Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
2000-11-13
2001-10-16
Shah, Mukund J. (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
06303781
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 reactor product comprising liquid melamine, CO
2
and NH
3
to a gas/liquid separator and subsequently transferring the melamine melt coming from the separator to a vessel where the melamine melt is cooled with a cooling medium.
Such a method is described, inter alia, in EP-A-747366. This describes a high-pressure process for preparing melamine from urea. In particular, EP-A-747366 describes how urea is pyrolysed in a reactor 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. The reactor product obtained contains liquid melamine, CO
2
and NH
3
and is transferred under pressure as a mixed stream to a gas/liquid separator. In this gas/liquid separator, which is kept at virtually the same pressure and temperature as the said reactor, the said reactor product is separated into a gaseous stream and a liquid stream. The gaseous stream contains CO
2
and NH
3,
waste gases and also melamine vapour. The liquid stream mainly comprises liquid melamine. The gaseous product is transferred to a scrubber unit, while the liquid melamine is transferred to a product-cooling unit. In the scrubber unit, the said CO
2
and NH
3
waste gases, which contain melamine vapour, are scrubbed with molten urea, at virtually the same pressure and temperature as the pressure and temperature of the reactor, to preheat the urea and to cool the said waste gases to a temperature of 177-232° C. and to remove the melamine present from the waste gases. Then the preheated molten urea, which contains the said melamine, is fed to the reactor. In the product-cooling unit, the liquid melamine is cooled with a liquid cooling medium, which forms a gas at the temperature of the liquid melamine in the product cooler, to produce a solid melamine product without scrubbing or further purification. In EP-A-747366 liquid ammonia is preferably used as the liquid cooling medium, the pressure in the product-cooling unit being above 4.14 MPa.
The purity of the melamine end product, according to EP-A-747366, is above 98.5 wt %, but it is not easy to achieve this level continuously on a constant level and on a commercial scale. This is a drawback, in particular when melamine is used in melamine-formaldehyde resins which are used in laminates and/or coatings. The impurities in the melamine end product, particularly when prepared on a commercial scale, consists primarly of melam, melem and oxygen containing impurities like ammelide, ammeline and urediomelamine.
REFERENCES:
patent: 3116294 (1963-12-01), Marullo et al.
patent: 3308123 (1967-03-01), Murata
patent: 3386999 (1968-06-01), Manes
patent: 5514796 (1996-05-01), Best et al.
patent: 5721363 (1998-02-01), Canzi et al.
patent: 8105027 (1983-06-01), None
Balasubramanian Venkataraman
DSM N.V.
Pillsbury & Winthrop LLP
Shah Mukund J.
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