METHOD FOR THE PROVISION OF CARBON DIOXIDE FOR THE SYNTHESIS OF UREA

Abstract

The invention relates to a process for separating carbon dioxide from CO.sub.2-containing gases and an apparatus for providing carbon dioxide for the synthesis of urea.

Claims

1.-15. (canceled)

16. A process for preparing urea, comprising: providing a gas stream comprising hydrogen, nitrogen and carbon dioxide; removing, with a solvent, at least part of the carbon dioxide of the gas stream to form a carbon dioxide-depleted gas stream; synthesizing ammonia from at least part of the hydrogen and at least part of the nitrogen which are present in the carbon dioxide-depleted gas stream; desorbing the carbon dioxide from the solvent; and synthesizing urea from at least part of the ammonia from said synthesizing step and at least part of the carbon dioxide from said desorbing step; wherein the desorbing of the carbon dioxide from said desorbing step is carried out at a higher pressure than the synthesizing of the urea in said synthesizing step.

17. The process of claim 16, wherein the solvent loaded with carbon dioxide is at least partially depressurized after said removing step and before said desorbing step in order to remove components which have been co-absorbed in said removing step from the solvent.

18. The process of claim 16, wherein the desorption of the carbon dioxide in said desorbing step is carried out at a higher pressure than the synthesis of the urea in said synthesizing step and the pressure difference between the pressure at which desorption is carried out and the pressure at which urea is synthesized in said synthesizing step is sufficiently high to compensate for any pressure drops of the carbon dioxide occurring before entry into the urea synthesis.

19. The process of claim 16, wherein the temperature at which said desorbing step is carried out is selected so that the carbon dioxide can desorb at the pressure prevailing in each case.

20. The process of claim 16, wherein the solvent by which the carbon dioxide is separated off in said removing step comprises an ammonia/water mixture.

21. The process of claim 16, wherein the desorption of the carbon dioxide in said desorbing step is carried out at a temperature in the range from about 130 C. to 270 C.

22. The process of claim 16, wherein the energy required for desorption of the carbon dioxide in said desorbing step is provided by steam.

23. The process as claimed in claim 22, wherein the steam is produced by process heat liberated in the synthesis of ammonia.

24. The process of claim 16, wherein at least part of the gas stream is provided by one or both of steam reforming and autothermal reforming.

25. The process of claim 16, wherein the synthesis of the urea in said synthesizing step comprises subreactions (i) and (ii): (i) formation of ammonium carbamate from ammonia and carbon dioxide; and (ii) conversion of the ammonium carbamate into urea and water; wherein energy for the subreaction (ii) is obtained at least partly from the subreaction (i).

26. An apparatus for preparing urea, comprising: an apparatus for providing a gas stream, the gas stream comprising hydrogen, nitrogen and carbon dioxide; a separation apparatus configured to remove at least part of the carbon dioxide of the gas stream by means of a solvent; an ammonia synthesis unit comprising an ammonia reactor configured to synthesize ammonia from at least part of the hydrogen and at least part of the nitrogen present in the gas stream and the carbon dioxide-depleted gas stream; a desorption apparatus configured to desorb the carbon dioxide from the solvent; and a urea synthesis unit comprising a urea reactor for the synthesis of urea from at least part of the ammonia synthesized in the ammonia synthesis unit and at least part of the carbon dioxide desorbed from the solvent; wherein the desorption apparatus is operated at a higher pressure than the urea synthesis unit.

27. The apparatus of claim 26, wherein the desorption apparatus is operated at a pressure which is above the pressure at which the urea synthesis unit is operated and the pressure difference between the pressure at which the desorption apparatus and the pressure at which the urea synthesis unit are operated is sufficiently high to compensate for any pressure drops of the carbon dioxide occurring before entry into the urea synthesis unit.

28. The apparatus of claim 26, wherein the apparatus for providing the gas stream comprises one or both of a steam reformer and an autothermal reformer.

29. The apparatus of claim 26, wherein the separation apparatus comprises an ammonia water scrub.

Description

[0058] FIG. 1 shows, by way of example, the flow paths in the apparatus of the invention.

[0059] A gas stream A (1), which comprises hydrogen, nitrogen, carbon monoxide, carbon dioxide and possibly further components which are inert in respect of the ammonia synthesis unit, e.g. methane or argon, is preferably provided in a steam reformer, optionally an autothermal reformer. At least part of the carbon dioxide present in the gas stream A (1) is removed in a separation apparatus (2), preferably by means of an ammonia-water scrub. The carbon dioxide-depleted gas stream B (4) formed here is fed to an ammonia synthesis unit (5) in which at least part of the hydrogen and at least part of the nitrogen of the gas stream B (4) are converted into ammonia. A product stream (6) which comprises predominantly ammonia, hydrogen and nitrogen is discharged from the ammonia synthesis unit (5). At least part of the ammonia of the product stream (6) is separated off in an ammonia removal apparatus (7) and fed as ammonia stream (9) to the urea synthesis unit (13). At least part of the ammonia can optionally be compressed after discharge from the ammonia removal apparatus (7). The ammonia-depleted product stream (8) is recirculated to the ammonia synthesis unit (5). Any excess ammonia stream (15) can be passed to a further use, as corresponds to the prior art.

[0060] The carbon dioxide-enriched solvent (3) leaving the separation apparatus (2) is optionally firstly introduced into an intermediate depressurization stage (16) in which any components (18) which have been coabsorbed in the separation apparatus (2) are removed from the solvent. The carbon dioxide-enriched solvent (3) is preferably compressed by means of a pump (17), with the target pressure preferably being from 1 bar to 30 bar above the urea synthesis pressure. The carbon dioxide-enriched and compressed solvent (3) is subsequently fed to a desorption apparatus (10) in which the carbon dioxide (12) is desorbed from the solvent stream (3), preferably at a pressure which is above the operating pressure of the urea synthesis unit (13). The regenerated solvent stream (11) which is discharged from the desorption apparatus (10) can be reused in the separation apparatus (2). At least part of the carbon dioxide stream (12) which is desorbed in the desorption apparatus (10) is fed into the urea synthesis unit (13) and reacted there with the ammonia stream (9) to form urea. The urea stream (14) leaves the urea synthesis unit (13) and is optionally processed further.

LIST OF REFERENCE NUMERALS:

[0061] 1. gas stream A [0062] 2. separation apparatus [0063] 3, 3, 3. carbon dioxide-enriched solvent [0064] 4. gas stream B [0065] 5. ammonia synthesis unit [0066] 6. product stream [0067] 7. ammonia removal apparatus [0068] 8. ammonia-depleted product stream [0069] 9. ammonia stream [0070] 10. desorption apparatus [0071] 11. solvent stream, regenerated [0072] 12. carbon dioxide stream [0073] 13. urea synthesis unit [0074] 14. urea stream [0075] 15. excess ammonia stream [0076] 16. intermediate depressurization stage [0077] 17. pump [0078] 18. coabsorbed components