Patent ID: 6702992
Filing Date: 2004-03-09
Classification: B01D,B01J,C07C,Y02P

Abstract:
Method for the modernisation of a plant for urea production comprising:a reactor (2) for urea synthesis; means (7, 8) for feeding ammonia and carbon dioxide to said reactor (2) for urea synthesis; a stripping unit (3) with carbon dioxide for subjecting a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving said reactor (2) 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 said flow comprising urea and residual carbamate in aqueous solution leaving said stripping unit (3) for separating the urea from the residual carbamate in aqueous solution; at least one vertical film condensation unit (4), comprising a tube bundle (23) for subjecting to partial condensation said flow comprising ammonia and carbon dioxide in vapour phase leaving said stripping unit (3), thus obtaining a liquid flow comprising carbamate in aqueous solution and a gaseous flow comprising ammonia and carbon dioxide in vapour phase; means (15, 14) 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 (2) for urea synthesis; means (7, 11, 14) for feeding a gaseous flow comprising one or more passivating agents, inert with respect to the synthesis reaction of urea, to said stripping unit (3) and from the stripping unit (3) to said reactor (2) for urea synthesis through said at least one condensation unit (4); said modernisation method being characterised in that it comprises the steps of: providing means (35) for feeding a minor portion of said flow comprising ammonia and carbon dioxide in vapour phase together with a minor portion of said gaseous flow comprising one or more passivating agents leaving said stripping unit (3) directly to said reactor (2) for urea synthesis; providing means (36) for feeding a major portion of said flow comprising ammonia and carbon dioxide in vapour phase together with a major portion of said gaseous flow comprising one or more passivating agents leaving said stripping unit (3) to said at least one condensation unit (4); providing in said at least one condensation unit (4) means (37) for subjecting to substantially total condensation said major portion of the flow comprising ammonia and carbon dioxide in vapour phase, obtaining a flow comprising urea and carbamate in aqueous solution.