Procédé de recyclage de l'anhydride carbonique et de l'ammoniac dans la synthèse de l'urée

摘要

In the synthesis of urea from ammonia and carbon dioxide at elevated temperature and pressure: (a) at least part of the unreacted gases is absorbed in a dilute aqueous solution of ammonium carbamate and ammonia at a first pressure which is lower than that in the reactor; (b) ammonia, carbon dioxide, and water are driven off from this solution at a second pressure, which is higher than the first but lower than that in the reactor, to leave a solution which is recycled as the dilute aqueous solution of (a); (c) the gases are condensed and recycled to the reactor. The reactor may be operated at 150-300 atmos. and 150-200 DEG C., the reaction mixture then passing through an expansion valve to a decomposer at 80-140 DEG C. to decompose most of the carbamate in the product. The gases are separated from the melt at 1-20, preferably 3-6 atmos., and 80-140 DEG C., the latter being worked up to urea and the former absorbed in an aqueous solution of ammonium carbamate and/or carbonate, and ammonia at substantially the same pressure and at 20-80, preferably 40-60 DEG C. The carbamate solution from the absorber is preheated to about 120-160 DEG C. and passed to a column from which ammonia, carbon dioxide, and a little water is driven off at the head and condensed under pressure, e.g. at 30-70 atmos. and 65-110 DEG C. Fresh ammonia may be added if the molar ratio NH3: CO2 is less than 3.5: 1. Water separation may also be effected in part of the column. The condensate is pumped back to the reactor. The solution from the base of the column is recycled in greater part to the absorber, the remainder being distilled to remove excess water, the ammonia and carbon dioxide present being returned to the absorber. If a smaller recovery of gases is sufficient, higher pressures are used in the decomposer, separator, and absorber, and the final distillation stage is omitted, or a lower decomposition temperature is used which results in a product of lower biuret content. Alternatively, two or more decomposers may be used to give minimum biuret content. In a further modification, particularly with a high feed ratio of NH3: CO2, e.g. 6: 1, the melt from the reactor is first expanded to 30-70 atmos. and part of the excess ammonia separated, together with carbon dioxide and water, and sent direct to the condenser for recycle to the reactor.