| 摘要 |
An improved process for the preparation of urea from carbon dioxide and excess ammonia at an elevated temperature and pressure to form a urea synthesis solution containing uncoverted ammonium carbamate and free ammonia. A portion of the ammonium carbamate contained in the urea synthesis solution is decomposed in a first decomposition zone by the supply of heat, and the first gas mixture thus obtained is separated and at least partially condensed in a first condensation zone. The residual area solution is heated in a second decomposition zone maintained at a pressure of between about 4 and 40 bar thereby decomposing a further portion of ammonium carbamate and the second gas mixture thus obtained is separated from the second residual urea containing stream. This second residual area stream is introduced into a further decomposition zone wherein remaining ammonium carbamate is substantially removed, and a third gas mixture thereby formed is processed to form a dilute aqueous ammonium carbamate solution. The remaining urea containing liquid phase is concentrated by evaporation in an evaporation zone. The evaporation zone includes a shell and tube heat exchanger wherein the urea containing liquid phase to be concentrated is introduced into the tube side of a first end of said exchanger, the second gas mixture is introduced into the shell side near the second end of said heat exchanger, and the dilute aqueous ammonium carbamate solution is fed into the shell side of said heat exchanger at a point between the first and second ends. Condensate formed in the shell side of the heat exchanger is discharged therefrom at the first end. In this manner, the heat of condensation of the condensing second gas mixture is transferred to the urea containing phase to be concentrated in the evaporation zone.
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