Patent ID: 7619114

Claim:
Method for revamping a urea plant of the type comprising: a urea synthesis reactor; means for feeding ammonia and carbon dioxide to the reactor for urea synthesis; a thermal stripping unit for subjecting a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution coming out from the reactor to a partial decomposition treatment of the carbamate and partial separation of the free ammonia, obtaining a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising urea, residual carbamate and ammonia in aqueous solution; a section operating at medium pressure for subjecting said flow comprising carbamate, urea and ammonia in aqueous solution to a substantially total separation treatment of the ammonia and partial decomposition of the carbamate; a recovery section operating at low pressure of a flow comprising urea and residual carbamate obtained in said section operating at medium pressure for the separation of the urea from the carbamate in aqueous solution, at least one horizontal condensation unit for subjecting the flow comprising ammonia and carbon dioxide in vapour phase coming out from the stripping unit to substantially total condensation, obtaining a flow of liquid comprising carbamate in aqueous solution; means for feeding the flow of liquid comprising carbamate in aqueous solution to the urea synthesis reactor; said revamping method being characterised in that it comprises the steps of: transforming said at least one horizontal condensation unit into a vertical condensation unit of the “submerged” type, comprising a tube bundle having a plurality of tubes for the condensation of carbamate, providing means for feeding said flow comprising ammonia and carbon dioxide in vapour phase and said condensation liquid comprising carbamate simultaneously and independently in each of the tubes of said tube bundle of said vertical condensation unit with circulation of said flow comprising ammonia and carbon dioxide in vapour phase and said condensation liquid comprising carbamate inside said tubes in equicurrent from the bottom towards the top, and providing means for feeding at least one part of the feed carbon dioxide into said stripping unit for use as stripping agent of said reaction mixture comprising urea, carbamate and free ammonia in aqueous solution coming out from the reactor.