Process for the preparation of urea, and urea and the urea solution thus prepared

摘要

Process for the preparation of urea in which a urea synthesis solution containing carbamate and free ammonia is formed in a high-pressure part in a synthesis zone at an NH3/CO2 molar ratio of up to 4:1, a temperature of at least 175 DEG C. and the corresponding pressure, a portion of the carbamate is decomposed in a first decomposition stage at synthesis pressure or lower pressure by a stripping treatment with carbon dioxide while heat is being supplied, and the gas mixture thus obtained is at least in part condensed and the condensate and the non-condensed portion of the gas mixture, if any, are returned to the synthesis zone, a further portion of the carbamate still present is decomposed in at least two further decomposition stages and the gas mixture formed is separated, in the first of the further decomposition stages a pressure of 12-30 bar being maintained and heat being supplied and in the second of the further decomposition stages a lower pressure being maintained, and the remaining urea-containing solution is processed further by evaporation to a concentrated urea solution and, if desired, solid urea. In the first decomposition stage a portion of the urea synthesis solution is subjected to a stripping treatment with carbon dioxide while heat is being supplied, and the remaining portion of the urea synthesis solution is countercurrently contacted with carbon dioxide under adiabatic conditions. The gas mixtures obtained in both operations are at least in part condensed in a first condensation zone. The solution obtained in the treatment of the urea synthesis solution with carbon dioxide under adiabatic conditions is supplied to the first of the further decomposition stages and the stripped urea synthesis solution to the second of the further decomposition stages.