Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
2001-09-06
2003-06-17
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
C544S203000
Reexamination Certificate
active
06579980
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 a melamine melt to a vessel where it is cooled with a cooling medium such as ammonia to produce solid high purity melamine.
Various methods for the production of melamine have been described in previous publications including, inter alia, 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
, is transferred under pressure as a mixed stream to a separator.
In this separator, 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, 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 4.14 MPa.
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.
Other methods have been suggested to overcome these drawbacks, including among them the applicant's earlier application, WO 98/55466, which used an external spray of liquid ammonia or cool ammonia gas spray to cool the melamine melt, which may be mixed with a minor amount of ammonia gas, as it was sprayed into the cooling vessel. Although this method represented a significant improvement over the prior art methods, the method described in WO 98/55466 still required an external spray of a cooling medium to solidify the melamine. The most efficient cooling of the melamine melt with an external spray, however, depends upon thorough atomization of the melamine melt (to maximize surface area) and thorough mixing of the atomized melamine melt and the cooling medium spray. A lack of uniformity in the melamine droplet size or spray pattern, and/or non-homogeneous mixing of the droplets and the cooling medium will produce less than optimum results.
Yet another method is taught in WO 97/20826 which provides for the solidifaction of the melamine through expansion and evaporation of dissolved ammonia. WO 97/20826 teaches the use of relatively high pressures, up to 40 MPa, at temperatures up to 60° C. above the melting point of melamine, followed by expansion of the melamine melt at a pressure between 20 MPa and amospheric pressure. In order to get a quantity of ammonia into solution sufficient to provide the desired cooling, the initial pressures are preferably high and the pressure drop during the relaxation step is large. In general, however, using higher pressures in a commercial plant necessitates increased capital investment in process vessels, piping, and pumps, and results in higher operating costs. It is preferable, therefore, to operate at the lowest possible pressure at which satisfactory results may be obtained.
The object of the present invention is to provide an improved method for preparing melamine from urea, in which melamine is obtained directly from liquid melamine melt 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 by cooling and solidifying via an incorporated cooling medium.
The applicant has found that high purity melamine can be produced from the melamine melt, which has a temperature between the melting point of melamine and 450° C., preferably less than 45° C., and more preferably less than 30° C. above the melting point, by incorporating sufficient excess ammonia into the melamine melt in an ammonia injection vessel to form a gas/liquid mixture having a gas/liquid mass ratio between 0.01 and 1.0, and preferably between 0.03 and 0.9. This gas/liquid mixture is then sprayed via a spraying means into an expansion vessel to cool and solidify the melamine by expanding and evaporating the incorporated ammonia in the reduced pressure expansion vessel. The expansion vessel includes an ammonia atmosphere that, although preferably held at a pressure between 0.5% and 60% of the pressure of the ammonia injection vessel, more preferably between 0.5% and 30% of the pressure of the ammonia injection vessel, is still above atmospheric pressure. The melamine powder thereby obtained may then be cooled further in the expansion vessel, or in a separate cooling vessel, and the pressure reduced to atmospheric pressure to obtain the final melamine powder product.
In the ammonia injection vessel, ammonia is injected into the melamine melt, the quantity of ammonia injected being more than necessary to saturate the melamine melt at equilibrium. The excess ammonia is maintained in the melamine melt as ammonia bubbles, the melamine melt and ammonia bubbles forming a two-phase gas/liquid mixture.
In the expansion vessel, the gas/liquid mixture is rapidly decompressed to cool and solidify the molten melamine. The expansion and vaporization of the excess ammonia in the gas/liquid mixture is sufficient to solidify the melamine without the need for any external cooling medium such as gas or liquid ammonia sprays, aqueous ammonia solutions, or other cooling means. Further cooling of the solid melamine may, however, be desirable and may be achieved by applying various techniques as disclosed in the prior art, particularly through the introduction of liquid ammonia or cool ammonia gas into the solid melamine.
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. At similar operating conditions, the melamine powder produced according to the present invention provides other advantages over the melamine produced by the prior art processes including reduced particle size, increased surface area, and increased porosity.
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:
6 CO(NH
2
)
2
® C
3
N
6
H
6
+6 NH
3
+3 CO
2
The production of melamine f
Balasubramanian Venkataraman
DSM N.V.
Pillsbury & Winthrop LLP
Raymond Richard L.
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