Patent EA-000931-B1 (EA)
Title: Process for producing pure melamine
Inventor: CANZI LORENZO
IPC: C07D251/60
Filed: 1996-12-04
Abstract: 1. A process for the production of highly pure melamine which comprises liquid, ammonia-containing melamine being rapidly depressurized from an ammonia partial pressure p1 between 400 and 50 bar to an ammonia partial pressure p2 between 200 bar and atmospheric pressure, where p1 is always greater than p2, at a temperature which is 0 to 60 degree C higher than the melamine solidification point dependent on the particular prevailing ammonia partial pressure, but is below 350 degree C, higher pressures permitting a greater temperature interval from the melamine solidification point than lower pressures, by which means pure melamine separates out in solid form, whereupon, in any sequence, the product is further depressurized if appropriate to atmospheric pressure, cooled to room temperature and the pure melamine is isolated.  2. The process as claimed in claim 1, wherein the pressure of melamine to be purified and present in the form of melt or liquid phase, as, for example, in the high pressure reactor pressing line designed for melamine synthesis by urea conversion bringing the pressure to an ammonia partial pressure to carry out rapid depressurizing between about 400 and 50 bar and at a temperature which is 0 to 60 degree C higher than the melamine solidification point dependent on the particular prevailing ammonia partial pressure. 3. The process as claimed in claim 1 or 2, wherein the pressure of melamine is rapidly depressurized from an ammonia partial pressure between about 400 and 80 bar and at a temperature which is 0 to 40 degree C higher than the melamine solidification point dependent on the particular prevailing ammonia partial pressure. 4. The process as claimed in claim 1 or 2, wherein the pressure of melamine is rapidly depressurized from an ammonia partial pressure between 300 and 100 bar and at a temperature which is 0 to 20 degree C higher than the melamine solidification point dependent on the particular prevailing ammonia partial pressure. 5. The process as claimed in claim 1, wherein the temperature is lowered. 6. The process as claimed in one of claims 2-5, wherein the temperature is lowered gradually at a cooling rate from 0.8 to 10 degree C/min. 7. The process as claimed in one of claims 2-6, wherein during the process ammonia is loaded. 8. The process as claimed in one of claims 2-7, wherein gaseous mixture NH3/CO2 formed in the process of urea conversion separate from liquid melamine, and CO2 dissolved in liquid melamine by introducing gaseous ammonia. 9. The process as claimed in one of claims 1-8, wherein prior to rapid depressurizing, the liquid melamine remains for a mean residence time from 5 min. to 20 hours. 10. The process as claimed in one of claims 1-9, wherein prior to rapid depressurizing, the liquid melamine remains for a mean residence time from 10 min. to 10 hours. 11. The process as claimed in one of claims 1-10, wherein prior to rapid depressurizing, the liquid melamine remains for a mean residence time from 30 min. to 4 hours. 12. The process as claimed in one of claims 1-11, wherein prior to rapid depressurizing the liquid melamine is saturated with ammonia. 13. The process as claimed in one of claims 1-12, wherein the liquid, ammonia-containing melamine is rapidly depressurized at a temperature close, if appropriate, to the temperature of melamine solidification or higher than that dependent on the particular prevailing ammonia partial pressure. 14. The process as claimed in one of claims 1-13, wherein the liquid, ammonia-containing melamine is rapidly depressurized to the pressure between atmospheric pressure and about 200 bar. 15. The process as claimed in one of claims 1-14, wherein the liquid, ammonia-containing melamine is rapidly depressurized to the pressure between atmospheric pressure and about 150 bar. 16. The process as claimed in one of claims 1-14, wherein the liquid, ammonia-containing melamine is rapidly depressurized to the pressure between atmospheric pressure and about 50 bar. 17. The process as claimed in one of claims 1-14, wherein the depressurizing is carried out in the reactor into which the liquid melamine is introduced. 18. The process as claimed in one of claims 1-14, wherein depressurizing is carried out by transferring or atomizing the liquid melamine in one or moe reactors. 19. The process as claimed in one of claims 17 or 18, wherein the ammonia atmosphere is present in the reactors. 20. The process as claimed in one of claims 18 or 19, wherein the temperature in the reactors is about the same as that in the receiver from which the depressurizing is carried out. 21. The process as claimed in one of claims 1-20, wherein after the depressurizing the solid melamine is remained under pressure between 200 bar and atmospheric during 1 to 20 minutes. 22. The process as claimed in one of claims 1-20, wherein after the depressurizing the solid melamine is remained under pressure between 200 bar and atmospheric during 10 minutes to 10 hours. 23. The process as claimed in one of claims 1-20, wherein after the depressurizing the solid melamine is remained under pressure between 200 bar and atmospheric during 30 minutes to 3 hours. 24. The process as claimed in one of claims 21 to 23, wherein after the depressurizing the solid melamine is remained at a temperature below about 290 degree C. 25. The process as claimed in one of claims 21 to 24, wherein the solid melamine is further remained at a temperature between 280 and 250 degree C. 26. The process as claimed in one of claims 21 to 24, wherein the solid melamine is cooled to room temperature using cold liquid medium, for example, liquid ammonia, by mixing with cold gases or by heat exchangers. 27. The process as claimed in one of claims 1-26, wherein the prpcess is carried out continuously. 28. The process as claimed in one of claims 1-13, wherein the process is caried out immediately after obtaining the melamine at a high pressure. 29. The process as claimed in one of claims 1, 9-28, wherein the melamine to be purified initially present in a solid state is heated to an ammonia partial pressure between 400 and 50 bar to the temperature above the temperature of the melamine solidification by about 0 to 60 degree C dependent on the particular prevailing ammonia partial pressure. 30. The process as claimed in one of claims 1, 9-29, wherein the melamine to be purified initially present in a solid state is heated to an ammonia partial pressure between 400 and 80 bar to the temperature above the temperature of the melamine solidification by about 0 to 40 degree C dependent on the particular prevailing ammonia partial pressure. 31. The process as claimed in one of claims 1, 9, wherein the melamine to be purified initially present in a solid state is heated to an ammonia partial pressure between 300 and 100 bar to the temperature above the temperature of the melamine solidification by about 0 to 20 degree C dependent on the particular prevailing ammonia partial pressure.