SONICATION IN A UREA OR MELAMINE SYNTHESIS PROCESS

Abstract

Process and respective reactor for urea or melamine synthesis, comprising sonication treatment of at least part of a reaction liquid mass or two-phase mixture contained inside said reactor.

Claims

1. A process for the synthesis of urea from ammonia and carbon dioxide, or for the synthesis of melamine from urea, inside a chemical reactor, said process comprises comprising a sonication treatment of at least part of a reaction liquid mass or two-phase mixture contained inside said chemical reactor.

2. The process according to claim 1, wherein said sound waves have a frequency in the range 1 kHz to 1 MHz.

3. The process according to claim 2, wherein said sound waves have a frequency equal to or greater than 20 kHz.

4. The process according to claim 1, said sound waves being generated by one or more electrical or mechanical sources.

5. The process according to claim 1, wherein the sonication treatment is performed in a sonication zone inside said chemical reactor.

6. The process according to claim 5, comprising the circulation of a liquid phase or liquid/vapour phase through at least one reaction zone and a recirculation zone, which are arranged coaxial and concentric inside said reactor, wherein the sonication treatment is performed in at least one of said two reaction and recirculation zones.

7. The process according to claim 6, for melamine synthesis, wherein the reaction zone is a primary reaction zone fed with urea melt.

8. A reactor for urea synthesis or melamine synthesis according to the process of claim 1, wherein it comprises at least one sonication sound waves source.

9. The reactor according to claim 8, wherein said sound waves source is located inside a sonication chamber.

10. The reactor according to claim 9, said sonication chamber being delimited, inside the reactor, by a cylindrical wall or by a vertical-axis tube.

11. The reactor according to claim 10, said chamber being coaxial with the reactor.

12. The reactor according to claim 8, wherein said source is designed to produce sound waves with a frequency in the range of 1 kHz to 1 MHz.

13. The reactor according to claim 8, comprising a reaction chamber and a recirculation chamber which are concentric and coaxial, wherein said at least one sound waves source is located inside one of said chambers.

14. The reactor according to claim 13, the reactor being intended for melamine synthesis, wherein the reaction chamber is delimited by a central tube and is directly in communication with an inlet for urea melt, and the recirculation chamber is an annular chamber around said reaction chamber and comprises heating elements.

15. The reactor according to claim 8, wherein it comprises a mechanical stirrer arranged so as to direct a liquid phase, contained inside the reactor, towards said sonication source.

16. Use of a sonication treatment inside a reactor for urea synthesis or inside a reactor for melamine synthesis, wherein said treatment provides transmission of sound waves to at least part of a liquid mass or two-phase mixture contained inside said reactor.

17. The process according to claim 3, wherein said sound waves have a frequency of 20 to 40 kHz.

18. The reactor according to claim 12, wherein said source is designed to produce sound waves with an ultrasound frequency of at least 20 kHz.

Description

DESCRIPTION OF THE FIGURES

[0037] FIG. 1 is a schematic illustration of a recirculating reactor for urea or melamine synthesis, designed to implement the invention.

[0038] FIG. 2 shows a more detailed diagram of a mode of implementing the invention and relates to a melamine reactor.

DETAILED DESCRIPTION

[0039] FIG. 1 shows a general diagram of the reactor 1 according to the invention. The reactor 1 comprises a shell 2 with a vertical axis 3 and an inner shell 4 which is open at the top and bottom. Said shell 4 preferably is cylindrical with a vertical axis, like the outer shell 2. More advantageously, said shell 4 is coaxial with the shell 2, i.e. has the same vertical axis 3.

[0040] Said inner shell 4 defines a chamber 5 inside which at least one sonication waves source 6 is installed.

[0041] The reactor 1 contains a liquid phase which recirculates through the chamber 5 and in this way is subjected to the action of the sound waves emitted by said source 6. The arrows 7 in FIG. 1 indicate the recirculation of the liquid; owing to said recirculation, the substantially entire liquid mass contained inside the reactor 1 is subjected to the sonication process, notwithstanding the volume of the chamber 5 is significantly smaller than the internal volume of the reactor 1.

[0042] FIG. 2 shows in greater detail an embodiment of the invention applied to melamine reactors. For the sake of simplicity, the details which are similar to those of FIG. 1 are indicated with the same reference numbers. In the reactor according to FIG. 2, the wall (or tube) 4 defines a central chamber 5 which forms the primary reaction zone fed with the urea melt 10 via a duct 11. The annular chamber 12, situated around the central chamber 5, forms a recirculation zone for the liquid. Heating elements 13, for example tubes, are housed inside said recirculation chamber 12.

[0043] The reactor in FIG. 2 comprises a further shell 14 delimiting an outer wall of the recirculation chamber 12 and also defining a secondary reaction chamber 15, between the shells 2 and 14, fed with gaseous ammonia 16 via a toroidal distributor 17 located on the bottom of said chamber.

[0044] The top part of the reactor comprises a top-end chamber 18 which collects the gases released inside the chambers 5, 12 and 15, mainly comprising NH.sub.3 and CO.sub.2. Said gases form a stream 19 of offgases which is carried away towards a scrubber.

[0045] The sound waves source 6 is situated preferably inside the central chamber 5. In this way the sonication affects the liquid phase which recirculates inside the chamber 5, mixing with the urea melt 10.

[0046] The reactor according to FIG. 2 operates in the following manner. The liquid melamine circulates with an ascending movement inside the reaction zone 5 and with a descending movement inside the recirculating zone 12, which is also heated by the action of the bodies 13. A recirculation in the direction of the arrows shown in the figures is thus created. It should be noted that, owing to the recirculating movement, the sonication affects all the liquid mass recirculating in the reactor, despite the fact that the source 6 is located only in a specific zone, i.e. inside the primary zone 5 which is directly fed with the urea melt 10.

[0047] During normal operation the liquid melamine reaches the level indicated in the figure by the line 20 and flows over the top edge of the shell 14, passing into the peripheral chamber 15. Inside said chamber 15, the liquid melamine undergoes stripping owing to the counter-flow of gaseous ammonia 16 fed from the toroidal distributor 17. The stripped melamine 21 is collected at the bottom of said chamber 15; the gases released during the processwhich contain CO.sub.2 and ammoniaare collected in the top chamber 18 and discharged from the line 19 towards a scrubber.

[0048] The invention may be subjected to variations. For example, with reference to FIG. 2, the sound waves source may be located inside the recirculation chamber 12. The sound waves source 6 may be of the electronical or mechanical type. Said source may be formed, in an equivalent manner, with a plurality of sources.

[0049] The example in FIG. 2 shows a melamine reactor which comprises internally the primary reaction zone, inside the central chamber 5 and the recirculation chamber 12, and the secondary or stripping reaction zone, inside the annular chamber 15. The invention, however, is also applicable to conventional reactors which comprise only the primary reaction zone. In this case, stripping with ammonia is performed in an external reactor. In other embodiments, which also fall within the scope of the present invention, the scrubber of the offgases 19 may be incorporated in the same reactor, for example incorporated in the top part of the reactor 1.