Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
2001-02-26
2002-03-12
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
06355797
ABSTRACT:
The application relates to a process for cooling liquid melamine by mixing with solid melamine.
The literature has already disclosed a multiplicity of processes for preparing melamine (Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A-16, pp 174-179). All industrially important processes begin from urea, which is reacted to form melamine, ammonia and CO
2
either at high pressure and non-catalytically or at low pressure with the use of a catalyst.
In the low-pressure processes, gaseous melamine is produced, in the high-pressure processes, essentially liquid melamine is produced. Gaseous melamine present is passed, together with the off-gases CO
2
and NH
3
, through a urea melt, with the off-gases cooling, the melamine dissolving in the urea and the urea being heated and fed to the reactor for the melamine synthesis. Gaseous melamine is also produced by the high-pressure process of WO 95/01345 (Kemira), the melamine melt finally obtained evaporating.
A big problem in the cooling and solidification of liquid melamine is that a temperature difference of over 300° C. must be passed through, and byproducts can form in the course of this. A familiar method for cooling is quenching with water or with steam, recrystallization generally being necessary in order to remove the various byproducts. If gas, for instance gaseous ammonia, is used for quenching, very high volumes of gas must be used and circulated. If liquid ammonia is used for quenching, for instance as in U.S. Pat. No. 4,565,867, although the heat of evaporation of the ammonia is used for cooling, likewise large amounts of gas must be circulated and continuously recompressed. Unexpectedly, a simple process has now been found in which the formation of byproducts is suppressed and in which large amounts of gas do not need to be circulated and recompressed.
The invention therefore relates to a process/ for cooling liquid melamine by mixing with solid melamine or with solid inert materials or with a mixture of solid melamine and solid inert materials.
Suitable solid inert materials can preferably be metal particles or glass particles, for example balls or rods of steel, in particular stainless steel, steel alloys or titanium alloys. It is also possible to cool additionally by feeding cold liquid NH
3
or gaseous NH
3
or by additional cooling elements and heat exchangers.
To mix the liquid melamine with solid melamine, not only can solid melamine be introduced into the liquid melamine, but also liquid melamine can be introduced into solid melamine, or the reaction partners encounter one another in a pressure-reduction and quenching vessel (quencher). It is preferable here if the liquid melamine is reduced in pressure on mixing. It is found to be advantageous to feed additional NH
3
during the mixing. The melamine is preferably cooled to below the melting point of melamine.
The liquid melamine to be cooled is under a certain ammonia pressure of about from 1 to 1000 bar. Since liquid melamine, depending on pressure and temperature, comprises byproducts such as melam, melem, melone, ureidomelamine, ammeline or ammelide, or has a tendency to eliminate NH
3
, it is preferably under ammonia pressure. The higher this ammonia pressure, the lower the content of byproducts. Depending on the melamine preparation process carried out, the liquid melamine to be cooled is advantageously under an ammonia pressure of from about 40 to 1000 bar, preferably from about 40 to 400 bar, particularly preferably under a pressure of from about 60 to 300 bar.
Liquid melamine can be cooled, for example, by introducing solid melamine into liquid melamine which is under a certain ammonia pressure. The solid melamine is heated on introduction of and mixing with the melt, while the melt cools. The ammonia pressure under which the melt is can in this case remain the same, be increased or be decreased. Preferably, in a continuous process it remains roughly constant.
The temperature of the melt or of the resulting mixture can, if appropriate, be decreased with the aid of additional cooling to below the solidification point of melamine, so that pure and solid melamine is formed in a gentle manner. If appropriate, the solid melamine formed still remains for a certain time under ammonia pressure, and is then depressurized.
However, it is also possible to decrease the temperature of the liquid melamine to be cooled only to the melamine solidification point, dependent on the respective ammonia pressure, or to just above it, in which case it is possible to add to the solid melamine ammonia also, for instance in liquid, gaseous or supercritical state, to saturate with ammonia the liquid melamine which can absorb more ammonia at lower temperature. This procedure can also be used, for example, if the liquid NH
3
-saturated melamine melt is then to be depressurized and solidified for instance in accordance with WO 97/20826.
The preferred possibility for cooling liquid melamine with solid melamine is to cool it below the solidification point.
It is possible in this case to mix the mixing partners, retaining the existing pressure, with subsequent pressure increase or under pressure decrease. Preferably, mixing is performed with pressure decrease.
It is possible to introduce solid melamine into liquid melamine or liquid melamine into solid melamine, or to introduce both mixing partners simultaneously into a quencher.
According to a preferred embodiment, solid melamine is charged into a vessel and liquid melamine is introduced, preferably with pressure decrease. Particularly preferably, the mixing is carried out in a fluidized bed.
At the beginning of the reaction, solid melamine or foreign material in the form of solid inert substances or a mixture of solid melamine and solid inert substances is introduced into the fluidized-bed reactor and used to build up the fluidized bed. As solid inert materials, use is preferably made of fluidizing bodies of metals or glass, for example balls or rods of steel, in particular stainless steel, steel alloys or titanium alloys. The fluidized bed is maintained by a gas, preferably ammonia. The temperature in the fluidized-bed reactor is below the melting point of melamine. Liquid melamine is injected. The finely divided liquid melamine forms a layer over the solid melamine particles or inert substance particles, causes these to grow and becomes solid. Owing to the agitation and friction of the particles in the fluidized bed, melamine is continuously abraded or knocked off from the particles. The larger and thus heavier melamine particles are discharged, for instance using a cyclone, as soon as they have reached a certain wanted particle size. Firstly, solid cold melamine can, to a small proportion, be fed continuously, so that the liquid melamine can deposit and solidify on it, secondly, depending on the mode of operating the fluidized-bed reactor and the other conditions prevailing in the fluidized bed, solid melamine particles form even in the gas space, which particles serve as crystallization nuclei and are covered with liquid melamine which then likewise solidifies. In this case, no solid melamine or virtually no solid melamine needs to be fed from the outside.
The solid melamine particles and inert substance particles in the fluidized bed can be cooled, and thus the desired temperature in the fluidized bed set, in a plurality of ways, for example by built-in cooling elements, by feeding solid cold melamine, by inert particles which, if appropriate, are ejected and, after external cooling, returned to the fluidized bed, by feeding cold liquid NH
3
or gaseous NH
3
, by the temperature and rate of the gas stream which maintains the fluidized bed, and by the enthalpy of evaporation of the ammonia present in the liquid melamine.
Some of this ammonia is recirculated to cool and maintain the fluidized bed. The ammonia is cooled, preferably before being returned to the fluidized bed, and if appropriate is liquefied. The other portion of the ammonia released can, depending on the existing pressure in the fluidized bed,
Agrolinz Melamin GmbH
Balasubramanian Venkataraman
Raymond Richard L.
Wenderoth Lind & Ponack LLP
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