PLANT AND PROCESS OF GRANULATING UREA AMMONIA SULPHATE

Abstract

Process and plant for the production of a urea ammonium sulfate granulate. In a first fluidized bed a first spraying liquid is sprayed comprising a urea melt and an aqueous solution of ammonium sulfate, e.g. a (near-) eutectic mixture. The bed temperature is below the crystallization temperature of the sprayed mixture. In a next compartment the granulate is fluidized to form a second fluidized bed. A second spraying liquid comprising a urea melt is sprayed into the second fluidized bed with a bed temperature higher than the bed temperature of the first fluidized bed and lower than the crystallization temperature of the sprayed second spraying liquid.

Claims

1. A process for the production of a urea ammonium sulfate granulate, using a granulator and providing a first fluidized bed of a granulate precursor material in a compartment of the granulator, the first fluidized bed continuously moving from a nuclei inlet to an outlet of the compartment; wherein a first spraying liquid comprising a urea melt and an aqueous solution of ammonium sulfate is sprayed into the first fluidized bed to produce an intermediate granulate material, the first fluidized bed having a bed temperature below the crystallization temperature of the sprayed mixture; wherein the intermediate granulate material is subsequently moved to a next compartment where it is fluidized to form a second fluidized bed while a second spraying liquid comprising a urea melt is sprayed into the second fluidized bed, the second fluidized bed having a bed temperature which is higher than the first fluidized bed and lower than the crystallization temperature of the sprayed second spraying liquid.

2. The process of claim 1, wherein the bed temperature of the second fluidized bed is maintained below the melting temperature of the dried mixture on the granulate precursor material.

3. The process of claim 1, wherein the bed temperature in the second compartment is 10-12° C. higher than the bed temperature in the first compartment.

4. The process of claim 1, wherein the bed temperature in the first compartment is about 95-100° C. and/or the bed temperature in the second compartment is about 105-112° C.

5. The process of claim 1, wherein the mean residence time of the granulate precursor material in the second fluidized bed is controlled to produce a final granulate having a residual moisture content of at most 0.3 wt %.

6. The process of claim 1, wherein exhaust air from the granulator is scrubbed using an aqueous solution of sulfuric acid converting ammonium and ammonium cyanate from the exhaust to ammonium sulfate in an aqueous solution, which is collected and, optionally after adjustment of the ammonium sulfate concentration, used to produce said mixture.

7. The process of claim 6, wherein the aqueous solution of sulfuric acid has a pH below 5, e.g., below 3.

8. The process according to claim 1, wherein the first spraying liquid comprises a eutectic or near-eutectic mixture of urea ammonium sulfate in an aqueous solution; and wherein the second spraying liquid is selected from the group comprising: a urea melt, or a hypereutectic mixture of a urea melt and at least 15 wt %, e.g., at least 18 wt. %, e.g. about 20 wt % of ammonium sulfate in an aqueous solution; or a hypoeutectic mixture of a urea melt and at most about 2 wt % of ammonium sulfate in an aqueous solution.

9. A granulate produced by the process of claim 1, the granulate comprising granulate particles having a nucleus covered by a first layer comprising urea ammonia sulfate and an outer second layer comprising urea substantially free of ammonia sulfate or an outer second layer comprising at least 15 wt %, e.g., at least 18 wt % of ammonium sulfate.

10. The granulator plant for the process according to claim 1, comprising a first compartment with sprayers connected to a source of a mixture of the first spraying liquid and a second compartment with sprayers connected to a source of the second spraying liquid.

11. The granulator plant according to claim 10, comprising a scrubber for scrubbing exhaust air from the granulator and a separator with an inlet connected to a discharge line for used scrubber liquid from the scrubber.

12. The granulator pant according to claim 11, wherein the discharge line for used scrubber liquid is connected to a supply line for supplying fresh urea melt.

13. The granulator plant according to claim 11, wherein the separator has an outlet connecting to a feed line for feeding the sprayers in the first granulator compartment.

14. The granulator plant according to claim 11, wherein the scrubber comprises: a first scrubber compartment with first sprayers and an inlet for granulator exhaust air; a second scrubber compartment with second sprayers, the second scrubber compartment being downstream of the first scrubber compartment, a first water collection reservoir for collecting used scrubber liquid from the first scrubber compartment and partly from the second scrubber compartment; a first circulation loop for returning scrubber liquid from the first water collection reservoir to the first sprayers; a second water collection reservoir for collecting part of the used scrubber liquid from the second scrubber compartment; a second circulation loop for returning scrubber liquid from the second water collection reservoir to the second sprayers; wherein the second circulation loop is connected to a source of an acid.

15. The granulator plant according to claim 14, wherein the first and second scrubber compartments are separated by means of a demister, such as a knit mesh or grid mesh.

Description

[0042] The invention will now be further explained with reference to the drawing.

[0043] FIG. 1: shows schematically an exemplary embodiment of a granulation plant according to the invention.

[0044] FIG. 1 shows a granulation plant 1 comprising a granulator 2 with a first compartment 3 and a second compartment 4. The first and second compartments 3, 4 comprise floors (not shown) with openings for the passage of fluidization air, which is blown from a blower 6. In this exemplary embodiment, the two compartments 3, 4 comprise sprayers 7, 8 extending from the floors and connected to a source of atomization air 9. In other embodiments, sprayers can extend from one or more side walls and/or extend downward. The sprayers 7 in the first compartment 3 are connected to a separator 11 forming a source of a eutectic or near-eutectic aqueous solution of urea ammonium sulfate, as explained hereafter. The sprayers in the second compartment are connected to a source of urea melt. The first compartment 3 has a nuclei inlet port 12 connected to a source of nuclei, and an intermediate granulate port 13 at an opposite side of the first compartment 3 leading to the second compartment 4. The second compartment 4 comprises a granulate outlet port 14 opposite to the intermediate granulate port 13.

[0045] In use, nuclei gradually flow from the nuclei inlet 12 of the first compartment 3 to the intermediate granulate port 13. The nuclei are fluidized by the fluidization air blown into the first compartment 3 via the openings in the floor to form a first fluidized bed 16. The urea ammonium sulfate solution is sprayed into the first fluidized bed 16 by the sprayers 7 on the granulator floor. While the water content evaporates, the urea ammonium sulfate deposits on the passing nuclei and crystallizes to form intermediate granules. The first fluidized bed 16 has a bed temperature of 95-100° C., which is well below the crystallization temperature of the sprayed urea ammonium sulfate, yet high enough to ensure sufficient evaporation of the moisture content.

[0046] Nuclei and intermediate granulate material in the first fluidized bed 16 moves continuously in the direction of the intermediate granulate port 13 where it enters the second compartment 4 to form a second fluidized bed 17. Here the sprayers 8 spray the pure urea melt into the second fluidized bed 17, which has a bed temperature of 105-112° C. This temperature is below the crystallization temperature of the urea melt, but is higher than the bed temperature of the first fluidized bed. The bed temperature is also lower than the melting point of the urea ammonium sulfate layer on the intermediate granulate (about 121° C. with a 0% water content).

[0047] The granulate leaving the second compartment 4 has a nucleus core, a urea ammonium sulfate inner layer and a urea outer layer, all with a residual moisture content of at most 0.3 wt %.

[0048] Exhaust air from the first and second compartments 3, 4 is transported to a wet scrubber 20 via a line 21. In the exemplary embodiment of FIG. 1, the wet scrubber 20 comprises a first scrubber compartment 22, a second scrubber compartment 23, a water separation compartment 24 and a clean air outlet 25 operatively connected to a discharge pump 26. The first and second scrubber compartments 22, 23 are separated by a first knit mesh or wire mesh demister 27. A further demister 28 separates the second scrubber compartment 23 from the water separation compartment 24. A third demister 29 separates the water separation compartment 24 from the clean air outlet 25. Below the first scrubber compartment 22 is a first water collection reservoir 31. Below the water separation compartment 24 is a second water collection reservoir 32. The second scrubber compartment 23 is partly above the first water collection reservoir 31 and partly above the second water collection reservoir 32. A first recirculation loop 34 returns water from the first water collection reservoir 31 to sprayers 35 in the first scrubber reservoir 31. A second recirculation loop 36 returns water from the second water collection reservoir 32 to sprayers 37 in the second scrubber reservoir 23. This second recirculation loop 36 is also connected to a source 38 of sulfuric acid to maintain the pH below 5. The second water collection reservoir 32 also has a fresh water inlet 41. All of the exhaust gas from the granulator 2 enters the scrubber via an inlet in the first scrubber compartment 22. As a result, the urea concentration will be substantially higher in the first water collection reservoir and the first recirculation loop. The urea concentration is substantially lower in the second recirculation loop, where the acid is added. This way, the chemical reaction between sulfuric acid and urea is limited.

[0049] The acidic water absorbs and separates ammonia and ammonium cyanate from the exhaust air entering the wet scrubber 20. The ammonia is solved as ammonium. The ammonium cyanate is not stable but tends to convert into urea or is hydrolyzed to ammonium and carbon dioxide. The ammonium forms ammonium sulfate with the sulfuric acid.

[0050] The first recirculation loop 34 comprises a branch line 42 towards a vacuum concentration unit with consecutively a connection to a supply line 43 for urea melt, a pump 44, a heat exchanger 45 and the separator 11. In the separator 11, a concentrated aqueous solution of urea ammonium sulfate is separated from the stream. The separated solution has an ammonium sulfate content of up to 12 wt %, a urea content of at least 80 wt % and a water content of about 1-5 wt %. This forms a eutectic or near-eutectic mixture with all ammonium sulfate being dissolved. The aqueous solution is returned via a return line 47 to the sprayers 7 in the first granulator compartment 3. This return line 47 is provided with an inlet 48 for additives, such as aluminium sulfate as a granulation aid.

[0051] Separated water vapour leaves the separator 11 and is lead to a vacuum condenser 49 where it is separated into water and process air. The condensed water is returned to the first water collection reservoir 31 via a return line 50 which comprises a seal pot 51 where the water is mixed with fresh sulfuric acid. The process air from the condenser 49 is mixed with the exhaust air from the granulator 2 and returned to the first scrubber compartment 22.

[0052] Tests were run with the arrangement of FIG. 1. The separator 11 was controlled to produce a urea ammonium sulfate solution with 5 wt % of water, up to 12 wt % of ammonium sulfate and at least 80 wt % of urea. Nuclei were supplied to form a fluidized bed continuously moving from the nuclei inlet 12 via the intermediate granulate port 13 to the granulate outlet 14 of the second compartment 4. The average bed temperature in the first compartment 3 was 90° C.

[0053] In the second compartment 4, a urea melt was sprayed with a 5 wt % water content. The sprayed volume of urea melt was about the same as the volume of urea ammonium sulfate sprayed in the first compartment 3. The average bed temperature in the second compartment 4 was about 112° C.

[0054] The resulting granulate product comprised a nucleus, coated by an inner layer of urea aluminum sulfate and an outer urea layer. The residual moisture content was below 0.3 wt %.

[0055] In an alternative embodiment, the sprayers of the second compartment can be fed by a source of a hypereutectic mixture of a urea melt and, e.g., 18-20 wt % of ammonium sulfate in an aqueous solution, or a hypoeutectic mixture of a urea melt and at most about 2 wt % of ammonium sulfate in an aqueous solution.