PROCESS FOR PRODUCING UREA AND BIURET

Abstract

A process for the production of urea wherein: production of pure urea (U) includes the concentration of an aqueous urea solution (23) by crystallization; a urea crystallization purge aqueous phase (32) comprising urea, water and biuret, which is purged from the urea crystallization process, is used in a high-biuret urea processing section (34) for additional production of urea with a high content of biuret.

Claims

1. A process for the production of urea wherein: the production of urea includes the concentration of an aqueous urea solution by means of a process of crystallization, which is performed in a urea crystallization section, obtaining solid crystals of urea and a urea crystallization purge aqueous phase comprising urea, water and biuret, which is purged from the urea crystallization process; at least a portion of said urea crystallization purge aqueous phase is further processed in a high-biuret urea processing section wherein a high-biuret urea is produced in a solid form, said high-biuret urea containing at least 55% by weight of biuret; wherein urea is produced at a high pressure of synthesis in a urea synthesis section, obtaining a synthesis effluent containing urea, water and unconverted ammonium carbamate; said effluent is processed in a recovery section at one or more pressure levels lower than the synthesis pressure to recover unconverted reagents which are recycled to the synthesis section, and said aqueous urea solution is obtained in the recovery section.

2. The process according to claim 1, wherein said high-biuret urea contains at least 70% by weight of biuret.

3. The process according to claim 1, wherein the sum of biuret and urea in the high-biuret urea is at least 80% by weight.

4. The process according to claim 1, wherein the urea crystallization purge aqueous phase is diluted with water before or during the processing in the high-biuret urea processing section.

5. The process according to claim 1, wherein the processing of the urea crystallization purge aqueous phase in the high-biuret urea processing section includes a step of crystallization wherein solid crystals containing biuret and urea are obtained.

6. The process according to claim 5, wherein processing in the high-biuret urea processing section includes separation of the crystals from a crystallization slurry and further processing of said crystals to remove residual water and obtain said high-biuret urea.

7. The process according to claim 1, wherein said urea crystallization purge aqueous phase contains at least 5% by weight of biuret, preferably at least 10% by weight.

8. The process according to claim 1, wherein said urea crystallization purge aqueous phase contains by weight 50% to 65% urea, 5% to 20% biuret, the balance being water and unavoidable impurities.

9. The process according to claim 1, wherein all the urea crystallization purge aqueous phase produced in the urea crystallization section is sent to the high-biuret urea processing section and no part of said urea crystallization purge aqueous phase is reintroduced into the urea synthesis section.

10. The process according to claim 1, wherein a residual aqueous phase withdrawn from the high-biuret urea processing section is recycled to the urea crystallization section.

11. The plant for producing pure urea and high-biuret urea with a process in accordance with claim 1, the plant comprising: a high-pressure urea synthesis section to obtain a synthesis effluent containing urea, water and unconverted ammonium carbamate; a recovery section to process said synthesis effluent at one or more pressure levels lower than the synthesis pressure to recover unconverted reagents which are recycled to said urea synthesis section, and to obtain an aqueous urea solution; a urea crystallization section arranged to process said aqueous urea solution and to obtain solid crystals of urea and a urea crystallization aqueous phase comprising urea, water and biuret; a processing section and a line arranged to feed waste water separated in the urea crystallization section to said processing section; the processing section being arranged to obtain, from said aqueous phase, a high-biuret urea in a solid form; wherein said processing section includes a crystallization section arranged to produce crystals containing biuret and a centrifuge arranged to separate the crystals from a crystallization mother liquor.

12. A method of modifying a urea plant, wherein: the plant includes a urea crystallization section arranged to produce solid urea by crystallization of an aqueous urea solution, obtaining solid crystals of urea and a crystallization aqueous phase comprising urea, water and biuret; the method including: the addition of a processing section arranged to obtain, from said aqueous phase, a high-biuret urea in the form of a solid product made predominantly of biuret and urea; the provision of a line arranged to feed part or all of the crystallization aqueous phase withdrawn from the urea crystallization section to the newly-installed processing section for the production of high-biuret urea; said modified urea plant being made according to claim 11.

Description

DESCRIPTION OF THE FIGURES

[0042] The invention and its advantages are now elucidated with the help of the figures wherein:

[0043] FIG. 1 illustrates a prior art urea production process involving crystallization.

[0044] FIG. 2 illustrates the production process of FIG. 1 modified according to the invention for the integrated production of conventional urea and high-biuret urea.

[0045] The process and items depicted in FIG. 1 are familiar to a skilled person. Fresh carbon dioxide 20 and ammonia 21 are reacted in the synthesis section 1 at a high pressure, e.g. above 100 bar, to form a reaction effluent 22 containing urea, water and unconverted reagent mostly in the form of ammonium carbamate.

[0046] Said effluent 22 is processed in the recovery section 2 at a lower pressure, for example in a low-pressure stage or in a medium-pressure stage followed by a low-pressure stage. Here the solution is heated to decompose the ammonium carbamate and gaseous ammonia and carbon dioxide removed for the solution are condensed to form a recycle solution 30 sent back to the synthesis section 1.

[0047] The recovery section 2 produces an aqueous solution 23 made predominantly of urea and water which after de-pressurization in a valve 13 is sent to the crystallization section 3.

[0048] A crystallizer 7 removes water from the solution and forms a stream 24 containing crystals of urea and an aqueous phase (mother liquor). The water 28 removed in the crystallizer 7 is sent to a waste water treatment unit 9 and forms a carbonate-containing stream 29 which can be recycled to the recovery section 2.

[0049] A portion 25 of said stream of crystals and liquor is sent via pump 10 to a centrifuge 8. Another portion 26 of the effluent of the crystallizer is recycled via line 26, pump 11 and heat exchanger 12.

[0050] In the centrifuge 8 solid crystals of urea 27 are separated from the liquor. The crystals are further dried in a drying unit 4 and melted in a melting unit 5. The so obtained pure urea melt feeds a finishing section 6 such as a prilling tower or a granulator where urea U is produced.

[0051] The urea U is a low-biuret urea, containing for example less than 0.9% or less than 0.6% of biuret in weight. Such a low content of biuret is required e.g. for use of the urea in the preparation of a diesel exhaust fluid or for use as foliar grade fertilizer.

[0052] The centrifuge 8 separates a stream of a crystallization aqueous phase 31 containing urea, water and biuret. A portion 33 of this liquor is reintroduced in the crystallizer 7 via line 26; a purge stream 32 is separated to avoid accumulation and precipitation of biuret. This stream 32 represents a waste of the urea crystallization process. FIG. 1 illustrates the aqueous purge stream 32 is sent to the synthesis section 1, which is common in the prior art but, as explained above, negatively affects the efficiency of conversion in the urea reactor.

[0053] FIG. 2 illustrates the plant modified in accordance with the invention. The crystallization waste stream 32 is sent to a high-biuret urea post-processing section 34 after dilution with water W.

[0054] In the section 34, a process of crystallization is performed. Said crystallization process produces a slurry including precipitated solid phase and a mother liquor.

[0055] The slurry is processed to separate the solid phase which may be dried to obtain high-biuret urea B. The remaining mother liquor 35 is recycled to the crystallization section 3, for example with the urea solution 23. It can be seen that the invention avoids contamination of the synthesis section 1 with biuret or urea.

Example

[0056] In a urea plant with a capacity of 790 metric tons per day, 33,000 kg/h of a urea solution (line 23) are sent to the urea crystallization section. Said solution is at 90 C. and contains 71.64% urea; 0.36% biuret; 28% water by weight. The crystallization in the section 3 is performed at 0.1 bar absolute pressure and about C.

[0057] 1520 kg/h of crystallization aqueous phase are removed with the purge line 32. This amount is regulated to keep a concentration of biuret not greater than 10%. The composition of the centrifuge mother liquor of line 31 is 63% urea, 27% water and 10% biuret.

[0058] The purge stream 32 is added with 1310 kg/h of water for dilution. In the processing section 34, 165 kg/h of solid high-biuret urea are obtained. The line 35 carries 3,990 kg/h of a residual mother liquor at about 3% of biuret.