Process and plant for the production of an aqueous solution comprising urea
View Patent ↗The present invention regards a process for the production of an aqueous solution comprising urea for use in the removal of nitrogen oxides from combustion gas or fumes, the process being characterized in that it comprises the steps of: —subjecting at least a part of a urea-concentrated aqueous solution comprising residual free ammonia, obtained directly from or downstream of a recovery section of a plant for the production of urea, to washing with carbon dioxide, so obtaining a first vapor phase comprising carbon dioxide and optionally ammonia and a urea-concentrated aqueous solution comprising carbamate and essentially lacking in free ammonia, and —diluting said urea-concentrated aqueous solution comprising carbamate and essentially lacking in free ammonia with water until the desired concentration of urea in aqueous solution is reached.
1. A process for the production of an aqueous solution comprising urea for use in the removal of nitrogen oxides from combustion gas or fumes, comprising the steps of:
obtaining a first urea-concentrated aqueous solution comprising residual free ammonia directly from or downstream of a recovery section of a plant for the production of urea;
washing with carbon dioxide at least a part of said first urea-concentrated aqueous solution comprising residual free ammonia, thus converting an amount of said residual free ammonia into carbamate in the solution and obtaining a first vapour phase comprising carbon dioxide, and a second urea-concentrated aqueous solution comprising carbamate that is essentially lacking in free ammonia, and
diluting said second urea-concentrated aqueous solution comprising carbamate and essentially lacking in free ammonia-with water until a desired concentration of urea in aqueous solution is reached.
2. The process according to claim 1 , wherein said first urea-concentrated aqueous solution comprising residual free ammonia is obtained directly from a urea recovery section of a urea plant.
3. The process according to claim 1 , wherein said first urea-concentrated aqueous solution comprising residual free ammonia comes from an evaporation section arranged downstream of said recovery section of the urea plant.
4. The process according to claim 3 , wherein said first urea-concentrated aqueous solution comprising residual free ammonia is obtained from an evaporator of said evaporation section used to concentrate a urea-concentrated aqueous solution comprising residual free ammonia coming from said recovery section of the urea plant.
5. The process according to claim 1 , wherein said desired concentration of urea in aqueous solution is in the range of 15%-35% by weight on the weight of the solution.
6. The process according to claim 1 , comprising the step of subjecting an aqueous solution comprising urea, ammonium carbamate and free ammonia obtained in a synthesis section of the urea plant to decomposition of the ammonium carbamate, optionally in the presence of a stripping agent, in said urea recovery section, and obtaining a second vapour phase comprising ammonia and carbon dioxide and said first urea-concentrated aqueous solution comprising residual free ammonia.
7. The process according to claim 1 , further comprising the step of feeding said first vapour phase comprising carbon dioxide to said urea recovery section.
8. The process according to claim 6 , further comprising the step of recycling said second vapour phase comprising ammonia and carbon dioxide, in condensate form, to the urea synthesis section.
9. The process according to claim 1 , comprising subjecting the entire said first urea-concentrated aqueous solution comprising residual free ammonia to washing with carbon dioxide.
10. The process according to claim 3 , further comprising the step of subjecting a part of said first urea-concentrated aqueous solution comprising residual free ammonia not subjected to washing with carbon dioxide to evaporation in said evaporation section, so obtaining melted urea.
11. The process according to claim 1 , wherein said first vapour phase containing carbon dioxide additionally contains an amount of ammonia.