Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Patent
1993-06-01
1994-09-27
Ford, John M.
Organic compounds -- part of the class 532-570 series
Organic compounds
Four or more ring nitrogens in the bicyclo ring system
544201, C07D25162, C07D25160
Patent
active
053508491
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the steam treatment of the gas stream leaving the synthesis reaction in the preparation of melamine from urea or the thermal decomposition products therefrom.
Such steam treatment is disclosed in U.S. Pat. No. 3,386,999 and has the object of producing, after quenching of the treated gas stream, highly pure melamine which is free of cyanic acid and other melamine precursors. In the single example of this patent publication the gas stream leaving the synthesis reactor is treated with steam for an unspecified period of time, the melamine obtained after a quenching and crystallisation is characterised by "dry and pure". Only 253.0 kg/hours melamine are obtained from 790 kg urea-feed per hour. A same process is described in U.S. Pat. No. 3,414,571: The gas stream leaving the synthesis reactor is treated with steam only. Further NL-A-8105027, example 3 discloses that under the practical conditions of conventional melamine synthesis processes, temperature 360.degree.-400.degree. C., and contact time 0.1-2.0 seconds, the conversion of cyanic acid is small if no catalyst is present and the uncatalysed process of U.S. Pat. No. 3,386,999 is of no practical use. Therefore NL-A-8105027 proposes the use of a catalyst in the steam treatment to decompose gaseous urea, thermal decomposition products thereof and melamine containing gas streams into carbondioxide and ammonia. Preferred catalysts are chosen from the group of active carbon, oxides and phosphates of boron and aluminum. EP-A-51156 claims the use of iron and copper oxide to catalyze the steam treatment of gas stream containing melamine, cyanic acid etc. At temperatures in the range 100.degree.-150.degree. C. a virtually complete conversion of HCN, melamine and isocyanic acid into ammonia and carbon dioxide is realized.
These known catalytic treatments make only sense when all or virtually all of the above-mentioned compounds are to be removed from off-gas streams. In practice, however, there appears to be a demand for selective removal of HCN, isocyanic acid, cyanamide and similar compounds from process gas streams still containing melamine in higher concentrations without significant amounts of melamine being converted. This applies, for instance, to the product stream leaving the synthesis reactor in the melamine process. This stream contains in the fluidizing gas, ammonia, the carbon dioxide and ammonia released in the reaction, besides higher concentrations of melamine product and, to a much lesser degree, isocyanic acid, cyanamide and any hydrogen cyanide formed. When melamine is recovered from the process stream by cooling with water, an aqueous medium or a cold gas stream, these compounds give rise to the formation or precipitation of undesirable by-products such as ureido melamine, melam, melem, cyanuric acid, melamine cyanurate, ammelide, guanidine and ammeline. These contaminate either the sublimated melamine or the melamine-containing suspension, and as a consequence either melamine of too low purity is obtained, or a further purification step, for instance by recrystallization, as disclosed in U.S. Pat. No. 3,496,177, becomes necessary. It may also prove necessary to prevent the build-up of impurities in the aqueous process stream obtained after separation, for instance by filtration, of the melamine, as this restricts recycling of this aqueous process stream in the process (see for instance U.S. Pat. No. 44,084,046). For the several variants of the known melamine synthesis processes and the gas and liquid streams occurring in these, the general process literature is referred to as well as the many patent applications relating to this literature, for instance U.S. Pat. No. 3,321,603, 3,300,493, 3,513,167, 3,700,672, 4,348,520 and GB-A-1309275. The above-mentioned restrictions raise the costs or necessitate the discharge of streams with a relatively high environment-burdening effect.
It is therefore the object of this invention to obtain a process for the treatment of gaseous product streams containing isocyanic acid, cyanamide
REFERENCES:
patent: 3386999 (1968-06-01), Manes
patent: 3414571 (1968-12-01), Haines
Janssen Hendrik J.
Schroijen Josephus C.
van de Moesdijk Cornelis G. M.
DSM N.V.
Ford John M.
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