MXPA00003720A - Process for the preparation of melamine - Google Patents

Abstract

Process for the preparation of melamine from urea via a high-pressure process in which dry melamine powder is obtained by transferring the melamine melt leaving the reactor to a vessel in which the melamine melt is cooled by means of ammonia characterized in that the melamine melt is sprayed into a cooling vessel and cooled by droplets of evaporating liquid ammonia which is sprayed into the same cooling vessel at a pressure above 0.1 MPa and a temperature between 50°C and the melting point of the melamine in the cooling vessel.

Description

PROCEDURE FOR THE PREPARATION OF MELAMINE DESCRIPTIVE MEMORY The invention relates to a process for the preparation of melamine from urea through a high pressure process, in which the solid melamine is obtained by transferring the molten melamine leaving the reactor to a vessel, where the molten melamine is cooled by ammonia. This process is described, inter alia, in US-A-4565867, which describes a high pressure process for the preparation of melamine from urea. US-A-4565867, in particular describes the pyrolysis of urea in a reactor at a pressure of 10.3 to 17.8 MPa and at a temperature of 354 to 427 ° C to produce a reaction product. The product of the reaction contains liquid melamine, C02, and NH3 and is transferred under pressure, as a mixture of streams, to a separator. In this separator, which is maintained at virtually the same pressure and temperature of said reactor, said product of the reaction is separated into a gas stream and a liquid stream. The gas stream contains gases released from C02 and NH3 as well as melamine vapor. The liquid stream substantially consists of liquid melamine. The gaseous product is transferred to a purification unit, while the liquid melamine is transferred to a product cooler. In the sewage unit, the above-mentioned gases C02 and NH3, which contain melamine vapor, are purified at virtually the same pressure as the reactor with molten urea in order to preheat the urea and cool said dissolved gases and remove the melamine what about the gases released. The preheated molten urea containing said melamine is then fed to the reactor. In the product cooler, the pressure of the liquid melamine is reduced and the melamine is cooled by a liquid cooling medium in order to produce a solid melamine product without washing or purifying it. In US-A-4565867, liquid ammonia is preferably used as a liquid cooling medium. A disadvantage of this method is that in a commercial scale production facility, the resulting melamine product is not homogeneous in both particle size and purity. Within the important parameters of quality are the color, the reactivity, and the type and concentration of impurities. In the production of melamine for the preparation of melamine-based resins, the purity and consistency of the product are very important. It is necessary to maintain a low and repeatable level of impurities, for example melem and ammelide for the transparency of melamine-based resins. The purpose of the present invention is to obtain an improved high pressure process for the preparation of melamine from urea wherein the melamine with a consistent product quality is obtained as a dry powder directly from the molten liquid melamine. Applicants have found that the melamine powder having the desired product quality can be obtained by using a process in which the melted melamine is sprayed in a cooling vessel where it cools very quickly upon contact with a few drops of ammonia which is spray simultaneously in the same cooling vessel, the cooling vessel having a pressure above 0.1 MPa and a temperature above 50 ° C and below the melting point of the melamine. The dried melamine powder produced, according to the present invention, is suitable for applications requiring high purity melamine without the need to purify it. The pressure in the cooling container is below 20 MPa and preferably below 200 ° C. In order to maximize the purity of the obtained solid melenin, it is preferable to cool the melted melamine as fast as possible by mixing rapidly and homogeneously with cold ammonia pressures. This method solidifies the molten melamine very fast and even prevents the molten melamine from contacting the wall of the cooling vessel. The contact between the molten melamine and the walls of the cooling vessel generates the formation of melamine lumps containing different levels of impurities that will limit the purity and consistency of the melamine product that can be obtained.

Applicants have further found that it is necessary to minimize any contact between the liquid ammonia and the walls of the cooling vessel. When the spraying of liquid ammonia has not completely evaporated before reaching the wall of the cooling vessel, the liquid ammonia can itself cause the formation of melamine lumps, which contain different levels of impurities that will limit the purity and consistency of the melamine product that can be obtained. To minimize the possibility of liquid ammonia reaching the wall of the cooling vessel, the present method sprays the liquid ammonia in the spray of molten melamine as droplets at a rate sufficient to provide a rapid and uniform mixing of ammonia sprays. and melamine towards the center of the cooling vessel. The small size of the ammonia droplets also increases the rate at which the melamine is cooled by the evaporation of the ammonia. To obtain the benefits of the rapid cooling provided by the present procedure, ammonia sprays should be located near the melamine inlet to the cooling vessel with the address, speed, amount of spray selected to achieve a rapid and uniform mixing of the sprays. ammonia and melamine to obtain a rapid solidification and cooling of the melamine without depositing lumps of melamine on the walls of the cooling vessel. In order to achieve the mixing of the melamine and ammonia sprays of the present process, it is understood that the ammonia spray nozzle and the melamine inlet will generally be located relatively close to each other in the cooling vessel. This need for the close position of the ammonia spray nozzles and the melamine inlet is not reflected in the cooling equipment generally used in the production of melamine of the most advanced technique, by putting into practice current practical procedures for the cooling of the melted melamine or melted melamine paste, the nature, location and rate at which the drying or cooling medium is fed to the cooling vessel is not critical, allowing the operation of said processes in containers having a wide range of physical configurations. However, in practicing the present process, the distance between the melamine inlet and the ammonia spray nozzle becomes important for successful operation. In practice, this distance is required to be less than 2 m, and most preferably less than 1.5 m, which allows satisfactory operation at reasonable ammonia feed conditions. A greater separation between the melamine inlet and the ammonia spray nozzles would cause an undesirable delay in melamine cooling, requires extreme ammonia feed conditions or both. In practicing the present process, the spraying of liquid ammonia and the spraying of molten melamine must be combined at high speeds, regimes and directions that are sufficient to produce the rapid and uniform mixing of the ammonia and melamine droplets. To obtain this mixing, it is preferable that the speed of the liquid ammonia is at least 6 m / s. This velocity (in m / sec) is determined by dividing the volume flow of a liquid (in m3 / sec) by the smallest cross-sectional area for the flow (in m2) in the spray nozzle. Similarly, it is preferred that the molten melamine be sprayed at a high speed. Although the ammonia spray nozzle (s) can be configured to spray the liquid ammonia in a wide variety of directions it is preferred that the nozzles are oriented to spray the ammonia droplets directly into the spray of water droplets. Melamine with the central axes of the ammonia nozzles located to interlace the central axis of the melamine nozzle. To minimize the distance that the ammonia spray must travel to achieve the spraying of molten melamine, it is preferable to orient the ammonia nozzles in such a way that their central axes are approximately perpendicular to the central axis of the molten melamine nozzle. By measuring along the central axis of the ammonia spray nozzle at the intersection with the central axis of the molten melamine nozzle, this configuration causes the ammonia spray distance to equalize the separation distance between the nozzles, preferably less than 2 m It will be understood that ammonia nozzles can also be oriented to provide an angle of intersection of less than 90 ° to produce a larger ammonia spray distance, but it is preferable less than 5 m, as long as other conditions are selected to ensure a necessary, fast and uniform mixing of the sprays of ammonia and molten melamine. It is preferable to use at least two ammonia spray nozzles to provide satisfactory cooling of the melted melamine. Although there is no maximum theoretical number of ammonia spray nozzles that can be used in the practice of the present process, it is anticipated that certain physical and economic considerations will discourage the excessive use of several ammonia spray nozzles. Two physical considerations of this type are the ability to place the spray nozzles in the cooling vessel without interfering in any way with adjacent spray nozzles and the potential for interaction between the liquid ammonia sprayings of adjacent spray nozzles resulting in the formation of larger drops. The larger ammonia droplets, which are less likely to be completely evaporated, are more oriented towards reaching the wall of the cooling vessel and producing the negative results mentioned above. In view of these considerations, the applicants believe that, in practice, the present method would normally operate with less than 25 ammonia spray nozzles. To evaluate the nature of mixing between the spraying of molten melamine and the ammonia spray, it is useful to consider the value of the impulse flow of ammonia and melamine sprays. The impulse flow value of the ammonia spray should be calculated by multiplying the flow of the mass through the ammonia nozzle (in kg / sec) by the velocity (as calculated previously) of the liquid ammonia flow (in m / s) through the ammonia spray nozzles. In the present invention, the impulse value of the ammonia spray is preferably at least 0.1 kg. m / sec2 and most preferably at least 0.2 kg. m / sec2. Similarly, the impulse flow value of the molten melamine spray is 5 kg. m / sec2 and most preferably at least 10 kg.m / s2. An ammonia spray nozzle suitable for use in the present method has been evaluated (using water in a mass flow equal to that predicted by liquid ammonia and atmospheric pressure, for convenience) and has been found to provide both a size distribution of the drops with a d5o less than 1.0 mm and the pressure of the drops through the spray nozzle between 30 KPa and 60 Kpa. One type of spray nozzles found to be satisfactory for this purpose are the spray nozzles SK SprayDry® from Spraying System Company of Wheaton, Illinois. During the cooling process the molten melamine spray drops are cooled and solidified into melamine powder by contacting the molten melamine spray with a spray of small drops of liquid ammonia. The volume of liquid ammonia used may be more than that needed in the solidification of the molten melamine to provide additional cooling of the solid melamine. To maximize the purity and consistency of the melamine produced, it is preferable that the cooling time (the duration of the ammonia spray (as measured above) divided by the sum of the liquid ammonia velocities and the molten melamine feeds ) must be less than 0.04 sec, and preferably less than 0.02 sec. The advantage of the method according to the present invention is that the melamine powder can be obtained on a commercial scale with a purity greater than 97.5 percent by weight and a constant level of several common impurities, that is, a constant product quality. The level of purity and the consistency of impurities means that melamine is sufficient for its use, virtually, in all melamine applications. In the preparation of melamine, urea is preferably used as a starting material in the form of a molten material. NH3 and CO2 are by-products obtained during the preparation of melamine which proceed according to the following reaction: 6 CO (NH2) 2 C3N6H6 + 6 NH3 + 3C02 ^ The preparation can be carried out at a high pressure preferably between 7 and 25 MPa, without the presence of a catalyst. The temperature of the reaction varies between 325 and 450 ° C and is preferably between 370 and 440 ° C. The byproducts of NH3 and C02 are usually returned to an adjacent urea plant. The purpose of the aforementioned invention is achieved in a plant suitable for the preparation of melamine from urea. A plant suitable for the present invention may comprise a sewage unit, a reactor in combination with a gas / liquid separator or with a separate gas / liquid separator, optionally a subsequent reactor, and a cooling and / or expansion vessel. In one embodiment of the method, the melamine is prepared from the urea in a plant consisting of a purifying unit, a melamine reactor, optionally in combination with a gas / liquid separator or a separate gas / liquid separator, optionally a rear reactor and a cooling vessel. The molten urea of a urea plant is fed to a filter unit at a pressure of 7 to 25 MPa, preferably 8 to 20 MPa and at a temperature above the boiling point of the urea preferably between 170 and 270 ° C. This treatment unit must also be provided with a cover that provides additional cooling in the depuration. The purification unit must even be provided with internal cooling bodies. In the purification unit the liquid urea makes contact with the reaction gases of the melamine reactor or the separated gas / liquid separator, installed downstream of the reactor or the subsequent reactor. In the case of the separated gas / liquid separator, the pressure and temperature may differ from the temperature and pressure in the melamine reactor. The reaction gases consist substantially of C02 and NH3 and also contain an amount of melamine vapor. The molten urea clears the melamine vapor out of the detached gases and carries this melamine to the reactor. In the process for the purification of the evolved gases, the urea is heated to 170-270 ° C at the temperature of the reactor, that is, from 370-440 ° C, to 170-270 ° C. The detached gases are removed from the top of the treatment unit and returned to a urea plant to be used as a new material for the production of urea. The preheated urea is taken out of the treatment unit together with the washed melamine and fed, by means of a high pressure pump, to the reactor, which has a pressure of 7 to 25 MPa and preferably of 8 to 20 MPa. Gravity can be used to transfer the molten urea to the melamine reactor by placing the filter unit above the reactor. In the reactor, the molten urea is heated to a temperature of 325 to 450 ° C, preferably from 370 to 440 ° C, at a pressure as described above, conditions under which the urea is converted to melamine, C02 and NH3. For the reactor, an amount of ammonia in the form of liquid or hot steam can be measured. The ammonia supplied serves to prevent the formation of melamine condensation products such as melam, melem and melon or to promote mixing in the reactor. The amount of ammonia fed to the reactor is from 0 to 10 moles per mole of urea, preferably from 0 to 5 moles of ammonia, and in particular from 0 to 2 moles of ammonia per mole per mole urea. The C02 and NH3 formed in the reaction as well as the additional amount of ammonia supplied, is collected in the separation section, for example, in the upper part of the reactor, but a separate gas / liquid separator downstream of the reactor is also possible. , and are separated in gaseous form from the liquid melamine. The resulting gas mixture is sent to the treatment unit to remove the melamine vapor and to preheat the molten urea. The liquid melamine is removed from the reactor and can be transferred to a subsequent reactor, in which the molten liquid melamine is brought into contact with the ammonia at a temperature between the melting point of melamine and 440 ° C. The residence time of the melted melamine in the cooling vessel is between two minutes and ten hours and preferably between 10 minutes and 5 hours. The pressure in the cooling vessel is preferably less than 5 MPa and in particular between 7 and 25 MPa, this pressure must preferably be maintained through the introduction of ammonia. The liquid melamine according to the present invention is transferred to a cooling vessel where solid melamine powder is released by cooling. The invention will be clarified with the following example.

EXAMPLE Melt melamine is introduced at a temperature of 395 ° C, through a spray device, into a high pressure vessel and cooled with liquid ammonia which is also sprayed into the vessel. The number of spray nozzles used is 4. The ammonia spray nozzles are directed in the direction of the spray cone of the melamine droplets. The distance between the entrance of liquid ammonia to the cooling vessel and the point of intersection with the central axis of the melamine spray cone with the central axis of the ammonia spray cone is 0.5 m. The temperature in the container varies between 176 and 182 ° C. The ammonia pressure in the container varies between 6.8 and 9.2 MPa. After 2 minutes, the product is cooled to room temperature. The final product contains less than 0.1 percent by weight of melem and less than 0.05 percent by weight of amelid. The product has a consistent quality.

Claims (7)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for preparing dry melamine powder from molten melamine comprising the following steps: producing melamine by reacting urea and NH3 in a high pressure process; feeding said molten melamine and liquid ammonia to a cooling vessel to produce a solid melamine product, cooling the melamine with the ammonia, characterized in that said melamine is sprayed into the cooling vessel, said cooling vessel having a temperature between 50 ° C and the melting point of melamine and a pressure between 0.1 MPa and 20 MPa; and in the liquid ammonia as the liquid cooling medium is sprayed into said cooling vessel, said liquid ammonia consists essentially of liquid ammonia droplets; mixing said molten melamine spray and said liquid ammonia spray, whereby said molten melamine is cooled and solidified.

2. A method according to claim 1, further characterized in that said ammonia droplets have a d50 < 1 mm

3. A method according to claim 1, further characterized in that the liquid ammonia has an implusion flow value of at least 0.1 kg. m / sec2.

4. - A method according to claim 3, further characterized in that the spray of molten melamine has a pulse flow value of at least 5 kg. m / sec2.

5. An apparatus for preparing dry melamine powder from molten melamine comprising: a means for reacting urea and NH3 to produce molten melamine; a cooling vessel, having an upper part, a lower part, and side walls connecting said upper and lower parts, said cooling container being positioned with said side walls essentially vertical; an inlet for melted melamine, said melamine inlet being positioned near the center of the top of said cooling vessel, said inlet comprising a spray head oriented to direct a spray of molten melamine towards the bottom of said cooling vessel; a plurality of inlets for liquid ammonia, said inlets comprising a plurality of spray heads, said plurality of ammonia inlets being positioned around the melamine inlet and oriented to direct a plurality of ammonia sprays in the spraying of molten melamine to produce solid melamine; and means for removing said solid melamine from the cooling vessel, said means being positioned near the bottom of said cooling vessel.

6. - An apparatus according to claim 5, further characterized in that each of the plurality of ammonia inputs are within two meters of said melamine inlet.

7. An apparatus according to claim 5, further characterized by each of the plurality of spray heads, when tested with water at approximately atmospheric pressure and in a mass flow equivalent to the mass flow of the liquid ammonia used for cooling and solidifying said melted melamine, provides a pressure drop of at least 30 KPa.