| 摘要 |
<p>A process for the displacement, at 80 to 250 kg/cm<2>, of the NH3 excess contained in urea solutions having a temperature of 150 to 230 DEG C and coming from a former stripping (isobaric with the synthesis of urea), performed in the presence of NH3, wherein said solutions contain also CO2, in amounts lower than 25% b.w. with respect to urea, and wherein said displacement is performed in countercurrent with CO2, within a falling-film heat-exchanger containing a tube bundle (CO2 stripper), isobaric with said synthesis, the amount of ammonium carbamate in the solution leaving the bottom of the CO2 stripper being substantially equal to the amount of ammonium carbamate in the solution entering the same stripper, characterized in that: a) the global NH3 : CO2 ratio, in the solution fed to the CO2 stripper, is from 2 to 8 b.w.; b) heat is substantially supplied only to the uppermost portion of the tubes of the CO2 stripper, wherein the ratio between the surface of the lowermost portion and the surface of the uppermost portion of the tubes is at least 1 : 1 and wherein the ratio between the residence time of the solution to be stripped in said lowermost portion and the residence time in said uppermost portion is at least 1 : 1, said residence time in the lowermost portion of the tubes being at least 3 s and said residence time in the uppermost portion being at the most 3 s.</p> |
