Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
2001-08-13
2002-04-30
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
C544S201000
Reexamination Certificate
active
06380385
ABSTRACT:
A multiplicity of processes for the preparation of melamine are already published in the literature. A preferred starting material in these is urea, which is reacted either at high pressure and non-catalytically or at low pressure and using a catalyst to form melamine, ammonia and CO
2
.
Although the known high-pressure processes, for instance those of Melamine Chemicals, Montedison or Nissan, in which the melamine is first formed as a liquid, have a lower energy consumption in comparison to low-pressure processes, if no purification stages are present, melamine contains impurities such as melam, melem, ammeline, ammelide or ureidomelamine, which interfere with some further melamine processing operations.
Melamine prepared by a high-pressure process is worked up, for example, according to U.S. Pat. No. 4,565,867 (Melamine Chemicals) by separating off the CO
2
and NH
3
waste gases from the liquid melamine, the pressure and temperature preferably being maintained at the same values as in the reactor. The liquid melamine is then fed to a product cooling unit, in which it is depressurized from 105-175 bar to about 14-42 bar and at the same time rapidly cooled and quenched with liquid ammonia from 350-430° C. to 48-110° C., by which means melamine separates out as a solid product.
According to U.S. Pat. No. 3,116,294 (Montecatini), the CO
2
and NH
3
waste gases are likewise separated off first, the liquid melamine is treated in countercurrent with NH
3
to remove CO
2
still dissolved, and the product is collected in a further reactor and allowed to dwell therein for a defined time. Finally, melamine is taken off from the second reactor and rapidly cooled by quenching with water or by mixing with cold gases.
However, the purity of melamine which is produced by one of these processes is insufficient for many applications, for instance in the preparation of melamine-formaldehyde resins for surface coatings, since, in particular, the melem content is too high.
According to U.S. Pat. No. 3,637,686 (Nissan), the crude melamine melt obtained by thermal decomposition of urea is rapidly cooled to 200-270° C. with liquid NH
3
or cold NH
3
gas, and is further cooled in a second step to 100-200° C. with aqueous NH
3
solution. The product must then be recrystallized in order to achieve a satisfactory melamine purity.
The object of the present invention was therefore to find a process which enables the preparation of pure melamine having a purity of up to greater than 99.8% and having a markedly reduced content of impurities, particularly melem and melam.
Unexpectedly, this object was able to be achieved by a process in which liquid, ammonia-containing melamine is rapidly depressurized at a temperature at or just above the solidification point of melamine dependent on the particular prevailing ammonia partial pressure, the solidification point, depending on the temperature at the beginning of depressurization and the desired final pressure, increasing by about up to 60° C., and solid melamine separating out.
The present invention therefore relates to process for the preparation of pure melamine, which comprises liquid, ammonia-containing melamine being rapidly depressurized from an ammonia partial pressure p
1
between 400 and 50 bar to an ammonia partial pressure p
2
between 200 bar and 20 bar, where p
1
is always greater than p
2
, at a temperature which is 0 to 60° C. higher than the melamine solidification point dependent on the particular prevailing ammonia partial pressure, higher pressures permitting a greater temperature interval from the melamine solidification point than lower pressures, by which means pure melamine separates out in solid form, whereupon, in any sequence, the product is further depressurized if appropriate to atmospheric pressure, cooled to room temperature and the pure melamine is isolated.
The process according to the invention is suitable for the purification of melamine which is produced in any known process of the prior art and, in particular, contains impurities such as melem and melam, the melamine being able to be present either as melt or in the liquid phase or in crystalline form.
If the melamine to be purified is already present as melt or as liquid phase, such as downstream of a high-pressure reactor for the synthesis of melamine by conversion of urea, the pressure and the temperature of the melt or the liquid melamine are brought to the initial ammonia partial pressure desired for the depressurization between about 400 and 50 bar, preferably between about 400 and 80 bar, particularly preferably between about 300 and 100 bar, and to the corresponding above-defined temperature, i.e. to a temperature which is about 0 to 60° C., preferably about 0 to 40° C., particularly preferably about 0 to 20° C., above the melamine solidification point dependent on the particular prevailing ammonia partial pressure. In this process it must be noted that at higher pressures the temperature difference between melamine solidification point and the temperature to be set at the beginning of depressurization can be greater than at lower pressures, since the solidification point of the melt at higher pressures is at lower temperatures than at low pressures. In order to achieve the temperature desired for the depressurization, the temperature is decreased if necessary. The temperature is particularly preferably below about 350° C. Cooling can be carried out either rapidly or slowly. Preferably, it is performed slowly at a cooling rate of 0.8 to 10° C./min. Since the melamine melt can absorb more ammonia at a lower temperature, ammonia is preferably fed during this operation. It is particularly advantageous to depressurize the liquid ammonia-containing melamine as far as possible close to or above the melamine solidification point dependent on the particular prevailing ammonia partial pressure.
It is further possible by means of the present invention to purify solid, contaminated melamine. The melamine to be purified, which is present in crystalline form or as powder, is first heated at an ammonia partial pressure between about 400 and 50 bar, preferably between about 400 and 80 bar, particularly preferably between about 300 and 100 bar, to a temperature which is about 0 to 60° C., preferably about 0 to 40° C., particularly preferably about 0 to 20° C., above the melamine solidification point dependent on the particular prevailing ammonia partial pressure. To melt solid melamine reliably, it is expedient firstly to heat it to about 370° C. and then to cool it to the desired depressurization temperature to ensure that the melamine is completely molten. Preferably, the desired depressurization temperature is below about 350° C.
Again it must be noted that the temperature difference at higher pressures can be greater than at lower pressures.
Preferably, the process of the invention is carried out directly after a melamine high-pressure process. Examples of high-pressure processes are, for instance, the Melamine Chemical, Montedison or Nissan process, as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A16, pp. 174-179. According to these processes, urea is usually converted in a temperature range from about 370 to 430° C. and at a pressure of about 70 to 300 bar. The melamine formed in these processes is finally obtained as a liquid phase.
According to the process of the invention, the initial ammonia partial pressure desired for the rapid depressurization is, if necessary, set between about 400 and 50 bar. In order to set the corresponding initial temperature for the depressurization, the liquid melamine obtained from the urea conversion process is cooled from the temperature prevailing in the reactor by means of suitable cooling apparatuses, for instance by means of heat exchangers, to the appropriate value, i.e. to a temperature which is about 0 to 60° C., preferably about 0 to 40° C., particularly preferably about 0 to 20° C.; above the melamine solidification point dependent on the particular ammonia partial pressure set
Canzi Lorenzo
Coufal Gerhard
Müllner Martin
Agrolinz Melanin GmbH
Balasubramanian Venkataraman
Shah Mukund J.
Wenderoth Lind & Ponack LLP
LandOfFree
Process for the preparation of pure melamine does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for the preparation of pure melamine, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the preparation of pure melamine will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2900690