| 摘要 |
An improved isobaric double-recycle process is disclosed for synthesizing urea with the formation of ammonium carbamate as an intermediate, comprising effecting reaction between ammonia and carbon dioxide at high NH3:CO2 molar ratios, a heat-treatment of the synthesis product at substantially the same pressure as that of the synthesis step and in the presence of a stripping gas, and two distinct isobaric recycles of the residual substances and of the substances in excess released from said synthesis product, said process being characterized in that: (a) said heat-treatment of the synthesis product is carried out in two consecutive stages which are isobaric or substantially isobaric with respect to the synthesis step, in the first of which stages said synthesis product is heated, whereby substantially all the residual ammonium carbamate is decomposed and the decomposition products are displaced together with part of the excess NH3, while in the second stage the remaining part of the NH3 excess is displaced by supplying supplemental heat and by injecting thereto a CO2 stream; and (b) the gas phase stripped in the first stage is immediately recycled to the synthesis step and the gas phase stripped in the second stage is subjected to a condensation and to a residual gas purge and then recycled, in the liquid state, to the synthesis step. The synthesis temperature may range from 170 DEG to 205 DEG C., the synthesis pressure from between 100 and 250 Kg/cm2, and the NH3:CO2 molar ratio during the synthesis from 2.5:1 to 10:1. Suitable apparatus for carrying out the process is also disclosed. |
