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.

Claims

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 The process according to claim wherein all or some of the urea aqueous solution withdrawn from the recovery section is stored in a urea solution tank and the urea aqueous solution subject to said reverse osmosis is taken from said tank.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.

Description

DESCRIPTION OF FIGURES

[0059] FIG. 1 is a scheme of a process for producing urea in an embodiment of the invention.

[0060] FIG. 2 is a scheme of a multiple-stage reverse osmosis section which can be used to implement the invention.

DETAILED DESCRIPTION

[0061] Referring to FIG. 1, a urea synthesis plant UP produces a urea aqueous solution 1 of urea. Said solution 1 is taken from a recovery section of the plant UP. The plant UP more in detail may include a high-pressure synthesis section—e.g. a CO.sub.2-stripping synthesis section—and a low-pressure recovery section from which the solution 1 is obtained.

[0062] Said solution 1 is stored in a urea solution tank T. The solution 2 taken from said tank 2 is sent to a reverse osmosis section RO including a membrane package which performs a reverse osmosis process to remove biuret from said solution 2.

[0063] A low biuret urea solution 3 is obtained from the section RO. This low-biuret solution 3 is sent to an evaporation section EV where water is removed and a highly concentrated solution 4 is obtained. This highly concentrated solution 4 is processed in a finishing section FIN to obtain solid urea U in the form of prills or granules.

[0064] A biuret-rich solution 5 is also produced in the section RO. Said solution 5 contains the biuret removed from the input solution 4 and has typically more than 1% biuret. Said solution 5 is recycled to the plant UP. A preferred use of the solution 5 in the plant is sending the solution 5 into a condenser of ammonia and CO.sub.2 vapours.

[0065] In another interesting application, water may be added to the stream 3 to produce a urea solution for use in SCR for removal of NOx.

[0066] FIG. 2 illustrates an exemplary embodiment of the section RO section.

[0067] An input solution F (e.g. the solution 2 of FIG. 1) is sent to a first reverse osmosis stage RO-1 together with internal recycle streams 20, 21. The stage RO-1 therefore receives a mixed stream 22 and produces a first permeate P1 and a first retentate R1.

[0068] The first permeate P1 is processed in a set of stages RO-1.1 and RO-1.2 wherein the permeate is progressively purified. Particularly the permeate P2 of the stage RO-1.1 is further purified in the stage RO-1.2 to produce a permeate P which is a first output of the process (e.g. the stream 3 of FIG. 1).

[0069] The first retentate R1 is processed in a set of stages RO-2.1 to RO-2.3. The retentate of each stage forms the input of the subsequent stage. The retentate R of the last stage RO-2.3 is another output of the process, for example the stream 5 of FIG. 1.

[0070] The stream P has the lowest amount of biuret whilst the stream R has the highest. The permeate streams of the stages RO-2.1 to RO-2.3 and the retentate streams of the stages RO-1.2 and RO-1.3 are streams with intermediate content of biuret; they can be recycled to the inlet of the first stage RO-1 via lines 20, 21 as shown in FIG. 2.

[0071] For example, in a preferred embodiment the streams of FIG. 2 have the following flow rates (m.sup.3/h) and mass fraction of biuret w.sub.B.

TABLE-US-00001 Stream m.sup.3/h W.sub.B F 95 0.50 22 290 0.77 P1 194 0.50 P2 107 0.34 P 88 0.25 R 7 4.0 20 90 1.05

[0072] The invention achieves the above mentioned goals of providing a cost-effective process for removing biuret from urea solutions and produce low-biuret urea.