Process for the preparation of urea

In a process for the preparation of urea from carbon dioxide and an excess of ammonia at an elevated temperature and pressure in a urea synthesis zone to form an aqueous urea synthesis solution containing urea, ammonium carbamate, and excess ammonia, and thereafter:

• heating and stripping said urea synthesis solution in a stripping zone at an elevated pressure with a first stripping gas selected from the group consisting of carbon dioxide, ammonia, inert gas, and mixtures thereof, thereby decomposing ammonium carbamate and separately removing a stripping zone off-gas containing ammonia, carbon dioxide, and water vapor from a first urea solution still containing residual ammonium carbamate;
• at least partially condensing said stripping zone off-gas in a first condensation zone to form a first ammonium carbamate solution;
• introducing said first urea solution into a decomposition zone at reduced pressure relative to said stripping zone wherein a further portion of ammonium carbamate is decomposed and separately removing a decomposition zone off-gas containing ammonia, carbon dioxide, and water vapor, from a second urea solution;
• further processing said second urea solution in a further processing zone to form a concentrated urea solution or solid urea product;
• and
• condensing said decomposition zone off-gas in a second condensation zone to form a second ammonium carbamate solution;
• the improvement comprising:
• introducing a portion of said second ammonium carbamate solution into a desorption zone maintained at a pressure of between about 2 and 40 bar wherein a desorption zone off-gas more concentrated with respect to ammonia and carbon dioxide than said second ammonium carbamate solution is separately removed from a residual liquid phase;
• condensing said desorption zone off-gas into a further portion of said second ammonium carbamate solution in a third condensation zone maintained at a pressure between about 2 and 40 bar to form a concentrated third aqueous ammonium carbamate solution;
• and
• recycling said third ammonium carbamate solution to said urea synthesis zone.