METHOD FOR PRODUCING MELAMINE WITH PURIFICATION OF MELAMINE OFF-GAS

Claims

1. A method for synthesizing melamine, in the implementation of which: a reaction is carried out with respect to the feed stream (1) of the urea melt in the melamine synthesis section (10) under conditions of non-catalytic melamine synthesis at high pressure to produce a crude melamine product (2) and an exhaust gas (3) containing ammonia and carbon dioxide; subjecting the exhaust gas (3) to a purification process by washing the exhaust gas (3) with a fusion (21, 121) of urea to obtain purified exhaust gas (4) and a fusion (5) of urea containing, as a result of the washing process, ammonia and melamine precursors, wherein the purification process is carried out in a single purification stage (6), and the exhaust gas (3) to be purified and the urea melt (21, 121) are introduced into this single purification stage (6), and the purified exhaust gas (4) is extracted from the said single purification stage (6), the urea melt (21, 121) used for the off-gas washing process at the cleaning stage includes a fresh urea melt (15) and a recycled urea melt (8), said recirculated urea melt (8) is removed from the purification stage (6) after contact with the exhaust gas (3) subjected to the cooling process and reintroduced into the purification stage (6) after the cooling process, the said cooling process is carried out in a shell-and-tube heat exchanger (11), where during the cooling process the recirculated urea melt (8), optionally with the addition of fresh urea melt (15), is cooled to a temperature of at least 165°C.

2. The method according to claim 1, wherein the cooling temperature of the recirculated urea melt 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 claim 1 or 2, wherein the temperature to which the recirculated urea melt is cooled is in any of the following ranges: from 165 to 245°C, or from 170 to 235°C, or from 175 to 225°C, or from 180 to 220°C.

4. The method according to any one of claims 1 to 3, wherein the fresh urea melt (15) is mixed with the recirculated urea melt (8) before the cooling process, or the fresh urea melt (15) is mixed with the recirculated urea melt (9) after cooling.

5. The method according to any one of claims 1 to 4, wherein the shell-and-tube heat exchanger (11) is installed in the recirculation line (19) outside the purification stage (6), and the fresh urea melt (15) is added to the recirculated urea melt (8, 9) at an input point located in the recirculation line (19) downstream of the shell-and-tube heat exchanger (11) or after it.

6. A method according to any of the preceding claims, wherein at the purification stage (6) the fresh urea melt (15) and the recirculated urea melt (8, 9), separately or mixed in a single stream (21, 121), are sprayed from the upper part of the purification stage (6), and the exhaust gas (3) to be purified is introduced into the lower section of the purification stage so that the exhaust gas (3) flows upward in countercurrent with the urea melt (15) and the recirculated urea melt (8).

7. A method according to any of the preceding claims, in which the recirculated urea melt (8), directed to the heat exchanger (11), with the possible addition of fresh urea melt (15), is cooled in the tube space of the heat exchanger (11), and the heat extracted in the heat exchanger (11) from the urea melt is used to generate steam (12) in the intertube space of this heat exchanger (11).

8. The method according to claim 7, wherein the temperature of the steam (12) generated in the intertube space of the heat exchanger (11) is from 160 to 240°C, preferably from 165 to 230°C, and preferably the steam (12) is saturated steam at a pressure of at least 6 barg.

9. The method according to any of the preceding claims, wherein the urea melt withdrawn from the lower part of the purification stage has a temperature in the range from 170 to 250°C, preferably from 175 to 240°C.

10. The method according to any of the preceding claims, wherein the melt (2) of crude melamine is subjected to further processing to obtain a solid melamine product (22), and the weight ratio between the recycled melt (8, 9) of urea and the solid melamine product (22) is between 11 and 30, or between 13 and 28, or between 14 and 22.

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

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

13. A method according to any of the preceding claims, wherein the synthesis of melamine comprises a conversion step and a stripping step, wherein in the conversion step a reaction is carried out with respect to the feed stream (1) of urea melt under melamine synthesis conditions at high pressure to produce a crude melamine product containing carbon dioxide, and in the stripping step the carbon dioxide-containing crude melamine product is stripped in the presence of gaseous ammonia (7).

14. The method according to claim 13, wherein the synthesis of melamine is carried out in a synthesis section (10) comprising a primary reactor followed by a secondary reactor, wherein the stripping step is carried out in the secondary reactor, and the off-gas (3) subject to the purification process comprises the off-gas removed only from the primary reactor, or the off-gas subject to the purification process comprises the off-gas removed from the primary reactor and the off-gas removed from the secondary reactor; or the synthesis of melamine is carried out in a synthesis section comprising a single reactor, and the off-gas subjected to the purification process is removed from this single reactor.

15. A 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 accumulated at the bottom of the purification stage.

16. A method according to any of the preceding claims, wherein the cleaning process is carried out at a pressure of from 50 to 200 bar.

17. A method for the combined synthesis of urea and melamine, in which: ammonia and carbon dioxide are reacted in the urea synthesis section to form a urea solution; this solution is processed in at least one regeneration section to obtain a purified urea solution; water is removed from this solution to form a urea melt (14); the said melt (14) of urea is used in the method for synthesizing melamine according to any of paragraphs 1-15; Melamine off-gas (3) produced during melamine synthesis is recycled to produce urea.