PROCESS AND PLANT FOR THE PRODUCTION OF A UREA SOLUTION FOR USE IN SCR PROCESS FOR REDUCTION OF NOX

Abstract

A process and a plant for the preparation of an acqueous solution of urea suitable for use in a SCR process for nitrogen oxides removal, wherein the urea solution from the recovery section of a urea plant is subject to at least one step of evaporation, separating a vapour stream containing water and ammonia, and obtaining a concentrated and substantially ammonia-free solution, and said concentrated solution is diluted to the concentration of urea suitable for use in the SCR process.

Claims

1. A process for the preparation of an aqueous solution of urea suitable for use in a SCR process for nitrogen oxides removal, starting from an aqueous stream containing urea of a urea synthesis process, comprising the steps of: obtaining said aqueous stream containing urea by dissociation of carbamate in a recovery section of a urea production plant which operates at a lower pressure than a synthesis section of the plant; subjecting said aqueous stream to at least one process step of evaporation, thereby separating a vapour stream containing water and ammonia, and obtaining a concentrated and substantially ammonia-free solution; and then diluting said concentrated solution to obtain a solution with a concentration of urea suitable for use in the SCR process.

2. The process according to claim 1, wherein the evaporation comprises a heating step and a subsequent separation under vacuum, where the vapour stream containing water and ammonia and the concentrated solution are separated.

3. The process according to claim 2, wherein said separation under vacuum is carried out at a pressure of 0.2 to 0.4 bar.

4. The process according to claim 1, wherein said aqueous stream from the recovery section is subject to a first evaporation step, obtaining a vapour phase containing water and ammonia and a concentrated urea solution in liquid phase; at least a portion of said concentrated urea solution is then subject to a second evaporation step, separating a vapour phase containing water and ammonia and obtaining a further concentrated and substantially ammonia-free urea solution; said further concentrated urea solution is then diluted with water to a predetermined concentration of urea.

5. The process according to claim 1, wherein the at least one step of evaporation is carried out in at least one shell-and-tube heat exchanger and the aqueous stream containing urea is fed to a tube side of said heat exchanger.

6. The process according to claim 1, wherein concentration of the solution after the dilution with water is 30 to 35 wt % urea.

7. The process according to claim 1, wherein the NH.sub.3 content of the diluted solution is less than 2000 ppm and preferably less than 200 ppm.

8. A process for the preparation of an aqueous solution of urea suitable for use in a SCR process for nitrogen oxides removal, starting from an aqueous stream containing urea of a urea synthesis process, comprising the steps of: obtaining said aqueous stream containing urea by dissociation of carbamate in a recovery section of a urea production plant which operates at a lower pressure than a synthesis section of the plant; subjecting said aqueous stream to at least one process step of evaporation, thereby separating a vapour stream containing water and ammonia, and obtaining a concentrated and substantially ammonia-free solution, wherein the concentrated solution has 200 to 500 ppm of ammonia; then diluting said concentrated solution to obtain a solution with a concentration of urea suitable for use in the SCR process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a simplified scheme of a plant suitable for the production of SCR solution according to the process of the invention.

[0031] FIG. 2 is a scheme of a preferred embodiment of the process of invention.

[0032] FIG. 3 is more detailed example of a preferred implementation of the scheme of FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0033] FIG. 1 shows a general block diagram of a plant for the production of urea. The plant comprises basically a high-pressure (HP) synthesis section 1, a recovery section 2 and a concentration section 3. The HP synthesis section 1 converts an ammonia input 4 and carbon dioxide input 5 into an acqueous stream 6, containing urea, ammonium carbamate (NH.sub.3).sub.2CO.sub.2 and free ammonia; the recovery section comprises medium and/or low-pressure equipments adapted to dissociate the carbamate and then recycle ammonia and carbon dioxide to the synthesis section 1 via the stream 7, and producing an acqueous stream 8 substantially containing urea, water and residual free ammonia; the concentration section 3 comprises equipments for removing water and obtaining urea melt 9 of high-purity, e.g. 99% or more, depending on the use.

[0034] All urea plants follow the basic scheme of FIG. 1; the equipments of sections 1, 2 and 3 are not described in detail as they are well known in the art.

[0035] Typically, stream 8 contains around 65-70% urea and around 30% water, with a residual 1-2% free ammonia. According to the invention, at least a portion of said acqueous stream 8 is fed to a further section 10 to produce an SCR solution 14, comprising 30-35% urea and adapted for use in SCR process for NOx reduction.

[0036] Referring to FIG. 1, a first portion 8a of the stream 8 is used to produce the SCR solution 14, while the remaining second portion 8b is sent to the concentration section 3. Said first portion 8a is fed to an evaporation and vacuum separation unit E, where a vapour phase 15 containing water and ammonia is separated, and a concentrated, ammonia-free solution 11 is obtained in the liquid phase. This solution 11 is fed to a mixer 12 it is diluted with water 13, up to a suitable concentration of urea which is preferably 30 to 35 wt %.

[0037] Stream 8b is optional. In a variant of the invention, the stream 8 is entirely directed to unit E and the stream 8b is taken from said unit E, according to dotted line in FIG. 1. In further variants, stream 8b can be directed to other uses than concentration in section 3.

[0038] Stream 13 is preferably of demineralised water. A treated condensate recovered elsewhere in the urea process may also be used.

[0039] FIG. 1 relates to a plant adapted to produce the SCR solution 14 together with the urea melt 9. In a plant specifically designed for the production of the SCR solution, the full stream 8 may be directed to the section 10, and the concentration section 3 may be absent.

[0040] FIG. 2 shows a preferred embodiment of the invention. The acqueous stream 8 or a portion thereof, such as the portion 8a, is fed to a first evaporation unit E.sub.1, obtaining a vapour stream 102 containing NH.sub.3 and water, and a concentrated solution 103. A portion 107 of said concentrated solution is fed to a second evaporation unit E.sub.2, obtaining a vapour phase 112 and a further concentrated solution with a very low ammonia content, indicated as stream 11. Said stream is fed to the mixing device 12. The remaining portion 114 of the concentrated solution from the first unit E.sub.1 is made available for other use(s), for example forming the stream 8b shown in dotted line in FIG. 1. In other embodiments, the full stream 103 may be directed to the second unit E.sub.2.

[0041] Vapour streams 102 and 112 may be joined, as shown, and directed e.g. to a vacuum package of the plant (not shown) This embodiment of the invention is particularly effective, by providing substantially two steps in series for concentration and free NH.sub.3 removal, in the evaporation units E.sub.1 and E.sub.2 respectively.

[0042] A particular embodiment of the invention is shown in greater detail in FIG. 3. The first unit E.sub.1 comprises a steam-heated evaporator 100 and a flash vessel 101. In the example, the evaporator 100 is a shell-and-tube heat exchanger, where the urea-containing stream 8 is fed into the tubes, heated by steam 120 condensing on the shell side, and recovered as condensate 121.

[0043] The output of tube bundle of the evaporator 100 is passed into the flash vessel 101, where the vapour phase 102 containing ammonia and water is separated, obtaining a concentrated solution 103 which is stored in a tank 104. A stream 105 is taken from said tank and fed to a pump 106. Delivery of said pump is split into a stream 107 for the production of the SCR solution, and a stream 114 for other use(s) such as the production of urea melt in concentration section 3.

[0044] The stream 107 of concentrated solution is fed to the second evaporation unit E.sub.2, comprising a heat exchanger 108 connected by a duct 109 to a vacuum separator 110. A vapour phase 112 containing water and urea is also separated from said separator 110, obtaining a further concentrated and substantially ammonia-free solution 113, sent to the mixer 12.

[0045] The second evaporation unit E.sub.2 comprises a shell-and-tube heat exchanger where the stream 107 is fed into the tubes, heated by steam 122 fed to the shell side, and recovered as condensate 123.

[0046] A suitable separator is preferably installed in the vessel 101 to separate the liquid phase 103 from the vapours 102. The separator 110 has preferably a bottom well 111 for collecting the liquid phase forming the concentrated solution 113.

[0047] It should be noted that shell-and-tube heat exchangers are preferred for the exchanger(s) 100 and/or 108, but any conventional heat exchanger may be used.

[0048] An example is as follows. A stream 8 containing 68 wt % urea is evaporated in the evaporator 100 and passed in the flash tank 101 at 0.5 bar abs and 95 C., obtaining a concentrated liquid stream 103 at 71 wt % urea. Stream 113 obtained in the second unit E.sub.2 has 90 wt % urea and 430 ppm of free ammonia; said stream is then diluted with water 13 in the mixer 12, obtaining a stream 14 having 32 wt % urea and <200 ppm of ammonia. This stream 14 is adapted for use as SCR solution, i.e. for the removal of NOx in a SCR process. A cooler for the solution may be installed downstream the mixing device 12.

[0049] Referring again to FIG. 1, a conventional plant for the production of urea, comprising the synthesis section 1, the recovery section 2 and the concentration section 3, can be modified in order to use at least part of the output of the section 2 to produce the SCR solution 14. The modification involves the provision of at least the section 10 and related piping and accessories, such as pumps, valves, etc. The section 10 in turn may comprise the units E.sub.1 and E.sub.2 as in FIGS. 2-3 and as described above. The modified urea plant, hence, is able to produce the SCR solution 14.