Process for producing biuret from urea

Abstract

A process for the production of biuret from urea wherein: a urea aqueous solution (24) withdrawn from the recovery section of a urea plant is processed to remove water and obtain a concentrated urea melt (25); said urea melt is processed under biuret-forming conditions to decompose urea into biuret and ammonia and obtain a high-biuret urea melt (26); said high-biuret urea melt (26) is diluted with water or with an aqueous stream obtaining a solution (28); the solution (28) is subject to crystallization and precipitation of a solid phase containing biuret which is separated from the aqueous phase.

Claims

1. A process for the production of biuret from urea comprising: a) ammonia and carbon dioxide are reacted in a synthesis section at a synthesis pressure to form urea and obtaining a reaction effluent containing urea, water and unconverted reagents; b) said reaction effluent is processed in a recovery section to recover unconverted reagents contained therein; c) a urea aqueous solution, withdrawn from the recovery section, is processed to remove water and obtain a concentrated urea melt; said urea aqueous solution containing no more than 100 ppm by weight of formaldehyde; d) said urea melt is processed under biuret-forming conditions to decompose urea into biuret and ammonia and obtain a biuret urea melt; e) said biuret urea melt is diluted with water or with an aqueous stream obtaining a solution; f) the solution obtained at step e) is subject to a process of crystallization including precipitation of a solid phase containing biuret and obtaining a slurry including precipitated solid phase and a mother liquor; g) the slurry obtained at step f) is processed to obtain a biuret-containing solid product and a mother liquor.

2. The process according with claim 1, wherein the step g) includes separation of a solid phase from the slurry and further processing of said solid phase to remove residual water.

3. The process according to claim 1, wherein the solid product obtained after step g) contains at least 55% by weight of biuret.

4. The process according to claim 1, wherein the sum of biuret and urea in the solid product obtained after step g) is at least 80% by weight.

5. The process according to claim 1, wherein step d) is performed by maintaining the urea melt in a reaction space.

6. The process according to claim 5, wherein the biuret-forming conditions of step d) include one or more of: a reaction temperature in the reaction space of 160? C. to 180? C.; a residence time in the reaction space that ranges from 30 min to 100 min; a pressure in the reaction space which is atmospheric pressure or slightly below atmospheric pressure.

7. The process according to claim 1, wherein the urea aqueous solution of step c) is free of formaldehyde.

8. The process according to claim 1, wherein: the urea aqueous solution of step c) is a first portion of a solution obtained from the recovery section; a second portion of urea aqueous solution from the recovery section is processed to remove water, separately from the first portion, obtaining a urea melt; said urea melt obtained from the second portion of the solution is processed for the production of low-biuret urea.

9. The process according to claim 1, wherein the solution obtained after dilution of step e) contains by weight 40% to 60% of water.

10. The process according to claim 1, wherein gaseous ammonia obtained at step d) is condensed obtaining an ammonia solution and ammonia contained in said solution is recycled to the urea process.

11. The process according to claim 10, wherein said gaseous ammonia is condensed with process condensate from a waste water treatment section and the ammonia solution is recycled to said waste water treatment section.

12. The process according to claim 10, wherein the ammonia solution is subject to ammonia stripping in a dedicated ammonia stripper, thus obtaining an aqueous process condensate and a carbonate recycle solution; said recycle solution is sent to the urea recovery section; a first portion of said process condensate is used for condensation of said gaseous ammonia.

13. The process according to claim 12, wherein: a waste water obtained at step c) is treated in said ammonia stripper and a second portion of said process condensate is used in step e) to dilute the biuret urea melt.

14. The process according to claim 1, wherein the mother liquor obtained at step f) is treated by adding an acid or carbon dioxide to reduce the pH of the liquor, and cause the precipitation of cyanuric acid contained in the liquor, and the precipitated cyanuric acid is removed.

15. The process according to claim 14, wherein the mother liquor is treated by absorption of gaseous carbon dioxide and the absorption is performed under pressure.

16. The process according to claim 1, wherein the mother liquor of step f) is mixed with the urea solution of step c) before the water removal step.

17. The process according to claim 1, wherein said urea aqueous solution contains no more than 50 ppm by weight of formaldehyde.

18. The process according to claim 1, wherein the solid product obtained after step g) contains at least 70% by weight of biuret.

19. The process according to claim 1, wherein step d) is performed by maintaining the urea melt in a reaction space in a continuously stirred condition.

20. The process according to claim 5, wherein the biuret-forming conditions of step d) include one or more of: a reaction temperature in the reaction space of 160? C. to 170? C.; a residence time in the reaction space that ranges from 30 min to 100 min; a pressure in the reaction space which is atmospheric pressure or slightly below atmospheric pressure.

21. The process according to claim 5, wherein the biuret-forming conditions of step d) include one or more of: a reaction temperature in the reaction space of 165? C.; a residence time in the reaction space that ranges from 30 min to 100 min; a pressure in the reaction space which is atmospheric pressure or slightly below atmospheric pressure.

22. The process according to claim 1, wherein the solution obtained after dilution of step e) contains by weight 50% of water.

23. The process according to claim 1, wherein the mother liquor obtained at step f) is treated by adding an acid or carbon dioxide to reduce the pH of the liquor to 7.2 or less, and cause the precipitation of cyanuric acid contained in the liquor, and the precipitated cyanuric acid is removed.

24. The process according to claim 14, wherein the mother liquor is treated by absorption of gaseous carbon dioxide and the absorption is performed at a pressure of about 5 bar abs.

25. The process according to claim 1, wherein the mother liquor of step f), after removal of cyanuric acid, is mixed with the urea solution of step c) before the water removal step.

Description

DESCRIPTION OF THE FIGURES

(1) The invention and its advantages are now elucidated with the help of the figures wherein:

(2) FIG. 1 illustrates a scheme of a first embodiment of combined production of low-biuret urea and high-biuret urea.

(3) FIG. 2 is a variant of FIG. 1;

(4) FIG. 3 is another variant of FIG. 1.

(5) FIG. 4 is a variant of FIG. 1 with dedicated ammonia stripper

(6) FIG. 1 illustrates a plant including a urea synthesis section 1, a recovery section 2, a high-biuret urea (HBU) section 3 and a low-biuret urea (LBU) section 4.

(7) The HBU section 3 includes a first evaporator 5, biuret reactor 6, crystallization section 7 and ammonia condenser 8.

(8) The LBU section 4 includes: a second evaporator 9, finishing section 10, waste water treatment section 11.

(9) In FIG. 1, the following process streams are illustrated. 20 fresh carbon dioxide. 21 input of ammonia. 22 effluent from the synthesis section, which is typically a solution of urea, water and unconverted ammonium carbamate. 23 urea solution from the recovery section 2, which is predominantly urea and water with unavoidable impurities. 24 first portion of the urea solution 23, directed to the HBU section 3. 25 urea melt obtained in the evaporator 5 and fed to the HBU reactor 6. Said urea melt 25 typically contains more than 99% urea, e.g. 99.5% or more. 26 high-biuret urea melt obtained in the reactor 6. This melt may contain for example 16% biuret. 27 dilution water. 28 solution obtained from dilution of the high-biuret urea melt 26. This solution may contain for example 50% water, the balance being biuret and urea. 29 solid product obtained in the crystallization section 7. 30 mother liquor from crystallization, which is sent back to the evaporator 5. 31 valve controlling the flow rate of the portion 24 of urea solution. 32 water removed in the evaporator 5, which is sent to the WWT section 11. 33 gaseous ammonia produced by the thermal decomposition of urea and removed from the HBU reactor 6, which is sent to the ammonia condenser 8. 34 water for condensation of the ammonia 33. 35 ammonia solution recycled to the WWT section 11. 36 hot steam for heating the biuret reactor 6. 37 second portion of the urea solution 23, which is directed to the LBU section 4. 38 low-biuret urea melt from the evaporator 9. 39 low-biuret urea, e.g. granules or prills, produced in the finishing section 10. 40 water removed from the urea solution in the evaporator 9 and directed to the WWT section 11. 41 recycle solution from the WWT section 11 sent to the recovery section 2. 42 carbamate-containing solution obtained in the recovery section 2 and sent back to the synthesis section 1, e.g. to a high-pressure condenser.

(10) Looking at FIG. 1 it can be appreciated that the urea solution 23 from the recovery section 2 is split into first portion 24 and second portion 37. The first portion 24 is used in the HBU section 3 for production of the high-biuret urea 29; the second portion 37 is used in the LBU section 4 for production of the low-biuret urea 39, for example fertilizer-grade urea.

(11) The high-biuret urea melt 26, having for example a content of biuret of about 70 wt %, is diluted with water 27 until it contains around 50% water. The so obtained aqueous solution 28 is processed in the crystallization section 7 to obtain precipitation of biuret. In the crystallization section 7, the solution may be suitably cooled, e.g. to around 5? C., to obtain precipitation.

(12) In the crystallization section 7, a slurry is obtained which is separated into a solid phase and a liquid phase represented by a mother liquor. Optionally the crystallization section 7 may include a drying section wherein the solid phase is processed to further remove water. Hence a solid product 29 and a mother liquor 30 are obtained. The solid product 29 may be a granular product or a powder.

(13) It has to be noted that each of the HBU section 3 and LBU section 4 has a dedicated evaporator 5, 9. The provision of separate evaporators avoids contamination with biuret of the line dedicated to the production of LBU.

(14) The water 32 removed from the evaporator 5 of the HBU section 3 and the ammonia condensate 35 are recycled to the WWT section 11, providing a first level of integration between the two sections 3 and 4.

(15) The mother liquor 30 is recycled internally in the HBU section 3, by joining the feed of the evaporator 5, to avoid contamination of the LBU section, particularly of the evaporator 9.

(16) FIG. 2 is a variant providing a second level of integration wherein process condensate from the WWT section 11 is used instead of fresh water for diluting the high-biuret melt and to promote condensation of the ammonia removed from the reactor 6.

(17) A first stream 43 of an aqueous process condensate from said WWT section 11 is used for condensation of ammonia instead of water 34; a second stream 44 of said process condensate is used to dilute the high-biuret melt 26 instead of water 27.

(18) FIG. 3 illustrates a third level of integration wherein a portion of the CO.sub.2 feed is used to remove cyanuric acid from the mother liquor 30 before it is recycled to the evaporator 5.

(19) Particularly, a stream 45 of CO.sub.2 taken from the CO.sub.2 feed is absorbed in the liquor 30, obtaining a liquor 46 at a lower pH wherein the cyanuric acid precipitates. Then cyanuric acid is removed from said liquor 46 in a centrifuge 47 obtaining cyanuric acid solution 48 and a purified liquor 49 which is recycled to the evaporator 5.

(20) FIG. 4 illustrates a further embodiment including a dedicated stripper 50 for the HBU unit 3. Said stripper 50 receives the water stream 32 and ammonia solution 35 and produces a process condensate 51. Said condensate 51 forms a condensation stream 543 and the dilution stream 544 whose function is similar to streams 43, 44 as above disclosed. Another part of said condensate 51 is sent to the WWT section 11 as stream 52.

(21) The stripper 50 additionally produces a second carbonate recycle solution 53 which is sent to the recovery section 2 in addition to the recycle solution 41.

(22) The stripper 50 illustrated in FIG. 4 may be implemented in all the embodiments of the invention, for example the embodiment of FIG. 3. The stripper 50 may be also integrated in the WWT section 11.