FIELD: production of inorganic compounds. SUBSTANCE: urea is produced by ammonia-carbon dioxide interaction in reaction space, reaction mixture containing urea, carbamate, and free ammonia in aqueous solution. In decomposition stage, carbamate in reaction mixture partially decomposes and free ammonia in aqueous solution is partially separated in the form of first stream including vapors of ammonia and carbon dioxide, and second stream is also formed consisting of urea and rest of carbamate in aqueous solution. The first stream is partially condensed to form stream of the first part of carbamate in aqueous solution which is recycled into reactor. Stream of urea and rest of carbamate in aqueous solution is introduced into urea-isolation section wherein urea is separated from the rest of carbamate to form second part of carbamate in aqueous solution which is also treated to partially decompose carbamate. In this step, second stream of ammonia vapors and carbon dioxide with impurities is separated and partially condensed to form third stream of carbamate in aqueous solution that is also recycled into reactor. Decomposition of the second carbamate solution stream is carried out at pressure equal to pressure in reaction space. Urea production plant comprises reactor; first desorber for partially decomposing carbamate in reaction mixture obtained from reactor; means for partially condensing vapors from desorber and recycling the first carbamate solution stream into reactor; urea isolation section wherein second carbamate solution stream is formed. In addition, plant contains second desorber for partially decomposing carbamate in the second stream from urea isolation section. Resulting vapors are directed into additionally installed condenser that is connected with reactor to provide recycling third part of carbamate in aqueous solution into reactor. Upgrade of plant consists in that reaction mixture from reactor is fed into the first desorber for partially decomposing carbamate and partially separating free ammonium in aqueous solution contained in reaction mixture followed by at least partial condensation of vapors removed from the first desorber in condenser and recycling first part of carbamate in aqueous solution into reactor. In this process, stream containing urea and rest of carbamate in aqueous solution from the first desorber is introduced into urea isolation section to separate urea from second part of carbamate in aqueous solution. In addition, second desorber is installed. EFFECT: increased purity and yield of product. 14 cl, 3 dwg, 3 ex
