MXPA97004638A - Procedure for the production of high pur melamine - Google Patents

Abstract

The present invention relates to a process for the production of high purity melamine starting with a conversion of urea effected under pressure, comprising, following the conversion reaction, effecting a further treatment of the melamine, characterized in that it comprises the steps of: a) separating the gas mixture of NH3 / CO2 from the liquid melamine, if appropriate, b) reducing the CO2 dissolved in the liquid melamine by introducing gaseous ammonia, c) allowing the liquid melamine to settle for an average residence time of up to 8 hours at a temperature between 430 ° C and the melting point of melamine and a partial pressure of ammonia of 50 to 400 bar and d) cooling in a slow and controlled manner by decreasing the temperature from the temperature present in a), b ) and / or c) from 330 to 270 ° C at a cooling rate of up to 150 ° C / min at a partial ammonia pressure of 50 to 400 bar, the higher pressures allowing cooling rates faster and, vice versa, lower pressures requiring slower cooling rates, after which the reaction vessel, in any desired sequence, is depressurized and cooled to room temperature and high purity melamine is obtained in the form of pol

Description

PROCEDURE FOR THE PRODUCTION OF HIGH PURITY MELAMINE DESCRIPTION OF THE INVENTION A multiplicity of processes for the production of melamine in the literature is known. A preferred starting material in this case is urea which is converted to melamine, ammonia and CO2 either at high pressure and not catalytically or at low pressure and with the use of a catalyst. Although known high pressure processes, for example, according to Melamine Chemicals, Montedison or Nissan, in which melamine is first formed as a liquid, they have a lower energy consumption compared to low pressure methods, melamine contains, if the purification steps are not present, impurities such as elama, melema, amelina, amelida or ureidomelatrina that interfere with some methods for the additional melamine process. The melamine produced by a high pressure process is prepared, for example, according to US 7 565 867 (Melamine Chemicals) by separating the C02 and NH3 gases from the liquid melamine, the pressure and temperature being maintained at the same values present in the reactor, then of which the liquid medium is fed to a product cooling unit, depressurized and rapidly cooled or quenched with a liquid medium, for example liquid anhydrous ammonia. According to US 3,116,294 (Montecatini), the separation gases of C02 and NH3 are also separated, and the liquid melamine is countercurrently treated with NH3 in order to remove C02, which is still dissolved, is collected in an additional reactor and let it rest in it for a certain time. Finally, the melamine is removed from the second reactor and cooled rapidly by quenching with water and mixing it with cold gases. However, the purity of the melamine, which has been produced by one of these methods is insufficient for many applications, for example in the production of melamine-formaldehyde resins for surface coatings, since, in particular, the content of melema It is too high. According to US 3,637,686 ÍKissan), the unpurified melamine melt obtained by the thermal decomposition of the urea is rapidly cooled from 200 to 270 ° C with liquid NH3 gas or cold NH3 gas and is further cooled from 100 to 200 ° C. in a second step with an aqueous NH3 solution. The product must then be recrystallized in order to obtain a satisfactory melamine purity.

EP-A-0612560 discloses a high pressure melamine reactor, the object of the invention being to combine in an individual high pressure reactor all the apparatuses that are necessary to prepare the melamine in the known industrial high pressure processes. However, only unpurified melamine can be prepared in this process. GB-A-0800722 discloses a sublimation process in which dehydrated urea is passed through a reactor in the presence of ammonia at 350 to 450 ° C and 20 to 10 ° C pressure atmospheres and then sublimated. US-A-3 484 440 also describes a sublimation process in which liquid melamine, before sublimation, is maintained for one hour at a pressure of 40-100 kg / cm2 and a temperature of 420 to 480 ° C together with the separation gases. Under these conditions, on the one hand, the by-products in the melt of melamine are first converted continuously into melamine, on the other hand, the melamine is continuously separated from the gas phase above it and which is saturated with melamine, and the melamine is separated in solid form in the separator using steam at 150 ° C. However, without the continuous separation of the gaseous melamine, it is not possible to prepare high purity melamine, since the by-products would accumulate.

The object of the present invention is therefore to find a method that allows the production of - high purity elastin without the additional purification steps, having a purity above 59.6% and a melema content below 100 ppm. Unexpectedly, it was possible to achieve this goal by a process in which the liquid elastin is cooled slowly and / or under control in the last stage. The present invention therefore relates to a process for the production of high purity melamine with a conversion of urea effected under pressure, comprising, following the conversion reaction, carrying out a subsequent treatment of the melamine, comprising the steps of a) separating the NH3 / C02 gas mixture from the liquid meiamine, if appropriate b) reducing the CO2 dissolved in the liquid melamine by introducing gaseous ammonia c) allowing the liquid melamine to settle for an average residence time of up to 8 hours at a temperature between 430 ° C and the melting point of melamine and a partial pressure of ammonia from 50 to 400 bar and d) cool slowly and controlled by decreasing the temperature from the present temperature in a) , b) and / or c) from 330 to 270 ° C at a cooling rate of up to 150 ° C / min at a partial ammonia pressure of 50 to 400 bar, higher pressures allowing cooling rates faster and, vice versa, lower pressures requiring slower cooling rates, after which the reaction vessel, in any desired sequence, is depressurized and cooled to room temperature and high purity melamine is obtained in the form of dust. The process according to the invention is suitable for the purification of melamine which is obtained from urea according to one of the known methods of high pressure, such as according to the process of Melamine Chemical, Montedison or Nissan, as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5a. Edition, vol. A16, pp 174-1"9. Urea is generally converted to these processes at a temperature regime of 370 to 430 ° C and a pressure of about 70 to 250 bar.The resulting melamine is finally obtained as a liquid phase. according to the novel process, in step a) the reaction mixture formed in the reactor comprising the liquid melamine phase and a gas phase of C02 / NH3 is fractionated in a suitable apparatus, for example in a gas separator, or the gas phase is separated from the liquid phase.The separator is maintained at a temperature above the melting point of the melamine, preferably the temperature and the pressure are approximately the same as those in the reactor. of C02 / NH3, which still contains melamine, is collected from above and processed in a known manner, for example by introducing it into a scrubber, and is reused after the gas separation, or at the same time, the NH3 gaseous can be int This reduces the C02 dissolved in the melamine (step b). The temperature during this is again at a value above the melting point of the melamine; preferably the temperature and the pressure are approximately the same values as those in the reactor. The introduction of NH3, and the duration of the introduction of the gaseous NH3 and the amount of NH3, depend on the desired final value of the C02 dissolved in the melamine. Ammonia can be introduced either in the gas phase or directly in the liquid melamine phase. In the next step, the liquid melamine is allowed to stand or remain if appropriate for a certain time in the presence of ammonia. In this case it is also possible to mix other gases, for example nitrogen. The average residence time in this step is from 0 to 8 hours, but under certain circumstances longer residence times are also possible. Preferably, the residence time is 10 minutes to 4 hours. During this time, a partial ammonia pressure of 50 to 400 bar is set, preferably 70 to 200 bar T The pressure in step (c) can also be set higher than in the reactor. The temperature in this step is at a value between the melting point of melamine and 430 ° C, preferably between the melting point of melamine and 400 ° C. After step c) or b) or a) the liquid melamine is cooled slowly and / or controlled. In this cooling the liquid melamine is cooled from the temperature present in step c) or b) to) at a temperature of between 27 ° C and 33 ° C, preferably from a temperature ≥ 370 ° at a temperature of up to approximately 290 ° C at a defined cooling rate. The cooling rate can be up to 150 ° C / min, preferably up to 1 ° C ° min, in particular up to 4 ° C "nrin is preferred. The lower limit of the cooling regime depends on the technical and economic conditions. It can be selected to be as low as desired according to the existing technical and economic conditions. Step d), like the previous steps, is carried out in the presence of ammonia. The ammonia partial pressure is 50 to 400 bar in this step, preferably around 70 to 200 bar. Again, a higher pressure than that of the reactor can also be set.

The cooling rate to be set is a function of the prevailing ammonia partial pressure, higher pressures permitting faster cooling rates and vice versa, lower pressures requiring slower cooling rates. The cooling rate if appropriate can be varied in the regime for cooling under control, in which case a constant cooling rate can not be set, but a defined cooling program. A defined cooling program means various cooling variants in which the cooling rate can assume different values at various temperatures, it being also possible to vary the pressure. For example, at the beginning of step d), a constant temperature can be set for a certain time and then cooling to the desired final temperature value can be effected at a cooling rate selected as a function of the pressure. Another possible variant is, for example, to alternate fixing phases, in which the temperature is maintained for a certain time at the temperature prevailing at that time, and the cooling phases. You can also set phases of slow cooling and faster in alternating sequence. The cooling phase can, if desired, also be carried out by simply cutting off the heat and allowing the mixture to stand at room temperature, which achieves a slow exponential cooling of the liquid melamine at a present temperature. The cooling program therefore has a plurality of different variants and can be adapted to particular conditions depending on the desired final value of impurities and as a function of the chosen process sequence. Above the regime (d) for cooling slowly and / or under control, for example in particular above the preferred regime in which step d) is carried out, which is above about 370 ° C, cooling it can, depending on the particular conditions, such as the sequence of the procedure or the available apparatus, be carried out either slowly, that is, at a cooling rate of up to 150 ° C / min as a function of the prevailing pressure, or more quickly. Below the cooling rate under control, that is, below 330 to 270 ° C, the reaction apparatus can be depressurized and the melamine can be cooled to room temperature at any cooling rate, after which it is obtained powder high purity meiamine. However, depending on the technical conditions, cooling can be carried out first and then the device can be depressurized.

Steps a) to d) in the novel process can be carried out if appropriate in separate containers or in apparatus suitable for the particular step. Nevertheless, other variants are also possible. Therefore, for example, steps a) and b), and steps c) and d), respectively, can be performed together on the same apparatus. A further possibility is that, following step a), the melamine is transferred to a delay vessel in which steps b) and c) are carried out and step d) is carried out in a separate vessel. The combination of steps a) to c) in a shared apparatus with a subsequent cooling apparatus for step d), is also a possible variant of the process. However, the mode of carrying out the procedure must be adapted to the particular conditions, for example, according to the equipment for the conversion of urea, the conditions of space, the requirement of time planned for the cooling phase, the residence time and other factors. The process according to the invention can, if required, be carried out both discontinuously and continuously. However, the process according to the invention is also suitable, in some modified form, for purifying contaminated melamine arising from any process known in the prior art and containing such contaminants as elina, amelide, melama, melema or ureidome .in. Therefore, it does not necessarily have to be coupled to a melamine plant. Therefore, the melamine of the mother liquor which, for example, arises in the current conventional melamine recrystallization processes can also be purified in this form. The present invention therefore also relates to a process for the production of high purity melamine, which comprises bringing the contaminated melamine to a partial ammonia pressure of 50 to 430 bar at a temperature that is between the melting point of the melamine and 430 ° C, allowing the liquid melamine to remain in this temperature regime for 0 to 8 hours and then carry out a slow controlled cooling, the temperature decreasing from 330 to 270 ° C at a cooling rate of up to 150 ° C / min at a partial pressure of ammonia of 50 to 400 bar, the higher pressures allowing faster cooling rates and, vice versa, the lower pressures requiring slower cooling rates, after which, in any desired sequence, the reaction vessel it is depressurized, cooled to room temperature, and high purity melamine powder is obtained.

By means of the process according to the invention, melamine is obtained at a purity of up to above 99.8%, such that additional purification steps such as recrystallization are no longer necessary. The content of the individual contaminants, in particular melema, can be kept so low in this case that these compounds do not interfere with any type of additional melamine process.

Example rsicl 1 to 5: X g of liquid melamine, obtained by conversion of urea on an industrial scale at 375 ° C and 70 to 75 bar were introduced into a container. A sample was taken from it and cooled rapidly and the initial content of the contaminants was determined. After separating the NH3 / C02 separation gases (step a), the liquid melamine was treated with NH3 for about 15 minutes at 370 ° C and 85 bars (step b). The liquid melamine was then allowed to stand for about 60 to 90 minutes at 370 ° C and an ammonia pressure of 85 bar (step c), and the liquid melamine was allowed to cool exponentially to 280 ° C changing the heat, so that the cooling rate < l ° C / min was achieved (step d). The reaction vessel was then depressurized and cooled slowly to room temperature. The particular amount of melamine, the initial content of the impurities of a-eiin (AN), amelide (AD), melema (ME), melama (MA) and ureidomelamine (UM) and the content of the contaminants after steps a) + b) + c) + d) (final) can be seen in Table 1.

Table 1: Example 1 Melamine AN AC ME ME UM < g > (pp- =) < ppß¡ íppa) í ps) (ppm) Initial 1096 5700 1400 17400 30CC 500 Final 100 < 50 < 50 < 300 < fifty Example 2: Initial 1710 4600 3000 S600 1500 soo Final 100 180 < 50 < 300 < 50 Example 3: Initial 410 4300. 1600 7200 2700 500 Final 180 < 50 < 50 < 300 < fifty Example 4: Initial 666 4300 1000 4500 1600 200 Final 100 c50 < 50 < 300 < fifty Example S: Initial 731 11400 2100 8600 2300 500 Final 130 < 50 < 50 < 300 < 50 Example 6: 1700 g of liquid melamine, produced analogously as in Examples [sic] I to 5, were separated from the NH3 / C02 separation gases, treated for about min with NH3 at a pressure of 84 bar and allowed to stand for 2 hours at 375 ° C and 85 bars. It was removed from the heat, the melamine was allowed to cool slowly to 290 ° C, depressurized and allowed to cool to room temperature. The final values of the contaminants in the melamine were: AN: 100 ppm ME: < 50 ppm UM: < 50 ppm AD: < 50 ppm MA: < 300 ppm Example 7: 1205 g of liquid melamine, produced analogously as in Examples [sic] 1 to 5, were separated from the NH3 / C02 separation gases, treated with NH3 for about 15 min at 85 bar and, without delay time , they were allowed to cool to 290 ° C after the heat had gone out. The reaction vessel was then depressurized and cooled to room temperature. The final values of the contaminants in the melamine were: AN: 270 ppm ME: < 200 ppm UM: < 100 ppm AD: < 50 ppm MA: < 580 ppm The 8: 300 g of melamine containing 8100 ppm of melema and about 65 g of liquid NH3 were heated to 360 ° C. The ammonia pressure was around 80 bars. The melamine was then allowed to stand under these conditions and then cooled slowly from 360 ° C to 330 ° C in 28 minutes (cooling rate of about 0.8 ° C / min). The final content of the contaminants, as a function of residence time, was, after depressurizing the reaction apparatus and cooling to room temperature: h ME ppm MA ppm AN ppm AD ppm 1 2250 < 300 400 50 2 430 < 300 320 < 50 4 160 < 300 310 < fifty Example 9: 300 g of melamine containing 8100 ppm of melema and the amount of liquid NH3 that is necessary to achieve a defined p pressure, were heated to 364 ° C, allowed to stand for 2 hours under these conditions and cooled from 360 to 330 ° C in about 5 minutes (cooling rate 6 ° C / min). The memema content as a function of the particular ammonia pressure was set, then the reaction apparatus was depressurized and cooled to room temperature: p (bar) ME ppm 80 1520 115 250 130 160 151 n: Examples rsicl 10: 300 mg of melamine containing 10,000 ppm of melema and 117 g of liquid ammonia were heated to 370 ° C. The ammonia pressure was 154 bar. The melamine was then allowed to stand under these conditions for 2 hours and cooled from 360 to 330 ° C with a cooling rate m. The content of melema after depressurizing the reaction apparatus and cooling to room temperature was, as a function of the cooling time: m (° C / min) ME ppm 7.5 210 0.8 10C Examples [sic] 11: 90 mg of melamine and the amount of ammonia needed to set an ammonia pressure of 150 bar were heated to 370 ° C in an autoclave, allowed to stand for 4 hours under these conditions and then cooled to 290 ° C at cooling rates average of 18 and 36 ° C / min that were achieved by defined air currents. The reaction vessel was then rapidly cooled to room temperature by immersion in cold water and depressurized. The contents of the initial and final melema were: ME (ppm) Initial 13000 Final (18 ° C / min) 280 Final (36 ° C / min) 600 Example 12: 124 mg of melamine and the amount of ammonia needed to set an ammonia pressure of 200 bar were heated to 370 ° C in an autoclave, allowed to stand for 3 hours under these conditions and then cooled to 320 ° C at a rate of average cooling of approximately 100 ° C / min. The product was then cooled rapidly to room temperature by immersion in cold water and depressurized. The contents of the initial and final melema were: ME (ppm) Initial 13000 Final 250 E ng 13. 300 mg of melamine containing 10,000 ppm of melema and the amount of ammonia needed to set an ammonia pressure of 200 bar were heated to 370 ° C in an autoclave, allowed to stand for 2 hours under these conditions and then cooled to 320 ° C. C at a cooling rate m and depressurized. The melema content was, as a function of the cooling rate: m (° C / min) ME ppm 0.9 < 5C 8 120

Claims (9)

1. A process for the production of high purity melamine starting with a conversion of urea effected under pressure, comprising, following the conversion reaction, effecting a further treatment of the melamine, characterized in that it comprises steps a) separating the gas mixture from NH3 / C02 of the liquid melamine, if appropriate b) reduce the dissolved C0 in the liquid melamine by introducing gaseous ammonia c) allow the liquid melamine to settle for an average residence time of up to 8 hours at a temperature between 430 ° C and the melting point of melamine and a partial pressure of ammonia from 50 to 400 bar; and d) cooling in a slow and controlled manner by decreasing the temperature from the temperature present in a), b) and / or c) from 330 to 270 ° C at a cooling rate of up to 150 ° C / min at a partial pressure of ammonia from 50 to 400 bar, higher pressures allowing faster cooling rates and vice versa; lower pressures require slower cooling rates, after which the reaction vessel, in any desired sequence, is depressurized and cooled to room temperature and high purity melamine in powder form is obtained.

2. The method according to claim 1, characterized in that the residence time in step c) is from 10 minutes to 4 hours.

3. The method according to claim 1, characterized in that the temperature in step c) is between 400 ° C and the melting point of the melamine.

4. The process according to claim 1, characterized in that the ammonia partial pressure in step c) is between 70 and 200 bar.

5. The process according to claim 1, characterized in that the reaction vessel is cooled in step d from 370 to 290 ° C slowly to a controlled cooling rate.

6. The method according to claim 1, characterized in that the cooling rate in step d) is up to 40 ° C / min.

7. The method according to claim 1, characterized in that step d) and step b are carried out simultaneously.

8. The method according to claim 1, characterized in that step a) and step b) are effected sequentially.

9. A process for the production of high purity melamine, characterized in that it comprises bringing the contaminated melamine to a partial pressure of ammonia of 5C at 400 bar at a temperature that is between the melting point of melamine and 430 ° C, allowing the melamine liquid to rest in this temperature regime [lacuna] up to 8 hours and then perform a slow, controlled cooling, the temperature decreasing from 330 to 270 ° C at a cooling rate of up to 150 ° C / min at a partial pressure of ammonia 50 to 400 bar, the higher pressures allowing faster cooling rates and, vice versa, the lower pressures requiring slower cooling rates, after which, in any desired sequence, the reaction vessel is depressurized and cooled to the temperature environment and "'high purity melamine in powder form is obtained.