METHOD OF PRODUCING MELAMINE WITH TWO-STAGE PURIFICATION OF MELAMINE OFF-GAS

Claims

1. A method for synthesizing melamine, comprising the steps of: introducing into the reaction a feed stream (1) of urea melt under conditions of melamine synthesis at high pressure in the absence of a catalyst to obtain a crude melamine product (2) and an exhaust gas (3) containing ammonia and carbon dioxide, performing a process for cleaning the exhaust gas (3) by washing the exhaust gas with a urea melt to obtain a purified exhaust gas (4) and a urea melt (5) containing ammonia and melamine precursors, wherein the cleaning process includes a first cleaning step performed in a first cleaning stage (6) and a second cleaning step performed in a second cleaning stage (7), wherein the purification steps are carried out sequentially in such a way that the exhaust gas (3) from the melamine synthesis is subjected to a first purification step, and the partially purified gas thus obtained is subjected to further processing in a second purification step, moreover: during the first cleaning step, the exhaust gas (3) comes into contact with a urea melt containing a recirculating urea melt (8) containing ammonia and melamine precursors, wherein the recirculating urea melt is withdrawn from the first stage (6) and reintroduced into the same first stage, characterized in that the recirculating melt (8) of urea is cooled in a shell-and-tube heat exchanger (11) to a temperature of at least 165°C before being reintroduced into the first stage (6).

2. The method according to claim 1, wherein the temperature to which the recirculated urea melt is cooled is not less than 170°C, or not less than 175°C, or not less than 180°C, and preferably not more than 245°C, or not more than 235°C, or not more than 225°C, or not more than 220°C.

3. The method according to item 1 or 2, wherein the temperature to which the recirculating melt (8) of urea is cooled is in any of the following ranges: 165-245°C, or 170-235°C, or 175-225°C, or 180-220°C.

4. The method according to any one of paragraphs 1-3, wherein the recirculating urea melt (8) is cooled in the tubular part of the heat exchanger (11), and the heat removed from the urea melt is used to produce steam (12) in the shell part of the heat exchanger (11).

5. The method according to claim 4, wherein the temperature of the steam (12) obtained in the shell part of the heat exchanger (11) is from 160°C to 240°C, preferably from 165°C to 230°C, and, preferably, the steam (12) is saturated steam at a pressure of at least 6 bar g.

6. The method according to any one of paragraphs 1-5, wherein at the first stage (6) of purification the exhaust gas (3) contacts in countercurrent with the urea melt containing the recirculated urea melt (8) saturated with ammonia and melamine precursors, then at the second stage (7) the exhaust gas exiting the first stage (6) contacts in countercurrent with the fresh urea melt (15), wherein the fresh urea melt is introduced into the second stage (7) and passes sequentially through the second stage (7) and the first stage (6).

7. The method according to any of the preceding claims, wherein the urea melt (5) withdrawn from the first purification stage has a temperature in the range from 170°C to 250°C, preferably from 175°C to 240°C.

8. The method according to any of the preceding paragraphs, wherein the second cleaning step is carried out in a temperature range from 135°C to 230°C.

9. The method according to any one of the preceding claims, wherein the crude melamine melt (2) is further processed to obtain a solid melamine product (22), and wherein the weight ratio between the recirculating urea melt (8, 9) and the solid melamine product (22) is from 11 to 30, or in the range from 13 to 28, or in the range from 14 to 22.

10. The method according to any of the preceding claims, wherein the feed stream (1) of urea melt comprises fresh urea melt (16) and a portion (17) of the urea melt (5) containing ammonia and melamine precursors, withdrawn from the first stage (6) of purification.

11. The method according to any of the preceding paragraphs, wherein carbon dioxide (18) is added to the exhaust gas (3) at the first stage (6) of purification.

12. The method according to any of the preceding claims, wherein the synthesis of melamine comprises a conversion step and a washing step, wherein in the conversion step the feed stream (1) of urea melt is reacted under melamine synthesis conditions at high pressure to obtain a crude melamine product containing carbon dioxide, and in the washing step the crude melamine product containing carbon dioxide is washed in the presence of gaseous ammonia (11).

13. The method according to claim 12, wherein: the synthesis of melamine is carried out in a synthesis section (10) containing a primary reactor followed by a secondary reactor, wherein the washing step is carried out in the secondary reactor, and the off-gas (3) subject to the purification process includes off-gas withdrawn only from the primary reactor, or the off-gas subject to the purification process includes off-gas withdrawn from the primary reactor and off-gas withdrawn from the secondary reactor, or wherein the synthesis of melamine is carried out in a synthesis section containing one reactor, and the off-gas subject to the purification process is withdrawn from this one reactor.

14. The method according to any of the preceding claims, wherein the off-gas (3) is introduced through an off-gas distributor above or below the liquid level of the urea melt (5) containing ammonia and melamine precursors.

15. The method according to any of the preceding paragraphs, wherein the cleaning process is carried out in a scrubber (20), and the fresh melt (15) of urea is divided into several sub-streams before being introduced into the second stage (7) and is introduced into several places in the second stage (7) of the scrubber.

16. A combined method for synthesizing urea and melamine, in which: ammonia and carbon dioxide are reacted to form a urea solution in the urea synthesis section, wherein the solution is treated in at least one regeneration section to obtain a purified urea solution, and water is removed from the solution to form a urea melt (14), the urea melt is used in the melamine synthesis method according to any one of paragraphs 1-15, melamine off-gases (3) formed during the melamine synthesis are recycled for the production of urea.