Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
1999-11-22
2001-05-22
Raymond, Richard L. (Department: 1614)
Organic compounds -- part of the class 532-570 series
Organic compounds
Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
active
06235902
ABSTRACT:
The invention relates to a method for preparing melamine from urea via a high-pressure process in which the solid melamine product is obtained by transferring the melamine melt from the reactor into a product-cooling vessel where the melamine melt is cooled with ammonia.
Such a method is disclosed, inter alia, in EP-A-747366 which describes a high-pressure process for preparing melamine from urea. In particular, EP-A-747366 describes how urea is pyrolyzed in a reactor, operating at a pressure of from 10.34 to 24.13 MPa and a temperature of from 354 to 454° C., to produce a reactor product. This reactor product, containing liquid melamine, CO
2
, and NH
3
and is transferred under pressure as a mixed stream to a separator.
In this separator, which is kept at virtually the same pressure and temperature as the reactor, the reactor product is separated into a gaseous stream and a liquid stream. The gaseous stream contains primarily CO
2
and NH
3
waste gases and a minor component of melamine vapor. The liquid stream mainly comprises a melamine melt. The gaseous stream is transferred to a scrubber unit, while the liquid stream is transferred to a product-cooling unit.
In the scrubber unit, operated at temperature and pressure conditions nearly identical to the reactor conditions, the gaseous stream is scrubbed with molten urea. The heat transfer achieved in the scrubber unit both preheats the molten urea and cools the gaseous stream to a temperature from 177 to 232° C. The molten urea also scrubs the gaseous stream to remove the melamine vapor from the waste gases. The preheated molten urea, along with the melamine that was scrubbed from the CO
2
and NH
3
waste gases, is then fed into the reactor.
In the product-cooling unit, the melamine melt is cooled and solidified with a liquid cooling medium to produce a solid high purity melamine product without the need for additional purification. The preferred liquid cooling medium is one that forms a gas at the temperature of the melamine melt and at the pressure in the product-cooling unit. EP-A-747366 identifies liquid ammonia as the preferred liquid cooling medium with the pressure in the product-cooling unit being above 41.4 bar. Although according to EP-A-747366 the purity of the solid melamine product obtained using the disclosed process was greater than 99 wt %, this degree of purity has proven difficult to maintain continuously on a commercial scale. The inability to maintain a purity greater than 99 wt % is a drawback that renders 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 may be consistently 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 obtained as a dry powder directly from the melamine melt by cooling the melamine melt with a liquid cooling medium.
We have found that high purity melamine can be continuously produced directly from the melamine melt coming from the separator. The melamine melt, which has a temperature between the melting point of melamine and about 450° C., is sprayed via spraying means into a solidification vessel. An ammonia atmosphere is maintained in the solidification vessel with the pressure of the ammonia being above 1 MPa, preferably above 1.5 Mpa, more preferably above 4.5 Mpa and even more preferably above 6 Mpa. The upper limit of the pressure of the amonia is below 40 Mpa, preferably below 25 Mpa and more preferably below 11 Mpa. As it enters the solidification vessel the melamine melt 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 melamine, preferably between 240° C. and the solidification point, and most 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 it may be cooled to a temperature above 200° C., preferably above 240° C., or, most preferably, above 270° C., over a period of between 6 seconds and 5 hours, preferably over a period of between 30 seconds and 2 hours. The melamine product may 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 above 99 wt %. The high purity melamine produced according to the present invention is suitable for virtually any melamine application, including melamine-formaldehyde resins used in laminates and/or coatings.
The preparation of melamine preferably uses urea as the raw material, the urea being fed into the reactor as a melt and reacted at elevated temperature and pressure. Urea reacts to form melamine, and the by-products NH
3
and CO
2
, according to the following reaction equation:
6CO(NH
2
)
2
→C
3
N
6
H
6
+6NH
3
+3CO
2
The production of melamine from urea can be carried out 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. The by-products NH
3
and CO
2
are usually recycled to an adjoining urea factory.
The above-mentioned 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 scrubber unit, a reactor having either an integrated gas/liquid separator or a separate gas/liquid separator, possibly a post-reactor, a first cooling vessel, and possibly a second cooling vessel. When a separate gas/liquid separator is used, the pressure and temperature of the separator are virtually identical to the temperature and pressure in the reactor.
In an embodiment of the invention, melamine is prepared from urea in an apparatus comprising a scrubber unit, a melamine reactor having either an integrated gas/liquid separator or a separate gas/liquid separator, a first cooling vessel, and a second cooling vessel. In this embodiment, the urea melt is fed into a scrubber unit operating at a pressure of from 5 to 25 MPa, preferably from 8 to 20 MPa, and at a temperature above the melting point of urea. This scrubber unit may be provided with a cooling jacket or internal cooling bodies to provide additional temperature control.
As it passes through the scrubber unit, the urea melt contacts the reaction waste gases coming from the melamine reactor or the separate gas/liquid separator. The reaction gases mainly consist of CO
2
and NH
3
and may include a minor amount of melamine vapor. The urea melt scrubs the melamine vapor from the CO
2
and NH
3
waste gases and carries this melamine along back to the reactor. In the scrubbing process, the waste gases are cooled from the temperature of the reactor, i.e. from 350 to 425° C., to from 170 to 240° C., the urea being heated to from 170 to 240° C. The CO
2
and NH
3
waste gases are removed from the top of the scrubber unit and may, for example, be recycled to an adjoining urea factory, where they can be used as raw materials for the urea production.
The preheated urea melt is drawn off from the scrubber unit, together with the melamine scrubbed from the waste gases, and transferred to the high pressure reactor operating at pressures between 5 and 25 MPa, and preferably between 8 and 20 MPa. This transfer may be achieved using a high-pressure pump or, where the scrubber is positioned above the reactor, gravity, or a combination of gravity and pumps.
In the reactor, the urea melt is heated to
Balasubramanian Venkataraman
DSM N.V.
Pillsbury & Winthrop LLP
Raymond Richard L.
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