PROCESS FOR PRODUCING FEED-GRADE UREA

Abstract

A process for the production of feed grade urea starting from a liquid urea melt, the process comprising processing the urea melt (6) to promote the formation of biuret until a concentration of at least 3% biuret, and subjecting the so obtained biuret-containing urea melt (8) to a shaping process to obtain a granular urea product.

Claims

1. A process for the production of feed grade urea starting from a liquid urea melt, the process comprising: processing the urea melt to form biuret from urea, until a target concentration of biuret in the melt is reached, said target concentration being 3% by weight or higher; subjecting the so obtained biuret-containing urea melt to a shaping process to obtain a granular urea product.

2. The process according to claim 1, wherein the target concentration of biuret in the melt is 5% by weight or greater, preferably in a range of 5% to 15%.

3. The process according to claim 1, wherein the step of processing the urea melt to form biuret includes maintaining the urea melt in a reaction space under biuret-forming conditions for a predetermined residence time until said target concentration of biuret is reached.

4. The process according to claim 3, wherein the biuret-forming conditions includes a temperature of 160° C. to 180° C., preferably 160° C. to 170° C. and more preferably 165° C.

5. The process according to claim 3, wherein the residence time is 5 min to 60 min, preferably 20 min.

6. The process according to claim 3, wherein said reaction space is maintained under a continuously stirred condition during the processing of the urea melt to form biuret.

7. The process according to claim 3, wherein said reaction space is maintained at atmospheric pressure.

8. The process according to claim 1, wherein the urea melt contains at least 96% urea by weight, preferably at least 99%.

9. The process according to claim 1, wherein no additive and particularly no formaldehyde is added to the urea melt before the shaping process.

10. The process according to claim 1, wherein the shaping process includes granulation or prilling.

11. The process according to claim 1, wherein the urea melt is produced in a urea synthesis plant wherein urea is synthesized from ammonia and carbon dioxide at a synthesis pressure obtaining a reaction effluent containing urea, water and unconverted ammonium carbamate; said reaction effluent is processed in one or more recovery sections at a lower pressure to recover unconverted reagents, obtaining a purified aqueous solution of urea; said aqueous solution is processed in an evaporation section to remove water and obtain the urea melt.

12. The process according to claim 11, wherein gaseous ammonia, removed from the reaction space during the formation of biuret is recycled to the urea synthesis plant as a source material for the synthesis of urea.

13. The process according to claim 12, wherein the gaseous ammonia is condensed with water and recycled as aqueous solution of ammonia.

Description

DESCRIPTION OF THE FIGURE(S)

[0033] FIG. 1 illustrates a scheme of an implementation of the invention. The following items and process streams are shown.

[0034] Item 1 is a synthesis section where ammonia (NH.sub.3) and carbon dioxide (CO.sub.2) are reacted at high temperature and high pressure to form urea. Said synthesis section 1 may include at least a reactor, a high-pressure stripper and a high-pressure condenser.

[0035] Stream 2 is an aqueous solution containing urea, water and unconverted ammonium carbamate. This solution can be withdrawn from the stripper of the synthesis section 1.

[0036] Item 3 is a recovery section where the unconverted reagents contained in the solution 2 are recovered and recycled back to the section 1. The unconverted reagents are normally recovered by one or more steps of heating the urea-containing solution for decomposition of ammonium carbamate into gaseous ammonia and CO2, and condensation of said reagents into a carbamate-containing solution which can be pumped back to the reactor or to the condenser of the section 1. The recovery section 3 may operate at one or more pressure levels.

[0037] Stream 4 is a purified solution obtained from the recovery section 3. This purified solution 4 contains urea, water and unavoidable impurities.

[0038] Item 5 is an evaporation section which removes water from the solution 4.

[0039] Stream 6 is a urea melt.

[0040] Item 7 is a continuously stirred reactor wherein the urea melt 6 is maintained at suitable biuret-forming conditions so that some urea decomposes into biuret and gaseous ammonia.

[0041] Item 8 is a biuret-containing stream withdrawn from the reactor 7.

[0042] Item 9 denotes gaseous ammonia removed from the reactor 7.

[0043] Item 10 is a urea shaping equipment, for example a granulator or a prilling tower.

[0044] Stream 11 is a granular urea product obtained in the urea shaping equipment 10.

[0045] Item 12 is a condenser wherein the gaseous ammonia 9 is condensed with the help of water 13.

[0046] The stream 14 denotes an aqueous solution of urea obtained in the condenser 12, which is recycled to the recovery section 3.

[0047] The item 15 is a heat exchanger where heat can be recovered from the biuret-containing melt, before the shaping process.

[0048] It must be noted that the urea solution 4 or the urea melt 6 may be a part of the solution or melt obtained from the recovery section 3 or evaporation 5, respectively. A remaining part of the urea solution or of the urea melt may be directed to a different use or process.

[0049] Example: urea melt 6 preferably containing 99.5% urea or more is kept in the reactor 7 for residence time of 20 min, at a temperature of 165° C. and atmospheric pressure. The so obtained melt 8 contains 8% by weight of biuret. This biuret-containing melt after cooling in the heat exchanger 15 is converted into a granular product. The ammonia 9 emerging from the reactor 7 is preferably condensed with water until a solution 14 containing at least 15% ammonia by weight is obtained, said solution 14 can be recycled to the urea plant.