UREA PLANT

Abstract

The invention relates to a urea plant with a CO.sub.2 and a NH.sub.3 feed, which comprises a purge line, characterized in that the purge line is connected with a fuel gas input line of a utility plant or an NH.sub.3 plant.

Claims

1. A method to reduce fuel consumption of an ammonia plant or a utility plant, which method comprises feeding a purge gas stream from a urea production plant, said urea production plant comprising a CO.sub.2 feed and an NH.sub.3 feed, into a fuel gas input line of said ammonia plant or utility plant; wherein said purge gas stream comprises impurities from said CO.sub.2 feed and NH.sub.3 feed.

2. The method of claim 1, wherein the purge gas stream contains oxygen at a concentration in the range of 0-10 mol %.

3. The method of claim 2, wherein the purge gas contains oxygen at a concentration in the range of 0-1 mol %.

4. The method of claim 3 wherein oxygen is essentially absent in the purge gas stream.

5. The method of claim 1, wherein the purge stream is fed into a reformer section in said ammonia plant.

6. The method of claim 1, wherein the purge stream is fed into a fuel gas supply of a steam boiler in said utility plant.

7. The method of claim 1, wherein the CO.sub.2 feed comprises H.sub.2, wherein the NH.sub.3 feed comprises H.sub.2 and wherein the CO.sub.2 feed comprises less than 0.05 vol % of oxygen.

8. The method of claim 1, wherein the urea production plant comprises synthesis equipment and synthesis piping and the material used for the synthesis equipment and synthesis piping a duplex ferritic-austenitic that contains, in % by weight: 0-0.05 C; 0-0.8 Si; 0.3-4 Mn; 28-35 Cr; 3-10 Ni; 1.0-4.0 Mo; 0.2-0.6 N; 0-1.0 Cu; 0-2.0 W; 0-0.010 S; 0-0.2 Ce, the remainder being Fe and normally occurring impurities and additives, having a ferrite content 30-70% by volume.

10. The method of claim 1 wherein said purge gas stream is derived solely from said urea production.

11. A urea production process, wherein urea is produced by reacting ammonia (NH.sub.3) and carbon dioxide (CO.sub.2), carried out in a urea plant comprising a CO.sub.2 and a NH.sub.3 feed, wherein the plant further comprises a purge line for a purge gas stream, wherein said purge gas stream comprises impurities from said CO.sub.2 feed and said NH.sub.3 feed, wherein the purge line is connected with a fuel gas input line of a utility plant or an NH.sub.3 plant, and wherein the method comprises using the purge gas as a fuel gas in said utility plant or NH.sub.3 plant.

12. A urea production process according to claim 11, wherein the oxygen concentration in the purge gas is in the range of 0-10 mol %.

13. A urea production process according to claim 11, wherein the oxygen concentration in the purge gas is in the range 0-1 mol %.

14. A urea production process according to claim 13, wherein the CO.sub.2 feed comprises H.sub.2, wherein the NH.sub.3 feed comprises H.sub.2, and wherein the CO.sub.2 feed comprises less than 0.05 vol % of oxygen.

15. A urea production process according to claim 14 wherein the purge stream is directed to the reformer section of said NH.sub.3 plant.

16. A urea production process according to claim 14, wherein said utility plant comprises a steam boiler, and wherein the purge stream is directed to the fuel gas supply of said steam boiler.

17. A urea production process according to claim 11, wherein oxygen is essentially absent in the purge gas.

18. A urea production process according to claim 11, wherein the plant comprises synthesis equipment and synthesis piping wherein as material for the synthesis equipment and synthesis piping a duplex ferritic-austenitic is used that contains, in % by weight: 0-0.05 C; 0-0.8 Si; 0.3-4 Mn; 28-35 Cr; 3-10 Ni; 1.0-4.0 Mo; 0.2-0.6 N; 0-1.0 Cu; 0-2.0 W; 0-0.010 S; 0-0.2 Ce, the remainder being Fe and normally occurring impurities and additives, the ferrite content being 30-70% by volume.

19. A process according to claim 11, wherein said purge line is configured to conduct purge gas derived only from said urea production.

Description

[0010] A further advantage of the invention is that methane emissions of the urea plant are reduced, which decreases surface ozone and slowing global climate warming.

[0011] The CO.sub.2 feed is generally provided with an additional oxygen stream, generally originating from air. The oxygen serves as an agent to prevent excessive corrosion of the synthesis equipment and the synthesis piping. As the oxygen does not contribute to the production of urea, it is vented with the purge gas. Excessive corrosion is prevented when the oxygen concentration in the purge gas is in the range of 5-20 mol %

[0012] As the NH.sub.3 feed and the CO.sub.2 feed comprise minor amounts of H.sub.2, the purge stream is very likely to be inflammable even before the addition of an oxidizing agent (e.g. air). Because of this inflammable character of the purge gas stream, it is unsafe to transport this gas (e.g. via pipelines) over some distance. In contrast, it is common practice up to now to vent this purge gas via shortest possible connections into the atmosphere.

[0013] By the use of a duplex ferritic-austenitic steel with a high content of Cr and N and a low content of Ni, as described in WO9500674, as a material of construction for the synthesis equipment and synthesis piping, oxygen needs no longer be supplied to the synthesis to prevent corrosion, or only in very low concentrations in the carbon dioxide feed e.g. <0.05 vol % of oxygen. Said duplex ferritic-austenitic steel is preferably a duplex, stainless steel alloy that contains, in % by weight: 0-0.05 C; 0-0.8 Si; 0.3-4 Mn; 28-35 Cr; 3-10 Ni; 1.0-4.0 Mo; 0.2-0.6 N; 0-1.0 Cu; 0-2.0 W; 0-0.010 S; 0-0.2 Ce, the remainder being Fe and normally occurring impurities and additives, the ferrite content being 30-70% by volume.

[0014] The use of said duplex ferritic-austenitic steel allows a reduction of the additional oxygen stream in the CO.sub.2 feed such that the oxygen concentration in the purge gas can be reduced to between 0-10 mol % or 0-1 mol %, without the risk of excessive corrosion taking place. Preferably oxygen is essentially absent in the purge gas. It is surprising that the purge gas stream, essentially without oxygen, now can be transported (e.g. by pipelines) without associated safety risks, to be used as a fuel gas.

[0015] The purge line of the urea plant of the invention is connected with a fuel gas input line of a utility plant or an NH.sub.3 plant. Preferably the purge stream is directed to the reformer section in an ammonia plant, or to the fuel gas supply of a steam boiler.

[0016] The invention will be elucidated hereinafter on the basis of FIG. 1, without being restricted to this embodiment.

[0017] FIG. 1 shows a urea plant (1) comprising a CO.sub.2 feed (10) and a NH.sub.3 feed (11). Solid urea leaves the plant via line 12. Water is purged via line 13. Purge line (14) is connected with a fuel gas input line of a utility plant or an NH.sub.3 plant (2).