MXPA99010739A - Method for preparing melamine - Google Patents

Abstract

Method for preparing melamine from urea via a high-pressure process in which solid melamine product is obtained directly from the melamine melt by transferring the melamine melt into a product cooling unit where it is cooled with ammonia. The melamine melt from the melamine reactor has a temperature between the melting point of melamine and 450°C and is sprayed, via spraying means, into a product cooling vessel containing an ammonia environment with an increased pressure, where the melamine melt solidifies to form melamine powder. The melamine powder, having a temperature of between 200°C and the solidification point of melamine, is maintained under an increased ammonia pressure for a contact time of between 6 seconds and 5 hours at a pressure. During this contact time, the powder melamine product may be maintained in the product cooling vessel, or a series of vessels, at a virtually constant temperature or may be cooled further to a temperature above 200°C.

Description

IMETODE TO PREPARE MELAMINE DESCRIPTIVE MEMORY The invention relates to a method for preparing melamine from urea by means of a high pressure process in which the solid melamine product is obtained by transferring the molten melamine bath from the reactor to the vessel to cool the product where it is obtained. cools the molten melamine bath with ammonia. Such a method is disclosed, among others, in EP-A-747366 which describes a high pressure process for preparing melamine from urea. In particular, EP-A-747366 describes how the urea is pyrolyzed in a reactor, operating at a pressure of 10.34 to 24.13 MPa and a temperature of 354 to 454 ° C, to produce a reactor product. This reactor product, which contains liquid melamine, CO2, and NH3, under pressure as a mixed stream, is transferred to a separator. In this separator, which is maintained at virtually the same pressure and temperature as the reactor, the reactor product is separated into a gas stream and a liquid stream. The gaseous stream contains mainly the waste gases CO2 and NH3 and a minor component of melamine vapor. The liquid stream comprises mainly a molten melamine bath. The gas stream is transferred to a purification unit, while the liquid stream is transferred to a chilling unit of the product. In the purification unit, operated under conditions of temperature and pressure almost identical to the conditions of the reactor, the gaseous current is purified with molten urea. The heat transfer carried out in the treatment unit preheats the molten urea and cools the gas stream to a temperature of 177 to 232 ° C. The molten urea also purifies the gas stream to separate the melamine vapor from the waste gases. The preheated fused urea is then introduced into the reactor, together with the melamine which was cleaned from the CO2 and NH3 waste gases. In the product chiller unit, the molten melamine bath is cooled and solidified with a liquid cooling medium to produce a high purity solid melamine product without the need for further purification. The preferred liquid coolant medium is one that forms a gas at the temperature of the molten bath of melamine and at the pressure of the product cooling unit. EP-A-747366 identifies liquid ammonia as a preferred liquid coolant medium with the pressure in the product cooling unit exceeding 41.4 bar. Although according to EP-A-747366 the purity of the solid melamine product which was obtained using the exposed process was greater than 99% by weight, this degree of purity has proved difficult to maintain continuously on a commercial scale. The inability to maintain a purity higher than 99% is a drawback that makes the melamine produced less suitable for more demanding applications, particularly melamine / formaldehyde resins used in laminates and / or coatings. It is the object of the present invention to provide an improved method for preparing melamine from urea, in which high purity melamine is conveniently obtained as a dry powder directly from the reactor product. More particularly, the object of the present invention is to provide an improved high pressure process for preparing melamine from urea, in which high purity melamine is obtained as a dry powder directly from molten melamine bath, cooling the molten bath of melamine with a liquid coolant medium. It has been discovered that high purity melamine can be continuously produced directly from the molten melamine bath from the separator. The molten melamine bath, which has a temperature between the melting point of the melamine and about 450 ° C, is sprayed by means of aspersion to a solidification vessel. An ammonia atmosphere is maintained in the solidification vessel, the ammonia pressure being greater than 1 MPa, preferably greater than 1.5 MPa, more preferably greater than 4.5 MPa and even more preferably higher than 6 MPa. The upper limit of the pressure is less than 40 MPa, preferably less than 25 MPa and more preferably less than 11 MPa. As it enters the solidification vessel, the molten melamine bath is cooled and solidified by contact with the liquid and gaseous ammonia to produce melamine powder having a temperature of between 200 ° C and the solidification point of the melamine, preferably between 240 ° C and the solidification point, and very > preferably between 270 ° C and the solidification point. Once solidified, the melamine powder is maintained under ammonia pressure for a contact time of between 6 seconds and 5 hours, preferably between 30 seconds and 2 hours. During this contact time, the temperature of the melamine product can remain virtually constant or can be cooled to a temperature above 200 ° C, preferably above 240 ° C, or more preferably above 270 ° C, for a period of time between 6 seconds and 5 hours, preferably during a period of between 30 seconds and 2 hours. The melamine product can be cooled in the solidification vessel or in a separate cooling vessel. The advantage of the method according to the present invention is the continuous production, on a commercial scale, of dry melamine powder with a purity greater than 99% by weight. The high purity melamine produced in accordance with the present invention is suitable for virtually any melamine application, including melamine-formaldehyde resins used in laminates and / or coatings. The melamine preparation preferably uses urea as the starting material, the urea being introduced to the reactor as a molten bath and reacting it at elevated temperature and pressure. The urea reacts to form melamine and the byproducts NH3 and CO2, according to the following reaction equation: 6 CO (NH2) 2? C3N6H6 + 6 NH3 + 3 CO2 The production of melamine can be carried out from urea, at high pressure, preferably between 5 and 25 MPa, without the presence of a catalyst, at reaction temperatures between 325 and 450 ° C, and preferably between 350 and 425 ° C. C. The NH3 and CO2 by-products are usually recirculated to an adjacent urea factory. The aforementioned objective of the invention is achieved by employing an apparatus suitable for the preparation of melamine from urea. An apparatus suitable for the present invention may comprise a sewage unit, a reactor having an integrated gas / liquid separator or a loose gas / liquid separator, possibly a postreactor, a first chill vessel and possibly a second chill vessel. When a loose gas / liquid separator is used, the pressure and temperature of the separator are virtually identical to the temperature and pressure in the reactor. In one embodiment of the invention, melamine is prepared from urea in an apparatus comprising a purifying unit, a melamine reactor having integrated gas / liquid separator or loose gas / liquid separator, a first cooling vessel and a second chiller vessel. In this embodiment, the molten urea bath is introduced to a treatment unit operating at a pressure of 5.25 MPa, preferably 8 to 20 MPa, and at a temperature above the melting point of the urea. This purification unit can be provided with a chiller cover or internal cooling bodies to provide additional temperature control. As it passes through the purification unit, the molten urea bath contacts the waste gases from the reaction from the melamine reactor or the loose gas / liquid separator. The reaction gases consist mainly of CO2 and NH3 and may include a lower amount of melamine vapor. The molten urea bath cleans the melamine vapor from the waste gases CO2 and NH3 and transports this melamine back to the reactor. In the purification process, the waste gases are cooled from the reactor temperature, ie from 350 or 225 ° C to 170 or 240 ° C, the urea being heated to 170 and up to 240 ° C. The waste gases CO2 and NH3 are extracted from the top of the treatment unit and can be recirculated, for example, to an adjacent urea factory, where they can be used as starting materials for the production of urea. The preheated molten urea bath is removed from the purification unit, together with the purified melamine from the waste gases and transferred to the high pressure reactor, achieving pressures between 5 and 25 MPa and preferably between 8 and 20 MPa. This transfer can be made using a high pressure pump or, if the scrubber is placed above the reactor, gravity or a combination of gravity and pumps. In the reactor, the molten urea bath is heated to a temperature between 325 and 450 ° C, preferably between about 350 and 425 ° C, at a pressure between 5 and 25 MPa, preferably between 8 and 20 MPa, to convert the urea to melamine, CO2 and NH3. In addition to the molten urea bath, a certain amount of ammonia can be metered into the reactor, for example as liquid, or hot steam. The additional ammonia, although optional, may serve, for example, to prevent the formation of melamine condensation products such as melam, mélem and melon, or to promote mixing in the reactor. The amount of additional ammonia supplied to the reactor can be up to 10 moles of ammonia per mole of urea, preferably up to 5 moles of ammonia per mole of urea and most preferably up to 2 moles of ammonia per mole of urea. The CO2 and NH3 produced in the reaction, as well as any additional ammonia supplied, accumulates in the separation section, for example in the upper part of the reactor or in a loose gas / liquid separator placed low reactor current and is separated from the reactor. the liquid melamine. If a loose gas / liquid downstream separator is used, it may be advantageous to add additional ammonia to this separator. The amount of ammonia in this case is 0.01-10 moles of ammonia per mole of melamine and preferably 0.1-5 moles of ammonia per mole of melamine. The addition of additional ammonia to the separator provides for the rapid separation of the carbon dioxide from the reactor product, thus preventing the formation of byproducts containing oxygen. As described above, the gas mixture withdrawn from the gas / liquid separator can be passed to the purification unit in order to extract the melamine vapor and preheat the molten urea bath. The molten melamine bath, which has a temperature between the melting point of the melamine and 450 ° C, is removed from the gas / liquid separator reactor downstream and is sprayed to the chiller vessel to obtain the solid product of melamine. Before spraying, however, the molten melamine bath of the reactor temperature can be cooled to a temperature closer, but still higher, to the melting point of the melamine. The molten melamine bath will be removed from the reactor at a temperature preferably above 390 ° C and more preferably above 400 ° C, and will be cooled to at least 5 ° C and preferably at least at 15 ° C, before sprinkling to the cooling vessel. Most preferably, the molten melamine bath will be cooled to a temperature that is 5-20 ° C higher than the solidification point of the melamine. The molten melamine bath in the gas / liquid separator can be cooled by a loose apparatus downstream of the gas / liquid separator. The cooling may take place by injection of cooling medium, for example ammonia gas having a temperature below the temperature of the molten melamine bath or passing the molten melamine bath through a heat exchanger. In addition, ammonia can be introduced into the molten melamine bath in such a manner that a gas / liquid mixture is sprayed into the spray medium. In this case, the ammonia is introduced at a pressure higher than that of the molten melamine bath and preferably at a pressure between 15 and 45 MPa. The residence time of the molten bath of melamine in the reactor and the spray medium is preferably at least 10 minutes and most preferably at least 30 minutes, and usually less than 4 hours. The molten melamine bath, possibly together with the ammonia gas, is transferred to a spray medium where it is sprinkled to a first chill vessel to solidify the melt bath of melamine at increased pressure and form a dry melamine powder. Thus, the melamine powder having a temperature between 200 ° C and the solidification point of melamine is formed, preferably between 240 ° C and the solidification point, and most preferably between 270 ° C and the solidification point. At increased pressure it means at a pressure greater than 1 MPa, preferably higher than 1.5 MPa, more preferably higher than 4.5 MPa and even more preferably higher than 6 MPa. The upper limit of the ammonia pressure is less than 40 MPa, preferably less than 25 MPa and more preferably less than 1 1 MPa. The melamine powder is then maintained under ammonia pressure for a period of contact between 6 seconds and 5 hours and preferably for a period between 30 seconds and 2 hours. During this contact time, the melamine powder can be kept at temperature or it can be further cooled to a temperature above 200 ° C, preferably above 240 ° C and most preferably above 270 ° C, for a period of time between 6 seconds and 5 hours, preferably during a period of between 30 seconds and 2 hours. If desired, this additional cooling may take place in the first chill vessel or in a second loose chill vessel. The invention will be explained in more detail with reference to the following loose example.

EXAMPLE Molten melamine bath having a temperature of 402 ° C, by means of a spraying device, was introduced into a high-pressure vessel and cooled with liquid ammonia which is being sprayed simultaneously into the vessel. The temperature in the vessel is 296 ° C and the ammonia pressure varies between 6.8 and 9.2 MPa. After 2 minutes, the "melamine powder at room temperature is cooled." The final product contained 0.5% by weight of melam and less than 0.2% by weight.

COMPARATIVE EXAMPLE The molten bath of melamine of 400 ° C, contained in a tube under ammonia pressure of 13.6 MPa, was quickly cooled to room temperature by contacting the closed tube with a mixture of ice and water. The final product contains 1.4% by weight of melam and 0.4% by weight of melam.

Claims (8)

NOVELTY OF THE INVENTION CLAIMS

1 .- Method for preparing melamine from urea by a high pressure process in which solid melamine is obtained by the molten bath of melamine from the reactor being transferred to the vessel where the molten bath of melamine is cooled with ammonia, characterized in that the molten bath of melamine coming from the melamine reactor and having a temperature between the melting point of melamine and 450 ° C, inside a vessel to an ammonia environment under a pressure is sprayed by spray means. greater than 1 MPa, the molten bath of melamine being converted to melamine powder having a temperature between 200 ° C and the solidification point of the melamine, the powder then remaining in contact with ammonia, for a period of 6 seconds-5 hours at a pressure greater than 1 MPa, with the options that the product remains virtually at the same temperature during said contact time or is cooled in such a way that The product has a temperature higher than 200 ° C for a period of 6 seconds to 5 hours and cooling takes place in the same vessel or in a loose cooling vessel.

2. Method according to claim 1, further characterized in that the molten bath of melamine is converted to melamine powder having a temperature between 240 ° C and the solidification point of the melamine.

3. Method according to claim 2, further characterized in that the molten bath of melamine is converted to melamine powder having a temperature between 270 ° C and the solidification point of the melamine.

4. Method according to claims 1-3, further characterized in that the powder remains in contact with ammonia for a period of 30 seconds to 2 hours.

5. Method according to claims 1-4, further characterized in that the product has a temperature higher than 240 ° C for a period of 6 seconds to 5 hours.

6. Method according to claim 5, further characterized in that the product has a temperature greater than 270 ° C for a period of 6 seconds to 5 hours.

7. Method according to claims 1-6, further characterized in that the product has a temperature higher than 240 ° C for a period of 30 seconds to 2 hours.

8. Method according to claim 7, further characterized in that the product has a temperature higher than 270 ° C for a period of 30 seconds to 2 hours.