Process for preparing urea
1989
Jonckers, K.
A process for the preparation of urea from carbon dioxide and a molar excess of ammonia at a pressure in the range of between 125 and 350 bar, after which the urea thus formed is separated from non-converted ammonium carbamate and excess ammonia and is processed into a concentrated urea solution. At least a portion of the non-converted ammonium carbamate present in the urea synthesis solution is decomposed to ammonia and carbon dioxide at the synthesis pressure or below by the simultaneous supply of heat and counter-current contact with a stripping gas, thereby forming a stripping zone gas mixture containing at least a portion of the carbamate decomposition products, together with a portion of the excess ammonia and the stripping gas. At least a portion of the stripping zone gas mixture is condensed in a first condensation zone to form an aqueous ammonium carbamate containing condensate. Part of the stripping zone gas mixture is directly passed to a second condensation zone. A further amount of carbamate still present in the stripped urea synthesis solution is decomposed in a decomposition step at a pressure of 4-40 bar, by subjecting this solution the heat exchange with the condensing gas mixture in the first condensation zone. The condensation heat of the gas mixture obtained in this heat exchange treatment is used in the concentration step of the urea solution formed, supplemented by condensation of steam. The amount of steam is controlled by controlling the amount of stripping zone gas mixture that is passed directly to the second condensation step.
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