Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Patent
1996-06-13
1998-03-24
Ford, John M.
Organic compounds -- part of the class 532-570 series
Organic compounds
Four or more ring nitrogens in the bicyclo ring system
544203, C07D25160, C07D25162
Patent
active
057314373
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/FI94/00307, filed Jul. 1, 1994.
The present invention relates to the preparation of melamine from urea. More precisely, the invention relates to the preparation of very pure melamine by a new high-pressure process.
It is known that melamine can be prepared from urea at a temperature of 390.degree.-410.degree. C. according to the following reaction formula: +6NH.sub.3 +3CO.sub.2 melamine mole, when the heating of the urea from 135.degree. C. (melting point of urea) to the reaction temperature is included.
Users require a very high purity of melamine; 99.8% and 99.9% are typical degrees of purity in product specifications. For this reason its production processes often include a complicated purification section involving a large quantity of apparatus.
There are two basic types of melamine production processes using urea as the raw material, namely, catalytic low-pressure processes and high-pressure processes in which no catalyst is used. In the former, the reactor pressure is approx. 10 bar or lower, in the latter higher than 80 bar (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A 16, p. 174).
In typical low-pressure processes, a fluid-bed reactor is used in which the catalyst is fluidized with gaseous ammonia or a mixture of ammonia and carbon dioxide. The melamine emerges in gaseous state from the reactor. The fact that corrosion is less than in high-pressure processes is regarded as one of the advantages of low-pressure processes. The best known users of low-pressure processes are BASF (Hydrocarbon Processing, September 1969, p. 184), Chemie Linz (Hydrocarbon Processing, November 1966, p. 146), and DSM/Stamicarbon (Chem. Eng., May 20, 1968, p. 124), each of which has developed its own process version.
In typical high-pressure processes the reaction takes place in a liquid phase. In this case the reactor is full of molten melamine mixed to some degree with molten raw material, i.e. urea, and intermediate reaction products. Also present in the mixture there are gas bubbles consisting of ammonia and carbon dioxide and a small amount of gaseous melamine. The required high amount of reaction heat is usually generated by intra-reactor heating elements, in which the heat is generated by means of electricity or, for example, a circulating hot salt melt.
Smaller apparatus size can be deemed to be one of the advantages of high-pressure processes over low-pressure processes. A reaction taking place in a liquid phase clearly requires less space. Furthermore, the process apparatus in which gas is treated remain moderate-sized owing to the high pressure. Another advantage is the high pressure of the obtained the product gas, a mixture of ammonia and carbon dioxide. This gas is often used for the preparation of urea and, being pressurized, it is better suited for this purpose as such.
The Montedison process (Ausind) is a typical high-pressure melamine production process (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A 16, p. 177). As in other melamine processes, urea melt and hot ammonia are introduced into a reactor. The reactor conditions are 70 bar and 370.degree. C. From the reactor the mixture of melamine melt and product gases is directed to a quencher, into which water containing ammonia and carbon oxide is also introduced. The temperature of the quencher is 160.degree. C. and its pressure 25 bar. From this quencher the reactor offgases are fed for further use, for example for the production of urea or fertilizers. The melamine is recovered from the slurry by a highly multiple-stage further treatment, which includes the removal of ammonia and carbon dioxide, the dissolving of the melamine in a large amount alkaline water, removal of color with activated carbon, crystallization, filtration, drying, and packaging.
The Montedison process has two significant disadvantages which the present invention does not have, for example, the product gas is obtained at a relatively low pressure, only 25 bar. Second, the number of process stages is very high
REFERENCES:
patent: 3484440 (1969-12-01), Kokubo et al.
patent: 4565867 (1986-01-01), Thomas et al.
patent: 5514797 (1996-05-01), Best
Shiroishi et al., "Technical Development of Melamine Manufacture by Urea Process", Chemical Economy & Engineering Review, vol. 8, No. 1, 2 (No. 92), pp. 34-39, 46, 47, 50, 51 (Jan. & Feb. 1976).
Ellwood, "Melamine Process Uses Low-Pressure Reactor to Achieve Low Costs", Chemical Engineering, pp. 124-126 (May 20, 1968).
Schwarzmann, "Make Melamine at Atmospheric Pressure", Hydrocarbon Processing, pp. 184-186 (Sep. 1969).
Schmidt, "New OSW Process Makes Melamine", Hydrocarbon Processing, vol. 45, No. 11, pp. 146-150 (Nov. 1966).
Oinas Pekka
Turunen Ilkka
Chong Suet M.
Ford John M.
Kemira Agro Oy
Nath Gary M.
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