A process for producing low-biuret urea

Abstract

A process for purifying a urea-containing aqueous stream, such as the aqueous stream from the recovery section of a urea plant, comprising a step of removing biuret from the urea-containing stream by reverse osmosis in one or more reverse osmosis stages.

1 . A process for purifying a urea-containing aqueous stream comprising a step of removing biuret from the urea-containing stream by reverse osmosis, wherein the urea-containing stream is an aqueous solution of urea obtained from a recovery section of a urea plant, preferably containing 60 to 90 wt % of urea. 
     
     
         2 . The process according to claim, wherein the reverse osmosis is performed with a thin-film composite membrane. 
     
     
         3 . The process according to of  claim 1 , wherein the reverse osmosis is performed with the urea-containing stream having a temperature of 60° C. to 90° C., preferably 70° C. to 80° C. 
     
     
         4 . The process according to  claim 1 , wherein the reverse osmosis is performed with one or more reverse osmosis stages in a cascade. 
     
     
         5 . The process according to  claim 4 , wherein the difference of pressure across the or each stage of the reverse osmosis process is 30 bar to 70 bar. 
     
     
         6 . The process according to  claim 1 , wherein the process produces a purified solution with a content of biuret lower than the input solution and having the same or substantially the same water to urea ratio as the input solution. 
     
     
         7 . The process according to  claim 1 , wherein the aqueous urea solution contains at least 25% wt of urea. 
     
     
         8 . The process comprising:
 reacting ammonia and carbon dioxide under urea-forming conditions and urea synthesis pressure in a urea synthesis section to form a urea-containing reaction effluent;   processing said urea-containing reaction effluent in a recovery section, including one or more recovery stages at a recovery pressure lower than said urea synthesis pressure, to remove unreacted ammonia and carbon dioxide from the reaction effluent and obtain a urea aqueous solution;   purifying said urea aqueous solution to remove biuret with a process according to  claim 1 .   
     
     
         9 . The process according to claim wherein all or some of the urea aqueous solution withdrawn from the recovery section is stored in a urea solution tank and the urea aqueous solution subject to said reverse osmosis is taken from said tank. 
     
     
         10 . The process according to  claim 9 , including a step of flash or pre-evaporation at a subatmospheric pressure of the urea aqueous solution before said solution is stored in said tank. 
     
     
         11 . The process according to  claim 9 , including a step of evaporation to remove water from a purified urea solution obtained with the reverse osmosis process. 
     
     
         12 . The process according to  claim 8 , wherein the osmosis process produces a permeate, which is a purified low-biuret urea-containing solution, and a retentate, which contains biuret removed from the input solution, and wherein at least part of said retentate is recycled to the recovery section. 
     
     
         13 . The process according to  claim 12 , wherein the retentate recycled to the recovery section is used in a condensation step of CO2- and ammonia-containing vapours as a means to improve condensation. 
     
     
         14 . The process according to  claim 12 , wherein the flow rate of the retentate recycled to the recovery section is not greater than 10% of the flow rate of the aqueous solution subject to the reverse osmosis purification process. 
     
     
         15 . The process according to  claim 8 , further including the production of biuret or feed-grade biuret and wherein at least part of said retentate is used to produce said biuret or said feed-grade biuret. 
     
     
         16 . The process according to  claim 8 , wherein urea is synthesized with a stripping process, preferably a CO2-stripping process. 
     
     
         17 . A plant for the synthesis of urea including:
 a urea synthesis section adapted to produce urea by reacting ammonia and carbon dioxide at a urea synthesis pressure;   a recovery section arranged for processing a urea-containing reaction effluent produced in the urea synthesis section, the recovery section operating at one or more recovery pressure(s) lower than said urea synthesis pressure, to remove unreacted ammonia and carbon dioxide from the reaction effluent and obtain a urea aqueous solution; and   a purification section arranged to remove biuret from the urea aqueous solution obtained in the recovery section, said purification section including one or more reverse osmosis stages where biuret is removed from the solution by means of a reverse osmosis process.   
     
     
         18 . The plant according to  claim 17 , wherein the one or more reverse osmosis stages are upstream a concentration section and a finishing section for the production of solid urea, so that the removal of biuret by reverse osmosis is performed before the solution is concentrated by removing water. 
     
     
         19 . The process according to  claim 3 , wherein the reverse osmosis is performed with the urea-containing stream having a temperature of 70° C. to 80° C. 
     
     
         20 . The process according to  claim 5 , wherein the difference of pressure across the or each stage of the reverse osmosis process is 35 bar to 50 bar. 
     
     
         21 . The process according to  claim 5 , wherein the difference of pressure across the or each stage of the reverse osmosis process is 40 bar or around 40 bar.