Process for the preparation of urea

Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing

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564 70, C07C12602

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active

045408130

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BRIEF SUMMARY
This invention relates to a process for preparing urea from ammonia and carbon dioxide at elevated temperatures and pressures whereby the net heat produced by the urea synthesis reaction can be more efficiently and effectively utilized.
One process for the preparation of urea which has found wide use in practical applications is described in European Chemical News Urea Supplement of Jan. 17, 1969, at pages 17-20. In the process there disclosed, the urea synthesis solution is formed in a reaction zone maintained at a high pressure and temperature, and is thereafter subjected to a stripping temperature at the synthesis pressure by heating this solution and contacting it countercurrently with a carbon dioxide stripping gas so as to decompose a major portion of the ammonium carbamate contained therein. The gas mixture thus formed, containing ammonia and carbon dioxide together with the stripping gas and a small quantity of water vapor, is removed from the remaining urea product stream and introduced into a condensation zone wherein it is condensed to form an aqueous ammonium carbamate solution. This aqueous carbamate solution, as well as the remaining non-condensed gas mixture, is recycled to the reaction zone for conversion to urea. The condensation of this gas mixture returned to the reaction zone provides the heat required for the conversion of ammonium carbamate into urea, and no heat need be supplied to the reaction zone from the outside.
The heat required for the stripping treatment is provided by the condensation of high-pressure steam on the outside of tubes in a vertical heat exchanger in which the stripping takes place. According to this publication, approximately 1,000 kg of steam at a pressure of about 25 bar is required per ton of urea. In practice, the consumption of high pressure steam (25 bar) has been reduced to about 850 kg per ton of urea, and the heat used in the stripping treatment can be partially recovered by condensation of the resulting gas mixture. This condensation, however, takes place at a relatively low temperature level with the result that steam of only 3-5 bar is produced, for which there is relatively little use in either this process or outside the process. For this reason, and particularly in view of continually increasing energy prices, it is highly desirable to reduce the consumption of high-pressure steam as much as possible and, depending on local conditions and needs, it may be desirable as well to avoid or greatly reduce the production of surplus low-pressure steam.
Various proposals have already been made to use the heat released in the formation of carbamate from ammonia and carbon dioxide to provide at least a part of the heat requirements in the stripping treatment. For various reasons, however, these proposals have not found practical application. For example, in British patent specification No. 1,147,734, a process is disclosed wherein the ammonium carbamate and urea synthesis is effected in two successive reaction zones. Fresh ammonia and at least a portion of the fresh carbon dioxide are fed into the first reaction zone, and the heat released by the formation of ammonium carbamate in this first reaction zone is transferred, via builtin heat exchange elements, to the urea synthesis solution formed in the second reaction zone at a pressure equal to or lower than that in the first reaction zone, resulting in the decomposition of unreacted ammonium carbamate. The decomposition products thus formed are stripped from the remaining urea solution by means of an inert gas.
In carrying out this known process, the fresh ammonia and carbon dioxide must be pre-heated to a high temperature, for instance to the synthesis temperature maintained in the first reaction zone, in order to have sufficient heat to effect the decomposition of ammonium carbamate in the stripping zone. This pre-heating of the reactants consumes a significant quantity of high-pressure steam. Of the surplus heat formed in the urea synthesis, however, only a small portion can be recovered as steam at a pre

REFERENCES:
patent: 3120563 (1964-02-01), Bongard
patent: 3406201 (1968-10-01), Baumann et al.
patent: 3759992 (1973-09-01), Mavrovic
patent: 3957868 (1976-05-01), Verstegen
patent: 4066693 (1978-01-01), Venderbos
patent: 4115449 (1978-09-01), Biermans et al.
patent: 4173615 (1979-11-01), Otsuka et al.
patent: 4301299 (1981-11-01), Inoue et al.
patent: 4354040 (1982-10-01), Inoue et al.

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