Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
1999-12-02
2001-08-14
Raymond, Richard L. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
active
06274731
ABSTRACT:
The invention relates to a method for preparing melamine from urea via a high-pressure process in which solid melamine is obtained by transferring the melamine melt coming from the reactor to a vessel where the melamine melt is cooled with an evaporating cooling medium.
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 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.
The object of the present invention is to obtain an improved method for preparing melamine from urea, in which melamine is obtained directly from the reactor product as a dry powder having a high degree of purity. More particularly, the object of the present invention is to obtain an improved high-pressure process for preparing melamine from urea, in which melamine is obtained directly from the liquid melamine melt as a dry powder having a high degree of purity via cooling.
Applicant has found that high purity melamine can be continuously produced from the melamine melt coming from the melamine reactor, which has a temperature between the melting point of melamine and 450° C., by spraying the melamine melt via spraying means into a vessel and cooling the melamine melt by contact with an evaporating cooling medium in an ammonia atmosphere with an increased ammonia pressure whereby melamine powder is obtained with a temperature below 270° C., releasing the ammonia pressure and cooling the melamine powder, at least for part of the cooling range, by agitating the powder mechanically and cooling either directly, indirectly or some combination.
An increased ammonia pressure means an ammonia pressure greater than 1 MPa, preferably greater than 1.5 MPa, more preferably greater than 4.5 MPa and even more preferably greater than 6 MPa. The ammonia pressure is below 40 MPa, preferably below 25 MPa and more preferably below 11 MPa.
Melamine powder has poor flow and fluidization characteristics and a low temperature equalization coefficient (poor thermal conductivity). Standard cooling methods such as a fluidized bed or a packed moving bed cannot, therefore, be readily implemented on a commercial scale. We have found, however, that the color of the melamine powder, in particular, is adversely affected if the melamine remains at a high temperature for too long. Effective control of the residence time at high temperature has, therefore, proved critical. It is therefore important to be able to cool the melamine powder effectively.
Surprisingly, it proved possible to cool melamine powder, despite its poor flow and thermal conductivity characteristics, by utilizing mechanical agitation coupled with direct and indirect cooling. The term indirect cooling describes those instances in which the mechanically agitated melamine powder contacts a cooled surface. The term direct cooling describes those instances in which the mechanically agitated melamine powder contacts a cooling medium such as ammonia or an airstream. A combination of both direct and indirect cooling mechanisms is obviously also possible.
The melamine powder formed by spraying the melamine melt into the solidification vessel is held under an increased ammonia pressure at a temperature above 200° C. for a contact time. The duration of this contact time is preferably between 1 minute and 5 hours, more preferably between 5 minutes 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. An increased ammonia pressure means an ammonia pressure greater than 1 MPa, preferably greater than 1.5 MPa, more preferably greater than 4.5 MPa and even more preferably greater than 6 MPa. The ammonia pressure is below 40 MPa, preferably below 25 MPa and more preferably below 11 MPa. 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 98.5 wt %, and generally above 99 wt %, that has very good color characteristics. 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 additional cooling vessels. 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
Slangen Hubertus J. M.
Tjioe Tjay T.
Balasubramanian Venkataraman
DSM N.V.
Pillsbury & Winthrop LLP
Raymond Richard L.
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