Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
2000-04-07
2001-07-17
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
Reexamination Certificate
active
06262261
ABSTRACT:
The invention relates to a process for the preparation of melamine from urea via a high-pressure process in which solid melamine is obtained by transferring the melamine melt leaving the reactor to a vessel in which the melamine melt is cooled by means of ammonia.
Such a process is described in, inter alia, U.S. Pat. No. 4,565,867, which describes a high-pressure process for the preparation of melamine from urea. U.S. Pat. No. 4,565,867 in particular describes the pyrolysis of urea in a reactor at a pressure of 10.3 to 17.8 MPa and a temperature of 354 to 427° C. for producing a reaction product. This reaction 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 said reactor, said reaction 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 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 said 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 so as to produce a solid melamine product without washing or further purification. In U.S. Pat. No. 4,565,867 use is preferably made of liquid ammonia as liquid cooling medium.
One disadvantage of this method is that in a commercial scale production installation the melamine product that is obtained is nonhomogeneous in both particle size and purity. Important quality parameters include color, reactivity, and the type and concentration of impurities. In the production of melamine for the preparation of melamine based resins, the purity and consistency of the product are very important. Maintaining a low and repeatable level of impurities, for example melem and ammelide, is necessary for the transparency of the melamine based resins.
The aim of the present invention is to obtain an improved high-pressure process for the preparation of melamine from urea in which melamine with a consistent product quality is obtained as a dry powder directly from the liquid melamine melt.
The applicants have now found that melamine powder having the desired product quality powder can be obtained by utilizing a process in which the melamine melt is sprayed into a cooling vessel where it is cooled very rapidly through contact with small droplets of ammonia which are sprayed simultaneously into the same cooling vessel, the cooling vessel having a pressure above 0.1 MPa and a temperature above 50° C. and below the melting point of the melamine. The dry melamine powder produced according to the present process is suitable for applications requiring high purity melamine without the necessity of further purification. The pressure in the cooling vessel is preferably below 20 MPa and more preferably below 15 MPa. The temperature in the cooling vessel is preferably below 270° C. and more preferably below 200° C.
In order to maximize the purity of the solid melamine obtained, it is preferred to cool the melamine melt as fast as possible through rapid and thorough mixing with the cold ammonia sprays. This method solidifies the molten melamine very quickly and thereby prevents the molten melamine from contacting the wall of the cooling vessel. Contact between the molten melamine and the walls of the cooling vessel results in the formation of large lumps of melamine containing different levels of impurities that will limit the purity and consistency of the melamine product that can be obtained.
The applicants have further found that it is necessary to minimize any contact between the liquid ammonia and the walls of the cooling vessel. When the liquid ammonia spray has not been completely evaporated before reaching wall of the cooling vessel, the liquid ammonia itself may trigger the formation of lumps of melamine containing different levels of impurities that will limit the purity and consistency of the melamine product that can be obtained.
In order to minimize the possibility that liquid ammonia will reach the cooling vessel wall, the present process sprays the liquid ammonia into the melamine melt spray as small droplets at a velocity sufficient to provide rapid and thorough mixing of the ammonia and melamine sprays toward the center of the cooling vessel. The small size of the ammonia droplets also increases the rate at which the melamine is cooled by evaporation of the ammonia. In order to obtain the benefits of the rapid cooling provided by the present process, the ammonia sprays should be located near the melamine inlet into the cooling vessel with the spray direction, velocity, and quantity selected to achieve thorough and rapid mixing of the ammonia and melamine sprays to obtain rapid solidification and cooling of the melamine without depositing lumps of melamine on the walls of the cooling vessel. To achieve the mixing of the melamine and ammonia sprays of the present process, it is understood that the ammonia spray nozzles and the melamine inlet will generally be positioned relatively near one another within the cooling vessel.
This need for the close positioning of the ammonia spray nozzles and the melamine inlet is not reflected in the cooling equipment generally used in current state of the art melamine production. In practicing current processes for the cooling of melamine slurries or melts, the nature, location, and rate at which the cooling or drying medium is fed into the cooling vessel is not critical, permitting operation of such processes in vessels having a broad range of physical configurations. In practicing the present process, however, the distance between the melamine inlet and the ammonia spray nozzle vessel becomes important for successful operation. In practice, it is prefered that this distance be less than 2 m, and more preferably, less than 1.5 m, which permits satisfactory operation at reasonable ammonia feed conditions. Greater separation between the melamine inlet and the ammonia spray nozzles would cause an undesirable delay in cooling the melamine, require more extreme ammonia feed conditions, or both.
In practicing the present process, the liquid ammonia spray and the melamine melt spray must be combined at velocities, rates, and directions which are sufficient to produce rapid and thorough mixing of the ammonia and melamine droplets. In order to obtain such mixing, it is preferred that the velocity of the liquid ammonia be at least 6 m/s. This velocity (in m/s) is determined by dividing the volume flow of the liquid (in m
3
/s ) by the smallest cross sectional area for flow (in m
2
) in the spray nozzle. Similarly, it is preferred that the melamine melt be sprayed at a high velocity.
Although the ammonia spray nozzle(s) may be configured to spray the liquid ammonia in a wide variety of directions, it is preferred that the nozzles be oriented to spray the ammonia droplets directly into the spray of melamine droplets with the central axes of the ammonia nozzles positioned to intersect the central axis of the melamine nozzle. In order minimize the distance the ammonia spray must travel to reach the melamine melt spray, it is preferred to orient the ammonia nozzles such that their central axis are approximately perpendicular to the central axis of the melamine melt nozzle. Measuring along the central axis of the ammonia spray nozzles to the intersection with the central axis of the melamine melt nozzle, this configuration sets the ammonia spray distance equal the separation distance between the nozzles, preferably less than 2 m. It will be
Best David E.
Tjioe Tjay T.
Balasubramanian Venkataraman
DSM NV
Ford John M.
Pillsbury & Winthrop LLP
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