FIELD: chemistry.
SUBSTANCE: invention relates to a method for combined synthesis of urea and melamine. In this method, urea is obtained in a urea synthesis apparatus having a urea synthesis section at high pressure, where an aqueous solution containing urea, ammonium carbamate and ammonia is obtained, and a urea extraction section at low pressure, and melamine is obtained in the melamine synthesis apparatus whose exhaust gases are by-products of the melamine synthesis process and come out of this apparatus at medium pressure and are recycled into the urea synthesis section at high pressure. The method involves the following steps: feeding at least a portion of said aqueous solution containing urea, ammonium carbamate and ammonia coming from the urea synthesis section into a processing section at medium pressure of the urea synthesis apparatus in order to extract ammonium carbamate and ammonia contained therein; dissociation of said portion of aqueous solution containing urea, ammonium carbamate and ammonia in the processing section at medium pressure to obtain aqueous solution of urea and a vapour phase containing ammonia, carbon dioxide and water; feeding said aqueous solution of urea obtained at the dissociation step in the processing section into a decomposition apparatus of the urea extraction section operating at given low pressure; decomposition of said aqueous solution of urea in the decomposition apparatus of the urea extraction section to obtain concentrated urea solution and a second vapour phase containing ammonia, carbon dioxide and water; condensation of said second vapour phase in the condenser of the urea extraction section which is linked to the decomposition apparatus to obtain a recycle aqueous solution of ammonium carbamate; feeding said gases from the melamine synthesis apparatus and said recycle ammonium carbamate solution into the condensation unit of the processing section at medium pressure of the urea synthesis apparatus; condensation of said exhaust gases and recycle aqueous carbamate solution in the condensation unit of the processing section at medium pressure to obtain concentrated aqueous carbamate solution; and directing said aqueous carbamate solution for recycling into said urea synthesis section at high pressure. Disclosed also is a combined apparatus for producing urea and melamine.
EFFECT: invention increases production capacity of apparatus for producing urea while simultaneously ensuring high degree of conversion of carbon dioxide to urea.
21 cl, 2 dwg
