Method for modernizing a urea production plant

1. Method for the modernisation of a plant for urea production of the type comprising:

a reactor for urea synthesis;

means for feeding ammonia and carbon dioxide to the reactor for urea synthesis;

a stripping unit with carbon dioxide for subjecting a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to a treatment of partial decomposition of carbamate and partial separation of free ammonia, thus obtaining a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising urea and residual carbamate in aqueous solution, respectively;

a recovery section for the flow comprising urea and residual carbamate in aqueous solution leaving the stripping unit for separating urea from the residual carbamate in aqueous solution;

at least one condensation unit of the film type, for subjecting to partial condensation said flow comprising ammonia and carbon dioxide in vapour phase leaving said stripping unit, thus obtaining a liquid flow comprising carbamate in aqueous solution and a gaseous flow comprising ammonia and carbon dioxide in vapour phase;

means for respectively feeding the flow comprising carbamate in aqueous solution and the gaseous flow comprising ammonia and carbon dioxide in vapour phase to said reactor for urea synthesis,

said modernization method further comprising the step of:

providing in said at least one condensation unit means for subjecting to substantially total condensation at least a portion of a flow comprising ammonia and carbon dioxide in vapour phase leaving said stripping unit, obtaining a flow comprising urea and carbamate in aqueous solution,

and being characterized by comprising the steps of:

providing a second stripping unit,

providing means for feeding, a first portion of the flow of reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to said stripping unit;

providing means for feeding a second portion of the flow of reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to said second stripping unit; and

providing means for feeding at least a portion of a flow comprising ammonia and carbon dioxide in vapour phase leaving said second stripping unit directly to the synthesis reactor.

2. Method according to claim 1 , wherein it further comprises the steps of:

providing means for feeding the entire flow comprising ammonia and carbon dioxide in vapour phase leaving said stripping unit to the bottom of said at least one condensation unit;

providing means for feeding the entire flow comprising ammonia and carbon dioxide in vapour phase leaving said second stripping unit directly to the reactor.

3. Method according to claim 1 , wherein it further comprises the step of:

providing means for feeding the flow comprising urea and residual carbamate in aqueous solution leaving said second stripping unit to said urea recovery section.

4. Method according to claim 1 , wherein said second stripping unit is of the type using carbon dioxide as a stripping agent and in that it further comprises the step of:

providing means for sending a portion of feed carbon dioxide to said second stripping unit.

5. Process for producing urea wherein it comprises the steps of:

reacting ammonia and carbon dioxide in a reaction space, obtaining a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution;

feeding a first portion of the reaction mixture to a stripping section with carbon dioxide and subjecting said first portion to a treatment of partial decomposition of carbamate and partial separation of free ammonia, obtaining a first flow comprising ammonia and carbon dioxide in vapour phase and a first flow comprising urea and residual carbamate in aqueous solution;

feeding a second portion of the reaction mixture to a second stripping section and subjecting said second portion to a treatment of partial decomposition of carbamate and partial separation of free ammonia, obtaining a second flow comprising ammonia and carbon dioxide in vapour phase and a second flow comprising urea and residual carbamate in aqueous solution

feeding said first flow and said second flow comprising urea and residual carbamate in aqueous solution to an urea recovery section;

feeding at least a portion of said first flow comprising ammonia and carbon dioxide in vapour phase to the at least one condensation unit and subjecting, said at least a portion to a substantially total condensation, obtaining a flow comprising urea and carbamate in liquid phase;

feeding at least a portion of said second flow comprising ammonia and carbon dioxide in vapour phase directly to the reaction space, and

feeding the flow comprising urea and carbamate in liquid phase to the reaction space.

6. Process according to claim 5 , wherein said first flow comprising ammonia and carbon dioxide in vapour phase coming from the stripping unit is fed to the at least one condensation unit and said second flow comprising ammonia and carbon dioxide in vapour phase coming from said second stripping unit is fed to the reaction space.

7. Process according to claim 5 , wherein said second portion of reaction mixture comprising urea, carbamate and free ammonia in aqueous solution fed to the second stripping section is a minor portion of the total flow of reaction mixture leaving the reaction space.

8. Process according to claim 7 , wherein said minor portion fed to the second stripping unit is about one third of the total flow of reaction mixture leaving the reaction space.

9. Plant for urea production, comprising:

a reactor for urea synthesis;

means for feeding ammonia and carbon dioxide to the reactor for urea synthesis;

a first stripping unit with carbon dioxide for subjecting a first portion of the reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to a treatment of partial decomposition of carbamate and partial separation of free ammonia, obtaining a first flow comprising ammonia and carbon dioxide in vapour phase and a first flow comprising urea and residual carbamate in aqueous solution;

at least one condensation unit of the submerged type; wherein it further comprises:

a second stripping unit,

means for feeding a first portion of the flow of reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to said first stripping unit;

means for feeding a second portion of the flow of reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to said second stripping unit, and

means for feeding at least a portion of a flow comprising ammonia and carbon dioxide in vapour phase leaving said second stripping unit directly to the synthesis reactor.

10. Plant according to claim 9 , wherein it comprises:

means for feeding the flow comprising ammonia and carbon dioxide in vapour phase leaving said stripping unit directly to said at least one condensation unit of the submerged type, and

means for feeding the flow comprising ammonia and carbon dioxide in vapour phase leaving said second stripping unit directly to the reactor.

11. Plant according to claim 9 , wherein it further comprises:

means for feeding the flow comprising urea and residual carbamate in aqueous solution leaving said second stripping unit to said urea recovery section.

12. Plant according to claim 9 , wherein said second stripping unit is of the type using carbon dioxide as a stripping agent and in that it further comprises:

means for sending a portion of feed carbon dioxide to said second stripping unit.