Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Patent
1980-03-24
1982-06-08
Doll, John
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C07C12602
Patent
active
043340962
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a process for synthesizing urea.
More particularly, the present invention relates to a process wherein the production cost of urea is reduced by efficiently returning reactants not converted to urea in the urea synthesis reaction into the reactor for the urea synthesis.
DESCRIPTION OF THE PRIOR ART
In the production of urea, the starting materials, i.e., ammonia and carbon dioxide, are substantially quantitatively converted to ammonium carbamate under usual reaction conditions, for example, comprising a pressure of 200 Kg/cm.sup.2 and a temperature of 190.degree. C., in the presence of excess ammonia, as shown in reaction formula (1). Ammonium carbamate is further converted to urea as shown in reaction formula (2) wherein the equilibrium conversion is determined by the reaction conditions and the actual conversion is determined by the rate of attaining the equilibrium conversion, which rate depends on the residence time in the reaction zone:
Namely, the excess ammonia and about 25-50%, based on the theoretical amounts, of the starting materials ammonia and carbon dioxide remain unconverted to urea.
A known process for separating urea from unconverted materials comprises heating an effluent from the reaction zone under reduced pressure to decompose ammonium carbamate into ammonia and carbon dioxide, thereby separating them in the form of a gas together with the excess ammonia, absorbing the ammonia and carbon dioxide in a recovered liquid formed in the subsequent step of decomposing and absorbing ammonium carbamate under a lower pressure, and returning the recovered liquid into the reaction zone.
Another known process comprises stripping an effluent from the reaction zone by subjecting it to gas-liquid contact with gaseous starting materials, ammonia or carbon dioxide, under a pressure which is substantially not reduced, thereby decomposing and separating unconverted ammonium carbamate and returning the decomposition products into the reaction zone. This process utilizes the fact that an effect equivalent to that obtained by pressure reduction is obtained by the gas-liquid contact.
SUMMARY OF THE INVENTION
The process of the present invention has been attained by comparison and study of those prior art processes quantitatively in detail, analyzing the steps of separating excess ammonia and decomposing/separating ammonium carbamate and returning those products into the reaction zone, individually and comprehensively in connection with the properties of the treated substances and investigating reationalization of the steps of urea production. It has been found, according to the process of the present invention, that the efficiency of the decomposition and separation can be improved unexpectedly as compared with those obtained by known processes wherein unconverted ammonium carbamate is decomposed and thereby separated under a high pressure.
If the technique of separating the gas formed by the decomposition from the liquid phase of the effluent from the reactor under a high pressure is improved, the decomposition is also accelerated consequently and favorable results can be obtained. For reducing the loss of power used for maintaining the pressure required in the synthesis zone and also for utilizing most of the high energy level of the process flow, ammonium carbamate is decomposed and thereby separated by introducing the effluent from the synthesis zone into an ammonium carbamate decomposition/separation zone without effecting pressure reduction. For accelerating the separation of the decomposed gas from the liquid phase under the high pressure, the free surface of the liquid effluent is enlarged in the process of the present invention. Residence time sufficient for the decomposition and separation is provided. In a stage in which the concentration of the unconverted substances is high and in which decomposition and separation are relatively large in amount, the decomposition and separation are effected by supplying only heat for effecting ammoni
REFERENCES:
patent: 3471558 (1969-10-01), Wentworth et al.
patent: 3607938 (1971-09-01), Braun
patent: 3984469 (1976-10-01), Guadalupi et al.
patent: 4137262 (1979-01-01), Guadalupi et al.
Fukui Akito
Inoue Shigeru
Konoki Keizo
Nobue Michio
Doll John
Mitsui Toatsu Chemicals Inc.
Toyo Engineering Corporation
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