GB1001188A - A process for the preparation of urea - Google Patents

Abstract

<FORM:1001188/C2/1> <FORM:1001188/C2/2> In a process for preparing urea from ammonia and carbon dioxide at elevated temperature and pressure, the reaction mixture discharged from the urea synthesis space and comprising urea, ammonium carbamate, excess ammonia and water, is subjected to pressure reduction whereby ammonium carbamate is decomposed with liberation of ammonia and carbon dioxide gases which together with water vapour are separated from the liquid phase and condensed to form an ammonium carbamate solution or suspension which is recycled to the urea synthesis space, and at least part of the recycled solution or suspension is formed by producing a first off-gas containing ammonia, carbon dioxide and water vapour by adiabatic expansion of the liquid phase and separating the off-gas from the liquid phase, heating the remaining liquid phase to liberate a second off-gas containing further ammonia, carbon dioxide and water vapour, and then combining and condensing together the first and second off-gases. In the embodiment of Fig. 2 reaction mixture under pressure from autoclave 1 is expanded via valve 2, heated in heater 3, gas and liquid phases are separated in separator 4, the liquid expanded through a valve 9, gas is separated in a separator 70 and passed to a condenser 8. The liquid is heated in a heater 6, passed to a separator 7 where a urea solution is obtained and removed through a valve 12, and gas is passed to condenser 8. The condensate formed in condenser 8 is pumped to a condenser 5 in which a higher pressure prevails, and gas from separator 4 also is passed to condenser 5. Condensate from condenser 5, consisting of ammoniacal ammonium carbamate solution is pumped back to autoclave 1. In another embodiment (Fig. 3) a separator 40 is inserted before heater 3 in the first pressure stage.