UREA PRILLING PROCESS

Abstract

A process of prilling of a urea melt comprising the use of calcium lignosulfonate or carboxymethyl starch as additive to obtain a formaldehyde-free urea product.

Claims

1. A process for the prilling of a urea melt, wherein: the prilling process includes distributing urea melt in the form of droplets in a prilling tower and urea droplets solidify as they fall down the tower in counter-current with upflowing air; the urea melt contains at least one of calcium lignosulfonate and carboxymethyl starch as a prilling additive, and the urea melt contains no added formaldehyde; 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 being processed in an evaporation section to remove water and obtain the urea melt, wherein said prilling additive is added to the purified aqueous solution before evaporation or is added to the urea melt.

2. The process according to claim 1, wherein the total amount of said prilling additive in the urea melt is 0.1% to 1.0% by weight on dry basis relative to the urea.

3. The process according to claim 1, wherein the total amount of said prilling additive in the urea melt is 0.3% to 0.7% by weight on dry basis relative to the urea.

4. The process according to claim 1, wherein the amount of calcium lignosulfonate in the urea melt is 0.7% by weight on dry basis relative to the urea.

5. The process according to claim 1, wherein the amount of carboxymethyl starch in the urea melt is 0.5% by weight on dry basis relative to the urea.

6. The process according to claim 1, wherein the urea melt contains, as a prilling additive, calcium lignosulfonate and carboxymethyl starch.

7. The process according to claim 1, wherein the urea melt contains more than 98% by weight of urea.

8. The process according to claim 1, wherein the urea melt is converted into droplets by means of one or more showerheads or by means of one or more rotating prilling bucket.

9. The process according to claim 1, wherein the so obtained urea prills have an average diameter not greater than 2.0 mm.

10. The process according to claim 1, wherein the so obtained urea prills are feed grade.

11. (canceled)

12. A process for the production of solid urea comprising: reaction of ammonia and carbon dioxide at a synthesis pressure obtaining a reaction effluent containing urea, water and unconverted ammonium carbamate; processing said reaction effluent at a lower pressure to recover unconverted reagents, obtaining a purified aqueous solution of urea; subjecting said aqueous solution of urea to an evaporation process to remove water and obtain the urea melt, the process further comprising: adding at least one of calcium lignosulfonate and carboxymethyl starch, as a prilling additive, to the purified urea aqueous solution or to the urea melt; wherein no formaldehyde is added to the purified aqueous solution or to the urea melt, so that the urea melt contains no added formaldehyde; converting the urea melt with the Drilling additive into a solid product by a prilling process wherein the prilling process includes distributing the urea melt in the form of droplets in a prilling tower wherein the droplets fall down the tower and solidify in the presence of a counter-current upflowing cooling air.

13. The process according to claim 12, wherein the prilling additive is added to the aqueous solution or to the urea melt in an amount which is 0.1% to 1.0% by weight on dry basis relative to the urea.

14. The process according to claim 12, wherein both calcium lignosulfonate and carboxymethyl starch are used as prilling additives, and they are added separately or as a mixture to the purified aqueous solution before evaporation or to the urea melt.

15. The process according to claim 12, wherein the urea is feed grade.

16. The process according to claim 6, wherein the urea melt contains, as the prilling additive, calcium lignosulfonate and carboxymethyl starch in a ratio of 1:1 by weight.

17. The process according to claim 7, wherein the urea melt contains more than 99% by weight of urea.

18. The process according to claim 9, wherein the so obtained urea prills have an average diameter not greater than 1.0 mm.

19. The process according to claim 13, wherein the prilling additive is added to the aqueous solution or to the urea melt in an amount which is 0.3% to 0.7% by weight on dry basis relative to the urea.

Description

DESCRIPTION OF THE FIGURE(S)

[0043] FIG. 1 illustrates a scheme of an implementation of the invention. The following items and process streams are shown.

[0044] Item 1 is a synthesis section where ammonia (NH3) and carbon dioxide (CO2) 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. [0045] 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. [0046] 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. [0047] Stream 4 is a purified solution obtained from the recovery section 3. This purified solution 4 contains urea, water and unavoidable impurities. [0048] Item 5 is an evaporation section which removes water from the solution 4. [0049] Stream 6 is a urea melt. [0050] Item 7 is a urea melt tank. [0051] Item 8 is an additive tank. [0052] Item 9 is an additive metering device. [0053] Stream 10 denotes the additive (calcium lignosulfonate and/or carboxymethyl starch) added to the urea melt from the tank 7. [0054] Stream 11 is urea melt containing the additive. [0055] Item 12 is a prilling tower. [0056] Item 13 is a prilling bucket installed in the prilling tower 12. [0057] Stream 14 is the solid product (urea prills) collected at the bottom of the prilling tower 12.

[0058] In alternative embodiments, the additive 10 may be added directly in the melt tank 7 or it may be added to the urea solution 4 before evaporation.

[0059] The additive 10 may be a mixture of calcium lignosulfonate and carboxymethyl starch. In some embodiments, calcium lignosulfonate and carboxymethyl starch may be added separately to the urea solution 4 and/or to the urea melt 6. For each additive a respective tank and a respective metering device can be provided. When added separately, the calcium lignosulfonate and carboxymethyl starch may be added at the same or a different location. The total amount of the two additives is preferably in the range 0.1% to 1.0% weight on dry basis relative to the urea.