METHOD FOR REVAMPING A HIGH PRESSURE MELAMINE PLANT

Abstract

A high pressure melamine plant comprising a synthesis section and a melamine purification section, wherein the synthesis section provides a first melamine-containing stream (6), and said melamine purification section comprises: a quencher (2) receiving said first melamine-containing stream (6) from the synthesis section; a stripper (3) fed with a second melamine-containing stream (7) from said quencher (2), wherein purified melamine (11) is collected and vapours (10) are extracted; a heat exchanger (12), wherein said vapours (10) are condensed providing a condensed stream (14); an absorber (4) fed with said condensed stream (14) providing an aqueous solution (15) comprising ammonia and carbon dioxide, at least a portion (15b) of said aqueous solution being exported from the purification section (200) of the plant.

Claims

1. A high pressure melamine plant comprising a synthesis section and a melamine purification section, wherein said synthesis section provides a first melamine-containing stream, and said melamine purification section comprises: a quencher receiving said first melamine-containing stream, which is a raw melamine melt, from the synthesis section; a stripper fed with a second melamine-containing stream from said quencher and also fed with a stripping medium, wherein purified melamine is obtained and vapours are extracted; a heat exchanger, wherein said vapours from the stripper are at least partially condensed by heat exchange with a cooling medium, obtaining a condensed stream; and an absorber fed with said condensed stream from said heat exchanger and providing an aqueous solution comprising ammonia and carbon dioxide, at least a portion of said aqueous solution being exported from the purification section of the plant; wherein said first melamine-containing stream is free, or substantially free, of melamine offgas.

2. The plant according to claim 1, wherein a first portion of said aqueous solution is recirculated inside the absorber to act as absorbing medium for said condensed stream, and a second portion of said aqueous solution is exported from the purification section of the plant.

3. The plant according to claim 1, comprising a line arranged to introduce a stream of water into the purification section, wherein: a first portion of said stream of water forms said cooling medium for the at least partial condensation of said stripper vapours; a second portion of said stream of water is directed to the quencher; said first portion of water, after leaving said heat exchanger, and said second portion of water act as quenching medium of said first melamine-containing stream.

4. The plant according to claim 3, comprising a line arranged to inject ammonia into said stream of water.

5. The plant according to claim 1, wherein said cooling medium leaving said heat-exchanger is at least partially supplied to the quencher, preferably after being heated in a further heat exchanger.

6. The plant according to any of the previous claim 1, wherein said synthesis section comprises a first melamine reactor, a second stripping reactor and a scrubber.

7. A process for purifying a melamine-containing stream from the synthesis section of a high pressure melamine plant, in a melamine purification section of said plant, wherein said melamine-containing stream is provided by said synthesis section free, or substantially free, of melamine offgas and is subjected to a quenching process which dissolves melamine to provide a second melamine-containing stream; said second stream is subjected to stripping with a stripping medium which provides purified melamine and vapours; said vapours are at least partially condensed by heat exchange with a cooling medium providing a condensed stream; and said condensed stream is subjected to an absorption process providing an aqueous solution, which is at least partially exported from said purification section.

8. The process according to claim 7, wherein said quenching process is carried out with a quenching medium which has a carbon dioxide content lower than 5% in weight, more preferably lower than 1%.

9. The process according to claim 7, wherein a first portion of a stream of water forms said cooling medium and a second portion of said stream of water is directed to the quenching process.

10. The process according to claim 9, wherein ammonia is injected into said stream of water.

11. The process according to claim 7, wherein said cooling medium used to condense the vapours from the stripping process is directed to the quenching process, preferably after being further heated.

12. A method for revamping a melamine purification section of a high pressure melamine plant comprising: a quencher fed with a first melamine-containing stream, which is a raw melamine melt, from the synthesis section and an aqueous solution comprising ammonia and carbon dioxide, wherein melamine is dissolved and off-gas are extracted; a stripper fed with a second melamine-containing stream from said quencher and with a stripping medium, wherein purified melamine is obtained and vapours are extracted, said vapours being added with a stream of water once extracted from the stripper; a first heat exchanger, wherein the vapours added with said stream of water are at least partially condensed by heat exchange with an aqueous solution comprising ammonia and carbon dioxide, providing a condensed stream; and an absorber fed with said condensed stream and a carbonate solution and providing an aqueous solution comprising ammonia and carbon dioxide, which is at least partially used for condensing the vapours in said first heat exchanger, heated in a second heat-exchanger and recycled to the quencher, said method being characterized by installing a line for at least partially feeding a stream of water, possibly added with ammonia, to the first heat exchanger for condensing said vapours; installing a line for at least partially sending the water leaving said heat exchanger to the quencher; installing a line for at least partially exporting the aqueous solution provided by said absorber from the purification section of the plant; discontinuing the line for feeding said aqueous solution to the first heat exchanger, for subsequent feeding to the second-exchanger and recycle to the quencher; discontinuing the line for adding said stream of water to the vapours extracted from the stripper; and discontinuing the line for introducing the carbonate solution into the absorber.

13. The method according to claim 12, wherein a line is installed for feeding a portion of said stream of water, possibly added with ammonia, to the quencher bypassing the first heat exchanger.

14. The method according to claim 12, a portion of the aqueous solution extracted from the absorber being directly supplied to the quencher, the method being characterized by discontinuing the line for directly supplying said aqueous solution to the quencher.

15. The method according to claim 12, wherein the line for extracting the off-gas from the quencher is discontinued.

16. A method for revamping a melamine purification section of a high pressure melamine plant comprising: a quencher fed with a first melamine-containing stream from the synthesis section and an aqueous solution comprising ammonia and carbon dioxide, wherein melamine is dissolved and off are extracted; a stripper fed with a second melamine-containing stream from said quencher and with a stripping medium, wherein purified melamine is obtained and vapours are extracted; a first heat exchanger, wherein said vapours are at least partially condensed by heat exchange with an aqueous solution, providing a condensed stream; and an absorber fed with said condensed stream from said first heat exchanger and providing an aqueous solution, which is at least partially used for condensing the vapours in said first heat exchanger, re-condensed in a second heat-exchanger and recycled to the quencher, said method being characterized by installing a line feeding a stream of water, possibly added with ammonia, to the quencher, at least a portion of said stream of water being heated inside said second heat exchanger; dismissing the stripper; dismissing the first heat exchanger; redirecting the stripping medium to the quencher; installing a line for exporting said second melamine-containing stream from said quencher; and dismissing the absorber.

17. The method according to claim 12, wherein a line for injecting ammonia to said stream of water is installed.

Description

DESCRIPTION OF THE FIGURES

[0058] FIG. 1 is a block scheme of the melamine plant according to a preferred embodiment of the invention.

[0059] FIG. 2 is a schematic diagram of the high-pressure section of the plant according to FIG. 1.

[0060] FIG. 3 is a block scheme of a melamine plant according to the prior art.

[0061] FIG. 4 is a block scheme of a melamine plant after revamping of the plant of FIG. 3, according to another embodiment of the invention.

DETAILED DESCRIPTION

[0062] With reference to FIG. 1, the melamine plant 1 comprises a high-pressure section 100 and a purification section 200 which operates at significantly lower pressure and temperature.

[0063] The high-pressure section 100 is fed with urea melt 5 and produces melamine 6. As the melamine 6 leaves the high-pressure section 100, the pressure is lowered from above 7 MPa to 0.4-2 MPa and melamine is supplied to the purification section 200.

[0064] Said purification section 200 essentially comprises a quencher 2, a stripper 3 and an absorber 4. In the quencher 2, melamine 6 is treated at around 160 C. with aqueous solutions of ammonia 13c, 13b to dissolve melamine. Melamine is collected from the bottom of the quencher 2 as an aqueous solution 7, further diluted with a stream of water 8 coming from the plant and fed to the stripper 3.

[0065] The stripper 3 is further fed with steam 9, which preferably comes from the plant, and with a further stream of water 19 coming from the plant. In the stripper 3, vapours 10 are extracted from the top and an ammonia- and carbon dioxide-free melamine solution 11 is collected from the bottom. Said solution 11 is then subjected to further purification, comprising for example: filtration, clarification with activated carbon, crystallization, solid separation of the melamine crystals and drying.

[0066] The vapours 10 are then sent to a heat exchanger 12, where they are condensed by thermal exchange with an aqueous solution of ammonia 13a.

[0067] The condensed vapours 14 are fed into the absorber 4, which provides an aqueous solution 15. Said aqueous solution is split into two portions: the first portion 15a is passed through a cooler 16 and recirculated inside the absorber 4 and the second portion 15b is exported from the purification section 200 and preferably recycled to the plant.

[0068] A stream of water 13 recycled from the plant and an ammonia stream 17 are mixed to provide an aqueous ammonia solution, which is split into portions 13a, 13b. The first portion 13a is fed to the above heat exchanger 12, where it is preheated to provide stream 13c, and the second portion 13b is supplied to the quencher 2.

[0069] Said stream 13c of preheated aqueous solution is fed to another heat exchanger 18, where it is further heated, and supplied to the quencher 2.

[0070] FIG. 2 shows the high-pressure section 100 of the plant 1 of FIG. 1 according to an embodiment of the invention, which comprises essentially a first melamine reactor 101, a second stripping reactor 102 and a scrubber 103.

[0071] In particular, the scrubber 103 is fed with the urea melt 5. The urea melt leaving the scrubber passes through a pump 20 and a first portion 5a of said urea melt is recirculated inside the scrubber 103 after passing through a heat exchanger 21, where it is advantageously cooled producing vapour, and a second portion 5b of said urea melt is supplied to the melamine reactor 101.

[0072] The melamine reactor 101 is also fed with a heat-carrier fluid 22 which supplies heat to the melamine reactor 101 in order to promote the endothermic reaction for conversion of urea into melamine. Typically said heat-carrier fluid 22 consists of molten salts which circulate inside suitable heating pipes of the reactor.

[0073] Raw melamine is separated as stream 23 and off-gas are extracted as stream 24.

[0074] Said raw melamine 23 is sent to the second stripping reactor 102, which is further fed with gaseous ammonia 25 as stripping agent. The purified melamine 6 leaving the stripping reactor 102 is fed to the quencher 2 of the purification section 200 of the plant, generally operating at a lower pressure than the synthesis section 100, and the off-gas 26 extracted therefrom are combined with the off-gas 24 from the melamine reactor 101, providing a stream 27.

[0075] Said stream 27 is introduced into the scrubber 103, where it undergoes washing with the urea melt streams 5 and 5a, respectively feeding and recirculating streams. A high pressure stream of anhydrous off-gas 28, mainly composed of ammonia and carbon dioxide, is extracted from the scrubber 103.

[0076] For example, the urea melt 5 is produced in a urea plant connected to the melamine plant comprising the section 100. The off-gas 28, in view of their content of NH3 and CO2 (which constitute the reagents for obtaining urea), are preferably recycled to said urea plant.

[0077] FIG. 3 shows a melamine plant according to the prior art, comprising a high-pressure section 100 and a purification section 200.

[0078] The high-pressure section 100 essentially comprises the melamine reactor 101, which is fed with the urea melt 5 and produces a stream 23 containing melamine and off-gas, which is supplied to the purification section 200.

[0079] The purification section 200 essentially comprises a quencher 2, a stripper 3 and an absorber 4. In the quencher 2, which is fed with said stream 23 containing melamine and off-gas and an aqueous solution 15d comprising ammonia and carbon dioxide, melamine 7 is collected from the bottom and off-gas 30 are extracted from the top.

[0080] The off-gas 30 leaving the top of the quencher 2 are saturated with water and conveyed to a condensation section (not shown); melamine 7 is diluted with a stream of water 8 and fed to the stripper 3, which is supplied with steam 9 and a further stream of water 19.

[0081] Vapours 10, which contain ammonia and carbon dioxide, are removed from the top of the stripper 3 and an ammonia- and carbon dioxide-free melamine solution 11 is collected from the bottom thereof. Said solution 11 is then subjected to further purification.

[0082] The vapours 10 are added with an amount of water 32, preferably recycled from the plant, condensed in a heat exchanger 12 and then absorbed in water within the absorber 4.

[0083] Said absorber is also fed with a carbonate solution 31, providing an aqueous solution 15. Said aqueous solution 15 is split into three portions: the first portion 15a is passed through a cooler 16 and recirculated inside the absorber 4, the second portion is recycled to the quencher 2 through a first line 15c and the third portion is recycled to the quencher 2 through a second line 15d.

[0084] The first line 15c provides direct supply of the above solution to the quencher 2, while the second line 15d provides: passage of the solution through the above mentioned heat exchanger 12 for condensing the off-gas 10 extracted from the stripper 3; passage through a further heat exchanger 18 where it is further heated; supply to the quencher 2.

[0085] Said plant of the prior art is advantageously revamped to provide the melamine plant illustrated in FIG. 1, by means of the following operations:

[0086] installation of a line 13 for recycling a stream of water from the plant;

[0087] installation of a line 17 for injecting ammonia into said stream of water;

[0088] installation of a line 13a for feeding a first portion of said stream of water containing ammonia to the first heat exchanger 12, wherein said stream of water 13a is preheated;

[0089] installation of a line 13b for feeding a second portion of said stream of water containing ammonia to the quencher 2;

[0090] installation of a line 13c for sending the preheated water to the second heat exchanger 18 and then to the quencher 2;

[0091] installation of a line 15b for at least partially exporting the aqueous solution provided by said absorber 4 from the purification section 200 of the plant;

[0092] discontinuing the line 15d for feeding said aqueous solution to the first heat exchanger 12, for subsequent feeding to the second-exchanger 18 and recycle to the quencher 2;

[0093] discontinuing the line 15c for directly supplying said aqueous solution to the quencher 2;

[0094] discontinuing the line 32 for adding said stream of water 32 to the vapours 10 extracted from the stripper 3;

[0095] discontinuing the line 31 for introducing the carbonate solution into the absorber 4;

[0096] discontinuing the line 30 for extracting the off-gas 30 from the quencher 2.

[0097] According to this embodiment, the revamping of the purification section 200 is carried out after revamping of the high pressure synthesis section to provide the synthesis section of FIG. 2.

[0098] According to another embodiment of the invention, the plant of the prior art is revamped to provide the melamine plant illustrated in FIG. 4, by means of the following operations:

[0099] redirecting the stripping medium (9) to the quencher (2);

[0100] installation of a line 13 for recycling a stream of water from the plant;

[0101] installation of a line 17 for injecting ammonia into said stream of water;

[0102] installation of a line 13c for feeding a first portion of said stream of water containing ammonia to the second heat exchanger 18 and then to the quencher 2;

[0103] installation of a line 13b for feeding a second portion of said stream of water containing ammonia to the quencher 2;

[0104] installation of a line for exporting said second melamine-containing stream 7 from said quencher 2, said stream 7 being sent to a further purification section (not shown);

[0105] installation of a pump 30 downstream the quencher 2, which receives said second melamine-containing stream 7 before being sent to said further purification section;

[0106] dismissing the stripper 3, the first heat exchanger 12, the absorber 4 and the cooler 16;

[0107] discontinuing the inlet and outlet lines to/from the stripper 3, the first heat exchanger 12, the absorber 4 and the cooler 16;

[0108] discontinuing the line 30 for extracting the off-gas 30 from the quencher 2.

[0109] According to this embodiment, the revamping of the purification section 200 is carried out after revamping of the high pressure synthesis section to provide the synthesis section of FIG. 2.