medium-pressure treatment section for feeding a recycle ammonium carbamate aqueous solution coming from said condenser of the urea recovery section to said first condenser of the medium-pressure treatment section;   connecting means between said first condenser and said second condenser of the medium-pressure treatment section for feeding a carbamate aqueous solution coming from said first condenser to said second condenser; and   connecting means between said dissociator and said second condenser of the medium-pressure treatment section for feeding a vapour phase comprising ammonia, carbon dioxide and water from said dissociator to said second condenser of the medium-pressure treatment section.   
   
   
       20 . The integrated plant according to  claim 19 , wherein said first condenser of the medium-pressure treatment section comprises a conventional tube bundle, in fluid communication, on the tube side, with the concentrated urea solution exiting the decomposer of the low-pressure urea recovery section and, on the shell side, with off-gases coming from the melamine synthesis section as well as with the recycle carbamate aqueous solution coming from the condenser of the low-pressure urea recovery section. 
   
   
       21 . The integrated plant according to  claim 12 , comprising connecting means between said single condenser or said second condenser of the medium-pressure treatment section and said urea synthesis section for feeding a carbamate solution coming from said single condenser or said second condenser in said urea synthesis section.