Method for preparing melamine

<td>REPUBLIC POLAND</td><td>(12) PATENT DESCRIPTION (19) PL (21) Application number: 351354</td><td>(11) 201689 (13) B1</td>

<td></td><td>(22) Date of notification: 02/03/2000</td><td>(51) Int.Cl. C07D 251/62 (2006.01)</td>

<td></td><td>(86) Date and number of the international application: 02.03.2000, PCT / NL00 / 00129</td><td>C07D 251/60 (2006.01)</td>

<td>patent Office</td><td>(87) Date and publication number of the international application:</td><td></td>

<td>Polish Republic</td><td>2000-09-14, WO00 / 53587 PCT Gazette No. 37/00</td><td></td>

(54)

Method for the production of melamine

<td>(30) Priority: 1999-03-08, EP, 99200675.9</td><td>(73) The right holder of the patent: DSM NV, Heerlen, NL</td>

<td>(43) Application announced: April 7, 2003 BUP 07/03</td><td>(72) Inventor (s): Tjay Tjien Tjioe, Sittard, NL</td>

<td>(45) The grant of the patent was announced:</td><td>(74) Representative:</td>

<td>April 30, 2009 WUP 04/09</td><td>Barbara Bogdan, POLSERVICE, Kancelaria Rzeczników Patentowych Sp. z o. o</td>

⁽⁵⁷⁾ The method of producing melamine from urea in the high-pressure process is that high-purity melamine is produced from the melamine / ammonia alloy, with a melamine temperature to 450 ° C and a pressure of 5 to 25 MPa, by introducing a sufficient excess of ammonia to the melt in the ammonia injection tank to form a two-phase gas / liquid mixture, the gas / liquid mixture having a gas / liquid mass ratio of 0.01 to 1.0, and a temperature above the melting point of melamine, and then spraying the gas / liquid mixture by spraying devices into an expansion vessel which is maintained above atmospheric pressure between 0.5 and 60% of the pressure of the ammonia injection vessel, cooling and solidifying melamine to a temperature between 100 ° C and the melamine solidification temperature, by depressurization and evaporation of the ammonia introduced into the expansion vessel.

PL 201 689 B1

Description of the invention

The invention relates to a process for the production of melamine from urea in a high pressure process, wherein the solid melamine is obtained by transferring the melamine melt to a tank where it is cooled with a cooling agent such as ammonia to obtain high purity solid melamine.

Prior publications have disclosed many different methods of producing melamine, including, inter alia, EP-A-747 366, which discloses a high pressure process for producing melamine from urea. In particular, EP-A-747 366 describes how 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 form a reactor product. This reactor product, containing liquid melamine, CO2 and NH3, is transferred under pressure as a mixed stream to the separator.

In this separator, the reaction product is separated into a gaseous stream and a liquid stream. The gaseous stream mainly contains CO2 and NH3 waste gases and melamine vapor. The liquid stream consists mainly of molten melamine. The gaseous stream is transferred to a scrubber and the liquid stream is transferred to the product cooling unit.

In the scrubber, the gaseous stream is washed with molten urea. Due to the heat exchange in the scrubber, the molten urea is both preheated and the gaseous stream is cooled down to a temperature of 177 to 232 ° C. The molten urea also leaches the gaseous stream to separate the melamine vapors from the exhaust gases. The preheated molten urea, together with the melamine washed out from the CO2 and NH3 off-gases, is then fed to the reactor.

In the product cooling unit, the melamine melt is cooled and solidified with a liquid cooling agent to produce a high purity solid melamine product that does not require further purification. Preferably, the liquid cooling medium is one which is in the form of a gas at the temperature of the melamine melt and at the pressure in the product cooling unit. In EP-A-747 366, the preferred cooling agent is ammonia at a pressure in the product cooling unit above 4.14 MPa.

Although according to EP-A-747 366 the purity of the solid melamine product was greater than 99% by weight using the disclosed process, it has been difficult to obtain this purity continuously on a commercial scale. The inability to maintain purity greater than 99% by weight is a disadvantage that makes the produced melamine less suitable for more demanding applications, especially as melamine-formaldehyde material used in laminates and / or coatings.

To overcome these drawbacks, other methods have also been suggested, including the method disclosed in WO 98/55466, in which method was used to cool the melamine melt using external spraying of liquid ammonia or spraying of cold liquid ammonia, whereby the melamine melt could be mixed together. with less ammonia gas compared to the amount sprayed into the cooling tank. Although this method represented a significant improvement over the prior art methods, this method, however, requires external spraying of a cooling agent to solidify the melamine. The effectiveness of cooling the melamine melt by external spraying, however, most depends on the thorough atomization of the melamine melt (to maximize the surface area) and thorough mixing of the melamine melt spray and the coolant spray. A lack of uniformity in the droplet size or spray pattern and / or non-uniform mixing of the droplets and cooling agent may produce sub-optimal results.

Yet another method is described in WO 97/20826, which consists in solidifying the melamine by depressurizing and evaporating the dissolved ammonia. This publication discloses that relatively high pressures, up to 40 MPa, can be applied, at temperatures up to 60 ° C above the melting point of melamine, and then expand the melamine melt at a pressure of 20 MPa to atmospheric pressure. In order for the amount of ammonia to be introduced into the solution to be sufficient to effect the required cooling, it is advantageous to use a high starting pressure and a large pressure drop during the expansion step. However, in general, the use of elevated pressure in equipment in industrial plants increases the investment costs, as the costs of tanks, pipes and pumps are higher. Therefore, it is more advantageous to operate at the lowest possible pressure, but which will produce the desired results.

PL 201 689 B1

The object of the invention is the production of melamine from urea, in which the melamine is obtained directly from the melted liquid melamine as a dry powder with a high degree of purity.

The method of producing melamine from urea in the high-pressure process is that high-purity melamine is produced from the melamine / ammonia alloy, with a melamine temperature to 450 ° C and a pressure of 5 to 25 MPa, by introducing a sufficient excess of ammonia to the melt in the ammonia injection tank to form a two-phase gas / liquid mixture, the gas / liquid mixture having a gas / liquid mass ratio of 0.01 to 1.0, and a temperature above the melting point of melamine, and then spraying the gas / liquid mixture by spraying devices into an expansion vessel which is maintained above atmospheric pressure between 0.5 and 60% of the pressure of the ammonia injection vessel, cooling and solidifying melamine to a temperature between 100 ° C and the melamine solidification temperature, by depressurization and evaporation of the ammonia introduced into the expansion vessel. Preferably, the gas / liquid mass ratio is from 0.03 to 0.9. Preferably, the temperature of the melamine melt sprayed in the expansion vessel is from the melting point of melamine to a temperature of 45 ° C, more preferably 30 ° C, higher than the melting point of melamine.

Preferably, the ammonia pressure in the expansion vessel is maintained from 0.5% to 30% of the pressure of the ammonia injection vessel.

Preferably, excess ammonia is introduced into the melt in the ammonia injection vessel, the vessel having some tubing between the reactor or gas / liquid separator and the expansion vessel.

Preferably, the outlet velocity of the gas / liquid mixture from the atomizing device is greater than 50 m / s.

Preferably, the ammonia pressure is released when the melamine is at a temperature lower than 200 ° C.

The obtained melamine powder can be further cooled in the expansion vessel or in a separate cooling vessel and the pressure reduced to atmospheric pressure to obtain the final melamine powder.

In the ammonia injection tank, ammonia is injected into the melamine melt, the amount of injected ammonia being greater than the amount necessary to saturate the melamine melt at equilibrium. The excess ammonia is maintained in the melamine melt in the form of ammonia bubbles, the melamine melt and the ammonia bubbles form a two-phase gas / liquid mixture.

In the expansion vessel, the gas / liquid mixture is rapidly decompressed to cool and solidify the melamine melt. The expansion and evaporation of the excess ammonia in the gas / liquid mixture is sufficient to solidify the melamine without external cooling agents such as spray gaseous or liquid ammonia, aqueous ammonia, or other cooling agents.

When further cooling is desired, various techniques disclosed in the prior art can be used, especially liquid ammonia or cold ammonia gas can be introduced into the solid melamine.

An advantage of the process according to the invention is that melamine can be produced in powder form on an industrial scale with a purity above 98.5% by weight, generally above 99% by weight, and which has very good color characteristics. The high purity melamine produced by the process of the present invention is suitable for any application including melamine formaldehyde material used in laminates and / or coatings. Under similar operating conditions, the melamine powder produced according to the process of the invention also has other advantages over the melamine produced by the methods known in the art, including smaller particle size, increased surface area and increased porosity. In the production of melamine, urea is used as a raw material, which is preferably introduced into the reactor in molten form and the reaction proceeds at elevated temperature and pressure. Urea reacts to form melamine and, as by-products, NH3 and CO2, according to the following equilibrium reaction:

CO (NH2) 2 θ C3N6H6 + 6 NH3 + 3 CO2

The production of melamine from urea is carried out under high pressure, from 5 to 25 MPa, in the absence of a catalyst, with a reaction temperature of from 325 to 450 ° C, preferably from 350 to 425 ° C. The by-products NH3 and CO2 are usually recycled to the accompanying urea plant.

In the process according to the invention, devices suitable for the production of melamine from urea are used. Apparatus suitable for this invention may include a scrubber, a reactor that also has an integrated gas / liquid separator or separate gas / liquid separator, an ammonia injection vessel, an expansion vessel, and optionally a secondary cooling vessel. It will be appreciated that the ammonia injection tank configuration is not limited and may vary depending on the system

In the factory, include some tubing between the reactor or gas / liquid separator and the expansion vessel.

In one embodiment of the invention, melamine is produced from urea in an apparatus comprising a scrubber, a melamine reactor having either an integrated gas / liquid separator or a separate gas / liquid separator, an ammonia injection vessel, an expansion vessel and optionally a cooling vessel. In this embodiment, molten urea is fed to the scrubber operating at a pressure of 5 to 25 MPa, preferably 8 to 20 MPa, at a temperature above the melting point of urea. The scrubber may be provided with a cooling jacket or an internal cooling unit to additionally control the temperature. After passing through the scrubber, the urea melt is contacted with reactive off-gases from the melamine reactor or gas / liquid separator. The reaction gases are mainly CO2 and NH3 and may contain melamine vapor. The molten urea scrubs the melamine vapor from the CO2 and NH3 off-gas and recirculates this melamine back into the reactor. In the scrubbing process, the off-gas is cooled from the reactor temperature, i.e. 350 to 425 ° C, to 170 to 240 ° C, the urea is heated from 170 to 240 ° C. The off-gas CO2 and NH3 are removed at the top of the scrubber and can, for example, be recycled to an adjoining urea plant where they are a raw material for urea production.

The preheated molten urea is withdrawn from the scrubber, together with the melamine scrubbed from the waste gas, and transferred to a high pressure reactor operating at a pressure of 5 to 25 MPa, preferably 8 to 20 MPa. Transmission may be by means of high pressure pumps, or by gravity, or by using a combination of gravity and pumps.

The urea melt is heated in the reactor to a temperature of 325 to 450 ° C, preferably 350 to 425 ° C, at a pressure of 5 to 25 MPa, preferably 8 to 20 MPa, converting the urea into melamine, CO2 and NH3. A certain amount of ammonia can be additionally metered into the molten urea as, for example, liquid or hot vapor. The additional ammonia, although only optional, may serve, for example, to prevent the formation of melamine condensation products, such as melam, melem or melon, or to facilitate agitation in the reactor. The amount of additional ammonia supplied to the reactor may be up to 10 moles of ammonia per mole of urea, preferably 5 moles of ammonia per mole of urea, and most preferably 2 moles of ammonia per mole of urea.

The CO2 and NH3 produced by the reaction, as well as additional ammonia, are collected in a separation section, for example at the top of the reactor or in a separate gas / liquid separator downstream of the reactor, and separated from the liquid melamine. If a separate co-current gas / liquid separator is used, it may be advantageous to dose additional ammonia to the separator. In this case, the amount of ammonia is 0.01-10 moles of ammonia per mole of melamine, preferably 0.1-5 moles of ammonia per mole of melamine. Adding additional ammonia to the separator facilitates the rapid separation of carbon dioxide from the reaction product, thereby preventing the formation of oxygen-containing by-products. As described above, the gas mixture removed from the gas / liquid separator can pass through a scrubber to wash the melamine vapor and preheat the urea melt.

The melted melamine, having a melting point of melamine to 450 ° C, is discharged from the reactor or from a cocurrent gas / liquid separator and optionally cooled and then fed to an ammonia injection vessel. In the ammonia injection vessel, excess ammonia is added to the melamine melt to form a gas / liquid mixture in which ammonia is present both in the solution and in a separate gas phase. Sufficient ammonia is added to produce a two-phase stream in which the gas / liquid mass ratio is from 0.01 to 1.0, preferably from 0.03 to 0.9. This gas / liquid mixture is then sprayed into the expansion vessel to form a solid melamine. The melted melamine may be cooled from the reactor temperature or the temperature of the gas / liquid separator to a temperature closer to, but still higher than, the melamine melting point before it is sprayed into the expansion vessel. The molten melamine may be cooled to a temperature preferably not more than 45 ° C, and even more preferably not more than 30 ° C, from the melting point before it is sprayed into the expansion vessel and withdrawn from the reactor at a temperature usually above 300 ° C. melamine. The lower the melt temperature prior to expansion, the less ammonia is required to cool and solidify the melamine melt in the expansion vessel. The melamine melt can be cooled in a gas / liquid separator, in an ammonia injection vessel or in an additional apparatus downstream of the reactor and upstream of the expansion vessel. It is envisaged that the cooling can be accomplished by injecting a cooling agent, for example ammonia below the melting point of melamine, or by passing the melamine melt through a heat exchanger.

PL 201 689 B1

The melamine / ammonia mixture is transferred to the spraying means as a two-phase mixture and then sprayed by spraying means into the expansion vessel, thereby solidifying the melamine to form a dry melamine powder. The atomizing agent may be a device that converts the gas / liquid mixture into droplets whereby the flow of the melt into the expansion vessel is very fast. The atomizing agent can be a nozzle or a valve. The outlet velocity of the gas / liquid mixture from the spraying agent is, in principle, greater than 20 m / s, preferably greater than 50 m / s. The outlet velocity is defined as the nominal volumetric flow of the mixture (in m³/ s) divided by the smallest areas of the flow cross-sectional area of ​​the nozzle or valve (in m²). The expansion vessel contains ammonia and operates under an increased ammonia pressure. The melamine droplets from the atomizing agent are cooled by the flow of energy from the melamine melt into the expanding and evaporating ammonia, and a melamine powder is produced. The melamine powder thus produced has a temperature of from 100 ° C to the solidification temperature of the melamine, preferably below 300 ° C.

In another embodiment of the invention, the melamine powder prepared by spraying a gas / liquid mixture in the expansion vessel is held in the expansion vessel at a predetermined contact time under elevated ammonia pressure and at a temperature above 200 °. The duration of the contact is preferably from 5 minutes to 2 hours. During contact, melamine can remain at the same temperature or it can be cooled to a temperature above 200 ° C. Additional cooling of the solidified melamine can be performed with additional cold ammonia gas or liquid ammonia either alone or in combination with mechanical mixing of the melamine powder with rotating screws or drums, turntables, turntables, segment turntables, rotating tubes and the like.

When the melamine powder is cooled to below 200 °, the ammonia pressure can be reduced. Preferably, the ammonia gas is completely removed (to an amount of less than 1000 ppm, preferably less than 300 ppm and most preferably less than 100 ppm) by blowing air through the melamine powder. The ammonia pressure may be reduced before or simultaneously with the cooling of the melamine powder from 200 ° C to ambient temperature.

The invention will be further explained in more detail by means of the following examples and comparative examples.

Example I.

To the melamine melt, saturated with ammonia at 359 ° C and 20.4MPa, was added additional ammonia gas at the same temperature. The liquid flow was 4.8 kg / hr, the additional ammonia flow was 1.4 kg / hr. This two-phase stream was expanded in the vessel, maintaining the ammonia pressure at 2.5 MPa, which caused the melamine melt to solidify. The molten melamine was further cooled with liquid ammonia and the vessel was depressurized. The melamine purity was 99.6%.

Example II

Additional ammonia gas at the same temperature was added to the melamine melt saturated with ammonia at 353 ° C and a pressure of 17.9MPa. The liquid flow was 4.8 kg / hr and the additional ammonia flow was 0.9 kg / hr. This two-phase stream was expanded in the vessel, maintaining the ammonia pressure at 1.8MPa, which caused the melamine melt to solidify. The molten melamine was further cooled with liquid ammonia and the vessel was depressurized. The melamine purity was 99.2%.

Comparative example A.

The same experiment as in Example 1 was repeated with no additional ammonia gas added. Thus, only a single melamine melt phase in the quench vessel was depressurized. The melamine purity was 98.7%.

Comparative example B

The same experiment as in Example II was repeated with no additional ammonia gas added. Thus, only a single melamine melt phase in the quench vessel was depressurized. The melamine purity was as high as 98.5%.

PL 201 689 B1