UREA FINISHING AND OFF-GAS TREATMENT PLANT AND PROCESS

Abstract

The invention pertains to a finishing process for urea-comprising material, a plant for finishing urea-comprising material, a method of modifying an existing plant, and a use. Methods are disclosed for preventing the clogging of the conduit for off-gas between the finishing section and the treatment section.

Claims

1. A finishing process for urea-comprising material comprising: subjecting a urea-comprising liquid stream to solidification in a urea finishing section, yielding a solid urea-comprising product and an off-gas stream comprising air, urea dust and ammonia, transporting said off-gas stream from an outlet of said urea finishing section to an off-gas treatment section through a conduit having a wall, wherein said off-gas has a temperature T.sub.1 at said outlet, subjecting said off-gas stream to a treatment to remove at least part of said urea dust and/or ammonia from said air in said treatment section, and maintaining a temperature of said wall of said conduit higher than T.sub.w,min, in at least one bend of the conduit, in at least one section where the diameter of the conduit changes and/or over at least 10%, of the length of the conduit, wherein T.sub.w,min=T.sub.150 C.

2. The finishing process of claim 1, wherein said wall of said conduit is provided with thermal insulation material and with heating elements.

3. The finishing process of claim 1, wherein the temperature of the wall is maintained at a temperature equal to or higher than T.sub.1.

4. The finishing process of claim 1, wherein the temperature of the off-gas stream in a zone less than 2 cm from said wall is higher than 60 C. preferably higher than 65 C., even more preferably higher than 70 C.

5. The finishing process of claim 1, wherein the difference in temperature between the gas stream near the wall of the conduit and the gas stream in the centre in cross-section of the conduit, at the same position in the length of the conduit, is less than 10 C., preferably less than 5 C.

6. The finishing process of claim 1, comprising maintaining the wall at a temperature of at least 60 C.

7. The finishing process of claim 1, wherein said solid urea-comprising product comprises urea particles, urea ammonium nitrate (UAN) particles, or urea ammonium sulfate (UAS) particles.

8. The finishing process of claim 1, wherein said solidification comprises prilling of urea-containing melt to give urea prills.

9. The finishing process of claim 1, comprising prilling urea in a forced draft urea prilling tower using cooling air and using a blower and/or fan, wherein the prilling tower has an outlet for off-gas at the top of said tower, wherein said off-gas has a temperature T.sub.1 at said outlet, subjecting said off-gas to dust scrubbing and optionally to acid scrubbing in an off-gas treatment section having an inlet for off-gas at 0 to 20 m elevation above ground level, supplying off-gas from said outlet at said top of the urea prilling tower to said inlet of said off-gas treatment section, and maintaining a temperature of said wall of said conduit higher than T.sub.w,min=T.sub.110 C.

10. A plant for finishing urea-comprising material, wherein the plant comprises a finishing section for solidifying a urea-containing liquid stream, an off-gas treatment section, and a conduit for off-gas from an outlet of said finishing section to an inlet of said treatment section, wherein said conduit comprises a wall, and wherein at least parts or all of said conduit are provided with thermal insulation and/or with one or more heating elements for maintaining a minimum temperature of said wall, wherein said thermal insulation and said one or more heating elements are configured for maintaining a temperature of said wall of said conduit higher than T.sub.w,min, in at least one bend of the conduit, in at least one section where the diameter of the conduit changes and/or over at least 10%, of the length of the conduit, wherein T.sub.w,min=T.sub.150 C., wherein T.sub.1 is the temperature of said off-gas at said outlet.

11. The plant of claim 10, wherein said conduit is provided with thermal insulation material and with heating elements, wherein the heating elements comprise electric tracing and/or steam tracing.

12. The plant of claim 10, wherein said finishing section is a urea prilling tower, and wherein the inlet for off-gas of said treatment section is at 0 to 20 m elevation above ground level.

13. The plant of claim 10, wherein the conduit is provided with thermal insulation material having a thermal conductivity of less than 1.0 W/(m.Math.K)) and having a thickness of at least 10 mm, preferably wherein the conduit is provided with thermal insulation material comprising one or more materials selected from the group of a polymeric material, a fibre based material, and an inorganic non-metal material.

14-15. (canceled)

16. The finishing process of claim 4, wherein the temperature of the off-gas stream in a zone less than 2 cm from said wall is higher than 65 C.

17. The finishing process of claim 4, wherein the temperature of the off-gas stream in a zone less than 2 cm from said wall is higher than 70 C.

18. The finishing process of claim 5, wherein the difference in temperature between the gas stream near the wall of the conduit and the gas stream in the centre in cross-section of the conduit, at the same position in the length of the conduit, is less than 5 C.

19. The plant of claim 13, wherein the conduit is provided with thermal insulation material having a thermal conductivity of less than 1.0 W/(m.Math.K)) and having a thickness of at least 10 mm, wherein the conduit is provided with thermal insulation material comprising one or more materials selected from the group of a polymeric material, a fibre based material, and an inorganic non-metal material.

20. The finishing process of claim 1, wherein said conduit has a length and wherein a temperature of said wall of said conduit higher than T.sub.w,min is maintained over at least 50% of the length of the conduit.

21. The finishing process of claim 1 wherein said conduit has a length and wherein a temperature of said wall of said conduit higher than T.sub.w,min is maintained over at least 90% of the length of the conduit.

22. The plant of claim 10, wherein said conduit has a length and wherein said thermal insulation and said one or more heating elements are configured for maintaining a temperature of said wall of said conduit higher than T.sub.w,min over at least 50% of the length of the conduit.

23. The plant of claim 10, wherein said conduit has a length and wherein said thermal insulation and said one or more heating elements are configured for maintaining a temperature of said wall of said conduit higher than T.sub.w,min over at least 90% of the length of the conduit.

Description

[0080] Embodiments of the invention will now be further illustrated in the following figures and example(s), which do not limit the invention or claims.

[0081] FIG. 1 shows deposits in a comparative conduit between a finishing section and a treatment section, observed after 10 days of continuous operation of pilot plant. The conduit was not thermally insulated or heated. A solid is formed in all the perimeter of the duct and is attributed to condensation and crystal growth.

[0082] FIG. 2 schematically illustrates an example embodiment of the invention, comprising a urea prilling tower A and a treatment section B (for dust and/or acid scrubbing) placed at ground level. Urea melt 1 is supplied to the top of the prilling tower A, and more in particular to the spraying device, e.g. a prilling bucket C. From spraying device, e.g. a prilling bucket C, the urea melt falls down inside the tower A, cools, crystallizes and solidifies into solid urea particles 2, using cooling air 3 and also giving off-gas 4. Off-gas 4 is supplied from the outlet of prilling tower A at the top of the tower to the scrubbing unit B through a duct D having a wall W. The off-gas is scrubbed in unit B to give the cleaned air stream 5 which is e.g. vented, and a liquid urea-containing purge stream 6. The purge stream 6 containing urea is disposed of for instance by recycle to the urea plant. In the invention, the duct D is provided with a heating and/or insulation element E for preventing heat loss in at least part of the wall W of the duct D. The heating and/or insulation element E is e.g. tracing, provided over at least part of the wall, e.g. using electric heating or a heating fluid 7, such as steam or condensate. Urea deposition on wall W by isocyanic acid condensation and reaction thereof with ammonia in off-gas stream 4 is avoided by the heating and/or insulation element E.

[0083] FIG. 3 shows two photographs of a duct between a prilling tower and a scrubber according to the invention, wherein the duct is thermally insulated. The photographs were taken after 2 weeks of discontinuous operation of a pilot plant. The deposited solids are located only at the bottom part of the duct and are attributed to settling of urea particles (urea dust) by gravity. In contrast to FIG. 1, no large clogs of solid deposits are formed. The settled particles do not adhere to the wall and are easily removed as shown in the right panel of FIG. 3.

EXAMPLE 1

[0084] In comparative plant 1, the scrubber was connected by a duct to a finishing section. The duct was not thermally insulated or heated. Depending on the outside temperature, the amount of fines <1 m was 10-70 wt. %, with higher amounts of fines at colder temperatures. Furthermore, at lower temperatures, the gas stream at the end of the duct contained less isocyanate and less ammonia at lower temperatures. The gas stream contained 10-90 mg isocyanate/Nm.sup.3 at the inlet of the scrubber, depending on the temperature at the inlet of the scrubber, which was in the range of 44 to 63 C.

[0085] For an inventive plant 2 with a thermally insulated duct between the finishing section (a prilling tower) and the scrubber, the amount of fines <1 m was normally 5 to 25%. The amount was not correlated with outside ambient temperature. The gas stream contained 100 to 220 mg isocyanate/Nm.sup.3 at the inlet of the scrubber, and was not correlated to the temperature of the off-gas at the inlet of the scrubber, i.e. the off-gas temperature downstream of the prilling tower and at the downstream end of the duct, which was in the range 58 to 70 C. and which was varied with the outside ambient temperature.

[0086] This indicates that isocyanate and ammonia react at lower temperatures in the non-insulated duct and that also submicron urea dust is formed. In the inventive plant 2, this is avoided.