Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
1999-01-28
2001-07-10
Ford, John M. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Four or more ring nitrogens in the bicyclo ring system
C544S201000, C544S198000
Reexamination Certificate
active
06258950
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for the preparation of solid melamine using a high-pressure process in which the melamine melt is transferred from the reactor to a vessel and is cooled using ammonia as to obtain melamine having a very high degree of purity (98.5 wt. % to 99.95 wt. %) as a dry powder directly from the reactor product.
DESCRIPTION OF THE PRIOR ART
Melamine (2,4,6-triaminosymtriazine) is a white crystalline product obtained by heating urea.
Purified crystalline melamine can be combined with formaldehyde to form melamine resin. Characteristics of subsequent products formed from the melamine resin are critically dependent upon the level of purity of the crystalline melamine used to form the resin. Obtaining crystalline melamine of very high purity is therefore an essential first step to melamine related product formulation.
The first step in melamine resin formation from crystalline melamine is the production of trimethylol melamine. This molecule can combine further with others of the same kind by a condensation reaction. Excess formaldehyde or melamine can also react with trimethyol melamine or its polymers, providing many possibilities of chain growth and cross-linking. The nature and degree of polymerization can be varied by pH and the degree of heat applied in the curing process. Impurities in the melamine also effect the nature of the polymerization reaction.
A major advantage of melamine resins is that they are more water resistant and heat resistant than urea resins. Melamine resins may be water-soluble syrups (low molecular weight) or insoluble powders (high molecular weight) dispersible in water. Melamine resins are widely used as molding compounds with &agr;-cellulose, wood flour, or mineral powders as fillers and with coloring materials. Melamine resins are also used in laminating, producing boil-proof adhesives, increasing the wet strength of paper, textile treatment, leather processing, and producing dinnerware and decorative plastic items. The use of melamine resins in general results in superior products over urea resin products.
Butylated melamine resins are formed by incorporating butyl or other alcohols during resin formation. These resins are soluble in paint and enamel solvents and in other surface coatings, often in combination with alkyds. They give exceptional curing speed, hardness, wear resistance, and resistance to solvents, soaps and foods.
Melamine-acrylic resins are water soluble and are used for formation of water-base industrial and automotive finishes. The use of melamine-acrylic resins provides smooth, durable surface finishes. However, as is the case with other melamine-based products, the superiority of melamine-acrylic resin products is related to the high level of purity of the initial crystalline melamine product.
A high level of purity is in particular required when melamine is used for the production of resins for coatings. Transparency and colourless are properties which are required for these applications.
A method of obtaining melamine crystals is described in U.S. Pat. No. 4,565,867 issued to Thomas et al., the complete disclosure of which is incorporated herein by reference. The Thomas reference discloses a high-pressure process for the preparation of melamine from urea. In particular, the pyrolysis of urea in a reactor at a pressure of about 10.3 MPa to about 17.8 MPa and a temperature of about 354° C. to about 427° C. for producing a reactor product is described.
This reactor product contains 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 CO
2
and NH
3
off-gases and also melamine vapour. The liquid stream substantially consists of liquid melamine. The gaseous stream product and the liquid stream product are treated differently. The gaseous product is transferred to a scrubber unit, while the liquid melamine is transferred to a product cooler. In the scrubber unit the above-mentioned CO
2
and NH
3
off-gases, which contain melamine vapour, are scrubbed, at virtually the same pressure as the reactor pressure, with molten urea so as to pre-heat the urea and cool said off-gases and remove the melamine that is present from the off-gases. The pre-heated molten urea, which contains melamine, is then fed to the reactor. In the product cooler the liquid melamine is reduced in pressure and cooled by means of a liquid cooling medium (preferably liquid ammonia) so as to produce a solid melamine product without washing or further purification.
The disadvantage of the above-mentioned Thomas method is that melamine having a purity which is insufficient for a number of critical applications, like resins for coatings. Thomas teaches a theoretical conversion yielding only 99.19 wt. % pure melamine. However, the example provided by the Thomas reference at column 9, line 61 through column 10, line 2, shows the Thomas method obtaining melamine with an even lower purity of only 98.0 wt. %. In the Thomas example, the melamine product remains 0.81 wt. % urea, 0.03 wt. % CO
2
, 0.05 wt. % melamine-related compounds and 0.07 wt. % organic solids (melem, melam, and other solids). However if the Thomas method is used in practice, the maximum purity is only 97.5 wt. %, measured by High Performance Liquid Chromatography (HPLC). Such a product is not pure enough for universal application.
A need therefore exists to provide an economical method to obtain highly pure melamine (98.5 wt. % to 99.95 wt. % and preferably 99.5 wt. % to 99.95 wt. %).
SUMMARY OF THE INVENTION
An object of the present invention is to obtain an improved high-pressure process for the preparation of melamine from urea in which melamine having a high degree of purity is obtained as a dry powder directly from the reactor product. More particularly it is an object of the present invention to obtain an improved high-pressure process for the preparation of melamine from urea in which melamine having a high degree of purity is obtained as a dry powder directly from the liquid melamine melt through cooling using ammonia.
The present invention provides a method of preparing highly pure solid melamine from urea melt obtained from a urea plant, the method comprising the combination of steps of:
(a) providing urea melt to a scrubber unit to effect separation of a liquid phase from a gas phase producing a urea melt mixture;
(b) transferring the urea melt mixture from the scrubber unit to a melamine reactor and heating the urea melt mixture to produce a melamine melt and off-gases; and
(c1) separating said off-gases from said elamine melt and
(c2) transferring the melamine melt to a first cooling vessel, the pressure in the cooling vessel being at a certain pressure preferably higher than 5 MPa and cooling the melamine melt to a temperature just above the melting point of melamine, preferably to between 1° C. to 30° C. and more preferably- to between 1° C. to 10° C. above the melting point of melamine.
(d) transferring the melamine melt to a second cooling vessel in order to convert the liquid melamine to a solid product, wherein in the second cooling vessel the melamine is further cooled using cold ammonia, preferably liquid ammonia to produce a solid pure melamine product.
Cold ammonia means ammonia with a temperature below the temperature of the melamine melt and is generally between 20 to 380° C., preferably between 50 to 300° C. to produce a solid pure melamine product.
During the further cooling in the second cooling vessel using ammonia, the melamine melt is cooled at least 10° C., preferably at least 50° C. and more preferably at least 100° C. Additional cooling may be obtained by expanding partly or as a whole the mixture of melamine melt and ammonia.
Optionally, the melamine melt in the process of conversion to a solid product can be expanded by lowering the pressure in the second cooling vessel to pr
Balasubramanian Venkataraman
DSM N.V.
Ford John M.
Pillsbury & Winthrop LLP
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